ELONGATED PERMANENT RING MAGNET WITH A PLURALITY OF AXIALLY DIRECTED MAGNETIZED ZONES AND MAGNETIC BEARING WITH SUCH A RING MAGNET

Abstract

The present invention is directed to a magnet for a magnetic bearing arrangement, a bearing arrangement comprising said magnet, and a vacuum pump comprising said bearing arrangement. In particular the invention can be particularly useful in a magnetic bearing arrangement that reduces stray magnetic fields for a turbomolecular vacuum pump, although it is understood that the invention is not limited to this field and other applications will be understood by the skilled person.

Claims

1. A single piece elongate permanent ring magnet for a magnetic bearing used in rotating machinery, whereby, in use said bearing has an axis of rotation parallel with, and centrally through the bore of the ring magnet, wherein the magnet comprises an even number of axially polarized zones between the axial ends of the magnet, with axially neighbouring polarized zones in mutual repulsion to each other.

2. The single piece elongate permanent ring magnet of claim 1, comprising at least four axially polarized zones.

3. The single piece ring magnet of claim 1, included in a magnetic bearing arrangement having a rotating magnet and a non-rotating magnet in which the single piece ring magnet concentrically surrounds the non-rotating magnet and wherein the non-rotating magnet is a single piece elongate permanent ring magnet comprising an even number of axially polarized zones between the axial ends of the magnet, with axially neighbouring polarized zones in mutual repulsion to each other and wherein the axially polarized zones on the rotating and non rotating magnet are orientated to provide a mutual repulsion between the rotating and non-rotating magnets.

4. The magnetic bearing arrangement of claim 3, wherein the rotating magnet with the single piece ring magnet concentrically surrounds the non-rotating magnet, and wherein the non-rotating magnet comprises an array of axially polarised a permanent ring magnets with axially neighbouring rings in mutual repulsion w each other and wherein the axially polarized zones on the rotating magnet and the axially polarized ring magnets of the non-rotating magnet array are orientated to provide a mutual repulsion between the rotating magnet and non-rotating magnet array.

5. The magnetic bearing arrangement of claim 3, wherein the baring arrangement is included in a high speed rotational device.

6. The magnetic bearing arrangement of claim 3, wherein the bearing arrangement is included in a turbomolecular pump.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0035] FIG. 4 illustrates a magnetic bearing arrangement 300 comprising the ring magnet 122 according to the present invention. The ring magnet 122 is a single piece cylindrical tubular magnet formed of a magnetic material, for example samarium-cobalt. When in use in the bearing arrangement 300 the magnet 122 rotates with the rotor of a turbomolecular pump (not shown) about the axis 102. The axis 102 passes parallel to and centrally through the bore of the magnet 122 as shown. The magnet illustrated has four axially polarized zones 122a, 122b, 122c and 122d with axially neighbouring polarized zones in mutual repulsion to each other, for example zones 122a and 122b meet each other with the same pole (south-south, as indicated by the tails of the arrows) as do 122b and 122c (North-North as indicated by the heads of the arrows).

[0036] As stated above, the advantage of producing a single piece ring magnet comprising an even number of axially polarised zones, compared to the use of separate ring magnets in an array 12 as shown in FIG. 2, is that there can be only one angular error, , from the ideal geometric (rotational) axis 102. As the angular error comes from either the compression, sintering or finishing stages of magnet production, every magnet produced will have its own errors. However, it is only possible to obtain a single, uniform angular

[0037] FIG. 4 also illustrates the relative position of the stationary, non-rotating magnet 144 of the bearing arrangement 300, i.e. the non-rotating magnet 144 is surrounded concentrically by the magnet 122. The magnet 144 can either be formed from an array of separate, axially polarized ring magnets as in the prior bearing arrangement (shown in FIG. 2) or also formed from a single piece magnet comprising an equal number of polarized zones (144a, 144b, 144c, and 144d) to that of the rotating magnet 122. However, it is preferable to use a single piece magnet for both the rotating and stationary magnets 122, 144 because, in addition to the reduced stray magnetic fields, they are also mechanically stronger, require less manufacturing time, and are easier to handle than two arrays of separate magnets. Thus turbomolecular pumps comprising the magnets of the present invention can be made more efficiently than previous devices.

[0038] The axially polarized zones 122a-d and 144a-d also are orientated to provide a mutual repulsion between the magnets 122, 144 and therefore create an almost frictionless bearing 300.

[0039] The bearing arrangement 300 is preferably used for the magnetic bearing of a high rotational speed machine, in particular in a turbomolecular pump to reduce stray fields in applications such as scanning electron microscopes.