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上海大学

机电工程与自动化学院

2010.8

GeneralConsiderationsofMachine

（机械概述）

1Thewordmachinehasbeengivenawidevarietyofdefinitions,butforthepurposeofthisarticleitisadevice,havingauniquepurpose,thatargumentsorreplaceshumanoranimaleffortfortheaccomplishmentofphysicaltasks.Toolsmayberegardasthesimplestclassofmachines.Theoperationofamachinemayinvolvethetransformationofchemical,thermal,electrical,ornuclearenergyintomechanicalenergy,orviceversa,oritsfunctionmaysimplybetomodifyandtransmitforcesandmotions.Allmachineshaveaninput,anoutput,andtransformingormodifyingandtransmittingdevice.

2Machinesthatreceivetheirinputenergyfromanaturalsource,suchasaircurrents,movingwater,coal,petroleum,oruranium,andtransformitintomechanicalenergyareknownasprimemovers.Windmills,waterwheels,turbines,steamengines,andinternal-combustionenginesareprimemovers.Inthesemachinestheinputsvary;

theoutputsareusuallyrotatingshaftscapableofbeingusedasinputtoothermachines,suchaselectricgenerators,hydraulicpumps,oraircompressors.Allthreeofthelatterdevicesmaybeclassifiedasgenerators;

theiroutputsofelectrical,hydraulic,andpneumaticenergycanbeusedasinputstoelectric,hydraulicorairmotors,Thesemotorscanbeusedtodrivemachineswithavarietyofoutputs,suchasmaterialsprocessing,packaging,orconveyingmachinery.Allmachinesthatareneitherprimemovers,generators,normotorsmaybeclassifiedasoperators.Thiscategoryalsoincludesmanuallyoperatedinstrumentsofallkinds,suchascalculatingmachinesandtypewriters.

3Iftheoperatorisapumpdrivenbyanelectricmotor,theflowofenergyfromtheprimemoveratthepowerplantthroughthegeneratorandthemotortotheoperatorisshowninFigure1.Theoperatorcanalsobedrivendirectlybyasmall,direct-connectedprimemover,suchasagasolineengine,asshownbythedottedlineinFigure1;

formostpower-drivenoperators,however,theflowofenergyfromtheprimemoverfollowsthesolidlines.

4Insomecases,machinesinallcategoriesarecombinedinoneunit.Inadiesel-electriclocomotive,forexample,thedieselengineistheprimemover,whichdrivestheelectricgenerator,which,inturn,supplieselectriccurrenttothemotorsthatdrivesthewheels.Thefollowingaresomeexamplessuppliedbyanautomobile.

5Inanautomobile,thebaseproblemisharnessingtheexplosiveeffectofgasolinetoprovidepowertorotatetherearwheels.Theexplosionofgasolineinthecylinderspushesthepistonsdown,andthetransmissionandmodificationofthistranslatory（linear）motiontorotarymotionofthecrankshaftisaffectedbytheconnectingrodsthatjoineachpistontocranks（Figure21）thatarepartofthecrankshaft.Thepiston,cylinder,crankandconnectingrodcombinationisknownasaslider-crankmechanism;

itisacommonlyusedmethodofconvertingtranslationtorotation（asinanengine）orrotationtotransmission（asinapump）.

6Toadmitthegasoline-airmixturetothecylindersandexhausttheburnedgases,valvesareused;

theseareopenandclosedbythewedgingactionofcams（projections）onarotatingcamshaftthatisdrivenfromthecrankshaftbygearsorachain.

7Inafour-stroke-cycleenginewitheightcylinders,thecrankshaftreceivesanimpulseatsomepointalongitslengtheveryquarterrevolution.Tosmoothouttheeffectoftheseintermittentimpulsesonthespeedofthecrankshaft,aflywheelisused.Thisisaheavywheel,attachedtothecrankshaft,thatbyitsinertiaopposesandmoderatesanyspeedfluctuations.

8Sincethetorque（turningforce）thatitdeliversdependsonitsspeed,aninternal-combustionenginecannotbestartedunderload.Toenableanautomobileenginetobestartedinanunloadedstateandthenconnectedtothewheelswithoutstalling,aclutchandatransmissionarenecessary.Theformermakesandbreakstheconnectionbetweenthecrankshaftandtransmission,whilethelatterchanges,infinitesteps,theratiobetweentheinputandoutputspeedsandtorquesofthetransmission.Inlowergear,theoutputspeedislowandtheoutputtorquehigherthantheenginetorque,sothatthecarcanbestartedmoving;

inhighgear,thecarismovingatasubstantialspeedandthetorquesandspeedsareequal.

9Theaxlestowhichthewheelsareattachedarecontainedintherearaxlehousing,whichisclampedtotherearsprings,andaredrivenfromthetransmissionbythedriveshaft.Asthecarmovesandthespringsflexinresponsetobumpsintheroad,thehousingmovesrelativetothetransmission;

topermitthismovementwithoutinterferingwiththetransmissionofthetorque,auniversaljointisattachedtoeachendofthedriveshaft.

