Why Balancing

Static unbalance

Two unbalances (shown as arrows in the adjacent sketch) may have the same direction and angular position. The same state of unbalance would be generated by a single unbalance with a magnitude which is twice as high, acting at the centre of gravity, i.e. for example at the center of the rotor.

If such a rotor is supported on two knife-edges, it will roll off on the knife-edges until its "heaviest point" points to the bottom. This type of unbalance is making itself felt even without rotation and is therefore referred to as "static unbalance." Static unbalance causes a shift of the mass center away from the geometrical center, which causes the rotor to vibrate during operation.

Static unbalance can be corrected in the center of gravity plane, either by removing the appropriate amount of material or by adding a weight at the opposite side.

Couple unbalance

Two unbalances (shown as arrows in the illustration) may have the same magnitude but, their direction may be offset by 180° relative to each other. This state of unbalance can no longer be determined by rolling a component on knife-edges as the rotor will not settle in a clearly defined position.

The rotating component will wobble about its vertical axis (perpendicular to the axis of rotation) as the two unbalances generate a moment in the workpiece. This type of unbalance is therefore referred to as couple unbalance.

To correct a couple unbalance, you need to generate an opposed moment, i.e. you need two correction weights arranged at a specific distance relative to each other. Once the balancing planes are defined, you either have to remove a specific amount of material or add an appropriate weight at the opposite side.

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 the horizontal or vertical balancing machine may be suitable for balancing.