機械外文翻譯--機構與機器.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.機構與機器一個系統(tǒng)，它按預先確定的方式來傳輸動力完成的具體的目標也許可以被認為是機器。一種機構也可以以類似的方式定義，但長期的機構通常是適用于一個系統(tǒng)的主要職能是傳遞運動。運動學是研究機構運動，而分析力和力矩的機械稱為動力學。一旦需要給出識別一個機構或機械裝置的特點，設計過程就開始了。通常需要仔細地分析位移，速度和加速度。這部分的設計過程后，其次是分析力和力矩。設計過程中可能會繼續(xù)很長時間后產(chǎn)生第一種模式，其中包括重新設計的組成部分，影響速度，加速度，力和力矩。年復一年的為了競爭成功，大部分的制造商必須不斷地修改他們的產(chǎn)品及其生產(chǎn)方法。提高生產(chǎn)速度，提高產(chǎn)品性能，重新設計的成本和減輕體重，運動分析和新的生產(chǎn)線往往是需要的。成功或許取決于正確的運動學和動力學的分析的問題。許多基本的連接裝置構造世紀以前已經(jīng)成為機器設計的組成部分，和我們使用這個術語形容當時的變化超過一年。因此，定義和專門的術語將不符合整個技術的文獻。在大多數(shù)情況下，但是，含義將是明確的背景下形成的重要性的描述。有幾個方面特別感興趣的研究機器運動學和動力學的定義如下。桿件一個桿件是一個嚴格的機構或其共同組成一個運動鏈。長期嚴格的桿件或有時只是使用一個理想化的桿件研究，由于機件拉緊不考慮微小撓度。一個完全不彎曲或不可拉長的桿件可能存在不僅是一種教科書式的模型，一個真正的機器的構件。對于典型的機械部分，最大尺寸的變化是只有長度部分的千分之一。當我們考慮多數(shù)機械裝置的運動特性時我們有理由忽視這個小小的運動。這個桿件定理中使用的一般意義上包括凸輪，齒輪，和其他構件除了曲柄、連桿和其他引腳連接組件。自由度自由度的數(shù)量的聯(lián)系是一些獨立的參數(shù)必須立場的每一個環(huán)節(jié)相對內(nèi)或固定桿件。如果即可改造的系統(tǒng)可以完全確定指定一個獨立的變量，該系統(tǒng)有一個自由度。多數(shù)實用的機械裝置就有一個自由度。一個無約束剛體有6個自由度：直線移動在三個坐標和旋轉運動三個坐標軸。如果該機構是限制于在一個平面運動，那有三個自由度：直線運動在兩個坐標方向和在平面內(nèi)的旋轉。高副和低副鏈接的剛體之間包括高副和低副兩個要素。這兩個因素中的低副是兩個理論表面之間的接觸，而這兩個因素中的高副是理論的點或線接觸（如果我們忽視了撓度）。低副是從設計的角度來看是可取的，由于聯(lián)合負荷以及由此產(chǎn)生的磨損分布在整個接觸面。因此，幾何變化或失敗而高接觸應力和過度磨損或許是可以避免的。機械裝置機械裝置是一個運動鏈系中的一環(huán)被認為是特定的目的是為了分析，但運動可能是其他的環(huán)節(jié)。如上所述，特定的桿件為指定的桿件不需要與實際相對固定在地球表面。如果運動學鏈主要目的是緩和或傳輸動力，其就通常被作為一種機械裝置，機器這種機構設計是為達到轉遞動力或力矩的目的通常是所謂的機器。發(fā)動機一個機器需要能量轉換而產(chǎn)生的機械動力通常稱為發(fā)動機。因此，曲軸，連桿，活塞和氣缸的自動的發(fā)動機由上面所述的發(fā)動機的定義，而其他的傳動部件，例如變速箱，差速器，和萬向聯(lián)軸器都被稱為為機械裝置。機器和發(fā)