Magnetic bearing assembly and vacuum pump

Abstract

A magnetic bearing which the axial position of stator magnet 1 can be adjusted 5, 6 to preload the bearing with an axial offset O with respect to the rotor magnet 2, and to compensate for axial displa

Description

The present invention relates to a magnetic bearing assembly for a vacuum pump and a vacuum pump with such a magnetic bearing assembly. Vacuum pumps and in particular turbomolecular pumps comprise a housing having an inlet and an outlet. In the housing a stator assembly is fixedly arranged at the housing and a rotor assembly comprising a rotor shaft with a plurality of pumping elements arranged at the rotor shaft. Further, a motor in particular an electromotor is provided to rotate the rotor shaft and therewith, to rotate the rotor assembly relative to the rotor shaft to convey a gaseous medium from the inlet to the outlet of the pump. In order to mount and rotably support the rotor shaft to the housing a bearing assembly is provided. In particular, in turbomolecular pumps the bearing assembly may comprise a magnetic bearing assembly and one mechanical bearing. Therein, the magnetic bearing assembly is typically arranged towards the inlet side, i.e. the end of the rotor assembly in the direction of the inlet of the vacuum pump, and the mechanical bearing is typically arranged towards the outlet side, i.e. the end of the rotor assembly in the direction of the outlet of the vacuum pump. The magnetic bearing assembly can be designed in particular as a passive magnetic bearing assembly comprising one static magnetic arrangement with a plurality of magnetic elements connected to the housing and one rotated magnetic arrangement with a plurality of magnetic elements connected to the shaft. Said magnetic bearing assembly is used to axially preload the mechanical bearing which can be in particular designed as a ball or roller bearing. Due to the different thermal expansion of the pump housing supporting the static magnetic arrangement and the shaft supporting the rotated magnetic arrangement during operation of the vacuum pump at high rotational speeds, an axial displacement between the magnetic elements of the static magnetic arrangement and the magnetic elements of the rotated magnetic arrangement is caused. Due to this axial displacement of the magnetic elements of the magnetic bearing assembly, its axial force on the mechanical bearing varies. As a result, the axial preload on the mechanical bearing is not constant over the operating condition. Negative operating states can occur, in which, for example, no axial preload force acts on the mechanical bearing any longer. In order to compensate for the axial displacement of the magnetic elements with respect to a predetermined offset and the corresponding variation of preload on the mechanical bearing, preload regulation devices are used. Known solutions for preload regulation adjust the preload on the mechanical bearing in a cold state of the vacuum pump. Therein, adjustment arrangements for preload regulation are used that typically comprise a fixation element arranged on top of the static magnetic arrangement and a preload element in particular a spring below the static magnetic arrangement. However, the predetermined axial offset between the magnetic elements of the rotated magnetic arrangement and the magnetic elements of the static magnetic arrangement will increase with increasing temperature of the rotor assembly during operation. Thus, the axial force on the rotor assembly in the axial direction towards the inlet of the vacuum pump will increase in a hot state and consequently the preload on the mechanical bearing will change which might cause malfunctioning of the vacuum pump during operation and / or damages to its components. Another problem of known magnetic bearing assemblies for preload regulation is that the stability of the preload on mechanical bearing is temperature dependent. Thus, it is a problem of known solutions that they cannot provide a sufficient combination of high axial expansion and / or compression with high axial forces in a small installation space. Another problem of known magnetic bearing assemblies for preload regulation is that in known devices electronic control units, or other electrically driven and / controlled components are employed to adjust preload regulation, which increases the complexity of the arrangement. Another problem of known solutions is that the efficiency of the solution might dependent on the installation position / orientation of the vacuum pump, decreasing the flexibility of the vacuum system design and limits the area of application of the vacuum pump. Further, another problem of known solutions is that since the maximum axial force on the mechanical bearing is limited by the bearing design, the pump design and operation range are often restricted because of the thermal elongation of the shaft. Thus, it is an object of the present invention to provide a magnetic bearing assembly and a vacuum pump with the magnetic bearing assembly, wherein the above problems are solved and preload on the mechanical bearing of the vacuum pump is easily and effectively regulated, thereby increasing the stability and reliability of the vacuum pump during operation. The problems are solved by a magnetic bearing assembly according to claim 1 and a vacuum pump according to claim 12. According to a first aspect of the present invention, a magnetic bearing assembly for a vacuum pump and in particular