How do we determine the allowable unbalance or tolerance for the part?

Determining the allowable unbalance or tolerance for a part is a critical step in ensuring the overall performance, lifespan, and safety of the system in which that part will operate. This tolerance value should be determined primarily by the part's designers or end users, as they best understand the speed, load, and criticality at which the part will operate. For example, the tolerance for a jet engine rotor will be much stricter than that of an electric fan motor. The allowable tolerance also directly impacts factors such as bearing life, noise level, energy consumption, and the overall operational reliability of the machine. Therefore, knowing the correct tolerance before starting the balancing process is crucial.

If you don't have this information or want to define a standard, you can refer to international standards such as ISO 2953. This standard defines acceptable balancing quality grades (e.g., G2.5, G6.3) for different applications. At MBS Balance, we offer our customers software and consulting services based on these standards, calculating the most appropriate tolerance based on the part's characteristics (weight, speed, etc.). This allows you to determine the correct starting point and ensure your part operates at the highest efficiency and safety levels. Remember, the right tolerance represents the ideal balance: neither too tight, which would increase costs, nor too loose, which would decrease performance.

The standard way to determine the tolerance numerically is as follows: a G quality grade is selected from the ISO 21940-11 (formerly ISO 1940-1) tables according to the part's application type, then the permissible specific unbalance is found with the formula e_per = (G × 1000) / w and multiplied by the rotor mass to obtain the total permissible unbalance (U_per = e_per × m). This value is distributed among the two correction planes according to the rules in the annex of the standard, taking the bearing positions into account.

A note: ISO 2953 is more for the acceptance and verification testing of balancing machines; the main reference for part tolerance is ISO 21940-11. Keeping the tolerance neither too tight (unnecessary cost) nor too loose (vibration and wear) depends on selecting the correct G grade. MBS Balance offers software and consultancy support that uses the part's mass, speed, and criticality information to calculate the target G grade and the tolerance per plane.