Dynamic characteristics of permanent magnetic thrust bearing used in vessels

The axial stiffness of a permanent magnetic thrust bearing (PMTB) decreases from 10(9) N/m to 10(7) N/m compared with the sliding bearing and the increment of thrust makes the stiffness diminish rapidly. In this paper, theoretical analysis and experiments were applied to research the axial dynamic characteristics of the PMTB. In theoretical analysis, the Runge-Kutta method is adopted to yield the time-domain response of the PMTB rotor under excitation of original velocity. The energy conservation method (ECM) is used to verify the calculating results, which proves that the Runge-Kutta method can obtain physically reasonable results. The analysis reveals that, due to the non-linearity of the PMTB, the rotor's vibration exhibits a "stripping" phenomenon compared with harmonic wave; and with the increment of axial static displacement (ASD), smaller excitation can make the PMTB generate this phenomenon. The stripping becoming obvious means the bearing is increasingly close to instability. In the experiments, a disc spring was used to constitute the testing system together with the PMTB. The curve fitting method was used to deal with the abstracted peak values from displacement and acceleration auto-power spectrum and further obtain the dynamic stiffness of the PMTB. The test results obviously show that the PMTB stiffness decreases with the ASD increment, and in general the dynamic stiffness is smaller than the static one, which proves the existence of the stripping phenomenon.