WO2019008372A1

WO2019008372A1 - Magnetic bearing

Info

Publication number: WO2019008372A1
Application number: PCT/GB2018/051906
Authority: WO
Inventors: Eng Keen KHOR
Original assignee: Edwards Limited
Priority date: 2017-07-07
Filing date: 2018-07-05
Publication date: 2019-01-10
Also published as: GB2564420A; GB201710949D0

Abstract

Magnetic bearing for a vacuum pump, comprising a stationary first magnetic element (16) with a number of axially arranged non-rotating permanent ring magnets (16a, 16b, 16c, 16d, 16e) and a second magnetic element (18) rotating relatively to the stationary first magnetic element (16) with an equal number of axially arranged rotating permanent ring magnets (18a, 18b, 18c, 8d, 18e) each facing the non-rotating ring magnets (16a, 16b, 16c, 16d, 16e), wherein the polarization of the non-rotating permanent ring magnets (16a, 6b, 16c, 16d, 16e) and the rotating permanent ring magnets (18a, 18b, 18c, 10 8d, 18e) are each arranged to form a Halbach-configuration such that facing non-rotating permanent ring magnets (16a, 16b, 16c, 16d, 16e) and rotating permanent ring magnets (18a, 18b, 18c, 18d, 18e) are in mutual repulsion.

Description

MAGNETI C BEARI NG

The present invention is directed to a magnetic bearing for a vacuum pump and a vacuum pump comprising said magnetic bearing.

Turbomolecular pumps are often employed as a component of the vacuum system used to evacuate devices such as scanning electron microscopes (SEMs) and lithography devices.

It is common for said turbomolecular pumps to comprise an oil free, passive permanent magnetic bearing arrangement, located in the high vacuum end of the pump, to provide a substantially friction free, dry bearing arrangement free of lubricating materials that might otherwise cause contamination in the evacuated volume.

As described in EP 2705263, known arrangements of passive permanent magnetic bearings employ a plurality of individual axially stacked ring magnets. Examples of such arrangements are shown in Figures 1 and 2.

Figure 1 illustrates a section of a typical turbomolecular pump 200 comprising a series of rotor blades 106 extending outwardly from a rotor shaft 108. A passive magnetic bearing arrangement 100, 110 is located at the high vacuum (inlet) end of the shaft 108. The bearing arrangement 100, 110 comprises a series of three permanent magnet rings 100 fixed to the pump housing surrounded concentrically by a series of three individual permanent magnet rings 110 which are fixed to, and rotate with, the rotor arrangement 106, 108 about the axis 102. A cross section of a further example of a passive permanent magnetic bearing arrangement 10 for a turbomolecular pump (not shown) is illustrated in more detail in Fig 2. In this example the bearing arrangement 10 comprises an array an array 14 of four inner non-rotating permanent magnetic rings 14a, 14b, 14c and 14d arranged such that the outer, rotating, array 12 surrounds the inner, static, array 14 in a concentric manner. The outer array 12 is attached to the rotor of a turbomolecular pump (not shown) with the static array 14 attached to the stator of said pump.

In this example the magnetization, (that is, the polarization), of the magnetic rings 12a to 12d and 14a to 14d in each array 12, 14 respectively is substantially aligned with the axis of rotation 102 of the pump rotor (not shown). The direction of magnetization (polarization) has been indicated by the arrows, with the head of each arrow indicating the north pole.

The magnets are arranged within each array such that they are in mutual repulsion with each other; that is proximate magnets in an array meet their nearest neighboring magnet in the same array with the same pole (e.g. magnets 12a and 12b meet each other with their south poles). The outer magnetic rings 12 a, 12d, 14a, 14d in each array have their north poles facing outermost. The magnets 12a to 12d and 14a to 14d in each array 12, 14 of the arrangement 10 are orientated to provide a mutual repulsion between the arrays 12, 14 and therefore create an almost frictionless bearing.

In order to enhance the stiffness of the magnetic bearing either more permanent ring magnets can be stacked or the magnetization of the already existing permanent ring magnets can be enhanced. Both approaches to enhance the stiffness of the magnetic bearing increase the amount of used magnetic material. Since most of the used permanent magnets for magnetic bearings contain rare earth materials, using more magnetic material would considerably increase the costs for the magnetic bearing. Due to the symmetric arrangement of the polarization of the individual permanent ring magnets, the magnetic fields of the permanent magnetic rings is also symmetric on the inner side closer to the rotational axis 102 and also on the outer side. However, for the bearing mechanism only one half of the magnetic field is used on this side which is facing the respective non-rotating or rotating permanent ring magnet. Further, the unconcentrated magnetic field may also interfere as stray field with other electronic equipment such in the scanning electron microscope or the devices for which the vacuum pump might be used.

