机械设计概论外文翻译.docx
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机械设计概论外文翻译
附录2:
英文资料
IntroductiontoMrchanicalDesign
Mechanicaldesignistheapplicationofscienceandtechnologytodeviseneworimprovedproductsforthepurposeofsatisfyinghumanneeds.Itisavastfieldofengineeringtechnologywhichnotonlyconcernsitselfwiththeoriginalconceptionoftheproductintermsofitssize,shapeandconstructiondetails,butalsoconsidersthevariousfactorsinvolvedinthemanufacture,markionganduseoftheproduct.
Peoplewhoperformthevariousfunctionsofmechanicaldesignaretypicallycalleddesigners,ordesigndesignisbasicallyacreativeactivity.However,inadditiontobeinginnovative,adesignengineermustalsohaveasolidbackgroundintheareasofmechanicaldrawing,kinematics,dynamics,materialsengineering,strengthofmaterialsandmanufacturingprocesses.
Asstatedpreviously,thepurposeofmechanicaldesignistoproduceaproductwhichwillserveaneedforman.Inventions,discoveriesandscientificknowledgebythemselvesdonotnecessarilybenfitpeople;onlyiftheyareincorporatedintoadesignedproductwillabenefitbederived.Itshouldberecognized,therefore,thatahumanneedmustbeidentifiedbeforeaparticularproductisdesigned.
Mechanicaldesignshouldbeconsideredtobeanopportunitytouseinnovativetalentstoenvisionadesignofaproduct,toanalyzethesystemandthenmakesoundjudgmentsonhowtheproductistobemanufactured.Itisimportanttounderstandthefundamentalsofengineeringratherthanmemorizemerefactsandarenofactsorequationswhichalonecanbeusedtoprovideallthecorrectdecisionsrequiredtoproduceagooddesign.Ontheotherhand,anycalculationsmademustbedonewiththeutmostcareandprecision.Forexample,ifadecimalpointismisplaced,anotherwiseaccrptabledesignmaynotfunction.
Gooddesignsrequiretryingnewideasandbeingwillingtotakeacertainamountofrisk,knowingthatifthenewideadoesnotworktheexistingmethodcanbereinstated.Thusadesignermusthavepatience,sincethereisnoassuranceofsuccessforthetimeandeffortexpended.Creatingacompletelynewdesigngenerallyrequiresthatmanyoldandwellestablishedmethodsbethrustaside.Thisisnoteasysincemanypeopleclingtofamiliarideas,techniquesandattitudes.Adesignengineershouldconstantlysearchforwaystoimproveanexistingproductandmustdecidewhatold,provenconceptsshouldbeusedandwhatnew,untriedshouldbeincorporated.
Newdesigngenerallyhave“bugs〞orunforeseenproblemswhichmustbeworkedoutbeforethesuperiorcharacteristicsofthenewdesignscanbeenjoyed.Thusthereisachanceforasuperiorproduct,butonlyathigherrisk.Itshouldbeemphasizedthat,ifadesigndoesnotwarrantradicalnewmethods,suchmethodsshouldnotbeappliedmerelyforthesakeofchange.
Duringthebeginningstagesofdesign,creativityshouldbeallowedtoflourishwithoutagreatnumberofconstraints.Eventhoughmanyimpracticalideasmayarise,itisusuallyeasytoeliminatethemintheearlystagesofdesignbeforefirmdetailsarerequiredbymanufacturing.Inthisway,innovativeideasarenotinhibited.Quiteoften,morethanonedesignisdeveloped,uptothepointwheretheycanbecomparedagainsteachother.Itisentirelypossiblethatthedesignwhichisultimatelyacceptedwilluseideasexistinginoneoftherejecteddesignsthatdidnotshowasmuchoverallpromise.
Psychologistsfrequentlytalkabouttryingtofitpeopletothemachinestheyoperate.Itisessentiallytheresponsibilityofthedesignengineertostrivetofitmachinestopeople.Thisisnotaneasytask,sincethereisreallynoaveragepersonforwhichcertainoperatingdimensionsandproceduresareoptimum.
Anotherimportantpointwhichshouldberecognizedisthatadesignengineermustbeabletocommunicateideastootherpeopleiftheyaretobeincorporated.Communicatingthedesigntoothersisthefinal,vitalstepinthedesignprocess.Undoubtedlymanygreatdesigns,inventions,andcreativeworkshavebeenlosttomankindsimplybecausetheoriginatorswereunableorunwillingtoexplaintheiraccomplishmentstoothers.Presentationisasellingjob.Theengineer,whenpresentinganewsolutiontoadministrative,management,orsupervisorypersons,isattemptingtosellortoprovetothemthatthissolutionisabetterone.Unlessthiscanbedonesuccessfully,thetimeandeffortspentonobtainingthesolutionhavebeenlargelywasted.
Basically,thereareonlythreemeansofcommunicationavailabletous.Therearethewritten,theoral,andthegraphicalfirms.Thereforethesuccessfulengineerwillbetechnicallycompetentandversatileinallthreeformsofcommunication.Atechnicallycompetentpersonwholacksabilityinanyoneoftheseformsisseverelyhandicapped.Ifabilityinallthreeformsislacking,noonewilleverknowhowcompetentthatpersonis!
