外文翻译轴联轴器和滚动轴承.docx
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外文翻译轴联轴器和滚动轴承
附录二外文原文和翻译
shafts、couplingsandrollingcontactbearings
keywords:
shafts、couplings、bearings
Virtuallyallmachinescontainshafts.Themostcommonshapeforshaftsiscircularandthecrosssectioncanbeeithersolidorhollow(hollowshaftscanresultinweightsavings).Rectangularshaftsaresometimesused,asinscrewdriverblades,socketwrenchesandcontrolknobstems.
Ashaftmusthaveadequatetorsionalstrengthtotransmittorqueandnotbeoverstressed.Italsobetorsionallystiffenoughsothatonemountedcomponentdoesnotdeviateexcessivelyfromitsoriginalangularpositionrelativetoasecondcomponentmountedonthesameshaft.Generallyspeaking,theangleoftwistshouldnotexceedonedegreeinashaftlengthequalto20diameters.
Shaftsaremountedinbearingsandtransmintpowerthroughsuchdevicesasgears,pullerys,camsandclutches.Thesedevicesintroduceforceswhichattempttobendtheshaft;hence,theshaftmustberigidenoughtopreventoverloadingofthesupportingbearings.Ingeneral,thebendingdeflectionofashaftshouldnotexceed0.01in.perftoflengthbetweenbearingsupports.
Inaddition,theshaftmustbeabletosustainacombinationofbendingandtorsionalloads.Thusanequivalentloadmustbeconsideredwhichtakesintoaccountbothtorsionandbending.Also,theallowablestressmustcontainafactorofsafetywhichincludesfatigue,sincetorsionalandbendingstressreversalsoccur.
Fordiameterslessthan3in.,theusualshaftmaterialiscold-rolledateelcontainingabout0.4percentcarbon.Shaftsareeithercold-rolledorforgedinsizesfrom3in.to5in.,shaftsareforgedandmachinedtosize.Pleasticshaftsarewidelyusedforlightloadapplications.Oneadvantageofusingplasticissafetyinelectricalapplications,sinceplasticisapoorconductorofelectricity.
Componentssuchasgearsandpulleysaremountedonshaftsbymeansofkey.Thedesignofthekeyandthecorrespondingkeywayintheshaftmustbeprperlyevaluated.Forexample,stressconcentrationsoccurinshaftsduetokeyways,andthematerialremovedtoformthekeywayfurtherweakenstheshaft.
Ifshaftsarerunatcirticalspeeds,severevibrationscanoccurwhichcanseriouslydamageamachine.Itisimportanttoknowthemagnitudeofthesecriticalspeedssothattheycanbeavoided.Asageneralruleofthumb,thedifferencebetweentheoperatingspeedandthecriticalspeedshouldbeatleast20percent.
Anotherimportantaspectofshaftdesignisthemethodofdirectlyconnectingoneshafttoanother.Thisisaccomplishedbydevicessuchasrigidandflexiblecouplings.
Acouplingisadeviceforconnectingtheedsofadjacentshafts.Inmachineconstruction,ouplingsareusedtoeffectasemipermanentconnectionbetweenadjacentrotatingshafts.Theconnectionispermanentinthesensethatitisnotmeanttobebrokenduringtheusefullifeofthemachine,butitcanbebrokenandrestoredinanemergencyorwhenwornpartsarereplaced.
Thereareseveraltypesofshaftcouplings,theircharacteristicsdependonthepurposeforwhichtheyareused.Ifanexceptionallylongshaftisrequiredinamanufacturingplantorapropellershaftonaship,itismadeinsectionsthatarecoupledtogetherwithrigidcouplings.Acommontypeofrigidcouplingconsistsoftwomatingradialflanges(disks)thatarecttachedbykey-drivenhubstotheedsofadjacentshaftsectionsandboltedtogetherthroughtheflangestoformarigidconnection.Alignmentoftheconnectedshaftsisusuallyeffetedbymeansofarabbetjointonthefaceoftheflanges.
Inconnectingshaftsbelongingtoseparatedevices(suchasanelectricmotorandagearbox),precisealigningoftheshaftsisdifficultandaflexiblecouplingisused.Thiscouplingconnectstheshaftsinsuchawayastominimizetheharmfuleffectsofshaftmisalignment.Flexiblecouplingsalsopermittheshaftstodeflectundertheirseparatesystemsofwithoneanother.Flexiblecouplingscanalsoservetoreducetheintensityofshockloadsandvibrationstransmittedfromoneshafttoanother.
