The unbalance of a rotor is caused by the radial distribution of its masses along its axis of rotation.
The consequence is that if the rotor is rigid, and this means that the values and relative positions of its masses do not change, the unbalance does not change with the speed. In a rigid rotor the operating speed does not modify mass distribution and consequently has no influence on the unbalance.
By adding a 20 gr mass at a defined radial position on a perfectly balanced disc an unbalance is generated. This unbalance does not change with the speed, it is just necessary to remove the added 20 gr mass independently of rotor speed.
For rigid rotors the balancing speed does not need to be specified because it is related only to machine sensitivity and not to the rotor unbalance which is under measurement.
Modern hard bearing balancing machines have the capability to measure the dynamic unbalance starting from 70 RPM.
The unbalance effect (centrifugal force) increases with speed, the electric signal increases at the same time and so machine sensitivity tends to increase because of a better ratio signal to noise.
Depending on the model and manufacturer optimum sensitivity values are obtainable starting from 400 to 600 RPM.
Note: don’t get confused between the cause (unbalance) with its effect (centrifugal force or vibration).
The effect increases with the speed while the cause (unbalance) in a rigid body does not change.
For further information about any aspect of balancing please visit www.cembhofmann.co.uk or call our expert team today on 0161872 3123.