机械外文翻译中英文Word格式文档下载.doc

机械外文翻译中英文Word格式文档下载.doc

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翻译：

@#@@#@英文原文@#@DefinitionsandTerminologyofVibration@#@vibration@#@Allmatter-solid,liquidandgaseous-iscapableofvibration,e.g.vibrationofgasesoccursintailductsofjetenginescausingtroublesomenoiseandsometimesfatiguecracksinthemetal.Vibrationinliquidsisalmostalwayslongitudinalandcancauselargeforcesbecauseofthelowcompressibilityofliquids,e.g.popesconveyingwatercanbesubjectedtohighinertiaforces（or“waterhammer”）whenavalveortapissuddenlyclosed.Excitationforcescaused,saybychangesinflowoffluidsorout-of-balancerotatingorreciprocatingparts,canoftenbereducedbyattentiontodesignandmanufacturingdetails.Atypicalmachinehasmanymovingparts,eachofwhichisapotentialsourceofvibrationorshock-excitation.Designersfacetheproblemofcompromisingbetweenanacceptableamountofvibrationandnoise,andcostsinvolvedinreducingexcitation.@#@Themechanicalvibrationsdealtwithareeitherexcitedbysteadyharmonicforces（i.e.obeyingsineandcosinelawsincasesofforcedvibrations）or,afteraninitialdisturbance,bynoexternalforceapartfromgravitationalforcecalledweight（i.e.incasesofnaturalorfreevibrations）.Harmonicvibrationsaresaidtobe“simple”ifthereisonlyonefrequencyasrepresenteddiagrammaticallybyasineorcosinewaveofdisplacementagainsttime.@#@Vibrationofabodyormaterialisperiodicchangeinpositionordisplacementfromastaticequilibriumposition.Associatedwithvibrationaretheinterrelatedphysicalquantitiesofacceleration,velocityanddisplacement-e.g.anunbalancedforcecausesacceleration（a=F/m）inasystemwhich,byresisting,inducesvibrationasaresponse.Weshallseethatvibratoryoroscillatorymotionmaybeclassifiedbroadlyas（a）transient;@#@（b）continuingorsteady-state;@#@and（c）random.@#@TransientVibrationsdieawayandareusuallyassociatedwithirregulardisturbances,e.g.shockorimpactforces,rollingloadsoverbridges,carsdrivenoverpotholes-i.e.forceswhichdonotrepeatatregularintervals.Althoughtransientsaretemporarycomponentsofvibrationalmotion,theycancauselargeamplitudesinitiallyandconsequenthighstressbut,inmanycases,theyareofshortdurationandcanbeignoredleavingonlysteady-statevibrationstobeconsidered.@#@Steady-StateVibrationsareoftenassociatedwiththecontinuousoperationofmachineryand,althoughperiodic,arenotnecessarilyharmonicorsinusoidal.Sincevibrationsrequireenergytoproducethem,theyreducetheefficiencyofmachinesandmechanismsbecauseofdissipationofenergy,e.g.byfrictionandconsequentheat-transfertosurroundings,soundwavesandnoise,stresswavesthroughframesandfoundations,etc.Thus,steady-statevibrationsalwaysrequireacontinuousenergyinputtomaintainthem.@#@RandomVibrationisthetermusedforvibrationwhichisnotperiodic,i.e.hasnomadeclear-severalofwhichareprobablyknowntosciencestudentsalready.@#@Period,Cycle,FrequencyandAmplitudeAsteady-statemechanicalvibrationisthemotionofasystemrepeatedafteranintervaloftimeknownastheperiod.Themotioncompletedinanyoneperiodoftimeiscalledacycle.Thenumberofcyclesperunitoftimeiscalledthefrequency.Themaximumdisplacementofanypartofthesystemfromitsstatic-equilibriumpositionistheamplitudeofthevibrationofthatpart-thetotaltravelbeingtwicetheamplitude.Thus,“amplitude”isnotsynonymouswith“displacement”butisthemaximumvalueofthedisplacementfromthestatic-equilibriumposition.@#@NaturalandForcedVibrationAnaturalvibrationoccurswithoutanyexternalforceexceptgravity,andnormallyariseswhenanelasticsystemisdisplacedfromapositionofstableequilibriumandreleased,i.e.naturalvibrationoccursundertheactionofrestoringforcesinherentinanelasticsystem,andnaturalfrequencyisapropertyofhesystem.@#@Aforcedvibrationtakesplaceundertheexcitationofanexternalforce（orexternallyappliedoscillatorydisturbance）whichisusuallyafunctionoftime,e.g.inunbalancedrotatingparts,imperfectionsinmanufactureofgearsanddrives.Thefrequencyofforcedvibrationisthatoftheexcitingorimpressedforce,i.e.theforcingfrequencyisanarbitraryquantityindependentofthenaturalfrequencyofthesystem.@#@ResonanceResonancedescribestheconditionofmaximumamplitude.Itoccurswhenthefrequencyofanimpressedforcecoincideswith,orisneartoanaturalfrequencyofthesystem.Inthiscriticalcondition,dangerouslylargeamplitudesandstressesmayoccurinmechanicalsystemsbut,electrically,radioandtelevisionreceiversaredesignedtorespondtoresonantfrequencies.Thecalculationorestimationofnaturalfrequenciesis,therefore,ofgreatimportanceinalltypesofvibratingandoscillatingsystems.Whenresonance