GB2570006A

GB2570006A - Magnetic bearing and vacuum pump with such a magnetic bearing

Info

Publication number: GB2570006A
Application number: GB1800343.4A
Authority: GB
Inventors: Keen Khor Eng
Original assignee: Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2018-01-09
Filing date: 2018-01-09
Publication date: 2019-07-10
Also published as: WO2019138213A1; GB201800343D0; DE212019000161U1

Abstract

Disclosed is a magnetic bearing 38 for supporting a rotating element with a non-rotating first magnetic element 42 that is in mutual repulsion with a rotating second magnetic element 44. At least one of the magnetic elements comprises a permanent magnetic made of a magnetic material whose relative magnetic permeability a first temperature is less than at a second higher temperature. The first temperature may be about 20ºC (ambient). The second temperature may be the maximum temperature of the rotating element. The magnetic material may be a neodymium magnet with a working temperature of 120-200ºC. Also disclosed is a turbomolecular vacuum pump 10 with said magnetic bearing. The use of neodymium magnets in a turbomolecular vacuum pump allows the magnetic strength for the rotor component to decrease as the rotor thermally expands. A roller bearing 32 is preloaded by offsetting the magnets of the magnetic bearing, but as the rotor thermally expands the preloading force is increased. The weakening magnetic strength of the neodymium somewhat mitigates this increase, meaning less force and thus less wear is experienced by the roller bearing.

Description

MAGNETIC BEARING AND VACUUM PUMP WITH SUCH A MAGNETIC BEARING

The present invention relates to a magnetic bearing for supporting a rotating element, in particular a rotor of a vacuum pump. Further, the present invention relates to a vacuum pump comprising such a magnetic bearing.

Known vacuum pumps comprise a housing with an inlet and an outlet. In the housing a motor-driven rotor is disposed, wherein the rotor comprises several rotor elements interacting with stator elements in order to convey a gaseous medium from the inlet to the outlet. In particular, for turbo molecular vacuum pumps, the rotational speed of the rotor is very high and may be up to several thousand revolutions per minute. This puts high requirements on the bearings with which the rotor is supported against the housing.

It is known to use magnetic bearings in order to support the rotating rotor of the vacuum pump against the housing. The magnetic bearing comprises a first non-rotating magnetic element which is connected to the housing, and a second rotating magnetic element which is connected to the rotor. First magnetic element and second magnetic element are arranged in close proximity while they are in mutual repulsion to each other to maintain a contactless bearing. This provides the advantage that no oil or grease is necessary which could contaminate the vacuum by outgassing. For cost reasons, usually only the bearing at the high vacuum end towards the inlet of the vacuum pump is built as magnetic bearing. The second bearing at the high pressure side towards the outlet of the vacuum pump may be built as roller bearing, since contamination of the vacuum by grease or oil in this area is negligible.

Upon operation of the vacuum pump, the temperature of the rotor may increase. Usually magnetic material which could be used for the magnetic bearing undergoes a weakening of the magnetic strength upon increase of temperature. Thus, in known magnetic bearings usually SmCo-magnets are used which provide permanent magnets which are relatively stable upon increase of temperature.

Usually the first bearing and the second bearing are arranged such that there is a preload to the roller bearing due to a small offset of the magnetic bearing to the neutral position, such that an axial force is employed towards the roller bearing/second bearing at the high pressure side. However, during operation the temperature of the rotor may rise such that the rotor is elongated due to thermal expansion. Thereby the preload to the roller bearing is increased which results in a fast wearout of the roller bearing. Thus, the lifetime of the roller bearing is reduced.

It is an object of the present invention to provide a magnetic bearing that is able for compensating thermal expansion. Further, it is an object of the present invention to provide a vacuum pump which is highly durable.

The above-mentioned problems are solved by the magnetic bearing in accordance with claim 1 as well by the vacuum pump in accordance to claim 7.

