Oil Whirl and Whip Instabilities - Within Journal Bearings.pdf

Current IssueArchive Subscribe Search: Home | Buyers Guide | Glossary | Events | Bookstore | Newsletters | Browse Topics GREASES HYDRAULICS INDUSTRIAL LUBRICANTS SYNTHETICS FILTRATION STORAGE/HANDLING OIL ANALYSIS Oil Whirl and Whip Instabilities - Within Journal Bearings Jam e s E. Be rry, Te chnical Associate s of C harlotte Tags: be aring lubrication Oil whirl is probably the most common cause of subsynchronous instability in hydrodynamic journal bearings. Typically, the oil film itself flows around the journal to lubricate and cool the bearing. This develops an average speed slightly less than 50 percent of the journal surface speed (Figure 1). Normally, the shaft rides on the crest of an oil pressure gradient, rising slightly up the side of the bearing somewhat off vertical at a given, stable attitude angle and eccentricity. The amount of rise depends on the rotor speed, rotor weight and oil pressure. With the shaft operating eccentrically relative to the bearing center, it draws the oil into a wedge to produce this pressurized load-carrying film. Figure 1. Oil Film Within a Journal 1 If the shaft receives a disturbing force such as a sudden surge or external shock, it can momentarily increase the eccentricity from its equilibrium position. When this occurs, additional oil is immediately pumped into the space vacated by the shaft. This results in an increased pressure of the load-carrying film, creating additional force between the oil film and shaft. In this case, the oil film can actually drive the shaft ahead of it in a forward circular motion and into a whirling path around the bearing within the bearing clearance. If there is sufficient damping within the system, the shaft can be returned to its normal position and stability. Otherwise, the shaft will continue in its whirling motion, which may become violent depending on several parameters. Related Articles Journal Bearings and Their Lubrication When to Clean and Repack Bearings How to Prevent Bearings from Overheating Can WD40 be Used as a Bearing Lubricant? Oil Whirl Instability Oil whirl demonstrates the following characteristics: 1. Oil whirl can be induced by several conditions including: light dynamic and preload forces excessive bearing wear or clearance a change in oil properties (primarily shear viscosity) an increase or decrease in oil pressure or oil temperature; improper bearing design (sometimes an over design for the actual shaft loading) fluid leakage in the shroud of blades and shaft labyrinth seals (so-called “Alford force” or “aerodynamic force”) change in internal damping (hysteretic, or material damping, or dry (coulomb) friction) gyroscopic effects, especially on overhung rotors with excessive overhang. www.machinerylubrication.com/Read/754/oil-whirl-whip White Papers 1/3 200 RPM. This kind of vibration is called dry whip. In these cases. Followed by Oil Whip from 9. but that the instability may occur at the first critical speed. When a shaft goes into oil whip. Sometimes machines exhibit oil whirl intermittently due to external vibratory forces transmitting into the unit or from sources within the machinery itself. oil whip may cause destructive vibration resulting catastrophic failure – often in a relatively short period of time.) 4. its dominant dynamic factors become mass and stiffness in particular. 6. the oil viscosity). excessive friction is generated between the Videos stationary bearing and rotating journal. thereby increasing the system damping and overall stability.000 RPM.800 RPM and remained in whirl until about 4. When the rotor speed increased to just above 9. (Reference 2 states that pure oil whirl occurs at 43 percent of shaft speed. The oil whirl was suddenly replaced by oil whip . or completely changing the bearing type to oil film bearings that are less susceptible to oil whirl (including axial-groove bearings.200 to 12. or even through the floor and foundation. whirl once again occurred just above 5. preloading the bearing by an internal oil pressure dam.machinerylubrication. This friction can excite vibration in the bearing and other components. Temporary corrective measures include changing the temperature of the oil (thus. Permanent corrective steps to resolve the oil whirl problem include installing a new bearing shell with proper clearances. Wear Debris Analysis: The Power of Knowing More using Automated SEM/EDX Top 7 Things a CMMS Can Do for Your Organization Buyers Guide Grease Guns Cleaners and Degreasers Maintenance Services and Products Poll Which factor do you consider to be the most influential on the life of engine oil? The engine's design and properties Where and how the vehicle is driven Selection of the engine oil and its properties Vote Figure 2.200 RPM. However. or to isolate the offending machine itself. the oil whip frequency remains the same. once the machine passed through resonance. which actually generated sufficient