1、精選優(yōu)質(zhì)文檔-傾情為你奉上精選優(yōu)質(zhì)文檔-傾情為你奉上專心-專注-專業(yè)專心-專注-專業(yè)精選優(yōu)質(zhì)文檔-傾情為你奉上專心-專注-專業(yè)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,

2、an explosion. But even a failed bearing leaves clues as to what went wrong. After a little detective work, action can be taken to avoid a repeat performance.1 .WHY BEARINGS FAILAn individual bearing may fail for several reasons; however, the results of an endurance test series are only meaningful wh

3、en the test bearings fail by fatigue-related 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 ca

4、n affect the conduct of a life test sequence.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 contamin

5、ants in the lubricant. Depending on bearing 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 s

6、ubstantially shortened endurances. This has 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 app

7、roximate. If this degree of filtration is 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 a

8、nother important consideration. It has long 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

9、 per million(ppm) may also have a detrimental 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

10、be considered during the evaluation of life 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 spec

11、ific purposes; for example, to provide antiwear 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-

12、related degradation. Care must be taken to 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 tes

13、t lubricant from a particular producer for 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

14、the consistency of the applied test conditions 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, an

15、d the test must be redone under better controls.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 pac

16、kage can alter the EHL conditions in the rolling 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 sh

17、ould be returned to the producer at regular 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 indus

18、trial environments where rather wide fluctuations 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

19、 in operating temperature levels in a lot 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

20、 test process.The deterioration of the condition 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 beari

21、ngs can produce damage to the shaft surface, 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 vibr

22、atory abrasion of the surface, which is accelerated 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.

23、The detection of bearing failure is also a 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 surf

24、ace and produce a spall of sufficient magnitude 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

25、with the complexity of the test system, the 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 repeate

26、dly terminate machine operation with a consistent 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 de

27、tection. In standard through-hardened bearing 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, h

28、owever, be the case with other experimental 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

29、 means of ensuring that an endurance test 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 examina

30、tions, and chemical evaluations as required. 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

31、 that the data are indeed valid. The “Damage 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 pho

32、tographs as well. The post-test analysis is, 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 investiga

33、tion each bearing is examined optically at 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 b

34、all bearing raceway. Figure 19.3 contains 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

35、subtle form of test alteration, where the 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 a

36、re not valid fatigue spalls and indicate the 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

37、 can be done in the selection process by recognizing 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 sufficientTo

38、rque requirements are determined by the lubricant，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，di

39、fferent lubricants have varying noise characteristics 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 reta

40、iner tolerance or eccentricities of the raceways 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

41、 less than 0.3um is used when minimal run 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

42、 many industrial products，and shields and 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 contam

43、inated, its lubricant deteriorates and operation 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 sh

44、ields illustrates some methods for controlling 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

45、when failed bearings are returned for analysis. 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

46、. Dust, for example, makes an irregular crackling. 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

47、 by their frequencies. Generally, defects 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

48、by a variety of inconsistencies, such as pockets 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

49、the grinding operation that produces balls 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 addi

50、tion 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, however, does not specify other bearing characteristics such as raceway quality, f

51、inish, or noise. Hence, a noise classification helps improve on the industry standard.(come from Lu,Zhengran . Study of the bearing capacity of fastener steel tube full hall formwork support using the theory ofstability of pressed pole with three-point rotation restraintJ . China Civil Engineering J

52、ournal 2012-5 )軸承壽命分析摘 要自然界苛刻的工作條件會導(dǎo)致軸承的失效，但是如果遵循一些簡單的規(guī)則，軸承正常運(yùn)轉(zhuǎn)的機(jī)會是能夠被提高的。在軸承的使用過程當(dāng)中，過分的忽視會導(dǎo)致軸承的過熱現(xiàn)象，也可能使軸承不能夠再被使用，甚至完全的破壞。但是一個被損壞的軸承，會留下它為什么被損壞的線索。通過一些細(xì)致的觀察工作，我們可以采取行動來避免軸承的再次失效。1 .軸承失效的原因軸承失效有以下多種原因，然而軸承的壽命實(shí)驗(yàn)卻是所有機(jī)械實(shí)驗(yàn)中最有意義的。實(shí)驗(yàn)者必須控制實(shí)驗(yàn)過程以確保結(jié)果。其他的失效模式在Tallian19.2中有詳細(xì)論述。下邊幾段就詳細(xì)論述了可以影響壽命試驗(yàn)結(jié)果的幾種失效模式。23章中

