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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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