Magnetic Forces in High- T c Superconducting Bearings

Abstract

Magnetic force measurements between a permanent rare-earth magnet and high-T_c superconducting material versus vertical and lateral displacements have been made. A large hysteresis loop results for large displacements, while minor loops result for small displacements. These minor loops seem to give a slope proportional to the magnetic stiffness, and are probably indicative of flux pinning forces.

Experiments of rotary speed versus time show a linear decay in a vacuum. Speeds up to 120,000 RPM for a levitated rotor have been achieved. Measurements of magnetic drag forces for a magnetic dipole moving over a high-T_c superconducting disc of YBCO show that the drag force reaches a constant value, independent of the speed. Damping of lateral vibrations of levitated rotors have been measured which indicates that transverse flux motion in the superconductor will create dissipation.

As a result of these force measurements we have been able to design an optimum shape for the superconductor bearing pads which gives good lateral and axial stability. Recent force measurements on melt-quench processed superconductors indicate a substantial increase in levitation force and magnetic stiffness over free sintered materials. In light of these results, the application of high-T_c superconducting bearings are beginning to show great promise.