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刮板输送机设计 学生姓名：敖煜 班级：0781053 指导老师：于斐 摘要：本次课题设计的题目是“刮板输送机的设计”。采用二维CAD软件Auto CAD 进行结构设计。本次的设计具体内容主要包括：刮板输送机传动总体设计；主要传动机构设计；主要零、部件设计；完成主要零件的工艺设计；设计一套主要件的工艺装备；撰写开题报告；撰写毕业设计说明书；翻译外文资料等。对于即将毕业的学生来说，本次设计的最大成果就是：综合运用机械设计、机械制图、机械制造基础、金属材料与热处理、公差与技术测量、理论力学、材料力学、机械原理、计算机应用基础以及工艺、夹具等基础理论、工程技术和生产实践知识。掌握机械设计的一般程序、方法、设计规律、技术措施，并与生产实习相结合，培养分析和解决一般工程实际问题的能力，具备了机械传动装置、简单机械的设计和制造的能力。关键词：刮板输送机；传动装置；设计；制造 指导老师签名： 0781053 CADAuto CAD 棻飻 学士学位论文原创性声明本人声明，所呈交的论文是本人在导师的指导下独立完成的研究成果。除了文中特别加以标注引用的内容外，本论文不包含法律意义上已属于他人的任何形式的研究成果,也不包含本人已用于其他学位申请的论文或成果。对本文的研究作出重要贡献的个人和集体，均已在文中以明确方式表明。本人完全意识到本声明的法律后果由本人承担。作者签名： 日期：学位论文版权使用授权书本学位论文作者完全了解学校有关保留、使用学位论文的规定，同意学校保留并向国家有关部门或机构送交论文的复印件和电子版，允许论文被查阅和借阅。本人授权南昌航空大学科技学院可以将本论文的全部或部分内容编入有关数据库进行检索，可以采用影印、缩印或扫描等复制手段保存和汇编本学位论文。 作者签名： 日期：导师签名： 日期：毕业设计（论文）任务书I、毕业设计(论文)题目：刮板输送机设计II、毕 业设计(论文)使用的原始资料(数据)及设计技术要求：原始数据： 刮板速度：0.1M/s，刮板宽度：390mm，机槽宽度：530mm，机长：42000mm设计技术要求：1.刮板输送机的性能计算2.刮板输送机的动力学分析 3.完成刮板输送机的总装图和零件图III、毕 业设计(论文)工作内容及完成时间：1. 开题报告、查阅资料、外文翻译（6000字符) 第1周第3周2. 刮板输送机的性能计算 第4周第6周3. 刮板输送机的动力学分析 第7周第9周4. 完成刮板输送机的总装图和零件图 第10周第13周5. 撰写毕业论文 第14周第16周6. 答辩准备及毕业答辩 第17周 、主 要参考资料：1 成大先机械设计手册第四版.北京：化学工业出版社，20022 李柱国机械设计与理论北京：科学出版社，20033 邱宣怀机械设计.第四版北京：高等教育出版社，19974 黄平，刘建素等. 常用机械零件及机构图册M. 北京. 化学工业出版社.19995机械设计手册联合编写组编.机械设计手册.中册.M.北京.化学工业出版社.19876 申永胜.机械原理教程M.北京.清华大学出版社M.2006 航空与机械工程 学院 机械设计制造及其自动化 专业类 0781053 班学生（签名）： 填写日期： 2011 年 01 月 03 日 指导教师（签名）： 助理指导教师(并指出所负责的部分)： 机械制造工程 系（室）主任（签名）： 姚坤弟附注:任务书应该附在已完成的毕业设计说明书首页。 I() II ()() 0.1M/s390mm530mm42000mm1.2. 3.III ()1. 桢6000) 132. 463. 794. 10135. 14166. 17 1 İ.磬2002 2 磬20033 .İ森磬19974 . M. . .19995.M.19876 .M.廪M.2006 0781053 2011 01 03 () : 毕业设计（论文）外文文献翻译毕业设计（论文）外文文献翻译 题目题目 如何延长轴承寿命 专业名称专业名称 机械设计制造及其自动化 班级学号班级学号 078105302 学生姓名学生姓名 敖煜 指导教师指导教师 于斐 填表日期填表日期 2011 年年 3 月月 5 日日 2 如何延长轴承寿命如何延长轴承寿命 摘要：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。通过一些细致的侦察工作，我们可以采 取行动来避免轴承的再次失效。 关键词：关键词： 轴承轴承 失效失效 寿命寿命 导致轴承失效的原因很多，但常见的是不正确的使用、污染、润滑剂使用不当、 装卸或搬运时的损伤及安装误差等。诊断失效的原因并不困难，因为根据轴承上留 下的痕迹可以确定轴承失效的原因。 然而，当事后的调查分析提供出宝贵的信息时，最好首先通过正确地选定轴承 来完全避免失效的发生。为了做到这一点，再考察一下制造厂商的尺寸定位指南和 所选轴承的使用特点是非常重要的。 1 1 轴承失效的原因轴承失效的原因 在球轴承的失效中约有 40%是由灰尘、脏物、碎屑的污染以及腐蚀造成的。污 染通常是由不正确的使用和不良的使用环境造成的，它还会引起扭矩和噪声的问题。 由环境和污染所产生的轴承失效是可以预防的，而且通过简单的肉眼观察是可以确 定产生这类失效的原因。 通过失效后的分析可以得知对已经失效的或将要失效的轴承应该在哪些方面进 行查看。弄清诸如剥蚀和疲劳破坏一类失效的机理，有助于消除问题的根源。 只要使用和安装合理，轴承的剥蚀是容易避免的。剥蚀的特征是在轴承圈滚道 上留有由冲击载荷或不正确的安装产生的压痕。剥蚀通常是在载荷超过材料屈服极 限时发生的。如果安装不正确从而使某一载荷横穿轴承圈也会产生剥蚀。轴承圈上 的压坑还会产生噪声、振动和附加扭矩。 类似的一种缺陷是当轴承不旋转时由于滚珠在轴承圈间振动而产生的椭圆形压 痕。这种破坏称为低荷振蚀。这种破坏在运输中的设备和不工作时仍振动的设备中 都会产生。此外，低荷振蚀产生的碎屑的作用就象磨粒一样，会进一步损害轴承。 与剥蚀不同，低荷振蚀的特征通常是由于微振磨损腐蚀在润滑剂中会产生淡红色。 消除振动源并保持良好的轴承润滑可以防止低荷振蚀。给设备加隔离垫或对底 3 座进行隔离可以减轻环境的振动。另外在轴承上加一个较小的预载荷不仅有助于滚 珠和轴承圈保持紧密的接触，并且对防止在设备运输中产生的低荷振蚀也有帮助。 造成轴承卡住的原因是缺少内隙、润滑不当和载荷过大。在卡住之前，过大的 摩擦和热量使轴承钢软化。过热的轴承通常会改变颜色，一般会变成蓝黑色或淡黄 色。摩擦还会使保持架受力，这会破坏支承架，并加速轴承的失效。 材料过早出现疲劳破坏是由重载后过大的预载引起的。如果这些条件不可避免， 就应仔细计算轴承寿命，以制定一个维护计划。 另一个解决办法是更换材料。若标准的轴承材料不能保证足够的轴承寿命，就 应当采用特殊的材料。另外，如果这个问题是由于载荷过大造成的，就应该采用抗 载能力更强或其他结构的轴承。 蠕动不象过早疲劳那样普遍。轴承的蠕动是由于轴和内圈之间的间隙过大造成 的。蠕动的害处很大，它不仅损害轴承，也破坏其他零件。 蠕动的明显特征是划痕、擦痕或轴与内圈的颜色变化。为了防止蠕动，应该先 用肉眼检查一下轴承箱件和轴的配件。 蠕动与安装不正有关。如果轴承圈不正或翘起，滚珠将沿着一个非圆周轨道运 动。这个问题是由于安装不正确或公差不正确或轴承安装现场的垂直度不够造成的。 如果偏斜超过 0.25，轴承就会过早地失效。 检查润滑剂的污染比检查装配不正或蠕动要困难得多。污染的特征是使轴承过 早的出现磨损。润滑剂中的固体杂质就象磨粒一样。如果滚珠和保持架之间润滑不 良也会磨损并削弱保持架。在这种情况下，润滑对于完全加工形式的保持架来说是 至关重要的。相比之下，带状或冠状保持架能较容易地使润滑剂到达全部表面。 锈是湿气污染的一种形式，它的出现常常表明材料选择不当。如果某一材料经 检验适合工作要求，那么防止生锈的最简单的方法是给轴承包装起来，直到安装使 用时才打开包装。 2 2 避免失效的方法避免失效的方法 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 4 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 anderon。