53、，從EHL的觀點(diǎn)討論了潤滑條件對壽命試驗(yàn)結(jié)果的影響，同時還有其他的潤滑條件會影響實(shí)驗(yàn)的結(jié)論，首先是潤滑劑的接觸面積，受到軸承的尺寸，轉(zhuǎn)速，潤滑劑的流動性等因素的影響，潤滑劑在軸承表面形成的潤滑層的厚度一般小于0.050.5um，大于這個薄層厚度的固體微粒會殘留在接觸面上，從而劃傷潤滑溝道和軸承的滾動面。從而大大縮短軸承的耐用性。關(guān)于這點(diǎn)Sayles和MacPherson以及其他人都有詳細(xì)的論證。因此，為了確保實(shí)驗(yàn)結(jié)果我們必須選用合適等級的潤滑劑。潤滑劑的選擇由工況決定，實(shí)驗(yàn)時也如此。如果工況選擇的范圍不確定，就必須考慮到接觸面積對實(shí)驗(yàn)結(jié)果的影響。23章中討論了不同的接觸面積對軸承失效壽命實(shí)驗(yàn)結(jié)

54、果的影響。潮氣是影響潤滑結(jié)果的另一個重要因素，長時間在水中和油中被腐蝕不但對外觀質(zhì)量有影響，還會影響到滾動表面的軸承壽命。關(guān)于這點(diǎn)Fitch等人19.7有過論證。而且，即使是僅有50100PPM（百萬分之一）的水汽含量也會產(chǎn)生有害影響，甚至產(chǎn)生表面看不出痕跡的腐蝕。這是由于軸承的溝道和滾動面之間會產(chǎn)生氫脆現(xiàn)象，從23章中也可以看出在潤滑實(shí)驗(yàn)中濕氣是如此重要的一個因素。因此在軸承壽命的試驗(yàn)結(jié)果中必須考慮到潮氣的影響。為了降低對壽命減少的影響，潮氣的含量最多不能超過40PPM。潤滑劑的化學(xué)成分也是需要考慮的。大多數(shù)商業(yè)潤滑油包含許多為特定目的而開發(fā)的專有添加劑。例如，為了提高抗磨損性能，為了能達(dá)到

55、極限壓力，或者耐熱性，還可以在邊際潤滑油膜的情況下提供邊界潤滑還能為邊界潤滑提供一個邊界潤滑層。這些添加劑同時也能即時的或者逐漸地影響滾動軸承的耐用性。為了避免添加劑成為加速壽命試驗(yàn)的條件，我們必須小心以確保測試潤滑劑的添加劑不會受到惡化。為了保證同組產(chǎn)品壽命試驗(yàn)的結(jié)果有連貫性，最好在整個壽命試驗(yàn)中都用同一供應(yīng)商的標(biāo)準(zhǔn)潤滑劑。為了得到一個合理的結(jié)果，統(tǒng)計學(xué)要求做很多組壽命試驗(yàn)。因此一個軸承的壽命試驗(yàn)需很長的時間。實(shí)驗(yàn)人員必須保證整個實(shí)驗(yàn)過程的連續(xù)性，由于任何微小的變化都會影響實(shí)驗(yàn)結(jié)果，因此這個過程是很復(fù)雜的。甚至這些微小的變化在造成重大變化之前都不會被注意到。一旦發(fā)生這樣的情況，就沒機(jī)會補(bǔ)救了

