It is known that modal balancing should be performed near the critical speed in order to separate the considered modal effect from the mixed response of several modes. But it is dangerous to operate a rotor system around a critical speed to have a modal balance. In this study, a new method was suggested to do modal balancing not on a critical speed but for below of critical speed. When all the critical speeds are well-separated, It can be assumed that the unbalance response of a rotor is mainly affected by the nearest critical speed (mode). A gradient of a monitored response below a critical speed was calculated with a micro computer and the gradient was used to do modal balancing at the corresponding critical speed. The results from the suggested new method is very effective and safe to have modal balancing.