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 则的劈啪声；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 1.610 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 1060 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 60300 次，它表明轴承上存在着密集的振痕或大面积的粗糙不平。 5 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 6 EXTENDING BEARING LIFE 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. Keywords: bearings failures life Bearings fail for a number of reasons，but the most common are misapplication，contamination，improper lubricant，shipping or handling damage，and misalignment. The problem is often not difficult to diagnose because a failed bearing usually leaves telltale signs about what went wrong However，while a postmortem yields good information，it is better to avoid the process altogether by specifying the bearing correctly in The first placeTo do this，it is useful to review the manufacturers sizing guidelines and operating characteristics for the selected bearing. Equally critical is a study of requirements for noise, torque, and runout, as well as possible exposure to contaminants, hostile liquids, and temperature extremes. This can provide further clues as to whether a bearing is right for a job. 1 Why bearings fail About 40% of ball bearing failures are caused by contamination from dust, dirt, shavings, and corrosion. Contamination also causes torque and noise problems, and is often the result of improper handling or the application environmentFortunately, a bearing failure caused by environment or handling contamination is preventable，and a simple visual examination can easily identify the cause Conducting a postmortem il1ustrates what to look for on a failed or failing bearingThen，understanding the mechanism behind the failure, such as brinelling or fatigue, helps eliminate the source of the problem. 7 Brinelling is one type of bearing failure easily avoided by proper handing and assembly. It is characterized by indentations in the bearing raceway caused by shock loadingsuch as when a bearing is dropped-or incorrect assembly. Brinelling usually occurs when loads exceed the material yield point(350,000 psi in SAE 52100 chrome steel)It may also be caused by improper assembly, Which places a load across the racesRaceway dents also produce noise，vibration，and increased torque. A similar defect is a pattern of elliptical dents caused by balls vibrating between raceways while the bearing is not turningThis problem is called false brinelling. It occurs on equipment in transit or that vibrates when not in operation. In addition, debris created by false brinelling acts like an abrasive, further contaminating the bearing. Unlike brinelling, false binelling is often indicated by a reddish color from fretting corrosion in the lubricant. False brinelling is prevented by eliminating vibration sources and keeping the bearing well lubricated. Isolation pads on the equipment or a separate foundation may be required to reduce environmental vibration. Also a light preload on the bearing helps keep the balls and raceway in tight contact. Preloading also helps prevent false brinelling during transit. Seizures can be caused by a lack of internal clearance, improper lubrication, or excessive loading. Before seizing, excessive, friction and heat softens the bearing steel. Overheated bearings often change color，usually to blue-black or straw coloredFriction also causes stress in the retainer，which can break and hasten bearing failure Premature material fatigue is