for turbomolecular vacuum pump is provided. The magnetic bearing assembly comprises a static magnetic arrangement to be connected to a stator of the vacuum pump and a rotated magnetic arrangement to be connected to a rotor of the vacuum pump arranged coaxially to each other along an axis of rotation. The static magnetic arrangement comprises a plurality of first magnetic elements and an adjustment arrangement. The rotated magnetic arrangement comprises a plurality of second magnetic elements, the plurality of second magnetic elements being arranged with a predetermined axial offset with respect to the plurality of first magnetic elements of the static magnetic arrangement to create a bearing preload. The adjustment arrangement is configured to compensate for the temperature dependent axial displacement of the rotated magnetic arrangement to maintain the predetermined axial offset. In other words, the adjustment arrangement is configured to compensate for the temperature dependent axial displacement of the rotated magnetic arrangement to maintain a substantially constant predetermined axial offset. In other words, the adjustment arrangement is configured to compensate for the temperature dependent axial displacement of the rotated magnetic arrangement from the static magnetic arrangement. That is, the adjustment arrangement is configured to compensate for the temperature dependent axial displacement of the rotated magnetic arrangement and change or increase from the predetermined axial offset. In this way, a temperature independent preload stability for the mechanical bearing of the vacuum pump can be provided. Further, since the magnetic bearing assembly according to the invention allows the magnetic elements to move relatively to the shaft based on the current operation point, new design options like longer shafts and heavier rotors become possible. Preferably, the adjustment arrangement comprises at least one preload member. Thereby, the preload of the mechanical bearing of the vacuum pump can be adjusted in a cold state, i.e. in a non-rotating state of the rotor arrangement. The adjustment arrangement comprises at least one compensation member. Thereby, the axial offset occurring between the vacuum pump housing and the vacuum pump rotor assembly due to temperature differences in a hot state of the vacuum pump can be compensated. Preferably, the at least one preload member is arranged at a first end axially next to the plurality of first magnetic elements and the at least one compensation member is arranged at a second end opposite to the first end axially next to the plurality of the first magnetic elements. Thereby, when the temperature of the rotor assembly increases, the heat will radiate and the temperature of the stator assembly including the preload member and the compensation member will also increase. The preload member then exerts an axial force in direction of the first end towards the inlet of the vacuum pump and compresses the compensation member which preferably comprises a temperature dependent stiffness. Thus, the first magnetic elements will be axially displaced towards the first end. In this way, the additional force on the rotor assembly and thereby the additional force on the mechanical bearing of the vacuum pump can be lowered. In this way, in a cold state, a bearing preload for the mechanical bearing of the vacuum pump is created and in a hot state, the temperature dependent axial displacement of the rotated magnetic arrangement from the predetermined axial offset can be compensated. Preferably, the at least one compensation member is arranged at a first side axially next to the plurality of first magnetic elements and the at least one preload member is arranged at a second side opposite to the first side axially next to the plurality of the first magnetic elements. Thereby, when the temperature of the rotor assembly increases, the heat will radiate and the temperature of the stator assembly including the preload member and the compensation member will also increase. The compensation member which is preferably configured to expand or to compress depending on a predetermined threshold temperature then exerts an axial force in direction of the first end towards the inlet of the vacuum pump and compresses the preload member which preferably comprises a temperature dependent stiffness. Thus, the first magnetic elements will be axially displaced towards the first end. In this way, the additional force on the rotor assembly and thereby the additional force on the mechanical bearing of the vacuum pump can be lowered. In this way, in a cold state, a bearing preload for the mechanical bearing of the vacuum pump is created and in a hot state, the temperature dependent axial displacement of the rotated magnetic arrangement from the predetermined axial offset can be compensated. The at least one compensation member is made of a shape memory alloy, wherein the shape memory alloy is configured to expand when its temperature exceeds a predetermined threshold. Additionally, the at least one compensation member may be made of a polymer with a temperature dependent Young's modulus or stiffness, preferably polypropylene, wherein the Young's modulus or stiffness of the polymer will decrease with increasing temperature. The compensation member can be provided with a material comprising high flexibility even for high axial forces in the range of several hundred Newton. In particular, the compensation member can be provided with