TABLE OF CONTENTS Passive superconducting bearings are based on the stable levitation of a permanent magnet over a bulk superconductor. This stable levitation is due to the pinning of magnetic flux surrounding the permanent magnet on defects within the superconductor. The interest in superconducting bearings strongly increased since bulk, melt-textured YBa 2Cu 3O 7 ? x (YBCO) is available, because this high-temperature superconductor exhibits large critical current densities at liquid nitrogen temperatures. Superconducting bearings can be used for rotating machines and for linear transportation systems. The main parameters of superconducting bearings as the levitation force and the magnetic stiffness will be discussed.
One of the most promising applications of bulk high- T c superconductors is the superconducting levitation which can be used in rotating superconducting magnetic bearings and in noncontact linear transportation. The advantage of superconducting magnetic bearings is that the levitation of a ferromagnetic permanent magnet over a bulk YBCO sample is completely passive. Contrary to this, no stable levitation is possible for two permanent magnets which interact via a 1/ r 2 force law the Earnshaw s theorem (Earnshaw 1842). Such an inverse square law is governed by the Laplace equation and allows only saddle-type equilibria resulting in unstable levitation. Therefore, conventional magnetic bearings composed of ferromagnetic permanent magnets require an active feedback control in order to stabilize the levitation. Inherently stable levitation can be achieved by introducing diamagnetic materials such as superconductors (Braunbeck 1939). The first...