The magnetic bearing in accordance to the present invention for supporting a rotating element, in particular a rotor of a vacuum pump, comprises a nonrotating first magnetic element and a second magnetic element rotating relatively to the first magnetic element. Therein, in particular the non-rotating first magnetic element is connected to a static element of a rotating device such as a housing of a vacuum pump, while the second magnetic element is connected to the rotating element, in particular a rotor of a vacuum pump. Thereby, the first magnetic element and the second magnetic element are arranged in close proximity to each other and in mutual repulsion to each other to maintain a contactless bearing which is also frictionless. Thus, no grease or oil is necessary, and the magnetic bearing can be employed in high vacuum environments without the risk of contaminating the vacuum by outgassing. The first magnetic element or the second magnetic element comprise a permanent magnet made of a magnetic material, wherein the magnetic material has a relative magnetic permeability μΓ(Τι) at a first temperature Ti and a relative magnetic permeability pr(T2) at a second temperature T2 which fulfill μΓ(Τι) < pr(T2), while the first temperature is smaller than the second temperature. The relative magnetic permeability is thereby defined as pr = Β/(Η*μ0) with B as the magnetic flux density, H as the magnetic field strength, and μο as the vacuum permeability. Thus, the relative magnetic permeability of the magnetic material employed as first magnetic element and/or second magnetic element weakens under an increase of temperature. Thus, thermal expansion of the rotating element is compensated due to a decrease of the magnetic strength of at least one of the magnetic elements. Thereby the preload to a second bearing supporting the rotating element, resulting from the thermal expansion of the rotor, is reduced by the inventive magnetic bearing.

In particular, first magnetic element and second magnetic element comprise the same magnetic material. Alternatively, the first magnetic element and second magnetic element may be built from different magnetic material, in order to tailor the effect of the reduction of the relative magnetic permeability in accordance to the needs of the rotating device, such as the vacuum pump.

In particular, the difference between μΓ(Τι) and μΓ(Τ2) is determined in accordance to the temperature difference between the rotating element and the nonrotating element during operation. At the beginning of the operation, the rotating element and the non-rotating element have the same temperature which is or may be almost equal to the ambient temperature. Thus upon start of operation, the temperature of the rotating element may rise which leads to thermal expansion of the rotating element. The temperature of the non-rotating element, such as the casing of the rotating device, is usually lower due to cooling, either by ambient air or by active cooling. Thus, the temperature difference leads to a different thermal expansion, whereby the thermal expansion of the non-rotating element is smaller than that of the rotating element. This results in an increase of the offset of the rotating element to the neutral position also increasing the preload to a second bearing. The difference between Pr(Ti) and μΓ(Τ2) compensates this difference of thermal expansion by tailoring the difference between μΓ(Τι) and μΓ(Τ2) correspondingly.

In particular, the difference between μΓ(Τι) and μΓ(Τ2) is determined in accordance to the difference of thermal expansion between the rotating element and the non-rotating element. By the difference of the thermal expansion, a preload is applied to the bearings which may lead to a fast wearout of used roller bearings. Thus, the difference between μΓ(Τι) and μΓ(Τ2) is tailored in accordance to the thermal expansion difference between the rotating element and the nonrotating element in order to compensate the difference of thermal expansion between the rotating element and the non-rotating element.

In particular, the relative magnetic permeability fulfills the condition ο*μΓ(Τι) < μΓ(Τ2), with c between 1.2 and 2, and preferably between 1.26 and 1.6. Most preferably, c is greater than 1.26, to achieve a most suitable reduction of the magnetic strength due to increased temperature. Thereby, the factor c may depend on the specific rotating device.

In particular, Ti corresponds to the ambient temperature of about 20°C, and T2 corresponds to the maximum temperature of the rotating element of about 90°C.

In particular, the magnetic material comprises neodymium (Nd). Usually, Nd is not used in magnetic bearings due to the strong thermal decrease of magnetic strength. On the other hand, magnets comprising Nd usually provide the highest magnetic strength at 20°C, even higher than for the samarium cobalt magnets. Thus in the present invention, Nd magnets provide the further advantage that the necessary offset at ambient temperatures can be smaller in order to reach sufficient preload on the roller bearing(s) of the rotating element. Thus, upon thermal expansion the offset to the neutral position under higher temperatures during operations is smaller than, for example, with samarium cobalt magnets. However, in order to provide sufficient radial stiffness of the rotating element, such as the rotor of the vacuum pump, the magnetic material comprising Nd should be tailored at operation temperatures to the radial stiffness provided by samarium cobalt magnets also at operation temperature. Thus, even for a weakened magnetic strength of Nd magnets under higher temperatures, preload to the second bearing is decreased as previously described, while simultaneously radial stiffness is maintained.

In particular, the magnetic material is one of the materials with maximum working temperature between 120°C - 200 °C. All these materials are specific Nd magnets suitable for the above-described purpose. In particular, the material grade is shown in the table below:

Further, the present invention relates to a vacuum pump, in particular a turbo molecular vacuum pump, comprising a stator and a rotor wherein the rotor is rotated by a motor and comprises several rotor elements in order to convey a gaseous medium from an inlet to an outlet. Thereby, the rotor is supported by at least two bearings, wherein at least one bearing is a magnetic bearing as previously described.

