Dynamic unbalance

In actual practice a rotor normally does not have a single unbalance, but an infinite number of unbalances distributed at random along its axis of rotation. These can be expressed as two resultant unbalances (shown as arrows in the illustration) acting in two arbitrary balancing planes. They differ in magnitude, and do not have a clearly defined angular position.

As this state of unbalance can only be determined comprehensively when the component is rotating, it is referred to as dynamic unbalance. It includes both a static component and a couple unbalance, whereby either of these two components may be dominant in the workpiece.

Due to the couple unbalance present, you need two balancing planes to be able to completely correct dynamic unbalance. Correction is achieved by removing the appropriate amount of material in these two balancing planes, or by adding weight on the opposite side.

Dynamic unbalance occurs in nearly all rotors. Therefore either horizontal or vertical balancing machines may be suitable for balancing.