WO2019138213A1

WO2019138213A1 - Magnetic bearing and vacuum pump with such a magnetic bearing

Info

Publication number: WO2019138213A1
Application number: PCT/GB2019/050036
Authority: WO
Inventors: Eng Keen KHOR
Original assignee: Edwards Limited
Priority date: 2018-01-09
Filing date: 2019-01-08
Publication date: 2019-07-18
Also published as: DE212019000161U1; GB201800343D0; GB2570006A

Abstract

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 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. The magnetic bearing comprises a non-rotating first magnetic element and a second magnetic element rotating relative to the first magnetic element. Thereby, the first magnetic element and the second magnetic element are in mutual repulsion to each other to maintain a contact-less bearing. Thereby, 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 μr(Τ₁) at a first temperature T₁ and a relative magnetic permeability μr(Τ2) at a second temperature T₂ which fulfill μr(Τ₁) < μr(Τ2) with T₁ < T₂.

Description

MAGNETIC BEARING AND VACUUM PUMP WITH SUCH A MAGNETIC

BEARI NG

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 and a method for fabricating 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 sec ond 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 in crease. Usually magnetic material which could be used for the magnetic bear- ing undergoes a weakening of the m agnetic strength upon increase of temper ature. Thus, in known m agnetic bearings usually Sm Co-m agnets are used which provide perm anent magnets which are relatively stable upon increase of tem perature in order to maintain radial stiffness even at higher tem peratures.

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 m agnetic bearing from the neutral position, such that an axial force is em ployed towards the roller bearing/second bearing at the high pressure side in order to m aintain radial stiffness of the rotor. However, during operation the tem perature of the rotor m ay 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 m agnetic bearing that is able for com pensating therm al expansion. Further, it is an object of the present invention to provide a vacuum pump which is highly durable.

The above-m entioned problem s are solved by the m agnetic bearing in accord ance with claim 1 and 9 as well by the vacuum pump in accordance to claim 10. Further, the given technical problem is solved by the m ethod of claim 14.

The m agnetic bearing in accordance to the present invention for supporting a rotating element, in particular a rotor of a vacuum pum p, comprises a non rotating first magnetic elem ent and a second magnetic elem ent rotating rela tively to the first m agnetic element. Therein, in particular the non-rotating first m agnetic elem ent is connected to a static elem ent of a rotating device such as a housing of a vacuum pump, while the second magnetic elem ent is connected to the rotating elem ent, in particular a rotor of a vacuum pum p. Therein, the first magnetic element and the second magnetic element are arranged in close proxim ity to each other and in m utual repulsion to each other to m aintain a contactless bearing which is also frictionless. Thus, no grease or oil is neces sary, and the magnetic bearing can be employed in high vacuum environments without the risk of contaminating the vacuum by outgassing. Preferably, the first magnetic element is disposed relative to the second magnetic element such that an axial force is applied to the rotating element. Therefore, the first magnetic element might be disposed offset from a neutral position relative to the second magnetic element. This applied axial force might induce a preload in a second bearing also supporting the rotating element. 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 m_G(Ti) at a first temperature Ti and a relative magnetic permeability m_G(T₂) at a second temperature T₂which fulfill m_G(Ti) < m_G(T₂), while the first temperature is smaller than the second temperature. The relative magnetic permeability is thereby defined as m_G = B/(H m₀) with B as the mag netic flux density, H as the magnetic field strength, and m₀ as the vacuum permeability. Thus, the relative magnetic permeability of the magnetic material employed as first magnetic element and/or second magnetic element increases while the magnetic field strength H weakens under an increase of temperature. By decrease of the magnetic field strength H an increase of an additional axial force due to the temperature increase from Ti to T₂ is compensated, wherein compensation might be understood here and in the following as full compen sation, partial compensation or even over-compensation. This additional axial force might be induced due to thermal expansion of the rotating element which 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 re duced by the inventive magnetic bearing and the preload to the second bearing can be maintained within the allowable limits to avoid skidding or fatigue of the second bearing I n particular, first m agnetic elem ent and second m agnetic elem ent com prise the same m agnetic material. Alternatively, the first m agnetic element and sec ond magnetic element m ay be built from different m agnetic m aterial, in order to tailor the effect of the reduction of the relative m agnetic perm eability in accordance to the needs of the rotating device, such as the vacuum pum p.