It is an object of the present invention to provide a magnetic bearing for a vacuum pump with an increased efficiency, while reducing stray fields.

A solution to the given problem is provided by the magnetic bearing in accordance with claim 1 and also by the vacuum pump of claim 7.

The magnetic bearing for a vacuum pump in accordance to the present invention comprises a stationary first magnetic element with a number of axially non-rotating permanent ring magnets. In use, the first magnetic element might be connected to the stator or the housing of the vacuum pump and thus being non-rotated. The number of non-rotating permanent ring magnets are stacked in an axial direction wherein the axial direction coincides in use with the rotational axis of the vacuum pump which is centrally through the bore of the non-rotating permanent ring magnets. Further, the magnetic bearing comprises a second magnetic element which is rotating relatively to the stationary first magnetic element. The second magnetic element comprises an equal number of axially arranged rotating permanent ring magnets. Thereby the number of the rotating permanent ring magnets is equal to the number of the non-rotating permanent ring magnets. In use, the second magnetic element might be connected to a rotor of the vacuum pump and rotated with the rotor. The rotating permanent ring magnets and the non-rotating permanent ring magnets are arranged concentrically wherein non-rotating and rotating permanent ring magnets facing each other. Thereby, the magnetic polarization of the non-rotating permanent ring magnets and also the rotating permanent ring magnets are each arranged to form a Halbach configuration such that facing non-rotating permanent ring magnets and rotating permanent ring magnets are in mutual repulsion. Due to the Halbach configuration, or also known as Halbach Array, the magnetic field created by the permanent ring magnets is asymmetric in a radial direction. Thus, the magnetic field of the non-rotating permanent ring magnets and the rotating permanent ring magnets can be concentrated on those sides, where the permanent ring magnets are facing each other. Due to the increase of magnetic field density, the magnetic bearing becomes stiffer without use of more magnetic material. Thereby costs can be reduced. Further, on the outermost sides of the permanent ring magnets, facing away from the magnetic bearing, the magnetic field is almost cancelled out. Thus, stray fields are greatly reduced which would have otherwise interfered with other technical equipment.

Preferably, the number of rotating permanent ring magnets and consequently also the number of non-rotating permanent ring magnets is at least three arranged in an axial direction. With five permanent ring magnets a Halbach Array can be created with sufficient concentration of the magnetic field on one side of the permanent ring magnets and cancellation of the magnetic field on the other side of the respective permanent ring magnets. Preferably, the number of used rotating permanent ring magnets and also the number of non- rotating permanent ring magnets can be further increased in order to enhance the bearing stiffness even further.

Preferably, the rotating permanent ring magnets and the non-rotating permanent ring magnets are along an axial direction alternating radially and axially polarized. Thus, for example along the axial direction of the magnetic bearing, the first permanent ring magnet might be radially or axially polarized. Then the second permanent ring magnet is respectively axially or radially polarized and so on. Preferably, along an axial direction the polarization of radially polarized rotating permanent ring magnets and non-rotating permanent ring magnets is alternating such that opposite facing in the sense of mirror images. Additionally, also the polarization of axially polarized rotating permanent ring magnets and non-rotating permanent ring magnets is also alternating along an axial direction. Thereby, from one radially polarized permanent ring magnet to the next radially polarized ring magnet, the direction of polarization changes by 180 degrees. Similarly, from one axially polarized permanent ring magnet to the next axially polarized ring permanent ring magnet, the direction of the polarization changes also by 180 degrees. In other words, the polarization of the permanent ring magnets changes from one permanent ring magnet to the next permanent ring magnet along an axial direction by 90 degrees. Thus, for example the fifth permanent ring magnet comprises the same direction of polarization as the first permanent ring magnet.

Preferably, the polarization of magnetization is angled relatively to the axial direction of the magnetic bearing. Then the polarization of the permanent ring magnets changes from one permanent ring magnet to the next permanent ring magnet along an axial direction by 90 degrees. Thus, for example the fifth permanent ring magnet comprises the same direction of polarization as the first permanent ring magnet. If the polarization of the non-rotating permanent ring magnets are angled relatively to the axial direction of the magnetic bearing also the polarization of the rotating permanent ring magnets are angled relatively to the axial direction to be in mutual repulsion with each other. In other words also in the case of an angled polarization the polarizations of the non-rotating permanent ring magnets and the rotating permanent ring magnets are mirror images. Preferably, the rotating permanent ring magnets and/or the non-rotating permanent ring magnets comprise ferrite/ ceramic, AINiCo, SmCo, NdFeB. In particular the ring magnets are formed by hard magnets which preferably comprise an intrinsic coercivity of more than 10kA/m.