a material comprising a temperature dependent variation and / or change in the materials Young's modulus, in particular, the temperature dependent variation and / or change in the materials Young's modulus is reversible. Thereby, under the same force applied the size and / or shape of the compensation member will vary depending on the temperature. Thus, the compensation member can provide a high axial force and thereby, the additional force on the rotor assembly and thereby the additional force on the mechanical bearing of the vacuum pump can be decreased. Preferably, the compensation member is made of a fibre-reinforced polymer or polymer composite material. Thereby, the compensation member can be provided with a material comprising a greater stiffness and / or strength and / or higher creep resistance. Thereby, the compensation member can be subjected to higher forces and / or its dimensions can be decreased. Preferably, the shape memory alloy comprises a transformation temperature between 20°C and 100°C. Thus, the compensation member can provide a high axial force and thereby, reduce the offset between the plurality of the first magnetic elements magnets and the plurality of the second magnetic elements. Thereby, the preload force and / or the additional force on the rotor assembly and thereby the additional force on the mechanical bearing of the vacuum pump can be decreased. Preferably, the at least one preload member is a spring, preferably a wave spring. Preferably, the spring is made from a material with a strong temperature dependency of the Young's modulus for temperatures between 20°C and 100°C. Preferably, the material is a plastic with a decrease of the Young's modulus of at least 0.4 % / K, in particular polypropylene. Preferably, the change in stiffness of the preload member is reversible. Thereby, a preload element can be provided that is reliable and cost effective. Preferably, at least one heating member configured to adjust the temperature of the static magnetic arrangement is provided. In particular, the at least one heating member is configured to adjust the temperature of the static magnetic element to be closer to the temperature of the rotated magnetic arrangement. Thus, the temperature dependent axial offset between the static magnetic arrangement and the rotated magnetic arrangement can be decreased. Preferably, the at least one heating member is arranged at the rotated magnetic arrangement, in particular, towards the same end as the compensation member. Preferably, the at least one heating member is configured to radiate heat. In particular, the at least one heating member is configured to radiate heat towards the static magnetic arrangement. Thereby, when the at least one heating member radiates heat the temperature of the stator assembly including the compensation member will increase. Thus, when the temperature of the compensation member increases the temperature dependent dimensional change of the compensation member will be amplified. Thus, the temperature dependent axial offset between the rotated magnetic arrangement and the static magnetic arrangement can be decreased. This will result in a lower preload on the magnetic bearing assembly. Preferably, the at least one heating member is arranged at the static magnetic arrangement, in particular axially next to the compensation member. Preferably, the at least one heating member is configured to generate heat. In particular, the at least one heating member is configured to generate heat on the stator assembly side, in particular, by electromagnetic induction caused by the second magnetic elements. Additionally or alternatively, the electromagnetic induction on the heating member can be caused by an additional magnet for heating. In particular, the additional magnet for heating is arranged at the first end or the second end axially next to the plurality of second magnets. Thereby, when the at least one heating member generates heat the temperature of the stator assembly including the compensation member will increase. Thus, when the temperature of the compensation member increases the temperature dependent dimensional change of the compensation member will be amplified. Thus, the temperature dependent axial offset between the rotated magnetic arrangement and the static magnetic arrangement can be decreased. This will result in a lower preload on the magnetic bearing assembly. Preferably, the at least one heating member is arranged at the static magnetic arrangement, in particular axially next to the compensation member. Preferably, the at least one heating member is configured to absorb heat. In particular, the at least one heating member is configured to absorb heat that is radiated from the rotor assembly side to the stator assembly side. Thereby, when the at least one heating member absorbs heat, the temperature of the stator assembly including the compensation member will increase. Thus, when the temperature of the compensation member increases the temperature dependent dimensional change of the compensation member will be amplified. Thus, the temperature dependent axial offset between the rotated magnetic arrangement and the static magnetic arrangement can be decreased. This will result in a lower preload on the magnetic bearing assembly. Preferably, the plurality of first magnetic elements and the plurality of second magnetic elements are permanent magnet rings. Thereby, the magnetic bearing arrangement can be operated without additional control