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1、精选优质文档-倾情为你奉上BEARING LIFE ANALYSISABSTRACTNature works hard to destroy bearings, but their chances of survival can be improved by following a few simple guidelines. Extreme neglect in a bearing leads to overheating and possibly seizure or, at worst, an explosion. But even a failed bearing leaves clu

2、es as to what went wrong. After a little detective work, action can be taken to avoid a repeat performance.专心-专注-专业1 .WHY BEARINGS FAILAn individual bearing may fail for several reasons; however, the results of an endurance test series are only meaningful when the test bearings fail by fatigue-relat

3、ed mechanisms. The experimenter must control the test process to ensure that this occurs. Some of the other failure modes that can be experienced are discussed in detail by Tallian 19.2. The following paragraphs deal with a few specific failure types that can affect the conduct of a life test sequen

4、ce.In Chapter 23, the influence of lubrication on contact fatigue life is discussed from the standpoint of EHL film generation. There are also other lubrication-related effects that can affect the outcome of the test series. The first is particulate contaminants in the lubricant. Depending on bearin

5、g size, operating speed, and lubricant rheology, the overall thickness of the lubricant film developed at the rolling element-raceway contacts may fall between 0.05 and 0.5 m . Solid particles and damage the raceway and rolling element surfaces, leading to substantially shortened endurances. This ha

6、s been amply demonstrated by Sayles and MacPherson 19.6 and others.Therefore, filtration of the lubricant to the desired level is necessary to ensure meaningful test result. The desired level is determined by the application which the testing purports to approximate. If this degree of filtration is

7、not provided, effects of contamination must be considered when evaluating test results. Chapter 23 discusses the effect of various degrees of particulate contamination, and hence filtration, on bearing fatigue life. The moisture content in the lubricant is another important consideration. It has lon

8、g been apparent that quantities of free water in the oil cause corrosion of the rolling contact surfaces and thus have a detrimental effect on bearing life. It has been further shown by Fitch 19.7 and others, however, that water levels as low as 50-100 parts per million(ppm) may also have a detrimen

9、tal effect, even with no evidence of corrosion. This is due to hydrogen embrittlement of the rolling element and raceway material. See also Chapter 23. Moisture control in test lubrication systems is thus a major concern, and the effect of moisture needs to be considered during the evaluation of lif

10、e test results. A maximum of 40 ppm is considered necessary to minimize life reduction effects.The chemical composition of the test lubricant also requires consideration. Most commercial lubricants contain a number of proprietary additives developed for specific purposes; for example, to provide ant

11、iwear properties, to achieve extreme pressure and/or thermal stability, and to provide boundary lubrication in case of marginal lubricant films. These additives can also affect the endurance of rolling bearings, either immediately or after experiencing time-related degradation. Care must be taken to

12、 ensure that the additives included in the test lubricant will not suffer excessive deterioration as a result of accelerated life test conditions. Also for consistency of results and comparing life test groups, it is good practice to utilize one standard test lubricant from a particular producer for

13、 the conduct of all general life tests.The statistical nature of rolling contact fatigue requires many test samples to obtain a reasonable estimate of life. A bearing life test sequence thus needs a long time. A major job of the experimentalist is to ensure the consistency of the applied test condit

14、ions throughout the entire test period. This process is not simple because subtle changes can occur during the test period. Such changes might be overlooked until their effects become major. At that time it is often too late to salvage the collected data, and the test must be redone under better con

15、trols.For example, the stability of the additive packages in a test lubricant can be a source of changing test conditions. Some lubricants have been known to suffer additive depletion after an extended period of operation. The degradation of the additive package can alter the EHL conditions in the r

16、olling content, altering bearing life. Generally, the normal chemical tests used to evaluate lubricants do not determine the conditions of the additive content. Therefore if a lubricant is used for endurance testing over a long time, a sample of the fluid should be returned to the producer at regula

17、r intervals, say annually, for a detailed evaluation of its condition.Adequate temperature controls must also be employed during the test. The thickness of the EHL film is sensitive to the contact temperature. Most test machines are located in standard industrial environments where rather wide fluct

