Static unbalance is present in a rotating body if the center of gravity of that body does not lie on the rotational axis. Such an unbalance (as shown in the illustration below) would cause the body to rotate, when resting on the shaft on level knife edge, due to the force of gravity.


Couple unbalance is present in a rotational body when the center of gravity lies on the rotational axis, but the principal inertia axis is not parallel with the rotational axis. Such an unbalance (as shown below) would not cause the body to rotate when the shaft rest on level knife edges. Weight A times its radius must be equal to Weight B times its radius and also must be 180° from Weight B.


Static and Couple unbalances can be removed by a two-plane correction. The illustration below shows two such planes. It is very important that the planes chosen for setting up the balancing machine should coincide with the planes in which final corrections will be applied.


The chart below gives an example of the force created when an unbalanced part is rotated and subjected to centrifugal force, along with its underlying formula. This illustrates how improper balancing can cause a great deal of stress on the subjected parts.

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