When a rotor assembly reaches the balancing machine it has normally been the subject of a number of machining and assembly operations. Each of these operations can contribute to the total unbalance of the rotor.

Here we see an example of an armature. This could have been a turbine, fan, pump or flywheel. The errors arise from the fact that none of the operations is perfect. Each one has a tolerance. If the shaft is bent then the laminations will run eccentric generating unbalance. If the OD is turned true it may help reduce the unbalance, but the slots for the wire will still be eccentric to the bearings. The commutator is an assembly of brass strips separated by insulation and mounted on a steel hub. Again there are several ways for the commutator to be out of balance.

The eccentricities, clearances and distortions add together in random ways so that a batch of supposedly identical rotors can have a wide range of initial unbalance values.

The balancing process is not a single operation but a 'measure, correct, audit' cycle.