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 centre 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 centre away from the geometrical centre, which causes the rotor to vibrate during operation.

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

Static unbalance occurs particularly on disc-shaped rotors. Therefore, a vertical balancing machine is usually the best choice for correcting static unbalance.