electronics and is thus, less complex. According to a second aspect of the present invention a vacuum pump preferably a turbomolecular vacuum pump with the magnetic bearing assembly according to the first aspect is provided. The vacuum pump comprises a housing, a stator assembly fixedly arranged at the housing, and a rotor assembly including a rotor shaft having at least one pump element arranged at the rotor shaft. The rotor assembly being configured to rotate relative to the stator assembly along an axis of rotation to convey a medium from an inlet to an outlet of the vacuum pump. The shaft being mounted in the housing by means of at least one magnetic bearing assembly according to the first aspect of the invention. Thereby, a vacuum pump with temperature independent preload stability can be provided that can be reliably operated in any installation position. Preferably, the vacuum pump comprises two bearing assemblies and one of the bearing assemblies is a mechanical bearing, preferably a roller bearing. Thereby, a stable and reliable bearing arrangement can be provided that is simple and effectively compensates for large axial displacements. Preferably, the magnetic bearing assembly according to the first aspect is arranged towards the inlet of the vacuum pump and the mechanical bearing assembly is arranged towards the outlet the vacuum pump. Thereby, the magnetic bearing arrangement can be installed in a standard configuration with the mechanical bearing to effectively adjust the preload on the mechanical bearing. In the following the present invention is described in more detail with reference to the accompanying figures. The figures show: Figure 1 a magnetic bearing assembly according to the state of art, Figure 2A a schematic drawing of a magnetic bearing assembly according to an embodiment of the present invention in a cold state, and Figure 2B a schematic drawing of the magnetic bearing assembly of Figure 2A in a hot state, Figure 3A a schematic drawing of a magnetic bearing assembly according to an embodiment of the present invention in a cold state, and Figure 3B a schematic drawing of the magnetic bearing assembly of Figure 3A in a hot state, Figure 4A a schematic drawing of the magnetic bearing assembly of figure 2A with a heating member according to an embodiment of the present invention, Figure 4B a schematic drawing of the magnetic bearing assembly of figure 3A with a heating member according to an embodiment of the present invention, Figure 5A a schematic drawing of the magnetic bearing assembly of figure 2A with another heating member according to an embodiment of the present invention, Figure 5B a schematic drawing of the magnetic bearing assembly of figure 3A with another heating member according to an embodiment of the present invention, Figure 6 a vacuum pump according to an embodiment of the invention. Figure 1 shows a known magnetic bearing assembly 40 comprising a static magnetic arrangement 1 connected to a stator 4 of the vacuum pump and a rotated magnetic arrangement 2 connected to a rotor 3 of the vacuum pump arranged coaxially to each other along an axis of rotation A of the rotor. Further, the static magnetic arrangement 1 comprises a preload element 5 arranged below two magnetic elements 11 and the rotated magnetic arrangement 2 comprises two magnetic elements 21 arranged with an axial offset 0 (indicated with an arrow in figure 1) with respect to the first magnetic elements 11 of the static magnetic arrangement 1. However, the number of magnetic elements 11, 21 is not limited to two and can be me more than two. In particular, a plurality of magnetic elements 11, 21 can be arranged at the static magnetic arrangement 1 and the rotated magnetic arrangement 2, respectively. Therewith, a bearing preload for the mechanical bearing in particular roller bearing of the vacuum pump is created in a cold state, i.e. in a non-rotating state of the rotor arrangement. When the vacuum pump is operated, the rotor arrangement rotates along its axis of rotation A and depending on the temperature differences within the vacuum pump the rotated magnetic arrangement 2 is axially displaced such that the axial offset 0 between the static magnetic arrangement 1 and the rotated magnetic arrangement 2 is changed and in particular is not constant during the cycle operation of the vacuum pump. As a consequence, the axial forces and thus, the corresponding preload on the mechanical bearing of the vacuum pump changes. Figure 2A shows a magnetic bearing 50 assembly according to an embodiment of the invention in a cold state. The magnetic bearing assembly 50 shown in figure 2A comprises a static magnetic arrangement 1 connected to a stator arrangement 4 of the vacuum pump and a rotated magnetic arrangement 2 connected to a rotor arrangement 3 of the vacuum pump arranged coaxially to each other along an axis of rotation A of the rotor arrangement. Further, the static magnetic arrangement 1 comprises an adjustment arrangement 5, 6 comprising a preload member 5 arranged at a first end axially next to two first magnetic elements 11 and one compensation member 6 that is arranged at another end opposite to the first end axially next to the two first magnetic elements 11. In particular, as can be seen in figure 2A the preload member 5 is arranged at the end towards the outlet of the vacuum pump and the compensation member 6 is arranged towards the inlet side of the vacuum pump. The number of first magnetic elements 11 shown in figure 2A is