Magnetic bearing assembly

Abstract

Stray magnetic fields created by non-symmetrical magnetisation of the permanent magnets in a rotating magnet array of a magnetic bearing assembly, when used in turbo molecular pumps, can cause significant problems to devices such as scanning electron microscopes. In order to minimise the stray fields, at least the dipole and quadrupole moments of each magnet in the array is first measured. The magnets in the array are then arranged relative to one another such that both the dipole and quadrupole moments are minimised, thus minimising the stray magnetic field when the magnet array rotates in the turbomolecular pump.

Description

technical field [0001] The invention relates to the field of magnetic bearings. In particular, it relates to an apparatus and method for reducing transverse stray magnetic fields caused by permanent magnetic bearing arrangements in high speed rotating machines such as turbomolecular pumps. Background technique [0002] Turbomolecular pumps are used as part of vacuum systems used to generate the high vacuum environments required for devices such as scanning electron microscopes (SEM) and lithography devices. [0003] Turbomolecular pumps commonly include an oil-free passive permanent magnetic bearing arrangement in the high vacuum end of the pump. [0004] figure 1 A section of a passive permanent magnetic bearing arrangement 10 of a turbomolecular pump (not shown) is shown in . In this example, the bearing arrangement 10 includes an array 12 of four outer rotating permanent magnet rings 12a, 12b, 12c and 12d and an array 14 of four inner non-rotating permanent magnet ring...

AI Technical Summary

Problems solved by technology

[0013] Although ferromagnetic shielding is commonly used to reduce such magnetic field emissions, such shielding is expensive and has only limited effectiveness

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