What is Balancing Tolerance and Why is it Important?

Balancing tolerance is a critical value that specifies the maximum permissible residual unbalance in a part. In other words, it is the highest level of unbalance allowed to remain in the part after the balancing process is complete. This value is determined based on the part's rotational speed, application, and overall operational requirements. For example, a very low and tight tolerance is needed for a high-speed turbine rotor, while a looser tolerance may be acceptable for a slower-operating fan. This tolerance is typically defined using international standards such as ISO 1940 and ensures the part operates safely and efficiently.

Determining the correct balancing tolerance and adhering to it is vital for a machine's longevity. Exceeding the tolerance increases the part's vibration levels, leading to rapid wear of bearings, shafts, and other machine components. Consequently, maintenance costs rise, energy efficiency drops, and potential failure risks emerge. Conversely, a part balanced to the correct tolerance level operates more quietly, more efficiently, and more safely. Therefore, balancing tolerance is not just a technical detail but a guarantee of machine performance and reliability.

To obtain the balancing tolerance numerically, the following path is followed via ISO 21940-11 (formerly ISO 1940-1): a G quality grade suitable for the application is selected, the permissible specific unbalance is found with e_per = (G × 1000) / w, and multiplied by the rotor mass to obtain the total permissible unbalance U_per = e_per × m. In two-plane balancing, this value is distributed among the planes with regard to the bearing positions. Since w in the formula is the operating speed, if the same part will run at a higher speed, its tolerance also tightens.

For the tolerance to be meaningful, the machine's minimum achievable residual unbalance (Umar) must be sufficiently smaller than the target; otherwise the tolerance cannot be reached safely. The part is accepted when the residual unbalance falls inside the circle representing this tolerance on the screen. MBS Balance machines automatically calculate the tolerance according to the entered G grade and show the tolerance circle; thus the accept/reject decision moves away from subjective interpretation and rests on a standard criterion.