Virtuallyallshaftscontainrollingcontactbearings.Theconcernofamachinedesignerwithballandrollerbearingsisfivefoldasfollows:
(a)lifeinrelationtoload;(b)stiffness,i.e.deflectionsunderload;(c)friction;(d)wear;(e)noise.Formoderateloadsandspeedsthecorrectselectionofastandardbearingonthebasisofloadratingwillusuallysecuresatisfactoryperformance.Thedeflectionofthebearingelementswillbecomeimportantwhereloadsarehigh,althoughthisisusuallyoflessmagnitudethanthatoftheshaftsorothercomponentsassociatedwiththebearing.Wherespeedsarehighspecialcoolingarrangementsbecomenecessarywhichmayincreasefrictionaldrag.Wearisprimarilyassociatedwiththeintroductionofcontaminants,andsealingarrangementsmustbechosenwithregardthehostilityoftheenvironment.
Becausethehighqualityandlowpriceofballandrollerbearingsdependsonquantityproduction,thetaskofthemachinedesignerbecomesoneofselectionratherthandesign.Rolling-contactbearingsaregenerallymadewithateelwhichisthrough-hardenedtoabout900HV,althoughinmanymechanismsspecialracesarenotprovidedandtheinteractingsurfacesarehardenedtoabout600HV.Itisnotsurprisingthat,owingtothehighstressesinvolved,apredominantformoffailureshouldbemetalfatigue,andagooddealofworkiscurrentlyinprogressintendedtoimprovethereliabilityofthistypeofbearing.Designcanbebasedonacceptedvaluesoflifeanditisgenerralpracticeinthebearingindustrytodefinetheloadcapacityofthebearingasthatvaluebelowwhich90lpercentofabatchwillexceedaliftofanemillionrevolutions.
Notwithstandingthefactthatresponsibilityforthebasicdesignofballandrollerbearingsrestswiththebearingmanufacturer,themachinedesignermustformacorrectappreciationofthedutytobeperformedbythebearingandbeconcernednotonlywithbearingselectionbutwiththeconditionsforcorrectinstallation.
Thefitofthebearingracesontotheshaftorontothehousingsisofcriticalimportancebecauseoftheircombinedeffectontheinternalclearanceofthebearingaswellaspreservingthedesireddegreeofinterferencefit.Inadequateinterferencecaninduceserioustroublefromfrettingcorrosion.Theinnerraceisfrequentlylocatedaxiallybyabuttingagainstashoulder.Aradiusatthispointisessentialfortheavoidanceofstressconcentrationandballracesareprovideswitharadiusorchamfertoallowspaceforthis.
Wherelifeisnotthedeterminingfactorindesign,itisusualtodeterminemaximumloadingbytheamounttowhichabearingwilldeflectunderload.Thustheconceptof“staticload-carryingcapacity”isunderstoodtomeantheloadthatcanbealppliedtoabearing,whichiseitherstationaryorsubjecttoslightswivelingmotions,withoutimpairingitsrunningqualitiesforsubsequentrotationalmotion.Thishasbeendeterminedbypracticalexperienceastheloadwhichwhenappliedtoabearingresultsinatotaldeformationoftherollingelementandracewayatanypointofcontactnotexceeding0.01percentoftherolling-elementdiameter.Thiswouldcorrespondtoapermanentdeformationof0.00025mmforaball25mmindiameter.
Thesuccessfulfunctioningofmanybearingdependsuponprovidingthemwithadequateprotectionagainsttheirenvironment,andinsomecircumstancestheenvirorationoftheberingsurfaces.Achievementofthecorrectfunctioningofsealsisanessentialpartofbearingdesign.Moreover,sealswhichareappliedtomovingpartsforanypurposeareofinteresttotribologistsbecausetheyarecomponentsofbearingsystemsandcanonlybedesignedsatisfactorilyonthebasisoftheapprolpriatebearingtheory.Notwithstandingtheirimportance,theamountofresearcheffortthathasbeendevotedtotheunderstandingoftheunderstandingofthebehaviorofsealshasbeensmallwhencomparedwiththatdevotedtootheraspectsofbearingtechnology.
