轴承(机械类毕业设计外文翻译)
轴承寿命分析
摘 要
自然界苛刻的工作条件会导致轴承的失效，但是如果遵循一些简单的规则，轴承正常运转的机会是能够被提高的。在轴承的使用过程当中，过分的忽视会导致轴承的过热现象，也可能使轴承不能够再被使用，甚至完全的破坏。但是一个被损坏的轴承，会留下它为什么被损坏的线索。通过一些细致的观察工作，我们可以采取行动来避免轴承的再次失效。
关键词：轴承 失效 寿命
1 .轴承失效的原因
轴承失效有以下多种原因，然而轴承的寿命实验却是所有机械实验中最有意义的。实验者必须控制实验过程以确保结果。其他的失效模式在Tallian[19.2]中有详细论述。下边几段就详细论述了可以影响寿命试验结果的几种失效模式。
23章中，从EHL的观点讨论了润滑条件对寿命试验结果的影响，同时还有其他的润滑条件会影响实验的结论，首先是润滑剂的接触面积，受到轴承的尺寸，转速，润滑剂的流动性等因素的影响，润滑剂在轴承表面形成的润滑层的厚度一般小于0.05~0.5um，大于这个薄层厚度的固体微粒会残留在接触面上，从而划伤润滑沟道和轴承的滚动面。从而大大缩短轴承的耐用性。关于这点Sayles和MacPherson以及其他人都有详细的论证。
因此，为了确保实验结果我们必须选用合适等级的润滑剂。润滑剂的选择由工况决定，实验时也如此。如果工况选择的范围不确定，就必须考虑到接触面积对实验结果的影响。
23章中讨论了不同的接触面积对轴承失效寿命实验结果的影响。
潮气是影响润滑结果的另一个重要因素，长时间在水中和油中被腐蚀不但对外观质量有影响，还会影响到滚动表面的轴承寿命。关于这点Fitch等人[19.7]有过论证。而且，即使是仅有50~100PPM（百万分之一）的水汽含量也会产生有害影响，甚至产生表面看不出痕迹的腐蚀。这是由于轴承的沟道和滚动面之间会产生氢脆现象，从23章中也可以看出在润滑实验中湿气是如此重要的一个因素。因此在轴承寿命的试验结果中必须考虑到潮气的影响。为了降低对寿命减少的影响，潮气的含量最多不能超过40PPM。
润滑剂的化学成分也是需要考虑的。大多数商业润滑油包含许多为特定目的而开发的专有添加剂。例如，为了提高抗磨损性能，为了能达到极限压力，或者耐热性，还可以在边际润滑油膜的情况下提供边界润滑还能为边界润滑提供一个边界润滑层。这些添加剂同时也能
即时的或者逐渐地影响滚动轴承的耐用性。为了避免添加剂成为加速寿命试验的条件，我们必须小心以确保测试润滑剂的添加剂不会受到恶化。为了保证同组产品寿命试验的结果有连贯性，最好在整个寿命试验中都用同一供应商的标准润滑剂。
为了得到一个合理的结果，统计学要求做很多组寿命试验。因此一个轴承的寿命试验需很长的时间。实验人员必须保证整个实验过程的连续性，由于任何微小的变化都会影响实验结果，因此这个过程是很复杂的。甚至这些微小的变化在造成重大变化之前都不会被注意到。一旦发生这样的情况，就没机会补救了。只能在更好的控制条件下重新做实验。
比如说：添加剂的稳定性会影响到整个实验的条件。现在已经知道了一些添加剂在长期使用时会造成大量的额外损耗。这些易退化的添加剂会影响轴承表面的润滑条件，从而影响轴承的寿命。
一般的对润滑剂做化学检测时是不会检测添加剂的成分的。因此，如果一种润滑剂用于长时间的轴承寿命实验的话，生产者应该定期更换实验的样品，比如一年一次。用来详细评估润滑剂的使用要求。
实验时还要控制的是适当的温度。润滑层（油膜）的厚度对温度的影响是相当敏感的，大多数装机实验是在标准的工业环境下进行的，在这一年实验时间中环境温度变化是非常大的。同时，个别轴承受温度变化的影响是会影响到整个系统的常规的制造公差的。因此，所有轴承受温度变化的影响会直接影响到寿命试验数据的准确性。因此为了保证实验数据的连贯性，必须监控并实时调节每个轴承的使用温度。因此对于轴承寿命试验时±3ºC的温度公差被认为是可接受的。
用于轴承寿命试验的硬件装备的磨损是另一个需要监控的恒量。用于重载实验的轴和轴承的内圈都会受到很大的载荷。反复拆装轴承会对轴的表面产生损害。这样的改变会影响几何形状的。轴外径和轴承内径都会受腐蚀的影响。腐蚀是由于震动产生的微粒被氧化而产生的。这样也会减少轴承寿命试验的时间。同时这样的机构也会在装配面上产生重大的几何形变，从而影响轴承内径，最终成为降低寿命的
重要原因。
轴承缺陷的检测也是寿命试验的重要考察因素。轴承缺陷最早是由原材料上的微小裂纹引起的。这样的缺陷在实验中是没法检测的。为了检测这个缺陷就需要使这个缺陷递增到能影响轴承参数的数量级别。比如说噪音，温度，震动等缺陷。可以在系统中应用这些技术方法来检验缺陷。而具有这样能力的系统可以从早期就检测出在多样化工作条件复杂系统中用来测试用的缺陷轴承。而当前还没有一个单一的系统能检测出所有的轴承缺陷。因此将来有必要选择一种能在轴承受到微小的伤害之前就停下机器的监控系统。
缺陷递增的速率是相当重要的。如果在实验结束时缺陷的程度和理论计算出的是一致的，唯一的区别就是实验中对缺陷的检测总是落后于理论计算的。标准的轴承钢在耐久性实验中缺陷的递增速度是相当快的。而且这个递增还不是主要因素，考虑到有代表性的耐久性实验的数据都是经统计学分析后得到的。有的也不一定，比如一些表面硬度不同的钢材或是专为实验用生产的钢材。因此在分析结果的时候就必须考虑是标准的轴承钢还是专门的实验用钢材。
耐久性实验最后结果的有效性是由元素-金相分析验证的。轴承会通过高倍光学显微镜，高倍电子扫描显微镜，高倍电子显微镜，化学元素分析等多种方法来分析。生产时出现的会导致缺陷的元素以及残留在表面发生化学变化以后会导致缺陷的元素（如S，P等有害元素）等都会影响轴承的寿命。这些检验方法都是用来保证实验得出的数据是真实有效的。Tallian将所有轴承失效的黑白图片汇编起来
【19.8】，可以为判断各种类型的失效提供依据。现在Tallian已经将其更新为【19.9】，其中加入了彩色图片。
元素-金相实验可以提供一个精确的证据，使实验结果处于可控制情况下，同时检测有疑点和争议的地方。当轴承从试验机上取下来的时候可以现做一个初步的研究，将会在30倍显微镜下观察失效的部分。而正常的显微图片请看19.2~19.6中的图片。、
图19.2是球轴承沟道的表面失效图片。图19.3是滚子轴承沟道由于未校准而造成表面开裂的图片。图19.4是一个球轴承由于外圈
表面锈蚀而导致外圈开裂的图片。图19.5是表面凹陷残骸的详细图片。图19.6是一个由于热变形造成的内圈游隙变化的图片。
最后的4张图片不是用正确的实验方法得到的有效的失效模式。然而，这些错误的数据需要从有效的失效数据中剔除掉，从而得到能正确评估寿命试验的有效数据。
2 .避免失效的方法
解决轴承失效问题的最好办法就是避免失效发生。这可以在选用过程中通过考虑关键性能特征来实现。这些特征包括噪声、起动和运转扭矩、刚性、非重复性振摆以及径向和轴向间隙。
扭矩要求是由润滑剂、保持架、轴承圈质量（弯曲部分的圆度和表面加工质量）以及是否使用密封或遮护装置来决定。润滑剂的粘度必须认真加以选择，因为不适宜的润滑剂会产生过大的扭矩，这在小型轴承中尤其如此。另外，不同的润滑剂的噪声特性也不一样。举例来说，润滑脂产生的噪声比润滑油大一些。因此，要根据不同的用途来选用润滑剂。
在轴承转动过程中，如果内圈和外圈之间存在一个随机的偏心距，就会产生与凸轮运动非常相似的非重复性振摆（NRR）。保持架的尺寸误差和轴承圈与滚珠的偏心都会引起NRR。和重复性振摆不同的是，NRR是没有办法进行补偿的。
