机械外文翻译--机构与机器.doc_第1页
机械外文翻译--机构与机器.doc_第2页
机械外文翻译--机构与机器.doc_第3页
机械外文翻译--机构与机器.doc_第4页
机械外文翻译--机构与机器.doc_第5页
已阅读5页,还剩1页未读 继续免费阅读

下载本文档

版权说明:本文档由用户提供并上传,收益归属内容提供方,若内容存在侵权,请进行举报或认领

文档简介

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

温馨提示

  • 1. 本站所有资源如无特殊说明,都需要本地电脑安装OFFICE2007和PDF阅读器。图纸软件为CAD,CAXA,PROE,UG,SolidWorks等.压缩文件请下载最新的WinRAR软件解压。
  • 2. 本站的文档不包含任何第三方提供的附件图纸等,如果需要附件,请联系上传者。文件的所有权益归上传用户所有。
  • 3. 本站RAR压缩包中若带图纸,网页内容里面会有图纸预览,若没有图纸预览就没有图纸。
  • 4. 未经权益所有人同意不得将文件中的内容挪作商业或盈利用途。
  • 5. 人人文库网仅提供信息存储空间,仅对用户上传内容的表现方式做保护处理,对用户上传分享的文档内容本身不做任何修改或编辑,并不能对任何下载内容负责。
  • 6. 下载文件中如有侵权或不适当内容,请与我们联系,我们立即纠正。
  • 7. 本站不保证下载资源的准确性、安全性和完整性, 同时也不承担用户因使用这些下载资源对自己和他人造成任何形式的伤害或损失。

评论

0/150

提交评论