force to overcome oil whirl. note that the shaft 1X RPM speed entered resonance. independent of the rotor RPM. these vibratory forces have the same frequency as the oil whirl frequency of that bearing and can contribute just the right magnitude of disturbing force at just the right tuned frequency to set the shaft into the whirl motion. At this point. scraping the sides of or grooving the bearing surface to disrupt the lubricant wedge. When this occurs. purposely introducing a slight unbalance or misalignment to increase the loading. its speed increased to 2X its first balance natural frequency. If this occurs. Development of Oil Whirl Just After Startup. www. or changing the oil pressure. Oil Whip Instability Oil whip occurs on those machines subject to oil whirl when the oil whirl frequency coincides with and becomes locked into a system’s natural frequency (often a rotor balance or critical speed frequency). Dry Whip Contact Us Share Dry whip occurs in journal bearing machines subjected to either a lack of lubrication or the use of the wrong lubricant. Dry whip can also be caused by journal bearings having excessive clearance as well as those having insufficient clearance. Oil whirl is considered severe when vibration amplitudes reach 40 to 50 percent of the normal bearing clearance. For example. 2. Oil whirl is easily recognized by its unusual vibration frequency which is generally 40 percent to 48 percent of shaft RPM. lobed bearings or tilting pad bearings). At this point. corrective action must be taken. and its amplitude is limited only by the bearing clearance. This vibration transmits from other machinery through attached structures such as piping and braces. The tilting pad bearing is a good choice because each segment or pad develops a pressurized oil wedge tending to center the shaft in the bearing. 7. Note that the oil whip frequency never changed even though the machine continued up in speed to 12. it may be necessary to either isolate this machine from surrounding machinery. At this time the oil whirl which was approximately 43 percent of RPM.Any of these conditions can induce oil whirl after a disturbing force induces an initial rotor deflection. was brought into coincidence with this critical speed. temporarily shifting the alignment by heating or cooling support legs. Note that the shaft went into whirl at a machine speed of approximately 1.a lateral forward precessional subharmonic vibration of the rotor. At this point. 3.com/Read/754/oil-whirl-whip 2/3 . refer to Figure 2. Left uncorrected.000 RPM 3 5. Figure 2 shows the development of oil whirl just after the shaft is brought up to speed.000 RPM. Pages 14-23. Get Free Report Lubrication Program Transformation Services Discover how Noria can tranform your lubrication program to best practices quickly and efficiently.” Analysis II – Concentrated Vibration Signature Analysis and Related Condition Monitoring Techniques. Machinery Lubrication (5/2005) Related Articles Journal Bearings and Their Lubrication When to Clean and Repack Bearings How to Prevent Bearings from Overheating Can WD40 be Used as a Bearing Lubricant? Get 60 Questions to Ask Your Lubricant Supplier absolutely FREE when you sign Email: up for our free Lube-Tips™ newsletter. “Interpreting Vibration Information from Rotating Machinery. Siegfried. 3. February 1986. this frequency will produce a high squealing noise similar to that generated by dry rolling element bearings..com/Read/754/oil-whirl-whip 3/3 . Chapter 6.machinerylubrication. Donald E. C lick Here Begin a Free Subscription Today Machinery Lubrication magazine delivers unbiased advice for improving lubrication practices and keeping critical equipment running at peak performance. “An Introduction to Machinery Analysis and Monitoring. References 1. 2002. geometries and lubricant properties. No.. Agnes. it must be taken care of quickly in order to prevent a potential catastrophic failure.: Pennwell Publishing Co. 1981. Bently. When dry whip is suspected. “Applications of Time Waveform Analysis. James E.” Sound and Vibration magazine. Like 670 people like this. Palmetier. Okla. 2. Volume 20. both the lubricant itself and the lubrication system should be closely inspected and the bearings should be checked to ensure they have proper clearances. It will generate a frequency specifically dependent upon the shaft and construction materials. George E. The frequency content itself will not be an integer multiple of the machine speed. Be the first of your friends. Normally. John S. Pages 141-151.” Tulsa.The dry whip condition is similar to rubbing a moistened finger over a dry pane of glass. Mitchell. 172-204. Pages 6-72 to 6-75. And when dry whip is suspected. 2. Zimmer. C lick Here Services Subscribe | Contact Us | Privacy Policy | RSS | Advertise Quick Links Home | Buyers Guide | Glossary | Events | Bookstore | Newsletters | Browse Topics © NORIA CORPORATION MACHINERY LUBRICATION | RELIABLE PLANT www. and Muszynska. Berry.