Influence of Axial Offset on the Nonlinear Behaviour of Passive Magnetic Bearings

This paper analyses the nonlinear behaviour of Passive Magnetic Bearings (PMBs), overcoming the conventional assumption of constant stiffness commonly adopted in their modelling. In case of rotors equipped with PMBs, the magnetostatic interaction between permanent magnets results in nonlinear radial stiffness with a strong impact on the system dynamics, requiring accurate modelling. In this work, the bearing stiffness is investigated as a function of the relative axial and radial position between rotor and stator magnets. Particular attention is given to the influence of axial offset between magnetic rings, which alters the magnetic field distribution and consequently affects the nature of stiffness nonlinearity. Furthermore, the impact of topological variations in the bearing configuration, specifically, the stacking of multiple magnetic rings, is examined to assess their influence on the stiffness characteristics. A lumped-parameter rotor model with nonlinear supports is employed to simulate system dynamics under different PMB configurations. The resulting steady-state frequency responses are analysed to identify the onset of nonlinear phenomena, while Fast Fourier Transform analyses provide insight into the frequency content associated with these effects. The findings highlight the crucial role of PMB topology in determining the effective stiffness and overall dynamic behaviour of PMB-supported rotors.