MULTI-STAGE VACUUM PUMP

Abstract

A multi-stage vacuum pump includes a plurality of rotor elements (14, 16) disposed on a common shaft (10) in a pump housing (12) for configuring multiple pump stages (18, 24, 26). The shaft is driven by an electric motor (40). An inner bearing element (42) is disposed between two rotor elements (14, 16) such that a mechanically favorable bearing arrangement is implemented using a simple configuration of the bearing elements (42, 44) as roller bearings. This is possible in particular due to the separation into two rotor elements (14, 16).

Description

The invention relates to a multi-stage vacuum pump.

Multi-stage vacuum pumps are configured e.g. as multi-inlet vacuum pumps having at least two inlets and one outlet. Said inlets are connected to different vacuum stages of the multi-inlet pump, wherein a different vacuum is generated at each inlet. Normally, in such an arrangement, the highest vacuum is generated by the inlet connected to the first stage of the multi-inlet pump, the second highest vacuum is generated by the inlet connected to the second stage, and so forth. Such vacuum pumps with a plurality of vacuum stages comprise, within a housing, a shaft which is driven by an electric motor, the latter being normally arranged around said shaft.

From the state of the art, there are known various types of bearing arrangements for said shaft. Particularly, it is known to support the shaft via two ball bearings, the electric motor being arranged between said two ball bearings. In the direction of the first stage, the shaft comprises a shaft extension. On said shaft extension, the rotor element is arranged. Thus, the rotor is arranged on the projecting shaft extension and accordingly is supported in a cantilevered manner. Since all of the forces acting on the rotor will be transmitted to the shaft at the cantilevered end of the shaft, the bearings are subjected to high stresses. Further, in shafts supported in this manner, the length of the shaft is limited because, otherwise, it would not be possible anymore to take up the occurring forces in the bearings, or one would have to use extremely complex and expensive bearings. A further disadvantage of this bearing arrangement resides in the relatively small bearing spacing.

In another bearing arrangement of a similar type, the electric motor is not arranged between the two bearings but externally of the bearing. Also here, the rotor is arranged on the cantilevered extension of the shaft and thus again has the disadvantages of a cantilevered bearing arrangement. This will cause an unfavorable position of the center of gravity and, as a result, high stresses acting on the bearing.

Further, from DE 603 13 493, it is known to support the shaft on both of its ends. Because of the large bearing spacing resulting from such an arrangement, the forces occurring in the bearings can be equalized and reduced. However, the bearing arranged on the suction side of the pump, i.e. in the region of the first stage, has to be designed as a magnetic bearing, which is of necessity due to the low pressure existing in this region. According to the state of the art, a grease-lubricated ball bearing is unsuited for use in this region because the low pressure would then cause the grease to be sucked out from the bearing. As usually the case in magnetic bearings, there is additionally provided a further ball bearing serving as a retainer bearing, while this bearing, however, is not used for take-up of forces but only as an emergency bearing. Due to the required provision of a permanent magnetic bearing and additionally of a retainer bearing, this type of bearing arrangement is expensive. Further, it is required to provide a star-shaped holding element for the permanent magnetic bearing, said holding element comprising flow passage openings. Since this star-shaped bearing shield is located in the region of the high vacuum stage, conductance losses will occur on extremely unfavorable sites in the course of the flow. Such occurrences will cause a deterioration of the maximum performance of the vacuum pump.

It is an object of the invention to provide an inexpensive and effective bearing arrangement for multi-stage vacuum pumps which particularly also allows for a reduction of the constructional length of the pump.

According to the invention, the above object is achieved by the features defined in claim 1.

As provided by the invention, the rotor is divided into at least two rotor elements. Thus, the two rotor elements are arranged separately from each other, and particularly are connected to the shaft separately from each other. Herein, depending on the configuration of the multi-stage vacuum pump and particularly on the arrangement of the inlets, one or also a plurality of stages can be provided per rotor element. By the inventive division of the rotor into two rotor elements, it is possible to arrange an inner bearing element, usually a rolling bearing such as a ball bearing, between the two rotor elements. Preferably, for this reason, one of the two rotor elements, particularly the rotor element forming or including the high vacuum stage, is arranged externally of the inner bearing element. The rotor element is thus arranged on a shaft extension which is projecting relative to the inner bearing element. Since, however, in contrast to the state of the art, it is not the whole rotor but only one of at least two rotor elements that is arranged on the cantilevered end of the shaft, the forces and moments introduced into the shaft at the cantilevered end thereof will be considerably smaller. The second rotor element can be arranged e.g. between an inner bearing element and an outer bearing element and particularly be fixedly connected to the shaft.