在工业中一般是根据具体的应用来选择不同类型和精度等级的轴承。例如，当要求振摆最小时，轴承的非重复性振摆不能超过0.3微米。同样，机床主轴只能容许最小的振摆，以保证切削精度。因此在机床的应用中应该使用非重复性振摆较小的轴承。
在许多工业产品中，污染是不可避免的，因此常用密封或遮护装置来保护轴承，使其免受灰尘或脏物的侵蚀。但是，由于轴承内外圈的运动，使轴承的密封不可能达到完美的程度，因此润滑油的泄漏和污染始终是一个未能解决的问题。
一旦轴承受到污染，润滑剂就要变质，运行噪声也随之变大。如果轴承过热，它将会卡住。当污染物处于滚珠和轴承圈之间时，其作
用和金属表面之间的磨粒一样，会使轴承磨损。采用密封和遮护装置来挡开脏物是控制污染的一种方法。
噪声是反映轴承质量的一个指标。轴承的性能可以用不同的噪声等级来表示。
噪声的分析是用安德逊计进行的，该仪器在轴承生产中可用来控制质量，也可对失效的轴承进行分析。将一传感器连接在轴承外圈上，而内圈在心轴以1800r/min的转速旋转。测量噪声的单位为anderons。即用um/rad表示的轴承位移。
根据经验，观察者可以根据声音辨别出微小的缺陷。例如，灰尘产生的是不规则的噼啪声；滚珠划痕产生一种连续的爆破声，确定这种划痕最困难；内圈损伤通常产生连续的高频噪声，而外圈损伤则产生一种间歇的声音。
轴承缺陷可以通过其频率特性进一步加以鉴定。通常轴承缺陷被分为低、中、高三个波段。缺陷还可以根据轴承每转动一周出现的不规则变化的次数加以鉴定。
低频噪声是长波段不规则变化的结果。轴承每转一周这种不规则变化可出现1.6~10次，它们是由各种干涉（例如轴承圈滚道上的凹坑）引起的。可察觉的凹坑是一种制造缺陷，它是在制造过程中由于多爪卡盘夹的太紧而形成的。
中频噪声的特征是轴承每旋转一周不规则变化出现10~60次。这种缺陷是由在轴承圈和滚珠的磨削加工中出现的振动引起的。轴承每旋转一周高频不规则变化出现60~300次，它表明轴承上存在着密集的振痕或大面积的粗糙不平。
利用轴承的噪声特性对轴承进行分类，用户除了可以确定大多数厂商所使用的ABEC标准外，还可确定轴承的噪声等级。ABEC标准只定义了诸如孔、外径、振摆等尺寸公差。随着ABEC级别的增加（从3增到9），公差逐渐变小。但ABEC等级并不能反映其他轴承特性，如轴承圈质量、粗糙度、噪声等。因此，噪声等级的划分有助于工业标准的改进。
BEARING LIFE ANALYSIS
ABSTRACT
Nature 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 clues as to what went wrong. After a little detective work, action can be taken to avoid a repeat performance.
KEY WORDS: bearings, failures , life
1 .WHY BEARINGS FAIL
An 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-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 can 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 contaminants 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 substantially 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 approximate. 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 another 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 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 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 specific 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-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 test 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 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, and 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 package 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 should 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 industrial 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 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 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 bearings 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 vibratory 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.
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 surface 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 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 repeatedly 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 detection. 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, however, 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 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 examinations, 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 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 photographs 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 investigation 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 ball 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 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 are 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 FAILURES
