Load cell application manual

GLOBAL Weighing/ 1 Marketing information Load cell application manual GLOBAL Weighing / 2 LOAD CELL APPLICATION MANUAL 1 GENERAL ASPECTS OF WEIGHING 1.0 Principle of electronic weighing 5 1.1 Mounting load cells 6 1.1.1 Mounting compression load cells 1.1.2 Mounting tension cells in principle 1.1.3 Mounting beams in principle 1.2 Stability and statically (un)defined systems 15 1.2.1 Statically undefined support 1.2.2 Stability of an object on load cells 1.2.3 Horizontal natural frequency and restoring force 1.3 General recommendations on the design of an electronic weighing installation 21 1.3.1 Design criteria 1.3.2 Stiff and rigid foundation 1.3.3 Traffic 1.3.4 Installation of vessels 1.3.5 Load cell selection 1.3.5.2 Installation of mounting kits 1.3.5.3 Protection of load cells against high temperature 1.3.5.4 Protection of load cells against overload 1.3.5.5 Protection against dynamic overload 1.4 Constraining 39 1.4.1 Using constrainers 1.4.2 Constraining of a suspended object 1.4.3 Orientation of the constrainers 1.4.4 Types of constrainers 1.4.4.1 MiniFLEXLOCK (general) 1.4.4.2 Rigidly clamped struts 1.4.4.3 Flexbeams 1.4.4.4 Pivoting rod 1.4.4.5 Rocking pin 1.4.5 Types of stops 1.4.5.1 Horizontal stops 1.4.5.2 Vertical stops (lift- off- protections) 1.5 Disturbing influences 58 1.5.1 Environmental influences 1.5.1.1 Wind forces 1.5.1.2 Heat and heat transfer 1.5.1.3 Freezing environmental conditions (ice, snow) 1.5.1.4 Dust, rain 1.5.2 Friction 1.5.3 Vibration, shock loading GLOBAL Weighing / 3 1.7 The weighing result 71 1.7.1 Terminology for load cells 1.7.1.2 Load cell and weighed installation 1.7.2 Influences from the construction 1.7.3 Approved installations 1.7.3.1 W&M regulations 1.7.3.2 Weighbridges 1.7.4 Standard accuracy: non- W&M installations 1.7.4.1 Installation specification 1.7.4.2 Use of the weighing installation 1.8 Installation and commissioning 80 1.8.1 Mechanical installation 1.8.2 Electrical installation 1.8.3 Calibration 1.8.3.1 Calibration of vessels of more than 5 tons 1.8.4 Corner point adjustment 1.8.5 Load cell check 2 MOUNTING THE LOAD CELLS 88 2.1 Mounting the compression load cell PR 6201 89 2.1.1 Mounting kit PR 6145 2.1.2 MiniFLEXLOCK PR 6143 2.3 Mounting the ultra flat PanCake load cell PR 6251 95 2.4 Mounting load beams 96 2.5 Mounting the S- type load cell 98 2.5.1 Mounting kits PR 6041/30, .../40 2.5.2 MiniFLEXLOCK PR 6043/30, .../40 2.5.3 Standard mounting kits for the tension load cell PR 6246 2.7 Mounting the compact load cell PR 6211 104 2.7.1 Mounting kits for the small type (30kg...300kg) 2.7.1.1 Mounting kit PR 6011/00 2.7.1.2 Rubber mounting kit PR 6011/03 2.7.1.3 MiniFLEXLOCK PR 6011/20 2.7.2 Mounting kits for the big type (500kg...10t) 2.7.2.1 Mounting kit PR 6011/10 2.7.2.2 MiniFLEXLOCK PR 6011/30 2.7.2.3 SeismoFLEX PR 6011/40 2.8 Accessories for the installation 112 2.8.1 Cable junction boxes 2.8.1.1 Plastic cable junction box PR 6130/08 2.8.1.2 Universal cable junction box PR 6130/60S, .../68S 2.8.2 Installation cable PR 6135 (PR 6136) 2.9 Constraining devices 115 2.9.1 Horizontal constrainers PR 6152/02 2.9.2 Constrainer PR 6143/80, .../83 GLOBAL Weighing / 4 3. TANK WEIGHING 118 3.1 Overview 118 3.2 Application examples 119 3.2.1 Some hints for installations with pivots 3.2.2 Installations with load cells only 3.4 Pipes, bellows ... 120 3.4.1 Pipes 3.4.1.1 Influences of stiff pipe connections 3.4.1.2 Calculation of the pipe stiffness 3.4.1.3 The constraining effect of pipes 3.4.2 Bellows 3.4.2.1 Influences of gas pressure 3.4.2.2 Influence of vertical bellows 3.5 Level control using pivots 128 3.5.1 3.5.1.1 Standardized pivots 3.5.1.2 Mounting hints 3.5.2 Calculation of an I beam pivot APPENDICES 139 A Alphabetic index 139 GLOBAL Weighing / 5 1.0 Principle of electronic weighing Definition. Weighing is the determination of mass. To weigh electronically an industrial object, this object is put on load cells. The load cells transform the weight of the object into an electric signal, which is led to an electronic measuring apparatus by means of an electric cable. Here the weight can be indicated, printed, and used for automatic control of an industrial process. Fig. 1.0-1 Main parts of a weighing system General remarks to the design of the weighing installation · Also after placing the load cells the object must remain a stable and reliable part of the industrial installation. · Only the weight of the object and no other vertical force shall flow through the load cells. · Vertical load cell position and simple mounting parts assure that this force flows through the primary axis of the load cell. · Parasitic vertical forces must be avoided or made very small. Examples are · friction of the object to the surroundings · forces caused by gas pressure · elastic forces of e.g. pipe connections · wind GLOBAL Weighing / 6 1.1 Mounting load cells Load cell mounting is the base for accurate weighing in the industrial surroundings, particularly for measurements with W&M installations. This chapter describes in the first place various types of bearings and their properties from the mechanical point of view. Afterwards the mounting principles for compression load cells, tension load cells and beams are shown in general. Load cells can be regarded as the bearings of a vessel. Usually the different types of bearings are classified according to their degrees of freedom. There is a number of possible arrangements, but only 3 methods are applied for load cell mounting. Each method is described with its advantages and disadvantages. The description of these methods shall enable you to criticize an installation and to choose an appropriate load cell for every purpose. Fig. 1.1-1 Mounting method 1: articulated column - examples: PR 6201 PR 6241 with mounting parts PR 6041/31S characteristics - spherical top and spherical bottom - load cell transfers only the weight, neither side loads nor momenta = measurement is unaffected by disturbing side forces and momenta - proper constraining is absolutely necessary to keep the system in a stable position especially if the centre of gravity is above the plane of the load cells - depending on the load cell construction restoring forces can be expected - highest measuring accuracy - principle usually applied for load cells in W&M installations or where high accuracy is required Fig. 1.1-2 Mounting method 2: articulated bearing - example: PR 6211 characteristics - spherical top or spherical bottom - load cell transfers side forces and weight, but no momentum = influences on the measurement can be expected - constraining necessary to eliminate the influence of the side forces on the result - medium accuracy GLOBAL Weighing / 7 Fig. 1.1.-3 Mounting method 3: load cell clamped - examples: MP 49 - load cell fixed with bolts to both the foundation and the object - high momenta caused by this mounting method - load cell transfers weight, side forces, and momenta measurement is influenced by all disturbing forces and momenta - no constraining necessary - lowest measuring accuracy 1.1.1 Mounting compression load cells in principle Fig. 1.1-4 Main installation parts for a compression type load cell 1. Standard mounting parts are advised: a special ‘load button’ and a special ‘bottom plate’. They allow - to have the most optimal material for the contact with the load cell top and the load cell bottom (standardized conditions) - easy exchange in case of wear - a standardized height between object and foundation 2. The ‘electric shunt’ as a protection against possible heavy stray currents in the structure has to be connected (further details are described in chapter 1.3) 3. ‘Shims’ are used in case of more than three load cells under the object; their purpose is to distribute the load evenly over all load cells (refer to ‘statically undefined systems’) 4. Make sure that both, the weighed object and the foundation, are rigid and stiff as to take the actual loads during the operation 5. Avoid vertical force shunts: pipes, heavy cables, contact to the surroundings etc. 6. Observe the external influences (wind, temperature) on the weighed object 7. Every object has to be properly constrained. Remember to install the lift-off-protection. GLOBAL Weighing / 8 8. Vertical position of the load cell The vertical position of the load cell can be adjusted by a horizontal movement of the foundation plate. The vertical position can be measured directly with a spirit level. Indirectly it can be checked by checking the square angle between foundation plate and load cell As to reach the best measuring results the following angles should preferably not be surpassed during the installation procedure: upper loading plate α < ±2° foundation plate β < ± 0.5° load cell tilt γ < ± 1° Those angles are also limits for the installation in use. Fig. 1.1-5 Permissible inclination 1.1.2 Mounting tension load cells in principle A safety hint If a break in suspension, support, load cell, or mounting part etc. represents a hazard to the life and health of men and animals, or if goods may be damaged, additional safety devices have to be provided. When taking the appropriate standards into consideration, the dimensions of all mounting and structural elements have to be calculated so that sufficient overload capacity is ensured for the design load. In particular, upright weighing objects have to be safeguarded against the weighing installation turning over or being shifted. Measuring principle Only the weight of the object should flow through the tension cells. For this reason, make sure that disturbing vertical forces caused by e.g. stiff pipe connections, wind, gas pressure, dirt are avoided. - the tension cells should be mounted in the upright position - (the name stands upright) - the cell should be mounted vertically within ±2° - the cell has to be mounted between two cardanic pivoting points - a torque around the axis should be avoided GLOBAL Weighing / 9 The tension load cells must be mounted in such a way that the movement and bending of the cable do not influence the weighing result. The correct positions are shown in the sketches above. Fig. 1.1-6 Mounting PR 6206 Fig. 1.1-7 Mounting the S type Remarks 1. in the wrong position corrosion could be feared by collected water, e.g. from rain, if the load cell is not made of stainless steel 2. especially in the case of a light object, the cable weight or the cable rigidity can influence the vertical position of tension cell. So, provide a good support of the cable. A tension cell is designed to measure only the force flowing through its ‘primary axis’. The weight G acts in vertical direction, therefore the tension cell has to be mounted vertically (if possible within ±2). This is not critical. A deviation of 2 changes the ‘span’ of the measure ment only 0.06%! (Explanation: with no friction in the pivots, the force through the primary axis is G/cos ) Different mounting methods In general two different ways to mount tension cells can be advised: - mounting with pivoting points (use of standard mounting kits) - mounting with long rods (this system is a very cheap alternative but needs a lot of free space) Method 1: tension load cells mounted with pivoting points The mounting should be done in such a way that one cardanic pivoting point is provided at the top end and another one at the bottom end of the tension cell (cardanic means that the tension cell can swing in all directions). This is to prevent that bending momenta or side forces act on the tension cell. Fig. 1.1-8 Installation of tension load cells = the danger of side forces on the load cell caused by thermal expansion of the object is eliminated = external horizontal forces are kept away from the tension cells. If necessary, those forces can be taken up by horizontal constrainers. GLOBAL Weighing / 10 Examples for such pivoting points are swivel bearings (fig. 1.1-9) and to a certain degree pairs of spherical washers (fig. 1.1-10). Fig. 1.1-9 Swivel eye Fig. 1.1-10 Mounting with spherical washers If the horizontal movement is not constrained, make sure that the part between the two pivots never touches a fixed point. This can cause extreme side forces on the load cell and result in its damage. Fig. 1.1-11 The load cell must not touch its surroundings Beside external forces on the load cell torsional momenta must be avoided. By external influences e.g. a stirring device a torsional momentum could occur on the tension cell. Mounting parts and constrainers can take care of this. Method 2: Long rod mounting Very long rods are flexible to transversal and rotational movements. This property can sometimes be used for a cheap mounting solution. The vessel is hung up with the help of long rods. The length between fixation points at the ceiling and the object respectively has to be more than 1.600mm. Two threaded rods of at least 900mm length are screwed into the load cell and locked with a spring washer. The material of these threaded rods has to have a tensile strength of at least 450 N/mm 2 (that is e.g. class 6.8). The fixation at the foundation and at the object has to be done with two pairs of spherical washers (DIN 6319) to avoid undesired pre-stresses. Also therefore, the nuts have to be tightened strongly only after the vessel is hanging in its final position. For security one can provide splitpens at the two ends. GLOBAL Weighing / 11 REMARK Half way in between the foundation and the object fixation points the bending momentum in the rod will always remain zero. Therefore this is the best position for the tension load cell to avoid bending influences on that cell. Fig. 1.1-12 Calculation for long rod mounting This mounting principle can easily be described, together with its reaction forces and moments. L 64 D E 12 = x F = k 3 4 • • • •π The rod stiffnesses cause here some disturbing effects on the tension cell. The bending stiffness of a steel rod with a length L and a circular diameter D can be calculated from This is the force (in newton), necessary for a lateral displacement of x mm. L D G = M = k 4 t t • • π φ The torsional stiffness of a steel rod with a length L and a circular diameter D is This is the torque around the vertical axis, necessary for a torsion angle of 1 radian. The following table shows both stiffnesses for several rod lengths at the standardised rod diameters for the different tension cell types. L = 1500 mm L = 2000 mm k k t k k t M12 0.7 N/mm 117.5 Nm/rad 0.3 N/mm 88.1 Nm/rad M16 2.3 N/mm 371.4 Nm/rad 1.0 N/mm 278.5 Nm/rad M20 x 1.5 5.7 N/mm 906.7 Nm/rad 2.4 N/mm 680.0 Nm/rad M30 x 2 28.8 N/mm 4,590 Nm/rad 12.2 N/mm 3,443 Nm/rad GLOBAL Weighing / 12 Examples for long rod mounting A vessel is normally suspended by three tension cells. The examples below show the effects for two situations of vessels moving. Case a: the vessel moves only sideways over 10 mm distance Case b: the vessel rotates around its axis with an amplitude of 10 mm at the radius R. Fig. 1.1-13 Vessel with long rod mounting Example 1 Vessel 1 gross weight 400 kg 3 pcs PR 6206/22 placed at a radius of R = 400 mm diameter of the rod D = 12 mm length of the rod L = 1500mm case a side force F = 7.3 N no torque case b side force F = 7.3 N torque M = 3 Nm Example 2 Vessel 2 gross weight 3000 kg 3 pcs PR 6206/13 placed at a radius of R = 600 mm diameter of the rod D = 16 mm length of the rod L = 2000mm case a side force F = 10 N no torque case b side force F = 10 N torque M = 5 Nm Example 3 vessel 3 gross weight 15000 kg 3 pcs PR 6206/53 placed at a radius of R = 1100 mm diameter of the rod D = 30 mm length of the rod L = 2000 mm case a side force F = 120 N no torque case b side force F = 120 N torque M = 32 Nm Conclusion In all examples the long rods are good enough and do not disturb the measurement. GLOBAL Weighing / 13 1.1.3 Mounting beams in principle Avoid the application of more than three beams to suspend a stiff object. In such a case the installation becomes statically undefined: it not certain that the load is equally distributed. Maybe even not all the beams carry the object. Overloading and subsequent damage to some of the beams can be the result. In case of a beam with a built-in-overload-protection a measuring error occurs. Fig. 1.1-14 Load beam (principle diagram) Further mounting directives 1. The foundation plate for fixing the beam must have a smooth, clean, flat surface and must be horizontal within ±0.5°. A deviation causes a sensitivity error, which, however, can be compensated by the measuring instrument. 2. The foundation must be rigid enough to avoid yielding under load. Change of the horizontallity by yielding of more than 1° between zero load and full load causes an additional non-linearity error of more than 10 -4 . 3. Use the prescribed mounting parts to suspend the object. However, the suspension rod might be longer if there is enough room for it. 4. Provisions must be present to adjust the vertical position of the suspension rods within J < ±2° to avoid big side forces on the beam. (F H = F G · tanJ, or F H = 0.035 · F G , J=2°; REMARK: This is easier for longer suspension rods.) 5. Grease all pivoting points to avoid too much friction (or even sticking by corrosion). 6. If the object is suspended with only one beam, adequate measures must be taken to avoid torsion of the object. 7. Provisions for lifting the object during installation must be present. Also provisions for suspending calibration masses. 8. Protect beams and especially the bellows against mechanical damage (falling tools etc.). 9. Avoid parasitic loads on the beam (e.g. on the bellows). Fig. 1.1-15 Load distribution (theory) Fig. 1.1-16 Choice of the mounting bolts GLOBAL Weighing / 14 The sketches on the previous page illustrate how to mount shear beams and advise the provisions to be taken. Some interesting details should be observed: - the mounting bolts S 1 and S 2 are not equally loaded - standard screws (4.6 and 5.6), which are widely used for constructions, are insufficient for mounting this shear beam property class 8.8 means conventional limit of elasticity ±640 N/mm 2 (no plastic deformation of the bolt up to this limit) tensile strength ±800 N/mm 2 (no destruction of the bolt up to this limit) - the nut (or the material) that acts together with the screw has to have the same properties nut: property class 8 material: low alloy steel (conventional limit of elasticity ±640 N/mm 2 ) GLOBAL Weighing / 15 1.2 Stability and statically (un)defined systems Combining load cells to weighing systems („scales“) is usually quite easy if a few points are observed: the stability of the installation, (¬ this chapter) the fixed position of the installation. (¬ chapter 1.4) A system with three load cells is always as stable as a three-leg-chair. A system with more than 3 load cells, e.g. 4 or even more load cells, must be shimmed to ensure an even load distribution. The term stability depends on the equilibrium of the installation. Definition EQUILIBRIUM - stable equilibrium system returns to its centre position after slight deviations Fig. 1.2-1 Stable equilibrium - neutral equilibrium system is in equilibrium in every position Fig. 1.2-2 Neutral equilibrium - unstable equilibrium the system does not return to its centre position after a slight excitation Fig. 1.2-3 Unstable equilibrium How can the state of the equilibrium of a weighing installation be detected? At first the position of the centre of gravity must be known: under or over the plane of the load cells. - definition centre of gravity In this point you can imagine the whole load of the weighing object concentrated. - definition plane of the load cells This is the plane directly above the load cell tops. Fig. 1.2-4 Some definitions GLOBAL Weighing / 16 1.2.1 statically undefined support Consider the two situations described below: A. Imagine you sit on a four-leg chair on an uneven floor. Probably only three legs carry your weight. (Or even sometimes only two!). The result is a rocking chair. Such a system is called a statically underdefined installation: the system has freedom to move (it can waggle). Problem: Only some of the load cells carry the weight. This can cause overloading. B. Imagine a vessel supported by three load cells and constrained with more than three constrainers in one plane. This system is completely fixed and has no freedom to move. Furthermore, the system can get clamped causing the constrainers to transfer also vertical forces. Such a system is called statically overdefined. Problem: Vertical force shunts can result in measuring errors. Both cases A and B can be collected under the term „statically undefined„ meaning the forces at the supporting points are not predictable with the help of the standard equations. Since both situations A and B can cause problems, every effort should be taken to minimize their negative results. Waggling should be avoided since it can damage load cells and its mounting parts by hammering. Additionally, it causes unnecessary movement of e.g. pipe connections with subsequent possible zero error. The above described situation could be improved 1. by increasing the vertical flexibility at the supporting points. More flexibility makes the necessity of the alignment (adjustment of the height) less, or the alignment less critical. It does not matter if the flexibility is caused by a non stiff object or by the flexibility of the foundation. Examples: Open trays or platforms are flexible objects. Softness of the subsoil can cause a flexible foundation, however, this is also the case if the load cells are placed on steel beams which bend under load. Such an installation with a flexible support, however, can cause other problems, e.g. in stability and accuracy. This area is covered in chapter 1.3. 2. by shimming, i.e. adjusting the height by fitting thin metal sheets between load cells and weighed object. 1.2.2 Stability of an object on load cells In principle an object on three or more load cells and with three constrainers, as described in chapter 1.4, gives a reliable, stable construction. The object could only collaps under the following improbable circumstances: • A foundation point cannot bear the vertical load. This is of course a very evident case which has not to be explained further. There is only one reason for it: a design error • Constrainers or their fixation points cannot take the actual horizontal loads. By breakdown, the plane of the load cell supporting points could make a transversal movement or a rotation. In an extreme case this can cause capsizing of the load cell. However, correct dimensioning of the constrainers overcomes this problem. Fig. 1.2-5 Possible object movement in case of wrong constraining GLOBAL Weighing / 17 • The weighing construction itself is not rigid enough to take the weight, the internal and external forces without a big deformation. Then capsizing of the load cells could occur by two effects: 1) if the plane of the constrainers is not put in the plane of the load cells and there is a big movement between these two planes Fig. 1.2-6 Constrainer installed above the plane of the load cells 2) if the plane of the constrainers is deformed. Fig. 1.2-7 Plane of the constrainers is deformed This can happen only if the object is deformed under the horizontal forces. = design error Conclusion: All the effects, described above, normally must not occur. It is the task of the mechanical designer to assure that his construction is strong and rigid enough to take vertical loads and horizontal forces. (If he fails, it is not the responsibility of GLOBAL Weighing who can only be made responsible for the measuring properties of the weighing installation.) 1.2.3 Horizontal natural frequency and restoring force Chapter 1.1 explained that the object should be free to move in the horizontal direction if no constrainers are installed. This is realized by pivoting or rolling constructional elements, which make the weighing installation a horizontally swinging system. Fig. 1.2-8 Stable equilibrium of suspended objects Suspended objects are in a stable equilibrium if their centre of gravity is below their suspension plane. In all stable cases the object can swing: the object is moving as indicated in fig. 1.2-9 and tries to return to its lowest position. The swinging object has a natural frequency of f 0 . If the object is moved out of its centre position over a distance of x, a restoring force F R tries to move it back to that centre position. These properties depend on the dimensions of the construction. GLOBAL Weighing / 18 Fig. 1.2-9 Restoring force of a suspended object - a: pendulum length - x: deflection The horizontal deflection x is counteracted by a horizontal force F R which can be calculated with the equation a x g m g m = FR • • ≈ • • α sin The object swings with a natural frequency f 0 a g 2 1 = f 0 • •π The shorter the pendulum length a, the higher the natural frequency, i.e. the better the stability of the installation. The 'pendulum length' is the distance between the upper and the lower pivoting point in case of two or more tension cells. pendulum length [mm] natural frequency [1/s] 150 1.30 200 1.10 500 0.70 1000 0.50 2000 0.35 5000 0.23 Example: With a pendulum length of a = 500 mm a force of F R is necessary to bring an object of 3t 12 mm out of its centre position. N 720 = s m 9.81 kg 3000 mm 500 mm 12 = F 2 R • • GLOBAL Weighing / 19 Supported objects (like compression load cells) can also work in a stable or unstable region. Fig. 1.2-10 Different types of equilibrium with supported load cells Fig. 1.2-11 Restoring force for a compression load cell A horizontal deflection is counteracted by a restoring force F R a a - R a x g m = F b R • • • Example: PR 6201 Datasheet states F R = 0.5% • F L per mm deflection This means for an installation with three load cells F R = 250 N/mm The object swings with a natural frequency f 0 in the horizontal direction a g a a - R 2 1 = f b 0 • • •π REMARKS 1. There is a general relationship between natural frequency and restoring force F R F g x f F G 2 0 R • • ≈ So the conclusion is that this force is smaller if the natural frequency is lower. 2. The object only returns to the centre position if the restoring force is bigger than the friction forces. 3. Constraining could be necessary if the disturbing horizontal forces are bigger than the restoring force. 4. With compression load cells the restoring force is bigger for a larger radius of the spherical segment. This had to be limited because a too big restoring force could lead to a not- allowed side force on the load cell. GLOBAL Weighing / 20 5. Under load the radius of the spherical segment is increased by deformation. This could lead to a bigger restoring force than specified in the datasheets. 6. In cases where the object can freely swing this moving can be used to check: - are there force shunts present? - is the friction small enough in the pivoting points 7. Swinging could be limited by ‘stops’ provided there is a stable equilibrium for the weighing system, and low friction in the pivoting points, and no contact between stops and weighing system during operation. GLOBAL Weighing / 21 1.3 General recommendations on the design of an electronic weighing installation The design of an electronic weighing installation covers two aspects: 1. the mechanical design influences the accuracy of the weighing 2. the cabling influences the weighing This chapters deals with both aspects. Using computers for the calculation of the stress applied to the constructions results in smaller machinery and lighter buildings if you compare them to constructions which were designed without the help of computers. Despite this fact the power of the machinery and the safety of the buildings remain unchanged or are even increased. As a general rule you could say that the strength of the materials is used in a higher degree. This means: - the cost for the construction is lower - in case of an emergency, the constructions have a lower safety factor 1.3.1 Design criteria The following statement is taken from one load cell operating manual: ‘The foundation and the steelwork for a weighing installation must be stable against the maximal expected load. It must be rigid enough to allow for an accurate measurement. Furthermore the foundation must be horizontal and plain below the load cells. The weight direction has to be vertical through the load cell as accurate as possible.’ These sentences and the like can be read throughout all our documentations for load cells and accessories. This chapter is to explain the meaning of these words. The design of machinery and buildings is done due to specific design concepts based on national and international regulations. All national standards for civil engineering require a construction to be resistant against the ultimate load. Definition ULTIMATE LOAD The design due to the ultimate load ensures that a machine or building does not break because of • fast burst • fatigue fracture • non permissible deflections • instability Most (inter)national standards refer to this design concept, e.g. ISO 18800. From the weighing point of view this concept is only a basic demand for each weighing system. The usual demand for a weighing system consists in another concept, called the ‘security from malfunction’ concept. To put it in other words, the machinery or a whole factory has to work all day long and to ensure the quality of their products. Furthermore, all dangerous and undesirable states of the production process have to be avoided such as standstill because of broken parts. During the last years a lot of different concepts has been developed: • design to ensure the safety of the workers • design so that the environment is affected as less as possible These considerations are quite general and abstract. They only inform you about the basic design concepts. The most important concept from the weighing point of view is the concept of ‘security from malfunction’: always ensure accurate weighing by proper design. The design rules given in this chapter are based on that concept. 