Elongated permanent ring management 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 bearing arrangement comprising: a non-rotating ring magnet comprising an array of axially polarized permanent ring magnets with axially neighboring axially polarized permanent ring magnets in the array in mutual repulsion to each other; a rotating ring magnet comprising a single elongate piece with an even number of axially polarized permanent magnetic zones between axial ends of the rotating ring magnet, with axially neighboring axially polarized permanent magnetic zones in mutual repulsion to each other; wherein the rotating ring magnet concentrically surrounds the non-rotating ring magnet whereby, the rotating ring magnet has an axis of rotation parallel with, and centrally through a bore of the rotating ring magnet; and wherein the axially polarized permanent magnetic zones on the rotating magnet and the array of axially polarized permanent ring magnets of the non-rotating ring magnet are orientated to provide a mutual repulsion between the rotating ring magnet and non-rotating ring magnet.

2. The bearing arrangement of claim 1, wherein the even number of axially polarized permanent magnetic zones is at least four.

3. The bearing arrangement of claim 1, wherein the bearing arrangement is included in a high speed rotation device.

4. The bearing arrangement of claim 1, wherein the bearing arrangement is included in a turbomolecular pump.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In order that the present invention may be well understood embodiments thereof, which are given by way of example only, will now be described with reference to the accompanying drawings, in which:

(2) FIG. 1 is a cross section of a turbomolecular pump of the prior art;

(3) FIG. 2 is a cross sectional illustration of a passive magnetic bearing arrangement for a turbomolecular pump of the prior art;

(4) FIG. 3a is an illustration of the asymmetric magnetisation of a permanent ring magnet of the prior art;

(5) FIG. 3b is an illustration of the asymmetric magnetisation of a permanent ring magnet of the prior art; and

(6) FIG. 4 is an illustration of a magnetic bearing arrangement comprising the magnet according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(7) 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).

(8) 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 error, , along the whole axial length of a single piece magnet. Thus, assuming that each of the polarized zones 122a, 122b, 122c, and 122d are of equal magnetic strength, the net transverse dipole moment along a single piece magnet 122 will be zero (as shown by the line 400 in FIG. 4).

(9) Although the magnet 122 is illustrated with 4 axially polarized zones, it may have any even number of axially polarized zones, for example 6, 8 10 12, according to the requirements of the particular bearing arrangement. The important feature is that there must be an even number of axially polarized zones for there to be a zero net transverse dipole moment, and thus virtually no stray fields produced when the magnet is rotating in the bearing arrangement of a turbomolecular pump.

(10) The magnet 122 is produced following the stages described above. However in order to produce a plurality of axially polarized zones, once the single piece magnet has been compressed in the aligning field to the desired shape, sintered and finished it is advantageous to charge the magnet to produce each of the polarized zones 122a, 122b, 122c and 122d simultaneously. In order to do this the as yet uncharged zones 122a, 122b, 122c and 122d (not shown) should each be surrounded by its own solenoid designed to axially polarize said zones. Ideally a solenoid is also placed into the internal bore of the magnet 122 so that each zone 122a, 122b, 122c and 122d has its own solenoid pair charging it. The solenoids/solenoid pairs preferably create each axially polarized zone to be of the same magnetic strength.

(11) 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.

(12) 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.

(13) 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.