机械类外文文献.docx
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机械类外文文献.docx
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机械类外文文献
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FundamentalsofMechanicalDesign
Mechanicaldesignmeansthedesignofthingsandsystemsofamechanicalnature—machines,products,structures,devices,andinstruments.Forthemostpartmechanicaldesignutilizesmathematics,thematerialssciences,andtheengineering-mechanicssciences.
Thetotaldesignprocessisofinteresttous.Howdoesitbegin?
Doestheengineersimplysitdownathisdeskwithablanksheetofpaper?
And,ashejotsdownsomeideas,whathappensnext?
Whatfactorsinfluenceorcontrolthedecisionswhichhavetobemade?
Finally,then,howdoesthisdesignprocessend?
Sometimes,butnotalways,designbeginswhenanengineerrecognizesaneedanddecidestodosomethingaboutit.Recognitionoftheneedandphrasingitinsomanywordsoftenconstituteahighlycreativeactbecausetheneedmaybeonlyavaguediscontent,afeelingofuneasiness,ofasensingthatsomethingisnotright.
Theneedisusuallynotevidentatall.Forexample,theneedtodosomethingaboutafood-packagingmachinemaybeindicatedbythenoiselevel,bythevariationsinpackageweight,andbyslightbutperceptiblevariationsinthequalityofthepackagingorwrap.
Thereisadistinctdifferencebetweenthestatementoftheneedandtheidentificationoftheproblem.Whichfollowsthisstatement?
Theproblemismorespecific.Iftheneedisforcleanerair,theproblemmightbethatofreducingthedustdischargefrompower-plantstacks,orreducingthequantityofirritantsfromautomotiveexhausts.
Definitionoftheproblemmustincludeallthespecificationsforthethingthatistobedesigned.Thespecificationsaretheinputandoutputquantities,thecharacteristicsofthespacethethingmustoccupyandallthelimitationsonthesequantities.Wecanregardthethingtobedesignedassomethinginablackbox.Inthiscasewemustspecifytheinputsandoutputsoftheboxtogetherwiththeircharacteristicsandlimitations.Thespecificationsdefinethecost,thenumbertobemanufactured,theexpectedlife,therange,theoperatingtemperature,andthereliability.
Therearemanyimpliedspecificationswhichresulteitherfromthedesigner'sparticularenvironmentorfromthenatureoftheproblemitself.Themanufacturingprocesseswhichareavailable,togetherwiththefacilitiesofacertainplant,constituterestrictionsonadesigner'sfreedom,andhenceareapartoftheimpliedspecifications.Asmallplant,forinstance,maynotowncold-workingmachinery.Knowingthis,thedesignerselectsothermetal-processingmethodswhichcanbeperformedintheplant.Thelaborskillsavailableandthecompetitivesituationalsoconstituteimpliedspecifications.
Aftertheproblemhasbeendefinedandasetofwrittenandimpliedspecificationshasbeenobtained,thenextstepindesignisthesynthesisofanoptimumsolution.Nowsynthesiscannottakeplacewithoutbothanalysisandoptimizationbecausethesystemunderdesignmustbeanalyzedtodeterminewhethertheperformancecomplieswiththespecifications.
Thedesignisaniterativeprocessinwhichweproceedthroughseveralsteps,evaluatetheresults,andthenreturntoanearlierphaseoftheprocedure.Thuswemaysynthesizeseveralcomponentsofasystem,analyzeandoptimizethem,andreturntosynthesistoseewhateffectthishasontheremainingpartsofthesystem.Bothanalysisandoptimizationrequirethatweconstructordeviseabstractmodelsofthesystemwhichwilladmitsomeformofmathematicalanalysis.Wecallthesemodelsmathematicalmodels.Increatingthemitisourhopethatwecanfindonewhichwillsimulatetherealphysicalsystemverywell.
Evaluationisasignificantphaseofthetotaldesignprocess.Evaluationisthefinalproofofasuccessfuldesign,whichusuallyinvolvesthetestingofaprototypeinthelaboratory.Herewewishtodiscoverifthedesignreallysatisfiestheneedorneeds.Isitreliable?
Willitcompetesuccessfullywithsimilarproducts?
Isiteconomicaltomanufactureandtouse?
Isiteasilymaintainedandadjusted?
Canaprofitbemadefromitssaleoruse?
Communicatingthedesigntoothersisthefinal,vitalstepinthedesignprocess.Undoubtedlymanygreatdesigns,inventions,andcreativeworkshavebeenlosttomankindsimplybecausetheoriginatorswereunableorunwillingtoexplaintheiraccomplishmentstoothers.Presentationisasellingjob.Theengineer,whenpresentinganewsolutiontoadministrative,management,orsupervisorypersons,isattemptingtosellortoprovetothemthatthissolutionisabetterone.Unlessthiscanbedonesuccessfully,thetimeandeffortspentonobtainingthesolutionhavebeenlargelywasted.
