The present invention relates to a rotor assembly for a vacuum pump and a vacuum pump with such a rotor assembly.

Vacuum pumps and in particular turbomolecular pumps comprise a housing having an inlet and an outlet. In the housing a rotor assembly comprising a rotor hub in particular a bell-shaped rotor hub is mounted on a rotor shaft. Therein, the rotor shaft is supported by a bearing arrangement and rotated by an electromotor. In particular, in turbomolecular pumps the bearing arrangement typically comprises at least one magnetic bearing and at least one mechanical bearing. Therein, the bearing towards the inlet side, i.e. the end of the rotor assembly in the direction of the inlet of the vacuum pump, may be built as a magnetic bearing. Whereas, the bearing towards the outlet side, i.e. the end of the rotor assembly in the direction of the outlet of the vacuum pump, may be built as a mechanical bearing in particular a roller bearing.

The rotor assembly further comprises a rotor element with a plurality of rotor discs interacting with a plurality of stator discs of a stator element in order to convey a gaseous medium from the inlet to the outlet. Each rotor/stator disc pair defining a single stage. Therein, the inlet stages typically comprise discs of especially large radial span to allow a large annular inlet area. The gaseous medium entering the inlet is captured by these upper stages and transferred to the lower compression stages having discs with shorter radial spans, where the gas is compressed to backing pressure or rough vacuum pressure.

In particular, in turbomolecular pumps the shaft supports the rotor hub onto which the rotor discs are typically press-fitted.

However, a problem of known rotor assemblies is that said press-fit generates very high axial forces during cycle operation of the pump due to deformation of the rotor hub in particular of a bell-shaped rotor hub, which leads to misalignment of the rotor discs.

Another problem of known rotor assemblies is the poor availability of rotor assembly raw material and thus, decreased flexibility in rotor assembly design since large element of the rotor must be machined from a one-piece raw material increasing costs and demands on quality of the raw material.

Thus, it is an object of the present invention to provide a rotor assembly and a vacuum pump, wherein the above problems of misalignment of the rotor elements is avoided and the flexibility of rotor design is increased.

The problem is solved by a rotor assembly according to claim 1 and a vacuum pump according to claim 14.

According to a first aspect of the present invention, a rotor assembly for a vacuum pump and in particular for turbomolecular vacuum pump is provided. The rotor assembly comprises a rotor shaft and a rotor hub, in particular a bellshaped rotor hub. The rotor hub comprises at least a first part and a second part separate from the first part. Therein, the first part comprises at least one first rotor element and the second part comprises at least one second rotor element. Thus, the first part and second part are two distinct elements joined to form the rotor hub.

The rotor assembly further comprises a centering element, wherein the first part and/or the second part are centered and aligned with respect to the rotor shaft by means of the centering element.

In this way, a modular rotor assembly can be provided that allows for flexible rotor design. In particular, the first rotor element at the inlet stage can comprise a large diameter and the compression stages can be regular sized. Thereby, alternative raw material, e.g. made from sheet material as opposed to drop forged material can be used in particular for the inlet stages. Further, in this way alternative manufacturing processes especially for the inlet rotor stages can be used. Moreover, due to the multi-part design the hazard potential in the event of rotor failure is reduced.

Preferably, the centering element is an extension preferably integrally built with the rotor shaft and extending through a first opening of the first part and/or a first opening of the second part. Thereby, the number of components necessary for the centering and aligning the two parts can be reduced and the assembly process of the two parts can be simplified.

Preferably, the at least one first part and the at least one second part are connected to the centering element via joining or bracing. Thereby, joints or joining points can be avoided.

Preferably, the centering element is a shaft journal. Thereby, the usual rotorshaft interface can be used to center and align the two parts and thus, the shaft journal is used for supporting the rotor hub and at the same time for centering and aligning the two parts of the rotor hub reducing costs and complexity of the overall design.

Preferably, the first part and the second part are fixed to the rotor shaft by means of at least two connecting elements extending through a respective second opening of the first part and/or the second part. Thereby, the two parts can be secured to the rotor shaft and the misalignment of the two parts in axial direction can be avoided.

