A shaft-rotor system which has a symmetrical nonlinear restoring force exerted on the rotor when it is radially displaced is analysed. Solutions which predict harmonic whirl are obtained and their stability is investigated. The effect of viscous friction in the rotating members is considered and, as a result of the nonlinearity, a whirl is shown to occur at frequencies which correspond to the free undamped vibrations of the system rather than at the synchronous speed.
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