The purpose of this study is to examine the torsion response of a rotor while in continuous contact with a stator for both forward synchronous whirling and backward dry-friction whirling. Experimentally obtained torsional strain data for both of these motions are presented, and the results indicate that the major contributions to the motions occur at the drive speed fd, twice the drive speed 2fd and the first torsional natural frequency ft for forward whirling. During backward whirling, the dominant response occurs at the drive speed fd and the sum of the whirl speed plus the drive speed at fw + fd. A distributed-parameter model in combination with a force-interaction model is used to capture the qualitative aspects of the system response. Simulations with this model reveal that the torsional vibrations are excited by stick-slip forces while undergoing backward whirling. Numerical and experimental results also show that motion at the first torsion natural frequency is the dominant component during forward whirling.

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