机械外文翻译机械设计.docx

机械外文翻译机械设计.docx

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机械外文翻译机械设计

附录一

Machinerydesigniseithertoformulateanengineeringplanforthesatisfactionofaspecifiedneedortosolveanengineeringproblem.Itinvolvesarangeofdisciplinesinmaterials,mechanics,heat,flow,control,electronicsandproduction.

Machinerydesignmaybesimpleorenormouslycomplex,easyordifficult,mathematicalornonmathematical,itmayinvolveatrivialproblemoroneofgreatimportance.Gooddesignistheorderlyandinterestingarrangementofanideatoprovidecertainresultsoreffects.Awell-designedproductisfunctional,efficient,anddependable.Suchaproductislessexpensivethanasimilarpoorlydesignedproductthatdoesnotfunctionproperlyandmustconstantlyberepaired.

Peoplewhoperformthevariousfunctionsofmachinerydesignaretypicallycalledindustrialdesigners.Heorshemustfirstcarefullydefinetheproblem,usinganengineeringapproach,toensurethatanyproposedsolutionwillsolvetherightproblem.[2]Itisimportantthatthedesignerbeginsbyidentifyingexactlyhowheorshewillrecognizeasatisfactoryalternative,andhowtodistinguishbetweentwosatisfactoryalternativesinordertoidentifythebetter.Soindustrialdesignersmusthavecreativeimagination,knowledgeofengineering,productiontechniques,tools,machines,andmaterialstodesignanewproductformanufacture,ortoimproveanexistingproduct.

Inthemodernindustrializedworld,thewealthandlivingstandardsofanationarecloselylinkedwiththeircapabilitiestodesignandmanufactureengineeringproducts.Itcanbeclaimedthattheadvancementofmachinerydesignandmanufacturingcanremarkablypromotetheoveralllevelofacountry’sindustrization.Ourcountryisplayingamoreandmorevitalroleintheglobalmanufacturingindustry.Toacceleratesuchanindustrializingprocess,highlyskilleddesignengineershavingextensiveknowledgeandexpertiseareneeded

Themajorpartofamachineisthemechanicalsystem.[3]Andthemechanicalsystemisdecomposedintomechanisms,whichcanbefurtherdecomposedintomechanicalcomponents.Inthissense,themechanicalcomponentsarethefundamentalelementsofmachinery.Onthewhole,mechanicalcomponentscanbeclassifiedasuniversalandspecialcomponents.Bolts,gear,andchainsarethetypicalexamplesoftheuniversalcomponents,whichcanbeusedextensivelyindifferentmachinesacrossvariousindustrialsectors.Turbineblades,crankshaftandaircraftpropelleraretheexamplesofthespecialcomponents,whicharedesignedforsomespecificpurposes.

Sometimes,designbeginswhenadesignerrecognizesaneedanddecidestodosomethingaboutit.Theneedisoftennotevidentatall;recognitionisusuallytriggeredbyaparticularadversecircumstanceorasetofrandomcircumstances,whicharisealmostsimultaneously.Identificationofneedusuallyconsistsofanundefinedandvagueproblemstatement.

Definitionofproblemisnecessarytofullydefineandunderstandtheproblem,afterwhichitispossibletorestatethegoalinamorereasonableandrealisticwaythantheoriginalproblemstatement.Definitionoftheproblemmustincludeallthespecificationsforthethingthatistobedesigned.Obviousitemsinthespecificationsarethespeeds,feeds,temperaturelimitations,maximumrange,expectedvariationinthevariables,anddimensionalandweightlimitations.

Thesynthesisisoneinwhichasmanyalternativepossibledesignapproachesaresought,usuallywithoutregardfortheirvalueorquality.Thisisalsosometimescalledtheideationandinventionstepinwhichthelargestpossiblenumberofcreativesolutionsisgenerated.Thesynthesisactivityincludesthespecificationofmaterial,additionofgeometricfeatures,andinclusionofgreaterdimensionaldetailtotheaggregatedesign.Analysisisamethodofdeterminingordescribingthenatureofsomethingbyseparatingitintoitsparts.Intheprocesstheelements,ornatureofthedesign,areanalyzedtodeterminethefitbetweentheproposeddesignandtheoriginaldesigngoals.

Evaluationisthefinalproofofasuccessfuldesignandusuallyinvolvesthetestingofaprototypeinthelaboratory.Herewewishtodiscoverifthedesignreallysatisfiestheneeds.Theabovedescriptionmaygiveanerroneousimpressionthatthisprocesscanbeaccomplishedinalinearfashionaslisted.Onthecontrary,iterationisrequiredwithintheentireprocess,movingfromanystepbacktoanypreviousstep,inallpossiblecombinations,anddoingthisrepeatedly.

Communicatingthedesigntoothersisthefinial,vitalpresentationstepinthedesignprocess.Basically,thereareonlythreemeansofcommunication.Thesearethewritten,theoral,andthegraphicalforms.Asuccessfulengineerwillbetechnicallycompetentandversatileinallthreeformsofcommunication.Thecompetentengineershouldnotbeafraidofthepossibilityofnotsucceedinginapresentation.Infact,thegreatestgainsareobtainedbythosewillingtoriskdefeat.

