机械专业毕业设计外文翻译振动的定义和术语.docx

机械专业毕业设计外文翻译振动的定义和术语.docx

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机械专业毕业设计外文翻译振动的定义和术语

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.Whenresonanceoccursinrotatingshaftsandspindles,thespeedofrotationisknownasthecriticalspeed.Hence,thepredictionandcorrectionoravoidance3ofaresonantconditioninmechanismsisofvitalimportancesince,intheabsenceofdampingorotheramplitude-limitingdevices,resonanceistheconditionatwhichasystemgivesaninfiniteresponsetoafiniteexcitation.

DampingDampingisthedissipationofenergyfromavibratingsystem,andthuspreventsexcessiveresponse.Itisobservedthatanaturalvibrationdiminishesinamplitudewithtimeand,hence,eventuallyceasesowingtosomerestrainingordampinginfluence.Thusifavibrationistobesustained,theenergydissipatedbydampingmustbereplacedfromanexternalsource.

Thedissipationisrelatedinsomewaytotherelativemotionbetweenthecomponentsorelementsofthesystem,andiscausedbyfrictionalresistanceofsomesort,e.g.instructures,internalfrictioninmaterial,andexternalfrictioncausedbyairorfluidresistancecalled“viscous”dampingifthedragforceisassumedproportionaltotherelativevelocitybetweenmovingparts.Onedeviceassumedtogiveviscousdampingisthe“dashpot”whichisalooselyfittingpistoninacylindersothatfluidcanflowfromonesideofthepistontotheotherthroughtheannularclearancespace.Adashpotcannotstoreenergybutcanonlydissipateit.

BasicMachiningOperationsandMachineTools

BasicMachiningOperations

Machinetoolshaveevolvedfromtheearlyfoot-poweredlathesoftheEgyptiansandJohnWilkinson’sboringmill.Theyaredesignedtoproviderigidsupportforboththeworkpieceandthecuttingtoolandcanpreciselycontroltheirrelativepositionsandthevelocityofthetoolwithrespecttotheworkpiece.Basically,inmetalcutting,asharpenedwedge-shapedtoolremovesarathernarrowstripofmetalfromthesurfaceofaductileworkpieceintheformofaseverelydeformedchip.Thechipisawasteproductthatiscomsiderablyshorterthantheworkpiecefromwhichitcamebutwothacorrespondingincreaseinthicknessoftheuncutchip.Thegeometricalshapeofthemachinesurfacedepednsontheshapeofthetoolanditspathduringthemachinigoperation.

Mostmachiningoperationsproducepartsofdifferinggeometry.Ifaroughcylindricalworkpiecerevolvesaboutacentralaxisandthetoolpenetratesbeneathitssurfaceandtravelsparalleltothecenterofrotation,asurfaceofrevolutionisproducedandtheoperationiscalledturning.Ifahollowtubeismachinedontheinsideinasimilarmanner,theoperationiscalledboring.Producinganexternalconicalsurfaceofuniformlyvaryingdiameteriscalledtaperturning.Ifthetoolpointtravelsinapathofvaryingradius,acontouredsurfacelikethatofabowlingpinacanbeproduced;or,ifthepieceisshortenoughandthesupportissufficientlyrigid,acontouredsurfacecouldbeproducedbyfeedingashapedtoolnormaltotheaxisofrotation.Shorttaperedorcylindricalsurfacescouldalsobecontourformed.

Flatorplanesurfacesarefrequentlyrequired.Thecanbegeneratedbyadialturningorfacing,inwhichthetoolpointmovesnormaltotheaxisofrotation.Inothercases,itismoreconvenienttoholdtheworkpiecesteadyandreciprocatethetoolacrossitinaseriesofstraight-linecutswithacrosswisefeedincrementbeforeeachcuttingstroke.Thisoperationiscalledplaningandiscarriedoutonashaper.Forlargerpiecesitiseasiertokeepthetoolstationaryanddrawtheworkpieceunderitasinplaning.Thetoolisfedateachreciprocation.Contouredsurfacescanbeproducedbyusingshapedtools.

Multiple-edgedtoolscanalsobeused.Drillingusesatwin-edgedflutedtoolforholeswithdepthsupto510timesthedrilldiameter.Whetherthedrilturnsortheworkpiecerotates,relativemotionbetweenthecuttingedgeandtheworkpieceistheimportantfactor.Inmillingoperationsarotarycutterwithanumberofcuttingedgesengagestheworkpiecemwhichmovesslowlywithrespecttothecutter.Planeorcontouredsurfacesmaybeproduced,dependingonthegeometryofthecutterandthetypeoffeed.Horizontalorverticalaxesofrotationmabeused,andthefeedoftheworkpiecemaybeinanyofthethreecoordinatedirections.

BasicMachineTools

Machinetoolsareusedtoproduceapartofaspecifiedgeometricalshapeandprecisesizebyremovingmetalfromaductilematerilaintheformofchips.Thelatterareawasteproductandvaryfromlongcontinuousribbonsofaductilematerialsuchassteel,whichareundesirablefromadisposalpointofview,toeasilyhandledwell-brokenchipsresultingfromcastiron.Machinetoolsperformfivebasicmetal-removalprocesses:

turning,planing,drilling,milling,andfrinding.Allothermetal-removalprocessesaremodificationsofthesefivebasicprocesses.Forexample,boringisinternalturning;