Universiti Tenaga Nasional, 2006 Mechanical Design and CAD LaboratoryCRANK AND SLOTTED LEVER QUICK RETURN MOTION EXPERIMENT OBJECTIVES The objective of this experiment is to investigate the kinematics motion of a Crank and Slotted Lever Quick Return mechanism. The investigation is to show that it is indeed a quick return mechanism and to evaluate the increase in efficiency that this would offer if applied to a machine tool. THEORY Definition of a Mechanism A mechanism is a simplified model, usually in the form of a line diagram, which is used to reproduce the motion occurring in a machine. The purpose of this reproduction is to enable the nature of the machine. The purpose of this reproduction is to enable the nature of the motion to be investigated without the encumbrance of the various solid bodies which form the machine elements The various parts of the mechanism are called links or elements. Where two links are in contact and a relative motion is possible, then they are known as a pair. An arbitrary set of a links which form a closed chain that is capable of relative motion, and that can be made into a rigid structure by the addition of a single link, is known as a kinematics chain. To form a mechanism from a kinematics chain one of the links must be fixed. However as any of the links can be fixed, it follows that there are as many mechanism as there are links in the chain. The technique obtaining different mechanism by fixing the various links in turn is known as inversion. Kinematics Pairs The relative motion between two links of a pair can take different form. Three types of a pairs are known as lower pairs and these are the frequently occurring ones: Sliding : such as occurs between a piston and a cylinder the link 1 is fixed. Strictly screw motion is a higher pair as it combines turning and sliding. This is known as Whiworth’s Quick Return Mechanism. 2006 Mechanical Design and CAD Laboratory Turning : such as occurs with a wheel on an axle Screw motion : such as occurs between a nut and a bolt All other cases are considered to be combinations of sliding and rolling are called higher pairs.crank mechanism is well known as the basis of a reciprocating engine. If we now fix link 2. we obtain the mechanism shown below. Slider – Crank Mechanism The slider. that is consider an inversion of the mechanism. As shown in the diagram below it consists of three turning pairs and one sliding pair 2 3 1 4 In the above diagram.Universiti Tenaga Nasional. 4 3 1 2 Expansion of a Revolute Pairs Consider the four bar linkage shown below:- . i. its center of rotation is at . The curved slider is thus still a revolute form and 3 are described by an angle and not by linear distance.Universiti Tenaga Nasional.e. 3. When the crank 2 form a complete revolution the block. The curved slider remains a revolute pair as long as its radius of curvature is finite. 2006 Mechanical Design and CAD Laboratory R3 3 R2 B 2 4 R1 A R4 OB The revolute pair R3 can be expanded so that it becomes a block. 3. E1 R3 B R2 2 E2 R1 A R4 OB The motion of 3 is still described by means of an angle referred to B. only transverses a small are from E1 to E2. If the radius of a curvature of a revolute pair becomes infinite. For Bar Chains:R2 3 R3 2 4 R1 R4 OB R2 3 R3 4 R4 2 R1 OB R2 3 R3 R4 4 2 R1 OB at ∞ Thus a prismatic pair may be considered as a revolute pair whose center is at infinity in the direction perpendicular to the generatrix. 2006 Mechanical Design and CAD Laboratory infinity.Universiti Tenaga Nasional. Then the revolute pair becomes prismatic pair variable change from an angular measurement to a linear distance measurement. . Now consider the crank and slotted lever quick return motion. This is very useful in the synthesis of a planar mechanism as the properties of a four bar mechanism become the properties of the slidercrank mechanism. OB at ∞ It is evident that we have a four bar chain with a prismatic pair as a limiting case of a revolute pair. 2006 Mechanical Design and CAD Laboratory Here we can see that a four bar mechanism when taken to the limit can be shown to become slider-crank mechanism. θ . Superimposed upon this is an inversion of the slider-crank chain.Universiti Tenaga Nasional. The slotted lever length. and the length of the links. AC. θ. is 240 mm. . It is a matter of a trigonometry to develop an expression for x in terms of the crank angle. On the apparatus x is 70 mm when θ is 0° and 180°. OB is 40 mm. 2006 Mechanical Design and CAD Laboratory The crank radius.Universiti Tenaga Nasional. Universiti Tenaga Nasional. 2006 Mechanical Design and CAD Laboratory APPARATUS Crank and Slotted Lever Quick Return Mechanism . Set the crank so that the pointer is at zero on the scale. Find an expression for theoretical distance (x) in term of θ. On both graphs. x. DISCUSSION 1. 2. What rotation angle is required for the cutting and return strokes? 3. Rotate the crank by 10° increments and for every increment. On the graph. 4. note the corresponding crosshead position. Plot a graph of crosshead velocity versus crank angle. show the return and cutting stroke. show the return and cutting stroke.Universiti Tenaga Nasional. Plot a graph of experimental crosshead position. Discuss the motion of the slider and verify that it is indeed a quick return mechanism. 5. a graph of theoretical crosshead position versus crank angle. 3. x. 4. RESULTS 1. 2006 Mechanical Design and CAD Laboratory PROCEDURES 1. How well does the experimental result agree with the predictions from the theory? 2. x. versus crank angle. Note the crosshead position. 2. What is the increase in efficiency (in term of the time required for each stroke in one revolution of crank) obtainable in the mechanism? . Plot on the same graph. (mm) . x. (mm) Theoretical Slider position. 2006 Mechanical Design and CAD Laboratory RESULT SHEET Crank and Slotted Lever Quick Return Motion Experiment Crank Angles.Universiti Tenaga Nasional. θ (degrees) 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340 350 360 Experimental Slider position. x.