In particular, at least one bearing is a roller bearing. Thereby the magnetic bearing is disposed at the high vacuum side of the vacuum pump, i.e. towards the inlet of the vacuum pump, while the roller bearing is disposed at the low vacuum side, i.e. towards the outlet of the vacuum pump in a region of a higher pressure. Thus, the frictionless/contactless magnetic bearing may be situated in the vacuum, whereby no grease or oil can contaminate the vacuum.

In particular, the difference between μΓ(Τι) and μΓ(Τ2) is determined in accordance to the temperature difference between rotor and stator during operation.

In particular, the difference between μΓ(Τι) and μΓ(Τ2) is determined in accordance to the difference of thermal expansion between rotor and stator during operation. Thus, if there is a difference between the thermal expansion between rotor and stator, the magnetic material of the magnetic bearing is tailored such that the magnetic strength of the magnetic bearing is reduced at operation temperature in order to reduce preload or the axial force towards the second bearing, which might be built as roller bearing. Thus, wearout of the roller bearing is reduced and a durable vacuum pump can be achieved.

Due to tailoring and exploiting of the effect that the magnetic strength of some magnetic materials is decreased at higher temperatures, preload between the magnetic bearing and a roller bearing of a vacuum pump due to thermal expansion can be reduced or even compensated. Thus, the roller bearing can always be operated at optimal preload conditions, reducing wearout of the bearing and enhancing the lifetime.

The present invention is further explained with respect to the embodiments shown in the accompanying figures.

The figures show:

Fig. 1 an exemplary embodiment of the vacuum pump

Fig. 2 a diagram for the axial force vs. the axial offset and

Fig. 3 a diagram for the radial stiffness vs. the axial offset.

The vacuum 10 comprises a housing 12 wherein with the housing several stator elements 14 are connected. In the housing a rotor 16 is disposed, wherein the rotor 16 comprises several rotor elements 18 interacting with the stator elements 14 in order to convey a gaseous medium from an inlet 20 to an outlet 22. Thereby the rotor 16 is rotated by an electro motor 24, comprising a stator 26 and a rotating element 28 connected to the rotor 16. Further, the rotor 16 is supported at the high pressure side 30 towards the outlet 22 by a roller bearing 32 against a first supporting element 34 of the housing 12. At the other end of the rotor 16, towards the inlet 20 of the vacuum pump 10, in the region 36 of low pressure and high vacuum, the rotor 16 is supported by a magnetic bearing 38 against a second supporting element 40 which is connected to the housing 12. The magnetic bearing 38 comprises a first non-rotating magnetic element 42 which is connected to the second supporting element 40. Further, the magnetic bearing 38 comprises a second rotating magnetic element 44 connected with the rotor 16. Thereby, the first magnetic element 42 and the second magnetic element 44 are arranged in close proximity to each other and in mutual repulsion to each other in order to provide a contactless and also frictionless bearing. Thereby, one of the first magnetic element 42 or second magnetic element 44 is slightly replaced by a small offset relative to a neutral position. Thereby an axial force in the direction of arrow 46 is generated in order to exert a preload to the roller bearing 32. The offset might be in the range of a view hundred pm. The relation between the axial offset and the generated force in the direction of arrow 46 is shown in the diagram of Fig. 2 for a samarium cobalt 5 magnet and an N32H magnet at ambient temperature.

If the temperature of the rotor 16 increases during operation of the vacuum pump 10, the axial offset is also increased due to thermal expansion of the rotor 16 which usually exceeds the thermal expansion of the housing 12. This leads to an increase of the generated preload force on roller bearing 32. This becomes also evident in view of the diagram of Fig. 2 showing the increased preload as "y-Force" on the roller bearing 32 due to the increase of the offset at operation temperature (denoted as "Hot")· However, due to the increased temperature also the magnetic strength of the employed magnetic material of the first magnetic element and/or the second magnetic element is decreased which also decreases the generated preload shown as difference between the curves in Fig. 2 for a certain material at different temperatures. However, in particular for the usually used SmCo5-magnets, this effect is not sufficient to compensate the generated perforce due to the thermal expansion of the rotor. In a specific example for the axial offset, in order to achieve 12.5 N of axial preload at ambient temperature, with SmCo5 as magnetic material the offset needs to be 130 pm while for N32H 85 pm as offset are sufficient in order to achieve 12.5 N of axial preload at ambient temperature. If the maximum offset increase due to thermal expansion is in both cases 100 pm, then the axial offset position at higher temperatures ("Hot") is for SmCo5 at about 230 pm, while for N32H magnets the axial offset at higher temperatures is about 185 pm. Thus, in accordance to the diagram of Fig. 2, the axial preload at higher temperatures is for the usually used SmCo5 as magnetic material for the magnetic elements is increased by 44% to 18 N, which will lead to a fast wearout of the roller bearing 32. Contrary, with a N32H magnet the increase of axial preload at higher temperatures is only about 16% to 14.5 N, which greatly enhances the lifetime of the roller bearing 32 since the roller bearing 32 can be operated close to the optimal axial preload.

