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DEFECT SEVERITY Bearing Behavior A properly selected and installed angular contact bearing should have anevenly distributed thrust load on theentire surface of both races. Under such conditions the bearing typically developssmall defects all the way around the race. The combination of a 360 degree load zone andmultiple small defects produces a series of harmonics of ball pass frequency. The numberof harmonics generated is related to the circumference of both races and/or the lengthof the load zone. A properly selected and installed deep groove ball bearing should carry aradial load in about one third of thecircumference. Under these conditions, the bearing develops a defect in the load zone thatgenerates a single ball pass frequency. Two defects can generate the second harmonic ofball pass frequency. If the load -- as "felt" by the bearing -- is on the rotor, as occurs in imbalance, a spall shouldoccur on the inner race (if the inner race is rotatingand the outer race is stationary). The load zone is always at the same spot on the innerrace. If the load is external to the rotor, as is the case with over-tight V belts, the defectshould occur on the outer race at the load zone, because the load zone is always at the sameplace on the outer race. If a thrust bearing is notevenly loaded over its circumference and has a radial load, it will behave like a radialbearing; that is, it will produce a single ball pass frequency. If a radial bearing has anabnormal thrust load, it will behave like a thrust bearing; that is, it will generate severalharmonics of the ball pass frequency. Bearings that are subjected to the type of loads for which they were not designed fail quickly.Excessive loads can be identified with spectralanalysis. This is accomplished by accurately measuring the generated BPFO or BPFI and thencalculating the contact angle. The contact angle identifies the amount of thrustload. A beginning defect on either race generates ball pass frequency. Early spallinformation can be identified when hairlinecracks develop. Manufacturing defects, such as holes in the race the size of a sharppencil point, can be identified. Ball pass frequency can be detected in unloaded machines,for example an electric motor not connected to a driven unit or a new motor on the shopfloor. After a defect has begun, itwill get larger, and the spectral bandwidth will get wider until the spectrum is modulatedwith the speed of the rotating unit. The ball pass frequency and the ball passfrequency plus or minus the unit speed may be generated. Modulation can continue untilthe ball pass frequency is no longer apparent. (In some cases the amplitudes of the sumand difference frequencies are equal to or exceed those of the ball pass frequency.)The spectrum then becomes a series of frequency peaks whose difference frequency isequal to the unit speed. These phenomena occur when a growing fatigue spall ispresent on the race. The limits of a significantchange in vibration level depend on the type of equipment involved and thecause of the vibration. For a coupled pump and motor that normally vibrate at 0.07 IPS, a changeof 0.05 IPS could be significant and should be inspected. On the other hand, a bucketelevator that normally vibrates within a range on either side of 0.5 IPS could be checked on aperiodic basis or when a change of 0.2 IPS occurs. At the other extreme, a cracked innerrace may not cause a noticeable change in the vibration amplitude. Defects in antifrictionbearings can be identified at 0.008 IPS; an increase of 0.1 IPS in a bearing could thus beimportant, whereas an increase of 0.1 IPS in an imbalance condition might cause little concern. Excessive vibration can bedefined as that level of vibration that experience has shown to be harmful to a particularpiece of equipment. For a directly coupled pump and motor, excessive vibration could be0.05 IPS at a bearing frequency. For a fan that weighs 2,000 pounds, 0.25 IPS at runningspeed could be excessive. At the other extreme, a 20 pound fan could vibrate at 2.0 IPS atrunning speed for long periods without incident. Bearings in rotating machineryshould be periodically checked with a frequency spectrum and time signal to detect and studydeveloping defects on the outer and inner races. An accurate method for thecalculation of bearing defect length is needed to allow a quantitative determination ofthe defect severity. With the defect size and progression of development determined, theremaining bearing life can be estimated. Empirical measurements indicatethat a 6313 bearing can generate six harmonics of BPFI if the bearing a) is in athrust load; b) is failing from a shallow flaking spall; and c) has a defect all the way around therace. The same bearing can generate seven harmonics of BPFO if the above threeconditions are met. When deep fatigue spalls arepresent, pulses are generated. The FTT contains several harmonics when pulses arepresent. Etching, corrosion, and fluting can also cause many harmonics. In spherical and tapered rollerbearings, the length of the defect on the outer race can be approximated by harmoniccontent. If the bearing is failing from a shallow flaking spall, each true harmonic of BPFOequals about 0.5 inches of defect length. For example, a spherical roller bearing thatis generating six harmonics of outer race ball pass frequency indicates an outer race defectabout three inches long. If the above knowledge and thefollowing information on the inner race are used to approximate defect size, andapplied to your diagnostics, it is helpful in assigning priorities for repair. Forexample, we may want to replace bearings with large defects before we replace bearings with small defects.
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发表于 2017-1-2 11:53
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