two. However, the number of first magnetic elements 11 is not limited to two and can be me more than two. In particular, a plurality of first magnetic elements 11 can be arranged at the static magnetic arrangement 1. Further, a rotated magnetic arrangement 2 is provided comprising two second magnetic elements 21 arranged with a predetermined axial offset 0 (indicated with an arrow in figure 2A) with respect to the two first magnetic elements 11 of the static magnetic arrangement 1. The number of second magnetic elements 21 shown in figure 2A is two. However, the number of second magnetic elements 21 is not limited to two and can be me more than two. In particular, a plurality of second magnetic elements 21 can be arranged at the rotated magnetic arrangement 2. Preferably, the plurality of first and second magnetic elements 11, 21 are permanent magnet rings. The number of permanent magnetic rings is not limited by the figures. In this way, in a cold state, i.e. in a non-rotating state of the rotor arrangement 3, a bearing preload for the mechanical bearing (shown in figure 4) in particular the roller bearing of the vacuum pump is created. Figure 2B shows the bearing arrangement 50 of figure 2A in a hot state, i.e. in a state when the vacuum pump is operated, and the rotor arrangement 3 is rotated by the rotor shaft (shown in figure 4) along its axis of rotation A. When operated at high rotational speeds the temperature of the vacuum pump might rise up to 130°C or higher. Depending in particular on the temperature difference between the rotor shaft and the vacuum pump housing as well as the materials used, considerable displacements in the axial direction can occur. Thus, when the axial offset 0 between the two first magnetic elements 11 of the stator arrangement 4 and the two second magnetic elements 21 of the rotor arrangement 3 increases with increasing temperature the axial force on the rotor arrangement 3 in the axial direction towards the first end, i.e. the side towards the vacuum pump inlet, will increase. In the embodiment shown in figures 2A, 2B a compensation member 6 is provided in addition to the preload member 5. In the embodiment shown in figures 2A, 2B the preload member 5 may be a flexible member in particular a spring with strongly temperature dependent stiffness. For example, this could be a wave spring made from a material with a strong temperature dependency of the Young's modulus between 20°C and 100°C. In particular, plastics with a decline of the Young's modulus of preferably at least 0.4 % / K could be used, for example polypropylene. In the embodiment shown in figures 2A, 2B, the compensation member 6 is in particular made from a polymer with a temperature dependent Young's modulus, preferably polypropylene. Thereby, when the Young's modulus of the compensation member 6 decreases with increasing temperature the stiffness of the compensation member 6 will decrease. Additionally, the geometry of the compensation member 6 may have a deformation dependent stiffness. Thus, in a cold state, the preload member 5 and the compensation member 6 are used in the magnetic bearing assembly 50 to adjust a desired preload on the mechanical bearing of the vacuum pump. In a hot state, i.e. when the temperature of the rotor assembly 3 is increased the temperature of the stator assembly 4 and of the adjustment member 5, 6 will rise as well due to radiation. Therefore, the stiffness of the compensation member 6 will decrease and the preload member 5 will exert an axial force in direction of the first end towards the inlet of the vacuum pump and thus, the first magnetic elements 11 will be axially displaced towards the first end. In this way, the additional force on the rotor assembly 3 and thereby the additional force on the mechanical bearing of the vacuum pump can be lowered. In particular, the change in stiffness of the compensation member 6 is reversible and thus, the elastic deformation of the compensation member 6 is reversible. Alternatively, a normal spring could be used as preload member 5 in combination with a compensation member 6 as described above. Figure 3A shows a magnetic bearing 70 assembly according to another embodiment of the invention in a cold state. The magnetic bearing assembly 70 shown in figure 3A comprises a static magnetic arrangement 1 connected to a stator arrangement 4 of the vacuum pump and a rotated magnetic arrangement 2 connected to a rotor arrangement 3 of the vacuum pump arranged coaxially to each other along an axis of rotation A of the rotor arrangement 3. Further, the static magnetic arrangement 1 of the stator arrangement 4 comprises an adjustment arrangement 5, 8 comprising a preload member 5 arranged at a first end axially next to the plurality of first magnetic elements 11 and one compensation member 8 that is arranged at another end opposite to the first end axially next to the plurality of the first magnetic elements 11. In particular, as can be seen in figure 3A the preload member 5 is arranged at the end towards the inlet of the vacuum pump and the compensation member 8 is arranged towards the outlet side of the vacuum pump. Also in the embodiment shown in figure 3A, a rotated magnetic arrangement 2 is provided comprising two magnetic elements 21 arranged with a predetermined axial offset 0 (indicated with an arrow in figure 3A) with respect to the two first magnetic elements 11 of the static magnetic arrangement 1. In particular, the plurality of first and second magnetic elements 11, 21 