軸承機械類畢業(yè)設(shè)計外文翻譯樣本

軸承壽命分析摘要自然界苛刻工作條件會導(dǎo)致軸承失效，但是如果遵循某些簡樸規(guī)則，軸承正常運轉(zhuǎn)機會是可以被提高。在軸承使用過程當(dāng)中，過度忽視會導(dǎo)致軸承過熱現(xiàn)象，也也許使軸承不可以再被使用，甚至完全破壞。但是一種被損壞軸承，會留下它為什么被損壞線索。通過某些細致觀測工作，咱們可以采用行動來避免軸承再次失效。核心詞：軸承失效壽命1.軸承失效因素軸承失效有如下各種因素，然而軸承壽命實驗卻是所有機械實驗中最故意義。實驗者必要控制實驗過程以保證成果。其她失效模式在Tallian[19.2]中有詳細闡述。下邊幾段就詳細闡述了可以影響壽命實驗成果幾種失效模式。23章中，從EHL觀點討論了潤滑條件對壽命實驗成果影響，同步尚有其她潤滑條件會影響實驗結(jié)論，一方面是潤滑劑接觸面積，受到軸承尺寸，轉(zhuǎn)速，潤滑劑流動性等因素影響，潤滑劑在軸承表面形成潤滑層厚度普通不大于0.05~0.5um，不不大于這個薄層厚度固體微粒會殘留在接觸面上，從而劃傷潤滑溝道和軸承滾動面。從而大大縮短軸承耐用性。關(guān)于這點Sayles和MacPherson以及其她人均有詳細論證。因而，為了保證明驗成果咱們必要選用適當(dāng)級別潤滑劑。潤滑劑選取由工況決定，實驗時也如此。如果工況選取范疇不擬定，就必要考慮到接觸面積對實驗成果影響。23章中討論了不同接觸面積對軸承失效壽命實驗成果影響。潮氣是影響潤滑成果另一種重要因素，長時間在水中和油中被腐蝕不但對外觀質(zhì)量有影響，還會影響到滾動表面軸承壽命。關(guān)于這點Fitch等人[19.7]有過論證。并且，雖然是僅有50~100PPM（百萬分之一）水汽含量也會產(chǎn)生有害影響，甚至產(chǎn)生表面看不出痕跡腐蝕。這是由于軸承溝道和滾動面之間會產(chǎn)生氫脆現(xiàn)象，從23章中也可以看出在潤滑實驗中濕氣是如此重要一種因素。因而在軸承壽命實驗成果中必要考慮到潮氣影響。為了減少對壽命減少影響，潮氣含量最多不能超過40PPM。潤滑劑化學(xué)成分也是需要考慮。大多數(shù)商業(yè)潤滑油包括許多為特定目而開發(fā)專有添加劑。例如，為了提高抗磨損性能，為了能達到極限壓力，或者耐熱性，還可以在邊際潤滑油膜狀況下提供邊界潤滑還能為邊界潤滑提供一種邊界潤滑層。這些添加劑同步也能即時或者逐漸地影響滾動軸承耐用性。為了避免添加劑成為加速壽命實驗條件，咱們必要小心以保證測試潤滑劑添加劑不會受到惡化。為了保證同組產(chǎn)品壽命實驗成果有連貫性，最佳在整個壽命實驗中都用同一供應(yīng)商原則潤滑劑。為了得到一種合理成果，記錄學(xué)規(guī)定做諸多組壽命實驗。因而一種軸承壽命實驗需很長時間。實驗人員必要保證整個實驗過程持續(xù)性，由于任何微小變化都會影響實驗成果，因而這個過程是很復(fù)雜。甚至這些微小變化在導(dǎo)致重大變化之前都不會被注意到。一旦發(fā)生這樣狀況，就沒機會補救了。只能在更好控制條件下重新做實驗。例如說：添加劑穩(wěn)定性會影響到整個實驗條件。當(dāng)前已經(jīng)懂得了某些添加劑在長期使用時會導(dǎo)致大量額外損耗。這些易退化添加劑會影響軸承表面潤滑條件，從而影響軸承壽命。普通對潤滑劑做化學(xué)檢測時是不會檢測添加劑成分。因而，如果一種潤滑劑用于長時間軸承壽命實驗話，生產(chǎn)者應(yīng)當(dāng)定期更換實驗樣品，例如一年一次。用來詳細評估潤滑劑使用規(guī)定。實驗時還要控制是恰當(dāng)溫度。潤滑層（油膜）厚度對溫度影響是相稱敏感，大多數(shù)裝機實驗是在原則工業(yè)環(huán)境下進行，在這一年實驗時間中環(huán)境溫度變化是非常大。同步，個別軸承受溫度變化影響是會影響到整個系統(tǒng)常規(guī)制造公差。因而，所有軸承受溫度變化影響會直接影響到壽命實驗數(shù)據(jù)精確性。因而為了保證明驗數(shù)據(jù)連貫性，必要監(jiān)控并實時調(diào)節(jié)每個軸承使用溫度。因而對于軸承壽命實驗時±3oC溫度公差被以為是可接受。用于軸承壽命實驗硬件裝備磨損是另一種需要監(jiān)控恒量。用于重載實驗軸和軸承內(nèi)圈都會受到很大載荷。重復(fù)拆裝軸承會對軸表面產(chǎn)生損害。這樣變化會影響幾何形狀。軸外徑和軸承內(nèi)徑都會受腐蝕影響。腐蝕是由于震動產(chǎn)生微粒被氧化而產(chǎn)生。這樣也會減少軸承壽命實驗時間。同步這樣機構(gòu)也會在裝配面上產(chǎn)生重大幾何形變，從而影響軸承內(nèi)徑，最后成為減少壽命重要因素。軸承缺陷檢測也是壽命實驗重要考察因素。軸承缺陷最早是由原材料上微小裂紋引起。這樣缺陷在實驗中是沒法檢測。為了檢測這個缺陷就需要使這個缺陷遞增到能影響軸承參數(shù)數(shù)量級別。例如說噪音，溫度，震動等缺陷?？梢栽谙到y(tǒng)中應(yīng)用這些技術(shù)辦法來檢查缺陷。而具備這樣能力系統(tǒng)可以從初期就檢測出在多樣化工作條件復(fù)雜系統(tǒng)中用來測試用缺陷軸承。而當(dāng)前還沒有一種單一系統(tǒng)能檢測出所有軸承缺陷。因而將來有必要選取一種能在軸承受到微小傷害之前就停下機器監(jiān)控系統(tǒng)。缺陷遞增速率是相稱重要。如果在實驗結(jié)束時缺陷限度和理論計算出是一致，唯一區(qū)別就是實驗中對缺陷檢測總是落后于理論計算。原則軸承鋼在耐久性實驗中缺陷遞增速度是相稱快。并且這個遞增還不是重要因素，考慮到有代表性耐久性實驗數(shù)據(jù)都是經(jīng)記錄學(xué)分析后得到。有也不一定，例如某些表面硬度不同鋼材或是專為實驗用生產(chǎn)鋼材。因而在分析成果時候就必要考慮是原則軸承鋼還是專門實驗用鋼材。耐久性實驗最后成果有效性是由元素-金相分析驗證。軸承會通過高倍光學(xué)顯微鏡，高倍電子掃描顯微鏡，高倍電子顯微鏡，化學(xué)元素分析等各種辦法來分析。