Passive Bearings

Passive magnetic bearings (PMB) achieve contact-free levitation of an object by permanent magnetic attractive or repulsive forces. Depending on the configuration, stabilization in radial, axial and tilt direction are possible. It is, however, not possible, to stabilized all degrees of freedom of a body by passive magnetic levitation, alone. This has been shown by Braunbeck who interpreted the prior findings of Earnshaw on the stability conditions in force fields for magnetic levitation. Diamagnetic materials such as superconductors are explicitly not considered in this statement. A very simple PMB design consists of permanent magnetic rings on the rotating shaft and the stator which stabilize the radial degree of freedom by repulsive forces.

Different configurations of attractive or repulsive permanent magnets are possible.

It is also possible to stabilize more than one degree of freedom with one permanent magnet as e.g. in disk-shaped, permanent rotors:

The stiffness values are determined by the geometrical bearing configuration and its material properties. The mere PMB provides close to zero damping which typically needs to be added by visco-elastic, electrodynamic or other damping elements. No active components such as actuators, coils or power electronics are needed in a PMB which makes it a cheap, small and mechanically simple magnetic bearing. The next picture shows a magnetic bearing concept with relatively low constructive complexity. The radial and tilt stabilization are achieved by two passive magnetic bearings. The active element is used to control the axial rotor position, which is unstable due to the Earnshaw Theorem.

PMBs show one characteristic weakness: their lack of damping. Due to the lack of damping of permanent magnetic bearings, additional measures are required to gain sufficient stability against disturbances and to successfully pass the resonance frequencies during run up. For applications with a limited temperature range, one possibility is to use a visco-elastically supported stator in order to damp the vibrations of the rotor. Alternatively, eddy current dampers can be used to constrain rotor vibrations. These damping measures demand considerable modelling effort but strongly improve the system performance: the vibrations caused by unbalance, magnetic tolerances or external excitations can be suppressed.