are permanent magnet rings. Again, in a cold state as shown in figure 3A, a bearing preload for the mechanical bearing (shown in figure 5) in particular the roller bearing of the vacuum pump is adjusted by the magnetic bearing arrangement 70. Figure 3B shows the bearing arrangement 70 of figure 3A in a hot state, i.e. in a state when the rotor arrangement 3 is rotated along its axis of rotation A at high rotational speeds. When operated at high rotational speeds, the axial offset between the two first magnetic elements 11 of the stator arrangement 4 and the two second magnetic elements 21 of the rotor arrangement 3 increases with increasing temperature. Then, the axial force on the rotor arrangement 3 in the axial direction towards the first end, i.e. the side towards the vacuum pump inlet, increases correspondingly. In the embodiment shown in figures 3A, 3B a compensation member 8 is provided in addition to the preload member 5. The preload member 5 is in the embodiment shown in figures 3A, 3B is a wave spring which may have or have not a temperature dependent stiffness. For example, this could be a wave spring made from a material with a strong temperature dependency of the Young's modulus between 20°C and 100°C. In particular, plastics with a decline of the Young's modulus of preferably at least 0.4 % / K could be used, for example polypropylene. The compensation member 8 shown in figures 3A, 3B is in particular made of a shape memory alloy. The stiffness of the shape memory alloy is temperature dependent and the memory shape alloy will expand when its temperature exceeds a predetermined threshold. In a hot state, i.e. when the temperature of the rotor assembly 3 is increased the temperature of the stator assembly 4 and of the adjustment member 5, 8 will rise as well. Therefore, when the temperature of the memory shape alloy 8 reaches or exceeds its transformation temperature, it will expand and thereby compress the preload member 6 that is for example a wave spring on the opposite side of the first magnetic elements 11 of the static magnetic arrangement 1. In this way, the additional force on the rotor assembly 3 and thereby the additional force on the mechanical bearing of the vacuum pump can be lowered. In particular, the change dimension of the compensation member 8 is reversible and thus, the compensation member 8 will transform back to its original dimensions once its temperature falls below a predetermined threshold. Figure 4A shows the bearing arrangement 50 of figure 2A with a heating member 7 according to an embodiment of the invention. In the embodiment shown in figure 4A the heating member 7 is arranged at the rotated magnetic arrangement 2, in particular towards the same end as the compensation member 6. In particular, both the compensation member 6 and the heating member 7 are arranged towards a second end opposite to the preload member 5 that is arranged at the first end. Preferably, the heating member 7 is configured to radiate heat, e.g. by resistive heating or the like. In particular, the heating member 7 is configured to radiate heat towards the static magnetic arrangement 1 including the compensation member 6. Thereby, when the heating member 7 radiates heat the temperature of the stator assembly 1 including the compensation member 6 will increase. Thus, when the temperature of the compensation member 6 increases the stiffness of the compensation member 6 will decrease and thereby, the compensation member 6 will be further compressed. Thus, the temperature dependent axial offset between the rotated magnetic arrangement 2 and the static magnetic arrangement 1 can be decreased, which leads to a lower preload on the magnetic bearing assembly. In this way, the additional force on the rotor assembly 3 and thereby the additional force on the mechanical bearing of the vacuum pump can be lowered. Figure 4B shows the bearing arrangement 70 of figure 3A with a heating member 7 according to an embodiment of the invention. In the embodiment shown in figure 4B the heating member 7 is arranged at the rotated magnetic arrangement 2, in particular towards the same end as the compensation member 8. In particular, both the compensation member 8 and the heating member 7 are arranged towards a first end opposite to the preload member 5 that is arranged at the second end. Preferably, the heating member 7 is configured to radiate heat, e.g. by resistive heating or the like. In particular, the heating member 7 is configured to radiate heat towards the static magnetic arrangement 1 including the compensation member 8. Thereby, when the heating member 7 radiates heat the temperature of the stator assembly 1 including the compensation member 8 will increase. Thus, when the temperature of the compensation member 8 increases it will further expand. Thus, the temperature dependent axial offset between the rotated magnetic arrangement 2 and the static magnetic arrangement 1 can be decreased, which leads to a lower preload on the magnetic bearing assembly. In this way, the additional force on the rotor assembly 3 and thereby the additional force on the mechanical bearing of the vacuum pump can be lowered. Figure 5A shows the bearing arrangement 50 of figure 2A with a heating member 9 according to an embodiment of the invention. In the embodiment shown in figure 5A the heating member 9 is arranged at the static magnetic arrangement 1, in particular axially next to the compensation member 6. In particular, both the compensation member 6 and the