Vibration

Vibration Analysis

Vibration

April 13, 2019 2 Minutes
Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The word comes from Latin vibrationem. The oscillations may be periodic, such as the motion of a pendulum—or random, such as the movement of a tire on a gravel road.

In many cases, however, vibration is undesirable, wasting energy and creating unwanted sound. For example, the vibrational motions of engines, electric motors, or any mechanical device in operation are typically unwanted.

Spring-mass system: mass, stiffness, damping
All machines have the three fundamental properties that combine to determine how the machine will react to the forces that cause vibrations, just like the spring-mass system.

A basic understanding of how a discrete spring-mass system responds to an external force can be helpful in understanding, recognizing and solving many problems encountered in vibration measurement and analysis.

All machines have the three fundamental properties that combine to determine how the machine will react to the forces that cause vibrations, just like the spring-mass system.

The three fundamental properties are:
(a) Mass (M)
(b) Stiffness (k)
(c) Damping (C).

These properties are the inherent characteristics of a machine or structure with which it
will resist or oppose vibration.
(a) Mass: Mass represents the inertia of a body to remain in its original state of rest or motion. A force tries to bring about a change in this state of rest or motion, which is resisted by the mass. It is measured in kg.

(b) Stiffness: There is a certain force required to bend or deflect a structure with a certain distance. This measure of the force required to obtain a certain deflection is called stiffness. It is measured in N/m.

(c) Damping: Once a force sets a part or structure into motion, the part or structure will have inherent mechanisms to slow down the motion (velocity). This characteristic to reduce the velocity of the motion is called damping. It is measured in N/(m/s).

As mentioned above, the combined effects to restrain the effect of forces due to mass, stiffness and damping determine how a system will respond to the given external force.

Simply put, a defect in a machine brings about a vibratory movement. The mass, stiffness and damping try to oppose the vibrations that are induced by the defect. If the vibrations due to the defects are much larger than the net sum of the three restraining characteristics, the amount of the resulting vibrations will be higher and the defect can be detected.