10Thedriveshaftisperpendiculartotherearaxles.Theright-angledconnectionisusuallymadewithbevelgearshavingaratiosuchthattheaxlesrotateatfromone-thirdtoone-fourththespeedofthedriveshaft.Therearaxlehousingalsoholdsthedifferentialgearsthatpermitbothrearwheelstobedrivenfromthesamesourceandtorotateatdifferentspeedswhenturningacorner.

11Likeallmovingmechanicaldevices,automobilescannotescapefromtheeffectsofthefriction.Intheengine,transmission,rearaxlehousing,andallbearing,frictionisundesirable,sinceitincreasethepowerrequiredfromtheengine;

lubricationreducesbutdoesnoteliminatethisfriction.Ontheotherhand,frictionbetweenthetiresandtheroadandinthebrakeshoesmakestractionandbrakingpossible.Thebeltsthatdrivethefan,generatorandotheraccessoriesarefriction-dependentdevices.Friction,isalsousefulintheoperationoftheclutch.Someofthedevicescitedabove,andothersthataredescribedbelow,arefoundinmachinesofallcategories,assembledinamultitudeofwaystoperformallkindsofphysicaltasks.Becauseofthisdiversityoffunctionandthelackofcommoncharacteristics,thisarticlewillnotbeconcernedwithspecificoperators.Neitherwillitdealwiththeoverallperformanceoftheprimemovers,norwiththeoperationofhydraulic,pneumatic,orelectricdevices.Itwillconsideronlytheoperationandstructureofthebasicmechanicaldevicesthataretheconstituentpartsofmachines.Thefunctionofmostofthesedevicesistotransmitandmodifyforceandmotion.Otherdevices,suchassprings,flywheels,shaftsandfasteners,performsupplementaryfunctions.

12Forthepurposeofthisarticleamachinemaybefurtherdefinedasadeviceconsistingoftwoormoreresistant,relativelyconstrainedpartsthatmayservetotransmitandmodifyforceandmotioninordertodowork.Therequirementthatthepartsofamachineberesistantimpliesthattheymaybecapableofcarryingimposedloadswithoutfailureorlossoffunction.Althoughmostmachinepartsaresolidmetallicbodiesofsuitableproportions,nonmetallicmaterials,springs,fluidpressureorgans,andtensionorganssuchasbeltsarealsoemployed.

13Themostdistinctivecharacteristicofamachineisthatthepartsareinterconnectedandguidedinsuchawaythattheirmotionrelativetooneanotherareconstrained.Relativetotheblock,forexample,thepistonofareciprocatingengineisconstrainedbythecylindertomoveonastraightpath;

pointonthecrankshaftareconstrainedbythemainbearingstomoveoncircularpaths;

nootherformsofrelativemotionarepossible.

14Onsomemachinesthepartsareonlypartiallyconstrained.Ifthepartsareinterconnectedbyspringsorfrictionmembers,thepathsofthepartsrelativetooneanothermaybefixed,butthemotionsofthepartsmaybeaffectedbythestiffnessofthesprings,frictionandthemassesoftheparts.

15Ifallthepartsofamachinearecomparativelyrigidmemberswhosedeflectionsunderloadarenegligible,thentheconstrainmentmaybeconsideredcompleteandtherelativemotionsofthepartscanbestudiedwithoutconsideringtheforcesthatproducethem.Foraspecifiedrotationalspeedofthecrankshaftofareciprocatingengine,forexample,thecorrespondingspeedsofpointsontheconnectingrodandthepistoncanbecalculated.Thedeterminationofthedisplacements,velocities,andaccelerationsofthepartsofamachineforaprescribedinputmotionisthesubjectmatterofkinematicsofmachines.Suchcalculationscanbemadewithoutconsideringtheforcesinvolved,becausethemotionsareconstrained.

16Accordingtothedefinition,bothforcesandmotionsaretransmittedandmodifiedinamachine.Thewayinwhichthepartsofamachineareinterconnectedandguidedtoproducearequiredoutputmotionfromagiveninputmotionisknownasthemechanismofthemachine.Thepiston,connectingrod,andcrankshaftinareciprocatingengineconstituteamechanismforchangingtherectilinearmotionofthepistonintotherotarymotionofthecrankshaft.

17Althoughbothforcesandmotionsareinvolvedintheoperationofmachines,theprimaryfunctionofamachinemaybeeithertheamplificationofforceorthemodificationofmotion.Aleverisessentiallyaforceincreaser,whileagearboxismostoftenusedasaspeedreducer.Themotionsandforcesinamachineareinseparable,however,andarealwaysinaninverseratio.Theoutputforceonaleverisgreaterthantheinputforce.Similarly,theoutputspeedofagearreducerislessthantheinputspeed,buttheoutputtorqueisgreaterthantheinputtorque.Inthefirstcaseagaininforceisaccompaniedbyalossinmotion,whileinthesecondcasealossinmotionisaccompaniedbyagainintorque.

18Althoughtheprimaryfunctionofsomemachinescanbeidentified,itwouldbedi