Preferably, the first magnet element and/or the second magnetic element are formed as a single piece. Thereby, the manufacture effort for the first magnetic element and/or the second magnetic element can be reduced.

The present invention further relates to a vacuum pump with at least one magnetic bearing as previously described. In particular, the vacuum pump comprises a stator and a rotor rotated relatively to the stator. The rotor comprises rotor elements in order to convey a gas from a pump inlet to a pump outlet. Thereby, the rotor is mounted by two bearings wherein as mentioned above, at least one bearing is built as magnetic bearing as previously described. Thereby, usually the magnetic bearing is arranged in the area of the vacuum and towards the inlet sided of the rotor. However, alternatively both bearings of the rotor can be built as magnetic bearings as previously described.

Preferably, in the second magnetic element, which is connected to the rotor of the vacuum pumps surrounds concentrically the first magnetic element which is connected to the housing of the pump or the stator of the pump.

Preferably, the vacuum pump is a turbo molecular pump or a molecular drag pump.

The present invention will be explained in further detail with reference to the accompanying drawings.

I n the figures:

Figure 1 shows the vacuum pump is a magnetic bearing;

Figure 2 shows a detailed view of a magnetic bearing arrangement; Figure 3 shows a detailed view of a magnetic bearing arrangement in accordance with the present invention and

Figure 4 shows a alternative embodiment with angled polarization.

Figures 1 and 2 are already described in the introduction part.

Figure 3 shows a schematic drawing of a magnetic bearing in accordance with the present invention. The magnetic bearing comprises a stationary first magnetic element 16 wherein the axial direction of the first magnetic element 16 coincides with the rotational axis 102 of the vacuum pump. The first magnetic element 16 comprises five axially arranged non-rotating permanent ring magnets 16a, 16b, 16c, 16d, 16e. The first magnetic element comprises a first inner side 20 and a first outer side 22. In the first outer side 22 is facing a second magnetic element 18, concentrically arranged with the first magnetic element 16 wherein also the axial axis of the second magnetic element 18 coincides with the rotational axis 102 of the vacuum pump. First magnetic element 16 and second magnetic element 18 are separated by a slit 24 in order to form a contact less bearing arrangement. The second magnetic element 18 also comprises five permanent ring magnets 18a, 18b, 18c, 18d, 18e. Further, the second magnetic element 18 comprises a second inner side 26 facing the first magnetic element 16 and a second outer side 28.

In a usual arrangement the first magnetic element 16 is connected to the stator or the housing of the vacuum pump and the second magnetic element 18 is rotated relative to the first magnetic element 16 and connected to the rotor of the vacuum pump. However, different configuration is also possible.

The polarizations of the permanent ring magnets 16a, 16b, 16c, 16d, 16e of the first magnetic elements 16 and the polarizations of the permanent ring magnets 18a, 18b, 18c, 18d, 18e of the second magnetic element 18 are arranged to be in mutual repulsion in order to create a frictionless bearing. Thereby, in the permanent ring magnets of the first magnetic element 16 and the second magnetic element 18 each form a Halbach Array. Therefore, the first permanent ring magnet 16a, 18a of the first magnetic element 16 and the second magnetic element 18 respectively is for example radially polarized. The second permanent ring magnets 16b, 18b are axially polarized and so on in an alternating manner. Alternatively, the first permanent ring magnet 16a, 18a of the first magnetic element 16 and the second magnetic element 18 respectively might be axially polarized. The second permanent ring magnets 16b, 18b are radially polarized and so on in an alternating manner.

In the shown embodiment of Fig. 3, the polarization between the radially polarized permanent ring magnets is alternated i.e. changed by 180 degrees. Thus, for example the radial polarization direction of the third permanent ring magnet 16c of the first magnetic element 16 is turned by 180 degrees compared to the radial polarization of the first permanent ring magnet 16a. Also the polarization direction from one axially polarized permanent ring magnet to the next axially polarized permanent ring magnet is also alternating. In other words, the direction of polarization is changed from one permanent ring magnet to the next permanent ring magnet by 90 degrees, while the direction of rotation between the first magnetic element 16 and the second magnetic element 18 is in the opposite order.