56、。只能在更好的控制條件下重新做實(shí)驗(yàn)。比如說：添加劑的穩(wěn)定性會影響到整個實(shí)驗(yàn)的條件?，F(xiàn)在已經(jīng)知道了一些添加劑在長期使用時會造成大量的額外損耗。這些易退化的添加劑會影響軸承表面的潤滑條件，從而影響軸承的壽命。一般的對潤滑劑做化學(xué)檢測時是不會檢測添加劑的成分的。因此，如果一種潤滑劑用于長時間的軸承壽命實(shí)驗(yàn)的話，生產(chǎn)者應(yīng)該定期更換實(shí)驗(yàn)的樣品，比如一年一次。用來詳細(xì)評估潤滑劑的使用要求。實(shí)驗(yàn)時還要控制的是適當(dāng)?shù)臏囟取櫥瑢樱ㄓ湍ぃ┑暮穸葘囟鹊挠绊懯窍喈?dāng)敏感的，大多數(shù)裝機(jī)實(shí)驗(yàn)是在標(biāo)準(zhǔn)的工業(yè)環(huán)境下進(jìn)行的，在這一年實(shí)驗(yàn)時間中環(huán)境溫度變化是非常大的。同時，個別軸承受溫度變化的影響是會影響到整個系統(tǒng)的常規(guī)的制造

57、公差的。因此，所有軸承受溫度變化的影響會直接影響到壽命試驗(yàn)數(shù)據(jù)的準(zhǔn)確性。因此為了保證實(shí)驗(yàn)數(shù)據(jù)的連貫性，必須監(jiān)控并實(shí)時調(diào)節(jié)每個軸承的使用溫度。因此對于軸承壽命試驗(yàn)時3C的溫度公差被認(rèn)為是可接受的。用于軸承壽命試驗(yàn)的硬件裝備的磨損是另一個需要監(jiān)控的恒量。用于重載實(shí)驗(yàn)的軸和軸承的內(nèi)圈都會受到很大的載荷。反復(fù)拆裝軸承會對軸的表面產(chǎn)生損害。這樣的改變會影響幾何形狀的。軸外徑和軸承內(nèi)徑都會受腐蝕的影響。腐蝕是由于震動產(chǎn)生的微粒被氧化而產(chǎn)生的。這樣也會減少軸承壽命試驗(yàn)的時間。同時這樣的機(jī)構(gòu)也會在裝配面上產(chǎn)生重大的幾何形變，從而影響軸承內(nèi)徑，最終成為降低壽命的重要原因。軸承缺陷的檢測也是壽命試驗(yàn)的重要考察因素

58、。軸承缺陷最早是由原材料上的微小裂紋引起的。這樣的缺陷在實(shí)驗(yàn)中是沒法檢測的。為了檢測這個缺陷就需要使這個缺陷遞增到能影響軸承參數(shù)的數(shù)量級別。比如說噪音，溫度，震動等缺陷?？梢栽谙到y(tǒng)中應(yīng)用這些技術(shù)方法來檢驗(yàn)缺陷。而具有這樣能力的系統(tǒng)可以從早期就檢測出在多樣化工作條件復(fù)雜系統(tǒng)中用來測試用的缺陷軸承。而當(dāng)前還沒有一個單一的系統(tǒng)能檢測出所有的軸承缺陷。因此將來有必要選擇一種能在軸承受到微小的傷害之前就停下機(jī)器的監(jiān)控系統(tǒng)。缺陷遞增的速率是相當(dāng)重要的。如果在實(shí)驗(yàn)結(jié)束時缺陷的程度和理論計算出的是一致的，唯一的區(qū)別就是實(shí)驗(yàn)中對缺陷的檢測總是落后于理論計算的。標(biāo)準(zhǔn)的軸承鋼在耐久性實(shí)驗(yàn)中缺陷的遞增速度是相當(dāng)快的。而且這個遞增還不是主要因素，考慮到有代表性的耐久性實(shí)驗(yàn)的數(shù)據(jù)都是經(jīng)統(tǒng)計學(xué)分析后得到的。有的也不一定，比如一些表面硬度不同的鋼材或