Analysis based on the Jeffcott model is presented to explain 1/2 speed and 1/3 speed whirling motion occurring in rotors which are subject to periodic normal-loose or normal-tight radial stiffness variations. The normal-loose stiffness variation results due to bearing-clearance effects, while normal-tight stiffness variations result from rubbing over a portion of a rotor’s orbit. The results demonstrate that 1/2 speed subharmonic motion can be explained as either a linear parametric-excitation phenomenon or as a stable nonlinear subharmonic motion. The 1/3 speed motion is shown to be possible due to the radial stiffness nonlinearity. A linear parametric-excitation analysis demonstrates that during a normal-light rubbing condition, Coulumb damping significantly widens the potential range of unstable speeds.

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