Preferably, the at least two connecting elements are bolts. Thereby, the standard bores can be used for bolting down the two parts to the rotor shaft simplifying the design. Preferably, the centering element is a pin extending into respective openings of the first part and/or the second part. Thereby, the pin can be used for the centering and alignment of the two parts with respect to each other and to the rotor shaft and also for pre-balancing and pre-mounting the rotor assembly.

Preferably, the centering element is a groove in a circular member which may extend at least partially around the rotor shaft above the first part. Therein, a protrusion of the first part may extend into the groove having a corresponding shape in order to create a form fit. In particular, the groove can be a groove extending in the axial direction into the surface of a pressure plate used to connect the first part and the second part to the rotor shaft, thereby intercepting parts of the centrifugal forces acting on the rotor hub.

Preferably, the centering element is a protrusion extending in axial direction between the first part and/or the second part into a groove or recess of the corresponding other part. Therein, the groove or recess has a form corresponding to the form of the protrusion to create a form fit. Thereby, the protrusion can be used for the centering and alignment of the two parts with respect to each other and to the rotor shaft and also for pre-mounting the rotor assembly.

Preferably, the centering element is a flange arranged at an extension integrally built with the rotor shaft. Thereby, a simple and cost-effective component can be used which also can be adjusted and/or replaced in case of malfunctioning or damage of the same.

Preferably, the first part and the second part are in mechanical contact with each other in the axial direction at an axial contact area. Thereby, gaps between the two parts in the axial direction and respective misalignment in the axial direction during rotation of the rotor assembly can be avoided due to redundant dimensioning/constraints. Preferably, the first part and the second part are separated from each other in radial direction by a radial gap. Thereby, an interaction of the two parts is only enabled in the axial direction and avoided in radial direction.

Preferably, the width of the radial gap is equal to or greater than 0,1 mm. Thereby, the two parts are separated in the radial direction such that physical interaction in the radial direction is disabled.

Preferably, the width of the radial gap is between 0,1 mm and 0,5 mm. Thereby, sufficient distance in the radial direction between the two parts can be achieved.

Preferably, the number of parts is greater than two. Thereby, the flexibility of the rotor assembly design can be increased.

Preferably, the rotor elements are integrally built with the first part and/or the second part, respectively.

According to a second aspect of the present invention a vacuum pump with the rotor assembly according to the first aspect is provided. The vacuum pump comprising a motor, a rotor shaft and a rotor hub, in particular a bell-shaped rotor hub, at least one magnetic bearing and may have one mechanical bearing. Alternatively, two magnetic bearing are implemented to support the rotor shaft. Further, the vacuum pump comprises a plurality of stator discs, and at least one rotor disc arranged at the at least one first rotor element and a plurality of rotor discs arranged at the at least one second rotor element. Thereby, a vacuum pump with increased flexibility in the rotor assembly design can be provided with reduced hazard potential in case of a rotor assembly damage or failure. Further, due to the possibility of larger inlet stages high pumping speed and high gas throughput for faster pump-down times can be achieved. Preferably, the at least one magnetic bearing is arranged at a first end of the rotor shaft, preferably the high vacuum side of the rotor shaft and the at least one mechanical bearing may be arranged at a second end of the rotor shaft opposite to the first end. Thereby, a bearing arrangement with reduced wear and low-vibration can be provided.

In the following the present invention is described in more detail with reference to the accompanying figures.

The figures show:

Figure 1 a rotor assembly according to the state of art

Figure 2 an embodiment of a rotor assembly according to the present invention,

Figure 3 an embodiment of a vacuum pump according to the present invention.

Figure 1 shows a known rotor assembly 100 comprising a rotor shaft 20 with a shaft journal 40 integrally built with the rotor shaft 20 and extending in axial direction towards the inlet side of the vacuum pump. The rotor assembly 100 further comprises a rotor hub 30 with an opening 312 thorough which the rotor hub 30 is mounted on the rotor shaft 20. In the example shown, the rotor hub 30 is a bell-shaped rotor hub. The rotor assembly 100 further comprises bolts 50 extending in axial direction through respective openings 313 of the rotor hub 30 for bolting down the rotor hub 30 to the rotor shaft 20 in axial direction.