1.3.2 Stiff and rigid foundation Definition RIGID foundation A rigid foundation does not deflect under load. The word RIGID describes a mechanical model since every real object loaded by a force deflects under it. There are differences in the degree of deflection. The recommendations are meant to assist you to ensure a design with the least deflection . GLOBAL Weighing / 22 The first important facility for a weighing system is a weighing frame (cp. fig. 1.3-1). It mechanically connects all load cells under a vessel etc. and ensures that they cannot move into different directions independently from each other. Since the load cells only transfer vertical compression forces the design has to ensure that in case of external forces the load cells cannot tilt. The constrainers take this task although they allow the load cell to move in case of thermal expansion. Fig. 1.3-1 Standard installation with two frames Where should the load cells be placed? Fig. 1.3-2 shows a first solution: the load cells are placed in the middle of the beams in the steelwork. Fig. 1.3-2 Solution 1: Load cells in the middle of long beams (not the best solution) Solution 1 (see fig. 1.3-2 above) As long beams are elastic they deflect under the load. Perhaps the load distribution is not even or the beams have different stiffnesses or ... This could result in clamped constrainers or tilted load cells or in a waggling installation with two load cells taking 90% of the whole load and the other two only 10%... The chance to run into problems with such a design is quite high. Fig. 1.3-3 Solution 2: Load cells placed on short beams (preferred installation) Solution 2 (see fig. 1.3-3 above) The load cells are placed on additional short beams. As the deflection rises with the third power of the beam length they do not deflection so much in this solution. An uneven load distribution is easier to compensate by shimming. For this reason solution 2 is to be preferred. GLOBAL Weighing / 23 But not only long beams can disturb your measurements because of their large deflections. Another source for troubles are weak floors. Avoid to place a weighing system on a floor which is quite weak. Try to find another solution by supporting the system from another floor below. (see fig. 1.3-4) Fig. 1.3-4 How to support a weighing system on the first floor A weighing system installed in an upper story of a building is influenced by a lot of different factors: 1. wind, wind induced movements, and vibrations of the building 2. machinery in a lower story can cause the building to vibrate at low frequencies 3. stiffness of the floor and the walls If possible such installations should be avoided. Fig. 1.3-5 Avoid weighing systems in upper stories Vessels supported by beams, which are not of equal stiffness, can tilt under the load. Therefore such an installation should be avoided. Fig. 1.3-6 Vessel supported by beams of different stiffness GLOBAL Weighing / 24 Another mistake in the design that is quite common is to interconnect several installations by putting them onto the same steelwork. Then the measuring result of one installation depends on the level inside the other vessel. For this reason, every vessel is recommended to be placed on a separate foundation without any interconnection to others. Fig. 1.3-7 suggests a design. Fig. 1.3-7 Every vessel stands independently There are a lot of ways to interconnect several vessels so that only a small number of wrong installations can be shown here. A common mistake is to place two vessels, which are loaded and unloaded separately, on only one supporting beam without additional columns in between. Fig. 1.3-8 Vessels interconnected through one soft beam (WRONG) Fig. 1.3-9 Avoid installation with a common supporting beam (WRONG) GLOBAL Weighing / 25 Fig. 1.3-10 Insufficient supporting columns (WRONG) Stiffening the steelwork Sometimes the systems is installed and does not work properly. The user asks our help and assistance to improve his installation. Usually, the steelworks can be improved by stiffening. To choose the right way of stiffening some details must be thought about: - how to stiffen the construction - where to place the stiffening beams In general there is a huge variety of methods to stiffen a construction: different shapes of beams, many possibilities how to place them ... This paragraph suggests two methods which have proven useful. Method 1: K-shape framework (fig. 1.3-11) Fig. 1.3-11 K shape framework (preferred) The K-shape framework has several advantages: - you can use rather small beams and achieve a high stiffness - the design allows traffic to pass under it - the design can be done in such a way that the beam supports directly the points where the load is induced Method 2: one single beam (fig. 1.3.12) Fig. 1.3-12 Single beam stiffening (not recommended) GLOBAL Weighing / 26 This method is quite easy because only one beam must be welded to the construction. However, there are certain disadvantages: - rather strong and heavy beams are needed - usually no traffic can pass below the construction - the buckling effect in the stiffening beam has to be observed Summary: The platform, the floor and the construction below the weighing object must be stiff and rigid to take all loads • compression or tension load (weight to be measured) • horizontal load (thermal expansion, displacement etc.) The next paragraphs describe some common error sources which are related to the design. Possible influences on the installation come from • moving vehicles and people on platforms which cause deflections and vibrations • motors on a weak platform or floor which transmits the induced vibrations • heavy machinery on the same platform which causes static deflections 1.3.3 Traffic Fig. 1.3-13 Traffic beside a vessel Fig. 1.3-14 Traffic above a suspended vessel You may say that this point is included in the chapter on the stiff floor. You are right. However, sometimes it is easier to identify an error when you have an idea what to look for. If there are failures in a weighing system, which only come up from time to time, the reason 'traffic' must be considered. 1.3.4 Installation of vessels Some general recommendations · Normally the load cell may be operated only up to temperatures up to 95°C (for exceptions see the relevant data sheets). Higher temperatures can destroy the cell. If the temperature in the weighing object or in the environments exceeds the a.m. value, the load cell should be protected: against conduction by heat protection plates and boxes against radiation by shields, screens · To achieve the highest measuring accuracy the load cell must be protected from high temperature changes. The allowed rate is 5K/h for W&M load cells and 15K/h for industrial types. Such effects may arise if you find wandering shadows etc. · The weighing object must be designed that wind is of no significance. Use lift-off-protections · The load cells must be protected from impact loads which are exerted by falling material or forklifters driving against the weighing object. The load cells can be protected by overload protections, like rubber springs · The weighing object must be free from vertical force shunts. The whole installation, like pipes and electrical cables, must be connected so flexible, that they do not exert a force on the weighing object. GLOBAL Weighing / 27 · No friction between weighing object and wall should disturb the weighing. Therefore the weighing object must stand free. · load distribution the weighing installation is equipped normally with three or four load cells 3 load cells - all cells bear the load (if possible: even load distribution) 4 load cells - higher stability than an installation with 3 load cells - uneven load distribution (needs shimming in almost every case) · use the adequate mounting kits the load cells must stay in vertical position (use spirit level) Installation drawing A drawing of the mechanical mechanical part of the installation with a clear view of: - load cell mounting - constraining - piping shall be made to be able to judge the measuring properties. The wall of the vessel must be so stiff that there is no bending under load. Fig. 1.3-15 Vessel with a stiff wall GLOBAL Weighing / 28 The construction of the weighing object with its surroundings must allow easy access to the process without in- fluencing it. Imagine a viewport in a vessel which can only be reached by ascending a ladder. The construction must ensure that this ladder is supported only by the surroundings of the weighing installation but not by the weighing object itself. Fig. 1.3-16 Observing the process In principle a weighed object has to be free from its surroundings to achieve an accurate measurement. However, in industrial applications there are often links between the object and the “outer world”. Examples are - pipes and tubes - pneumatic and hydraulic hoses - electric cables - bellows and slabs - dirt and stones in a gap between object and its surroundings In general these “links” have the following three effects: 1. An undefined part of the weight of the link acts as a disturbing force on the object. Mostly this is a constant force, which can be treated as a part of the dead weight. However, sometimes it can be variable, e.g. in the case of an electric cable with a changing position or in the case of a pipe with changing contents. If this force is not constant, it can introduce a non-reproducibility error. 2. The stiffness effect of the link. In principle this can give a span error and a zero error. The effect is described and calculated in detail in chapter 3.4 3. The friction effect of the link This leads to an undefined zero-error (non-reproducibility and hysteresis effect). The friction effect can be caused by friction in pivoting points, by friction of parts pressed against the object, or by internal friction in the material of hoses and the like. Fig. 1.3-17 Principle of installation for pipes GLOBAL Weighing / 29 Fig. 1.3-18 Avoid the influence of storage silos 1.3.5 Load cell selection An industrial vessel can be put on load cells or be suspended by tension cells or load beams. The sketch below gives a rough idea of the application areas of important GLOBAL Weighing load cell types. The load cells have nominal capacities between 10 kg and 300 t. Fig. 1.3-19 Programme overview The first step during the design phase is to decide if the weighing object should be suspended or stand upright. A suspended design usually offers a higher stability for the installation whereas an upright standing vessel can more easily be added to an existing concept. The second step is to decide how many load cells should be used. When choosing the number of load cells for your weighing installation keep the following arguments in mind: - Less supporting points require that strength and stiffness of both the construction and the foundation must be increased - If more than 3 load cells are used, the support is statically undefined. This fact makes it necessary to shim the load cells in such a way that the load distribution becomes even (cp. chapter 1.2). - If an object is suspended by tension cells or load beams and the centre of gravity is below the supporting points, it is sometimes possible to use less than 3 constrainers. The third step is to choose the nominal load L n of the applied load cell. The following aspects should be considered: - All load cells supporting the object have to be of the same nominal load. This is necessary for proper summing up all loads by the parallel circuiting of the load cells. - The maximum actual load at the supporting points is calculated under 1) extreme conditions (e.g. storm, overloading) 2) normal measuring conditions These load values are compared with the 'load limits' in the load cell specification. GLOBAL Weighing / 30 ¬ the extreme load may never exceed the limiting load L l ¬ if possible choose the nominal load L n higher than the maximum load under measuring conditions in order to be sure of all guaranteed load cell properties. The fourth step is the choice of the accuracy class. The purpose of the weighing installation defines the accuracy with which the measurements must be done. If the installation has to be approved by the local W&M authorities, you have to choose a load cell type which already got its test certificate. In other cases, when you have to fulfil a more or less clear accuracy desire of the customer, you must choose based on your experience. You could try to improve your own intuition on this subject by studying this load cell application manual. (refer to chapter 1.7) Selection in short - choose L n bigger than the maximum load during measuring - check, if disturbing forces never give a bigger load than L u - choose the load cell type for the desired accuracy - check, if the load cell output voltage is sufficient for the desired net scale Selection example Vessel on three load cells - technical data dead load 2.8t net weight 7.0t gross weight 9.8t - with even load distribution maximum load per load cell is 3.3t - extreme wind causes an extra load of 0.8t. So extreme maximum load is 4.1t. ([ lower than L u ) - total nominal load is 3 º 5t = 15t With the used PR 1592 indicator the minimum scale span is 20% so 0.2 º 15t = 3t. ([ this is lower than the net weight) - for batching without W&M requirements we choose PR 6201/53D1 As you see, for normal cases the selection is very easy... Fig. 1.3-20 load limits (according to VDE/VDI 2637) GLOBAL Weighing / 31 1.3.5.2 Installation of the mounting kits All load cells can easily be installed by using the specially designed mounting kits. The correct adaptation to a construction is shown in fig. 1.3-21. Especially supporting steel beams must be stiffened as shown. Furthermore the constrainer must be mounted in longitudinal direction of the steel beam: the side forces are only transferred in this orientation. Fig. 1.3-21 Gussets stiffen the supporting I beam 1.3.5.3 Protection of load cells from high temperature The first hint concerns the environmental temperature and heat sources next to the load cell. Most load cells can only be operated up to 95°C (exception: PR 6211LT may be operated up to 155°C). If higher temperatures are expected and load cells like PR 6201 are used, they must be protected from being overheated. Fig. 1.3-21 gives some advice. The heat protection shield is made of sheet metal and protects the load cell from heat radiation (direct sunlight, hot oven etc.). A heat protection plate is often made of ceramics and protects the load cell from a heat flow caused by a temperature difference. Fig. 1.3-22 Different heat protection devices 1.3.5.4 Protection from overload Another complication may arise from falling loads. The load cell specification contains the value L u , which tells the nominal load. Above this limit a zero shift may occur. The value L l is the limiting load. Above this value there could occur damage to all measuring properties. When selecting a load cell care must be taken that the nominal load of the load cell is big enough to prevent overloading in the weighing installation. Sometimes, however, this is not possible. E.g. if a very small measuring range, compared to possibly occurring big forces, has to be used. In such cases an overload protection can avoid damage to the load cell. Some load cells, e.g. PR 6211D1 with a capacity of 300kg or lower, have an overload protection already built-in. If the load cell is loaded by a weight bigger than 1.5 L n the measuring element touches the stop and cannot deflect further. This avoids the damage of the load cell. Fig. 1.3-23 shows the behaviour of this internal overload protection. GLOBAL Weighing / 32 Fig. 1.3-23 Load cell PR 6211 with built-in overload protection In figure 1.3-24 the principle of an external protection of a load cell against overload is shown. Fig. 1.3-24 Static overload protection with a prestressed spring The device of figure 1.3-24 makes use of a prestressed spring, which causes that under normal conditions (without overload) the platform is interconnected rigidly with the load cell. This is the big advantage of such a construction. However, the disadvantage is that it does not give a protection against dynamic overload. This is explained in chapter 1.3.5.5. Figure 1.3-25 gives the principle of a protection device with a free spring. Fig. 1.3-25 Principle of an overload protection with a spring GLOBAL Weighing / 33 With this construction there are some problems to take care of: - the platform is no longer a rigid element, but easily swings up and down and even sideways - if the spring is tall, this horizontal movement makes the load cell position unstable (the load cell can capsize) - with some spring materials the deflection under a constant load increases with time. In these cases it is difficult to adjust the clearance between the stop and the platform. One should be sure that the platform touches the stop under all conditions at say 140% of L n and it remains free under normal weighing conditions say up to 100% of L n . Fig. 1.3-26 Devices for overload protection Figure 1.3-26 suggests some spring elements for static overload protection. A rubber element is usually suitable up to capacities of 2t; the capacity of the element which consists of cup springs depends on the type of the cup springs. 1.3.5.5 Protection from dynamic overload If the load on a weighing installation is slowly changing, we speak of (quasi-)static loading. In that case we can take measures against overload as described above (chapter 1.3.5.4). However, if the load increases suddenly (e.g. by falling goods), the loading is called impact loading. This could damage thew load cells. The reason for this is that the very short impact loading causes a shock wave travelling through the platform to the interior of the load cell. This can even happen if there are measures taken against static overloading, because the impact time interval is too short for the protection springs to come into action. Therefore we have to take care that the shock wave cannot travel through the protection springs. Fig. 1.3-27 Falling mass In order to get a better understanding of the dynamic forces, we take a simple model and try to calculate its behaviour under dynamic loading. A mass m is falling from a height h on the platform (with mass M). We assume that the point of contact can be represented by a spring with a constant k C (see fig. 1.3-27). At the moment of collision the mass has the velocity v 0 h g v ⋅ ⋅ · 2 0 Then the mass becomes part of the vibrating system (consisting of mass m, platform mass M, and platform stiffness k C ) and makes a vibration movement of half a sine during half a period before it loses contact with the platform. The natural frequency of the vibrating system be a 0 and the time interval be At. 0 0 , ω π ω · ∆ + · t M m k c To make a practical estimation of the time interval, we can bring the formula in the following form h g x v x t ⋅ ⋅ ⋅ · ⋅ · ∆ 2 0 0 0 π ω GLOBAL Weighing / 34 Example For a height of h = 0.1 m and a deflection x 0 = 1 mm you get At = 1.6 ms. This value could be a good estimate for the impact time interval. In that very short time the springs under the platform must work to keep the impact away from the load cells. The protection devices described above deflect during that impact interval a distance x. The device with pre-stressed spring (fig. 1.3-24) with an excess load of e.g. 0.3 L n and a platform mass M = 0.4 L n the platform is accelerated by 2 5 . 7 4 . 0 3 . 0 s m g a · ⋅ · That results in a platform travel of mm t a x 015 . 0 ) ( 2 1 2 · ∆ ⋅ ⋅ · The same platform load means a load of 1.5 L n on the device with the free spring (fig. 1.3-25). With the same platform mass the acceleration is calculated 2 5 . 37 4 . 0 5 . 1 s m g a · ⋅ · And the platform travel mm t a x 075 . 0 ) ( 2 1 2 · ∆ ⋅ ⋅ · The platform travel in both cases is so small that the protection springs never come into action during the impact interval. The energy of the impact load is transported with sound velocity through the platform, the springs, and the load cells. This wave of mechanical energy means local deformation, everywhere the wave passes, with the possibility of local destructive effects. As you see, the devices protecting from static overload do not protect from dynamic overloading. Some design advice should help you to avoid such situations · Make the falling height as small as possible. This minimizes the velocity at the collision point. · Make the distance between the position of the impact and the load cells as big as possible. The shock wave loses energy by damping effects inside the material before it reaches the load cells. · Put a layer of soft material, e.g. wood or rubber, on top of the platform where the impact takes place. This increases the impact time interval At, because of the lower k C value. The result will be a smaller value of the impact force F 0 . · Put a rubber spring between platform and load cell in the path of the shock wave. The basic idea is that a wave cannot easily propagate from one medium to another if there is a big difference between the acoustical impedances. The acoustical impedance of a material can be found from E z ⋅ · ∞ ρ o specific mass of the material E Young's modulus some values z ac = 3.9 º 10 7 Ns/m 3 steel z ac = 6.7 º 10 4 Ns/m 3 rubber On the border between two media a big part , of the wave is reflected. The reflection coefficient can be calculated by GLOBAL Weighing / 35 2 2 1 2 1 ) ( z z z z + − · γ Example: reflection coefficient calculation z 1 = 3.9 º 10 7 Ns/m 3 steel z 2 = 6.7 º 10 4 Ns/m 3 rubber 9932 . 0 , ) 1 2 1 ( 2 2 1 ≈ − − · γ γ z z 1.3.6 Measures against electrical damage General recommendations - The cable of the load cells must be paralleled in a cable junction box. Avoid the penetration of moisture into the cable. - Protect the complete weighing object against overvoltage; otherwise the load cells could be damaged by lightning etc. - The complete cabling must be screened. The screen must only be connected to the ground at one single point to avoid erroneous currents. - All electric arc welding next to the weighing object has to be done as carefully as possible so that the load cells can not be damaged. - Avoid the installation of power cables next to the measuring cables (min distance 1m). If a weighing object is not protected from overvoltage, e.g. resulting from electric welding or lightning strikes in the neighbourhood, heavy currents could flow through the weighing structure and in particular through the load cells. Apart from magnetic or static induction effects in the cabling between the load cells and the measuring instrument such an event can damage the load cells. Two effects can mainly cause the damage Voltage effect The potential difference between load cell body and ground gets high, because of the current flowing through the structure. The strain gauge filament, however, remains on ground potential, because it is directly connected to the measuring instrument. The high voltage difference between filament and billet material can destroy the insulation layer. Such a load cell no longer operates. Current effect Especially for the possible very short pulses of flowing current, the skin effect can cause a very high current density through the surface layer of the billet where the strain gauges are located. In practice we encountered completely burned and even evaporated strain gauge filaments, perhaps due to this effect. If there is any chance of heavy currents in the weighing structure the following measures have to be taken: - The earthing screw of the load cell should be connected electrically to the central earthing junction, where the measuring instrument is also earthed (± 6mm² Cu). - Possible electrical resistances at the load cell contact points with object and foundation should be shunted. Use the flexible copper strap, delivered with the load cells for interconnecting upper loading plate and foundation plate. ATTENTION: the strap should be installed as close to the load cell as possible, but the strap must not transfer mechanical forces to the load cell - Electric welding in the neighbourhood of the load cells is not allowed if the load cell measuring cable is already connected to the measuring instrument. GLOBAL Weighing / 36 1.3.6.1 Earthing Earthing of an electronic weighing installation serves three purposes: safety, prevention of interference, and damage prevention. 1. safety All electrical equipment connected to the mains must not cause a danger of life if being touched. The legal regulations are to observed. 2. Interference Capacitive coupling of the outer world to the measuring circuit can disturb the measurement. This can be avoided with cable screens which have to be at the same potential as the measuring circuit. Therefore the following measures are to be taken - provide armoured steel conduits for the cabling - least distance between power cables and measuring cables 1m. Mind that all screens have to be earthed at one point only, to avoid that otherwise stray currents still could change their potential. 3. Damage prevention It is a very dangerous situation if the strain gauge filament has a too big voltage difference to the load cell body. For that reason the load cells have an earthing screw which can be interconnected with the ‘central earth rail’. Heavy stray currents can be expected if e.g. the weighed object is situated outdoors or at a big distance from the electronic measuring equipment (weighbridge, big bunkers, etc.) In those cases you have to take the following practical measures: a) An ‘earthing tube’ or ‘earthing plate’ with an earth resistance of <5O should be put into the ground in the neighbourhood of the weighed object. b) This earthing electrode has to be connected with the 'central earth rail', mounted in the neighbourhood of the cable junction box. c) now make the following connections: (the thick lines in the wiring diagram) - all load cell earthing screws with the central earth rail with conductors of a t least 6mm 2 copper - the ‘measuring earth terminal’ of the electronic measuring instruments with the central earth rail (equalizing line) NOTES: • Mostly 16mm 2 is enough. You can make a rough check, if you can estimate the possible current flowing through the equalizing line. The voltage drop may not exceed 100V, which is the maximum allowed voltage between strain gauge filament and load cell body to avoid electrical damage. • The measuring cable PR 6135 is mounted in a steel pipe. This pipe could be used as a shunt resistance to the equalizing line to lower its resistance. Attention: Never use cable screenings for this purpose! Fig. 1.3-28 Earthing of a weighing installation GLOBAL Weighing / 37 1.3.6.2 Measures against lightning There are different methods to safeguard an electrical installation against lightning strikes. The most simple solution for our weighing equipment is the installation of surge arrestors (cp. fig. 1.3-29) Fig. 1.3-29 Cable junction box with overvoltage protection 1.3.6.3 Measures against damage caused by welding Welding with load cells in position: During the construction phase of a plant it is recommended to install load cell dummies in order not to damage the load cells by welding or by wrong handling. If after the installation of the load cells some welding must be done please follow the instructions below: 1. disconnect the load cells from the electronics 2. position mass connection and welded joint at one side of the load cell as shown in the sketch values for the parameters a and b are given below under the headline ‘welding during operation’ since they are only necessary in this case 3. after these preparations welding may be done Fig. 1.3-30 Welding near an installed load cell GLOBAL Weighing / 38 In some plants welding takes place during operation from time to time. Therefore, it could be impossible to disconnect the load cells. Always keep in mind that welding near load cells in operation can cause damage. There are no guarantees that they work properly after the welding. If it seems to be absolutely necessary to weld during operation keep the following distances: a > 600mm, b < 600mm a > 3 º b Fig. 1.3-31 No welding in this situation GLOBAL Weighing / 39 1.4 Constraining A weighing object is mounted on load cells. They transfer the vertical load but no horizontal forces at all because of their design principle (articulated column). In order to avoid influences from external horizontal forces on the measurement additional safety devices must be installed to fix the position of the weighing object. We find different types of horizontal forces: - constant loads, quasi constant loads (e.g. thermal expansion) - vibration (motors, stirrers, vibration feeders...) - shock loading, impacts There exist two different types of devices for taking the disturbing horizontal forces: Constrainers Constrainers are used to eliminate horizontal loads constantly. They even operate during the weighing. Constrainers do not permit any movement of the weighing object in the constrained direction. They completely take the load. Stops Stops only limit the possible movement of the weighing object. For this reason their adjustment is critical. They are used to prevent a weighing object from movements which can damage it. During weighing they must not touch the weighing object to avoid force shunts. If such a device either constrainer or stop shows internal ‘clearance’ (space for motion), horizontal forces and impact loads can cause a horizontal object velocity. This means kinetic energy to the object. At the end of the free object travel, this kinetic energy causes a deformation of the weakest points between the object and the foundation. This ‘hammering effect’ can be very destructive to some parts of the construction, e.g. if it causes permanent deformations or even fracture at these points. 