機械外文翻譯--機構(gòu)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(gòu)與機器一個系統(tǒng)，它按預(yù)先確定的方式來傳輸動力完成的具體的目標(biāo)也許可以被認為是機器。一種機構(gòu)也可以以類似的方式定義，但長期的機構(gòu)通常是適用于一個系統(tǒng)的主要職能是傳遞運動。運動學(xué)是研究機構(gòu)運動，而分析力和力矩的機械稱為動力學(xué)。一旦需要給出識別一個機構(gòu)或機械裝置的特點，設(shè)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(shè)計過程后，其次是分析力和力矩。設(shè)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(shè)計的組成部分，影響速度，加速度，力和力矩。年復(fù)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(shè)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(xué)和動力學(xué)的分析的問題。許多基本的連接裝置構(gòu)造世紀(jì)以前已經(jīng)成為機器設(shè)計的組成部分，和我們使用