The rotor assembly 100 further comprises a pressure plate 80 for reducing tensions on the rotor assembly 100. The pressure plate 80 is fixed to the rotor hub 30 via fixing elements, e.g. bolts 50. The rotor assembly 100 is rotated by an electromotor (not shown in figure 1) and may be rotatably supported by a roller bearing (not shown in figure 1) at the outlet side of the rotor assembly 100, i.e. at the end of the rotor assembly 100 towards the outlet (not shown in figure 1) and opposite to the axial end of the rotor assembly 100 towards the inlet of the vacuum pump. In addition, the rotor assembly 100 is rotatably supported by a magnetic bearing 90 arranged at the inlet side of the rotor assembly 100 within a contour of the rotor hub 30. As can be seen in figure 1, the rotor assembly 100 further comprises a plurality of rotor elements 29 with rotor discs/rotor elements 311 extending radially from the rotor hub 30.

The rotor elements 29 are typically press-fitted to the rotor hub 30 and are therefore subject to high axial forces due to material deformation of the rotor hub 30 when the rotor assembly 100 rotates at high speeds during operation of the vacuum pump. Subsequently, the rotor elements 29 will be misaligned with respect to each other and the rotor shaft 20.

Therein, the rotor hub 30 of the prior art is built as one piece and thus must be manufactures from a single piece raw material increasing costs and availability of raw material of a sufficient quality.

Figure 2 shows a rotor assembly 10 according to the invention. The rotor assembly 10 comprises a rotor shaft 20 and a rotor hub 30 mounted thereon. In the embodiment shown in figure 2, the rotor hub 30 is a bell-shaped rotor hub. However, in another embodiment the rotor hub 30 might have another shape like e.g. a cylindrical shape.

According to the invention, the rotor hub 30 comprises two separate parts 31, 32. Therein, the first part 31 and the second part 32 are separated from each other in the axial direction at a centering element 40. Further, the first part 31 comprises a plurality of first rotor elements 311 and the second part 32 comprises a plurality of second rotor elements 321. The first and second part 31, 32 are connected to the centering element 40 via joining, clamping or bracing. In this way, the two parts 31, 32 do preferably not comprise any joints that might suffer from tearing due to the deformation of the rotor hub 30.

With the centering element 40, the two parts 31, 32 of the rotor hub are centered and aligned with respect to each other and the rotor shaft 20.

The rotor assembly 10 further comprises bolts 50 extending in axial direction through a respective second openings 313 of the first part 31 of the rotor hub

30 and through a respective second openings 323 of the second part 32 of the rotor hub 30. Therein, the number of bolts 50 is at least two. However, the number of bolts 50 is not restricted to two and might be greater than two depending on different parameters like e.g. the size of the centering element 40 and/or the two parts 31, 32.

With the bolts 50 the first and second part 31, 32 are bolted down to the rotor shaft 20 in axial direction.

The rotor assembly 10 further comprises a pressure plate 80 for reducing tensions on the rotor assembly 10. The pressure plate 80 is fixed to the first part

31 with respective fixing elements, e.g. by the bolts 50 extending in axial direction through respective openings of the pressure plate 80.

The rotor assembly 10 is rotated by an electromotor (not shown in figure 2) and may be rotatably supported by a roller bearing (not shown in figure 2) at the outlet side of the rotor assembly 10, i.e. at the end of the rotor assembly 10 towards the outlet (not shown in figure 2) and opposite to the axial end of the rotor assembly 10 towards the inlet of the vacuum pump. Further, the rotor assembly 10 is rotatably supported by at least one magnetic bearing 90 arranged in the inlet side of the rotor assembly 10 within a contour of the second part 32 of the bell-shaped hub 30. Further, the first part 31 and the second part 32 are in mechanical contact with each other in the axial direction at an axial contact area 60 and are separated from each other in radial direction by a radial gap 70. Preferably, the radial gap has a width of 0,1 mm up to 0,5 mm. In this way, the first part 31 and the second part 32 are only in contact with each other in the axial direction in particular by the axial contact area 60 and do not contact each other in the radial direction.