1.4.1 Using constrainers Fundamentally an object has 6 degrees of freedom: 3 degrees of translation, 3 degrees of rotation. The installation must be designed in such a way that load cells and constrainers together eliminate all the degrees of freedom. 1. load cells and 5 constrainers (see fig. 1.4-1, left sketch) This is the theoretical solution: one degree is eliminated by the load cells, the others are taken by the constrainers. Complications can arise if thermal expansions disturb the object. The expansion causes additional forces on the load cells. Additionally it is often not practicable to use 5 constrainers because of the lack of fixing points. Furthermore it is quite difficult to adjust the constrainers correctly without clamping. Fig. 1.4-1 Different arrangements of load cells, constrainers and lift-off-protections 2. load cells and 3 constrainers and 2 lift-off-protections (see fig. 1.4-1, middle) This is another theoretical solution: it is based upon the fact that the load cells take only one direction of a vertical load (downwards). The direction vertical upwards is taken by the lift-off-protection. 3. load cells and 3 constrainers (see fig. 1.4-1, right sketch) In quiet installations it is sometimes possible to omit the devices which take rotations around the horizontal axis. GLOBAL Weighing / 40 Some installations do not require all suggested constrainers, e.g. - if the object is quiet (no permanent vibrations, only small horizontal forces ...) 2 constrainers can even be omitted - constrainers can limit the resolution of the installed weighing equipment - with objects, suspended by tension cells or by load beams, the centre of gravity is lower than the point of suspension, which causes a stable equilibrium. Fig. 1.4-2 Standard installation diagrams 4. using more constrainers than necessary This can cause big forces in the constrainers because the system is statically undefined (overdetermined). More constrainers than necessary can cause undesired big axial forces in all constrainers. This happens if not all parts of the construction have the same temperature, or if the fixation points of the constrainers to the foundation make a very small movement by the same reason. As an illustration of this effect we will look to a weighing platform with four constrainers instead of the necessary three ones (fig 1.4-3) Constrainers like pivoting rods or flexbeams are in fact rods which will change in length a little bit under the influence of an axial force. They behave like springs with a very big axial stiffness. We assume that the longitudinal constrainers have an axial stiffness k B and the two lateral constrainers k A . Further we assume that the platform is completely rigid. By some effect the fixation point of the constrainer at point ‘1' moves over a distance ∆l. The problem is now to calculate the forces F A and F B which will work on the platform (and their reactions in the constrainers respectively). Because of the equilibrium of the object the following conclusions are made: - sum of the forces in y- direction =0: forces F B have the same value - sum of forces in x- direction =0: forces F A have the same value - sum of momenta = 0: F A × a = F B × b GLOBAL Weighing / 41 Fig. 1.4-3 Weighing platform with 4 constrainers The values of the displacements can easily be obtained: k F + = x k F - = x A A 2 A A 3 The displacements x 2 and x 3 cause a rotation of the object over angle α a b k F 2 = a x - x = 2 A A 3 2 • • α This rotation causes a displacement in the y- direction of point ‘1' relative to point ‘2' of bºa. The object translates over y 2 . k F + = y B B 2 In total the displacement of point ‘1' is ) ) a b ( k 2 + k 1 ( F = b + y = y 2 A B B 2 1 • • •α The spring at point ‘1' is compressed over a distance ∆l - y 1 . This results in a spring force ) y - l ( k = F 1 B B ∆ • Combination of the last two equations gives ) ) a b ( k k + (1 2 l k = F 2 A B B B • • ∆ • F A can be found now with the momentum equilibrium F a b = F B A • A practical example A weighbridge is constrained with four flexbeams. The stiffnesses were calculated k A = 3.0 º 10 5 N/mm k B = 3.6 º 10 5 N/mm Further is a = 6,000 mm, b = 1,500 mm l = 1,100 mm (constrainer length) GLOBAL Weighing / 42 If the constrainer at point ‘1' has a temperature difference of 1°C with the other longitudinal constrainer, then the dilatation is about mm 10 1.3 = 1100mm 10 1.2 = l -2 -5 • • • ∆ The force in the constrainers is F B = 220 kg F A = 60 kg In most cases the influences of the lateral constrainers can be neglected as you can see from the calculation above. Therefore you can simplify the above equation l k 2 1 F B B ∆ • • ≈ Conclusion Already small temperature differences can cause big forces in the constrainers. Therefore with too many constrainers much care should be given to their horizontal position. 1° non- horizontallity already causes a vertical force shunt of about 1.7 per cent of the axial force (that is in the example 4kg zero error for one constrainer). Only a relatively non-rigid platform construction could decrease the disturbing effect, but this is not very open to a serious calculation and you cannot reckon on this! “Feeling the constrainers“ In the chapter 1.5 you find some possible causes for external forces, which can act on the constrainers. However it is interesting to know what axial constrainer forces can be expected if there are no external forces. This is especially important if you want to judge by „feeling the constrainers“ of the proper functioning of the weighing installation. “Feeling the constrainers“ results, e.g. with pivoting rods or rocking pins, in a rough estimate. We consider several mounting conditions and ask for their influences: a. standard conditions without external forces These conditions are - base plates and upper loading plates horizontally mounted - primary axis of load cell vertical In this case, only the effect of the friction in the pivoting point at the load cell top can cause an axial constrainer force of at maximum ±1% of the weight of the object. (N.B. only for PR 6201/54 2%) b. effect of non-horizontal plates Only about 0.1%/degree of the vertical load on the load cell c. effect of non vertical load cell For PR 6201 there is a the restoring force of nearly 1%/degree. (about 6%/degree for PR 6201/54) d. effect of non-alignment of the load cells in a horizontal plane no effect at all The main conclusion from that points is: If you feel that the constrainers are clamped and a possible foundation vibration does not influence the friction effect, then there is something wrong in the installation. GLOBAL Weighing / 43 Some possible error sources - bad constrainer mounting rocking pins mounted without enough clearance rocking pins mounted with too much distance between the two pins of a pair the constrainers are mounted in a bad pattern (at least two constrainers in one line, three or more constrainers pointing to the same point) - unsuspected external side force a stone or other part is clamped between the object and its surroundings a rigid pipe connection is expanded by temperature an internal pressure inside bellows produces a side force - dirt or wear at the load cell contact surfaces greasing these places could prevent from increasing the friction flattening of the spherical segment, or impression in the base plate, or dirt can be the causes for an increasing restoring force. Influence of constrainers on the resolution In order to avoid vertical force shunts the adequate constrainer must be chosen for each installation. The con- strainer reaction in vertical direction must be as small as possible. For this reason, the vertical stiffness k y of the constrainer is limited. You can calculate it by comparing it with the deflection either of the load cell or of the construction. This depends on the mounting position of the constrainer. A z MR h g k n y • • ≤ explanation of the variables k y permissible vertical stiffness of the constrainer g gravitational field strength (» 9.81 m/s 2 ) h n static deflection under load 1. for load cell » 0.5mm 2. for the construction: value given by manufacturer MR measuring range 1. number of load cells x capacity of single load cell (in kg) 2. real measuring range z number of constrainers A constrainer influence on measurement (= 10 -4 = 0.01%) Examples 1. 4 load cells, 50t, 3 constrainers, f st = 0.5mm k y ± 131 N/mm 2. weighbridge 60t, 1 constrainer connected to the centre of a weighing platform, deflection 3mm k y ± 20 N/mm 1.4.2 Constraining a suspended object Generally there are no special rules for constraining a suspended object. As however its centre of gravity is usually below the load cells such installations are more easily stable than those with compression cells. Omitting constrainers with quiet suspended objects With quiet suspended objects constraining seems not always necessary. However to prevent undesired torque on the tension cells, rotation around a vertical axis should be limited. This can be done with two constrainers (see fig. 1.4-4a). If the pendulum length is short, the fixation point at the object is a quasi fixed ‘stable point' (fig. 1.4-4b). Then only one constrainer can be sufficient in case of the object suspended by one tension cell. If two or more tension cells with small ‘pendulum length’ are used no constrainers are needed to limit the rotation (fig. 1.4-4c). The larger the distance d the better the constraining against rotation around a vertical axis. GLOBAL Weighing / 44 Fig. 1.4-4 How to avoid torque on load cells Pendulum length and restoring momentum (rotation around a vertical axis) The principle description of the restoring force is given in chapter 1.2. This paragraph concentrates on the interaction between restoring force F H and constraining. The restoring force F H depends on the horizontal movement x of the installation and the pendulum length l. F l x = F G H • For the calculation it is necessary to know the connection between horizontal movement and rotation around the fixation point of the constrainer. This is described in fig. 1.4-5. 1.4-5 Geometrical conditions The object rotation is d x = θ Calculating the restoring force F R and the restoring moment M R results in F l d = M , F l d = F G 2 H G H • • • • θ θ The object rotation is in both cases counteracted by a restoring momentum, which is bigger for bigger values of d 2 /l. It depends on the external momenta acting on the object and the possible free torsional movement (‘clearance’) which is in the tension cell suspension if a certain value of d 2 /l is allowed. GLOBAL Weighing / 45 1.4.3 Orientation of the constrainers The mounting direction of constraining rods must be carefully observed. The rod takes high forces and transmits them into the steelwork. Fig.1.4-6 shows the right way of installation: the steelworks only take tension and compression forces. Fig. 1.4-6 Correct constrainer orientation (constrainer and steel beam parallel) Fig. 1.4-7 shows a difficult way to install: the steelworks take momenta which act around the weak axes of an I- beam. For a simple silo without any devices like motors, stirrers etc. this installation does not normally cause difficulties. But if there are active devices like motors, stirrers etc. you may run into problems because the small movements act like forced oscillations. In this case the I-beams take side loads. They operate like torsional springs: they save and restore the kinetic energy via the constraining rod. Fig. 1.4-7 Wrong constrainer installation 1.4.4 Types of constrainers Constraining can be done with several devices. They can be classified - integrated devices for load cell mounting and constraining (e.g. MiniFLEXLOCK) - special constraining devices (e.g. horizontal constrainers, rocking pin) As a general rule you could say that the integrated devices are usually applied for all types of tank weighers. They have become the standard devices now. The special devices are used in truck scales or in case that the integrated devices are not capable of taking the side forces. The special devices are described in more detail in the following paragraphs. GLOBAL Weighing / 46 1.4.4.1 MiniFLEXLOCK (general overview) The mounting kit „MiniFLEXLOCK“ has been introduced by GWT as an integrated solution for mounting and constraining in the beginning of the 80s. It is available for all load cell families. This mounting and constraining device offers several advantages: • constraining and mounting combined in one single kit • constraining at the optimal point (the plane of the load cells) • avoiding problems which arise from bad constrainer installation • MiniFLEXLOCK is designed for every standard installation • easy exchange of load cells • available for every compression load cell family (Þ identical principle for installation and constraining) • different side loads possible • the different arrangements of the MiniFLEXLOCK are discussed in chapter 1.4.1 Further information about the different MiniFLEXLOCK is provided in chapter 2 where the important mounting kits are described. 1.4.4.2 Rigidly clamped struts The rigidly clamped strut is the worst of all constrainers because must be adjusted very exactly to avoid vertical force shunts. The main reason for using a rigidly clamped strut is that there is no clearance which could result in damage or at least wear in case of shocks or vibration. The strut is simply a long horizontal rod which is clamped rigidly at both ends. This construction can be made very stiff in the horizontal direction but flexible enough in the vertical direction. However, there are some design problems: - The vertical stiffness can be reduced by making the rod length as big as possible This is, however, counteracted by the fact that the longer the rod is the smaller the permissible load is. (For slender rods, the permissible axial compression force is limited by the collapsing effect.) - During installation the clamping has to be done with big care, because misalignment would result in a relatively big vertical reaction force. - An arbitrary deformation of object or foundation could cause misalignment thus leading to arbitrary zero errors. For choosing the right strut dimensions it is important to realize that the effect of a rigidly clamped strut is the same as that of a pipe. Therefore you must calculate the allowed vertical stiffness of the strut. (Chapter 1.4.1 provides a formula.) Where to fix the constrainers to the object? - Avoid to use these struts out of the plane of the load cells! If these constrainers are placed out of this plane and e.g. thermal dilatation deforms the distance to this plane, you get zero shifts. They are caused by the deflection of the constrainer which then gives a vertical reaction on the object. - Mount the struts horizontally! Avoid pretension during mounting. Therefore, make the alignment easy with spherical rings at both ends as is shown in the fig. 1.4-8. Use spring washers to make sure that the connection is tightened rigidly. Fig. 1.4-8 Recommended fixation of rigidly clamped struts GLOBAL Weighing / 47 To make the use of rigidly clamped struts easier, we calculated some standard struts. If there is a need for higher capacities, you can calculated the dimensions with the standard methods of strength analysis. Material steel, zinc plated, property class 8.8 length 500mm diameters M12, M16, M20x1.5, M30x2 For each rod you need the following accessories: 4 sets of spherical rings (DIN 6319) 4 nuts 4 spring washers strut 12 strut 16 strut 20 strut 30 thread M12 M16 M20x1.5 M30x2 vertical stiffness (N/mm) 18 61 173 893 effect of misalignment (N/°) 141 479 1359 7014 collapsing load (kN) 1.3 7 15 75 permissible deflection (mm) 9 6 5 3 REMARKS - The values above are calculated for a clamped length of 450mm. For other lengths L these values have to be multiplied by for vertical stiffness (450mm/L) 3 for misalignment effect (450mm/L) 2 for collapsing load (450mm/L) 2 - These struts can be used for installations with 3 load cells with a nominal capacity of at least L n strut 100kg strut 12 500kg strut 16 1t strut 20 5t strut 30 Example A weighing object with 3 load cells is to be constrained. The user expects shock loading and chooses rigidly clamped struts for this purpose. load cell type PR 6201/24D1 side forces 10kN constrainer length 750mm 1) collapsing load strut 30 kN 10 > kN 27 = ) 750mm 450mm ( kN 75 2 • 2) vertical stiffness strut30 Stiffness of the strut mm N 192 = ) 750mm 450mm ( mm N 893 = c 3 CONS • Stiffness of the load cell mm kN 1177 = 20,000kg 3 0.5mm s m 9.81 = c 2 LC • • If you compare constrainer stiffness c CONS and load cell stiffness c LC , you see that the constrainer stiffness is negligible. GLOBAL Weighing / 48 3) effect of vertical misalignment strut30 ° • ° N 2525 = ) 750mm 450mm ( N 7014 = f 2 In this case a vertical misalignment of 1° would mean a vertical displacement of 13mm. Such displacements can exist if the supporting construction is not rigid enough. 1.4.4.3 Flexbeams In cases where high constraining forces (more than 20kN) are expected in the weighing installation the rocking pin, which can withstand 200kN, is a possible solution. However, if shock loading occurs, this is one cause of wear of the rocking pins. Examples for shock loading are truck and railway weighbridges. Here frequently vehicles are braking on the platform, causing sudden horizontal forces on the platform and the constrainers. Therefore, on these installations regular maintenance is necessary to readjust the clearance of the rocking pins after the wear of their contact surfaces. We got good experiences with a special constrainer type, which is designed to solve the problem of the horizontal shock loading. Fig. 1.4-9 Flexbeam A U-shaped steel beam ending in two flat leaf springs is clamped at both ends at A and B. This construction is stiff in the constraining X-direction and it is flexible in the Y-direction and around the Z-axis and the X-axis. At first sight this looks to be a good constrainer, but it can for that purpose only function correctly if it is also flexible in the Z-direction and around the Y-axis. The solution for this is that both clamping points are made of I-shaped supports, which are flexible in the Z-direction and around the Y-axis. Fig. 1.4-10 Characteristic dimensions of flexbeams GLOBAL Weighing / 49 The table below suggests some standard flexbeams so that you need not to calculate them each time you must use them. F m [kN] beam UNP L [mm] l [mm] k Y [N/mm] Y m [mm] HEA l S [mm] k Z [kN/mm] kt Y [kNm/rad] 20 80 1,000 197 3.1 36.6 120 48 1.3 1.4 20 80 600 197 8.5 19.8 120 48 1.3 1.4 30 80 1,000 167 3.6 31.7 140 65 2.9 2.7 50 100 1,000 136 5.6 26.5 140 108 4.9 4.6 75 100 1,000 74 10.3 15.3 160 149 2.8 5.6 100 120 1,000 96 15.0 16.7 160 198 3.7 7.5 150 160 1,000 92 25.7 15.0 180 298 3.8 9.9 200 200 1,000 120 35.9 16.8 200 366 4.3 14.0 300 240 1,500 101 31.6 20.0 240 476 4.8 23.0 How to use the table: 1. you must make an estimate of the maximum constrainer force (e.g. braking force) which occurs in the installation. Now, look up in the column F m (the maximum allowed axial force) which combination UNP/HEA fulfills the job 2. a beam length L of 1000mm is practical in most cases 3. reducing the beam length L increases the stiffness as can be seen in the case of F m = 20kN. (N.B. You cannot improve this by increasing the spring length l because this would decrease the permissible axial force) Influence of torsion (kt Y ) In the ideal case the stiffness kt Y should be zero. To understand what happens because this is not so, we will look to the platform with three flexbeams (fig. 1.4-11). Fig. 1.4-11 Platform with three flexbeams Without constrainer III the platform could rotate around point A. Moving point B over x mm in the indicated direction gives a rotation of x/a rad. Then the supports of I and II are twisted, resulting in a momentum kt a x 2 = M y • • on the platform. This gives a reaction force at B of kt a 2 = a M = F y 2 B • per mm movement. Now, with constrainer III in its position but with a small movement at B (e.g. caused by thermal expansion) this results in the above calculated force in constrainers II and III. Example: With 150kN flexbeam from table above, a = 2000mm the force in flexbeam III is 5N/mm at point B. Conclusion: the flexibility of the support is good enough. GLOBAL Weighing / 50 Influence of flexion (k z ) In the ideal case this stiffness k z should be zero. To understand what happens because this is not so we look to the platform with two flexbeams (fig. 1.4-12) Fig. 1.4-12 Platform with two flexbeams If by some reason point A moves over a distance of x mm in the indicated direction (e.g. thermal dilatation of constrainer II), both supports of the flexbeam I are deformed: - twisting over an angle of x/L (L = beam length) gives a momentum M on the platform [Nm] kt L x 2 = M y • • This is reacted by a force in the flexbeam II of ] mm N [ kt L 2 = f Y 2 t • - flexion over a distance of x/2, resulting in a reaction force in the flexbeam II of ] mm N [ k 0.5 = F Z f • Example With the 150 kN flexbeam from the table above we have kt Y = 0.99 × 10 7 Nmm/rad k Z = 0.38 × 10 4 N/mm L = 1,000 mm Then F t + F f = 1920 N/mm The flexibility of the support is good enough. Example: combined railway weighbridge (max. capacity 60t) Fig. 1.4-13 Combined railway weighbridge A braking force of 280 kN is expected. This should be taken up by two constrainers under the two main beams, to simplify the construction. For exact positioning of the rail ends on the bridge, it is necessary that there is a lateral constrainer A at both ends of the weighbridge. GLOBAL Weighing / 51 Constrainer B With the table above the constrainer for F m = 150 kN could be chosen. Its technical data are beam UNP 160, L = 1000 mm, l = 92 mm support HEA 180, l S = 298 mm, k Y = 25.7 N/mm Constrainer A The designer wishes a shorter length than L = 1,000 mm. He chose UNP 140 L = 890 mm, l = 140 mm The figures for vertical stiffness, maximum allowed force and the allowed maximum deflection are calculated. k Y = 15.2 N/mm F m = 92 kN Y m = 20.5 mm The supports For the supports he chose HEA 140. l S = 140 mm, t s = 5.5 mm, h s = 133 mm - 2 × 8.5 mm = 116 mm With an allowed shear stress t = 84 N/mm 2 the axial force in the constrainer is limited to only 65 kN instead of the value for F m as calculated above. The torsion stiffness around the vertical axis and the lateral stiffness are kt Y = 0.57 × 10 7 Nmm/rad k Z = 0.3 × 10 4 N/mm These stiffnesses are smaller than the corresponding figures in the table for a constrainer of 100 kN. So there is no need to repeat the kind of calculation as is done ... Concluding we can accept the chosen dimensions: Fig. 1.4-14 Constrainer dimensions for railway weighbridge (example) A solution with only one central longitudinal constrainer B is preferred from the viewpoint of measuring quality. In principle four constrainers are too many. The reason is explained in detail in chapter 1.4.1: you remember, a temperature difference of only 1°C between both constrainers B can cause an axial force in these constrainers of 0.23t. The result is a zero-error if the constrainers are not exactly horizontal. Therefore much attention has to paid to the horizontal position of all constrainers under this weighbridge. GLOBAL Weighing / 52 Flexbeams (calculation method) The calculation is divided into two paragraphs: paragraph A describes the calculation of the beam itself paragraph B explains how to dimension the supports. A Calculation of the beam and its properties Figure 1.4-15 shows the mechanical model used for the flexbeam calculation: a stiff part ending in two flexible leaf springs witha length l. These springs are held in a horizontal position at A and B. If B is moved downwards relative to A, there are working vertical forces F Y and momenta M on the device at the points A and B. Fig. 1.4-15 Flexbeam dimensions 1. vertical stiffness of the beam Elasticity calculation teaches that 12 t b = I , L l I E 2 = Y F = k 3 2 Y Y • • • • Spring dimensions b width t thickness l length E Young's modulus (steel: E = 2.1 × 10 5 N/mm 2 ) 2. allowed axial force In principle, this force is limited by two restrictions: either the stiff beam or the flexible beam can collapse. The calculation has to ensure the safety of both the cases. a) collapsing of the beam i L = b min λ for steel Fe 360 (St37) and λ b < 100.8 the collapsing load we find [N] A ) mm N 1.154 - mm N (174.6 = F b 2 2 X • • λ REMARK i min and A are to be found in the data for the used steel profile GLOBAL Weighing / 53 b) collapsing of the springs t l 3.5 = s • λ for steel Fe 360 (St37) and λ s < 100.8 the collapsing load will be [N] t b ) mm N 1.154 - mm N (174.6 = F s 2 2 X • • • λ The permissible axial load is, of course, the smallest value of both the calculated values F X ! General remark about the slenderness ratio In cases that l > 100.8, the EULER formula must be used, resulting in a smaller permissible force. This condition should be avoided. 3. permissible vertical constrainer deflection The limitation is caused by the permissible bending stress in the springs s b . t l) + (L l L E 2 = Y 2 b • • • • σ max B Calculation of the support and its properties Fig. 1.4-16 Flexbeam support dimensions 1. permissible force in X- direction This can be found from a shear calculation [N] t l = F s s sX • • τ For mild steel Fe360 the tables give us a value t = 92 N/mm 2 2. stiffness around the Y-axis of the support This is expressed as a momentum per radian and found by h G l t = kt s s 3 s 3 Y • • • η q 3 = 0.333 G modulus of gliding Fe 360 (St37): G = 81,000 N/mm 2 GLOBAL Weighing / 54 3. stiffness of the support in Z- direction This is found by calculating the bending stress (, assuming that the ‘beam is clamped at both sides’). 12 t l = I , h I E 12 = k 3 s s 3 s Z • • • 1.4.4.4 Pivoting rod The rod has two pivoting ends and is placed horizontally between the object and the fixed surroundings (or foundation). In principle there are two effects, which could can a vertical reaction force on the object: 1. bearing friction Fig. 1.4-17 Reaction D DG to an upward movement of the object If the object would be moved upwards or downwards, the bearing will react with a friction momentum M F . This causes a vertical reaction force of l M 2 = G F • ∆ counteracting the movement. The friction momentum in the bearings is nearly proportional to the axial force in the rod. Therefore also the disturbing vertical reaction is proportional to the axial force in the rod (and practically zero if there is no axial force). 2. non horizontal rod position Fig. 1.4-18 Non horizontal pivoting rod If the rod is placed under an angle a with the horizontal and a side force is working on the object, then the resultant of this force F H and the rod reaction is about 1.7% of F H per degree. If heavy vibration or shock loading is expected, choose an overdimensioned type of constrainer. Also be sure that the pivoting rods are mounted with the smallest possible play, to avoid any hammering effects. This concerns the bearings, their axles, and all the screw thread connections. GLOBAL Weighing / 55 1.4.4.5 Rocking pins Unlike the above described types of constrainers rocking pins are mounted with a clearance. They cannot take any shock loading. Fig. 1.4-19 Principle of the rocking pin The two end surfaces of each pin are part of a sphere with a diameter equal to the length of the pin. This sphere can roll between two flat vertical contact surfaces. In case of big horizontal forces during measurement, the contact surfaces should be placed vertical within ±1° to avoid a disturbing vertical component. The horizontal position of the rocking pin is not critical at all. The rocking pin is used in pairs, because it can only take up a compression and not a tensile force. The two pins of one pair shall always be mounted in one line and close to each other. The latter to avoid clamping if the distance d (see fig. 1.4-19) changes by thermal expansion or another deformation. In cases of falling lumps of material, or of stirring devices in reactors, shocks can occur. The rocking pin is not suitable in case of big shock loads because the pins must be mounted with an axial play. Big shocks will therefore have a „hammering effect“ on the contact surfaces. This causes damage („wear“) on the long run. Therefore regular inspection and pin adjustment to the least possible play is necessary. In case of big horizontal shocks a constraining device without play should be selected, e.g. rigidly clamped struts. 1.4.5 Types of stops In general two different usages of stops must be separated. Horizontal stops are often applied to replace constrainers, e.g. in weighbridges. Vertical stops, however, serve as lift-off protections and are applied additionally to constrainers in the horizontal plane. Both types have some properties in common: · Stops are safety devices. · Stops may not take forces during the weighing operation. · Stops only act in one direction (horizontal stops mostly take compression forces, vertical stops mostly tension forces). · Stops are sensitive to frequent shocks and heavy vibration (‘hammering’ can damage their contact surface). 