Levitation Characteristics of an Aged Superconducting Magnetic Bearing

Superconducting magnetic bearings are friction-free devices and therefore in principle suitable for long-term operation, as no wear is observed. However, other degradation mechanisms can influence the operation. Up to now, it has not been clear to what extent degradation of either the bulk superconductors or the permanent magnets impacts the overall bearing performance on long timescales. Therefore, we studied the bearing properties of a 20-year-old rotational superconducting magnetic bearing, which was cooled down occasionally in an open liquid nitrogen bath for presentation. Otherwise, the bearing was stored under ambient conditions. To characterize the current status, we measured the bearing’s static and dynamic stiffness in radial and axial directions. Comparing our results to the values measured after the setup of the bearing revealed a stiffness degradation of up to 77%. This decrease is mainly attributed to the degradation of the bearing’s superconducting bulks and the permanent magnets. Analysis of both components showed clear signs of degradation. The permanent magnetic rotor’s magnetic field is around 19% smaller compared to the original state. The superconducting bulks now only inhomogeneously trap magnetic flux. Critical current calculation based on this data revealed a significant reduction compared to the original measurements. Nonetheless, the bearing allows for a stable levitation.