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(jī)構(gòu)與機(jī)器一個(gè)系統(tǒng)，它按預(yù)先確定的方式來(lái)傳輸動(dòng)力完成的具體的目標(biāo)也許可以被認(rèn)為是機(jī)器。一種機(jī)構(gòu)也可以以類似的方式定義，但長(zhǎng)期的機(jī)構(gòu)通常是適用于一個(gè)系統(tǒng)的主要職能是傳遞運(yùn)動(dòng)。運(yùn)動(dòng)學(xué)是研究機(jī)構(gòu)運(yùn)動(dòng)，而分析力和力矩的機(jī)械稱為動(dòng)力學(xué)。一旦需要給出識(shí)別一個(gè)機(jī)構(gòu)或機(jī)械裝置的特點(diǎn)，設(shè)計(jì)過(guò)程就開始了。通常需要仔細(xì)地分析位移，速度和加速度。這部分的設(shè)計(jì)過(guò)程后，其次是分析力和力矩。設(shè)計(jì)過(guò)程中可能會(huì)繼續(xù)很長(zhǎng)時(shí)間后產(chǎn)生第一種模式，其中包括重新設(shè)計(jì)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(jìng)爭(zhēng)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(jì)的成本和減輕體重，運(yùn)動(dòng)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(yùn)動(dòng)學(xué)和動(dòng)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(jī)器設(shè)計(jì)的組成部分，和我們使用這個(gè)術(shù)語(yǔ)形容當(dāng)時(shí)的變化超過(guò)一年。因此，定義和專門的術(shù)語(yǔ)將不符合整個(gè)技術(shù)的文獻(xiàn)。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(gè)方面特別感興趣的研究機(jī)器運(yùn)動(dòng)學(xué)和動(dòng)力學(xué)的定義如下。桿件一個(gè)桿件是一個(gè)嚴(yán)格的機(jī)構(gòu)或其共同組成一個(gè)運(yùn)動(dòng)鏈。長(zhǎng)期嚴(yán)格的桿件或有時(shí)只是使用一個(gè)理想化的桿件研究，由于機(jī)件拉緊不考慮微小撓度。一個(gè)完全不彎曲或不可拉長(zhǎng)的桿件可能存在不僅是一種教科書式的模型，一個(gè)真正的機(jī)器的構(gòu)件。對(duì)于典型的機(jī)械部分，最大尺寸的變化是只有長(zhǎng)度部分的千分之一。當(dāng)我們考慮多數(shù)機(jī)械裝置的運(yùn)動(dòng)特性時(shí)我們有理由忽視這個(gè)小小的運(yùn)動(dòng)。這個(gè)桿件定理中使用的一般意義上包括凸輪，齒輪，和其他構(gòu)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(dú)立的參數(shù)必須立場(chǎng)的每一個(gè)環(huán)節(jié)相對(duì)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(gè)獨(dú)立的變量，該系統(tǒng)有一個(gè)自由度。多數(shù)實(shí)用的機(jī)械裝置就有一個(gè)自由度。一個(gè)無(wú)約束剛體有6個(gè)自由度：直線移動(dòng)在三個(gè)坐標(biāo)和旋轉(zhuǎn)運(yùn)動(dòng)三個(gè)坐標(biāo)軸。如果該機(jī)構(gòu)是限制于在一個(gè)平面運(yùn)動(dòng)，那有三個(gè)自由度：直線運(yùn)動(dòng)在兩個(gè)坐標(biāo)方向和在平面內(nèi)的旋轉(zhuǎn)。高副和低副鏈接的剛體之間包括高副和低副兩個(gè)要素。這兩個(gè)因素中的低副是兩個(gè)理論表面之間的接觸，而這兩個(gè)因素中的高副是理論的點(diǎn)或線接觸（如果我們忽視了撓度）。低副是從設(shè)計(jì)的角度來(lái)看是可取的，由于聯(lián)合負(fù)荷以及由此產(chǎn)生的磨損分布在整個(gè)接觸面。因此，幾何變化或失敗而高接觸應(yīng)力和過(guò)度磨損或許是可以避免的。機(jī)械裝置機(jī)械裝置是一個(gè)運(yùn)動(dòng)鏈系中的一環(huán)被認(rèn)為是特定的目的是為了分析，但運(yùn)動(dòng)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(shí)際相對(duì)固定在地球表面。如果運(yùn)動(dòng)學(xué)鏈主要目的是緩和或傳輸動(dòng)力，其就通常被作為一種機(jī)械裝置，機(jī)器這種機(jī)構(gòu)設(shè)計(jì)是為達(dá)到轉(zhuǎn)遞動(dòng)力或力矩的目的通常是所謂的機(jī)器。發(fā)動(dòng)機(jī)一個(gè)機(jī)器需要能量轉(zhuǎn)換而產(chǎn)生的機(jī)械動(dòng)力通常稱為發(fā)動(dòng)機(jī)。因此，曲軸，連桿，活塞和氣缸的自動(dòng)的發(fā)動(dòng)機(jī)由上面所述的發(fā)動(dòng)機(jī)的定義，而其他的傳動(dòng)部件，例如變速箱，差速器，和萬(wàn)向聯(lián)軸器都被稱為為機(jī)械裝置。機(jī)器和發(fā)