The semi-floating ring bearings (SFRBs) are developed to attenuate rotor vibrations by providing damping, but the subsynchronous oscillations cannot be avoided. The subsynchronous oscillations of the rotor are the self-excited whirl motions for the instability of the oil films. Their frequencies are usually less than the half of the rotor speed, but difficult to be predicted because the effects of SFRB’s parameters on the subsynchronous oscillation frequency are not clear. In this paper, an analytical expression is proposed to estimate the subsynchronous whirl frequency of SFRB. To verify the accuracy of the predicting expression, the two-dimensional nonlinear responses of the SFRB are calculated by the Finite Element (FE) method. After confirming the accuracy, the predicting expression is generalized to the rotor-SFRBs (flexible rotor supported by SFRBs). The nonlinear responses of rotor-SFRBs are simulated. Two typical subsynchronous oscillations excited by the two SFRBs respectively are observed, and their frequencies coincide the prediction of the expression as expected.

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