References:
1Erickson.BeltandApplicationforEngineers.MarcelDekker.Inc,1997
2South,Mancuso.MechanicalPowerTransmissionComponents.1994
轴、联轴器和滚动轴承
关键词:
轴、联轴器、轴承
实际上,几乎所有的机器中都装有轴。
轴最常见的形状是圆形,其截面可以是实心的,也可以是空心的(空心轴可以减轻重量)。
有时也采用矩形轴,例如,螺丝起子的头、套筒扳手和控制旋扭的杆。
为了在传递转矩时不发生过载,轴应该具有适当的抗扭强度。
轴还应该具有足够的抗扭刚度,以使同一个轴上的两个传动零件之间的相对转角不回过大。
一般来说,在轴的长度等于其直径的20倍时,扭转角不应该超过1度。
轴安装在轴承中,通过齿轮、皮带轮、凸轮、和离合器等零件传递动力。
通过这些零件传来的里可能会使轴产生弯曲变形。
因此,轴应该有足够的刚度以防止支撑轴承受力过大。
总而言之,在两个轴承支承之间,轴在每英尺长度上的弯曲变形不应该超过0.01英寸。
此外,轴还必须能承受弯矩和扭矩的组合作用。
因此,要考虑扭矩与弯矩的当量载荷。
因为扭矩和弯矩会产生交变应力,在许用应力中也应该有一个考虑疲劳现象的安全系数。
直径小于3英寸的轴可以采用含碳量大约为0.4%的冷轧钢,直径在3~5英寸之间的轴可以采用冷轧钢或锻造毛坯。
当直径大于5英寸大,则要采用锻造毛坯,然后机械加工到所要求的尺寸。
轻载时,广泛采用塑料轴。
由于塑料是电的不良导体,在电器中用它做轴比较安全。
齿轮和皮带轮等零件通过键联接在轴上。
在对键及轴上与之相应的键槽进行设计时,必须进行认真的计算。
例如,轴上的键槽会引起应力集中,由于键槽的存在使轴的横截面积间隙哦啊,会进一步减弱轴的强度。
如果轴以临界速度转动,将会发生强烈的振动,可能会毁坏整台机器。
知道这些临界速度的大小是很重要的,因为这样可以避开它。
一般凭经验来说,工作速度与临界速度之间至少应该相差20%。
轴的设计工作的另一个重要方面是轴与轴之间的直接联接方法。
这是有刚性或者弹性联轴器来实现的。
联轴器是用来把两个相邻轴端联接起来的装置。
在机械结构中,联轴器被用来实现相邻的两根转轴之间的半永久性联接。
在机器的正常使用期间内,这种联接一般不必拆开,在这种意义上,可以说联轴器的联接是永久性的。
但是在紧急情况下,或者需要更换以磨损的零件时,可以先把联轴器拆开,然后再联接上。
联轴器有几种类型,它们的特性随其用途而定。
如果制造工厂中或者船舶的螺旋浆需要一根特别长的轴,可以采用分段的方式将其制造出来,然后采用刚性联轴器将各段联接起来。
一种常用的联轴器是由两个配对的法兰(盘)组成,这两个法兰盘借助靠键传动的轴套联接到相邻两节轴的两端,然后用螺栓穿过法兰联接起来形成刚性接头。
相互联接的两根轴通常是靠法兰面的槽口来对准的。
在把属于不同的设备(例如一个个电动机和一个变速箱)的轴联接起来的时候,要把这些轴精确地对准是比较困难的,此时可以采用弹性联轴器。
这种联轴器联接轴的方式可以把由于被联接的轴之间的轴线不重合所造成的有害影响减少到最低程度。
弹性联轴器也允许被联接的轴在它们各自的载荷系统作用下产生偏斜或在轴线方向自由移动(浮动)而不至于相互干扰。
弹性联轴器也可以用来减轻从一根轴传到另一根轴上的冲击载荷和振动的强度。
而几乎所有的轴都装有滚动轴承。
对于球轴承和滚子轴承,一个机器设计人员应该考虑下面五个方面:
(a)寿命与载荷的关系;(b)刚度,也就是在载荷作用下的变形;(c)摩擦;(d)磨损;(e)噪音。
对于中等载荷和转速,根据额定负荷选择一个标准轴承,通常都可以保证其具有令人满意的工作性能。
当载荷较大时,轴承零件的变形,尽管它通常小于轴和其他与轴承一起工作的零部件的变形,将会变得重要起来。
在转速高的场合需要有专门的冷却装置,而这可能会增大摩擦阻力。
磨损主要是由于污染物的进入引起的,必须选用密封装置以防止周围环境的不良影响。
因为大批量生产这种方式决定了球轴承和滚子轴承不但质量高,而且价格低