What is residual unbalance and what is the need of residual unbalance?

In industry, complete balancing of a rotor is difficult due to the following reasons.

1. Complete balancing is time-consuming which increases the downtime of equipment.
2. Complete zero balancing demands the highest accuracy of the balancing machine (in the case of shop floor).

Since it is impractical to achieve zero unbalance, ISO standards provide tolerance limits. These tolerance limits decide the permissible residual unbalance.