38 Definitions and Terminology of Vibration
All matter-solid ,liquid and gaseous - is capable of vibration ,e.g. vibration of gases occurs in tail ducts of jet engines causing troublesome noise and sometimes fatigue cracks in the metal .Vibration in liquids is almost always longitudinal and can cause large forces because of the low compressibility of liquids , e. g. pipes conveying water can be subjected to high inertia forces (or “water hammer”) when a valve or tap is suddenly closed. Excitation forces caused , say , by changes in flow of fluids or out-of-balance rotating or reciprocating parts, can often be reduced by attention to design and manufacturing details . Aypical machine has many moving parts,each of which is a potential source of vibration or shock-excitation. Designers face the problem of compromising between an acceptable amount of vibration and noise, and costs involved in reducing excitation.
The mechanical vibrations dealt with are either excited by steady hamonic forces (i. e. obeying sine and cosine laws in case of forced vibrations) or, after an initial disturbance, by no extemal force apart from gravitational force callde weight (i. e. in cases of natural or free vibrations). Hamonic vibrations are said to be “simple” if there is only one frequency as represented diagrammatically by a sine cosine wave of displacement against time.
Vibration of a body or material is periodic change in position or displacement fiom a static-equilibriun position. Associated with vibration are the interrelated physical quantities of acceleration, velocity and displacement – e. g. an unbalanced force causes acceleration (a = F/m) in a system which , by resisting, induces vibration as a response . We shall see that vibratory or oscillatory motion may be classified broadly as (a) transient; (b) continuing or steady-state; and (c) random .
Transient Vibrations die away and are usually associated with irregular disturbances , e . g. shock or impact forces, rolling loads over bridges , cars driven over pot holes – i. e . forces which do not repeat at regular intervals. Although transients are temporary components of vibrational motion , they can cause large amplitudes initially and consequent high stress but , in mang cases , they are of short duration and can be ignored leaving only steady-state vibrations to be considered .
Steady-State Vibrations are often associated with the continuous operation of machinery and , although perioduc , are not necessarily hamonic or sinusoidal . Since vibrations require energy to produce them , they reduce the efficiency of machines and mechanisms because of dissipation of energy , e. g . by friction and consequent heat-transfer to surroundings , sound waves and noise , stress waves through frames and foundations , etc . Thus , steady-atate vibrations always require a continuous energy-input to maintain them.
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所有的物质固态,液态和气态 - 是振动的能力,例如气体振动发生在喷气发动机尾管造成麻烦的噪音和金属有时疲劳裂纹。振动在液体中几乎总是纵向和可能导致由于液体,E.低压缩性大部队湾管道输水,可经受高惯性力(或“水锤”)时,阀门或水龙头突然关闭。激励的力量造成的,就是说,在液体或旋转外的平衡或往复流部件的变化,通常可以通过注意减少到设计和制造细节。 Aypical机有许多移动部件,这是每个振动或冲击,激发潜在来源。设计师面临着之间的振动和噪音是否可以接受金额妥协的问题,涉及的成本减少激励的机械振动与稳定不是由(调和部队兴奋即在强迫振动情况下服从正弦和余弦法处理),或在经过最初的滋扰没有Tiles分力量,除了引力callde重量(即在自然情况下或免费振动)。调和振动据说是“简单”如果只有一个频率为代表的位移与时间正弦余弦波图解。
参与减少激励一个机构或材料振动是周期性变化的位置或位移〜1911一静态平衡约束的地位。与振动相关的是相互关联的物理量的加速度,速度和位移 - 大肠杆菌湾1不平衡力的原因加速度(a = F的/米的制度,由抵制,诱使作为响应振动)。我们将看到,振动或振荡运动大致可分为(1)暂时性;(二)继续或稳定状态;及(c)随机的。
瞬态振动消失,通常是不规则的扰动引起的,电子商务。湾冲击或影响力,超过桥梁轧制负荷,汽车驾驶超过锅洞 - 岛é。势力不要重复定期。虽然瞬态振动议案临时组成部分,他们最初可能会导致大振幅和随之而来的压力,但在高莽情况下,它们是短暂的,可以忽略不计只剩稳态振动加以考虑。稳态振动往往与连续操作的机器,虽然perioduc,不一定调和或正弦。由于振动产生的能量需要他们,他们的机器和减少因能源,大肠杆菌耗散机制的效率克。通过帧和基金会等摩擦和由此产生的热量转移到周围环境,声波和噪音,应力波。因此,稳定,atate振动能量总是需要不断输入,以维持他们。
参考资料:Google翻译