What is Vibration? (part 1)

DLI Engineering
Posted 10-20-03

In its simplest form, vibration can be considered to be the oscillation or repetitive motion of an object around an equilibrium position. The equilibrium position is the position the object will attain when the force acting on it is zero. This type of vibration is called "whole body motion", meaning that all parts of the body are moving together in the same direction at any point in time.

The vibratory motion of a whole body can be completely described as a combination of individual motions of six different types. These are translation in the three orthogonal directions x, y, and z, and rotation around the x, y, and z-axes. Any complex motion the body may have can be broken down into a combination of these six motions. Such a body is therefore said to possess six degrees of freedom. For instance, a ship can move in the fore and aft direction (surge), up and down direction (heave), and port and starboard direction (sway), and it can rotate lengthwise (roll), rotate around the vertical axis (yaw), and rotate about the port-starboard axis (pitch).

Suppose an object were restrained from motion in any direction except one. For instance, a clock pendulum is restricted from motion except in one plane. It is therefore called a single degree of freedom system. Another example of a single degree of freedom system is an elevator moving up and down in an elevator shaft.

The vibration of an object is always caused by an excitation force. This force may be externally applied to the object, or it may originate inside the object. It will be seen later that the rate (frequency) and magnitude of the vibration of a given object is completely determined by the excitation force, direction, and frequency. This is the reason that vibration analysis can determine the excitation forces at work in a machine. These forces are dependent upon the machine condition, and knowledge of their characteristics and interactions allows one to diagnose a machine problem.