caused by a high load or excessive preloadWhen these conditions are unavoidable，bearing life should be carefully calculated so that a maintenance scheme can be worked out Another solution for fighting premature fatigue is changing materialWhen standard bearing materials，such as 440C or SAE 52100，do not guarantee sufficient life，specialty materials can be recommended. In addition，when the problem is traced back to excessive loading，a higher capacity bearing or different configuration may be used Creep is less common than premature fatigueIn bearingsit is caused by excessive clearance between bore and shaft that allows the bore to rotate on the shaftCreep can be expensive because it causes damage to other components in addition to the bearing 8 0ther more likely creep indicators are scratches，scuff marks，or discoloration to shaft and boreTo prevent creep damage，the bearing housing and shaft fittings should be visually checked Misalignment is related to creep in that it is mounting relatedIf races are misaligned or cockedThe balls track in a noncircumferencial pathThe problem is incorrect mounting or tolerancing，or insufficient squareness of the bearing mounting siteMisalignment of more than 1/4can cause an early failure Contaminated lubricant is often more difficult to detect than misalignment or creepContamination shows as premature wearSolid contaminants become an abrasive in the lubricantIn addition。insufficient lubrication between ball and retainer wears and weakens the retainerIn this situation，lubrication is critical if the retainer is a fully machined typeRibbon or crown retainers，in contrast，allow lubricants to more easily reach all surfaces Rust is a form of moisture contamination and often indicates the wrong material for the applicationIf the material checks out for the job，the easiest way to prevent rust is to keep bearings in their packaging，until just before installation 2 Avoiding failures The best way to handle bearing failures is to avoid themThis can be done in the selection process by recognizing critical performance characteristicsThese include noise，starting and running torque，stiffness，nonrepetitive runout，and radial and axial playIn 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 usedLubricant viscosity must be selected carefully because inappropriate lubricant，especially in miniature bearings，causes excessive torqueAlso，different lubricants have varying noise characteristics that should be matched to the application. For example，greases produce more noise than oil Nonrepetitive runout(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 raceways and ballsUnlike repetitive runout, no 9 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 runout 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 applications Contamination is unavoidable in 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 problems Once a bearing is contaminated, 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 shields illustrates some methods for controlling contamination Noise is as an indicator of bearing qualityVarious noise grades have been developed to classify bearing performance capabilities Noise analysis is done with an Anderonmeter, 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 m/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 10 in the race. Detectable pockets are manufacturing flaws and result when the race is mounted too tightly in multiplejaw 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 runout. 