Vibration is the motion of a particle or a body or a system of connected bodies displaced from a position of equilibrium. or in other words, it is defined as to and from motion of a body about the mean position.
1] Simple pendulum motion.
2] The motion of mass attached at the end of spring.
If the bob is swung to the left by an angle $\theta$ and released, it swings back to the mean position shown by the firm vertical line but it does not stop there. it continues on the right traveling thorough a certain angle and then reverses itself. this swinging of the pendulum to and fro from the mean position.
Causes of VIBRATIONS :
The main causes of Vibration are as follows:
1] Unbalanced forces in the machine : These forces are produced from within the machine itself.
2] Dry friction between the two mating surfaces: This produces what are known as self excited vibration.
3] External excitations : These excitation's may be periodic, random or of the nature of an impact produced external to the vibrating system.
4] Earthquakes : These are responsible for the failure of many buildings, dams, etc.
5] Winds : These may cause the vibrations of transmission and telephone lines under certain conditions.
EFFECTS OF VIBRATIONS:
The effects of Vibrations are:
1] excessive stresses
2] undesirable noise
3] looseness of parts and
4] partial or complete failure of parts.
Inspite of these harmful effects the vibration phenomenon does have some uses also, e.g. in musical instruments, vibrating screens, shakers, stress relieving, etc.
CLASSIFICATION OF VIBRATION:
Some of the important types of vibration are as follows :
1] Free and forced vibration :
After disturbing the system, the excitation is removed, then the system vibrates on its own, this type of vibration is known as free Vibration. Simple pendulum is one of the examples. the Vibration which is under the influence of external force is called forced Vibration. Machine tools, electric bells, etc are the suitable examples.
2] Linear and Non linear Vibration :
If, in a vibratory system mass, spring and damper behave in a linear manner, then the Vibrations caused are known as linear in nature. Linear vibrations are governed by linear differential equations.
on the other had, if any of the basic components of a vibratory system behaves non linearly, the vibration is called as non linear, linear vibrations become non linear for very large amplitude of vibration. it does not follow the law of superposition.
3] Damped and Undamped Vibration :
If the vibratory has damper, the motion of the system will be opposed by it and the energy of the system will be dissipated in friction. this type of vibration is called as damped vibration.
On the contrary, the system having no damper is known as undamped vibration.
4] Deterministic and Random Vibration:
If in the vibratory system the amount of external excitation is known in magnitude, it causes deterministic vibration. contrary to it the non deterministic Vibrations are known as random vibrations.
5] Longitudinal, Transverse and Torsional Vibrations:
If the vibration takes place along the longitudinal axis, it is known as longitudinal vibration.
If the vibration takes place perpendicular to the longitudinal axis, it is known as transverse vibration.
If the vibration takes place about the longitudinal axis, it is known as torsional vibration.
6] Transient Vibration :
In ideal systems, the free vibrations continue indefinitely as there is no damping, the amplitude of vibration decays continuously because of damping (in a real system) and vanishes ultimately, such vibration in a real system is called transient vibration.