Residual Unbalance Check / Procedure

Once a rotor has met the balance specifications, it is desirable to perform a residual unbalance check to verify that the rotor is balanced correctly at each plane. This not only proves that the rotor is balanced, but it will verify that the machine is working properly. A residual unbalance check is accomplished by taking a known amount of weight at a known radius, and a known angle, and verifying that the balance machine properly tracks the weight.

For example, if the ISO 1940-1 balance tolerance for a rotor (m=150 kg, n=3000 rpm, quality grade G2.5) is 1193 gram-mm. For a 100 mm correction radius, then the tolerable unbalance would be 5.9 grams/each plane. After the component (or rotor) has been balanced, assume that the balancing machine displays a 4.7 gram unbalance at 90°, and a radius of 100 mm. This residual unbalance can be verified by placing a 10 gram weight sequentially at 12 locations (30° apart) at the 100 mm radius.

The balance machine reading (grams at degrees) are recorded at each position, and the twelve measured vectors are plotted in a polar coordinate format.

To determine the residual unbalance, take the highest amplitude reading, subtract the lowest reading, and divide the difference by two as in the following equation:

Residual Unbalance = (Max – Min)/2

This residual unbalance should be equal to the final display on the balancing machine. If these residuals are equal, the balancing machine is functioning properly.