Fig. 4 shows a further embodiment of the present invention wherein same or similar features are indicated by the same reference number. In Fig. 4 the direction of polarization of each of the permanent ring magnets of the first magnetic element 16 and the second magnetic element 18 are angled relatively to the axial direction 102 of the magnetic bearing still forming a Halbach Array. Thereby the direction of polarization is mirror imaged between the non-rotating permanent ring magnet and the rotating permanent ring magnet. The angle between the polarization and the axial direction is between 0 degree and 90 degree and preferably 45 degree. Thus in all embodiments in accordance with the present invention by the Halbach Array, the first magnetic element 16 comprises a stronger magnetic field on the first outer side 22 while at the first inner side 20 the magnetic field is almost cancelled to zero. Simultaneously, the second magnetic element 18 comprises a strong magnetic field on the second inner side 26 which is facing the first outer side 22 of the first magnetic element 16, while on the second outer side 28 of the second magnetic element 18, the magnetic field is cancelled almost to zero. Thus, the magnetic field between the first magnetic element 16 and the second magnetic element 18 is enhanced and thereby the stiffness of the magnetic bearing is improved without use of additional magnetic material. Additionally, since the magnetic field on the second outer side 28 of the second magnetic element 18 is reduced, stray fields are reduced which may interfere with other electronic equipment. Thus, by the present invention a stiffer magnetic bearing is achieved without use of more magnetic material which reduces costs. Simultaneously, influences of other electronic equipment due to magnetic stray fields are also reduced.

Claims

CLAI MS

1. Magnetic bearing for a vacuum pump, comprising a stationary first magnetic element (16) with a number of axially arranged non-rotating permanent ring magnets (16a, 16b, 16c, 16d, 16e) and a second magnetic element (18) rotating relatively to the stationary first magnetic element (16) with an equal number of axially arranged rotating permanent ring magnets (18a, 18b, 18c, 18d, 18e) each facing the non- rotating ring magnets (16a, 16b, 16c, 16d, 16e), wherein the polarization of the non-rotating permanent ring magnets (16a, 16b, 16c, 16d, 16e) and the rotating permanent ring magnets (18a, 18b, 18c, 18d, 18e) are each arranged to form a Halbach-configuration such that facing non-rotating permanent ring magnets (16a, 16b, 16c, 16d, 16e) and rotating permanent ring magnets (18a, 18b, 18c, 18d, 18e,) are in mutual repulsion.

2. Magnetic bearing according to claim 1 , characterized in that the number of rotating permanent ring magnets (18a, 18b, 18c, 18d, 18e) and also the number of non-rotating permanent ring magnets (16a, 16b, 16c, 16d, 16e) is at least three preferably at least five arranged in an axial direction.

3. Magnetic bearing according to claim 1 or 2, characterized in that rotating permanent ring magnets (18a, 18b, 18c, 18d, 18e) and non-rotating permanent ring magnets (16a, 16b, 16c, 16d, 16e) are in an axial direction alternating radially and axially polarized.

4. Magnetic bearing according to claim 3, characterized in that the direction of polarization of radially and axially polarized rotating permanent ring magnets (18a, 18b, 18c, 18d, 18e) and non-rotating permanent ring magnets (16a, 16b, 16c, 16d, 16e) is alternating in an axial direction.

5. Magnetic bearing according to claim 1 or 2, characterized in that the direction of polarization rotating permanent ring magnets (18a, 18b, 18c, 18d, 18e) and n on -rotating permanent ring magnets (16a, 16b, 16c, 16d, 16e) is angled relatively to the axial direction, wherein preferably the angle is between 0 degree and 90 degree.

6. Magnetic bearing according to any of claims 1 to 5, characterized in that the rotating permanent ring magnets (18a, 18b, 18c, 18d, 18e) and/or the non-rotating permanent ring magnets (16a, 16b, 16c, 16d, 16e) comprise Ferrite-ceram ic, AINiCo, SmCo, NdFeB.

7. Magnetic bearing according to any of claims 1 to 6, characterized in that the first magnetic element (16) and/ or the second magnetic element (18) are formed as single piece.

8. Vacuum Pump with at least one magnetic bearing in accordance with any of claims 1 to 7, wherein in particular the second magnetic element (18) surrounds the first magnetic element (16) concentrically.

9. Vacuum pump according to claim 8, characterized by two magnetic bearings in accordance with any of claims 1 to 7.

10. Vacuum pump according to claims 8 or 9, characterized in that the vacuum pump is a turbomolecular pump or a molecular drag pump.