机械外文翻译--机构与机器.doc

MechanismandMachinesAsystemthattransmitsforcesinapredeterminedmannertoaccomplishspecificobjectivesmaybeconsideredamachine.Amechanismmaybedefinedinasimilarmanner,butthetermmechanismisusuallyappliedtoasystemwheretheprincipalfunctionistotransmitmotion.Kinematicsisthestudyofmotioninmechanism,whiletheanalysisofforceandtorquesinmachinediscalleddynamics.Oncetheneedforamachineormechanismwithgivencharacteristicsisidentified,thedesignprocessbegins.Detailedanalysisofdisplacements,velocities,andaccelerationsisusuallyrequired.Thispartofthedesignprocessisthenfollowedbyanalysisofforceandtorques.Thedesignprocessmaycontinuelongafterfirstmodelhavebeenproduceandincluderedesignsofcomponentthataffectvelocities,accelerations,force,andtorques.Inordertosuccessfullycompeteformyeartoyear,mostmanufacturersmustcontinuouslymodifytheirproductandtheirmethodsofproduction.Increasesinproductionrate,upgradingofproductperformance,redesignforcostandweightreduction,andmotionanalysisofnewproductlinesarefrequentlyrequired.Successmayhingeonthecorrectkinematicanddynamicanalysisoftheproblem.Manyofthebasiclinkageconfigurationshavebeenincorporateintomachinesdesignedcenturiesago,andthetermweusetodescribethenhavechangeovertheyear.Thus,definitionsandterminologywillnotbeconsistentthroughoutthetechnicalliterature.Inmostcases,however,meaningswillbeclearformthecontextofthedescriptivematter.Afewtermsofparticularinteresttothestudyofkinematicanddynamicsofmachinesaredefinebelow.LinkAlinkisoneoftherigidbodiesormembersjoinedtogethertoformakinematicchain.Thetermrigidlinkorsometimessimplylinkisanidealizationusedinthestudyofthatdoesnotconsidersmalldeflectionsduetostrainsinmachinemembers.Aperfectlyrigidorinextensiblelinkcanexistonlyasatextbooktypeofmodelofarealmachinemember.Fortypicalmachinepart,maximumdimensionchangesareofonlyaone-thousandthofthepartlength.Wearejustifiedinneglectingthissmallmotionwhenconsideringthemuchgreatermotioncharacteristicofmostmechanisms.Thewordlinkisusedinageneralsensetoincludecams,gears,andothermachinemembersinadditiontocranks,connectingrodsandotherpin-connectedcomponents.Degrees-of-freedomThenumberofdegrees-of-freedomofalinkageisthenumberofindependentparametersrequiredtopositionofeverylinkrelativetotheframeorfixedlink.Iftheinstantaneousconfigurationofasystemmaybecompletelydefinedbyspecifyingoneindependentvariable,thatsystemhasonedegree-of-freedom.Mostpracticalmechanismshaveonedegree-of-freedom.Anunconstrainedrigidbodyhassixdegrees-of-freedom:translationinthreecoordinatesandrotationaboutthreecoordinateaxes.Ifthebodyisrestrictedtomotioninaplane,therearethreedegrees-of-freedom:translationintwocoordinatedirectionsandrotationwithintheplane.LowerandHigherPairsConnectionsbetweenrigidbodiesconsistoflowerandhigherpairsofelements.Thetwoelementsofalowerpairhavetheoreticalsurfacecontactwithoneanother,whilethetwoelementsofahigherpairhavetheoreticalpointorlinecontact(ifwedisregarddeflections).Lowerpairsaredesirablefromadesignstandpointsincetheloadatthejointandtheresultantwearisspreadoverthecontactsurface.Thus,geometricchangesorfailureduetohighcontactstressesandexcessivewearmaybeprevented.MechanismAmechanismisakinematicchaininwhichonelinkisconsideredfixedforthepurposeofanalysis,butmotionispossibleinotherlinks.Asnotedabove,thelinkdesignatedasthefixedlinkneednotactuallybestationaryrelativetothesurfaceoftheearth.Akinematicchainisusuallyidentifiedasamechanismifitsprimarypurposeisthemodificationortransmissionofmotion.MachineAmechanismdesignedforthepurposeoftransmittingforcesortorquesisusuallycalledamachine.EngineAmachinethatinvolvesconversionofenergytoproducemechanicalpoweriscommonlycalledanengine.Thus,thecrankshaft,connectingrod,piston,andcylinderofanautomotiveenginewouldbeanenginebytheabovedefinitions,whileotherdrivetraincomponentssuchasthetransmission,differential,anduniversaljointwouldbeconsideredmachines.Machinesandenginesmayhavethesameconfigurationasothermechanismsthatdonotconvertenergyandarenotintendedtotransmitsignificantlevelsofforceortorque.Thus,forthepurposeofkinematicanalysis,theabovedistinctionbetweenmechanism,machine,andenginemaybeofonlyacademicimportance.AMechanismhasbeendefinedas“acombinationofrigidorresistantbodiessoformedandconnectedthattheymoveuponeachotherwithdefiniterelativemotion.”Mechanismsformthebasicgeometricalelementsofmanymechanicaldevicesincludingautomaticpackagingmachinery,typewriters,mechanicaltoys,textilemachinery,andothers.Amechanismtypicallyisdesignedtocreateadesiredmotionofarigidbodyrelativetoareferencemember.Kinematicdesignofmechanismsisoftenthefirststepinthedesignofacompletemachine.Whenforcesareconsidered,theadditionalproblemsofdynamics,bearingloads,stresses,lubrication,andthelikeareintroduced,andthelargerproblembecomesoneofmachinedesign.Thefunctionofamechanismistotransmitortransformmotionfromonerigidbodytoanotheraspartoftheactionofamachine.Therearethreetypesofcommonmechanicaldevicesthatcanbeusedasbasicelementsofamechanism.GearSystemsGearsystems,inwhichtoothedmembersincontacttransmitmotionbetweenrotatingshafts.Gearsnormallyareusedforthetransmissionofmotionwithaconstantangularvelocityratio,althoughnoncirculargearscanbeusedfornonuniformtransmissionofmotion.CamSystemsCamsystems,whereauniformmotionofaninputmemberisconvertedintoanonuniformmotionoftheoutputmember.Theoutputmotionmaybeeithershaftrotation,slidertranslation,orotherfollowermotionscreatedbydirectcontactbetweentheinputcamshapeandthefollower.Thekinematicdesignofcamsinvolvestheanalyticalorgraphicalspecificationofthecamsurfaceshaperequiredtodrivethefollowerwithamotionthatisaprescribedfunctionoftheinputmotion.PlaneandSpatialLinkagesTheyarealsousefulincreatingmechanicalmotionsforapointorrigidbody.Linkagescanbeusedforthreebasictasks.(1)Rigidbodyguidance.Arigidbodyguidancemechanismisusedtoguidearigidbodythroughaseriesofprescribedpositionsinspace.