I n particular, the difference between m_G(Ti) and m_G(T₂) or analogously the dif ference between the m agnetic field strength at the first tem perature Ti and at the second tem perature T₂ is determ ined or set in accordance to the tem pera ture difference between the rotating elem ent and the non-rotating elem ent during operation such that by increase of the tem perature from Ti to T₂ in crease of the additional axial force due to the tem perature increase from Ti to T₂ is com pensated. At the beginning of the operation, the rotating element and the non-rotating element have the same tem perature which is or m ay be al m ost equal to the am bient temperature. Thus upon start of operation, the tem perature of the rotating element m ay rise which leads to thermal expansion of the rotating elem ent. 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 tem perature difference leads to a different thermal expansion, whereby the therm al expansion of the non-rotat ing elem ent is sm aller than that of the rotating element. This results in an increase of the offset of the rotating elem ent relative to the neutral position increasing the applied axial force and also the preload to a second bearing. The m agnetic m aterial properties m_G(Ti) and m_G(T₂) com pensates this difference of thermal expansion by tailoring the difference between m_G(Ti) and m_G(T₂) corre spondingly. Hence, a specific magnetic m aterial which shows a weakening of the magnetic strength is used for the bearing in order to tailor the thermal properties of the bearing and utilize these therm al properties to com pensate for therm al expansion of the rotor. Therein the weakening of the m agnetic strength with increasing tem perature was usually considered as disadvanta geous and therefore well known magnetic bearings use m agnetic m aterials which show only as little as possible weakening at all. I n the present invention this effect is utilized to compensate for another thermal effect, i.e. thermal expansion.

In particular, the difference between m_G(Ti) and m_G(T₂) is determined or set in accordance to the difference of thermal expansion between the rotating ele ment and the non-rotating element. By the difference of the thermal expan sion, a preload is applied to the bearings which may lead to a fast wearout, fatigue or skidding of used roller bearings. Thus, the difference between m_G(Ti) and m_G(T₂) is tailored in accordance to the thermal expansion difference be tween the rotating element and the non-rotating element in order to compen sate the difference of thermal expansion between the rotating element and the non-rotating element such that the decrease of the magnetic strength com pensates for the additional axial force due to the thermal expansion.

In particular, the relative magnetic permeability fulfills the condition c_*p_r(Ti) < m_G(T₂), 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 mag netic 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 ad vantage that the necessary offset at ambient temperatures can be smaller in order to reach sufficient preload on the roller bearing(s) of the rotating ele ment. Thus, upon thermal expansion the offset to the neutral position under higher tem peratures during operations is smaller than, for exam ple, with sa m arium cobalt m agnets. However, in order to provide sufficient radial stiffness of the rotating element, such as the rotor of the vacuum pump, the magnetic m aterial comprising Nd should be tailored at operation tem peratures to the radial stiffness provided by sam arium cobalt magnets also at operation tem perature. Thus, even for a weakened m agnetic strength of Nd m agnets under higher tem peratures, preload to the second bearing is decreased as previously described, while sim ultaneously radial stiffness is maintained.

I n particular, the m agnetic m aterial is one of the materials with maxim um working temperature between 120°C - 200 °C. All these materials are specific Nd magnets suitable for the above-described purpose. I n particular, the m ate rial grade is shown in the table below:

Further, the present invention relates to a vacuum pum p, in particular a turbo m olecular vacuum pum p, com prising a stator and a rotor wherein the rotor is rotated by a m otor and comprises several rotor elem ents in order to convey a gaseous m edium 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. Therein, by the at least one magnetic bearing an axial force is applied to the rotor as preload to the second bearing, wherein increase of the relative m agnetic permeability or decrease of the m agnetic field strength with the temperature change from Ti to T₂, preload is adjusted and preferably kept constant. Thus, increase of the preload at the second bearing due to the tem perature increase is reduced.

I n particular, at least one bearing is a roller bearing. Thereby the m agnetic bearing is disposed at the high vacuum side of the vacuum pum p, i.e. towards the inlet of the vacuum pum p, 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 m ay be situated in the vacuum , whereby no grease or oil can contam inate the vacuum .

I n particular, the difference between m_G(Ti) and m_G(T₂) or the field strength H at the tem peratures Ti and T₂ is determ ined or set in accordance to the tem perature difference between rotor and stator during operation.

I n particular, the difference between m_G(Ti) and m_G(T₂) or the field strength H at the tem peratures Ti and T₂ is determ ined or set in accordance to the differ ence of thermal expansion between rotor and stator during operation. Thus, if there is a difference between the therm al expansion between rotor and stator, the magnetic material of the m agnetic bearing is tailored such that the mag netic strength of the m agnetic bearing is reduced at operation tem perature in order to com pensate preload or the axial force towards the second bearing, which m ight be built as roller bearing. Thus, wearout or skidding of the roller bearing is reduced and a durable vacuum pum p can be achieved.