Since, according to the invention, the inner bearing element is preferably not arranged in the region of the high vacuum but instead is arranged within the outer rotor element comprising the high vacuum stage, the bearing will not be subjected to the extremely low pressures which prevail in the region of the high vacuum. This offers the inventive advantage that especially grease-lubricated bearings such as e.g. ball bearings can be used. Particularly, the provision of a ball bearing has the advantage that ball bearings have a distinctly smaller constructional size. Further, the provision of a preferably grease-lubricated ball bearing in this region advantageously obviates the need for an additional emergency bearing. In magnetic bearings, such an emergency bearing would be positively required because, otherwise, no emergency running properties would be guaranteed in case of failure of the magnetic bearing.

Especially due to the relatively large bearing spacing, the forces acting on the bearing will distribute more favorably. This is of advantage under the aspect of rotor dynamics. Further, possible angular deviations of the shaft will be smaller, resulting in advantages for the mechanics of the bearing. Due to the benefit of reduced angular deviations, smaller gaps can be realized so that the efficiency of the pump can be improved and higher final pressures can be achieved. According to a particularly preferred embodiment, the inner bearing element is fixed via a holding element. Said holding element is formed with at least one throughflow opening. For fixing the bearing in position, particularly for fixing the outer bearing shell in case of rolling bearings, the holding element is preferably connected to the pump housing. With special preference, the holding element comprises a plurality of throughflow openings and particularly has a star-shaped configuration. The individual throughflow openings, preferably arranged in a regular pattern and preferably having identical shapes, are with preference configured as partial ring segments. Since, when viewed in the conveying direction, the inner bearing element is arranged within the rotor element comprising the high vacuum stage, the medium will flow through the throughflow openings only when exiting from the high vacuum stage and respectively when entering the next stage. The conductance losses caused by the holding element will thus be distinctly lower than in case of an arrangement wherein such a holding element is provided in the region of the high vacuum stage, i.e. in the gas-entrance region of the high vacuum stage.

According to a particularly preferred embodiment, at least two rotor elements are provided, wherein both the inner bearing element and the holding element are arranged between these two rotor elements.

According to a further particularly preferred embodiment, the inner bearing element is—along the axial direction—at least partially arranged within a rotor element. Particularly, this rotor element is the rotor element comprising the high vacuum stage. Since also in this embodiment the rotor element as seen in flow direction is arranged before the inner bearing element, the inner bearing element is also herein arranged between two rotor elements, particularly between the two fastening regions of the rotor elements to the shaft. Due to the resultant, at least partial covering of the inner bearing element by a part of the rotor element in the axial direction, the cantilevered shaft extension can be made shorter. This will further improve the bearing mechanics.

Preferably, an outer bearing element is arranged in such a manner that, between the two bearing elements, a rotor element is arranged, the latter particularly being fixedly connected to the shaft. Thus, when two rotor elements are provided, the outer bearing element is preferably arranged outside of the rotor element forming the lowest stage. In this arrangement, it is particularly preferred that the drive unit is arranged between the outer bearing element and the rotor element forming the lowest vacuum stage. This has the advantage of allowing for a very large bearing spacing between the two bearing elements, resulting in improved bearing mechanics.

In embodiments comprising e.g. three or more inlets and a corresponding number of vacuum stages, it is preferred that the outer bearing element is arranged between two rotor elements. These will preferably be the two rotor elements forming the lowest vacuum stages, while, optionally, a given rotor element can also form a plurality of vacuum stages.

Particularly in case of a multi-stage vacuum pump with more than two inlets and in case of the herein preferred arrangement of the outer bearing element between two rotor elements, the outer bearing element is fixed by a holding element. Said holding element is preferably provided with through openings and is designed corresponding to the holding element of the inner bearing element.

The inner bearing element is preferably designed as a rolling bearing. It is, however, also possible to provide a magnetic bearing, particularly a permanent magnetic bearing, while optionally also a retainer bearing can be provided.

According to a particularly preferred embodiment, the multi-stage vacuum pump of the invention is a vacuum pump of the multi-inlet type. This pump is provided, apart from the main inlet, with at least one additional inlet. Preferably, each of said additional inlets is arranged between two adjacent vacuum pumps. In usual multi-inlet vacuum pumps, a pressure of 1×10⁻⁵ mbar to 1×10⁻⁹ mbar can be generated on the high vacuum side. At a first intermediate inlet, a pressure of 1×10⁻² mbar to 1×10⁻⁵ mbar can be reached. In case that a second intermediate inlet is provided, a pressure of 1×10⁻² mbar to 5×10⁻¹ mbar can be reached thereat.

The invention will be explained in greater detail hereunder by way of preferred embodiments.

In the drawings, the following is shown:

FIG. 1 is a cross-sectional schematic diagram of a first embodiment comprising two rotor elements,

FIG. 2 is a cross-sectional schematic diagram of a first embodiment comprising three rotor elements, and

FIG. 3 is a schematic plan view of a holding element.

