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畢業(yè)設(shè)計(jì) 論文 外文文獻(xiàn)翻譯 題目 如何延長軸承壽命 專業(yè)名稱 機(jī)械設(shè)計(jì)制造及其自動化 班級學(xué)號 078105302 學(xué)生姓名 敖煜 指導(dǎo)教師 于斐 填表日期 2011 年 3 月 5 日 2 如何延長軸承壽命 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(gè)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)致的偵察工作 我們可以采 取行動來避免軸承的再次失效 關(guān)鍵詞 軸承 失效 壽命 導(dǎo)致軸承失效的原因很多 但常見的是不正確的使用 污染 潤滑劑使用不當(dāng) 裝卸或搬運(yùn)時(shí)的損傷及安裝誤差等 診斷失效的原因并不困難 因?yàn)楦鶕?jù)軸承上留 下的痕跡可以確定軸承失效的原因 然而 當(dāng)事后的調(diào)查分析提供出寶貴的信息時(shí) 最好首先通過正確地選定軸承 來完全避免失效的發(fā)生 為了做到這一點(diǎn) 再考察一下制造廠商的尺寸定位指南和 所選軸承的使用特點(diǎn)是非常重要的 1 軸承失效的原因 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(yù)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?qū)⒁У妮S承應(yīng)該在哪些方面進(jìn) 行查看 弄清諸如剝蝕和疲勞破壞一類失效的機(jī)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(shí)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(dāng)軸承不旋轉(zhuǎn)時(shí)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(yùn)輸中的設(shè)備和不工作時(shí)仍振動的設(shè)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(jìn)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(shè)備加隔離墊或?qū)Φ?3 座進(jìn)行隔離可以減輕環(huán)境的振動 另外在軸承上加一個(gè)較小的預(yù)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(shè)備運(yùn)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(nèi)隙 潤滑不當(dāng)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(lán)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(yù)載引起的 如果這些條件不可避免 就應(yīng)仔細(xì)計(jì)算軸承壽命 以制定一個(gè)維護(hù)計(jì)劃 另一個(gè)解決辦法是更換材料 若標(biāo)準(zhǔn)的軸承材料不能保證足夠的軸承壽命 就 應(yīng)當(dāng)采用特殊的材料 另外 如果這個(gè)問題是由于載荷過大造成的 就應(yīng)該采用抗 載能力更強(qiáng)或其他結(jié)構(gòu)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(nèi)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(nèi)圈的顏色變化 為了防止蠕動 應(yīng)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(guān) 如果軸承圈不正或翹起 滾珠將沿著一個(gè)非圓周軌道運(yùn) 動 這個(gè)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(zhì)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(guān)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_(dá)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(dāng) 如果某一材料經(jīng) 檢驗(yàn)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(shí)才打開包裝 2 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(guān)鍵性能特征來實(shí)現(xiàn) 這些特征包括噪聲 起動和運(yùn)轉(zhuǎn)扭矩 