軸承與齒輪的失效分析機械課程畢業(yè)設計外文文獻翻譯@中英文翻譯@外文翻譯

中國地質大學長城學院 本科畢業(yè)設計外文資料翻譯 系 別： 工程技術系 專 業(yè)： 機械設計制造及自動 姓 名： 葛祥勇 學 號： 05211635 2015 年 4 月 20 日 外文原文 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 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 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 1/4can 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 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 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 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. 外文資料翻譯譯文 軸承與齒輪的失效分析 在球軸承的失效中約有 40%是由灰塵、臟物、碎屑的污染以及腐蝕造成的。污染通常是 由不正確的使用和不良的使用環(huán)境造成的，它還會引起扭矩和噪聲的問題。由環(huán)境和污染所產(chǎn)生的軸承失效是可以預防的，而且通過簡單的肉眼觀察是可以確定產(chǎn)生這類失效的原因。 通過失效后的分析可以得知對已經(jīng)失效的或將要失效的軸承應該在哪些方面進行查看。弄清諸如剝蝕和疲勞破壞一類失效的機理，有助于消除問題的根源。 只要使用和安裝合理，軸承的剝蝕是容易避免的。剝蝕的特征是在軸承圈滾道上留有由沖擊載荷或不正確的安裝產(chǎn)生的壓痕。剝蝕通常是在載荷超過材料屈服極限時發(fā)生的。如果安裝不正確從而使某一載荷橫穿軸承圈也會產(chǎn)生剝蝕。軸承圈上 的壓坑還會產(chǎn)生噪聲、振動和附加扭矩。 類似的一種缺陷是當軸承不旋轉時由于滾珠在軸承圈間振動而產(chǎn)生的橢圓形壓痕。這種破壞稱為低荷振蝕。這種破壞在運輸中的設備和不工作時仍振動的設備中都會產(chǎn)生。此外，低荷振蝕產(chǎn)生的碎屑的作用就象磨粒一樣，會進一步損害軸承。與剝蝕不同，低荷振蝕的特 征通常是由于微振磨損腐蝕在潤滑劑中會產(chǎn)生 淡紅色。 消除振動源并保持良好的軸承潤滑可以防止低 荷振蝕。給設備加隔離墊或對底座進行隔離可以減輕環(huán)境的振動。另外在軸承上加 一個較小的預載荷不僅有助于滾珠和軸承圈保持緊密的接觸，并且對防止在設備運輸中 產(chǎn)生的低荷振蝕也有幫助。 造成軸承卡住的原因 是缺少內隙、潤滑不當和載荷過大。在卡住之前，過大的摩擦和熱量使軸承鋼軟化。 過熱的軸承通常會改變顏色，一般會變成藍黑 色或淡黃色。摩擦還會使保持架受力，這會破 壞支承架，并加速軸承的失效。 材料過早出現(xiàn)疲勞破壞是由重載后過大的預載引起的。如果這些條件不可避免，就應仔細計算軸承壽命，以制定一個維護計劃。 另一個解決辦法是更換材料。若標準的軸承材料不能保證足夠的軸承壽命，就應當采用特殊的材料。另外，如果這個問題是由于載荷過大造成的，就應該采用抗載能力更強或其他結構的軸承。 蠕動不象過早疲勞那樣普遍。軸承的蠕動是由于軸和內圈之間的間隙過大造成的。蠕動的害處很大，它不僅損害軸承，也破壞其他零件。 蠕動的明顯特征是劃痕、擦痕或軸與內圈的顏色變化。為了防止蠕動，應該先用肉眼檢查一下軸承箱件和軸的配件。 蠕動與安裝不正有關。如果軸承圈不正或翹起，滾珠將沿著一個非圓周軌道運動。這個問題是由于安裝不正確或公差不正確或軸承安裝現(xiàn)場的垂直度不夠造成的。如果偏斜超過 0.25，軸承就會過早地失效。 檢查潤滑劑的污染比檢查裝配不正或蠕動要困難得多。污染的特征是使軸承過早的出現(xiàn)磨損。潤滑劑中的固體雜 質就象磨粒一樣。如果滾珠和保持架之間潤滑不良也會磨損并削弱保持架。在這種情況下，潤滑對于完全加工形式的保持架來說是至關重要的。相比之下，帶狀或冠狀保持架能較容易地使?jié)櫥瑒┑竭_全部表面。 銹是濕氣污染的一種形式，它的出現(xiàn)常常表明材料選擇不當。如果某一材料經(jīng)檢驗適合工作要求，那么防止生銹的最簡單的方法是給軸承包裝起來，直到安裝使用時才打開包裝。 避免失效的方法 解決軸承失效問題的最好辦法就是避免失效發(fā)生。這可以在選用過程中通過考慮關鍵性能特征來實現(xiàn)。這些特征包括噪聲、起動和運轉扭矩、剛性、非重復性振擺以 及徑向和軸向間隙。 扭矩要求是由潤滑劑、保持架、軸承圈質量（彎曲部分的圓度和表面加工質量）以及是否使用密封或遮護裝置來決定。潤滑劑的粘度必須認真加以選擇，因為不適宜的潤滑劑會