In the embodiment shown in figure 2, the centering element 40 is an extension preferably integrally built with the rotor shaft 20 in particular the centering element is the shaft journal that is and extending through a first opening 312 of the first part 31 and a first opening 322 of the second part 32. The first opening 312 of the first part 31 and the first opening 322 of the second part 32 each comprise a corresponding diameter configured to provide a form fit for the first part 31 and the second part 32 such that the first part 31 and the second part 32 are centred and aligned with respect to each other and the shaft 20.

In another embodiment the centering element 40 is one or more pins connected to the first or second part and extending into respective openings of the respective other part 31, 32.

According to another embodiment, the centering element 40 is a groove in the pressure plate 80 extending at least partially around the rotor shaft 20 above the first part 31, wherein a protrusion of the first part 31 having a corresponding shape extends into the groove in order to create a form fit. In particular, the groove can be a groove extending in the axial direction into the surface of the a pressure plate 80 used to connect the first part 31 and the second part 32 to the rotor shaft 20, thereby intercepting parts of the centrifugal forces acting on the rotor hub 30. In another embodiment, the centering element 40 is a protrusion extending in axial direction between the first part 31 and/or the second part 32 into a groove or recess or the respective other part. Therein, the groove or recess has a form corresponding to the form of the protrusion to create a form fit.

In another embodiment, the centering element 40 is a flange arranged at an extension integrally built with the rotor shaft 20.

Figure 3 shows an embodiment of a vacuum pump 200 in particular a turbomo- lecular pump with a rotor assembly according to the present invention. The vacuum pump 200 comprises a motor configured to drive the rotor shaft 20. The rotor shaft 20 and a bell-shaped rotor hub 30 are arranged in the vacuum pump housing. However, according to another embodiment the rotor hub 30 might have another shape, e.g. a cylindrical shape. The two magnetic bearings 90 and the two mechanical bearings 230 are configured to support the rotor shaft 20 within the housing. As can be seen in figure 3, a plurality of stator discs 240 are fixedly arranged at the housing and one first rotor disc 250 is arranged at the first rotor element 311 of the first part 31. Further, a plurality of second rotor discs 260 are arranged at the second rotor element 321 of the second part 32 of the bell-shaped hub 30. Therein, the one magnetic bearing 90 is arranged at towards a first end, towards the inlet of the vacuum pump in particular at the high vacuum side of the rotor shaft 20. Whereas, the mechanical bearing 230 is arranged at another end of the rotor shaft 20 opposite to the first end, in particular towards the outlet side of the vacuum pump. In the embodiment shown in figure 3, the centering and alignment of the two parts 31, 32 is achieved by means of the centering element 40, in this case the shaft journal.

According to the present invention, the rotor assembly can be provided in a two- part, i.e. modular design, wherein the centering and alignment of the two parts 31, 32 of the rotor hub 30 is achieved easily and reliably by the centering element 40 integrally formed with the rotor shaft 20 and hence, the problem of misalignment caused by press-fitted rotor discs is avoided. Further, due to the modular design of the rotor assembly 10 the vacuum pump 200 is provided with a particularly large inlet stage diameter but with regularly sized compression stages. Thus, the pumping speed and the weight of the rotor assembly are optimised, only the first inlet stage is enlarged in diameter. Further, at the same time the hazard potential in the event of rotor failure due to multi-part design is reduced.

Reference list

10 rotor assembly according to the invention

20 rotor shaft

29 rotor element

30 rotor hub

31 first part

32 second part

312, 322 first opening

313, 323 second opening

40 centering element

50 connecting element

60 axial contact area

70 radial gap

80 pressure plate

90 magnetic bearing

100 rotor assembly according to the state of the art

200 vacuum pump according to the invention

230 mechanical bearing

240 stator disc

250 first rotor disc

260 second first rotor disc

311 first rotor element

321 second rotor element