精品中文翻译机械元件的设计定.docx

精品中文翻译机械元件的设计定.docx

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精品中文翻译机械元件的设计定

DesignofMachineElementsandnoiseControl

Thisarticle,asthetitleindicates,willnotdealwiththebroaderaspectsofthedesignofcompletemachines,butwillattempttoexplainthefundamentalprinciplesrequiredforthecorrectdesignoftheseparateelementswhichcomposethemachine.

Theprinciplesofdesignareofcourseuniversal.Thesametheoryorequationsmaybeappliedtoaverysmallpart,asinaninstrument,ortoalargerbutsimilarpartusedinapieceofheavyequipment.Innocase,however,shouldmathematicalcalculationsbelookeduponasabsoluteandfinal.Theyareallsubjecttotheaccuracyofthevariousassumptionswhichmustnecessarilybemadeinengineeringwork.Sometimesonlyaportionofthetotalnumberofpartsinamachinearedesignedonthebasisofanalyticcalculations.Theformandsizeoftheremainingpartsarethenusuallydeterminedbypracticalconsiderations.Ontheotherhand,ifthemachineisveryexpensive,orifweightisafactor,asinairplanes,designcomputationsmaythenbemadeforalmostalltheparts.

Thepurposeofthedesigncalculationisofcoursetoattempttopredictthestressordeformationinthepartinorderthatinmaysafelycarrytheloadswhichwillbeimposeduponit,andthatitmaylastfortheexpectedlifeofthemachine.Allcalculationsare,ofcourse,dependentonthephysicalpropertiesoftheconstructionmaterialsasdeterminedbylaboratorytests.Arationalmethodofdesignattemptstotaketheresultsofrelativelysimpleandfundamentaltestsandapplythemtoallthecomplicatedandinvolvedsituationsencounteredinpresent-daymachinery.

Inaddictin,ithasbeenamplyprovedthatsuchdetailsassurfacecondition,manufacturingtolerances,andheattreatmenthaveamarkedeffectonthestrengthandusefullifeofamachinepart.Thedesignanddraftingdepartmentsmustspecifycompletelyallsuchparticulars,andthusexercisethenecessaryclosecontroloverthefinishedproduct.

Traininginrapidandaccuratenumericalworkisinvaluabletothedesigner.Thedesignershouldkeepanaccuratenotebook,asitisfrequentlynecessaryforhimtorefertoworkwhichhehasdoneinthepast.Asketch,carefullydrawntoscale,isalsoanecessity,andprovidesaconvenientplaceforputtingdownaportionofthedatausedinconnectionwiththeproblem.Itgoeswithoutsayingthatalldata,assumptions,equations,andcalculationsshouldbewrittendowninfullinordertobeintelligiblewhenreferredtoatalaterdate.

InfluenceofMachineToolonMachiningAccuracy

Amachinetoolhasthefollowingfunctionsinthemachiningprocess:

Clampingofthetoolandworkpieceandestablishingtheircorrectrelativeposition.

Supplyingtheworkingmotions（primaryandauxiliarymotion）　andmaintainingthecorrectrelativepositionoftheworkpieceandtoolduringthemachiningoperation.

Supplyingtheenergyforcuttingorerosionwork.

Machiningaccuracyisevaluatedbyconsideringthedifferencebetweenthedesired（nominal）dimensionsandpropertiesandthoseofthefinishedproduct.Thesmallerarethemachiningerrors,thegreateristheaccuracy.

Themachinetoolinfluencesmachiningerrorsby:

Errorsinworkpiecepositioninginrelationtothetool.Thismaybecausedbyerrorsintheproductionandassemblyofmachinetoolpartsandassembliesandbyerrorsinthepositioningandclampingoftheworkpieceandtoolrelativetothemachinetool.Itiscalledinshortthegeometricinfluenceonmachiningaccuracy；

Errorsintheworkingmotionsofthetoolandworkpieceduringthemachiningprocess,orkinematicinfluenceonmachiningaccuracy;

Errorsinthedisplacementofmachinetoolpartsandassemblies,workpieceandtoolasaresultofforcesappearinginthemachiningprocess,calleddynamicinfluenceonmachiningaccuracy.

Errorsinthedisplacementofmachinetoolpartsandassemblies,workpieceandtoolasaresultofthermalexpansioncausebyheatgeneratedinthemachiningprocessandasaresultoffrictioninthedrivingmechanisms.Thisiscalledthermalinfluenceonmachiningaccuracy.

Geometricerrorsonamachinetoolshouldwhereverpossiblebemeasureddirectly.Theirevaluationdependsontheabsolutemagnitudeoftheerrorandonthesenseofdeviationfromnominaldimensions.Errorswhosesenseindicatesthat,asthewearsetsin,theywilldecrease,areconsideredtobelessharmful.

Theimportanceofgeometricerrorsfortotalmachiningaccuracyisrelativelygreat.Thiscanbedemonstratedonthebasisofastatisticalcomparisonoferrorsmeasuredonfinishedproductswitherrorscalculatedfromgeometricdeviationsonly.Somestatisticalreferencedenotestheshareofgeometricerrorsintotalmachiningerrorsvarieswithinarangeof50%～75%.Thusaconclusioncanbedrawnthatgeometricerrorsinmachinetoolsshouldbereducedasfaraspossible,butstillwithinlimitsofaneconomicallyjustifiedaccuracy.

