Answer to Question #211390 in Mechanical Engineering for Paul

Question #211390

Difference between Dynamic vibration and Static vibration

Expert's answer

Static unbalance is the simplest form of unbalance. It is equivalent to a single heavy spot in the rotor. It is called static because it will be present even when the rotor is not spinning. Static balancing is necessary to prevent a stationary object from rolling. When an object’s center of gravity is on its axis of rotation, it will remain stationary. But if the center of gravity is not on its axis of rotation, it tends to roll, and a braking force is necessary to keep it stationary. A rotating body at rest is in static balance if it continues to be at rest at all angular positions of its axis. Static unbalance can be corrected with a single plane balance.

A rotor will actually have an infinite number of imbalances distributed at random along its axis of rotation. This can be expressed as 2 resultant unbalances acting in 2 arbitrary balance planes. They differ in magnitude and do not have clearly defined angular positions. As this state of unbalance can only be determined comprehensively when the component is rotating, it is referred to as dynamic unbalance. Dynamic unbalance can be corrected with a 2 plane balance.


A rotating object must have static balance before it can have dynamic balance. But the object can have dynamic imbalance even when it is statically balanced. If that is the case, the object will tend to vibrate while rotating even though it has no tendency to roll while stationary.

In other words, dynamic balance can be thought of as the ability of an object to balance while in motion. A perfectly dynamically balanced body requires just the force supporting its weight to make it stay in place while rotating; a dynamically unbalanced body in rotation requires external forces to counter the wobble from vibrations.

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