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1、Unit5MechanicalVibration機(jī)械振動(dòng)Amechanicalvibrationisanoscillatory,periodicallyrepeatedmotionofaparticleorbodyaboutapositionofequilibrium.Anengineerisfrequentlyconfrontedwiththeproblemofmechanicalvibrationssincetheyareencounteredtosomedegreeinalmostalltypesof
2、machineryandstructures.Mostvibrationsareundesirableinmachinesandstructuresbecausetheyproduceexcessivestressesorrepeatedstresses;causeaddedwear;increasebearingloads;inducefatigue;createacutepassengerdiscomfortinplanes,ships,trains,buses,andautomobiles;andab
3、sorbenergythatcouldotherwisedousefulwork.ThecollapseoftheTacomaNarrowsBridgein1940isanexampleofstructuralfailureduetoexcessivestressesproducedbyvibrations.Theaccuracyofprecisioninstruments,tools,andmachinesmaybeimpairedbyexcessivevibrations.Rotatingmachine
4、partsrequirecarefulbalancinginordertopreventdamagefromvibrations.Whenpartofpropellerofanairplaneisshotofforbreaksoffinflightthepropellerisnolongersymmetrical,andthevibrationsfromtheenginemayteartheenginefromtheplaneunlessitcanbestoppedintime.Thevibrationsp
5、roducedinanautomobilebytheengineorbydrivingonroughroadssetuprepeatedstressesincertainpartsthatcaneventuallyleadtofailureofthemembers.機(jī)械振動(dòng)是一個(gè)質(zhì)點(diǎn)或物體以一個(gè)平衡位置為中心做往復(fù)周期的振蕩運(yùn)動(dòng)。由于工程師幾乎要面對(duì)所有的機(jī)械和結(jié)構(gòu)中，因此機(jī)械振動(dòng)就成了一個(gè)經(jīng)常有遇要到的問(wèn)題。在機(jī)器和結(jié)構(gòu)中，大多數(shù)振動(dòng)是不希望存在的，因?yàn)檎駝?dòng)會(huì)產(chǎn)生附加應(yīng)力或交變應(yīng)力，引起額外磨損，增大軸承
6、載荷，導(dǎo)致疲勞破壞，使飛機(jī)、船、火車(chē)及汽車(chē)上的乘客產(chǎn)生嚴(yán)重的不很舒服感，并且振動(dòng)會(huì)吸收本可以做有用功的能量。1940年的TacomaNarrowsBridge倒塌就是一個(gè)由振動(dòng)引起附加應(yīng)力而導(dǎo)致結(jié)構(gòu)破壞的例子。附加振動(dòng)可能損壞精密儀表、工具和機(jī)械的精度。為了防止來(lái)自振動(dòng)的破壞，旋轉(zhuǎn)機(jī)械需要很好的平衡。當(dāng)飛機(jī)的螺旋槳在飛行過(guò)程中損壞或斷裂時(shí)，螺旋槳就將不再對(duì)稱(chēng)，除非可以及時(shí)停止，否則發(fā)動(dòng)機(jī)產(chǎn)生的振動(dòng)就會(huì)把它從飛機(jī)上撕裂下來(lái)。汽車(chē)由發(fā)動(dòng)機(jī)或在崎嶇不平的路上行駛時(shí)產(chǎn)生的振動(dòng)會(huì)在一定的部位產(chǎn)生交變應(yīng)力，最終導(dǎo)致桿件
7、疲勞失效。Vibrationsaresometimesusedtoproducedesirableeffects.Forexample,vibratorsareusedtocompactconcreteintheformsandtoseparatethegrainfromthechaffinthreshingmachines.Instrumentswhichfunctionproperlyonanairplanewithaconventionalenginemaytendtostickwhenuseding
8、lidersorjet-poweredplanesbecauseofthelackofvibration.Insuchinstances,avibratorissometimesinstalledontheinstrumentpanel.有時(shí)候振動(dòng)也能產(chǎn)生有益的效果。比如，振動(dòng)可以壓實(shí)物體，用脫粒機(jī)將谷粒與谷殼分離。那些在具有常規(guī)發(fā)動(dòng)機(jī)的飛機(jī)上可正常運(yùn)行的儀器，當(dāng)用于滑翔機(jī)或噴氣機(jī)時(shí)，因?yàn)槿狈φ駝?dòng)，可能會(huì)趨于粘滯