1.4.5.1 Horizontal stops Though stops are no constrainers use the same rules for their positions in the horizontal plane. Keep in mind that horizontal stops only take compression forces. Therefore, stops must be used in pairs to keep the weighing object in its position. Sometimes weighing objects are equipped with stops instead of constrainers. · If a suspended object is in a very stable equilibrium and under quiet conditions, sometimes constrainers are omitted (see above chapter 1.4.2). This could be of an advantage, for high accuracy measurement, to avoid any possible friction. · When weighing objects on compression load cells like weighbridges, constrainers are often replaced with stops. The restoring force caused by the spherical segments is thought to give a sufficient stable equilibrium and to assure the weighing object to be free from the stops during the weighing. GLOBAL Weighing / 56 Fig. 1.4-20 Weighbridge with 4 stops In these installations, however, trouble can occur, usually if the maintenance is neglected: · excessive permanent movements of the weighing object can cause wear at the load cell billet top and at the spherical segment · the stops can be damaged or destroyed by ‘hammering’ · the accuracy can decrease because of friction, non-vertical load cell position, and damage to the load cell A solution for this situation cannot always be found... This depends on a lot of circumstances: · the magnitude of the restoring force per mm object travel · the magnitude of the side forces acting · the maximum permissible side force on the load cell · the permissible measuring error and, therefore, the necessary vertical load cell position · the friction at the load cell top · the possible wear at the load cell contact points by the object movements. To avoid the above mentioned negative effects (wear, stop damaged, accuracy decreased), the optimal adjustment of the stops is important. There are two opposite arguments for adjusting the gap distance of stops: 1) wide gap distance · useful to avoid friction between object and stop If the object touches the stop, this could result in a reproducibility error by friction forces. Therefore, the gap between object and stop must be big enough. · avoiding destruction at the stops · allowing a big object travel Þ can cause not acceptably high restoring forces on the load cell and possible wear at the billet top 2) short gap distance · avoiding destructive hammering at the stop in case of sudden side forces (like braking forces) 1.4.5.2 Vertical stops (lift-off protections) Vertical stops (they are also named lift-off protections) are used in combination with horizontal constrainers: the horizontal constrainers keep the weighing object in the right horizontal position and the vertical stops avoid its tilting or capsizing. Normally, the constrainers in the horizontal plane are mounted as suggested by the standard installation diagrams in fig. 1.4-2. Beside every load cell a vertical stops is installed. Fig. 1.4-21 Compression load cells, constrainers, and lift-off-protections GLOBAL Weighing / 57 Fig. 1.4-21 shows weighing installations on compression load cells with different numbers of constrainers and stops: · installations with 5 constrainers (fig. 1.4-21, left sketch) The disadvantage of this type of design is not obvious: if the weighing objects expands under temperature, some of the constrainers in the second plane above the plane of the load cells can get clamped, transfer disturbing forces to the load cells and thus cause measuring errors. · installations with 3 constrainers in the horizontal plane and lift-off protections beside them (fig. 1.4-21, right sketch). This type of installation avoids the production of disturbing forces by constrainers in a second plane above the plane of the load cells. The vertical stops only make sure that the installation cannot tilt. The simplest type of a vertical stops consists of a rod and two nuts. The width of the clearance is determined by max. permissible movement where the load cell does not leave its position. (PR 6201: 2mm) Fig. 1.4-22 Simple lift-off-protection with a rod A second type of lift-off protection is shown in fig. 1.4-23. Fig. 1.4-23 Lift-off-protection Most of our MiniFLEXLOCK mounting kits provide a threaded hole which makes the installation of a lift-off protection very easy (fig. 1.4-20). If there is a need for a lift-off protection with a higher capacity, you can find the strength of an appropriate rod by simply choosing from the table given below. thread permissible force M12 10.5kN M16 19.6kN M20 30.6kN M24 44.1kN M30 70.1kN M36 102.1kN M42 140.1kN M48 184.1kN The properties of the bolts are assumed to fulfil at least the conditions below limit of elasticity 240N/mm 2 ultimate load 400N/mm 2 Such bolts are standard bolts and used in combination with steel type Fe360 or St37. GLOBAL Weighing / 58 1.5 Disturbing forces The load cell errors are specified and well-known by nearly everyone who uses load cells. Besides these errors various other influences exist like mounting faults, environmental influences, and influences from the design could affect the measurement. 1. load cell errors These errors are normally intensively discussed in case of the malfunction of an installation. However, their influence compared to mounting faults and external influences usually is small. 2. mounting faults A lot of trouble concerning weighing installations is caused by improper mounting of the load cells: · side forces and momenta affect the load cell · bad application of the load to the load cell (use the right mounting parts) · non vertical load cell position (use spirit level) These errors are usually rather easy to detect and to correct. 3. external influences These influences must be taken into consideration during the design phase in order to choose the right load cell type and the adequate way of constraining. Depending on the type of construction the errors listed below can occur • parasitic vertical forces on the installation • temperature on the load cell • dynamic forces on the installation (vibrations) • dust, snow, rain • wind forces • pipes, bellows Advice concerning the items listed under position 2 is given in the chapters directly related to the load cell mounting (see section 2). This chapter 1.5 deals with the possible disturbing influences coming from the environment of an installation (item 3) and how to fight against them. 1.5.1 Environmental influences 1.5.1.1 Wind forces Wind forces influence mainly outdoor installations. They affect the strength of constrainers, lift-off protection elements and the strength and the stability of the complete construction. These parts have to withstand the occurring forces. Normally, wind forces do not affect the weighing process itself because the design load chosen appears only during a very heavy storm. While such a storm is blowing tank weighers or truck scales are out of use. In order to determine the value of the wind forces acting on a vessel we must consider some basic facts on fluid mechanics: A. How can we describe the wind? B. How do the shape and the dimensions of the vessel influence the forces? C. How does the surrounding of the construction influence the forces? A. Description of the wind Taking a closer look at the blowing wind we find out that the description of the wind needs the knowledge of two factors: a the velocity of the wind b the properties of the air (density, viscosity) GLOBAL Weighing / 59 Fig. 1.5-1 Velocity distribution The velocity of the wind is not constant in value when we consider different heights (see sketch on the right hand side). Directly above the ground the velocity of the wind equals zero because of the friction between wind and ground. The velocities increase according to a parabolic function of higher order. In most cases the parameters of the function describing this flow are unknown so that it is impossible to calculate the velocities. Therefore a more practical approach has to be used: a rectangular velocity distribution is chosen to approximate a real measured velocity distribution. Such a choice is called the design load for this construction. The state of the air depends on e.g. the barometric pressure, the air density and the temperature. The effect of the compressibility of the air may be neglected. Even during a very heavy storm the velocity of the wind is rather slow compared to the sonic speed. Therefore we can assume the properties of the air to be independent from the velocity. For the calculation we need values for the properties density o A and viscosity q A . The barometric pressure p decreases with an increasing height above the ocean. Therefore the barometric pressure at this level has is chosen. The same behaviour applies for the density o A as you can find out from the ideal gas equation: p ~ o A . DESIGN LOADS Values for the design loads are specified either by the customer or by the national standards and regulations. In case of wind the state for the design load is (usually) a very heavy storm with a wind velocity of 12 Beaufort or above. s m v g e 40 . . · The values for air density and air viscosity given are valid for Germany 2 6 2 10 73 . 17 ) 10 ( , 25 . 1 ) 10 ( m Ns C m kg C A A − ⋅ · ° · ° η ρ B. Shape and size of the vessel Many experiments were undertaken to measure the forces which the flowing air exerts on an upright standing vessel. The results show that the forces depend on - the shape of the vessel (circular vessel, square vessel, ...) - the smoothness of the vessel's surface - the size of the vessel The two influences 'shape of the vessel' and 'smoothness of the surface' are assembled into one number that is characteristic for that particular installation. The number is called 'drag coefficient' or 'friction coefficient' and is referred to as c w . For a normal weighed object the influence of the smoothness is negligible. Therefore the number represents only the influence of the air velocity. friction coefficient c w upright standing circular vessel 0.8 ...1.0 There is another parameter influencing the magnitude of the wind force - the size of the vessel. GLOBAL Weighing / 60 In order to make the calculation as easy as possible an exposed area is defined Definition exposed area The exposed area of the weighed object is calculated by multiplying its diameter and height. H height D diameter A area osed exp ⋅ · The exposed cross sectional area was defined in the beginning of this century when the computers were not invented. C. Area of installation Depending on the area where the weighing object is placed you have to calculate with different air velocities. Example 1: weighing object in a town Various buildings slow the air velocity down. This case is chosen for the standard design load. Fig. 1.5-2 Weighing object near a town Example 2: weighing object on a hill or near the sea The wind blows without any hindrance at the weighing object. There is nothing that slows the air velocity down. Therefore you have to assume a higher design load as usual. Fig. 1.5-3 Weighing object on a hill wind velocity, impact pressure Supposed values for the wind velocity are not known, some useful assumptions have to be made. The table below is intended to help you to find such assumptions. Beaufort air velocity of wind impact pressure use 10 < 28.4 m/s 0.5 kN/m 2 12 < 36.9 m/s 0.85 kN/m 2 > 12 42.0 m/s 1.1 kN/m 2 standard value near the town > 12 45.6 m/s 1.3 kN/m 2 standard value outside a town > 12 51.4 m/s 1.65 kN/m 2 GLOBAL Weighing / 61 After having considered all relevant data the calculation scheme can be introduced. calculation scheme for the wind forces acting on an upright standing vessel Fig. 1.5-4 Schematic sketch for calculation F W = c W º q º A F W wind force cross sectional area of vessel A = D · · H D diameter H height impact pressure q = 0.5 · · r r L · · v² o L density of the air v velocity of the air (rectangular velocity distribution) drag coefficient c W = 0.8 ... 1 (upright vessel) GLOBAL Weighing / 62 Example 1. Horizontal forces Calculate the horizontal forces for a paper pulp digester according to the sketch below. The diameter D is 3m, the height H is 10m. Consider two cases: 1. Beaufort 5 v = 8 m/s 2. Beaufort 12 v = 40 m/s (hurricane) Fig. 1.5-5 Paper pulp digester The friction coefficient for this installation is assumed to be c w =0.8. The density of the air shall be o A = 1.25 kg/m 3 . case 1: q 1 = 0.5 • 1.25 kg/m 3 • (8m/s) 2 = 0.04 kN/m 2 F w1 = 0.8 • 0.04 kN/m 2 • 3m • 10m = 0.96 kN q 2 = 0.5 • 1.25 kg/m 3 • (40m/s) 2 = 1 kN/m 2 F w2 = 0.8 • 1 kN/m 2 • 3m • 10m = 24 kN Interpretation of the results: - this installation must be protected against capsizing - a vertical component of the wind force can cause a measuring error as the wind does not always blow exactly horizontal; the error is found to be a temporary zero shift - errors bigger than 1‰ of the net weight can normally be expected only with very heavy wind ( > 5 Beaufort) Example 2. A vertical lift-force Wind can cause an upthrust F l on a lying tank. This is caused by the high air velocity at the upper side. Calculate for a horizontal cylindrical tank of 3m in diameter and 10m in length the `lift`-force. The tank is filled with sand (o s = 0.5kg/dm³). Consider two cases: 1. Beaufort 5 u= 8 m/s 2. Beaufort 12 u= 40 m/s (hurricane) Fig. 1.5-6 Horizontal tank GLOBAL Weighing / 63 Volume of the tank: V T = /4 * 3² * 10 m³ = 70.7 m³ Weight of the sand: F C = 70.7 m³ * 0,5 kg/dm³ * 9.81 m/s² = 347 kN impact pressure q 1 q 1 = 0.5 • 1.25 kg/m 3 • (8m/s) 2 = 0.04 kN/m 2 lifting force F l1 F l1 = 0.8 • 0.04 kN/m 2 • 3m • 10m = 0.96 kN impact pressure q 2 q 2 = 0.5 • 1.25 kg/m 3 • (40m/s) 2 = 1 kN/m 2 lifting force F l2 F l2 = 0.8 • 1 kN/m 2 • 3m • 10m = 24 kN F 1 = 0.96 kN = 2.8‰ F C F 2 = 29.04 kN = 6.9% F C Interpretation of the results: - negligible under normal conditions Example 3. A vertical ‘suction’-force Sometimes the vessel is protruding out of a building. E.g. to discharge into a truck (see figure below). If the wind is free to blow under the building, it causes that the air pressure under the building is lower than indoors. A vertical force F v is the result. In this cases A denominates the cross section of the vessel. The vertical force is calculated by F v = p • A. Fig. 1.5-7 Discharging into a lorry Calculate the forces for three vessels with diameters of 2m, 4m and 6m respectively. Assume a difference pressure of Ap = 0.0025bar (= 2.5 cm H 2 O). diameter of vessel vertical force F v 2 m 78.5 kg 4 m 314 kg 6 m 707 kg Interpretation of the results: - This effect can very seriously influence the measuring accuracy if you consider W&M installations. Non- reproducibility error by arbitrary temporary zero shifts. GLOBAL Weighing / 64 Possible solutions to avoid these effects are: - make it impossible for the wind to blow under the building (e.g. by flexible doors) - make the opening in the floor where the vessel comes through much larger than the vessel. This makes the pressure difference smaller. 1.5.1.2 Heat and heat transfer Heat in its various forms can cause troubles to many installations, outdoors and indoors. Sometimes the reason is obvious, sometimes the reason is well hidden. Example 1: A customer complains about a storage silo outdoors. The silo is standing next to a high production hall. He observed a change in weight throughout the day although the silo was neither loaded nor unloaded. In the morning, the silo stands in the shade. As the sun is rising, the shadow of the building wanders and in the afternoon the silo stands directly in the sun. In this example the reason for the varying weight values is quite obvious: the weight changes because of the temperature sensibility of the load cells. Example 2: A customer complains about a hopper scale indoors. The weigher is standing in a large hall together with many other vessels. The temperature in the hall is kept constantly at 20°C. Accidentally the weight changes although the contents of the hopper do not change. A visit at site produced the followings results: the outside wall had a door that was usually closed. For some reasons the door happened to be opened. This caused a draught of air which affected the load cells in such a way that the weight indication changed. In order to get a better understanding how heat and high temperatures can trouble load cells and scales the physical effects are considered on the pages below. Three different physical mechanisms describe the heat transfer: - conduction - convection - radiation mechanism 1: CONDUCTION Fig. 1.5-8 Heat conduction If the weighing object has a higher temperature than its surroundings, a heat flow through the load cell is forced. This flow can have different results. 1. damage of the load cell The glue has a limited temperature stability. If a particular value is exceeded the glue softens and the application of the strain gauge to the measuring element loosens or even gets lost. Therefore the upper limit given for the storage temperature must not be exceeded. 2. change in characteristic values A heat flow through the load cell could result in the same effect as a too high rate of temperature change. Possible solutions to overcome these complication are - use of high temperature load cells PR 6211 LT GLOBAL Weighing / 65 (compensated temperature range up to +180°C) - use of heat protection plates - sometimes: use of tension cells with long suspension rods mechanism 2: CONVECTION Fig. 1.5.9 Heat convection apply flowing air with a high temperature to a vessel. This results in an increase in temperature in the vessel. Furthermore the load cells are affected. This can result in a high heat increase during a short time. The detection of erroneous readings caused by conduction is often a little bit complicated since this error happens to appear under changing circumstances and at different times. If such an error is identified it may be difficult to find a satisfying solution. mechanism 3 RADIATION Fig. 1.5-10 Heat radiation and protection shield A heat flow caused by radiation depends on the ratio of two surface temperatures: the surface temperature of the hot object and the surface temperature of the heated object. The heat flow does not depend on any medium but only on the different temperatures. Errors caused by heat radiation are usually easy to detect. You find always an extremely hot object that causes the complications. Possible sources for the radiation are - the sun in outdoor installations - an arc furnace or another container with hot object (e.g. molten metal) Mostly heat radiation can be fought by placing a sheet of metal between the object and the load cell. As stated above all three heat transfer mechanisms can cause trouble to a weighing object. The heat convection and the heat radiation affect only the load cell by changing its temperature. Because heat conduction exist normally with hot weighing objects this mechanism affects both, the vertical position and the temperature stability of a load cell. For this reason, the next paragraph introduces a calculation scheme for the temperature expansion of a beam. The physical behaviour of such a beam is described by - the temperature difference between beam and its surrounding - a material dependent constant: the thermal expansion coefficient of the particular material GLOBAL Weighing / 66 material expansion coefficient k [mm/m/100K] Fe 360 11 concrete 1.1 1.4301 1.6 aluminium 2.3 For easier comprehension and calculation the expansion coefficient is given in the form above whereas tables normally state it in the form k*10 -6 K -1 . Calculation scheme for the thermal expansion of a beam Fig. 1.5-11 Schematic sketch for calculation Al = l • k • AT Al thermal expansion temperature difference T o - T a T o operating temperature T a ambient temperature thermal expansion coefficient beam length GLOBAL Weighing / 67 Example 1: I-beam IPB 100 Calculate the expansion of an I-beam IPB100 which is heated from an ambient temperature of 20°C to 70°C. What forces are generated internally in the beam if it is assumed to be clamped at both sides? material constructional steel Fe 360 beam length l = 2m ambient temperature T = 20°C, operating temperature T = 70°C cross sectional area A = 2,600mm 2 Young's modulus E = 210,000 N/mm 2 Solution 1. strain T = k • AT T = 1.1 * 50 mm/m = 0.55mm/m = 0.55*10 -3 2. expansion Al = l • k •AT Al = 2m * 0.55mm/m = 1.1mm 3. force F = A • E • k • AT F = 2,600mm 2 *210,000N/mm*0.55*10 -3 = 300kN The force is caused in case of a rigidly clamped beam. Please observe that the values are higher than usually expected. Such high forces could affect seriously the measuring result in that case the transducer is influenced by side forces because of the mounting principle. Remember, GLOBAL Weighing suggests for this reason mounting kits where the load cell acts as articulated column. Example 2 Tank weigher Consider the sketch below. The tank weigher is made from stainless steel. It is calibrated under ambient temperature (20°C) and operated at a temperature of 250°C. Calculate the expansion under operating conditions. Fig. 1.5-12 Tank weigher R = 1.25m stainless steel = k = 1.6 mm/m/100K temperature difference AT = 250°C-20°C = 230°C T = 1.6mm/m * 230°C/100K= 3.68 mm/m expansion of the radius Ar = 1.25 * 3.68 mm = 4.6mm = high expansion max. permissible value for PR 6201: 10mm max. permissible value for PR 6241: 3.9mm GLOBAL Weighing / 68 1.5.1.3 freezing environmental conditions (ice, snow) The load cells are specified to operate even at temperatures down to –30°C. However, if the air temperature changes continuously between e.g. +10°C and –10°C, ice could be deposited on the pivoting points of the weighing installation. Critical points are: 1. load cell and mounting parts - contact point between the spherical load cell top and the hollow of the load button - contact point between spherical segment of the load cell and the base plate - the cardanic suspension of tension cells or load beams - the cardanic pivoting points of the constrainers 2. installation - the gap between the weighing object and its surroundings In all these cases the joint can cause a measuring error because the contact points freeze. If you have a suspicion that cold weather had affected one installation, you should check the whole installation on the points mentioned above. Anti-freezing agents, like glycol, can help you to overcome the situation. What errors are caused if ice or snow is deposited on an outdoor vessel? If the silo is heated or held at a constant temperature above zero, additional loads caused by ice or snow do not need to be observed. In all other cases you should be careful about them. In some countries legal regulations exist, which give design loads for ice and snow deposition. These regulations assume a thickness of the ice layer, e.g. 3cm. This equals a 2 7 m kN l ice for load design i · In the same way, the design loads for snow are given, 2 2 5 . 5 ... 5 . 0 m kN m kN l snow for load design s · Example: outdoor silo The silo has a capacity of 50t. diameter D = 6m design load l S = 4kN/m 2 Solution ) 5 . 11 ( 113 4 ) 6 ( 4 2 2 t kN m kN m m s ≈ · ⋅ ⋅ · π The additional load is more than 20% of the silo capacity. 1.5.1.4 rain, dust It is unlikely that either rain or dust can form a layer, which is thick enough to influence the weighing: the dust is blown away by the wind, the rain drips down from the top of the vessel. 1.5.2 Friction In order to get an accurate result a weighing object has to stand free from its surroundings and any links connected to it must be as flexible as possible (refer to chapter 1.3). However, after some time of operation the links can harden and become stiff. If the scale is not cleaned regularly, dust can be assembled between the weighing object and its surroundings. Both effects can cause friction. Friction results in unpredictable zero shifts: non-reproducibility and hysteresis affect the accuracy. Causes for friction - internal friction in the pivoting points - parts pressed against the object - internal friction in the hoses, tubes, cables, . GLOBAL Weighing / 69 For this reason, a regular check on the parts connected to the scale and a regular cleaning in a dusty area is recommended. 1.5.3 Vibrations, shock loading Vibrations and shock loading (resulting from falling material) can severely influence a weighing object. Vibration The source of the vibration can be the scale itself, e.g. a mixing device. The second possibility is an external source, e.g. a motor. In this case, the vibrations are transmitted through the construction. To lower the amplitudes of the vibration you can install damping devices such as rubber mounting kits or additional cups springs. The choice depends on the excitation frequency and the capacity of the load cells. For load cells with capacities above 5t it is nearly impossible to find rubber mounting kits. Position of the damping elements: - if the vibrations come from outside of the scale, the damping element must be placed between the floor and the load cell - if the vibrations come from the weighing object itself, the damping element must be placed between the load cell and the weighing object How can a possible influence be estimated? 1. natural frequency f 0 of the damping element stat x g f ⋅ · π 2 1 0 2. ratio of excitation frequency f ex and natural frequency f 0 0 f f ex · η 3. max. vibration amplitude 2 1 η − · stat peak x s Example: 900kg Platform with PR 6211/32D1 and rubber mounting kit PR 6011/03 technical data of the rubber mounting kit: 1mm deflection under 2250N load stiffness of the rubber element: mm kN mm N c s 25 . 2 1 2250 · · deflection under nominal load mm mm N N c w x s LC stat 33 . 1 2250 3000 · · · natural frequency f 0 of the damping element ) 822 ( 7 . 13 33 . 1 81 . 9 2 1 2 0 rpm Hz mm s m f · · ⋅ · π GLOBAL Weighing / 70 min. frequency with 0 isolation ) 1160 ( 3 . 19 2 0 min rpm Hz f f · · ⋅ · Vibrations produced by equipment like motors with higher speed than the min. frequency are damped. However, vibrations between 822rpm and 1160rpm are enlarged. GLOBAL Weighing / 71 1.7 The weighing results The definitions given in paragraph 1.7.1 concern only the load cell properties. 1.7.1 Terminology for load cells error difference between the measured value and the real value Hysteresis The maximum difference between the loading and the unloading curve related to C n . Non-linearity Maximum deviation from the best straight line in relation to rated output. The best line goes through the common point of pre-load and the loading/unloading curve and splits these curves into two parts with the same positive and negative values Combined error Half distance between the limitation of the band, which covers loading and unloading curve, and where the centre line goes through the common point of pre-load and the loading curve. Repeatability The difference between load cell output readings taken from consecutive tests under identical loading and environmental conditions. Creep The change in load cell output occurring with time while under constant load and with environmental conditions and other variables also remaining constant. Zero signal temperature coefficient Relative output signal variation without load related to rated output C n and a temperature variation of 10K, where the variation of temperature per hour is maximum 5K/h. Rated output temperature coefficient Relative variation of the real rated output C i due to temperature variation related to C n and a variation of 10K in the nominal temperature range Accuracy class The accuracy is simply the worst value of following values • combined error • repeatability • creep (30min) F cr30 • zero signal temperature effect • rated output temperature effect 1.7.1.2 load cell and weighing installation Some words describing the properties of load cells and weighing installations have different meanings depending on their actual context. Take i.e. the word „accuracy“. load cell „accuracy“ is the worst value of the errors listed below • combined error • repeatability • creep (30 min) • zero signal temperature coefficient • rated output temperature coefficient GLOBAL Weighing / 72 weighing installation 1) „accuracy“ refers to the O.I.M.L. curves for the different classes. It describes the deviation of the weight indication from the real value of the weight. 2) The „accuracy“ of the weighing installation includes some more than only the load cell: • accuracy of the load cell • the quality of mounting • the steelworks, the concrete around the load cells • the cabling • the electronic Direct influence is only possible on load cell and electronic. The other parts are done by the customer. In some cases he might need support from GLOBAL Weighing. 1.7.2 Influences from the construction The chapter on the design provides you with hints how to improve steelwork. As such constructions are very complicated, it is usually not possible to give an easy way to calculate the influences on a difficult design. Maybe a rule of thumb can help you: calculate the following ratio capacity cell load installed error weighing If this value is about 0.001 = the installation should be in good condition. If this value is about 0.01 = the influence of the construction is dominant. 