外文翻译--机械设计及加工工艺.doc

英文原文MechanicalDesignandManufacturingProcessesMechanicaldesignistheapplicationofscienceandtechnologytodeviseneworimprovedproductsforthepurposeofsatisfyinghumanneeds.Itisavastfieldofengineeringtechnologywhichnotonlyconcernsitselfwiththeoriginalconceptionoftheproductintermsofitssize,shapeandconstructiondetails,butalsoconsidersthevariousfactorsinvolvedinthemanufacture,marketinganduseoftheproduct.Peoplewhoperformthevariousfunctionsofmechanicaldesignaretypicallycalleddesigners,ordesignengineers.Mechanicaldesignisbasicallyacreativeactivity.However,inadditiontobeinginnovative,adesignengineermustalsohaveasolidbackgroundintheareasofmechanicaldrawing,kinematics,dynamics,materialsengineering,strengthofmaterialsandmanufacturingprocesses.Asstatedpreviously,thepurposeofmechanicaldesignistoproduceaproductwhichwillserveaneedforman.Inventions,discoveriesandscientificknowledgebythemselvesdonotnecessarilybenefitpeople；onlyiftheyareincorporatedintoadesignedproductwillabenefitbederived.Itshouldberecognized,therefore,thatahumanneedmustbeidentifiedbeforeaparticularproductisdesigned.Mechanicaldesignshouldbeconsideredtobeanopportunitytouseinnovativetalentstoenvisionadesignofaproduct,toanalyzethesystemandthenmakesoundjudgmentsonhowtheproductistobemanufactured.Itisimportanttounderstandthefundamentalsofengineeringratherthanmemorizemerefactsandequations.Therearenofactsorequationswhichalonecanbeusedtoprovideallthecorrectdecisionsrequiredtoproduceagooddesign.Ontheotherhand,anycalculationsmademustbedonewiththeutmostcareandprecision.Forexample,ifadecimalpointismisplaced,anotherwiseacceptabledesignmaynotfunction.Gooddesignsrequiretryingnewideasandbeingwillingtotakeacertainamountofrisk,knowingthatifthenewideadoesnotworktheexistingmethodcanbereinstated.Thusadesignermusthavepatience,sincethereisnoassuranceofsuccessforthetimeandeffortexpended.Creatingacompletelynewdesigngenerallyrequiresthatmanyoldandwell-establishedmethodsbethrustaside.Thisisnoteasysincemanypeopleclingtofamiliarideas,techniquesandattitudes.Adesignengineershouldconstantlysearchforwaystoimproveanexistingproductandmustdecidewhatold,provenconceptsshouldbeusedandwhatnew,untriedideasshouldbeincorporated.Newdesignsgenerallyhavebugsorunforeseenproblemswhichmustbeworkedoutbeforethesuperiorcharacteristicsofthenewdesignscanbeenjoyed.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.Thesearethewritten,theoral,andthegraphicalforms.Thereforethesuccessfulengineerwillbetechnicallycompetentandversatileinallthreeformsofcommunication.Atechnicallycompetentpersonwholacksabilityinanyoneoftheseformsisseverelyhandicapped.Ifabilityinallthreeformsislacking,noonewilleverknowhowcompetentthatpersonis!Thecompetentengineershouldnotbeafraidofthepossibilityofnotsucceedinginapresentation.Infact,occasionalfailureshouldbeexpectedbecausefailureorcriticismseemstoaccompanyeveryreallycreativeidea.Thereisagreatdealtobelearnedfromafailure,andthegreatestgainsareobtainedbythosewillingtoriskdefeat.Inthefinalanalysis,therealfailurewouldlieindecidingnottomakethepresentationatall.Tocommunicateeffectively,thefollowingquestionsmustbeanswered:(1)Doesthedesignreallyserveahumanneed?(2)Willitbecompetitivewithexistingproductsofrivalcompanies?(3)Isiteconomicaltoproduce?(4)Canitbereadilymaintained?