Since in particular Nd magnets comprise a higher magnetic strength at room temperature than samarium cobalt magnets as described above, the necessary axial offset at room temperature to achieve the desired axial preload can be smaller. Thus in accordance to Fig. 3, the radial stiffness of a magnetic bearing comprising N32H as magnetic material is about 37 N/mm at operating temperature ("Hot"), which is comparable to the radial stiffness of a magnetic bearing with SmCo5 magnets, or even slightly better. Thus by employing a magnetic material in the magnetic bearing which shows stronger weakening under higher temperatures, the axial preload can be kept close to the optimum of the roller bearing 32. Simultaneously, the radial stiffness is comparable or even slightly better due to the use of magnets which show a stronger magnetic field at ambient temperature.

Claims

1. Magnetic bearing for supporting a rotating element comprising: a non-rotating first magnetic element and a second magnetic element rotating relatively to the first magnetic element, wherein the first magnetic element and the second magnetic element are in mutual repulsion to each other to maintain a contact less bearing, wherein the first magnetic element and/or the second magnetic element comprise a permanent magnet made of a magnetic material, wherein the magnetic material has a relative magnetic permeability pr(Ti) at a first temperature Ti and a relative magnetic permeability pr(T2) at a second temperature T2 which fulfill μΓ(Τι) < pr(T2) with Ti <T2.

2. Magnetic bearing according to claim 1, characterized in that the difference between pr(Ti) and pr(T2) is determined in accordance to the temperature difference between rotating element and the non-rotating element during operation.

3. Magnetic bearing according to claim 1 or 2, characterized in that the difference between pr(Ti) and pr(T2) is determined in accordance to the thermal expansion of the rotating element during operation.

4. Magnetic bearing according to any of claims 1 to 3, characterized in that c-pr(Ti) < pr(T2) with c between 1.2 and 2, preferably between 1.26 and 1.6 and most preferably greater than 1.26.

5. Magnetic bearing according to any of claims 1 to 4, characterized in that Ti corresponds to the ambient temperature of about 20°C and T2 corresponds to the maximum temperature of the rotating element.

6. Magnetic bearing according to any of claims 1 to 5, characterized in that the magnetic material comprises Nd.

7. Magnetic bearing according to any of claims 1 to 6, characterized in that the magnetic material is a magnetic material with a working temperature of above 120°C, preferably between 120°C to 200°C.

8. Magnetic bearing according to any of claims 1 to 7, characterized in that the magnetic material is one of the magnetic material grades: any of N27 to N50 with Magnet Type Suffix H for a maximum working temperature of 120°C, any of N27 to N48 with Magnet Type Suffix SH for a maximum working temperature of 150°C, any of N27 to N45 with Magnet Type Suffix UH for a maximum working temperature of 180°C and any of N27 to N42 with Magnet Type Suffix EH for a maximum working temperature of 200°C .

9. Vacuum pump, in particular turbomolecular vacuum pump, comprising a stator and a rotor, wherein the rotor is rotated by a motor and comprises rotor elements in order to convey a gaseous medium from an inlet to an outlet, wherein the rotor is supported by at least two bearings, wherein at least one bearing is a magnetic bearing in accordance with any of the claims 1 to 8.

10. Vacuum pump according to claim 9, characterized in that at least one bearing is a roller bearing.

11. Vacuum pump according to claim 9 or 10, characterized in that the difference between μΓ(Τι) and μΓ(Τ2) is determined in accordance to the temperature difference between rotor and stator during operation.

12. Vacuum pump according to any of claims 9 to 11, characterized in that the difference between μΓ(Τι) and μΓ(Τ2) is determined in accordance to the thermal expansion of the rotor during operation.