1.7.3 Approved installations 1.7.3.1 W&M regulations In most countries, legal regulations on the required accuracy are given by the national services of Weights & Measures in accordance with proposals of the O.I.M.L. (Organisation Internationale de Métrologie Légale) at Paris. The O.I.M.L. issues a lot of regulations concerning load cells and scales 1. Test reports for load cells (these test reports are no approvals!) Metrological regulation for load cells R 60 The load cell test reports (according to OIML R60 or national regulations) contain values for E max , n LC , Y, ... which are necessary for the design of the installation 2. Non-automatic weighing instruments (NAWI) Non-automatic weighing instruments R 76 GLOBAL Weighing mainly deals with non-automatic scales, i.e. truck scales, batching systems. The W&M systems are subdivided into four precision classes: I precision speciale laboratory II precision fine laboratory III precision moyenne weighbridges IIII precision ordinaire used in factories where asphalt or concrete is made The classes C and D concern GLOBAL Weighing systems. These classes will be discussed in more detail. GLOBAL Weighing / 73 The permissible number of scale divisions is class C 500 to 10000 class D 50 to 1000 The permissible error for the installation during initial verification is shown in fig. 1.7-1 as well as the maximum error for trade use. Fig. 1.7-1 Permissible errors for W&M weighing systems (class III) Fig. 1.7-2 Comparison between W&M systems (class III and class IIII) · The technical data are valid for the temperature range from -10°C to +40°C. Outside this range there is an additional deviation of 1d/5K permissible. The certificates describe different types of scales, for instance · weighbridges · tank weighing · crane weighing · In-Motion-Weighing · ... The W&M authorities only certify complete ‘applications’, but no single load cells. A certificate contains all parts which are necessary for the installation of the scales such as load cells and its accessories, special mechanical constructions for weighbridges, the indicator and its accessories, ... GLOBAL Weighing / 74 1.7.3.2 Weighbridges 1. Terminology v (load cell verification interval) the load cell interval used in the test of the load cell for accuracy classification (expressed in units of mass) v min (minimum load cell verification interval) the smallest load cell verification interval into which the load cell measuring range can be divided E max maximum load d (actual scale interval) value expressed in units of mass of - the difference between the values corresponding to two consecutive scale marks for analogue indication, or - the difference between two consecutive indicated values for digital indication. e (verification scale interval) value used for the classification and verification of an instrument (expressed in units of mass) Calculation procedure for the values of an assizable weighing system (according to EN 45 501) 1. Maximum capacity of the load cell The maximum load of the scale must not exceed the sum of the capacities of the load cells N R Max Q E Max ⋅ ⋅ ≥ N number of load cells in the scale E max maximum capacity of a single load cell R transmission factor of a lever system (for systems without levers R = 1) Q correction factor scales with levers Q = 1.1...1.3 scales without levers 1 - N N + L = Q De Max L De dead load Max maximum load to be measured 2. number of load cell divisions n nLC ≥ 3. smallest verification interval e N R Y E e or N R v e • • ≥ • ≥ max min 4. number indicator divisions n nind ≥ 5. minimum output voltage e N R U E S u exc • • • ≤ ∆ max min S rated output, e.g. 1mV/V U exc supply voltage GLOBAL Weighing / 75 6. permissible input resistance R N R R Lmax LC Lmin ≤ ≤ R Lmin min. permissible input resistance of electronics R Lmax max. permissible input resistance of electronics Example weighbridge for trucks (3000d) weighing range 60t weight of the deck 35t 4 load cells accuracy 20kg 1. required load cell capacity (T) 117,000kg 1.7 1 50,000kg 4 95,000kg 1.7 1 - 4 4 + 35,000kg + 60,000kg 35,000kg = Q ≈ • • ≤ ≈ chosen: PR6221/54C3 E max = 14,000 n LC = 3,000 ± 3000 = n 2. minimum permissible step for the load cell kg 7.1kg 4 1 14,000 50,000kg e 20 ≤ ≈ • • ≥ minimum permissible step for indicator = d ± 10kg Therefore a step of 20kg is permissible. V = 20kg 4 1 20V 50,000kg V mV u µ 4 2 min • • • ≤ ∆ 3. The indicator be a PR 1713 with n ind = 5000. The minimum input voltage for PR 1713 is 1.2V per d; this condition is fulfilled, too. 4. Permissible input resistance (T) = 4 0 Ω Ω ≤ Ω 270 108 75 PR 1713 has a lower limit for the input resistance of 75Ω and no upper limit. 1.7.4 standard accuracy: non W&M application The user often asks us to give some figures about “the accuracy of a process weighing system”. Properly speaking, there is no such thing as the accuracy of an installation. We can only speak of an installation according to e.g. ‘O.I.M.L. Class III, 3000d’, which means that the installation can pass some well defined tests. However, we can speak about the weighing accuracy of a certain weight measurement. This gives information on how far we can rely on the weighing result. It gives, in other words, the tolerance on the weighing result. GLOBAL Weighing / 76 To predict the possible error, it is not only necessary to know the measuring properties of the installation. Also the environmental situation and the loading situation for that special measurement must be known. Some definitions tolerance zone of possible errors influence factor specified load cell property (result on output of disturbing influence or loading situation) disturbing influence condition which influences load cell output, independent of the force on the load cell loading situation history of loading until the moment of measurement The error on the measured result of a weighing is built up by several aspects 1.7.4.1 Installation specification The load cell specification gives the primary information for the possible measuring errors. However, you also need to know what part of the calibration curve is used in the particular installation. Or in other words, how the load cell output is transformed to a scale indication. For a given installation the important parameters are: - installed load n º L n (n: number of load cells) - dead load D - scale range S (the scale begins with zero) Load cell errors expressed in relation to the scale range become: - a zero error of z% of L n ¬ error (n º L n ) º z%/S of scale range - a span error of s% ¬ zero error D/S º s% of scale range and a span error s% of the measured net load The influence factors connected to the loading situation are specified as a% of L n . The operational loads on the load cells, however, never are lower than D kg and mostly never will become more than D + S kg. Therefore these errors will be smaller in a practical installation, as will be explained now: Normally the non-linearity can be described as quadratic deviation between the calibration curve and a straight line. That means, that for a smaller part of the calibration curve, the non-linearity error as a percentage of the used part is also smaller. Example: If the non-linearity error were exactly quadratic, then with a specified non-linearity of 1% of L n , this becomes only S/(n º L n ) º 1% of the scale range. Of course, the slope of the calibration curve over the part S is not the same as for the complete curve. Calibration of the installation, however, makes this not relevant. Hysteresis will also be smaller than specified, if the load is only changing over a small part of the calibration curve. To simplify the matter for the error calculation we will use the specified figures for non-linearity, hysteresis and combined error with the remark that they are related to the scale range and not with L n . The specified creep is the effect that occurs if the load is changed from zero to L n (the rated load of the load cell), or inverse. Naturally, this effect is less, if the load is changed over a smaller part of the calibration curve. Also here we will use the specified figure for the error calculation, but with the remark that the figure is related to the scale range instead of L n . Repeatability is always existing and does not change with the loading situation. Related to the scale range the figure becomes bigger than the specified figure. Example: If the specified repeatability is r% of L n , this will be (n º L n )/S º r% of the scale range. 1.7.4.2 Use of the weighing installation Not every error type mentioned above will always contribute to the final weighing result. That depends on the use which is made of the installation. This will be illustrated by the following three typical examples. Case 1 normal weighing The aim is to make a rapid, accurate measurement of certain mass quantity. Before loading the platform or vessel (load carrier), the zero point on the scale is checked and/or adjusted. After the mass is placed and the GLOBAL Weighing / 77 indication is stable, the reading is done. The procedure eliminates all zero errors. A variant to this procedure is ‘negative weighing’. In that case the mass to be weighed is taken out of the vessel. This also eliminates zero errors. In both cases we have to do with 1. TK c (temperature influence on span) 2. F u (hysteresis) 3. F lin (non-linearity) 4. F v (repeatability) Remarks: a) Knowledge of the conditions during the calibration of the installation can be important. If the temperature is about the same as during calibration, the temperature effect is small. If the weighed loads are in the neighbourhood of test load during calibration, the non-linearity error will be very small. Excluding F lin means that we have to calculate with F u . In that case that the measurements are done in an arbitrary way all over the scale range, F lin cannot be excluded and we can calculate with F comb as the combined effect of hysteresis and non-linearity. b) Repeatability is always existing. We have to it with twice. Case II Storage silos Here the installation is used to measure how much material is present at different moments. The load is changed continuously, but never made zero. In this case it is impossible to readjust the zero point before a measurement. Moreover, the measurements are done over a very long period and under very different environmental conditions. Therefore all influence factors play their role. Remark: We have, of course, the case I again, if we use this installation for a short term measurement of the incoming or outgoing quantities. Case III Batching The aim of this measurement is to weigh small quantities, (e.g. additives in a batch), which are added to or subtracted from a big load. The loading therefore only changes in small steps, which excludes the hysteresis effect. Also zero effects are not present, because the value of the small quantity is found by subtracting two subsequent readings. Non-linearity plays ist role, because the slope of the calibration curve at the point of the actual load can be very different from the mean value. The error expressed in kg, however, will be very small because the measured incremental quantity is very small. The conclusion is that in this case we have only to do with the repeatability error F v , and that twice! Other error sources Until now we discussed the weight indication for an ideal installation with: - no mechanical disturbing influences, such as wind, pipes, friction etc - no electrical interference on the cabling - an ideal electronic equipment with negligible measuring errors - an exact reading of the weighing result with no human errors and no rounding off problems for digitalisation There are installations where not all of these sources of error can be eliminated or kept the desired minimum. Installation cost and technical problems can limit our possibilities! In that case it will be clear that all these error sources should be taken into account if the accuracy of the final weighing result is discussed or even has to be guaranteed! Note The rounding-off error in digital equipment can be very important. Per reading it can be ±0.5d at maximum. (d: scale division). If the weighing result is the difference of two readings this extra error will be ±1d at GLOBAL Weighing / 78 maximum (however, not probable ...). If the scales have 1000d this is ±0.1% of the scale range. Especially for incremental weighing this could be a reason for choosing scales with many divisions. Totalizing the partial errors This is an uncertain point in the error calculation. As is said already the specified influence factors are maximum and absolute values. We will call the maximum errors found with them the 'partial errors'. Now the problem is that we want to come to the most probable total error. therefore we could totalize them by pure adding and find the total error F tot max (called ‘maximum’). But then we know that this F tot max is certainly too big. The probability that this maximum value is reached is practically zero. Another approach is to take the root out of the sum of the squares of the partial errors. This could give a better result. However, there is no mathematical proof for that. (It could even be stated that such a calculation is only allowed for a big number of measurements made under the same circumstances. Here we have to do with one measurement under arbitrary conditions done with the aid of a small number of load cells out of a very big number of manufactured load cells! This is not the same.) We call this second approach F totp (‘probable’). Because there is no mathematical proof for the use of this approach, we do not use it. Remark: If we use more than one load cell in the installation this has an ameliorating effect on the influence factors. For the combination the factors are the same as the specified ones if the first approach is used. With the second approach they are multiplied with 1/_n. Example 1 normal weighing Vessel with three load cells PR 6201/23D1 installed total 6000kg dead load 3000kg net scale range 2000kg The calibration was done at 1800kg net and at 15°C. What is the tolerance if a measurement is done of 1500kg at a stable temperature of 35°C? Technical data TK c 0.03%/10K TK 0 0.08%/10K * F v 0.01% * F cr30 0.02% * F cr4h 0.05% * F lin 0.05% * F u 0.03% * In this case the relevant influence factors are TK c , F u . The temperature has a difference of 20°C compared to calibration. The partial errors are temperature on span (35°C - 15°C)/10°C º 0.0003 º 1500kg = 0.9kg linearity error 0.0005 º (2000kg/6000kg) 2 º 6000kg = 0.33kg repeatability at zero 0.0001 º 6000kg = 0.6kg repeatability at load 0.0001 º 6000kg = 0.6kg The total error is calculated as maximum error 0.9 kg + 0.3 kg + 0.6 kg + 0.6 kg = 2.43 kg REMARK For a weight of 1,800kg, the maximum error is 2.37 kg GLOBAL Weighing / 79 Example 2 Storage silo We use the same vessel as in example 1. What is the tolerance for a measurement of 1500kg at a temperature of 20°C? Now all influence factors play a role. temperature effect on span (20°C-15°C)/10°C º 0.0003 º (3000kg + 1500kg) = 0.68kg temperature effect on zero (20°C-15°C)/10°C º 0.0008 º 6000kg = 2.40kg linearity error 0.0005 º (2000kg/6000kg) 2 º 6000kg = 0.33kg repeatability at zero 0.0001 º 6000kg = 0.6kg creep 0.0005 º (3000kg + 1500kg) = 2.25kg The total error is calculated as maximum 6.23 kg Example 3 Batching The same vessel as in example 1. What is the accuracy with which we can measure an addition of 100 kg to a load of about 1500 kg? With a quadratic non-.linearity there could be calculated that the maximum deviation of the slope of the calibration curve over the scale range will be ±0.07% of the mean slope. Taking this as a possible span error for the measurement of the small quantity of 100 kg then we have the following partial errors: span error due to non-linearity 0.0007 × 100 kg = 0.07kg repeatability before the addition 0.0001 × 6000 kg = 0.6kg repeatability after the addition 0.0001 × 6000 kg = 0.6kg The total error is calculated as maximum 1.3 kg GLOBAL Weighing / 80 1.8 Installation and commissioning The possible measuring accuracy of a weighing installation depends on the positioning of the load cells and especially on their mounting. Load cells are accurate measuring devices and must be handled with care. Do not lift the load cell at its cable. Avoid shock loads on the load cell (falling goods, heavy shocks). During the building phase of a system the load cell should be replaced by a dummy to keep damages caused by welding away from it. 1.8.1 Mechanical installation Aligning the load cells is not necessary in case of only three load cells because such an installation is mechanically defined (refer to chapter 1.2). In case of more than 3 load cells in one installation the load cells must be adjusted in height to avoid waggling and to get an even load distribution. Each load cell of the particular installation shall take nearly the same load. This can be achieved by shimming: you insert thin sheets of metal (thickness: 0.5mm...2mm) between upper loading plate of the mounting kit and the construction. The criterion for the best alignment is a load distribution which also under different loading conditions ensures enough load on every load cell. To find out the actual load distribution, you must measure the forces on all load cells separately (this is especially advised in the case of a very big object with more than four load cells). Alignment procedure 1. Disconnect all load cell output leads 2. Measure the outputs of the different load cells separately with a measuring instrument. 3. Compare the indications. 4. If an indication is too low, then you need another shim at that particular load cell. 1.8.2 Electrical installation Load cell cabling instructions Fix the load cell cable in such a way that the load cell does not move when being pushed or pulled at the cable. Shortening the load cell cable changes the factory calibration of the load cell. Therefore the shortening is forbidden. The load cell cables and the extension cables should be carried in armoured steel conduits, which also have a magnetic shielding function. All cables for measuring purposes have to to be mounted separately from other cables. They shall be put at a distance of at least 1m from all power cables. The permissible length between load cell and measuring instrument mainly depends on local circumstances. Normally the limit for W&M purposes is 300m. Ensure that no moisture enters the cables or the cable connections before and during the mounting, installation and operation. The screens of the load cell cables should remain insulated and only interconnected at the ‘screen terminal’ situated in the cable junction box. The special GLOBAL Weighing cables PR 6135 or PR 6136 should be preferably used to interconnect the cable junction box with the electronic measuring instrument. The screens of this cable should also be interconnected at the ‘screen terminal’ in the cable junction box. Earthing of the total screen should be done at the ‘screen terminal’ of the measuring instrument. GLOBAL Weighing / 81 Earthing Earthing of an electronic weighing installation serves three purposes: safety, prevention of interference, and damage prevention. 1. safety All electrical equipment connected to the mains must not cause a danger of life if being touched. The legal regulations are to observed. 2. Interference Capacitive coupling of the outer world to the measuring circuit can disturb the measurement. This can be avoided with cable screens which have to be at the same potential as the measuring circuit. Therefore the following measures are to be taken - provide armoured steel conduits for the cabling - least distance between power cables and measuring cables 1m. Mind that all screens have to be earthed at one point only, to avoid that otherwise stray currents still could change their potential. 3. Damage prevention It is a very dangerous situation if the strain gauge filament has a too big voltage difference to the load cell body. For that reason the load cells have an earthing screw which can be interconnected with the ‘central earth rail’. Heavy stray currents can be expected if e.g. the weighed object is situated outdoors or at a big distance from the electronic measuring equipment (weighbridge, big bunkers, etc.) In those cases you have to take the following practical measures: a) An ‘earthing tube’ or ‘earthing plate’ with an earth resistance of <5Ω should be put into the ground in the neighbourhood of the weighed object. b) This earthing electrode has to be connected with the ‘central earth rail’, mounted in the neighbourhood of the cable junction box. c) now make the following connections: (the thick lines in the wiring diagram) - all load cell earthing screws with the central earth rail with conductors of a t least 6mm 2 copper - the ‘measuring earth terminal’ of the electronic measuring instruments with the central earth rail (equalizing line) NOTES: • Mostly 16mm 2 is enough. You can make a rough check, if you can estimate the possible current flowing through the equalizing line. The voltage drop may not exceed 100V, which is the maximum allowed voltage between strain gauge filament and load cell body to avoid electrical damage. • The measuring cable PR 6135 is mounted in a steel pipe. This pipe could be used as a shunt resistance to the equalizing line to lower its resistance. Attention: Never use cable screenings for this purpose! Fig. 1.8-1 Earthing of a weighing installation GLOBAL Weighing / 82 1.8.3 Calibration Reasons for a calibration procedure on site Weighing is the determination of the mass of an object (refer to chapter 1.1). Mass is an invariable property of an object. It does not change with its position on the earth nor in space. However weighing with load cells measures a force. The weight F G is the force with which the earth attracts the mass. This force depends on the position on earth (see table). The characteristic value for the weight is the gravitational field strength at that specific spot. city gravitational field strength g [m/s 2 ] Hamburg 9.814 Helsinki 9.819 Johannesburg 9.786 Madrid 9.800 Melbourne 9.800 Milan 9.806 Paris 9.809 Singapore 9.781 Teddington 9.812 The weight is calculated with the formula m g = F G • F G Force in Newton m mass in kg g local gravitational field strength in m/s 2 Please, notice the different values of g. The table shows a difference of 0.4% between the extremes in g. In the load cell factory in Hamburg the output of the load cells is calibrated within ±0.25%. If the desired weighing accuracy must be better than ±0.25%, a calibration procedure on site is indispensable. Example: scales used for trade transactions. Check on proper installation Before the final calibration, it is necessary to check if all disturbing effects are reduced to a minimum. Always keep in mind that errors are seldom generated by the load cells but are introduced by the mechanical or electrical installation of the equipment. 1. Visual inspection mechanically: pipes, hoses, dirt, stones, friction by touching the outer world, vibration of foundation, bellows, wind, suction, etc electrically: bad ground-connection, interference, moisture in cable connections 2. Stability test Put a stable load on vessel or platform and observe the stability of the weight indication. This should be excellent. 3. Resolution Put on a very small load of about 0.5d value and observe if the change in weight indication is big enough. Repeat this after taking the small test load away. 4. Hysteresis test Check how the weight indication comes back after placing and removing a big load. Errors with these tests should be well within the desired accuracy limits on different places of the scales. Possible causes of error can systematically be traced by: a. disconnecting or removing parts which link the weighing object and its surroundings (pipes, hoses and the like) b. looking for changing material contents in pipes and hoses connected to the object c. looking for changing gas pressure on bellows (see chapter 3.3) GLOBAL Weighing / 83 d. switching off electric motors on the object or in its neighbourhood e. measuring the insulation resistance in the cabling f. in case of severe doubt, the load cells can be checked electrically (see sheet 1.851) Final calibration The purpose is to bring the indication on the measuring instruments (in mass units) in accordance with the weighed mass. This is done by changing the span of the electronic indicating instrument. The test weights used for the final calibration can be - calibrated test weights (weight stones with a small tolerance) - some material, weighed on another weighbridge - a known quantity of fluid of known specific gravity (and non volatile!). In all cases you should know (or have a good estimate of) the tolerance of the test weights, which should be at least a factor 3 (O.I.M.L.) better, but preferably 5 or 10 times better than the desired installation accuracy. If you have to check against an O.I.M.L.-tolerance curve, special attention must be given to the critical points at 500d and 2,000d. Remark that correction of a small error at 500d, by changing the instrument span, has a bigger influence at the higher scale values. Example: a correction of 0.4d at 500d changes the indication at the 2,000d scale position by 1.6d = 0.4d 500 2000 • On the other hand a correction for an error of 0.4d at 2000d changes the indication at 500d only by 0.1d = 0.4d 2000 500 • 1.8.3.1 Calibration of vessels of more than 5 tons O.I.M.L. and most regulations require that a weighing installation is suitable for testing. This means among other things that a load carrier must be designed in such a way that test loads can applied in an easy and safe way. In the case of bunkers, hoppers, etc. a (detachable) carrier should be supplied for 10% of the weighing capacity, but at least for 5t of calibrated weights. There is no general rule for the special load carrier which has to be provided. Depending on the local circumstances, this may be designed on top of the vessel, hooked around the vessel, or hanging under the vessel. However, be sure that a normal even distribution of the load is possible. Calibration of the vessel may be done with a certain amount of calibrated weights (e.g. 5t) plus arbitrary weights. Remark: the arbitrary weights should have such a composition that their masses will not change during the calibration procedure (think of evaporation, hygroskopic effects, leakage, loss of material). Calibration procedure for non- W&M installations If there are no legal requirements the procedure above described can be followed. However, if necessary a smaller amount of calibrated weights may be used. Calibration procedure 1. Adjust the weight indicator to read zero (compensation for tare weight) 2. Put on the obligatory amount of calibrated weights; note the indicator reading 3. remove the calibrated weights 4. fill the vessel with material until exactly the previous reading 5. repeat the steps 2...4 until 100% of the weighing capacity is reached 6. adjust the span of the indicator, if necessary 1.8.4 Corner point adjustment The unloaded platform is put on the disconnected load cells. The load cells are supplied with 12V or 20V depending on the electronic used. The outputs of all load cells are measured separately. Shims are put between the upper loading plates and the platform until the difference between the readings is not more than 0.2mV (this amount corresponds with about 5% of the load on the load cells due to the weight of the platform). GLOBAL Weighing / 84 O.I.M.L. and most national regulations ask for a corner point test. During this examination a test load (, which is defined by these regulations) is placed successively on the corners of the ‘load carrying device’, e.g. the platform. The different readings should be the same within prescribed limits. The reading is the result of the electrical addition of the load cell outputs caused by the parallel circuiting of the load cells. The accuracy of this addition is influenced by the equality of the value C n /R a of the different load cells. C n rated output in mV/V R a output resistance in O As for an example the output of two load cells in parallel can be calculated from ) R C L + R C L ( R + R R R a 2 2 a 1 1 a a a a 2 1 2 1 2 1 ⋅ ⋅ ⋅ ⋅ Normally the factory adjustments on the rated output C n and the output resistance R a of the load cells are good enough to fulfil all legal requirements. The corner point test results can be improved by looking for the load cell with the lowest output and thereafter decreasing the output of all other load cells until that lowest output value. Procedure 1. Take the readings of the corner point test. 2. Determine the lowest reading. 3. Calculate the deviations of the other readings compared to the lowest reading. 4. Insert a resistor in the supply line of the deviating load cells to lower the C n - value of these load cells. 