18、uations in ambient temperature are experienced over a period of a year. In addition, the heat generation rates of individual bearings can vary as a result of the combined effects of normal manufacturing tolerances. Both of these conditions produce variations in operating temperature levels in a lot

19、of bearings and affect the validity of the life data. A means must be provided to monitor and control the operating temperature level of each bearing to achieve a degree of consistency. A tolerance level of3C is normally considered adequate for the endurance test process.The deterioration of the con

20、dition of the mounting hardware used with the bearings is another area requiring constant monitoring. The heavy loads used for life testing require heavy interference fits between the bearing inner rings and shafts. Repeated mounting and dismounting of bearings can produce damage to the shaft surfac

21、e, which in turn can alter the geometry of a mounted ring. The shaft surface and the bore of the housing are also subject to deterioration from fretting corrosion. Fretting corrosion results from the oxidation of the fine wear particles generated by the vibratory abrasion of the surface, which is ac

22、celerated by the heavy endurance test loading. This mechanism can also produce significant variations in the geometry of the mounting surfaces, which can alter the internal bearing geometry. Such changes can have a major effect in reducing bearing test life.The detection of bearing failure is also a

23、 major consideration in a life test series. The fatigue theory considers failure as the initiation of the first crack in the bulk material. Obviously there is no way to detect this occurrence in practice. To be detectable the crack must propagate to the surface and produce a spall of sufficient magn

24、itude to produce a marked effect on an operating parameter of the bearing: for example, noise, vibration, and/or temperature. Techniques exit for detecting failures in application systems. The ability of these systems to detect early signs of failure varies with the complexity of the test system, th

25、e type of bearing under evaluation, and other test conditions. Currently no single system exists that can consistently provide the failure discrimination necessary for all types of bearing life tests. It is then necessary to select a system that will repeatedly terminate machine operation with a con

26、sistent minimal degree of damage.The rate of failure propagation is therefore important. If the degree of damage at test termination is consistent among test elements, the only variation between the experimental and theoretical lives is the lag in failure detection. In standard through-hardened bear

27、ing steels the failure propagation rate is quite rapid under endurance test conditions, and this is not a major factor, considering the typical dispersion of endurance test data and the degree of confidence obtained from statistical analysis. This may not, however, be the case with other experimenta

28、l materials or with surface-hardened steels or steels produced by experimental techniques. Care must be used when evaluating these latter results and particularly when comparing the experimental lives with those obtained from standard steel lots.The ultimate means of ensuring that an endurance test

29、series was adequately controlled is the conduct of a post-test analysis. This detailed examination of all the tested bearings uses high-magnification optical inspection, higher-magnification scanning electron microscopy, metallurgical and dimensional examinations, and chemical evaluations as require

30、d. The characteristics of the failures are examined to establish their origins and the residual surface conditions are evaluated for indications of extraneous effects that may have influenced the bearing life. This technique allows the experimenter to ensure that the data are indeed valid. The “Dama

31、ge Atlas” compiled by Tallian et al. 19.8 containing numerous black and white photographs of the various bearing failure modes can provide guidance for these types of determinations. This work was subsequently updated by Tallian 19.9, now including color photographs as well. The post-test analysis i

32、s, by definition, after the fact. To provide control throughout the test series and to eliminate all questionable areas, the experimenter should conduct a preliminary study whenever a bearing is removed from the test machine. In this portion of the investigation each bearing is examined optically at

33、 magnifications up to 30 for indications of improper or out-of-control test parameters. Examples of the types of indications that can be observed are given in Figs. 19.2-19.6.Figure 19.2 illustrates the appearance of a typical fatigue-originated spall on a ball bearing raceway. Figure 19.3 contains

34、a spalling failure on the raceway of a roller bearing that resulted from bearing misalignment, and Fig. 19.4 contains a spalling failure on the outer ring of a ball bearing produced by fretting corrosion on the outer diameter. Figure 19.5 illustrates a more subtle form of test alteration, where the

