Unit 5 Mechanical Vibrations A mechanical vibration is an oscillatory，periodically repeated motion of a particle or body about a position of equilibrium .An engineer is frequently confronted with the problem of machinery and structures. 机械振动是质点或物体在其平衡位置附近所作的震荡的，周期性的往复运动。 工程师经常面临机械振动的问题，因为在某种程度上他们在几乎所有的机械和结构中遇到过这些问题 Most vibrations are undesirable in machines and structures because they produce excessive stresses or repeated stresses; 大多数的震动在机械和结构中是不希望存在的，因为震动会产生附加应力或者交变应力。 cause added wear;increase bearing loads; induce fatigue;create acute passenger discomfort in planes，ships，trains，buses，and automobiles;and absorb energy that could otherwise do useful work. 引起额外磨损，增大轴承载荷，导致疲劳破坏，使飞机、船、火车及汽车上的乘客产生严重的不舒服感，并且振动会吸收本可以做有用功的能量。 The collapse of the Tacoma Narrows Bridge in 1940 is an example of structural failure due to excessive stresses produced by vibrations. The accuracy of precision instruments，tools，and machines may be impaired by excessive vibrations. 1940 年（发生的）塔克马窄桥的垮塌事件就是一个因为震动产生的附加应力导致结构失效的例子。 精密仪器、工具和电脑的准确性可能被附加的震动所损坏 Rotating machine parts require careful balancing in order to prevent damage from vibration original text 旋转机械零件需要仔细的平衡,以防止振动的造成的伤害。 When part of the propeller of an airplane is shot off or breaks off in flight the propeller is no longer symmetrical，and the vibrations from the engine may tear the engine from the plane unless it can be stopped in time. original text 当飞机的螺旋桨的（某）部分在飞行中被打掉或断裂，（这时）螺旋桨不再是对称的，来自引擎的振动会把引擎从飞机上扯下来,除非它可以被及时的停止。 ...
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