外文翻译机械运动和动力学精品.docx
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外文翻译机械运动和动力学精品
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外文原文
Kinematicsanddynamicsofmachinery
Oneprincpleaimofkinemaricsistocreatthedesignedmotionsofthesubjectmechanicalpartsandthenmathematicallycomputethepositions,velocities,andaccelerations,whichthosemotionswillcreatontheparts.Since,formostearthboundmechanicalsystems,themassremainsessentiallyconstantwithtime,definingtheaccelerationsasafunctionoftimethenalsodefinesthedynamicforcesasafunctionoftime.Stress,inturn,willbeafunctionofbothappliedandinerialsforces.sinceengineeringdesignischargedwithcreatingsystemswhichwillnotfailduringtheirexpectedservicelife,thegoalistokeepstresseswithinacceptablelimitsforthematerialschosenandtheenvironmentalconditionsencountered.Thisobviselyrequiesthatallsystemforcesbedefinedandkeptwithindesiredlimits.Inmechinery,thelargestforcesencounteredareoftenthoseduetothedynamicsofthemachineitself.Thesedynamicforcesareproportionaltoacceletation,whichbringsusbacktokinematics,thefoundationofmechanicaldesign.Verybasicandearlydecisionsinthedesignprocessinvovlingkinematicswiiprovetroublesomeandperformbadly.
Anymechanicalsystemcanbeclassifiedaccordingtothenumberofdegreeoffreedomwhichitpossesses.thesystemsDOFisequaltothenumberofindependentparameterswhichareneededtouniquelydefineitsposioninspaceatanyinstantoftime.
Arigidbodyfreetomovewithinareferenceframewill,inthegeneralcase,havecomplexmotoin,whichissimultaneouscombinationofrotationandtranslation.Inthree-dimensionalspace,theremayberotationaboutanyaxisandalsosimultaneoustranslationwhichcanberesoledintocomponentionalongthreeaxes,inaplane,ortwo-dimentionalspace,complexmotionbecomesacombinationofsimultaneousalongtwoaxesintheplane.Forsimplicity,wewilllimitourpresentdiscusstionstothecaseofplanarmotion:
Purerotationthebodypessessesonepoint(centerofrotation)whichhasnomotionwithrespecttothestationaryframeofreference.Allotherpointsonthebodydescribearcsaboutthatcenter.Areferencelinedrawnonthebodythroughthecenterchangesonlyitsangulaiorientation.
Puretranslationallpointsonthebodydescribeparallelpaths.Areferencelinedrawnonthebodychangesitslinearposionbutdoesnotchangeitsangularoriention.
Complexmotionasimulaneouscombinationofrotionandtranslationm.anyreferencelinedrawnonthebodywillchangebothitslinearpisitionanditsangularorientation.Pointsonthebodywilltravelnon-parallelpaths,andtherewillbe,ateveryinstant,acenterofrotation,whichwillcontinuouslychangelocation.
Linkagesarethebacisbuildingblocksofallmechanisms.Allcommonformsofmechanisms(cams,gears,belts,chains)areinfactvariationsoflinkages.Linkagesaremadeupoflinksandkinematicpairs.
Alinkisan(assumed)rigidbodywhichpossessesatleasttwoormorelinks(attheirnodes),whichconnectionallowssomemotion,orpotentialmotion,betweentheconnectedlinks.
Thetermlowerpairisusedtidescribejiontswithsurfacecontact,aswithapinsurroundedbyahole.Thetermhigherpairisusedtodescribejiontswithpointorlinecontact,butifthereisanyclerancebetweenpinandhole(astheremustbeformotion),so-calledsurfacecontactinthepinjiontactuallybecomeslinecontact,asthepincontactsactuallyhascontactonlyatdiscretepoints,whicharethetopsofthesurfaces’asperities.Themainpracticaladvantageoflowerpairsoverhigherpairsistheirbetterabilitytotraplubricantbetweentheirenvlopingsurface.Thisieespeciallytruefortherotatingpinjoint.Thelubricantismoreeasilysqueezedoutofahigherpair.assresult,thepinjointispreferredforlowwearandlonglife.
Whendesigningmachinery,wemustfirstdoacompletekinematicanalysisofourdesign,inordertoobtaininformationabouttheaccelerationofthemovingparts.wenextwantteusenewton’ssecondlawtocaculatethedynamicforces,buttodosoweneedtoknowthemassesofallthemovingpartswhichhavetheseknownacceletations.Thesepartsdonotexityet!
aswithanydesigninordertomakeafirstpassatthecaculation.wewillthenhavetoitneratetobetteranbettersolutionsaswegeneratemoreinformation.
Afirstestimateofyourparts’massescanbeobtainedbyassumingsomereasonableshapesandsizeforallthepartsandchoosingapproriatematerials.Thencaculatethevolumeofeachpartandmultipyitsvolumebymaterial’smassdensity(notweightdensity)toobtainafirstapproximationofitsmass.thesemassvaluescanthenbeusedinNewton’sequation.
