In high-speed balancing of flexible rotors with overhangs, it is important to determine ahead of time if the overhang needs to be restrained while the rotor is being tested in the bunker. This is done by adding a stub shaft to the rotor and a third pedestal to the overall system. Since the bearing properties are rarely known precisely and have a considerable influence on the rotor's dynamics, the model complexity increases tremendously if a third pedestal is needed during the balancing operation. From a balance engineer's viewpoint, it is therefore important to know ahead of time, without modeling the entire rotor, if the overhangs are likely to cause a problem during the balancing operation. This paper presents a criterion for identifying rotors with shaft overhangs that are likely to exhibit dangerous behavior while balancing. The proposed approach provides quick estimates of the L-mode frequency of the overhang using the influence coefficients and Dunkerley's formula. Two additional criteria based on further simplifications of the influence coefficient approach are also presented. Numerical results from four industrial rotors recently balanced indicate that the proposed methods are effective in determining the need for a third pedestal without having to resort to extensive rotor modeling.
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