Due to tailoring and exploiting of the effect that the magnetic strength of some m agnetic m aterials is decreased at higher tem peratures, preload between the m agnetic bearing and a roller bearing of a vacuum pum p due to thermal ex pansion can be reduced or even com pensated. Thus, the roller bearing can always be operated at optim al preload conditions, reducing wearout of the bearing and enhancing the lifetime. Further, the present invention relates to a method for fabricating a magnetic bearing the magnetic bearing being used for supporting a rotating element, in particular a rotor of a vacuum pump. The method comprises the steps of:

• providing a non-rotating first magnetic element and a second magnetic element rotating relatively to the first magnetic element, wherein the first magnetic element and/or the second magnetic element comprise a permanent magnet made of a magnetic material;

• arrange the first magnetic element and the second magnetic element are in mutual repulsion to each other to maintain a contact less bearing;

• determining a first temperature Ti as being preferably the temperature of the rotating element and/or the non-rotating element at the beginning of the operation;

• determining a second temperature T₂ as being preferably the tempera ture of the rotating element during operation; and

• providing the magnetic material of the first magnetic element and/or the second magnetic element with a relative magnetic permeability m_G(Ti) at Ti and a relative magnetic permeability m_G(T₂) at T₂ which fulfills m_G(Ti) < m_G(T₂) with Ti < T₂, in order to compensate an increase of the preload to a second bearing supporting the rotating element under an increase of the temperature from Ti to T₂.

Thus the magnetic material is selected in accordance to the temperatures T1 and T₂ in order to maintain proper preload at a second bearing. Thus the mag netic bearing of the present invention is able to compensate for thermal ex pansion of the rotor by adapting the relative magnetic permeability. In other words, the magnetic field strength at different temperatures is tailored such that the decrease with temperature of the magnetic field strength compensates the axial force induced by the thermal expansion. I n particular, the m agnetic bearing in accordance with the m ethod above fur ther comprises one or more features of the magnetic bearings described above and em bodiments thereof described below or in the accompanying claim s.

The present invention is further explained with respect to the embodim ents shown in the accom panying figures.

The figures show:

Fig. 1 an exem plary embodim ent 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 1 0 com prises a housing 12 wherein with the housing several stator elem ents 14 are connected. I n the housing a rotor 16 is disposed, wherein the rotor 1 6 com prises several rotor elem ents 18 interacting with the stator ele m ents 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 m otor 24, com prising a stator 26 and a rotating elem ent 28 connected to the rotor 16. Further, the rotor 1 6 is supported at the high pressure side 30 towards the outlet 22 by a roller bearing 32 against a first supporting elem ent 34 of the housing 12. At the other end of the rotor 16, towards the inlet 20 of the vacuum pum p 10, in the region 36 of low pressure and high vacuum , the rotor 16 is supported by a m agnetic bearing 38 against a second supporting elem ent 40 which is connected to the housing 12. The magnetic bearing 38 com prises a first non-rotating m agnetic elem ent 42 which is connected to the second supporting element 40. Further, the m agnetic bearing 38 comprises a second rotating magnetic element 44 connected with the rotor 16. Thereby, the first m agnetic element 42 and the second magnetic element 44 are arranged in close proxim ity to each other and in m utual repulsion to each other in order to provide a contactless and also frictionless bearing. Thereby, one of the first magnetic elem ent 42 or second m agnetic elem ent 44 is slightly replaced by a sm all 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 m ight 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 SmCo5 m agnet and an N32H magnet at ambient tem perature.