5. These resistors must have the value ] [ impedance cell load [kg] load t cornerpoin actual [kg] deviation = R Ω • Note: The resistors should only be fitted with the instrument switched off! Example of an adjustment According to the data sheet the output resistance of the load cells is R a = 610Ω. The test load of the corner point test is 10,000kg. corner actual load [kg] deviation [kg] 1 9,994 ----- 2 10,016 +22 3 10,004 +10 4 10,000 + 6 Practical execution of the adjustment The resistors can be made of manganine wire. E.g. with 0.5mm the resistance is 476mm/ Ω. corner resistor value manganine wire load cell 1 no resistor --- load cell 2 1.3Ω 619 mm load cell 3 0.6Ω 286 mm load cell 4 0.4Ω 190 mm The calculated resistors are mounted in the cable junction box at the indicated positions (see fig. 1.8-2). GLOBAL Weighing / 85 Fig. 1.8-2 Load cell wiring diagram 1.8.5 Load cell check The check described in this chapter is intended to give you a first idea what may have happened to a load cell which seems to be defective. A more detailed check is only possible under the conditions of a test stand, i.e. in the factory in Hamburg. Before you do any checks at least the following questions must be answered: • What kind error was observed? (occurrence of the error: accidentally, slowly increasing, suddenly, ...) • How long was the system in operation until the error occurred? The more you know about the error and the circumstances in the installation the easier you find right source of the problems (be it the load cell, be it the construction ...). The load cell check consists of two parts 1. a visual check 2. a simple electrical check Both the checks are important. Very often you can judge by the look of the load cell what caused the system to fail. Visual check The visual check is quite simple: describe the load cell and its mounting parts. Especially focus on the points mentioned below. - load cell top and load cell bottom size and shape of the black contact spots (large, round, ...) surface (smooth, ...) - mounting parts size and shape of the black spots surface - cable sheath cuts ... Simple electrical check on load cells You need the following instruments - 12V DC supply (e.g. indicator) - digital multimeter, resolution 10V DC - display of weighing indicator GLOBAL Weighing / 86 A standard procedure is used to check the technical data of the load cell. It helps you to find the diagnosis when the load cell shows deviations. During the checks you measure the load cell data - tolerance on zero point D 0 - input and output resistance (R in , R out ) - insulation resistance (R iso ) The load cell must be completely unloaded for all described checks. Check 1 tolerance on zero point (D 0 ) Fig. 1.8-3 Tolerance on zero point The permissible values depend on load cell type. You find them in the data sheets and the instruction manuals for the particular type. For your convenience the values are calculated with a load cell supply voltage of 12V (see fig. 1.8-3) - PR 6201, PR 6221 < ±1% of 1mV/Vº 12V = ±0.12mV - PR 6211, PR 6221, PR 6241, PR 6246 < ±1% of 2mV/Vº 12V = ±0.24mV For a supply voltage of 20V the values change to ±20mV or ±40mV, of course. Check 2 bridge resistances Fig. 1.8-4 Bridge resistances The exact values differ with the load cell type and its accuracy class. Please refer to the data sheets or the instruction manuals to find them out. Example: PR 6201/14C3 - input resistance R in = 650O±6O (measure red and blue core) - output resistance R out = 610O±0.5O (measure green and grey core) Additionally it is recommended to measure the further combinations red and grey, red and green, blue and green, blue and grey GLOBAL Weighing / 87 Check 3 insulation resistance R iso Measure each core and also the screen (if existing) with the housing. If you look into our data sheets, the specification says „better than 5000MΩ“. But it should be not too easy to measure such values with a standard digital multimeter because such an instrument displays “OL” (“Open Loop”) if the insulation resistance is higher than 20MΩ. Interpretation of the results: 1. If a load cell fails in check 1, it was usually overloaded. Further information can be gained from check 2: if the resistances between the cores (part 2 of check 2) differ significantly was the load cell tilted? did the housing touch the bottom plate of the mounting kit? 2. A load cell passes check 1 but fails in check 2. In that case you can be sure that the strain gauge broken. Possible causes are · welding · lightning strokes · vibration · temperature too high for load cell 3. A load cell failed in check 3. An „insulation resistance“ between 10O and 50kO indicates that the strain gauge and/or its insulation is destroyed. Values between 50kO and 200kO indicate the penetration of moisture in the load cell. In such a case you can measure a galvanic voltage between housing and cores. REMARK: the weight indication returns to a correct value when the load cell is completely insulated from the ground potential (not valid in case of the penetration of moisture) 4. Perhaps the load cell passed all the tests above. It could be sensitive to knocking (use a light hammer, about 100g). Connect the load cell like in fig. 1.8-3, put some paper or plastics between load cell and hammer, and knock slightly on the housing, the measuring element and the small connection box. Does the indication remain stable? Perhaps not all wires in the cable junction box are correctly connected? (bad soldering, screwed connection not fastened...) GLOBAL Weighing / 88 2. MOUNTING THE LOAD CELLS To enable easy mounting combined with simple constraining, standard mounting kits are provided for all load cell families in our programme. Most kits are available either in constructional steel or in stainless steel. - N- versions are always made of constructional steel (Fe360, St37), are zinc plated, and have a yellow chromated surface - S- versions are always made of stainless steel (e.g. B.S. 304S15, X8CrNi 18 10, 1.4301) Some mounting kits are integrated mounting kits: they combine a standard mounting solution with constraining, e.g. the MiniFLEXLOCK. Furthermore they have a threaded hole in the lower mounting plate which can be used for installing a lift-off protection. You can use following combinations of rod and lower mounting plate: - minimal property class of the rods 4.6 (i.e. limit of elasticity 240 N/mm 2 , ultimate load 400 N/mm 2 ) - minimal rigidity of stainless steel A2 thread maximum tension force (kN) M6 2.5 M8 4.5 M12 10.5 M16 19.6 M20 30.6 The lift-off protection must not touch the weighing object during the execution of a weighing. It is a safety device, a stop and not a constrainer. Therefore the clearance, as shown in fig. 2.0-1, is important. There are drilling templates (true-to-scale representations!) available for every compression cell mounting kit. They provide easy installation because you can already drill the holes for fixing the mounting kit in the workshop and need not to do it at site. Please, keep to the warnings on their covers: do not transmit the drilling templates by telefax and do not copy them with a photocopier because the dimensions will not be correct on the copy. Example: A distance of 65mm can vary between 62mm and 67mm depending on the type of telefax- or photocopy- machine used. Sometimes it may be necessary to protect the mounting bolts from losening, e.g. in an installation where heavy vibrations occur. This can be achieved by cementing the bolts to the nuts. A possible cement is LOCTITE 274. Install the flexible copper strap which is delivered together with the load cell. Make sure that before installing the load cell all welding at the construction is finished. Mount the load cell vertically so that the weighing result is not disturbed by the tilted load cell. Fig. 2.0-1 Some indispensable parts and accessories GLOBAL Weighing / 89 2.1 Mounting the compression load cell PR6201 The mounting kits are intended to be used together. They are equal in height for the load cells in the range between 0.5t up to 50t. The way how to arrange the mounting kits is discussed in chapters 1.3 and 1.4. (3 load cells installed with PR6143/.., the others with PR6145/..). Sometimes the stainless steel version, especially the base plate and the load disc, are checked with magnets and found to be magnetic; and that is correct: both articles are magnetic, although they are made of stainless steel. So do not tell the customer all parts from stainless steel are non-magnetic! Furthermore, even non- magnetic stainless steel can become magnetic after special treatment (e.g. deep drawing, pressing). 2.1. 1 Mounting kit PR 6145 The standard mounting plate kit PR6145 permits easy and service-friendly installation of the PR6201/.. load cell series. type order number capacity PR 6145/00 N 9405 361 45001 0.5...50t PR 6145/00 S 9405 361 45002 0.5...20t PR 6145/08 N 9405 361 45081 100t PR 6145/10 N 9405 361 45101 200t REMARK: If PR 6201/24D1, PR 6201/24C3, PR6201/34 and PR6201/54 are used with stainless steel version PR6145/00S, you must order the additional stainless steel bottom plate PR6143/54S. ASSUMPTIONS · Ensure that the foundation is horizontal (check using spirit level), flat and rigid for the expected loads. · Ensure even load distribution. The foundations should be at equal height and the contact surfaces of the object (vessel or platform) should be located in parallel. · The bore holes of top and bottom mounting plate must coincide. MOUNTING INSTRUCTIONS · Mount lower and upper mounting plate at foundation or object support with screws. It is indispensable to ensure that the plates are located in parallel and vertically upon each other · Link upper and lower mounting plate with the flexible copper strap (fig. 2.1-1, pos.2) packed with the load cell. Two screws M10x15 with washer A10 are included in the mounting kit · Clean the load cell seating in the mounting plate from dirt. · Grease load cell top and bottom with the included grease. · Insert the load cell only, when all welding work near the load cell and mounting work at the object is finished. · When using PR6145/00S, the bottom disc marked with ‘SS’ and the light beige O-ring have to be used. The standard bottom disc (without marking) and the dark grey O-ring are not required · Position the load button correctly type capacity position of load button PR 6201 0.5t ... 10t, 20t L small hollow down, circular groove up PR 6201 20tD1/C3, 30t, 50t small hollow up, circular groove down GLOBAL Weighing / 90 INSPECTION AFTER MOUNTING · check if the mounting plates are vertical upon each other, and in parallel · the load cell must be in vertical position (if necessary, release the mounting screws slightly and correct the position of the mounting plates, without obstructing the holes for the bolts) · ensure the right position of the load button Fig. 2-1-1 Dimensions of mounting plate kit PR 6145/.. Type Nominal load Dimensions in mm a b c d e f g h i PR 6145/00N 0.5t...50t 15 190.5 15 150 115 14 65 100 18 PR 6145/00S 0.5t...50t 15 190.5 15 150 115 14 65 100 18 PR 6145/08 100t 30 290 30 180 145 18 95 130 18 PR 6145/10 200t 40 385 40 220 185 24 135 180 14 GLOBAL Weighing / 91 2. 1.2 Mounting kit MiniFLEXLOCK PR 6143 MiniFLEXLOCK PR6143 is a mounting kit for PR6201 load cells with a load rating of 0.5t to 50t, which also provides horizontal constraining of the object to be weighed. The kit is prepared for construction of a protection against lifting. In order to ensure the required space for movement of the weighing facility, max. three MiniFLEXLOCK kits may be used for constraining an object. When using a higher number of load cells, the remaining load cells must be installed with mounting kit PR6145/00. type order number load cell PR 6143/00N 9405 361 43001 0.5t...50t PR 6143/00S 9405 361 43002 0.5t...20t PR 6143/10N 9405 361 43101 0.5t...50t PR 6143/10S 9405 361 43102 0.5t...50t ASSUMPTIONS · ensure that the foundation is horizontal (check using spirit level), flat and rigid for the expected load · ensure even load distribution the foundations should be at equal height and the contact surfaces of the object (vessel or platform) should be located in parallel · the bore holes of top and bottom mounting plate must coincide. MOUNTING INSTRUCTIONS · insert the dummy load cell or, if no dummy is provided, the load cell with the load button · position the load button correctly type capacity position of load button PR 6201 0.5 ... 10t, 20t L small hollow down, circular groove up PR 6201 20t D1/C3, 30t, 50t small hollow up, circular groove down · When using a stainless steel MiniFLEXLOCK (version S), the bottom disc marked with „SS“ and the food- resistant O-ring (light beige) must be used. In this case, the standard bottom disc (without stamp) and the dark grey standard O-ring are not required. · Connect upper and lower mounting plate with flexible copper strap (packed with the load cell). Two bolts M8 and washers for locking are delivered with the mounting kit. · Re-mount the upper mounting plate at the auxiliary plate. · Mount lower and upper mounting plate by means of bolts and washers to foundation and weighing object support (vessel, platform, etc.). For constraining unit PR6143/00, bolts M12, min. rigidity class 5.8 (to ISO 898), and for PR6143/10, bolts M14, min. rigidity class 8.8 are required. It is indispensable to ensure that the plates are located in parallel and vertically upon each other · When all constraining units and mounting kits are mounted at the weighing facility, remove the auxiliary mounting plates (tighten the relevant bolts in the threaded holes of the mounting plates) and align the mounting plates so that the dummies stand untiltedly and in exact vertical position. If required, release the mounting bolts slightly and adjust the mounting plates. For this purpose, the holes offer some free space. · tighten the mounting bolts crosswisely in the order 1...6 (fig. 2.1-2). GLOBAL Weighing / 92 Fig 2.1-2 Tighten the bolts in the order 1...6 Bolt Property class Mounting torque PR 6143/00N M12 5.8 104Nm PR 6143/00S M12 A2 104Nm PR 6143/10N M14 8.8 165Nm PR 6143/10S M14 A2 165Nm · Only when all welding work near the load cell and mounting and alignment at the object to be weighed are finished, lift the object and replace the dummy by the load cell take care that the load button is positioned correctly. The load cell must stand vertically and untiltedly after installation. INSPECTION AFTER MOUNTING · Check if the mounting plates are vertical upon each other, and in parallel · The load cell must be in vertical position (if necessary, release the mounting screws slightly and correct the position of the mounting plates, without obstructing the holes for the bolts) · Ensure the right position of the load button · Check, if the constrainer is free of load, i.e. if it can be rotated around its longitudinal axis. If this is not the case, release the counter-nuts (Fig. 14, part 5) of the constrainer, and turn the hexagon (6), until the constrainer is free of load. Subsequently, re-tighten the counter nuts and position the swivel bearing eyes vertically. · Check, if there is vertical space for movement and the required space for thermal expansion. The permissible tolerances without considerable effect on the accuracy are given in Fig. 15. The space for movement required for displacement of the object to be weighed due to thermal expansion, vibration, etc. can be used only if load cell and constraining unit are installed exactly. To prevent vertical force shunts, all mechanical connections (pipes, cables, bellows) of the object to be weighed to its surrounding construction must be as flexible as possible. The overall load must be supported by the load cells. GLOBAL Weighing / 93 Fig. 2.1-3 Lift-off protection with PR 6143 SERVICE INSPECTION - Check if the constrainer is not clamped. - Check if the safety clips are present and if the axle is fixed Fig. 2.1-4 Safety clip and axle Fig. 2.1-5 Dimensions of MiniFLEXLOCK PR 6143/00 GLOBAL Weighing / 94 Fig. 2.1-6 Dimensions of MiniFLEXLOCK PR 6143/10 GLOBAL Weighing / 95 2.3 Mounting the ultra flat PanCake load cell PR 6251 PanCake is the ideal load cell for level-by-weight (cf. 3.5). Its performance is better than the often used tank radar system or ultrasonic level measuring devices. PanCake is easy and simple to install (“just bolt it”) under the foot of a tank. There is no need for additional expensive mounting kits. If you have no access from the upper side of the foot to the load cell, you may apply the transition plate to mount the PanCake (fig. 2.3-1). Fig. 2.3-1 Transition plate PR 6051/00S for PanCake All supporting and connecting plates (for foundation and vessel) must be horizontal, flat and rigid. The plate which is in contact with the load cell top must be tempered to a hardness of 42 HRc. The standard base plates PR 6051/1x fulfil this requirement. The little pins help to position the load cell correctly, but they are not designed to withstand horizontal side loads. Fig. 2.3-2 Base plates PR 6051/10S, .../11S for PanCake GLOBAL Weighing / 96 2.4 Mounting bending beams A safety instruction for suspended loads If a break in suspension, support, load cell, or mounting part etc represents a hazard to the life and health of men or animals, or if goods may be damaged, additional safety devices must be provided. Observe the overload protection as the beams have quite low capacities and are easy to overload! Keep in mind that a 10kg load cell will be damaged if it is loaded with 30kg. In other words, no one must step onto it; otherwise it is damaged. Fig. 2.4-1 Mounting kit PR 6007 The use of the mounting kit PR 6007 ensures a simple and safe suspension of the weighing object. The stable cover protects the thin and sensible bellows. By turning the protection cover to 180°, the load end of the cell is either protected or freely accessible. The pre-stretched steel rope provides easy restoring to the stable position and a counteracting against side forces and torque momenta of the object. Spherical washers minimize the risk of tilting at the suspension points. The overload stop protects the load cell against damages due to overload. GLOBAL Weighing / 97 MOUNTING INSTRUCTIONS - Already mount load cell and mounting kit in the workshop Pay attention to the arrow on the front side of the load cell, showing the direction of the load. - Clean the support surface and the mounting surface of the load cell and mount the load cell by means of two bolts. - Mount the kit on the supporting construction and fasten the plate with four bolts. - Lift the object to be weighed and mount the steel rope. Take care that the spherical washers are correctly positioned. The castle nut has to be secured by a split. - If the object is suspended by more than 1 load cell, adjust the mounting kits in such a way that it is level. - Adjust the overload stop! Load the object with 130% full load. The overload stop must touch the load cell under these conditions. Lock the overload stop safely. GLOBAL Weighing / 98 2.5 Mounting the S-type load cells 2.5.0 Differences between PR 6246 N and PR 6241D1, PR 6246D1 2.5.0.1 General information about the replacement PR 6246N had been developed as a universal load cell for tension and compression use. For the W&M version of class C3 it proved to be necessary to separate tension from compression type. Therefore it was decided to create two different families: one for tension applications (PR 6246) and the other for compression use (PR 6241). In some countries there was a certain demand for a load cell useful for W&M systems of class IIII. So, the N versions were upgraded to D1 versions. Direct replacement of PR 6246 N in an existing installation is possible as you see from the table below installed load cell replacement for tension mode replacement for compression mode PR 6246 N, 100kg...500kg PR 6246 D1, 100kg...500kg (PR 6241 D1, 100kg...500kg) PR 6241 D1, 100kg...500kg (PR 6246 D1, 100kg...500kg) PR 6246 N, 1t...3t PR 6246 D1, 1t...3t PR 6246 D1, 1t...3t PR 6241 D1, 3t The above table shows that the small versions of PR 6241 D1 and PR 6246 D1 may be used in either mode if they are not in an W&M system. But you may not use the compression type load cells PR 6241 with capacities of 1t up to 5t in tension mode. This applies for all installations, W&M and non- W&M. In new installations the following load cells should preferably be used tension mode PR 6246 D1, 100kg...3t compression mode PR 6241 D1, 100kg...5t You have to use the load cells from the list below for approved installations tension mode PR 6246 D1, 100kg...3t compression mode PR 6241 D1, 100kg...5t You may not use the types with the small capacities in other modes as indicated by the two arrows on the load cell. GLOBAL Weighing / 99 2.5.0.2 Colour codes of the load cell cable The colour code of the load cell cable wires was also changed from PR 6246 N to PR 6241 D1, PR 6246 D1 types. PR 6246 N was defined as a „universal load cell“; its preferred operation direction was „compression mode“. Now the definitions are - PR 6241 D1 is a compression load cell. The output signal is positive if the load cell is loaded with a compression load. - PR 6246 D1is a tension load cell. The output signal is positive if the load cell is loaded with a tension load. Fig. 2.5-2 Connection for load cell operated in specified mode Fig. 2.5-3 Connection for load cell operated in reverse mode 2.5.1 Mounting kits for compression load cell PR 6241 2.5.1.1 Mounting kits PR 6041/30, PR 6041/40 Mounting kits PR 6041/30 N + S and PR 6041/40 N + S are used for unconstrained installation of GLOBAL Weighing series PR 6241 compression load cells. The kit can be used for load cell types PR 6241 D1 and PR 6241 C3. Mounting kits PR 6041/30 and PR 6041/40 N + S comprise an upper and a lower mounting plate and a load button set. Fig. 2.5-4 Mounting kit PR 6041/30 GLOBAL Weighing / 100 The load cell is installed with the load buttons fitted between the mounting plates. Due to their spherical head surface, the load buttons permit limited load cell inclination, in order not to prevent free movement with thermal expansion of the object (vessel, etc.) to be weighed. During transport and mounting, the mounting kit is fixed by means of an auxiliary mounting plate, which must be removed after installation. The mounting kit includes a load cell dummy, which is used for installation and alignment of the mounting kit and replaced by the load cell after finishing mounting, welding and alignment. Before installing the mounting kit with the load cell, ensure that the foundation is horizontal (check with spirit level), flat and rigid for the expected load. In order to ensure even load distribution, the foundations should be at equal height and the supporting surfaces of the object to be weighed (vessel or platform) should be in parallel. The bore holes of upper and lower mounting plate must coincide. MOUNTING INSTRUCTIONS · Screw lower and upper mounting plate (with auxiliary plate fitted and load cell dummy in position) at foundation and support of object to be weighed. When all mounting kits or constraining units are mounted at the weighing facility, remove the auxiliary mounting plates (screw the relevant screws in the threaded holes of the mounting plates) and align the mounting plates so that the load cell dummy is uncanted and exactly horizontal. If necessary, release the mounting screws somewhat and adjust the mounting plates (the screw holes offer some space). Now, tighten the mounting screws correctly. · Connect upper and lower mounting plate using the flexible copper strap delivered with the load cell; two mounting screws M8 x 10 with spring washers are included in the mounting kit. · Replace the load cell dummy with the load button set only after finishing the mounting and alignment at the object to be weighed including all welding near the load cell. When installing, take care that the mounting parts are exempt of dirt (sand, etc.). The load cell shall be positioned centrally between the limiting pins and shall be vertical and untilted after installation. Fig. 2.5-5 Mounting kit PR 6041/40 GLOBAL Weighing / 101 2.5.1.2 MiniFLEXLOCK PR 6043/30, PR 6043/40 Mounting kits PR 6043/30 N + S and PR 6043/40 N + S are used for constrained installation of GLOBAL Weighing series PR 6241 compression load cells. The kit can be used for load cell types PR 6241 D1 and PR 6241 C2 + C3. To ensure an invariably high weighing accuracy, the foundation for the mounting kit must be horizontal (use spirit level), flat and rigid for the loads to be supported. In order to prevent partial overloading of the load cells, the load must be distributed as symmetrical as possible. Therefore, the foundations of the mounting kits must be aligned and the supporting surfaces of the weighing object (vessel or platform) must mounted in parallel. Fig. 2.5-6 MiniFLEXLOCK PR 6043/30 MOUNTING INSTRUCTIONS · Connect upper and lower mounting plate with flexible copper strap (packed with the load cell). Two bolts M8 and washers for locking are delivered with the mounting kit. · Mount lower and upper mounting plate by means of bolts and washers to foundation and weighing object support (vessel, platform, etc.). · When all constraining units and mounting kits are mounted at the weighing facility, remove the auxiliary mounting plates (tighten the relevant bolts in the threaded holes of the mounting plates) and align the mounting plates so that the dummies stand uncantedly and in exact vertical position. If required, release the mounting bolts slightly and adjust the mounting plates. For this purpose, the holes offer some free space. · Only when all welding work near the load cell and mounting and alignment at the object to be weighed are finished, lift the object and replace the dummy by the load cell. Take care that the pressure piece is positioned correctly. The load cell must stand vertically and untiltedly after installation. · Check, if the constrainer is free of load, i.e. if it can be rotated around its longitudinal axis. If this is not the case, release the counter-nuts of the constrainer, and turn the hexagon, until the constrainer is free of load. Subsequently, re-tighten the counter nuts and position the swivel bearing eyes vertically. GLOBAL Weighing / 102 Fig. 2.5-7 MiniFLEXLOCK PR 6043/40 After mounting, check · if the load cell stands untiltedly in the mounting kit, · if the upper mounting plate is positioned horizontally, and · if there is vertical space for movement and the required space for thermal expansion. The space for movement required for displacement of the object to be weighed due to thermal expansion, vibration, etc. can be used only if load cell and constraining unit are installed exactly. To prevent vertical force shunts, all mechanical connections (pipes, cables, bellows) of the object to be weighed to its surrounding construction must be as flexible as possible. The overall load must be supported by the load cells. 2.5.2 Mounting kits for the tension load cell PR 6241 2.5.2.1 Swivel bearing kit PR 6046 A safety instruction If a break in suspension, support, load cell, or mounting part etc represents a hazard to the life and health of men or animals, or if goods may be damaged, additional safety devices must be provided. The swivel bearing kit is used for suspension of objects to be weighed on load cells PR 6246. The kit is easy to install and permits free suspension of the object with short pendulum length and very low friction which may render constraining unnecessary. PR 6246/22...52 PR 6246/13...33 PR 6046/00 possible impossible PR 6046/11 impossible possible · When fitting the swivel bearing bolts into the load cell thread, following the hints in the operating instructions concerning screw-in depth, maintaining of the load cell and torque is indispensable. GLOBAL Weighing / 103 · Take care that the load cell is mounted in correct position, the connecting cable must not be weighed together with the load. The letters must be upright. · Secure the swivel bearing bolt by counter nuts to avoid self-releasing under the circumstances of the application. If heavy vibration must be expected, the threads must be protected with e.g. Loctite 274. Fig. 2.5-8 Swivel bearing kits PR 6046/00 and PR 6046/11 GLOBAL Weighing / 104 2.7 Mounting the compact load cell PR 6211 The load buttons are available as spare parts. load cell capacity order code PR 6211/31...32 30kg...300kg 5312 693 98068 PR 6211/52...53LT 500kg...5t 5312 693 98069 PR 6211/52...53D1 500kg...10t 5312 693 98085 2.7.1 Mounting kits for the small type (30...300kg) 2.7.1.1 Mounting kit PR 6011/00 Mounting kit PR 6011/00 is designed especially for easy and reliable installation of small PR 6211/31.../32 series compression load cells with a nominal load of 30 kg to 300 kg. By means of a rod, 3 nuts and 2 washers, the mounting kit facilitates the construction of a protection against lifting. Mounting kit comprises two mounting plates. The load cell is seated in a recess of the lower mounting plate. A pressure piece is located between load cell head and upper mounting plate. Its teflon surface permits easy movement within the recess in the upper mounting plate. Thereby, thermal expansions and small displacements of the object to be weighed can be absorbed without a measurement error. Fig. 2-7.1 Mounting kit PR 6011/00 The space for movement is limited by the size of the recess. If the load button presses against the edge of the recess with the constraining missing, horizontal forces in this direction are compensated via the load cell. In this case, heavy horizontal forces cause an inclination of the load button and may cause measurement errors and even mechanical damage. Therefore, take care that the load buttons are located centrally in the recess with the object in rest. Before installing the mounting kit, ensure that the foundation is horizontal (check with spirit level), flat and rigid for the expected load. To ensure even load distribution, the foundations should be equally high and the supporting surfaces of the object to be weighed (vessel or platform) should be in parallel. It can be used for mounting kit and constraining unit. The holes in upper and lower mounting plate must coincide. To facilitate load cell and mounting kit installation and alignment, dummies can be used. These dummies are inserted when installing the mounting kits. As their head fits exactly into the recess of the upper mounting plate, GLOBAL Weighing / 105 they ensure accurate alignment of mounting kits and weighing installation. The dummies are replaced by the load cells only, when all mechanical and alignment work, as well as welding near the load cell are finished. MONUNTING INSTRUCTIONS · Link upper and lower mounting plate using the flexible copper strap packed with the load cell. Two mounting screws M6 x 6 with fan washer are included in the mounting kit. · Screw lower and upper mounting plate to foundation or object support. Taking care that the plates are in parallel and vertically upon each other is indispensable. The above-mentioned dummy ensures that they are centered. · Replace the dummy by the load cell only, when all welding work near the load cell and mounting at the object are finished. Handle the load cell carefully to prevent damage to the membrane on the bottom. Place the object to be weighed carefully on to the load cells and take care that the load button is seated correctly in the recess of the upper mounting plate. · Check, if the load button is seated centrally in the recess of the upper mounting plate. For this, check distance A (Fig. ???) to protective ring around the load cell: it must be 6 mm. If necessary, release the mounting screws by a few turns and shift the mounting plates as far as permitted by the holes for the screws. Only if load cells and mounting kits are installed exactly, the space for movement, which is required for object displacement due to thermal expansion, vibration, etc., can be used fully without limiting the measurement accuracy. To prevent vertical force shunts, all mechanical connections of the object to be weighed to the surrounding construction (pipes, cables, bellows) must be as flexible as possible. The overall load must be supported only by the load cells. Tighten all screws and nuts well. When heavy vibrations are expected, we recommend using a protection e.g. by means of Loctite 274. A protection against lifting, can be realized using a rod M6 (rigidity ± 4.6), three nuts and two washers are required. Note that the rod must have sufficient space for movement in bore hole. 2.7.1.2 Rubber mounting kit PR 6011/03 Rubber mounting kit PR 6011/03 is used where the load cell is subjected to shock and vibration which can cause measurement errors. In order to minimize these effects, screw the rubber mounting kit on to the upper mounting plate of mounting kit PR 6011/00 or constraining unit PR 6011/20 with shock or vibration originating from the object to be weighed (vessel with stirring unit, shaking facility, etc. or filling with bulk material). With vibrations issued from the foundation, fit the rubber mounting kit below the lower mounting plate. In case rubber mounting kits are used, all load cells must be fitted with such a kit. The rubber mounting kit, screwed on to or below mounting kit or constraining unit, must not be used for compensation of horizontal forces. GLOBAL Weighing / 106 Fig. 2.7-2 Rubber mounting kit PR 6011/03 Bolt the rubber mounting kit to the mounting plate of the mounting kit using three screws M6 x 10 with spring washer. A threaded pin M12 is provided for fixing at the object to be weighed (or foundation). MOUNTING INSTRUCTIONS · Link weighing object and foundation using the flexible copper strap packed with the load cell. · Rubber mounting kits must be screwed to the relevant mounting plate of the mounting kit before mounting (see above). The required screws M6 x 12 and fan washers are included. Max. tightening torque for the screws: 7Nm. Subsequently, mount the overall kit with dummy, if possible. · The rubber mounting kit must not be canted. To prevent vertical force shunts, all mechanical connections of the object to be weighed to the surrounding construction (pipes, cables, bellows) must be as flexible as possible. The overall load must be supported only by the load cells. Tighten all screws and nuts well. When heavy vibrations are expected, we recommend using a protection e.g. by means of Loctite 274. 