Itisabsolutelyessentialthatadesignengineerknowhowandwhypartsfailsothatreliablemachinesthatrequireminimummaintenancecanbedesigned．Sometimesafailurecanbeserious，suchaswhenatireblowsoutonanautomobiletravelingathighspeed．Ontheotherhand，afailuremaybenomorethananuisance．Anexampleisthelooseningoftheradiatorhoseinanautomobilecoolingsystem．Theconsequenceofthislatterfailureisusuallythelossofsomeradiatorcoolant，aconditionthatisreadilydetectedandcorrected．

Thetypeofloadapartabsorbsisjustassignificantasthemagnitude．Generallyspeaking，dynamicloadswithdirectionreversalscausegreaterdifficultythanstaticloads，andtherefore，fatiguestrengthmustbeconsidered．Anotherconcerniswhetherthematerialisductileorbrittle．Forexample，brittlematerialsareconsideredtobeunacceptablewherefatigueisinvolved．

Manypeoplemistakinglyinterpretthewordfailuretomeantheactualbreakageofapart．However，adesignengineermustconsiderabroaderunderstandingofwhatappreciabledeformationoccurs．Aductilematerial，howeverwilldeformalargeamountpriortorupture．Excessivedeformation，withoutfracture，maycauseamachinetofailbecausethedeformedpartinterfereswithamovingsecondpart．Therefore，apartfails（evenifithasnotphysicallybroken）wheneveritnolongerfulfillsitsrequiredfunction．Sometimesfailuremaybeduetoabnormalfrictionorvibrationbetweentwomatingparts．Failurealsomaybeduetoaphenomenoncalledcreep，whichistheplasticflowofamaterialunderloadatelevatedtemperatures．Inaddition，theactualshapeofapartmayberesponsibleforfailure．Forexample，stressconcentrationsduetosuddenchangesincontourmustbetakenintoaccount．Evaluationofstressconsiderationsisespeciallyimportantwhentherearedynamicloadswithdirectionreversalsandthematerialisnotveryductile．

Ingeneral，thedesignengineermustconsiderallpossiblemodesoffailure，whichincludethefollowing．

Thepartsizesandshapesselectedalsomusttakeintoaccountmanydimensionalfactorsthatproduceexternalloadeffects，suchasgeometricdiscontinuities，residualstressesduetoformingofdesiredcontours，andtheapplicationofinterferencefitjoints．

Camsareamongthemostversatilemechanismsavailable．Acamisasimpletwo-memberdevice．Theinputmemberisthecamitself，whiletheoutputmemberiscalledthefollower．Throughtheuseofcams，asimpleinputmotioncanbemodifiedintoalmostanyconceivableoutputmotionthatisdesired．Someofthecommonapplicationsofcamsare

——Camshaftanddistributorshaftofautomotiveengine

——Productionmachinetools

——Automaticrecordplayers

——Printingmachines

——Automaticwashingmachines

——Automaticdishwashers

Thecontourofhigh-speedcams（camspeedinexcessof1000rpm）mustbedeterminedmathematically．However，thevastmajorityofcamsoperateatlowspeeds（lessthan500rpm）ormedium-speedcamscanbedeterminedgraphicallyusingalarge-scalelayout．Ingeneral，thegreaterthecamspeedandoutputload，thegreatermustbetheprecisionwithwhichthecamcontourismachined．

Thefollowingdesignpropertiesofmaterialsaredefinedastheyrelatetothetensiletest．

StaticStrength．Thestrengthofapartisthemaximumstressthatthepartcansustainwithoutlosingitsabilitytoperformitsrequiredfunction．Thusthestaticstrengthmaybeconsideredtobeapproximatelyequaltotheproportionallimit，sincenoplasticdeformationtakesplaceandnodamagetheoreticallyisdonetothematerial．

Stiffness．Stiffnessisthedeformation-resistingpropertyofamaterial．Theslopeofthemoduluslineand，hence，themodulusofelasticityaremeasuresofthestiffnessofamaterial．

Resilience．Resilienceisthepropertyofamaterialthatpermitsittoabsorbenergywithoutpermanentdeformation．Theamountofenergyabsorbedisrepresentedbytheareaunderneaththestress-straindiagramwithintheelasticregion．

Toughness．Resilienceandtoughnessaresimilarproperties．However，toughnessistheabilitytoabsorbenergywithoutrupture．Thustoughnessisrepresentedbythetotalareaunderneaththestress-straindiagram，asdepictedinFigure2．8b．Obviously，thetoughnessandresilienceofbrittlematerialsareverylowandareapproximatelyequal．

Brittleness．Abrittlematerialisonethatrupturesbeforeanyappreciableplasticdeformationtakesplace．Brittlematerialsaregenerallyconsideredundesirableformachinecomponentsbecausetheyareunabletoyieldlocallyatlocationsofhighstressbecauseofgeometricstressraiserssuchasshoulders，holes，notches，orkeyways．

Ductility．Aductilitymaterialexhibitsalargeamountofplasticdeformationpriortorupture．Ductilityismeasured