机械运动和动力学中英文对照外文翻译文献.docx
- 文档编号:4244038
- 上传时间:2023-05-06
- 格式:DOCX
- 页数:13
- 大小:26.46KB
机械运动和动力学中英文对照外文翻译文献.docx
《机械运动和动力学中英文对照外文翻译文献.docx》由会员分享,可在线阅读,更多相关《机械运动和动力学中英文对照外文翻译文献.docx(13页珍藏版)》请在冰点文库上搜索。
机械运动和动力学中英文对照外文翻译文献
中英文对照翻译
英文资料
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.any
referencelinedrawnonthebodywillchangebothitslinearpisitionanditsangularorientation.Pointsonthebodywilltravelnon-parallelpaths,andtherewillbe,ateveryinstant,acenterofrotation,whichwillcontinuouslychangelocation.
Linkagesarethebacisbuildingblocksofallmechanisms.Allcommonforms
ofmechanisms(cams,gears,belts,chains)areinfactvariationsoflinkages.Linkagesaremadeupoflinksandkinematicpairs.
Alinkisan(assumed)rigidbodywhichpossessesatleasttwoormorelinks(attheirnodes),whichconnectionallowssomemotion,orpotentialmotion,betweentheconnectedlinks.
Thetermlowerpairisusedtodescribejiontswithsurfacecontact,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.siecethisdidnotameasureoftheactualstrain,anothermethodofdeterminingductilityissometimesused.afterthespecimanhasbeenfractured,measurementsaremadeoftheareaofthecrosssectionatthefracture.Ductilitycanthenbeexpr
- 配套讲稿:
如PPT文件的首页显示word图标,表示该PPT已包含配套word讲稿。双击word图标可打开word文档。
- 特殊限制:
部分文档作品中含有的国旗、国徽等图片,仅作为作品整体效果示例展示,禁止商用。设计者仅对作品中独创性部分享有著作权。
- 关 键 词:
- 机械运动 动力学 中英文 对照 外文 翻译 文献