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. PAGE PAGE 10 078105302 2011 3 5 綨 1 40% İİ 豸豸 豸豸 顣 仯 齫0.25 档 2 NRRNRRNRR 磬0.3 1800r/minanderonum/rad 飬磬黮 仯 仯仯1.610棨 İİ 仯1060仯60300 ABECABEC繫ABEC39ABEC EXTENDING BEARING LIFE AbstractNature 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. Keywords: bearings failures life Bearings fail for a number of reasonsbut the most common are misapplicationcontaminationimproper lubricantshipping or handling damageand misalignment. The problem is often not difficult to diagnose because a failed bearing usually leaves telltale signs about what went wrong Howeverwhile a postmortem yields good informationit is better to avoid the process altogether by specifying the bearing correctly in The first placeTo do thisit is useful to review the manufacturers sizing guidelines and operating characteristics for the selected bearing. Equally critical is a study of requirements for noise, torque, and runout, as well as possible exposure to contaminants, hostile liquids, and temperature extremes. This can provide further clues as to whether a bearing is right for a job. 1 Why bearings fail About 40% of ball bearing failures are caused by contamination from dust, dirt, shavings, and corrosion. Contamination also causes torque and noise problems, and is often the result of improper handling or the application environmentFortunately, a bearing failure caused by environment or handling contamination is preventableand a simple visual examination can easily identify the cause Conducting a postmortem il1ustrates what to look for on a failed or failing bearingThenunderstanding the mechanism behind the failure, such as brinelling or fatigue, helps eliminate the source of the problem. Brinelling is one type of bearing failure easily avoided by proper handing and assembly. It is characterized by indentations in the bearing raceway caused by shock loadingsuch as when a bearing is dropped-or incorrect assembly. Brinelling usually occurs when loads exceed the material yield point(350,000 psi in SAE 52100 chrome steel)It may also be caused by improper assembly, Which places a load across the racesRaceway dents also produce noisevibrationand increased torque. A similar defect is a pattern of elliptical dents caused by balls vibrating between raceways while the bearing is not turningThis problem is called false brinelling. It occurs on equipment in transit or that vibrates when not in operation. In addition, debris created by false brinelling acts like an abrasive, further contaminating the bearing. Unlike brinelling, false binelling is often indicated by a reddish color from fretting corrosion in the lubricant. False brinelling is prevented by eliminating vibration sources and keeping the bearing well lubricated. Isolation pads on the equipment or a separate foundation may be required to reduce environmental vibration. Also a light preload on the bearing helps keep the balls and raceway in tight contact. Preloading also helps prevent false brinelling during transit. Seizures can be caused by a lack of internal clearance, improper lubrication, or excessive loading. Before seizing, excessive, friction and heat softens the bearing steel. Overheated bearings often change colorusually to blue-black or straw coloredFriction also causes stress in the retainerwhich can break and hasten bearing failure Premature material fatigue is caused by a high load or excessive preloadWhen these conditions are unavoidablebearing life should be carefully calculated so that a maintenance