The best way to handle bearing failures is to avoid them．This can be done in the selection process by recognizing critical performance characteristics．These include noise，starting and running torque，stiffness，non-repetitive run out，and radial and axial play．In some applications, these items are so critical that specifying an ABEC level alone is not sufficient．
Torque requirements are determined by the lubricant，retainer，raceway quality(roundness cross curvature and surface finish)，and whether seals or shields are used．Lubricant viscosity must be selected carefully because inappropriate lubricant，especially in miniature bearings，causes excessive torque．Also，different lubricants have varying noise characteristics that should be matched to the application. For example，greases produce more noise than oil．
Non-repetitive run out(NRR)occurs during rotation as a random eccentricity between the inner and outer races，much like a cam action．NRR can be caused by retainer tolerance or eccentricities of the
raceways and balls．Unlike repetitive run out, no compensation can be made for NRR.
NRR is reflected in the cost of the bearing．It is common in the industry to provide different bearing types and grades for specific applications．For example，a bearing with an NRR of less than 0.3um is used when minimal run out is needed，such as in disk—drive spindle motors．Similarly，machine—tool spindles tolerate only minimal deflections to maintain precision cuts．Consequently, bearings are manufactured with low NRR just for machine-tool applications．
Contamination is unavoidable in many industrial products，and shields and seals are commonly used to protect bearings from dust and dirt．However，a perfect bearing seal is not possible because of the movement between inner and outer races．Consequently，lubrication migration and contamination are always problems．
Once a bearing is contaminated, its lubricant deteriorates and operation becomes noisier．If it overheats，the bearing can seize．At 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 surfaces．Fending off dirt with seals and shields illustrates some methods for controlling contamination．
Noise is as an indicator of bearing quality．Various noise grades have been developed to classify bearing performance capabilities．
Noise 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 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 μm/rad.
With experience, inspectors can identify the smallest flaw from their sound. 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 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 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 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 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, however, does not specify other bearing characteristics such as raceway quality, finish, or noise. Hence, a noise classification helps improve on the industry standard.