Definition: In general, unbalance is understood as an unsymmetrical distribution of mass of a rotating body. This phenomenon can also occur in pumps.

To avoid an unbalance in pumps, the rotating assembly is mechanically balanced before it is being installed, which corrects the geometric mass distribution. As a result, the rotation axis of the pump rotor becomes the free axis without inertial effects during the rotation. In addition, there is static balancing in which the mass center of gravity of the pump rotor is placed as precisely as possible in the rotation axis of the pump shaft.

Nevertheless, unbalances can still occur during operation. It can be caused by a number of factors such as emerizing or aggressive fluids handled, geometric unevenness or a fluctuating energy transfer.