剛性 非重復(fù)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(zhì)量 彎曲部分的圓度和表面加工質(zhì)量 以及是否使用密封或遮護(hù)裝置來決定 潤滑劑的粘度必須認(rèn)真加以選擇 因?yàn)椴贿m 宜的潤滑劑會產(chǎn)生過大的扭矩 這在小型軸承中尤其如此 另外 不同的潤滑劑的 4 噪聲特性也不一樣 舉例來說 潤滑脂產(chǎn)生的噪聲比潤滑油大一些 因此 要根據(jù) 不同的用途來選用潤滑劑 在軸承轉(zhuǎn)動過程中 如果內(nèi)圈和外圈之間存在一個(gè)隨機(jī)的偏心距 就會產(chǎn)生與 凸輪運(yùn)動非常相似的非重復(fù)性振擺 NRR 保持架的尺寸誤差和軸承圈與滾珠的偏 心都會引起 NRR 和重復(fù)性振擺不同的是 NRR 是沒有辦法進(jìn)行補(bǔ)償?shù)?在工業(yè)中一般是根據(jù)具體的應(yīng)用來選擇不同類型和精度等級的軸承 例如 當(dāng) 要求振擺最小時(shí) 軸承的非重復(fù)性振擺不能超過 0 3 微米 同樣 機(jī)床主軸只能容 許最小的振擺 以保證切削精度 因此在機(jī)床的應(yīng)用中應(yīng)該使用非重復(fù)性振擺較小 的軸承 在許多工業(yè)產(chǎn)品中 污染是不可避免的 因此常用密封或遮護(hù)裝置來保護(hù)軸承 使其免受灰塵或臟物的侵蝕 但是 由于軸承內(nèi)外圈的運(yùn)動 使軸承的密封不可能 達(dá)到完美的程度 因此潤滑油的泄漏和污染始終是一個(gè)未能解決的問題 一旦軸承受到污染 潤滑劑就要變質(zhì) 運(yùn)行噪聲也隨之變大 如果軸承過熱 它將會卡住 當(dāng)污染物處于滾珠和軸承圈之間時(shí) 其作用和金屬表面之間的磨粒一 樣 會使軸承磨損 采用密封和遮護(hù)裝置來擋開臟物是控制污染的一種方法 噪聲是反映軸承質(zhì)量的一個(gè)指標(biāo) 軸承的性能可以用不同的噪聲等級來表示 噪聲的分析是用安德遜計(jì)進(jìn)行的 該儀器在軸承生產(chǎn)中可用來控制質(zhì)量 也可 對失效的軸承進(jìn)行分析 將一傳感器連接在軸承外圈上 而內(nèi)圈在心軸以 1800r min 的轉(zhuǎn)速旋轉(zhuǎn) 測量噪聲的單位為 anderon 即用 um rad 表示的軸承位移 根據(jù)經(jīng)驗(yàn) 觀察者可以根據(jù)聲音辨別出微小的缺陷 例如 灰塵產(chǎn)生的是不規(guī) 則的劈啪聲 滾珠劃痕產(chǎn)生一種連續(xù)的爆破聲 確定這種劃痕最困難 內(nèi)圈損傷通 常產(chǎn)生連續(xù)的高頻噪聲 而外圈損傷則產(chǎn)生一種間歇的聲音 軸承缺陷可以通過其頻率特性進(jìn)一步加以鑒定 通常軸承缺陷被分為低 中 高三個(gè)波段 缺陷還可以根據(jù)軸承每轉(zhuǎn)動一周出現(xiàn)的不規(guī)則變化的次數(shù)加以鑒定 低頻噪聲是長波段不規(guī)則變化的結(jié)果 軸承每轉(zhuǎn)一周這種不規(guī)則變化可出現(xiàn) 1 6 10 次 它們是由各種干涉 例如 軸承圈滾道上的凹坑 引起的 可察覺的凹 坑是一種制造缺陷 它是在制造過程中由于多爪卡盤夾的太緊而形成的 中頻噪聲的特征是軸承每旋轉(zhuǎn)一周不規(guī)則變化出現(xiàn) 10 60 次 這種缺陷是由在 軸承圈和滾珠的磨削加工中出現(xiàn)的振動引起的 軸承每旋轉(zhuǎn)一周高頻不規(guī)則變化出 現(xiàn) 60 300 次 它表明軸承上存在著密集的振痕或大面積的粗糙不平 5 利用軸承的噪聲特性對軸承進(jìn)行分類 用戶除了可以確定大多數(shù)廠商所使用的 ABEC 標(biāo)準(zhǔn)外 還可確定軸承的噪聲等級 ABEC 標(biāo)準(zhǔn)只定義了諸如孔 外徑 振擺等 尺寸公差 隨著 ABEC 級別的增加 從 3 增到 9 公差逐漸變小 但 ABEC 等級并不 能反映其他軸承特性 如軸承圈質(zhì)量 粗糙度 噪聲等 因此 噪聲等級的劃分有 助于工業(yè)標(biāo)準(zhǔn)的改進(jìn) 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 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 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 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 8 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 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 9 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 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