生產(chǎn)時浮現(xiàn)會導(dǎo)致缺陷元素以及殘留在表面發(fā)生化學(xué)變化后來會導(dǎo)致缺陷元素（如S，P等有害元素）等都會影響軸承壽命。這些檢查辦法都是用來保證明驗得出數(shù)據(jù)是真實有效。Tallian將所有軸承失效黑白圖片匯編起來【19.8】，可覺得判斷各種類型失效提供根據(jù)。當(dāng)前Tallian已經(jīng)將其更新為【19.9】，其中加入了彩色圖片。元素-金相實驗可以提供一種精準(zhǔn)證據(jù)，使實驗成果處在可控制狀況下，同步檢測有疑點和爭議地方。當(dāng)軸承從實驗機上取下來時候可以現(xiàn)做一種初步研究，將會在30倍顯微鏡下觀測失效某些。而正常顯微圖片請看19.2~19.6中圖片。、圖19.2是球軸承溝道表面失效圖片。圖19.3是滾子軸承溝道由于未校準(zhǔn)而導(dǎo)致表面開裂圖片。圖19.4是一種球軸承由于外圈表面銹蝕而導(dǎo)致外圈開裂圖片。圖19.5是表面凹陷殘骸詳細圖片。圖19.6是一種由于熱變形導(dǎo)致內(nèi)圈游隙變化圖片。最后4張圖片不是用正的確驗辦法得到有效失效模式。然而，這些錯誤數(shù)據(jù)需要從有效失效數(shù)據(jù)中剔除掉，從而得到能對的評估壽命實驗有效數(shù)據(jù)。2.避免失效辦法解決軸承失效問題最佳辦法就是避免失效發(fā)生。這可以在選用過程中通過考慮核心性能特性來實現(xiàn)。這些特性涉及噪聲、起動和運轉(zhuǎn)扭矩、剛性、非重復(fù)性振擺以及徑向和軸向間隙。扭矩規(guī)定是由潤滑劑、保持架、軸承圈質(zhì)量（彎曲某些圓度和表面加工質(zhì)量）以及與否使用密封或遮護裝置來決定。潤滑劑粘度必要認真加以選取，由于不適當(dāng)潤滑劑會產(chǎn)生過大扭矩，這在小型軸承中特別如此。此外，不同潤滑劑噪聲特性也不同樣。舉例來說，潤滑脂產(chǎn)生噪聲比潤滑油大某些。因而，要依照不同用途來選用潤滑劑。在軸承轉(zhuǎn)動過程中，如果內(nèi)圈和外圈之間存在一種隨機偏心距，就會產(chǎn)生與凸輪運動非常相似非重復(fù)性振擺（NRR）。保持架尺寸誤差和軸承圈與滾珠偏心都會引起NRR。和重復(fù)性振擺不同是，NRR是沒有辦法進行補償。在工業(yè)中普通是依照詳細應(yīng)用來選取不同類型和精度級別軸承。例如，當(dāng)規(guī)定振擺最小時，軸承非重復(fù)性振擺不能超過0.3微米。同樣，機床主軸只能容許最小振擺，以保證切削精度。因而在機床應(yīng)用中應(yīng)當(dāng)使用非重復(fù)性振擺較小軸承。在許多工業(yè)產(chǎn)品中，污染是不可避免，因而慣用密封或遮護裝置來保護軸承，使其免受灰塵或臟物侵蝕。但是，由于軸承內(nèi)外圈運動，使軸承密封不也許達到完美限度，因而潤滑油泄漏和污染始終是一種未能解決問題。一旦軸承受到污染，潤滑劑就要變質(zhì)，運營噪聲也隨之變大。如果軸承過熱，它將會卡住。當(dāng)污染物處在滾珠和軸承圈之間時，其作用和金屬表面之間磨粒同樣，會使軸承磨損。采用密封和遮護裝置來擋開臟物是控制污染一種辦法。噪聲是反映軸承質(zhì)量一種指標(biāo)。軸承性能可以用不同噪聲級別來表達。噪聲分析是用安德遜計進行，該儀器在軸承生產(chǎn)中可用來控制質(zhì)量，也可對失效軸承進行分析。將一傳感器連接在軸承外圈上，而內(nèi)圈在心軸以1800r/min轉(zhuǎn)速旋轉(zhuǎn)。測量噪聲單位為anderons。即用um/rad表達軸承位移。依照經(jīng)驗，觀測者可以依照聲音辨別出微小缺陷。例如，灰塵產(chǎn)生是不規(guī)則噼啪聲；滾珠劃痕產(chǎn)生一種持續(xù)爆破聲，擬定這種劃痕最困難；內(nèi)圈損傷普通產(chǎn)生持續(xù)高頻噪聲，而外圈損傷則產(chǎn)生一種間歇聲音。軸承缺陷可以通過其頻率特性進一步加以鑒定。普通軸承缺陷被分為低、中、高三個波段。缺陷還可以依照軸承每轉(zhuǎn)動一周浮現(xiàn)不規(guī)則變化次數(shù)加以鑒定。低頻噪聲是長波段不規(guī)則變化成果。軸承每轉(zhuǎn)一周這種不規(guī)則變化可浮現(xiàn)1.6~10次，它們是由各種干涉（例如軸承圈滾道上凹坑）引起?？刹煊X凹坑是一種制造缺陷，它是在制造過程中由于多爪卡盤夾太緊而形成。中頻噪聲特性是軸承每旋轉(zhuǎn)一周不規(guī)則變化浮現(xiàn)10~60次。這種缺陷是由在軸承圈和滾珠磨削加工中浮現(xiàn)振動引起。軸承每旋轉(zhuǎn)一周高頻不規(guī)則變化浮現(xiàn)60~300次，它表白軸承上存在著密集振痕或大面積粗糙不平。運用軸承噪聲特性對軸承進行分類，顧客除了可以擬定大多數(shù)廠商所使用ABEC原則外，還可擬定軸承噪聲級別。ABEC原則只定義了諸如孔、外徑、振擺等尺寸公差。隨著ABEC級別增長（從3增到9），公差逐漸變小。但ABEC級別并不能反映其她軸承特性，如軸承圈質(zhì)量、粗糙度、噪聲等。因而，噪聲級別劃分有助于工業(yè)原則改進。BEARINGLIFEANALYSISProceedingsoftheNinthInternationalSymposiumonMagneticBearings.Kentucky.USA.,(August):3-61.WHYBEARINGSFAILAnindividualbearingmayfailforseveralreasons；however，theresultsofanendurancetestseriesareonlymeaningfulwhenthetestbearingsfailbyfatigue-relatedmechanisms.Theexperimentermustcontrolthetestprocesstoensurethatthisoccurs.SomeoftheotherfailuremodesthatcanbeexperiencedarediscussedindetailbyTallian[19.2].Thefollowingparagraphsdealwithafewspecificfailuretypesthatcanaffecttheconductofalifetestsequence.InChapter23，theinfluenceoflubricationoncontactfatiguelifeisdiscussedfromthestandpointofEHLfilmgeneration.Therearealsootherlubrication-relatedeffectsthatcanaffecttheoutcomeofthetestseries.Thefirstisparticulatecontaminantsinthelubricant.Dependingonbearingsize，operatingspeed，andlubricantrheology，theoverallthicknessofthelubricantfilmdevelopedattherollingelement-racewaycontactsmayfallbetween0.05and0.5m.Solidparticlesanddamagetheracewayandrollingelementsurfaces，leadingtosubstantiallyshortenedendurances.Thishasbeenamplydemonstratedbyandandothers.Therefore，filtrationofthelubricanttothedesiredlevelisnecessarytoensuremeaningfultestresult.Thedesiredlevelisdeterminedbytheapplicationwhichthetestingpurportstoapproximate.Ifthisdegreeoffiltrationisnotprovided，effectsofcontaminationmustbeconsideredwhenevaluatingtestresults.Chapter23discussestheeffectofvariousdegreesofparticulatecontamination，andhencefiltration，onbearingfatiguelife.Themoisturecontentinthelubricantisanotherimportantconsideration.Ithaslongbeenapparentthatquantitiesoffreewaterintheoilcausecorrosionoftherollingcontactsurfacesandthushaveadetrimentaleffectonbearinglife.IthasbeenfurthershownbyFitch[19.7]andothers，however，thatwaterlevelsaslowas50-100partspermillion(ppm)mayalsohaveadetrimentaleffect，evenwithnoevidenceofcorrosion.Thisisduetohydrogenembrittlementoftherollingelementandracewaymaterial.SeealsoChapter23.Moisturecontrolintestlubricationsystemsisthusamajorconcern，andtheeffectofmoistureneedstobeconsideredduringtheevaluationoflifetestresults.Amaximumof40ppmisconsiderednecessarytominimizelifereductioneffects.Thechemicalcompositionofthetestlubricantalsorequiresconsideration.Mostcommerciallubricantscontainanumberofproprietaryadditivesdevelopedforspecificpurposes；forexample，toprovideantiwearproperties，toachieveextremepressureand/orthermalstability，andtoprovideboundarylubricationincaseofmarginallubricantfilms.Theseadditivescanalsoaffecttheenduranceofrollingbearings，eitherimmediatelyorafterexperiencingtime-relateddegradation.Caremustbetakentoensurethattheadditivesincludedinthetestlubricantwillnotsufferexcessivedeteriorationasaresultofacceleratedlifetestconditions.Alsoforconsistencyofresultsandcomparinglifetestgroups，itisgoodpracticetoutilizeonestandardtestlubricantfromaparticularproducerfortheconductofallgenerallifetests.Thestatisticalnatureofrollingcontactfatiguerequiresmanytestsamplestoobtainareasonableestimateoflife.Abearinglifetestsequencethusneedsalongtime.Amajorjoboftheexperimentalististoensuretheconsistencyoftheappliedtestconditionsthroughouttheentiretestperiod.Thisprocessisnotsimplebecausesubtlechangescanoccurduringthetestperiod.Suchchangesmightbeoverlookeduntiltheireffectsbecomemajor.Atthattimeitisoftentoolatetosalvagethecollecteddata，andthetestmustberedoneunderbettercontrols.Forexample，thestabilityoftheadditivepackagesinatestlubricantcanbeasourceofchangingtestconditions.Somelubricantshavebeenknowntosufferadditivedepletionafteranextendedperiodofoperation.ThedegradationoftheadditivepackagecanaltertheEHLconditionsintherollingcontent，alteringbearinglife.Generally，thenormalchemicaltestsusedtoevaluatelubricantsdonotdeterminetheconditionsoftheadditivecontent.Thereforeifalubricantisusedforendurancetestingoveralongtime，asampleofthefluidshouldbereturnedtotheproduceratregularintervals，sayannually，foradetailedevaluationofitscondition.Adequatetemperaturecontrolsmustalsobeemployedduringthetest.ThethicknessoftheEHLfilmissensitivetothecontacttemperature.Mosttestmachinesarelocatedinstandardindustrialenvironmentswhereratherwidefluctuationsinambienttemperatureareexperiencedoveraperiodofayear.Inaddition，theheatgenerationratesofindividualbearingscanvaryasaresultofthecombinedeffectsofnormalmanufacturingtolerances.Bothoftheseconditionsproducevariationsinoperatingtemperaturelevelsinalotofbearingsandaffectthevalidityofthelifedata.Ameansmustbeprovidedtomonitorandcontroltheoperatingtemperaturelevelofeachbearingtoachieveadegreeofconsistency.Atolerancelevelof3Cisnormallyconsideredadequatefortheendurancetestprocess.Thedeteriorationoftheconditionofthemountinghardwareusedwiththebearingsisanotherarearequiringconstantmonitoring.Theheavyloadsusedforlifetestingrequireheavyinterferencefitsbetweenthebearinginnerringsandshafts.Repeatedmountinganddismountingofbearingscanproducedamagetotheshaftsurface，whichinturncanalterthegeometryofamountedring.Theshaftsurfaceandtheboreofthehousingarealsosubjecttodeteriorationfromfrettingcorrosion.Frettingcorrosionresultsfromtheoxidationofthefinewearparticlesgeneratedbythevibratoryabrasionofthesurface，whichisacceleratedbytheheavyendurancetestloading.Thismechanismcanalsoproducesignificantvariationsinthegeometryofthemountingsurfaces，whichcanaltertheinternalbearinggeometry.Suchchangescanhaveamajoreffectinreducingbearingtestlife.Thedetectionofbearingfailureisalsoamajorconsiderationinalifetestseries.Thefatiguetheoryconsidersfailureastheinitiationofthefirstcrackinthebulkmaterial.Obviouslythereisnowaytodetectthisoccurrenceinpractice.Tobedetectablethecrackmustpropagatetothesurfaceandproduceaspallofsufficientmagnitudetoproduceamarkedeffectonanoperatingparameterofthebearing：forexample，noise，vibration，and/ortemperature.Techniquesexitfordetectingfailuresinapplicationsystems.Theabilityofthesesystemstodetectearlysignsoffailurevarieswiththecomplexityofthetestsystem，thetypeofbearingunderevaluation，andothertestconditions.Currentlynosinglesystemexiststhatcanconsistentlyprovidethefailurediscriminationnecessaryforalltypesofbearinglifetests.Itisthennecessarytoselectasystemthatwillrepeatedlyterminatemachineoperationwithaconsistentminimaldegreeofdamage.Therateoffailurepropagationisthereforeimportant.Ifthedegreeofdamageattestterminationisconsistentamongtestelements，theonlyvariationbetweentheexperimentalandtheoreticallivesisthelaginfailuredetection.Instandardthrough-hardenedbearingsteelsthefailurepropagationrateisquiterapidunderendurancetestconditions，andthisisnotamajorfactor，consideringthetypicaldispersionofendurancetestdataandthedegreeofconfidenceobtainedfromstatisticalanalysis.Thismaynot，however，bethecasewithotherexperimentalmaterialsorwithsurface-hardenedsteelsorsteelsproducedbyexperimentaltechniques.Caremustbeusedwhenevaluatingtheselatterresultsandparticularlywhencomparingtheexperimentalliveswiththoseobtainedfromstandardsteellots.Theultimatemeansofensuringthatanendurancetestserieswasadequatelycontrolledistheconductofapost-testanalysis.Thisdetailedexaminationofallthetestedbearingsuseshigh-magnificationopticalinspection，higher-magnificationscanningelectronmicroscopy，metallurgicalanddimensionalexaminations，andchemical evaluationsasrequired.Thecharacteristicsofthefailuresareexaminedtoestablishtheiroriginsandtheresidualsurfaceconditionsareevaluatedforindicationsofextraneouseffectsthatmayhaveinfluencedthebearinglife.Thistechniqueallowstheexperimentertoensurethatthedataareindeedvalid.The“DamageAtlas”compiledbyTallianetal.[19.8]containingnumerousblackandwhitephotographsofthevariousbearingfailuremodescanprovideguidanceforthesetypesofdeterminations.ThisworkwassubsequentlyupdatedbyTallian[19.9]，nowincludingcolorphotographsaswell.Thepost-testanalysisis，bydefinition，afterthefact.Toprovidecontrolthroughoutthetestseriesandtoeliminateallquestionableareas，theexperimentershouldconductapreliminarystudywheneverabearingisremovedfromthetestmachine.Inthisportionoftheinvestigationeachbearingisexaminedopticallyatmagnificationsupto30forindicationsofimproperorout-of-controltestparameters.ExamplesofthetypesofindicationsthatcanbeobservedaregiveninFigs.19.2-19.6.Figure19.2illustratestheappearanceofatypicalfatigue-originatedspallonaballbearingraceway.Figure19.3containsaspallingfailureontheracewayofarollerbearingthatresultedfrombearingmisalignment，andFig.19.4containsaspallingfailureontheouterringofaballbearingproducedbyfrettingcorrosionontheouterdiameter.Figure19.5illustratesamoresubtleformoftestalteration，`wherethespallingfailureoriginatedfromthepresenceofadebrisdentonthesurface.Figure19.6givesanexampleofatotallydifferentfailuremodeproducedbythelossofinternalbearingclearanceduetothermalunbalanceofthesystem.Thelastfourfailuresarenotvalidfatiguespallsandindicatetheneedtocorrectthetestmethods.Furthermore，thesedatapointswouldneedtobeeliminatedfromthefailuredatatoobtainavalidestimateoftheexperimentalbearinglife.2.AVOIDINGFAILURESThebestwaytohandlebearingfailuresistoavoidthem．Thiscanbedoneintheselectionprocessbyrecognizingcriticalperformancecharacteristics．Theseincludenoise，startingandrunningtorque，stiffness，non-repetitiverunout，andradialandaxialplay．Insomeapplications，theseitemsaresocriticalthatspecifyinganABEClevelaloneisnotsufficient．Torquerequirementsaredeterminedbythelubricant，retainer，racewayquality(roundnesscrosscurvatureandsurfacefinish)，andwhethersealsorshieldsareused．Lubricantviscositymustbeselectedcarefullybecauseinappropriatelubricant，especiallyinminiaturebearings，causesexcessivetorque．Also，differentlubricantshavevaryingnoisecharacteristicsthatshouldbematchedtotheapplication.Forexample，greasesproducemorenoisethanoil．Non-repetitiverunout(NRR)occursduringrotationasarandomeccentricitybetweentheinnerandouterraces，muchlikeacamaction．NRRcanbecausedbyretainertoleranceoreccentricitiesoftheracewaysandballs．Unlikerepetitiverunout，nocompensationcanbemadeforNRR.NRRisreflectedinthecostofthebearing．Itiscommonintheindustrytoprovidedifferentbearingtypesandgradesforspecificapplications．Forexample，abearingwithanNRRoflessthan0.3umisusedwhenminimalrunoutisneeded，suchasindisk—drivespindlemotors．Similarly，machine—toolspindlestolerateonlyminimaldeflectionstomaintainprecisioncuts．Consequently，bearingsaremanufacturedwithlowNRRjustformachine-toolapplications．Contaminationisunavoidableinmanyindustrialproducts，andshieldsandsealsarecommonlyusedtoprotectbearingsfromdustanddirt．However，aperfectbearingsealisnotpossiblebecauseofthemovementbetweeninnerandouterraces．Consequently，lubricationmigrationandcontaminationarealwaysproblems．Onceabearingiscontaminated，itslubricantdeterioratesandoperationbecomesnoisier．Ifitoverheats，thebearingcanseize．Attheveryleast，contaminationcauseswearasitworksbetweenballsandtheraceway，becomingintheracesandactingasanabrasivebetweenmetalsurfaces．Fendingoffdirtwithsealsandshieldsillustratessomemethodsforcontrollingcontamination．Noiseisasanindicatorofbearingquality．Variousnoisegradeshavebeendevelopedtoclassifybearingperformancecapabilities．Whichisusedforqualitycontrolinbearingproductionandalsowhenfailedbeari