Basically,thereareonlythreemeansofcommunicationavailabletous.Therearethewritten,theoral,andthegraphicalforms.Thereforethesuccessfulengineerwillbetechnicallycompetentandversatileinallthreeformsofcommunication.Atechnicallycompetentpersonwholacksabilityinanyoneoftheseformsisseverelyhandicapped.Ifabilityinallthreeformsislacking,noonewilleverknowhowcompetentthatpersonis!
Thecompetentengineershouldnotbeafraidofthepossibilityofnotsucceedinginapresentation.Infact,occasionalfailureshouldbeexpectedbecausefailureorcriticismseemstoaccompanyeveryreallycreativeidea.Thereisagreattobelearnedfromafailure,andthegreatestgainsareobtainedbythosewillingtoriskdefeat.Inthefindanalysis,therealfailurewouldlieindecidingnottomakethepresentationatall.
IntroductiontoMachineDesign
Machinedesignistheapplicationofscienceandtechnologytodeviseneworimprovedproductsforthepurposeofsatisfyinghumanneeds.Itisavastfieldofengineeringtechnologywhichnotonlyconcernsitselfwiththeoriginalconceptionoftheproductintermsofitssize,shapeandconstructiondetails,butalsoconsidersthevariousfactorsinvolvedinthemanufacture,marketinganduseoftheproduct.
Peoplewhoperformthevariousfunctionsofmachinedesignaretypicallycalleddesigners,ordesignengineers.Machinedesignisbasicallyacreativeactivity.However,inadditiontobeinginnovative,adesignengineermustalsohaveasolidbackgroundintheareasofmechanicaldrawing,kinematics,dynamics,materialsengineering,strengthofmaterialsandmanufacturingprocesses.
Asstatedpreviously,thepurposeofmachinedesignistoproduceaproductwhichwillserveaneedforman.Inventions,discoveriesandscientificknowledgebythemselvesdonotnecessarilybenefitpeople;onlyiftheyareincorporatedintoadesignedproductwillabenefitbederived.Itshouldberecognized,therefore,thatahumanneedmustbeidentifiedbeforeaparticularproductisdesigned.
Machinedesignshouldbeconsideredtobeanopportunitytouseinnovativetalentstoenvisionadesignofaproductistobemanufactured.Itisimportanttounderstandthefundamentalsofengineeringratherthanmemorizemerefactsandequations.Therearenofactsorequationswhichalonecanbeusedtoprovideallthecorrectdecisionstoproduceagooddesign.Ontheotherhand,anycalculationsmademustbedonewiththeutmostcareandprecision.Forexample,ifadecimalpointismisplaced,anotherwiseacceptabledesignmaynotfunction.
Gooddesignsrequiretryingnewideasandbeingwillingtotakeacertainamountofrisk,knowingthatisthenewideadoesnotworktheexistingmethodcanbereinstated.Thusadesignermusthavepatience,sincethereisnoassuranceofsuccessforthetimeandeffortexpended.Creatingacompletelynewdesigngenerallyrequiresthatmanyoldandwell-establishedmethodsbethrustaside.Thisisnoteasysincemanypeopleclingtofamiliarideas,techniquesandattitudes.Adesignengineershouldconstantlysearchforwaystoimproveanexistingproductandmustdecidewhatold,provenconceptsshouldbeusedandwhatnew,untriedideasshouldbeincorporated.
Newdesignsgenerallyhave“bugs”orunforeseenproblemswhichmustbeworkedoutbeforethesuperiorcharacteristicsofthenewdesignscanbeenjoyed.Thusthereisachanceforasuperiorproduct,butonlyathigherrisk.Itshouldbeemphasizedthatifadesigndoesnotwarrantradicalnewmethods,suchmethodsshouldnotbeappliedmerelyforthesakeofchange.
Duringthebeginningstagesofdesign,creativityshouldbeallowedtoflourishwithoutagreatnumberofconstraints.Eventhoughmanyimpracticalideasmayarise,itisusuallyeasytoeliminatethemintheearlystagesofdesignbeforefirmdetailsarerequiredbymanufacturing.Inthisway,innovativeideasarenotinhibited.Quiteoften,morethanonedesignisdeveloped,uptothepointwheretheycanbecomparedagainsteachother.Itisentirelypossiblethatthedesignwhichultimatelyacceptedwilluseideasexistinginoneoftherejecteddesignsthatdidnotshowasmuchoverallpromise.
Psychologistsfrequentlytalkabouttryingtofitpeopletothemachinestheyoperate.Itisessentiallytheresponsibilityofthedesignengineertostrivetofitmachinestopeople.Thisisnotaneasytask,sincethereisreallynoaveragepersonforwhichcertainoperatingdimensionsandproceduresareoptimum.
Anotherimportantpointwhichshouldberecognizedisthatadesignengineermustbeabletocommunicateideastootherpeopleiftheyaretobeincorporated.Initiallythedesignermustcommunicateapreliminarydesigntogetmanagementapproval.Thisisusuallydonebyverbaldiscussionsinconjunctionwithdrawinglayoutsandwrittenmaterial.Tocommunicateeffectively,thefollowingquestionsmustbeanswered:
(1)Doesthedesignreallyserveahumanneed?
(2)Willitbecompetitivewithexistingproductsofrivalcompanies?
(3)Isite
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