Thecompetentengineershouldnotbeafraidofthepossibilityofnotsuccessdinginapresentation.Infact,occasionalfailureshouldbeexpectedbecausefailureorcriticismseemstoaccompanyeveryreallycreativeidea.Thereisagreatdealtobelearenedformafailure,andthegreatestgainsareobtainedbythosewillingtoriskdefeat.Inthefinalanalysis,therealfailurewouldlieindecidingnottomakethepresentationatall.Tocommunicateeffectively,thefollowingquestionsmustbeanswered:
(1)Doesthedesignreallyserveahumanneed?
(2)Willitbecompetitivewithexistingproductsofrivalcompanies?
(3)Isiteconomicaltoproduce?
(4)Canitbereadilymaintained?
(5)Willitsellandmakeaprofit?
Onlytimewillprovidethetrueanswerstotheprecedingquestions,buttheproductshouldbedesigned,manufacturedandmarketedonlywithinitialaffirmativedesignengineeralsomustcommunicatethefinalizeddesigntomanufacturingthroughtheuseofdetailandassemblydrawings.
Quiteoften,aproblemwilloccurtduringthemanufacturingcycle.Itmaybethatachangeisrequiredinthedimensioningortolerancingofapartsothatitcanbemorereadilyproduced.Thisfallsinthecategoryofengineeringchangeswhichmustbeapprovedbythedesignengineersothattheproductfunctionwillnotbeadverselyaffected.Inothercases,adeficiencyinthedesignmayappearduringassemblyortestingjustpriortoshipping.Theserealitiessimplybearoutthefactthatdesignisalivingprocess.Thereisalwaysabetterwaytodoitandthedesignershouldconstantlystrivetowardsfindingthatbetterway.
SomeRulesforMechanicalDesign
Designingstartswithaneed,realorimagined.Existingapparatusmayneedimprovementsindurability,efficiently,weight,speed,orcost.Newapparatusmaybeneededtoperformafunctionpreviouslydonebymen,suchascomputation,assembly,orservicing.Withtheobjectivewhollyorpartlydefined,thenextstepindesignistheconceptionofmechanismsandtheirarrangementsthatwillperformtheneededfunctions.Forthis,freehandsketchingisofgreatvalue,notonlyasarecordofone’sthoughtsandasanaidindiscussionwithothers,butparticularlyforcommunicationwithone’sownmind,asastimulantforcreativeideas.
Whenthegeneralshapeandafewdimensionsoftheseveralcomponentsbecomeapparent,analysiscanbegininearnest.Theanalysiswillhaveasitsobjectivesatisfactoryorsuperiorperformance,plussafetyanddurabilitywithminimumweight,andacompetitivecost.Optimumproportionsanddimensionswillbesoughtforeachcriticallyloadedsection,togetherwithabalancebetweenthestrengthoftheseveralcomponents.Materialsandtheirtreatmentwillbechosen.Theseimportantobjectivescanbeattainedonlybyanalysisbasedupontheprinciplesofmechanics,suchasthoseofstaticsforreactionforcesandfortheoptimumutilizationoffricition;ofdynamicsforinertia,acceleration,andenergy;ofelasticityandstrengthofmaterialsforstressanddeflection;andoffluidmechanicsforlubricationandhydrodynamicdrives.
Finally,adesignbaseduponfunctionandreliabilitywillbecompleted,andaprototypemaybebuilt.Ifitstestsaresatisfactory,andifthedeviceistoproducedinquantity,theinitialdesignwillundergocertainmodificationsthatenableittobemanufacturedinquantityatalowercost.Duringsubsequentyearsofmanufactureandservice,thedesignislikelytoundergochangesasnewideasareconceivedorasfurtheranalysisbasedupontestsandexperienceindicatealterations.Salesappeal,customersatisfaction,andmanufacturecostareallrelatedtodesign,andabilityindesignisintimatelyinvolvedinthesuccessofanengineeringventure.
Tostimulatecreativethought,thefollowingrulesaresuggestedforthedesigner.
1.Applyingenuitytoutilizedesiredphysicalpropertiesandtocontrolundesiredones.Theperformancerequirementsofamachinearemetbyutilizinglawsofnatureorpropertiesofmatter(e.g.,flexibility,strength,gravity,inertia,buoyancy,centrifugalforce,principlesoftheleverandinclinedplane,friction,viscosity,fluidpressure,andthermalexpansion),alsothemanyelectrical,optical,thermal,andchemicalphenomena.However,whatmaybeusefulinoneapplicationmaybedetrimentalinthenext.Flexibilityisdesiredinvaluespringsbutnotinthevalvecamshaft;frictionisdesiredattheclutchfacebutnotintheclutchbearing.Ingenuityindesignshouldbeappliedtoutilizeandcontrolthephysicalpropertiesthataredesiredandtominimizethosethatarenotdesired.
2.Provideforfavorablestressdistributeandstiffnesswithminimumwerght.
Oncomponentssubjectedtofluctuatingstress,particularattentionisgiventoareductioninstressconcentration,andtoanincreaseofstrengthatfillets,threads