Unit5mechanicalvibrations..doc

Unit 5 Mechanical Vibration 机械振动 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 mechanical vibrations since they are encountered to some degree in almost all types of machinery and structures. Most vibrations are undesirable in machines and structures because they produce excessive stresses or repeated stresses; 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. 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. Rotating machine parts require careful balancing in order to prevent damage from vibrations. When part of 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. The vibrations produced in an automobile by the engine or by driving on rough roads set up repeated stresses in certain parts that can eventually lead to failure of the members. 机械振动是一个质点或物体以一个平衡位置为中心做往复周期的振荡运动。由于工程师几乎要面对所有的机械和结构中，因此机械振动就成了一个经常有遇要到的问题。在机器和结构中，大多数振动是不希望存在的，因为振动会产生附加应力或交变应力，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很舒服感，并且振动会吸收本可以做有用功的能量。1940年的Tacoma Narrows Bridge倒塌就是一个由振动引起附加应力而导致结构破坏的例子。附加振动可能损坏精密仪表、工具和机械的精度。为了防止来自振动的破坏，旋转机械需要很好的平衡。当飞机的螺旋桨在飞行过程中损坏或断裂时，螺旋桨就将不再对称，除非可以及时停止，否则发动机产生的振动就会把它从飞机上撕裂下来。汽车由发动机或在崎岖不平的路上行驶时产生的振动会在一定的部位产生交变应力，最终导致杆件疲劳失效。 Vibrations are sometimes used to produce desirable effects. For example, vibrators are used to compact concrete in the forms and to separate the grain from the chaff in threshing machines. Instruments which function properly on an airplane