unit 5 mechanical vibrations

1、Unit5MechanicalVibration機械振動Amechanicalvibrationisanoscillatyperiodicallyrepeatedmotionofaparticlebodyaboutapositionofequilibrium.Anengineerisfrequentlyconfrontedwiththeproblemofmechanicalvibrationssincetheyareencountere

2、dtosomedegreeinalmostalltypesofmachinerystructures.Mostvibrationsareundesirableinmachinesstructuresbecausetheyproduceexcessivestressesrepeatedstressescauseaddedwearincreasebearingloadsinducefatiguecreateacutepassengerdis

3、comftinplanesshipstrainsbusesautomobilesabsbenergythatcouldotherwisedousefulwk.ThecollapseoftheTacomaNarrowsBridgein1940isanexampleofstructuralfailureduetoexcessivestressesproducedbyvibrations.Theaccuracyofprecisioninstr

4、umentstoolsmachinesmaybeimpairedbyexcessivevibrations.Rotatingmachinepartsrequirecarefulbalancingindertopreventdamagefromvibrations.Whenpartofpropellerofanairplaneisshotoffbreaksoffinflightthepropellerisnolongersymmetric

5、althevibrationsfromtheenginemayteartheenginefromtheplaneunlessitcanbestoppedintime.Thevibrationsproducedinanautomobilebytheenginebydrivingonroughroadssetuprepeatedstressesincertainpartsthatcaneventuallyleadtofailureofthe

6、members.機械振動是一個質(zhì)點或物體以一個平衡位置為中心做往復周期趨于粘滯。在這種情況下，就需在儀器的儀表板上安裝振動器。Whenaparticlebodythatissupptedbyasystemofspringsashaftabeamanyotherelasticsystemisdisturbedfromitspositionofequilibriumbytheapplicationsuddenremovalofanaddit

7、ionalfce，theparticlebodywillvibrate.Somecommonexampleare(1)theverticalvibratymotionofbodyAinFig.1.19(a)whenitisdisplacedverticallyfromitsequilibriumpositiononahelicalspringthenreleased(2)theverticalvibratymotionofbodyBof

8、Fig.1.19(b)whenitisdisplacedfromitsequilibriumpositiononaflexiblespringboard(cantileverbeam)ofnegligiblemassreleased(3)themotionofthependulumbobCinFig.1.19(c)asitswingsinaverticalplanewhensupptedbyacdofnegligiblemass.當質(zhì)點

9、或物體是由彈簧系統(tǒng)支撐時，由于實際應用和額外力的移走，軸、梁或其他彈性系偏離平衡位置，這時質(zhì)點或物體將開始振動。一些常見的例子有：圖1.19（a）中，A物體在螺旋彈簧上被垂直拉離平衡位置時釋放，它就會垂直振動；圖1.19（b）中，當B物體在一個懸臂梁（忽略質(zhì)量）上偏離平衡位置并釋放時，它的垂直振動；圖1.19（c）中，當擺錘C由細線（忽略質(zhì)量）固定在豎直平面時的搖擺運動。Fig.1.20(a)showsabodyWsuspendedfr