2.7.1.3 MiniFLEXLOCK PR 6011/20 Constraining unit PR 6011/20 MiniFLEXLOCK is designed especially for easy and reliable installation of small PR 6211/31.../32 series compression load cells with a nominal load of 30 kg to 300 kg. The constraining unit constrains the object to be measured in longitudinal direction, i.e. the unit absorbs horizontal forces of max. 450 N. The required space for normal thermal expansion is ensured in Y-direction with the constraining unit and in all directions with the mounting kit. To ensure the required space for movement of the measuring facility, max. three MiniFLEXLOCK units PR 6011/20 may be used for constraining a vessel. When using 4 or more load cells, the remaining load cells must be installed using mounting kit PR 6011/00 in order not to prevent compensation for expansion and to obtain an equal mounting height. By means of a rod, 3 nuts and 2 washers, the constraining unit facilitates the construction of a protection against lifting. Constraining unit comprises two mounting plates. The load cell is seated in a recess of the lower mounting plate . A load button is located between load cell head and upper mounting plate. Its teflon surface permits easy movement within the recess in the upper mounting plate. Thereby, thermal expansions and small displacements of the object to be weighed can be absorbed without a measurement error. GLOBAL Weighing / 107 The space for movement is limited by the size of the recess. If the load button presses against the edge of the recess with the constraining missing, horizontal forces in this direction are compensated via the load cell. In this case, heavy horizontal forces cause an inclination of the load button and may cause measurement errors and even mechanical damage. Therefore, take care that the load buttons are located centrally in the recess with the object in rest position and that horizontal forces are absorbed by the constrainers of three constraining units PR 6011/20. Fig. 2.7-3 MiniFLEXLOCK PR 6011/20 Before installing a constraining unit, ensure that the foundation is horizontal (check with spirit level), flat and rigid for the expected load. To ensure even load distribution, the foundations should be equally high and the supporting surfaces of the object to be weighed (vessel or platform) should be in parallel. It can be used for mounting kit and constraining unit. The holes in upper and lower mounting plate must coincide. To facilitate load cell and mounting kit installation and alignment, dummies can be used. These dummies are inserted when installing the mounting kits. As their head fits exactly into the recess of the upper mounting plate, they ensure accurate alignment of mounting kits and weighing installation. The dummies are replaced by the load cells only, when all mechanical and alignment work, as well as welding near the load cell are finished. MOUNTING INSTRUCTIONS · To facilitate installation of the mounting plates of the constraining unit, the constrainer can be removed. For this, just withdraw the swivel eyes from the swivel balls. · Link upper and lower mounting plate using the flexible copper strap packed with the load cell. Two mounting screws M6 x 6 with fan washer are included in the mounting kit. · Screw lower and upper mounting plate to foundation or object support. Taking care that the plates are in parallel and vertically upon each other is indispensable. The above-mentioned dummy ensures that they are centred. · Re-mount the constrainers of the constraining units. With deviating length of the constrainer, release the counter-nuts and adjust the length by turning the hexagon. Re-tighten the counter nuts. · Replace the dummy by the load cell only, when all welding work near the load cell and mounting at the object are finished. Handle the load cell carefully to prevent damage to the membrane on the bottom. Place the object to be weighed carefully on to the load cells and take care that the load button (1) is seated correctly in the recess of the upper mounting plate. · Check, if the load button is seated centrally in the recess of the upper mounting plate. For this, check distance A to protective ring around the load cell: it must be GLOBAL Weighing / 108 6 mm. If necessary, release the mounting screws by a few turns and shift the mounting plates as far as permitted by the holes for the screws. The distances B and C, measured in longitudinal constrainer direction, must not differ by more than 3mm. I.e. the load button must not press against the recess edge. If necessary, adjust the constrainer length as described above. Tighten the mounting screws well. Only if load cells and mounting kits are installed exactly, the space for movement, which is required for object displacement due to thermal expansion, vibration, etc., can be used fully without limiting the measurement accuracy. To prevent vertical force shunts, all mechanical connections of the object to be weighed to the surrounding construction (pipes, cables, bellows) must be as flexible as possible. The overall load must be supported only by the load cells. Tighten all screws and nuts well. When heavy vibrations are expected, we recommend using a protection e.g. by means of Loctite 274. A protection against lifting, can be realized using a rod M6 (rigidity ³ 4.6), three nuts and two washers are required. Note that the rod must have sufficient space for movement in bore hole. 2.7.2 Mounting kits for the big type (500kg...10t) 2.7.2.1 Mounting kit PR 6011/10 Mounting kit PR 6011/10 was specially designed for non-constrained installation of PR 6211/52.../14 series load cells with 500kg...10t nominal capacity. This easy-to-install mounting kit permits reliable mounting.. The load button is located between the load cell head and the upper mounting plate. A slide plate cemented into a recess on the upper mounting plate rests on the teflon-plated top of this load button. The load button can move easily in the recess so that thermal expansions and little displacements of the weighing object can be absorbed without causing a measurement error. The space for movement is limited by the dimensions of the recess. If the load button presses against the recess edge, horizontal forces in this direction are induced into the load cell. In this case, strong horizontal forces will cause an inclination of the load button, which can result in a measurement error or even in mechanical damage. Therefore, ensure that the load buttons are located centrally in the recess at rest position of the weighing object, and that constraining and/or horizontal limitations are provided, when strong horizontal forces must be taken into account. Fig. 2.7-4 Mounting kit PR 6011/10 GLOBAL Weighing / 109 Before installing the mounting kit with the load cell, ensure that the foundation is horizontal (check using spirit level), flat and rigid for the expected load. To ensure even load distribution, the foundation should be at equal height and the contact surfaces of the object (vessel or platform) should be located in parallel. The bore holes of top and bottom mounting plate must coincide. MOUNTING INSTRUCTIONS · Mount lower and upper mounting plate at foundation or object support with screws. It is indispensable to ensure that the plates are located in parallel and vertically upon each other. If necessary, adjust using a suitable U-shaped profile. · Link upper and lower mounting plate with the flexible copper strap packed with the load cell; two screws M8 x 10 with washer are included in the mounting kit. · Only when all welding work near the load cell and mounting work at the weighing object is finished, clean the load cell seating in the mounting plate from dirt, remove the load cell from the styrofoam packing, and position it in the recess. Handle the load cell carefully to prevent damaging the membrane on the bottom of the load cell. Put the weighing object carefully on the load cells and ensure that the load button is positioned correctly in the recess of the upper mounting plate. The upper and lower mounting plate must be vertical upon each other. · Check, if the upper mounting plate is seated centrally. For this, check distance A around the load cell to protection ring. If necessary, release the mounting screws slightly and correct the position of the mounting plates, without obstructing the holes for the bolts. Any accidentally installed limitations must be corrected accordingly. 2.7.2.2 MiniFLEXLOCK PR 6011/30 Constraining unit PR 6011/30 N + S MiniFLEXLOCK is a mounting kit for the GLOBAL Weighing series PR 6211 load cells for compression measurements up to a nominal load from 0.5 to 10t with horizontal constraining of the object to be weighed. Permitting quick and simple installation with a load cell, the unit compensates horizontal forces up to 5 kN in longitudinal constrainer direction whilst giving sufficient space in Y- direction for movement of the object due to normal thermal expansion. Fig. 2.7-5 MiniFLEXLOCK PR 6011/30 GLOBAL Weighing / 110 To ensure the required space for movement of the measuring facility, max. three MINI FLEXLOCK units PR 6011/30 may be used for constraining a vessel. When using 4 or more load cells, the remaining load cells must be installed using mounting kit PR 6011/10 in order not to prevent compensation for expansion and to obtain an equal mounting height. Using a rod, 3 nuts and two washers, the constraining unit permits easy installation of a lift-off protection. Constraining unit PR 6011/30 comprise two mounting plates and a constrainer. The load cell is seated in a recess of the lower mounting plate. A load button is located between load cell head and upper mounting plate. Its teflon surface permits easy movement within the recess in the upper mounting plate. Thereby, thermal expansions and small displacements of the object to be weighed can be absorbed without a measurement error. The space for movement is limited by the size of the recess. If the load button presses against the edge of the recess with the constraining missing, horizontal forces in this direction are compensated via the load cell. In this case, heavy horizontal forces cause an inclination of the load button and may cause measurement errors and even mechanical damage. Therefore, take care that the load buttons are located centrally in the recess with the object in rest position and that horizontal forces are absorbed by the constrainers of three constraining units PR 6011/30. Before installing the constraining unit, ensure that the foundation is horizontal (check with spirit level), flat and rigid for the expected load. To ensure even load distribution, the foundations should be equally high and the supporting surfaces of the object to be weighed (vessel or platform) should be in parallel. The holes in upper and lower mounting plate must coincide. Mounting the constraining unit is done by means of threaded bolts M8. To facilitate load cell and mounting kit installation and alignment, dummies can be used. These dummies are inserted when installing the mounting kits. As their head fits exactly into the recess of the upper mounting plate, they ensure accurate alignment of mounting kits and weighing installation. The dummies are replaced by the load cells only, when all mechanical and alignment work, as well as welding near the load cell are finished. MOUNTING INSTRUCTIONS · Remove the upper screws of the auxiliary plate. Lift the upper mounting plate , remove the plastic pipe and insert the load cell dummy or if not available the load cell into the recess of the lower mounting plate. Position the upper mounting plate onto the load cell head and fix the screws of the auxiliary plate. · Link upper and lower mounting plate using the flexible copper strap packed with the load cell. Two mounting screws M8 x 10 with spring washer are included in the load cell delivery. · Use bolts M8 to mount bottom and top mounting plate at foundation and object support. When all mounting kits are fixed remove auxiliary plate. Screw the screws for the auxiliary plate into the thread holes of the mounting plates. · Tighten the fixing bolts crosswisely. Take care that the plates are in parallel and vertically upon each other. The above-mentioned dummy ensures that they are centred. If the load cell is used for the alignment, check - if the upper mounting in non-constrained direction (Y axis) is positioned centrally to the bottom one in order to allow movement due to thermal expansion during operation. Distances A and B between protective ring and load cell must be equal. - if the constrainer is without load, i.e. if it can be turned by hand. If this is not possible: · Release the fixing bolts of the mounting plates and lock nuts of the constrainer. · Turn the central hexagon, until the constrainer is free of load. · Re-tighten lock nuts and fixing bolts crosswisely. · Check, if the constrainer is free of load. GLOBAL Weighing / 111 Distances C and D, measured in longitudinal constrainer direction, may not differ by more than 3.5 mm, i.e. load button must not be pressed against the edge of the recess in the upper mounting plate. Check, if distances A and B in Y direction are equal. If not repeat above mentioned adjustments. · Replace the dummy by the load cell only, when all welding work near the load cell and mounting at the object are finished. Handle the load cell carefully to prevent damage to the membrane on the bottom. Place the object to be weighed carefully on to the load cells and take care that the load button is seated correctly in the recess of the upper mounting plate. Check, if the distances A and B between protective ring and load cell are equal. Check, if the constrainer is without load, i.e. if it can be turned by hand If not perform above mentioned adjustments. Only with load cell and constraining unit installed correctly, full use can be made of the space required for movement of the object due to thermal expansion, vibration, etc., without limitation of the measurement accuracy. Tighten all screws and nuts well. When heavy vibrations are expected, we recommend using a protection e.g. by means of Loctite 274. To prevent vertical force shunts, all mechanical connections of the object to its surrounding construction (pipes, cables, bellows) must be as flexible as possible. The overall load must be supported only by the load cells. A protection against lifting can be realized using a rod M12 (rigidity ± 4.6), three nuts and two washers. Note that the rod must have sufficient space for movement in bore hole. Note that 1mm (±0.5mm) space for lifting is indispensable. GLOBAL Weighing / 112 2.8 Accessories for the installation Load cell cable, cable junction box and extension cable transmit the weight (a DC signal of some millivolts) from the load cell to the electronic indicator. The importance of excellent load cell and cable data is obvious for everyone. Best weighing results, however, also require a careful design of the cable junction box. Commercially available standard boxes from companies like Rose and Weidmüller which are often applied in electrical installations suffer from various shortcomings, e.g. insufficient insulation resistance, low protection class, low resistance to irradiation... 2.8.1 Cable junction boxes Since the junction boxes PR 6130/08 and PR 6130/6X are especially designed for weighing purposes, they overcomes the a.m. disadvantages and some more: - The printed circuit board inside the box has some recesses which restrain the propagation of parasitic currents; in this way, the extremely high insulation resistances of some thousands of MegOhms is maintained. à The user gets a stable and reliable weighing result, even under harsh conditions. Dirt and dust inside the junction box do not influence the weighing accuracy. - The cable junction boxes are protected against ingression of dust and water acc. to IP 65 (NEMA 4X). à The cable junction box may be installed in harsh environment. - A valve allows that moisture which penetrated the box to escape easily. à Reduction of humidity ensures reliable operation. - A drilling template comes together with the box, making installation quite an easy matter. - All cable glands are now according to the new regulation. à metric cable glands 2.8.1.1 Plastic cable junction box PR 6130/08 Cable junction box PR 6130/08 is designed for the use in most industrial applications. It is easy to mount and connects up to 8 load cells in parallel to an extension cable PR 6135 or to a corresponding cable with approx. 8...12mm outside diameter. The cable junction box is made from plastic and meets the protection class IP65 (or NEMA 4X). The unit is suitable for installation at a vertical wall, whereby the cable entries are positioned at the bottom side. The two M25 cable glands are for four load cell cables each; please close the open entries with the plugs supplied with the box. The M20 cable entry is fitted for the extension cable. This cable junction box may not be used in hazardous area application. Fig. 2.8-1 Cable junction box PR 6130/08 (dimensions) GLOBAL Weighing / 113 - The cable is preferably introduced from the bottom side. Fig. 2.8-2 Connection of extension cable and load cell cables - Fit the cable screenings with crimps and connect them to the screw terminals marked K5/K6; they are not connected electrically to the housing. - The screening of the extension cable must be connected to earth (protective earth or potential equalisation) on the side of the weight indicator. 2.8.1.2 Stainless steel cable junction box PR 6130/60 (and PR 6130/68) Cable junction boxes PR 6130/60, PR 6130/68 are suitable for all industrial and all W&M weighing systems (corner adjustment built-in). They may also be installed in hazardous areas. Fig. 2.8-3 Cable junction boxes PR 6130/60, PR 6130/68 (dimensions) GLOBAL Weighing / 114 - The cable is preferably introduced from the bottom side. - Connect the cores to the terminals according to the colour markings. - Fit the cable screenings with crimps and connect them to the screw terminals marked yellow; they are not connected electrically to the housing. - The screening of the extension cable must be connected to earth (protective earth or potential equalisation) on the side of the weight indicator. - The housing earth connection or potential equalisation cable must be fitted below the earth screw (fig.2.8-3, part 1) on the housing outside. - The adhesive label packed with the instrument must be fitted on the cover below the type label. - The cable junction box is suitable for connection of intrinsically safe circuits. The circuits are: - the connected load cells (passive) - the extension cable to one interface with one (active), intrinsically safe circuit, e.g. PR 1626/60 in connection with an evaluating instrument, e.g. indicator PR 1713. The intrinsically safe circuit comprises power supply, sense and measurement voltage circuit. - Connecting several active, intrinsically safe circuits in the cable junction box is not permissible! 2.8.2 Extension cables PR 6135 (PR6136) The installation cable connects the cable junction box and the weighing electronic over long distances (several hundreds of metres). In all industrial and W&M applications, the grey cable PR 6135 is used, for hazardous area applications the special cable PR 6136 is recommended (blue sheath). The cable supply the voltage necessary for the operation of the load cell, and transfer the output back to the weighing electronic. The 6-wire-technique eliminates errors caused temperature changes and ensures a constant supply voltage. The overall screen guarantees a high degree of resistance against errors resulting from electric fields. The additional separate screening of the measuring wires is a second barrier for the mV output signal against fields. Some important properties of the extension cable: - UV resistant, no silicon, flame proof, no halogen - multiple screening ensures highly reliable signal transmission - guarantees the safe transmission of the measuring signal over long distances GLOBAL Weighing / 115 2.9 Constraining devices GLOBAL Weighing delivers mounting kits and MiniFLEXLOCK for every load cell family. It is recommended to use these mounting kits because they include the two functions CONSTRAINING and STANDARDIZED INSTALLATION. If for some reason additional constraining is needed, one of the units listed below may be applied. 2.9.1 Horizontal constrainers PR 6152/02 Fig. 2.9-1 Horizontal constrainer PR 6152/02 GLOBAL Weighing horizontal constrainers PR 6152/02 are used for constraining weighing installations. They are particularly suitable for platform weighers with nominal capacities from 10t and more. Correct constraining of the object prevents measurement errors due to horizontal cross-forces or due to displacement of the centre of gravity. Moreover, constraining protects against damage caused by horizontal forces. When installing, take care to provide sufficient space for thermal expansion: any restraint in vertical direction may lead to important measurement errors. Horizontal constrainers PR 6152/02 eliminate forces up to 200kN in pressure direction. In order to absorb horizontal forces acting in opposite direction a second horizontal constrainer must be installed. Therefore PR 6152/02 is only available in pairs. Fig. 2.9-2 Operating principle of horizontal constrainer GLOBAL Weighing / 116 In order to avoid absorption of vertical forces, the horizontal constrainer should be mounted in a correct horizontal position. With the weighing installation unloaded, the flange at the platform may be somewhat higher, in order to be positioned horizontally or slightly below the flange at the foundation when the platform is loaded. Thereby, a minimum symmetrical deviation of the axial line from the horizontal line is provided. Optimum space and force absorption are ensured when the constrainer is installed without compression or extension, i.e. if the thrust piece in the middle can just be turned manually so that there is virtually no space left for shock generation. Mounting the PR 6152/02 The figure suggests two consoles for mounting a pair of rocking pins. - the consoles have an angle of exactly 90°. They are mounted on a horizontal plane, and therefore the necessary vertical position of the contact surfaces is easily obtained. - The consoles can be bolted to the construction to allow for a very small axial play. Preferred is a value of 0.5 mm; the pins can just be 'rotated' by hand. - If necessary, the centre piece, which is bolted th the object, can be adjusted for verticallity with shims. - As explained in chapter 1.4.4.5, the horizontal position of the rocking pin is not critical at all. - the pins are fixed with 2 bolts M10 x 50 1 threaded rod M10 x 70 Fig. 2.9-3 Mounting example for horizontal constrainer PR 6152/02 The simple consoles can only be used if the mounting height of the load cells used is smaller than about 200 mm. In cases with bigger load cells or if the horizontal force which has to be taken is > 20 kN, we have to do with a very heavy and big object. In those cases there should be made a special design for the mounting of the constrainers, fitting to the rest of the installation optimally. Fig. x suggests a heavier type of console for horizontal loads up to about 100 kN. The design has to fulfil the following requirements: - both ends of the PR 6152/02 have to be mounted against smooth and clean surfaces which must be vertical within ±0.5° - the consoles have to be adjustable in the horizontal direction, in such a way that PR 6152/02 has an axial play of about 0.5 mm - the deformation the axial direction of the constrainer under maximum horizontal load has to be smaller than 0.5 mm - if the consoles are bolted, the bolts have to exert enough pression of the console against the underlying construction that friction can take the horizontal force - if the consoles are welded, provisions have to be made (e.g. shims between the PR 6152/02 and the vertical surfaces) to make the adjustment of the 0.5 mm axial play possible GLOBAL Weighing / 117 2.9.2 Constrainer PR 6143/80 and PR 6143/83 GLOBAL Weighing constrainer PR 6143/8. can be used universally for constraining of weighing installations. Correct constraining of an object prevents measurement errors and protects against damage due to horizontal forces, without impairing the freedom of the object to move in vertical direction. Fig. 2.9-2 Constrainer PR 6143/8x Note that thermal expansion and displacement may affect the freedom of movement of the object whereby considerable measurement errors may be caused. (cf chapter 1.4) constrainer type max. constrainer force PR 6143/80 2 kN PR 6143/83 20 kN In order to avoid absorption of vertical forces, install the constrainer in a correct horizontal position whereby a deviation of < 1° (= height difference < 8mm) is negligible. The nominal distance between the axles of PR 6143/8. is 500mm. After installation, this distance is adjustable ±6mm by turning the constrainer, to provide optimum freedom of movement. Distance K (see drawing) must not exceed 10mm. GLOBAL Weighing / 118 3. Tank weighing 3.1 Overview The general aspects of weighing are discussed in chapter 1. Some aspects are especially of interest for tank scales, hopper scales and the like: chapter 1.3 General recommendations on the design chapter 1.4 Constraining chapter 1.5 Disturbing influences chapter 1.7.4 Standard accuracy: non W&M application This chapter defines three types of weighing installations and discusses their accuracy aspects. The standard mounting parts for the load cells are described in chapter 2. This chapter 3 discusses some special aspects which appear only in tank scales, hopper scales etc. A first subject are the connections of tanks with pipes to other objects of a process. Wrong dimensioning can cause measuring errors. Chapter 3.4 explains how to avoid them. Sometimes load cells are replaced by pivots to save money. Chapter 3.5 explains how to design pivots in order to avoid errors. GLOBAL Weighing / 119 3.2 Application examples 3.2.1 Some hints for installations pivots Suppose there is a vessel with 3 point bearing, two bearings have pivots and the third one has a load cell. In which case do you need additional constraining? From the mechanical point of view, a pivot is a clamped connection. No degrees of freedom are left for a rigid object, neither rotational nor translational. If the object is rigid enough and no side forces act on the object, you do not need additional constrainers, whereas in case of a long non-rigid tank additional constrainers are absolutely necessary. Additional constraining is recommended in particular if the load cell is tilted by side forces. Further recommendations - Links like pipes, hoses etc. should be placed at the side of the pivot to avoid errors resulting from thermal expansion of these links. - Pivots and load cells together should be used with liquids only. Fig. 3.2-1 Horizontal tank with pivot and load cell 3.2.2. Installations with load cells only - Installations including more than four load cells to weigh an object should be carefully designed so that the load distribution is as even as possible. The load cells should preferably placed at equal distances on the circumference. (see fig. 3.2.2) Fig. 3.2-2 Distribution of more than four load cells - Large vessels usually demand constrainers for heavy side forces, which are caused e.g. by wind. Standard constrainers like PR 6152/02 can withstand forces up to 200kN. Higher forces can be employed e.g. to flexbeams. (see chapter 1.4.4.3) - Suspended installations If a break in suspension, support, load cell, or mounting part etc. represents a hazard to the life and health of men and animals, or if goods may be damaged, additional safety devices have to be provided. GLOBAL Weighing / 120 3.4 Pipes, bellows All vessels are integrated in continuous and discontinuous industrial processes as e.g. storage or blending vessels. Pipes convey the materials from the storage to the process bins, which are equipped with a weighing system. The reading informs about the actual state and the progress of the process. In order to achieve the required accuracies, the system design must ensure that no undesired external forces confuse the output signal: - environmental influences on vessel and load cell (temperature, air pressure etc.) - support or suspension points - each device connected to the vessel, e.g. pipes and hoses, must be examined carefully if its movements can affect the measuring result. Accurate design avoids undesirable force shunts and hence errors in the measurement. Chapter 1.3 presents some information about the piping design (as far as the weighing point of view is concerned) whereas this chapter covers the topics · influences by pipes · models for systems with vessels and pipes · how to use bellows · calculation of pipe stiffness. 