Unbalance is defined as an unequal distribution of mass causing the mass axis to differ from the bearing axis.  During rotation, the unequal mass along with the radial acceleration due to rotation create a centrifugal force.  This results in force on the bearings and/or vibration of the bearings.

Balancing is a procedure in which the mass distribution of a rotor is assessed and if necessary, adjusted via addition or subtraction of weight to ensure that the vibration of the journals and/or forces on the bearings are within specified limits. Vibration is a mechanical movement where an object oscillates about an equilibrium point.  It commonly produces unwanted sound and wastes energy.  Vibration in rotating equipment can greatly reduce the life of the equipment and the bearings.  

Before studying the basic principles of balancing, one must keep in mind that there are many causes of vibration other than unbalance.  In some cases, balancing may result in only a partial or temporary reduction in vibration, while in other cases balancing is the only effective course of action.  Mechanical unbalance, which produces a force at 1 X RPM, has been found to be one of the most common causes of machinery vibration, present to some degree on nearly all rotating machines.

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