(2)Pathgenerationmechanismwillguideapointonarigidbodythroughaseriesofpointsonaspecifiedpathinspace.(3)Functiongeneration.Amechanismthatcreatesanoutputmotionthatisaspecifiedfunctionoftheinputmotion.Mechanismsmaybecategorizedinseveraldifferentwaystoemphasizetheirsimilaritiesanddifferences.Onesuchgroupingdividesmechanismsintoplanar,spherical,andspatialcategories.Allthreegroupshavemanythingsincommon；thecriterionwhichdistinguishesthegroups,however,istobefoundinthecharacteristicsofthemotionsofthelinks.Aplanarmechanismisoneinwhichallparticlesdescribeplanecurvesinspaceandallthesecurveslieinparallelplanes；i.e.thelociofallpointsareplanecurvesparalleltoasinglecommonplanarmechanisminitstruesizeandshapeonasingledrawingorfigure.Theplanefour-barlinkage,theplatecamandfollower,andtheslider-crankmechanismarefamiliarexamplesofplanarmechanisms.Thevastmajorityofmechanismsinusetodayareplanar.Asphericalmechanismisoneinwhicheachlinkhassomepointwhichremainsstationaryasthelinkagemovesandinwhichthestationarypointsofalllinkslieatacommonlocation；i.e.,thelocusofeachpointisacurvecontainedinasphericalsurface,andthesphericalsurfacesdefinedbyseveralarbitrarilychosenpointsareallconcentric.Themotionsofallparticlescanthereforebecompletelydescribedbytheirradialprojections,or“shadows,”onthesurfaceofaspherewithproperlychosencenter.Hookesuniversaljointisperhapsthemostfamiliarexampleofasphericalmechanism.Spatialmechanisms,ontheotherhand,includenorestrictionsontherelativemotionsoftheparticles.Themotiontransformationisnotnecessarilycoplanar,normustitbeconcentric.Aspatialmechanismmayhaveparticleswithlociofdoublecurvature.Anylinkagewhichcontainsascrewpair,forexample,isaspatialmechanism,sincetherelativemotionwithinascrewpairishelical.机构与机器一个系统，它按预先确定的方式来传输动力完成的具体的目标也许可以被认为是机器。一种机构也可以以类似的方式定义，但长期的机构通常是适用于一个系统的主要职能是传递运动。运动学是研究机构运动，而分析力和力矩的机械称为动力学。一旦需要给出识别一个机构或机械装置的特点，设计过程就开始了。通常需要仔细地分析位移，速度和加速度。这部分的设计过程后，其次是分析力和力矩。设计过程中可能会继续很长时间后产生第一种模式，其中包括重新设计的组成部分，影响速度，加速度，力和力矩。年复一年的为了竞争成功，大部分的制造商必须不断地修改他们的产品及其生产方法。提高生产速度，提高产品性能，重新设计的成本和减轻体重，运动分析和新的生产线往往是需要的。成功或许取决于正确的运动学和动力学的分析的问题。许多基本的连接装置构造世纪以前已经成为机器设计的组成部分，和我们使用这个术语形容当时的变化超过一年。因此，定义和专门的术语将不符合整个技术的文献。在大多数情况下，但是，含义将是明确的背景下形成的重要性的描述。有几个方面特别感兴趣的研究机器运动学和动力学的定义如下。杆件一个杆件是一个严格的机构或其共同组成一个运动链。长期严格的杆件或有时只是使用一个理想化的杆件研究，由于机件拉紧不考虑微小挠度。一个完全不弯曲或不可拉长的杆件可能存在不仅是一种教科书式的模型，一个真正的机器的构件。对于典型的机械部分，最大尺寸的变化是只有长度部分的千分之一。当我们考虑多数机械装置的运动特性时我们有理由忽视这个小小的运动。这个杆件定理中使用的一般意义上包括凸轮，齿轮，和其他构件除了曲柄、连杆和其他引脚连接组件。自由度自由度的数量的联系是一些独立的参数必须立场的每一个环节相对内或固定杆件。如果即可改造的系统可以完全确定指定一个独立的变量，该系统有一个自由度。多数实用的机械装置就有一个自由度。一个无约束刚体有6个自由度：直线移动在三个坐标和旋转运动三个坐标轴。如果该机构是限制于在一个平面运动，那有三个自由度：直线运动在两个坐标方向和在平面内的旋转。高副和低副链接的刚体之间包括高副和低副两个要素。这两个因素中的低副是两个理论表面之间的接触，而这两个因素中的高副是理论的点或线接触（如果我们忽视了挠度）。低副是从设计的角度来看是可取的，由于联合负荷以及由此产生的磨损分布在整个接触面。因此，几何变化或失败而高接触应力和过度磨损或许是可以避免的。机械装置机械装置是一个运动链系中的一环被认为是特定的目的是为了分析，但运动可能是其他的环节。如上所述，特定的杆件为指定的杆件不需要与实际相对固定在地球表面。如果运动学链主要目的是缓和或传输动力，其就通常被作为一种机械装置，机器这种机构设计是为达到转递动力或力矩的目的通常是所谓的机器。发动机一个机器需要能量转换而产生的机械动力通常称为发动机。因此，曲轴，连杆，活塞和气缸的自动的发动机由上面所述的发动机的定义，而其他的传动部件，例如变速箱，差速器，和万向联轴器都被称为为机械装置。机器和发