机械制造类专业英语（第2版）模拟考试卷5.docx

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2024-09-01 发布于山东
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机械制造类专业英语（第2版）模拟考试卷5.docx

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Oneprincipleaimofkinematicsistocreate(design)thedesiredmotionsofthesubjects’mechanicalpartsandthenmathematicallycomputethepositions,velocities,andaccelerations,whichthosewillcreateontheparts.Since,formostearthboundmechanicalsystems,themassremainsessentiallyconstantwithtime,definingtheaccelerationsasafunctionoftimethenalsodefinesthedynamicforcesasafunctionoftime.Stresses,inturn,willbeafunctionofbothappliedandinertial(ma)forces.Sinceengineeringdesignischargedwithcreatingsystemswhichwillnotfailduringtheirexpectedservicelife,thegoalistokeepstresseswithinacceptablelimitsforthematerialschosenandtheenvironmentalconditionsencountered.Thisobviouslyrequiresthatallsystemforcesbedefinedandkeptwithindesiredlimits.Inmachinery,thelargestforcesencounteredareoftenthoseduetothedynamicsofthemachineitself.Thesedynamicforcesareproportionaltoacceleration,whichbringsusbacktokinematics,thefoundationofmechanicaldesign.Verybasicandearlydecisioninthedesignprocessinvolvingkinematicprinciplescanbecrucialtothesuccessofanymechanicaldesign;adesignwhichhaspoorkinematicswillprovetroublesomeandperformbadly.

Anymechanicalsystemcanbeclassifiedaccordingtothenumberofdegreeoffreedom(DOF)whichitprocesses.Thesystem’sDOFisequaltothenumberofindependentparameterswhichareneededtouniquelydefineitspositioninspaceatanyinstantoftime.

Arigidbodyfreetomovewithinareferenceframewill,inthegeneralcase,hascomplexmotion,whichisasimultaneouscombinationofrotationandtranslation.Inthree-dimensionalspace,theremayberotationaboutanyaxisandalsosimultaneoustranslationwhichcanberesolvedintocomponentsalongthreeaxes.Inaplane,ortwo-dimensionalspace,complexmotionbecomesacombinationofsimultaneousrotationaboutoneaxis(perpendicular