Field balancing is a technique used to balance a rotating part in place without removing the part from the machine. The advantages of field balancing are apparent, in that time can be saved by not removing the rotating part from the machine and sending it to a shop for balancing.
Imbalance occurs when a shaft’s center of mass does not rotate at the center of rotation. This results in a heavy spot on the rotor. This heavy spot produces a centrifugal force that forces the rotor to rotate off center and causes a high vibration amplitude at 1x the rotor speed in spectral and time waveform vibration data.
Some of the causes of rotor imbalance can be from eccentricity, improper key length, stress relief distortion, thermal distortion, corrosion or uneven wear, deposit buildup, asymmetrical design, assembly errors, and rotor damage.
For more information please watch the below video that further describes what imbalance is and the methods used to correct it.