Introduction to Vibration Measurement
Vibratory response of a system can be expressed through a number of parameters.
Induced Strain / Stress
Types of Vibration -
Random - Such vibration lasts only for a short while,as a response to a random event.
Such vibration is also known as stochastic vibration,which in mathematical terminology means a function whose value in the immediate future cannot be predicted.
Such phenomena are caused by single or random excitation,and the system responds to the same in its own typical manner.
Periodic - Such vibration stay as a steady repeating phenomenon and has a specific period over which it repeats itself.It is always response to an alternating force.
Example - Machinery Vibration
Such vibration displays a periodic time function,which follows well defined mathematical rules,like a sine wave or the sum of many sine wave.It can thus be reasonably predicted.Thus the concepts of frequency and its inverse,the time-period are associated with periodic vibrations.
What are the effects of Vibration
Vibration occurs in every rotating and reciprocating machine.
Possible causes are : rotor imbalance,misalignment,faulty bearings or gear,eddy currents,electromagnetic force.
If unchecked,these causes create large and destructive periodic forces,which lead to metal fatigue,and eventual failure of the machinery.
Vibration of machine requires kinetic energy which is wasted energy.
Operator fatigue,noise and other nuisance.
How to measure Vibration?
Displacement : microns,pk-pk
All vibration is the to and from movement of a body
Amount=Magnitude=Displacement from one end to another.
Units of displacements are microns.
Displacement emphasizes low frequency vibration
Velocity : mm/sec,true RMS
If the first differential displacement dD/dt is computed, it gives the rate of change of displacement,which is velocity.This is the most commonly measured parameter.
Units of velocity are mm/sec and this is normally true RMS.
Velocity places even emphasis on low and high frequencies of vibration.
Most common mode of measurement.
Acceleration : m/sec2
If the second differential displacement d2D/dt2 is computed,it yields acceleration (Rate of change of velocity).
Units of acceleration is m/sec2
Acceleration emphasizes high frequencies and de-emphasizes low frequencies.
Frequency analysis is the technique which is used for identifying the frequencies of the various components of the vibration,and their associated magnitudes.Each frequency corresponds to a specific mechanical cause.