35、spalling failure originated from the presence of a debris dent on the surface. Figure 19.6 gives an example of a totally different failure mode produced by the loss of internal bearing clearance due to thermal unbalance of the system.The last four failures are not valid fatigue spalls and indicate t

36、he need to correct the test methods. Furthermore, these data points would need to be eliminated from the failure data to obtain a valid estimate of the experimental bearing life.2 .AVOIDING FAILURESThe best way to handle bearing failures is to avoid themThis can be done in the selection process by r

37、ecognizing critical performance characteristicsThese include noise，starting and running torque，stiffness，non-repetitive run out，and radial and axial playIn some applications, these items are so critical that specifying an ABEC level alone is not sufficientTorque requirements are determined by the lu

38、bricant，retainer，raceway quality(roundness cross curvature and surface finish)，and whether seals or shields are usedLubricant viscosity must be selected carefully because inappropriate lubricant，especially in miniature bearings，causes excessive torqueAlso，different lubricants have varying noise char

39、acteristics that should be matched to the application. For example，greases produce more noise than oilNon-repetitive run out(NRR)occurs during rotation as a random eccentricity between the inner and outer races，much like a cam actionNRR can be caused by retainer tolerance or eccentricities of the ra

40、ceways and ballsUnlike repetitive run out, no compensation can be made for NRR.NRR is reflected in the cost of the bearingIt is common in the industry to provide different bearing types and grades for specific applicationsFor example，a bearing with an NRR of less than 0.3um is used when minimal run

41、out is needed，such as in diskdrive spindle motorsSimilarly，machinetool spindles tolerate only minimal deflections to maintain precision cutsConsequently, bearings are manufactured with low NRR just for machine-tool applicationsContamination is unavoidable in many industrial products，and shields and

42、seals are commonly used to protect bearings from dust and dirtHowever，a perfect bearing seal is not possible because of the movement between inner and outer racesConsequently，lubrication migration and contamination are always problemsOnce a bearing is contaminated, its lubricant deteriorates and ope

43、ration becomes noisierIf it overheats，the bearing can seizeAt the very least，contamination causes wear as it works between balls and the raceway，becoming imbedded in the races and acting as an abrasive between metal surfacesFending off dirt with seals and shields illustrates some methods for control

44、ling contaminationNoise is as an indicator of bearing qualityVarious noise grades have been developed to classify bearing performance capabilitiesNoise analysis is done with an Ander-on-meter, which is used for quality control in bearing production and also when failed bearings are returned for anal

45、ysis. A transducer is attached to the outer ring and the inner race is turned at 1,800rpm on an air spindle. Noise is measured in andirons, which represent ball displacement in m/rad.With experience, inspectors can identify the smallest flaw from their sound. Dust, for example, makes an irregular cr

46、ackling. Ball scratches make a consistent popping and are the most difficult to identify. Inner-race damage is normally a constant high-pitched noise, while a damaged outer race makes an intermittent sound as it rotates.Bearing defects are further identified by their frequencies. Generally, defects

47、are separated into low, medium, and high wavelengths. Defects are also referenced to the number of irregularities per revolution.Low-band noise is the effect of long-wavelength irregularities that occur about 1.6 to 10 times per revolution. These are caused by a variety of inconsistencies, such as p

48、ockets in the race. Detectable pockets are manufacturing flaws and result when the race is mounted too tightly in multiple jaw chucks.Medium-hand noise is characterized by irregularities that occur 10 to 60 times per revolution. It is caused by vibration in the grinding operation that produces balls

49、 and raceways. High-hand irregularities occur at 60 to 300 times per revolution and indicate closely spaced chatter marks or widely spaced, rough irregularities.Classifying bearings by their noise characteristics allows users to specify a noise grade in addition to the ABEC standards used by most manufacturers. ABEC defines physical tolerances such as bore, outer diameter, and run out. As the ABEC class number increase (from 3 to 9), tolerances are tightened. ABEC class,