I f the tem perature 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 therm al 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” ) . This increased offset is caused by their therm al expansion of the rotor. Thus, the increased offset could by translated to the tem perature itself since therm al expansion is linear in tem perature in a first approximation . Flowever, due to the increased tem perature also the m agnetic strength of the em ployed magnetic material of the first m ag netic element and/or the second m agnetic 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. Flowever, in particular for the usually used SmCo5-magnets, this effect is not sufficient to com pensate the generated perforce due to the therm al expansion of the rotor since Sm Co5 are particularly selected for their good tem perature stability. I n a specific ex ample for the axial offset, in order to achieve 12.5 N of axial preload at am bient tem perature, with SmCo5 as m agnetic m aterial the offset needs to be 130 pm while for N32FI 85 pm as offset are sufficient in order to achieve 12.5 N of axial preload at ambient tem perature. I f the maxim um offset increase due to ther m al expansion is in both cases 100 pm , then the axial offset position at higher tem peratures (“ Hot”) is for Sm Co5 at about 230 pm , while for N32FI magnets the axial offset at higher tem peratures is about 185 pm . Thus, in accordance to the diagram of Fig. 2, the axial preload at higher tem peratures is for the usually used SmCo5 as m agnetic m aterial for the magnetic elem ents is in creased 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 tem peratures is only about 16% to 14.5 N, which greatly enhances the lifetim e of the roller bearing 32 since the roller bearing 32 can be operated close to the optim al axial preload. Thus, the effect of the weakening of the m agnetic field strength of permanent m agnets, which was considered as disadvantage for m agnetic bearings up to now, is utilized in the present invention to com pensate for another thermal effect such as therm al expansion of the rotor and the re sulting increased preload at the second bearing.

Since in particular Nd magnets com prise a higher magnetic strength at room tem perature 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 m agnetic bearing comprising N32H as magnetic m aterial is about 37 N/m m at operating temper ature (“Hot”) , which is com parable to the radial stiffness of a magnetic bearing with Sm Co5 magnets, or even slightly better. Thus by em ploying a magnetic m aterial in the m agnetic bearing which shows stronger weakening under higher tem peratures, the axial preload can be kept close to the optim um of the roller bearing 32. Sim ultaneously, the radial stiffness is comparable or even slightly better due to the use of m agnets which show a stronger magnetic field at am bient temperature.

Claims

CLAI MS

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 p_r(Ti) at a first temperature Ti and a relative magnetic permeability p_r(T₂) at a second temperature T₂ which fulfill p_r(Ti) < p_r(T₂) with Ti < T₂, in order to compensate the increase of the applied axial force due to the temperature increase from Ti to T₂.

2. Magnetic bearing according to claim 1, characterized in that the difference between m_G(Ti) and p_r(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 dif ference between m_G(Ti) and m_G(T₂) is determined in accordance to the ther- mal expansion of the rotating element during operation such that the de- creased magnetic permeability compensates for the axial force induced by the thermal expansion.

4. Magnetic bearing according to any of claims 1 to 3, characterized in that c-M_r(Ti) < p_r(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 preferably 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 Suf- fix 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. 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 first magnetic element is disposed relative to the second magnetic element to apply an axial force to the rotating element, wherein the magnetic material has a first magnetic field strength at a first temperature Ti and a second magnetic field strength at a second temper- ature T₂ with Ti < T₂, wherein the first magnetic field strength is larger than the second magnetic field strength, in order to compensate an in- crease of an axial force due to the temperature increase from Ti to T₂ by the decrease of the magnetic field strength with temperature.

10. 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 9, wherein by the at least on magnetic bearing an axial force is applied via the rotor to the second bearing as a preload force, wherein by the increase of the magnetic permeability with temperature or de- crease of magnetic field strength with temperature the preload is adjusted and preferably kept substantially constant.

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

12. Vacuum pump according to claim 10 or 11, characterized in that the dif- ference between p_r(Ti) and p_r(T₂) is determined in accordance to the tem- perature difference between rotor and stator during operation.

13. Vacuum pump according to any of claims 10 to 12, characterized in that the difference between p_r(Ti) and p_r(T₂) is determined in accordance to the thermal expansion of the rotor during operation such that the de- creased magnetic permeability compensates for the axial force induced by the thermal expansion.

14. A method for producing a magnetic bearing, the magnetic bearing being used for supporting a rotating element, in particular a rotor of a vacuum pump, by providing a non-rotating first magnetic element and a second magnetic element rotating relatively to the first magnetic element, wherein the first magnetic element and/or the second magnetic element comprise a per- manent magnet made of a magnetic material, arrange the first magnetic element and the second magnetic element are in mutual repulsion to each other to maintain a contact less bearing, wherein the method further comprising: determining a first temperature Ti as being preferably the temperature of the rotating element and/or the non-rotating element at the beginning of the operation, determining a second temperature T₂ as being preferably the temperature of the rotating element during operation, and providing the magnetic material of the first magnetic element and/or the second magnetic element with a relative magnetic permeability p_r(Ti) at Ti and a relative magnetic permeability p_r(T₂) at T₂ which fulfills m_G(Ti) < p_r(T₂) with Ti < T₂, in order to compensate an increase of the preload to a second bearing supporting the rotating element under an increase of the temperature from Ti to T₂.