机械专业英语翻译.doc

河南理工大学本科毕业设计 1 EXTENDING BEARING 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 place To 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 environment Fortunately 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 bearing Then understanding 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 loading such 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 河南理工大学本科毕业设计 2 steel It may also be caused by improper assembly Which places a load across the races Raceway 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 turning This 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 colored Friction also causes stress in the retainer which can break and hasten bearing failure Premature material fatigue is caused by a high load or excessive preload When 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 material When 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 河南理工大学本科毕业设计 3 Creep is less common than premature fatigue In bearings it is caused by excessive clearance between bore and shaft that allows the bore to rotate on the shaft Creep can be expensive because it causes damage to other components in addition to the bearing 0ther more likely creep indicators are scratches scuff marks or discoloration to shaft and bore To prevent creep damage the bearing housing and shaft fittings should be visually checked Misalignment is related to creep in that it is mounting related If races are misaligned or cocked The balls track in a noncircumferencial path The problem is incorrect mounting or tolerancing or insufficient squareness of the bearing mounting site Misalignment of more than 1 4 can cause an early failure Contaminated lubricant is often more difficult to detect than misalignment or creep Contamination shows as premature wear Solid contaminants become an abrasive in the lubricant In addition insufficient lubrication between ball and retainer wears and weakens the retainer In this situation lubrication is critical if the retainer is a fully machined type Ribbon 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 application If 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 them This can be done in the selection process by recognizing critical performance characteristics These include noise starting and running torque stiffness nonrepetitive runout and radial and 河南理工大学本科毕业设计 4 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 Nonrepetitive runout 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 runout 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 runout 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 河南理工大学本科毕业设计 5 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 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 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 run out As the ABEC class number increase from 3 to 9 tolerances are tightened ABEC class however does not specify 河南理工大学本科毕业设计 6 other bearing characteristics such as raceway quality finish or noise Hence a noise classification helps improve on the industry standard 如何延长轴承寿命 1 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 河南理工大学本科毕业设计 7 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 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 河南理工大学本科毕业设计 8 生过大的扭矩 这在小型轴承中尤其如