Imbalance
A condition of a rotating part where the center of mass does not lie on the center of rotation. Imbalance of a rotor causes a centripetal force at the frequency of the rotation rate to be applied to the bearings. If it is large, it can severely shorten the life of the bearings, besides causing undue vibration of the machine. Forces caused by imbalance are proportional to the square of the RPM, and this means that high-speed machines must be balanced to a higher standard than low-speed machines.
Imbalance exists in several forms. Static Imbalance is the condition where the principal inertia axis of a rotor is offset from and parallel to the axis of rotation. A rotor with static imbalance will seek a position with the heavy spot at the bottom if placed on level knife-edges. Static imbalance can theoretically be corrected by the addition of a single correction mass.
Couple imbalance is the condition where the principal inertia axis intersects the rotation axis of the rotor at the center of gravity. A rotor with couple imbalance will be stable in any position on knife edges, but will produce out-of-phase imbalance forces on the bearings when rotated. Correction of couple imbalance requires the addition of two correction masses.
Dynamic imbalance is a combination of these two types, and is the most common type found in practice. In dynamic imbalance, the principal inertia axis neither intersects nor is parallel to the axis of rotation. Correction of dynamic imbalance requires at least two correction masses.
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