Howwillweknowwhetherourchosensizesandshapesoflinksareevenacceptable,letaloneoptimal?
unfortunately,wewillnotknowuntillwehavecarriedthecomputationsallthewaythroughacompletestressanddeflectionanalysisoftheparts.Ititoftenthecase,especiallywithlong,thinelementssuchasshaftsorslenderlinks,thatthedeflectionsoftheparts,redesignthem,andrepeattheforce,stress,anddeflectionanalysis.designis,unavoidably,aniterativeprocess.
Itisalsoworthnothingthat,unlikeastaticforcesituationinwhichafaileddesignmightbefixedbyaddingmoremasstotheparttostrenthenit,todosoinadynamicforcesituationcanhaveadeleteriouseffect.moremasswiththesameaccelerationwillgenerateevenhigherforcesandthushigherstresses!
themachinedesigeroftenneedtoremovemass(intherightplaces)formpartsinordertoreducethestessesanddeflectionsduetoF=ma,thusthedesignerneedstohaveagoodunderstandingofbothmaterialpropertiesandstessanddeflectionanalysistoproperlyshapeandsizepartsforminimummasswhilemaximzingthestrengthandstiffnessneededtowithstandthedynamicforces.
Oneoftheprimaryconsiderationsindesigninganymachineorstrucreisthatthestrengthmustbesufficientlygreaterthanthestresstoassurebothsafetyandreliability.Toassurethatmechanicalpartsdonotfailinservice,itisnecessarytolearnwhytheysometimesdofail.Thenweshallbeabletorelatethestresseswiththestrenthstoachievesafety.
Ideally,indesigninganymachineelement,theengineershouldhaveathisdisposalshouldhavebeenmadeonspecimentshavingthesameheattreatment,surfaceroughness,andsizeastheelementheprossestodesign;andthetestsshouldbemadeunderexactlythesameloadingconditionsasthepartwillexperienceinservice.thismeansthat,ifthepartistoexperienceabendingandtorsion,itshouldbetestedundercombinedbendingandtorsion.Suchtestswillprovideveryusefulandpreciseinformation.theytelltheengineerwhatfactorofsafetytouseandwhatthereliabilityisforagivenservicelife.wheneversuchdataareavailablefordesignpurposes,theengineercanbeassurethatheisdoingthebestjustifiediffailureofthepartmayendangerhumanlife,orifthepartismanufacturedinsufficientlylargequantities.Automobilesandrefrigrerators,forexample,haveverygoodreliabilitiesbecausethepartsaremadeinsuchlargequantitiesthattheycanbethoroughlytestedinadvanceofmanufacture,thecostofmakingtheseisverylowwhenitisdividedbythetotalnumberofpartsmanufactrued.
Youcannowappreciatethefollowingfourdesigncategories:
(1)failureofthepartwouldendangerhumanlife,orthepartismadeinextremelylargequantities;consequently,anelaboratetestingprogramisjustifiedduringdesign.
(2)thepartismadeinlargeenoughquantitiessothatamoderateseruesoftestsisfeasible.
(3)Thepartismadeinsuchsmallquantitiesthattestingisnotjustifiedatall;orthedesignmustbecompletedsorapidllythatthereisnotenoughtimefortesting.
(4)Theparthasalreadybeendesigned,manufactured,andtestedandfoundtobeunsatisfactory.Analysisisrequiredtounderstandwhythepartisunsatisfactoryandwhattodotoimproveit.
Itiswiththelastthreecategoriesthatweshallbemostlyconcerned.thismeansthatthedesignerwillusuallyhaveonlypublishedvaluesofyieldstrenth,ultimatestrength,andpercentageelongation.withthismeagerinformationtheengieerisexpectedtodesignagainststaticanddynamicloads,biaxialandtriaxialstressstates,highandlowtemperatures,andlargeandsmallparts!
Thedatausuallyavailablefordesignhavebeenobtainedfromthesimpletensiontest,wheretheloadwasappliedgraduallyandthestraingiventimetodevelop.Yetthesesamedatamustbeusedindesigningpartswithcomplicateddynamicloadsappliedthousandsoftimesperminute.nowondermachinepartssometimesfail.
Tosumup,thefundamentalproblemofthedesigneristousethesimpletensiontestdataandrelatethemtothestrengthofthepart,regardlessofthestressortheloadingsituation.
Itispossiblefortwometaltohaveexactlythesamestrengthandhardness,yetoneofthesemetalsmayhaveasupeiorabilitytoaborboverloads,becauseofthepropertycalledductility.Dutilityismeasuredbythepercentageelongationwhichoccursinthematerialatfrature.Theusualdivdinglinebetweenductilityandbrittlenessis5percentelongation.Amaterialhavinglessthan5percentelongationatfractureissaidtobebrittle,whileonehavingmoreissaidtobeductile.
Theelongationofamaterialisusuallumeasuredover50mmgaugelength.siecethisidnotameasureoftheactualstrain,anothermethodofdeterminingductilityissometimesused.afterthespecimanhasbeenfractured,measurementsaremadeoftheareaofthecrosssectionatthefracture.Ductilitycanthenbeexpr