Inthecaseofmachinetoolswithcomplexworkingmotions,thekinematicaccuracyofthemachinetoolisofgreatimportance.Thiscanbeobservedwhenmachininggears,threadsandothercurvedsurfaces.

Thekinematicaccuracyofamachinetoolresultsfromtheaccuracyofcouplingofitsinternalkinematicchains.

Akinematicerrorisdefinedbydeviationsofthefinalandinitiallinkofakinematicchainfromnominalpositionsordisplacements.

Kinematicerrorsmaybecalculatedormeasuredbymeansofspecialinstrumentscalledkinematicmeters.

Checkingofkinematicerrorshasnotyetbecomeawidespreadpracticeandtherarenostandardsestablishingtheirpermissiblevalues,justastherearenostandardsconcerninggeometricerrors.Oneofthemainreasonswhythemeasurementofkinematicaccuracyhasnotspreadisthatrelativelycomplicatedandexpensiveequipmentisnecessary,togetherwithtime-consumingoperations.

Thenoisecontrolproblemcanbebrokendownintothreesegmentsincludingthedesignconcept,thedesignfixandtheacousticfix.Thelattertowfixeswillprobablyoverlapinmostcasesandinteractwitheachotherwhilethedesignconceptwillsettheconstraintsfortheoverallproblem.Thedefinitionoftheproblemisusuallygeneralinmatureandarisesmostoftenfromacomplaintofsomeoneorsomegroupsegmentofthepopulation.

Identifyingthesourcesdoesnotusuallyrequiresoundmeasurementsatthisjuncturealthoughthosemeasurementswillberequiredpriortoattemptingthoseproceduresindicatedinthe“DesignFix”column.

Ofgreatimportanceinthedesignconceptstageisthecorrectidentificationofthepopulationaffectedbythesource,thatis,thereceiveandthepathofpropagationtothem.Havingidentifiedthepeopleaffectedbythenoisesource,itisnecessarytoexaminetheacceptablenoiselimitationstobeimposeduponoracceptedbythoseindividuals.Aretheselimitsthosewhichareestablishedorimposedbyexistingcriteriaeitherlegalorotherwiseoraretherepeculiarconditionswhichmakesthisproblemunique,forexample,isthereaconditionedpopulationinagenerallyhighnoiseenvironmentsuchasthatwhichexistsinmajorurbancenters?

Priortoadesignfixandaspartofthedesignconcept,therestraintstothedesignmustbeexamined.Aretherelegallimitations?

IstheproblemsuchthatthemaximumnoiseexposureforestablishedtimeperiodshasbeenpromulgatedbysuchlawsastheFederalOccupationalSafetyandHealthAct?

IsthemaximumsourcelevelspecifiedbyafederalagencysuchastheaircraftnoiseemissionlimitsestablishedbytheFederalAviation?

Theoperationandmaintenancerequirementsmustbecriticallyreviewed.Constraintsimposedbyrequiredaccesstointeriorparts,say,tosafetyswitchesforeitheroperationormaintenance,mustbeviewedatthedesignstage,andparticularlyifthedesigninvolvestheattemptednoisereductionlfanexistingpieceofmachinery.Consultationwithexperiencedoperatorsatthisstagecansavemanypotentialredesignrequirementsafterthedesignisfixed.

Wemustaddressthesocialaspectslftheproblem.Theestheticslfthefinaldesignshouldbeconsideredhere.Willthefinaldesignenhancethevisualparametersoftheproblem?

Willtheresultlftheacousticfixbeamoreappealingproduct?

Giantnoisecontroldevicesareoftenfacedbyengineersinvolvedinthedesignofpowerplantfacilitiestobelocatedclosetotheircustomersininhabitedareas.Theymustdecide,forexamplebetweentheforceddraftcoolingtowerwithitslargeareabutlowheightprofileandtheinherentlyquieternaturaldraftcoolingsystemwhichrequiresastructureusuallymorethan50000fthigh.Thesocialimplicationsof“nosolution"shouldbeexaminedasagainstatotalofpartialreductionlfthesourcenoise.Tosomeextenttheno-solutionconceptwillhaveabearingontheeconomicconstraintsimposedonthedesignconcept.

Veryearlyintheapproachtoasolutiontoaproblemofcontrollinganoisesource,theeconomicconstraintsmustbespelledout.Noisecontroldevicescanoftenbeveryexpensive.Lowfrequencyattenuationismorethanthetreatmentofhighfrequencysounds.Thiscostmustbeweighedagainstbenefitstobeobtained.Noisecontroldollarsareoftenconsiderednonproductivedollarsatfirstglancebycontrastwith,forinstance,heatrecoverydeviceswhichnotonlysaveenergybutpayforthemselvesinfuelcosts.However,whenthesedollarsareweighedagainstpotentialsavingsincompensatoryawardsforhearinglossorasthepriceofagoodpublicrelationshipwithneighbors,theytakeonalessfrighteningaspect.Whenacompanywithaseverenoiseproblemwhichishazardoustotheiremployeesisfacedwitha$500perdiemfineorathreat