scheme can be worked out Another solution for fighting premature fatigue is changing materialWhen standard bearing materialssuch as 440C or SAE 52100do not guarantee sufficient lifespecialty materials can be recommended. In additionwhen the problem is traced back to excessive loadinga higher capacity bearing or different configuration may be used Creep is less common than premature fatigueIn bearingsit is caused by excessive clearance between bore and shaft that allows the bore to rotate on the shaftCreep can be expensive because it causes damage to other components in addition to the bearing 0ther more likely creep indicators are scratchesscuff marksor discoloration to shaft and boreTo prevent creep damagethe bearing housing and shaft fittings should be visually checked Misalignment is related to creep in that it is mounting relatedIf races are misaligned or cockedThe balls track in a noncircumferencial pathThe problem is incorrect mounting or tolerancingor insufficient squareness of the bearing mounting siteMisalignment of more than 1/4can cause an early failure Contaminated lubricant is often more difficult to detect than misalignment or creepContamination shows as premature wearSolid contaminants become an abrasive in the lubricantIn additioninsufficient lubrication between ball and retainer wears and weakens the retainerIn this situationlubrication is critical if the retainer is a fully machined typeRibbon or crown retainersin contrastallow lubricants to more easily reach all surfaces Rust is a form of moisture contamination and often indicates the wrong material for the applicationIf the material checks out for the jobthe easiest way to prevent rust is to keep bearings in their packaginguntil just before installation 2 Avoiding failures The best way to handle bearing failures is to avoid themThis can be done in the selection process by recognizing critical performance characteristicsThese include noisestarting and running torquestiffnessnonrepetitive runoutand radial and axial playIn some applications, these items are so critical that specifying an ABEC level alone is not sufficient Torque requirements are determined by the lubricantretainerraceway quality(roundness cross curvature and surface finish)and whether seals or shields are usedLubricant viscosity must be selected carefully because inappropriate lubricantespecially in miniature bearingscauses excessive torqueAlsodifferent lubricants have varying noise characteristics that should be matched to the application. For examplegreases produce more noise than oil Nonrepetitive runout(NRR)occurs during rotation as a random eccentricity between the inner and outer racesmuch like a cam actionNRR can be caused by retainer tolerance or eccentricities of the raceways and ballsUnlike repetitive runout, 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 examplea bearing with an NRR of less than 0.3um is used when minimal runout is neededsuch as in diskdrive spindle motorsSimilarlymachinetool spindles tolerate only minimal deflections to maintain precision cutsConsequently, bearings are manufactured with low NRR just for machine-tool applications Contamination is unavoidable in many industrial productsand shields and seals are commonly used to protect bearings from dust and dirtHowevera perfect bearing seal is not possible because of the movement between inner and outer racesConsequentlylubrication migration and contamination are always problems Once a bearing is contaminated, its lubricant deteriorates