(5)Willitsellandmakeaprofit?Onlytimewillprovidethetrueanswerstotheprecedingquestions,buttheproductshouldbedesigned,manufacturedandmarketedonlywithinitialaffirmativeanswers.Thedesignengineeralsomustcommunicatethefinalizeddesigntomanufacturingthroughtheuseofdetailandassemblydrawings.Quiteoften,aproblemwilloccurduringthemanufacturingcycle3.Itmaybethatachangeisrequiredinthedimensioningortolerancingofapartsothatitcanbemorereadilyproduced.Thisfailsinthecategoryofengineeringchangeswhichmustbeapprovedbythedesignengineersothattheproductfunctionwillnotbeadverselyaffected.Inothercases,adeficiencyinthedesignmayappearduringassemblyortestingjustpriortoshipping.Theserealitiessimplybearoutthefactthatdesignisalivingprocess.Thereisalwaysabetterwaytodoitandthedesignershouldconstantlystrivetowardsfindingthatbetterway.Designingstartswithaneed,realorimagined.Existingapparatusmayneedimprovementsindurability,efficiently,weight,speed,orcost.Newapparatusmaybeneededtoperformafunctionpreviouslydonebymen,suchascomputation,assembly,orservicing.Withtheobjectivewhollyorpartlydefined,thenextstepindesignistheconceptionofmechanismsandtheirarrangementsthatwillperformtheneededfunctions.Forthis,freehandsketchingisofgreatvalue,notonlyasarecordofonesthoughtsandasanaidindiscussionwithothers,butparticularlyforcommunicationwithonesownmind,asastimulantforcreativeideas.Whenthegeneralshapeandafewdimensionsoftheseveralcomponentsbecomeapparent,analysiscanbegininearnest.Theanalysiswillhaveasitsobjectivesatisfactoryorsuperiorperformance,plussafetyanddurabilitywithminimumweight,andacompetitiveeast.Optimumproportionsanddimensionswillbesoughtforeachcriticallyloadedsection,togetherwithabalancebetweenthestrengthoftheseveralcomponents.Materialsandtheirtreatmentwillbechosen.Theseimportantobjectivescanbeattainedonlybyanalysisbasedupontheprinciplesofmechanics,suchasthoseofstaticsforreactionforcesandfortheoptimumutilizationoffriction；ofdynamicsforinertia,acceleration,andenergy；ofelasticityandstrengthofmaterialsforstressanddeflection；andoffluidmechanicsforlubricationandhydrodynamicdrives.Finally,adesignbaseduponfunctionandreliabilitywillbecompleted,andaprototypemaybebuilt.Ifitstestsaresatisfactory,andifthedeviceistobeproducedinquantity,theinitialdesignwillundergocertainmodificationsthatenableittobemanufacturedinquantityatalowercost.Duringsubsequentyearsofmanufactureandservice,thedesignislikelytoundergochangesasnewideasareconceivedorasfurtheranalysisbasedupontestsandexperienceindicatealterations.Salesappeal,customersatisfaction,andmanufacturecostareallrelatedtodesign,andabilityindesignisintimatelyinvolvedinthesuccessofanengineeringventure.Tostimulatecreativethought,thefollowingrulesaresuggestedforthedesigner.1.Applyingenuitytoutilizedesiredphysicalpropertiesandtocontrolundesiredones.Theperformancerequirementsofamachinearemetbyutilizinglawsofnatureorpropertiesofmatter(e.g.,flexibility,strength,gravity,inertia,buoyancy,centrifugalfor,principlesoftheleverandinclinedplane,friction,viscosity,fluidpressure,andthermalexpansion),alsothemanyelectrical,optical,thermal,andchemicalphenomena.However,whatmaybeusefulinoneapplicationmaybedetrimentalinthenext.Flexibilityisdesiredinvalvespringsbutnotinthevalvecamshaft；frictionisdesiredattheclutchfacebutnotintheclutchbearing.Ingenuityindesignshouldbeappliedtoutilizeandcontrolthephysicalpropertiesthataredesiredandtominimizethosethatarenotdesired.2.Provideforfavorablestressdistributeandstiffnesswithminimumweight.Oncomponentssubjectedtofluctuatingstress,particularattentionisgiventoareductioninstressconcentration,andtoanincreaseofstrengthatfillets,threads,holes,andfits.Stressreductionaremadebymodificationinshape,andstrengtheningmaybedonebyprestressingtreatmentssuchassurfacerollingandshallowhardening.Hollowshaftsandtubing,andboxsectionsgiveafavorablestressdistribution,togetherwithstiffnessandminimumweight.Sufficientstiffnesstomaintainalignmentanduniformpressurebetweencontactingsurfacesshouldbeprovidedforcrank,cam,andgearshafts,andforenclosuresandframescontainingbearingsupports.Thestiffnessofshaftsandothercomponentsmustbesuitabletoavoidresonantvibrations.3.Useequationstocalculateandoptimizedimensions.Thefundamentalequationsofmechanicsandtheothersciencesaretheacceptedbasesforcalculations.Theyaresometimesrearrangedinspecialformstofacilitatethedeterminationoroptimizationofdimensions,suchastheBeamandsurfacestressequationsfordetermininggear-toothsize.Factorsmaybeaddedtoafundamentalequationforconditionsnotanalyticallydeterminable,e.g.,onthinsteeltubes,anallowanceforcorrosionaddedtothethicknessbasedonpressure.Whenitisnecessarytoapplyafundamentalequationtoshapes,materials,orconditionswhichonlyapproximatetheassumptionsforitsderivation,itisdoneinamannerwhichgivesresultsonthesafeside.Insituationswheredataareincomplete,equationsofthesciencesmaybeusedasproportioningguidestoextendasatisfactorydesigntonewcapacities.4.Choosematerialsforacombinationofproperties.Materialsshouldbechosenforacombinationofpertinentproperties,notonlyforstrengths,hardness,andweight,butsometimesforresistancetoimpact,corrosion,andloworhightemperatures.Costandfabricationpropertiesarefactors,suchasweldability,machineability,sensitivitytovariationinheat-treatingtemperatures,andrequiredcoating.5.Selectcarefullybetweenstockandintegralcomponents.Apreviouslydevelopedcomponentsisfrequentlyselectedbyadesignerandhiscompanyfromthestocksofpartsmanufacturers,ifthecomponentmeettheperformanceandreliabilityrequirementsandisadaptablewithoutadditionaldevelopmentcoststotheparticularmachinebeingdesigned.However,itsselectionshouldbecarefullymadewithafullknowledgeofitsproperties,sincethereputationandliabilityofthecompanysufferifthereisafailureinanyoneofthemachinesparts.Inothereasesthestrength,reliability,andcostrequirementsarebettermetifthedesignerofthemachinealsodesignsthecomponent,withtheparticularadvantageofcompactnessifitisdesignsintegralwithothercomponents,e.g.,gearstobeforgedinclustersorintegralwithashaft.6.Provideforaccuratelocationandnoninterferenceofpartsinassembly.Agooddesignprovidesforthecorrectlocatingofpartsandforeasyassemblyandrepair.Shouldersandpilotsurfacesgiveaccuratelocationwithoutmeasurementduringassembly.Shapescanbedesignedsothatpartscannotbeassembledbackwardsorinthewrongplace.Interferences,asbetweenscrewsintappedholes,andbetweenlinkagesmustheforeseenandpretended.Inaccuratealignmentandpositioningbetweensuchassembliesmustbeavoided,orprovisionmustbemadetominimizeanyresultingdetrimentaldisplacementsandstresses.Thehumanracehasdistinguisheditselffromallotherformsoflifebyusingtoolsandintelligencetocreateitemsthatservetomakelifeeasierandmoreenjoyable.Throughthecenturies,boththetoolsandtheenergysourcestopowerthesetoolshaveevolvedtomeettheincreasingsophisticationandcomplexityofmankindsideas.Intheirearliestforms,toolsprimarilyconsistedofstoneinstruments.Consideringtilerelativesimplicityoftheitemsbeingmadeandthematerialsbeingshaped,stonewasadequate.Whenirontoolswereinvented,durablemetalsandmoresophisticatedarticlescouldbeproduced.ThetwentiethcenturyhasseenthecreationofproductsmadefromtheMostdurable,consequently,themostunmachinablematerialsinhistory.Inanefforttomeetthemanufacturingchallengescreatedbythesematerials,toolshavenowevolvedtoincludematerialssuchasalloysteel,carbide,diamond,andceramics.Asimilarevolutionhastakenplacewiththemethodsusedtopowerourtools.Initially,toolswerepoweredbymuscles；eitherhumanoranimal.Howeverasthepowersofwater,wind,steam,andelectricitywereharnessed,mankindwasabletofurtherextendedmanufacturingcapabilitieswithnewm