heating member 9 are arranged towards a second end opposite to the preload member 5 that is arranged at the first end. Preferably, the heating member 9 is configured to generate heat. In particular, the at least one heating member 9 is configured to generate heat on the stator assembly 4 side, in particular, by electromagnetic induction caused by the second magnetic elements 21. Additionally or alternatively, the electromagnetic induction on the heating member 9 can be caused by an additional magnet for heating 21' arranged at the second end axially next to the plurality of second magnets 21. Thus, when the temperature of the compensation member 6 increases the stiffness of the compensation member 6 will decrease and thereby, the compensation member 6 will be further compressed. Thus, the temperature dependent axial offset between the rotated magnetic arrangement 2 and the static magnetic arrangement 1 can be decreased, which leads to a lower preload on the magnetic bearing assembly. In this way, the additional force on the rotor assembly 3 and thereby the additional force on the mechanical bearing of the vacuum pump can be lowered. Figure 5B shows the bearing arrangement 70 of figure 3A with a heating member 9 according to an embodiment of the invention. In the embodiment shown in figure 5B the heating member 9 is arranged at the static magnetic arrangement 1, in particular axially next to the compensation member 8. In particular, both the compensation member 8 and the heating member 9 are arranged towards a first end opposite to the preload member 5 that is arranged at the second end. Preferably, the heating member 9 is configured to generate heat. In particular, the at least one heating member 9 is configured to generate heat on the stator assembly 4 side, in particular, by electromagnetic induction caused by the second magnetic elements 21. Additionally or alternatively, the electromagnetic induction on the heating member 9 can be caused by an additional magnet for heating 21' arranged at the first end axially next to the plurality of second magnets 21. Thus, when the temperature of the compensation member 8 increases by the heat generated by the heating member 9, it will further expand. Thus, the temperature dependent axial offset between the rotated magnetic arrangement 2 and the static magnetic arrangement 1 can be decreased, which leads to a lower preload on the magnetic bearing assembly. In this way, the additional force on the rotor assembly 3 and thereby the additional force on the mechanical bearing of the vacuum pump can be lowered. Figure 6 shows an embodiment of a vacuum pump 100 according to the invention. In the embodiment shown in figure 6, the vacuum pump 100 is in particular a turbomolecular vacuum pump comprising a housing 10 with a stator assembly 4 fixedly arranged at the housing 10. The stator assembly comprises a plurality of stator discs 18. The vacuum pump 100 further comprises a rotor assembly 3 including a rotor shaft 30 with a plurality of rotor discs 16 arranged at the rotor shaft 30. Therein, the rotor assembly 3 is configured to rotate relative to the stator assembly 4 along the axis of rotation A to convey a gaseous medium from the vacuum pump inlet 22 to the outlet 20 of the vacuum pump. The shaft 30 is mounted in the housing 10 by means of a magnetic bearing assembly 70 according to the embodiment shown in figures 3A, 3B and one mechanical bearing 26 in particular a roller bearing. Alternatively, the shaft 30 is mounted in the housing 10 by means of a magnetic bearing assembly 50 according to the embodiment shown in figures 2A, 2B and one mechanical bearing 26 in particular a roller bearing. Alternatively, the shaft 30 is mounted in the housing 10 by means of a magnetic bearing assembly 50, 70 and an active magnetic bearing. As can be seen in figure 6, the magnetic bearing assembly 70 is arranged towards the inlet 22 of the vacuum pump and the mechanical bearing assembly 26 is arranged towards the outlet 20 the vacuum pump. In a cold state, the preload of the mechanical bearing is adjusted by means of the adjustment arrangement 5, 8 of the magnetic bearing assembly 70. During operation of the vacuum pump and in particular when the temperature of the vacuum pump increases, i.e. in a hot state, due to the different temperatures of the rotor assembly 3 and the stator assembly 4, axial displacement of the rotor assembly 3 with respect to the stator assembly 4 occurs and thus the preload on the roller bearing changes. By means of the adjustment arrangement 5, 8 these axial displacements are substantially completely compensated and the axial preload on the mechanical bearing is kept substantially constant during operation of the vacuum pump 100. Thus, a magnetic bearing assembly and a vacuum pump with the magnetic bearing assembly can be provided, wherein the preload on the mechanical bearing can easily and effectively be regulated and stability and reliability of the vacuum pump can be optimized. Reference list 1 static magnetic arrangement 2 rotated magnetic arrangement 3 rotor assembly 4 stator assembly 5 preload member 6, 8 compensation member 7, 9 heating member 10 vacuum pump housing 11 first magnetic elements 16 rotor disc 18 stator disc 20 vacuum pump outlet 21 second magnetic elements 21' additional magnet for heating 22 vacuum pump inlet 26 mechanical bearing 30 rotor shaft 40 bearing assembly 50 bearing assembly 70 bearing assembly 100 vacuum pump A axis of rotation 0 predetermined axial offset