3.4.1 Influences of pipe connections In many installations the pipes are directly, rigidly fixed at the vessels and at other supporting points in the building. This creates no real problem under stable ambient conditions, e.g. constant temperature, and in case of rigid supporting points under vessel and pipes. The latter condition is yet quite often not fulfilled since many weighers are installed on platforms or inside frameworks, which must be regarded as elastic. At least all outdoor vessels are subjected to temperature changes between day and night. Both thermal expansions and movements between the supporting points of pipe and vessel cause reaction forces in the load cells, thus leading to incorrect measurements (e.g. zero point errors, span errors). These errors cannot be compensated neither mechanically nor electrically because they depend on the actual state of the complete system including loading and environmental conditions. Fig. 3.4-1 illustrates the possible forces and displacements: a rigid pipe can lift some weight off the vessel and cause a displacement (δ P ). A force F P along the pipe can push the vessel to the side and exert the forces ∆F which need not to be of the same magnitude. The charging makes the vessel grow in diameter as it is not stiff. Fig. 3.4-1 Vessel with pipe connections As long as the ambient conditions remain stable, pipes can fix a vessel on a rigid foundation in its position, meaning that they can replace constrainers. On the other hand, varying temperatures influence the dimensions of the pipes: increasing temperatures lengthen stiff pipes and widen the vessel, causing a change in the height of the connection point. As a result some of the weight can be lifted off the load cells when pipes and vessel are rigidly connected. This effect decreases the span and lowers the weight indication below the indication achieved without pipes. During the calibration procedure the instrument can be re-adjusted to overcome this error. The pipe stiffnesses, however, cannot be assumed a constant value: they change during operation, causing a span error, an additional non-linearity error and a non-reproducibility error. GLOBAL Weighing / 121 Beside changes at the connecting point of pipe and vessel, the height of the supporting points of the pipe can change by some reason like temperature dilatation, deformation of the foundation and interconnections between vessels (refer to the design chapter 1.3). With too stiff pipes this results in a zero shift. An installation with rigid pipe connections in combination with other standard constrainers can be statically undefined (overdetermined), i.e. they (constrainers and pipes) can clamp the vessel. A pipe outside the plane of the constrainers exerts a moment on the weighing object (see chapter 1.4), which increases or decreases the loads on the load cells. In case of thermal expansion, forces of unknown magnitude arise producing a poor measuring result. Conclusion: It is evident to decide in advance whether to constrain the vessel by pipes or by standard constrainers only. In the first case, all rules written down in chapter 1.4 for constrainers apply to pipes in the same way whereas in the second case the pipes must be flexible enough not to clamp the weighing system. Expansion devices like bellows are installed to achieve this. In most cases the latter method will be preferred for ease of mounting and a clear, transparent design. Fig. 3.4-2 Thermal dilatation Fig. 3.4-3 deformation of foundation Fig. 3.4-4 The pipe is connected to another vessel with flexible foundation The following examples illustrate the possible effects and show their magnitude. Example 2 vessels interconnected (fig. 3.4-4) 1. Influence of a pipe on the span We assume a vessel standing on a rigid floor. A pipe with vertical stiffness of C = 216 N/mm is connected. There are three load cells of 10t, which means a load cell stiffness of 300,000 N per 0.5mm. The influence of the pipe on the span is therefore % . % mm N mm N 036 0 100 600000 216 · ⋅ ⋅ GLOBAL Weighing / 122 A span error is no problem if the circumstances do not change; otherwise the indication will change depending on the pipe loading. 2. Influence of the vessel temperature on zero We assume that the vessel is made of aluminium, the height is 5m, and the temperature of the vessel changes from 10°C to 100°C. Then the vessel will grow vertically over ( ) 5 000 100 10 24 10 6 1 108 , . mm C C C mm ⋅ ° − ° ⋅ ⋅ − ° · The pipe will deflect the same amount causing a vertical force F v N 2,333 = 216 mm 8 10 = F mm N . v ⋅ Assuming a net scale range of 12t (or about 120 kN) this means a zero error of % . % , 94 1 100 kN 120 N 333 2 · ⋅ 3. Zero error if the pipe is connected to a second vessel We assume that the second vessel is put on load cells and the weight in that vessel changes over e.g. 50% of their totalized nominal load. then the second vessel moves over at least 50% of 0.5mm or 0.25mm in vertical direction. (Possibly this movement is bigger by elasticity of the foundation of the second vessel.) Pipe deflection over 0.25mm causes a vertical force of N mm N mm . 54 216 25 0 · ⋅ which means a zero error on the first vessel of % . % N , N 045 0 100 000 120 54 · ⋅ 3.4.2 Describing systems with vessels and pipes The objective of this paragraph is to show how to transform a system consisting of vessels, pipes, load cells and accessories into a mechanical model. After the identification of all important influences which can cause incorrect measurements, their description enables the user to calculate the reactions of the system. A clear and complete system description can only be obtained if the supporting points and the foundations are taken into consideration beside vessel and pipes... Two different types of foundations are distinguished: (i) flexible foundations like steelworks and (ii) stiff foundations like concrete foundations. The flexible foundation is characterized by the property that it deflects under load. The longer the supporting beams are, the higher their deflection under load since the deflection increases with the third power of the beam length. As a rule of thumb you can say that the deflection can rise up to 0.25% of the beam length. On the other hand, a stiff foundation does not respond to vertical forces with vertical deflections except for case of damage. Thus the foundation can completely be described by the possible vertical deflection of the supporting points. They can only move if the foundation is not rigid, if the permissible load is exceeded, or if the load distribution differs from the expected one. Vessels and pipes are made of steel and therefore are elastic so that they can be deformed. Since the various elements are interconnected, all the deflections can influence the correct weighing result. Some examples illustrate this mutual influence: Material in a pipe tries to pull it down to the ground. Insufficient supporting conveys these forces to the vessel and causes a reaction force in the load cells. A vessel filled to its limits widens and tries to pull all connected pipes downwards. Stiff pipes, however, can lift some of the weight off the vessel. GLOBAL Weighing / 123 So you must expect expansions of the vessel and vertical movements of the pipe but also axial expansions of the pipes. The vertical pipe movements are described by the vertical displacements of the supporting / suspending points, by its elasticity and geometric dimensions. The pipe supports can deflect when traffic passes; but usually the pipe deflects first. The pipe expands under temperature and bends under deflection of the supporting points. If the pipe does not hang freely, it transfers forces. If you exaggerate, you can compare a vessel with a balloon: it widens when filled. A mechanical model which describes the movements and elasticities of the system can be built from the standard elements “spring” (stiffness c) and “rigid connection” (stiffness = 4). The springs work in compression and/or bending mode. (load cells: compression or tension; pipes: bending). 3.4.3 Calculation of the pipe stiffness The calculation of the stiffness is based on the mechanical model shown in fig. 3.4-5. Fig. 3.4-5 Model for the stiffness calculation of the pipe Like in many strain calculation the main influencing factors are - Young's modulus E - the length l of the pipe - the cross sectional area of the pipe, cross section A and moment of inertia I Young's modulus is a constant and typical for the each material. material Young's modulus steel 210,000 N/mm 2 copper 110,000 N/mm 2 aluminium 70,000 N/mm 2 The cross sectional area A of a pipe and the moment of inertia I are easy to calculate: [ ] 2 2 - 2 D 4 = mm d A , _ ¸ ¸ ⋅ π [ ] 4 4 - 4 D 64 = mm d I , _ ¸ ¸ ⋅ π The formula for the pipe stiffness is achieved by applying the elasticity calculation: 1 ] 1 ¸ ⋅ ⋅ mm N l I E K C 3 = Substituting the expression for the momentum of inertia I results in ( ) 1 ] 1 ¸ ⋅ ⋅ ⋅ mm N l E d D K C 3 4 4 - 64 = π K is called the clamping factor and characterizes the mounting conditions of the pipe. This factor has to be determined for some interesting cases. The theory claims that the clamping factor K equals 12 if the pipe is straight and clamped at one side and free at the other one. Experiments, however, show that a value of K = 10 is more realistic. GLOBAL Weighing / 124 In some installations and special designs other values for the clamping factor K can be assumed: 1. Bend in the vertical plane h/l K 0.2 8 0.5 6.3 1 4.8 5 3.4 Fig. 3.4-6 Pipe with a bend in the vertical plane 2. Bend in the horizontal plane b/l K 0.2 7.1 0.5 4.3 1 1.8 5 0.06 Fig. 3.4-7 Pipe with a bend in the horizontal plane 3.4.4 Calculation of the influence on the measuring result The actual stiffness is compared with a permissible stiffness under the condition that the pipe must not influence the weighing result. The permissible stiffness is calculated using the load cell stiffness. ] mm N [ E n h g C max a ⋅ ⋅ · E max nominal load of one load cell [kg] n number of load cells ----- h max. deflection under load [mm] g gravitational field strength =9.81m/s 2 The actual stiffness C t is the sum of the calculated values C for all pipes connected to the vessel to be weighed. Compare the totalized pipe stiffness C t and the allowed stiffness C a pi t C C Σ · A C C a t ≤ If the totalized pipe stiffness C t is bigger than C a , measures have to be taken. - The allowed influence on the ‘span’ The stiffness of the pipe reduces the span of the installation. a typical value for the allowed influence is 1 per cent. This means that the totalized pipe stiffness should not be more than 1 per cent of the total of all load cells together. The resulting decrease in span of 1 per cent can easily be corrected with the measuring instrument during the calibration procedure. GLOBAL Weighing / 125 If the pipe stiffnesses were constant there would not be a measuring error caused by the pipes. However, a change in pipe stiffness causes a span error. Example: Assuming that the pipe stiffness is 1 per cent of the permissible stiffness. A 10 per cent change would result in a span error of 0.1 per cent. For high accuracy weighing, it is recommended to choose a value lower than 1 per cent for the permissible influence. If the calculated totalized pipe stiffness C t is bigger than the permissible pipe stiffness C a then the following measures could be taken: - localize the stiffest pipe - lower its stiffness You could try to make the pipe longer or try to make the clamping less rigid. You could suggest to install bellows. 3.4.5 Bellows Pipes are often fixed with bellows to avoid destructive forces. They are mainly used to compensate for axial, lateral, and angular expansion due to temperature. This is achieved by transforming the force in the pipe to a movement. Four types of bellows are distinguished: · axial type Axial bellows compensate thermal expansion along the axis of the pipe Fig. 3.4-8 Axial bellows · lateral type Lateral bellows compensate movements in the plane rectangular to the axis of the pipe. Fig. 3.4-9 Lateral bellows GLOBAL Weighing / 126 · angular type Angular bellows must be used in groups of two or three. Two angular bellows substitute one lateral bellows. Three angular bellows form a 3 point system. Fig. 3.4-10 Angular bellows · universal type Universal bellows compensate thermal expansions along the axis and rectangular to the axis of the pipe. When using bellows you must observe some restrictions. · Bellows must not be loaded with torsional momenta. · Only low frequency vibrations are permitted · The number of movements is limited (e.g. 5000). If the pipe is made flexible at both ends, the stiffness is reduced to a minimum because there is no bending in the pipe itself. Practically this is done with bellows. 3.4.6 Influences of gas pressure If the contents of a weighed vessel are under gas pressure, the pipe connections ask for some extra care. Gas pressure in the pipe gives: · no influence in flexible horizontal pipes · even no influence if that pipe is vertically connected to the vessel with a stiff part · however, if the connection is made via bellows, a vertical disturbing force can arise. If the effective area of the bellows is A, then this force F v is A p F v ⋅ ∆ · example 1: With an pressure of Ap = 2 bar and a diameter D = 150mm the weight indication is increased with F v = 3.5 kN. kN . m . m N mm bar F v 5 3 15 0 4 10 2 150 4 2 2 2 2 5 2 2 · ⋅ ⋅ ⋅ · ⋅ ⋅ · π π GLOBAL Weighing / 127 example 2: A hopper is filled with dusty material. Bellows are provided to protect the environment. During the filling process the internal pressure increases temporarily by Ap = 100 Pa. The diameter of the bellows is 1.5m. The weight indication changes with F v = 177N. N m . m N m . Pa F v 177 5 1 4 10 5 1 4 100 2 2 2 2 2 2 · ⋅ ⋅ · ⋅ ⋅ · π π 3.4.7 Influence of vertical bellows The contents of this hopper are not correctly measured because the bellows reduce the weight by the grey shaded column above. The material inside this column is supported by the outlet pipe and not by the weighing object. Fig. 3.4-11 Hopper with vertical bellows If you exchange the positions of valve and bellows, you get correct results for charging the hopper. When discharging the above described effect happens to the measurement again: the (grey shaded) column of material above the bellows is immediately subtracted from the contents of the hopper. Conclusion: Such a bellows installation should be avoided. For this reason, place the bellows horizontal. GLOBAL Weighing / 128 3.5 Level control using pivots Fig. 3.5-1 Schematic diagram of a system for level control In order to reduce the cost price of a weighing installation sometimes not all bearings are equipped with load cells. Some of them are replaced by pivots. Two combinations are possible: · one load cell and two pivots in case of a three point bearing · two load cells and two pivots in case of a four point bearing Because of this replacement you cannot expect a too high accuracy. So, level control can be used if the customer only requires level control. Level control is a method which determines the momentum instead of the weight. The weight F G is found by calculating the momentum equilibrium around the pivoting axis. G l a F ⋅ · This equation shows which effects can disturb the measuring result: · the centre of gravity wanders A change in the position of the centre of gravity changes the distance a to the pivoting axis. · not carefully designed pivots give disturbing moments · the distances a and l could change · the friction moment could be too big · the stiffness of the pivot could be too big · Horizontal forces on the object, out of the plane of the pivots, disturb the moment M. Some aspects must be observed when designing a weighing system with pivots: · the weighing vessel should have a big diameter and a low height · the centre of gravity must have a constant distance to the pivots · the weighing object must be stiff · the temperature must be constant (constant distance of the centre of gravity) The following sketches show situations in which the designer of the weighing system failed. Fig. 3.5-2 Wandering centre of gravity Fig. 3.5-3 Wandering centre of gravity GLOBAL Weighing / 129 3.5.1.1 Standardized pivots To avoid the necessity to calculate pivots each time you need them, two standard types are proposed here. Their length is 200mm; their other dimensions are given in fig. 3.5-4. Two different I-beam types were chosen: HEB 100 maximum permissible vertical load: 16 tons HEM 100 maximum permissible vertical load: 33 tons Remark: Risk of buckling makes that load decreases if a big horizontal force is expected. The flexibility of the pivots is small enough to use them in combination with load cells with a nominal load of 200 kg or more. The main properties of an ‘I-beam’-pivot are: · maximum allowed vertical load This has nothing to do with the measuring properties, but only a question of strength of the device. The beam should be strong enough to bear the load. · Influence of side force on the permissible load The pivot could be used as a constraining element. With the normal load cell mounting parts there are no forces due to temperature expansion and the ‘internal’ side forces on the pivot will be small. With external influences like wind or a calamity, however, the side force on the pivot could be the reason for its collapse if we do not take this side force into account in the strength calculation. In the table, the effect of the side force is expressed in a formulae for the maximum allowed load. · the stiffness of the pivot the torsional stiffness of the beam around the pivoting axis will, in principle, influence the measurement. This is because a vertical load cell movement results in a reacting moment of the pivot and therefore in a disturbing vertical reaction on the load cell. In the table, the beam stiffness is given as the reacting moment per bending angle of the I-beam (expressed in Nmm/rad). To get a better idea of of how this works out in a practical case, also a disturbing ‘reaction force’ is listed. The listed value will be generated in the following practical case: distance between the load cell and the pivot: d=2000mm vertical movement of the load cell x=0.5mm The bending stress in the beam for that case is also put in the table. Putting it generally: reaction = (x/d²) x stiffness stress is proportional to the bending angle of the I-beam Example: with a distance of 4000mm and a movement of 1mm the reaction will be the half of the value in the table and the stress remains the same. GLOBAL Weighing / 130 In practice you could claim that the pivot is good for measuring forces bigger than 500 x this listed reaction. Example: we take two standard pivots HEB 100 (of 200mm) for weighing a small object with one load cell. As there are two pivots the total reaction will be twice 1.7N. Therefore the set-up is suitable for measuring with the load cell a mass bigger than 500 x ( 2 x 1.7N))/10 = 170kg If all three supporting points take the same load, the installation is good for an object with a minimum total net mass of 3 x 170kg or about 500 kg I-beam type HEB 100 HEM 100 beam length 200 mm a [m] 200 mm a [m] weight [kg] 4.1 20.4 × a 8.4 41.8 × a height [mm] 100 100 120 120 flange width [mm] 100 100 106 106 body thickness [mm] 6 6 12 12 pivot stiffness [Nmm/rad] 1.35 × 10 7 6.75 × 10 7 × a 1.08 × 10 8 5.4 × 10 8 × a reaction [N] 1.7 8.4 × a 13.5 67.5 × a stress [N/mm2] 2.8 2.8 5.6 5.6 permissible load [t] L+28H < 16.8 L+28H < 84 × a L+14H < 33.6 L+14H < 168 × a Remarks · A standard minimum length of 200mm of the pivoting beams is chosen for ease of mounting and to have a wide application range for HEB 100. A smaller length (down to a minimum of 100mm) is possible, but makes the device less robust. · If the object is very small the construction could be simplified by combining two pivots to just one pivot of more than 200mm length. · Also if there are extreme side forces, (e.g. example 1 on page 3) we are forced to choose a bigger length than 200mm. · We chose profiles for the smallest possible height and a relatively thick body. This makes the buckling tendency very low. Profiles with a bigger height will be more flexible around the pivoting axis, but will be less resistant against a horizontal force. · Sometimes we meet a beam with welded strengthening plates as shown in the sketch (designed in an attempt to decrease the buckling tendency). This is not necessary with the two proposed standard pivots. The design is more expensive and the local stresses during bending of the beam body will be bigger than without these plates. GLOBAL Weighing / 131 3.5.1.2 Mounting recommendations for level control with pivots 1) For an adequate pivot function the two pivoting beams under one object should be positioned in line as accurate as possible. Together they form a knife edge. Fig. 3.5-4 Installation of two pivots in line 2) The pivots should be bolted to the object and to the foundation to take all possible horizontal forces 3) excessive bending of the pivoting beams should be avoided during the mounting procedure of the object 4) The easiest way to mount the load cell is using the mounting kit PR 6145. Sometimes it may be recommended to use mounting kit MiniFLEXLOCK PR 6143. 3.5.2 Calculation of an I-beam pivot In chapter 3.5.1 we suggested to design the pivot as an I-beam and gave some standardized pivots. Supposed there is no chance to use a standard pivot a special one has to be calculated. For the calculation you need to know some characteristic values: - the thickness of the stick - the height of the stick Fig. 3.5-5 characteristic values of an I beam For the calculation you have to determine the slenderness ratio l 1 1 5 3 t h . ⋅ · λ With this ratio the permissible stress for the material is found. The material was chosen as Fe 360 because its often used and usually easy to get. The higher the stick the higher the danger that the buckling effect affects the pivot. For this reason the permissible force lowers with higher stick. GLOBAL Weighing / 132 Fig. 3.5-6 Loading of an I beam We did not take a possible side force into account. However a side force perpendicular to the pivot, acting at the same time, decreases the permissible vertical force. In this chapter a method is presented which takes both forces into account. Moreover, the calculation method for the pivot stiffness of an I-beam is described. In principle the pivot is loaded by forces and momenta (refer to fig. 3.5-6) - a vertical compression force F v - a horizontal force F 1 (caused by external forces on the object) - a pivoting moment M (caused by e.g. load cell deflection) Fig. 3.5-7 Permissible stress for an I beam The permissible horizontal force F 1max is limited by the allowed bending stress b in the beam. 1 2 3 h t l H b max ⋅ ⋅ ⋅ · σ Note: b should be kept low, because this stress is additional to the compression stress caused by the vertical force. l t F v b ⋅ − ·140 σ The permissible vertical force can be calculated on buckling. The permissible buckling stress s as taken from the graph should, however, be decreased by the bending stress b , caused by the side force. 2 1 3 t l h H b ⋅ ⋅ ⋅ · σ Because this can reduce very strongly the permissible vertical force, it is advised to choose an I-beam for which l<30 and therefore =140N/mm². In those cases: bending stress + compression ± 140 N/mm² 2 2 1 140 3 mm N t l F t l h H ≤ ⋅ + ⋅ ⋅ ⋅ GLOBAL Weighing / 133 Example: for HEM 100 with l = 250 mm h 1 = 56 mm t = 12 mm Now, with this equation: F v + 14 F 1 < 420 kN The pivot stiffness can be defined as the moment in Nmm, which is necessary th rotate the pivot over 1 radian. This can be calculated from 1 h E I k p ⋅ · To get a better insight in this value, we could measure this stiffness as the vertical reaction on a vertical movement of a point with a horizontal distance of one mm to the pivoting line. The pivot stiffness is then expressed as the vertical reaction force R a in newton, when that point deflects vertically over z mm. The pivoting action deforms the beam, giving a bending stress s 1 . This can be calculated from: Φ ⋅ ⋅ ⋅ · E h t 1 1 2 1 σ GLOBAL Weighing / 134 Appendix A A E accuracy class 71 earthing 36, 80-81 accuracy aspects 71-79 effect of horizontal force 129-130 acoustical impedance 34 electrical check 85-87 alignment 16, 80 electrical damage 86 electrical shunt 37 B equilibrium 17 base plate 89-90 error 71 batching accuracy 77 error types 71 bellows 125-126 error estimation 75-79 braking force 50 buckling effect 131 F falling material 33, 69-70 C feeling constrainers 42-43 cable 35, 80-81 flexible object 27 cable junction box 112-114 freedom of movement 39 cable length 80 freezing 68 cable protection 35 friction 68-69 cable screen 36, 80-81 flexbeams 48-54 cabling 35 flexbeam supports 53-54 calculation pivoting beam 131-133 flexbeam dimensions 49 calculation of flexbeams 52-54 flexbeams in practice 50-51 calibrated weights 82-83 flexbeam calculation 52-54 calibration 82-83 capsizing , 56-57 G cardanic pivoting points 68 gas pressure 126-127 central earth rail 36, 81 centre of gravity 15 H clamped constrainer 42-43 hammering 39 clamping factor 123-124 heat 64-65 collapsing load 47, 52 heat protection 31, 64-65 collision 26 horizontal stop 55-56 conduction 64 hysteresis 71 constrainer 39-57 constrainer pattern 39 I constrainer plane 15 ice 68 constrainer number 39 impact 33 constrainer, too many 40 installation design 21-38 constraining 39-57 insulation 87 constraining and temperature 41 insulation resistance 87 corner adjustment 83-85 interference 36, 81 corner point test 83-85 L D lifting force 62-63 design 21-38 lift-off-protection 56-57 destructive load 30 links 28-29 dimensioning a pivoting beam 131-133 load distribution 84 dirt 68 load cell tilt 8 disturbing effects 58-70 load cell type and capacity 29 dynamic overload 33-35 load cell cabling 36 GLOBAL Weighing / 135 M stray current 35 measuring error 71 strut stiffness 47 measuring range suction force 63 measuring accuracy 75-79 suspended object 8-12, 43-44 MiniFLEXLOCK 46 suspension rod 10-12 moisture 35, 80 swivel bearing 102-103 mounting kit T N temperature 64-67 natural frequency 17-20 terminology 71-75 nominal load 30 thermal expansion 66 non-linearity 71 tilting 8, 56-57 non-reproducibility 71 torque on tension cell 8-12 O V omitting constrainers 39 vibration 69-70 overload 31-35 overload protection 32 W object stability 15-16 waggling 16 weak foundation 23 P weight 82 pendulum length 18 weights & measures (W&M) 72-75 pipe 120-127 welding 37-38 pipe calculation 123-125 wind 58-63 pipe stiffness 123-125 wind velocity 58-59 pivot stiffness 130, 133 pivot weighing recommendations 119 Z pivoting rod 54-55 zero error 121-122 preload 32 principle of flexbeams 48 R radiation (heat) 65 railway weighbridge 50-51 reproducibility 71 restoring force 17-20 rigid mounting 6-7 rocking pin 55 rod suspension 10 S safety device 39 shim 7 shock 33-35 shock wave 33-35 shock loading 33-35 side load 39 stability 16-17 stable equilibrium 15 standardized pivots 130 static overload 31-32 statically undefined 16 stiffness of flexbeam support 53-54 stop 39, 55-57 GLOBAL Weighing / 136 GLOBAL Weighing / 4