Chapter50-VibrationVIBRATIONMichaelJ.GriffinVibrationisoscillatorymotion.Thischaptersummarizeshumanresponsestowhole-bodyvibration,hand-transmittedvibrationandthecausesofmotionsickness.Whole-bodyvibrationoccurswhenthebodyissupportedonasurfacewhichisvibrating(e.g.,whensittingonaseatwhichvibrates,standingonavibratingfloororrecumbentonavibratingsurface).Whole-bodyvibrationoccursinallformsoftransportandwhenworkingnearsomeindustrialmachinery.Hand-transmittedvibrationisthevibrationthatentersthebodythroughthehands.Itiscausedbyvariousprocessesinindustry,agriculture,miningandconstructionwherevibratingtoolsorworkpiecesaregraspedorpushedbythehandsorfingers.Exposuretohand-transmittedvibrationcanleadtothedevelopmentofseveraldisorders.Motionsicknesscanbecausedbylowfrequencyoscillationofthebody,sometypesofrotationofthebodyandmovementofdisplaysrelativetothebody.MagnitudeOscillatorydisplacementsofanobjectinvolvealternatelyavelocityinonedirectionandthenavelocityintheoppositedirection.Thischangeofvelocitymeansthattheobjectisconstantlyaccelerating,firstinonedirectionandthenintheoppositedirection.Themagnitudeofavibrationcanbequantifiedbyitsdisplacement,itsvelocityoritsacceleration.Forpracticalconvenience,theaccelerationisusuallymeasuredwithaccelerometers.Theunitsofaccelerationaremetrespersecondpersecond(m/s2).TheaccelerationduetotheEarth’sgravityisapproximately9.81m/s2.Themagnitudeofanoscillationcanbeexpressedasthedistancebetweentheextremitiesreachedbythemotion(thepeak-to-peakvalue)orthedistancefromsomecentralpointtothemaximumdeviation(thepeakvalue).Often,themagnitudeofvibrationisexpressedintermsofanaveragemeasureoftheaccelerationoftheoscillatorymotion,usuallytheroot-mean-squarevalue(m/s2r.m.s.).Forasinglefrequency(sinusoidal)motion,ther.m.s.valueisthepeakvaluedividedby.Forasinusoidalmotiontheacceleration,a(inm/s2),canbecalculatedfromthefrequency,f(incyclespersecond),andthedisplacement,d(inmetres):a=(2πf)2dThisexpressionmaybeusedtoconvertaccelerationmeasurementstodisplacements,butitisonlyaccuratewhenthemotionoccursatasinglefrequency.Logarithmicscalesforquantifyingvibrationmagnitudesindecibelsaresometimesused.WhenusingthereferencelevelinInternationalStandard1683,theaccelerationlevel,La,isexpressedbyLa=20log10(a/a0),whereaisthemeasuredacceleration(inm/s2r.m.s.)anda0isthereferencelevelof10-6m/s2.Otherreferencelevelsareusedinsomecountries.FrequencyThefrequencyofvibration,whichisexpressedincyclespersecond(hertz,Hz),affectstheextenttowhichvibrationistransmittedtothebody(e.g.,tothesurfaceofaseatorthehandleofavibratorytool),theextenttowhichitistransmittedthroughthebody(e.g.,fromtheseattothehead),andtheeffectofvibrationinthebody.Therelationbetweenthedisplacementandtheaccelerationofamotionarealsodependentonthefrequencyofoscillation:adisplacementofonemillimetrecorrespondstoaverylowaccelerationatlowfrequenciesbutaveryhighaccelerationathighfrequencies;thevibrationdisplacementvisibletothehumaneyedoesnotprovideagoodindicationofvibrationacceleration.Theeffectsofwhole-bodyvibrationareusuallygreatestatthelowerendoftherange,from0.5to100Hz.Forhand-transmittedvibration,frequenciesashighas1,000Hzormoremayhavedetrimentaleffects.Frequenciesbelowabout0.5Hzcancausemotionsickness.Thefrequencycontentofvibrationcanbeshowninspectra.Formanytypesofwhole-bodyandhand-transmittedvibrationthespectraarecomplex,withsomemotiono...
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