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Vibration

Vibration Definition
If a force displaces an elastic body from its position, the body will develop a restoring force which tends to return it to that state. therefore when the displacing force is removed, the body will move towards the original position Owing to its mass or rather, to its inertia the body on its return movement will not stop at its previous position of equilibrium but will be carried beyond causing a displacement in the opposite direction. the restoring force thus developed in the opposite direction will reverse the action and the process will continue until these consecutive movements called oscillations or vibrations.

What is meant by Vibration?
All bodies having mass and elasticity are capable of vibration. When an external force is applied to the body, the internal forces are set up in the body which tend to bring the body in the original position. The internal forces which are set up are the elastic forces which tend to bring the body in the equilibrium position.


Causes of Vibration:
The main causes of vibration are:
  1. Unbalanced centrifugal force in the. System due to faulty design and poor manufacturing.
  2. Elastic nature of the system.
  3. External excitation applied to the system
Amplitude:
The amplitude of the oscillation is the maximum distance that the oscillating object moves away from the equilibrium position. 

Frequency of Vibration:
The number of vibrations per second or the rate at which they occur is known as the frequency of vibration. 

Natural frequency of Vibration:
If an elastic body is allowed to vibrate freely it will do so at a certain rake known as its natural frequency of vibration

Forced Vibration:
When displacing force act repeatedly on an elastic body they cause vibrations known as forced vibrations

Harmonic Vibration:
When the forced vibration occurs at the same rate as the natural frequency of vibration of the body or some multiple of it called a harmonic vibration

Damping:
It is resistance provided to the vibrating body and vibrations related to it are called damped vibration.

Resonance:
When displacing forces acts repeatedly on an elastic body they cause vibrations known as forced vibrations. when these forced vibrations occur at the same rate as the natural frequency of vibration of the body or some multiple of it called harmonic vibration the free vibration will be reinforced or amplified by forced vibration this condition is known as resonance.

Vibration Analysis:
The most commonly used method for rotating machines is called vibration analysis. Measurements can be taken on machine bearing casings with seismic or piezo-electric transducers to measure the casing vibrations, and on the vast majority of critical machines, with eddy-current transducers that directly observe the rotating shafts to measure the radial (and axial) vibration of the shaft. The level of vibration can be compared with historical baseline values such as former startups and shutdowns, and in some cases established standards such as load changes, to assess the severity. 

Elimination of Unnecessary Vibrations:
Unwanted vibrations can be reduced by:
1. Removing external excitation if possible.
2. Using shock absorbers.
3. Dynamic absorbers.
4. Proper balancing of rotating parts.
5. Removing manufacturing defects and material inhomogeneities.
6. Resting the system on proper vibration isolators.

Importance of Vibration Study:
The imp of vibration study is to reduce or eliminate vibration effects over mechanical components by designing them suitably. Proper design and manufacture of parts will reduce. Unbalance in engines which causes excessive and unpleasant stress in a rotating system because of vibration, proper design of machine parts will reduce and tear due to vibration and loosening parts. The proper designing and material distribution prevent the locomotive leaving the track due to excessive vibration which may result in an accident or heavy loss. Proper designing of structure buildings can prevent the condition of resonance which causes dangerously large oscillations which may result in failure of that part.


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