Claims

1. A magnetic bearing assembly for a vacuum pump, comprisinga static magnetic arrangement (1) to be connected to a stator of the vacuum pump and a rotated magnetic arrangement (2) to be connected to a rotor of the vacuum pump arranged coaxially to each other along an axis of rotation (A),the static magnetic arrangement (1) comprising a plurality of first magnetic elements (11) and an adjustment arrangement (5, 6, 8),the rotated magnetic arrangement (2) comprising a plurality of second magnetic elements (21), the plurality of second magnetic elements (21) being arranged with a predetermined axial offset (0) with respect to the plurality of first magnetic elements (11) of the static magnetic arrangement (1) to create a bearing preload,wherein the adjustment arrangement (5, 6, 8) is configured to compensate for the temperature dependent axial displacement of the rotated magnetic arrangement (2) to maintain the predetermined axial offset (0),wherein the adjustment arrangement (5, 6, 8) comprises at least one compensation member (6, 8),the at least one compensation member being made of a shape memory alloy wherein the shape memory alloy is configured to expand when its temperature exceeds a predetermined threshold.

2. The bearing assembly according to claim 1, wherein the adjustment arrangement (5, 6, 8) comprises at least one preload member (5).

3. The bearing assembly according to claim 2, wherein the at least one preload member (5) is arranged at a first end axially next to the plurality of first magnetic elements (11) and the at least one compensation member (6) is arranged at a second end opposite to the first end axially next to the plurality of the first magnetic elements (11).

4. The bearing assembly according to claim 2, wherein the at least one compensation member (8) is arranged at a first side axially next to the plurality of first magnetic elements (11) and the at least one preload member (5) is arranged at a second side opposite to the first side axially next to the plurality of the first magnetic elements (11).

5. The bearing assembly according to any of the claims 2-4, wherein the at least one preload member (6) is a spring.

6. The bearing assembly according to any of the claims 1-5, wherein at least one heating member (7, 9, 21') configured to adjust the temperature of the static magnetic arrangement (1) is provided.

7. The bearing assembly according to any of the claims 1-6, wherein the plurality of first magnetic elements (11) and the plurality of second magnetic elements (21) are permanent magnet rings.

8. A vacuum pump, comprisinga housing (10),a stator assembly (4) fixedly arranged at the housing (10), anda rotor assembly (3) including a rotor shaft (30) having at least one pump element (16) arranged at the rotor shaft (30),the rotor assembly (3) being configured to rotate relative to the stator assembly (4) along an axis of rotation (A) to convey a medium from an inlet (22) to an outlet (20) of the vacuum pump,the shaft (30) being mounted in the housing (10) by means of at least one magnetic bearing assembly (50, 70) according to any of the claims 1-7.

9. The vacuum pump according to claim 8, wherein the vacuum pump comprises two bearing assemblies (50, 70, 26) and one of the bearing assemblies (50, 70, 26) is a mechanical bearing assembly (26).

10. The vacuum pump according to claim 8, wherein the vacuum pump comprises two bearing assemblies (50, 70, 26) and one of the bearing assemblies (50, 70, 26) is an active magnetic bearing assembly.

11. The vacuum pump according to claim 9 or 10, wherein the magnetic bearing assembly (50, 70) according to any of the claims 1 - 7 is arranged towards the inlet (22) of the vacuum pump and the mechanical bearing assembly (26) or the active magnetic assembly (26) is arranged towards the outlet (20) the vacuum pump.

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