In the strongly simplified schematic representation of a first embodiment of the multi-stage vacuum pump of the invention (FIG. 1), a shaft 12 is arranged in a pump housing 10. Said shaft 12 carries the rotor elements 14,16 which according to the invention are separated or detached from each other. Said rotor elements are fixedly connected to shaft 12.

In the illustrated embodiment, rotor element 14 forms a first vacuum stage 18 in which the highest vacuum is generated. The gas which is to be conveyed is suctioned via a first inlet opening 20. In the illustrated embodiment, the first vacuum stage is a vacuum stage formed by a turbomolecular pump. A stator 22 connected to housing 10 cooperates with said rotor 14.

The second rotor element 16 in the illustrated embodiment is arranged to form two vacuum stages 24,26. Also the second vacuum stage 24 is formed by a turbomolecular pump while, also herein, a stator 28 is connected to housing 10. The third stage 26 is a Holweck stage wherein the helical extension 30 is arranged in engagement with a corresponding helical recess. The second vacuum stage 24 will suction the medium through an inlet opening 34, and the third vacuum stage 26 will suction the medium through an inlet opening 36. The suctioned medium will be conveyed by from all three stages 18,24,26 to the discharge opening 38.

In the illustrated embodiment, an electric motor 40 for driving said shaft 12 is located in the region of the third stage. As provided according to a preferred embodiment, said electric motor 40 is arranged to surround shaft 12. In the axial direction, said Holweck stage preferably surrounds the electric motor 40.

Support of shaft 12 is realized by an inner bearing element 42 and an outer bearing element 44. Said inner bearing element 42 is arranged between the two rotor elements 14,16. In case that the inner bearing element 42 is a rolling bearing, an inner bearing ring is e.g. pressed onto shaft 12. An outer bearing ring is fixed via a holding element 46. Said holding element 46, shown in plan view in FIG. 3, comprises a plurality of through openings 48 formed as partial ring segments and particularly arranged in a regular configuration, for passage therethrough of the medium conveyed by the first stage 18.

In the illustrated embodiment, outer bearing element 44 is arranged outside the lowest, i.e. third stage 26. Also bearing element 44 preferably is designed as a rolling bearing.

Along the axial direction 50, the inner bearing element of the illustrated embodiment is arranged within rotor element 14. For this purpose, rotor element 14 comprises a recess 52 having a substantially circular cross section.

In the context of the second preferred embodiment, constructional components similar to or identical with those described above are designated with the same reference numerals. For ease of survey, no pump housing is illustrated. The gas flow is indicated by arrows 54, 56, 58, 60. An alternative gas flow is indicated by arrows 62, 64, 56, 58, 60.

In the embodiment shown in FIG. 2, three rotor elements 14, 66, 68 are provided on the shaft 64. In the illustrated embodiment (FIG. 2), all rotor elements 14, 66, 68 are shown as rotor elements of molecular pumps while, of course, the rotor elements can also be of a different type. Further, also in this embodiment, single rotor elements can form a plurality of stages.

As in the embodiment shown in FIG. 1, the inner bearing element 42 is arranged between two rotor elements 14,66 and again is fixed or connected to the housing by a holding element 46 (FIG. 3). In the illustrated embodiment, the drive motor 40 is arranged between the two rotor elements 14,66.

The second, i.e. outer bearing element 44 is arranged between the two rotor elements 66,68 (FIG. 2) and, in the illustrated embodiment, is fixed via a holding element 46.

Via a first flow path which is indicated by the arrows 54, 56, 58, 60, the flow will successively pass through the individual pump stages formed by the rotor elements 14, 66, 68.

Within the second flow path which is indicated by arrows 62, 64, 56, 58, 60, there is additionally performed a suctional intake of gas through a further inlet opening in the direction marked by arrow 62. Via a bypass or connection channel (arrow 64), the gas which is sucked both in the direction of arrow 62 and in the direction of arrow 54 will be conveyed to the next stage (rotor element 66).

The arrows 54, 62, 56 and 68 correspond to inlet openings and outlet openings, respectively. Arrow 60 corresponds to the discharge opening.

Claims

1. A multi-stage vacuum pump, comprising: a plurality of rotor elements arranged on a common shaft in a pump housing to form a plurality of pump stages,a drive element for driving said shaft, andan inner bearing element arranged between two rotor elements.

2. The multi-stage vacuum pump according to claim 1, wherein said inner bearing element is fixed via a holding element comprising at least one throughflow opening.

3. The multi-stage vacuum pump according to claim 2, wherein said inner bearing element is connected to the pump housing via said holding element.

4. The multi-stage vacuum pump according to claim 2, wherein said holding element is substantially round.

5. The multi-stage vacuum pump according to claim 2, wherein said holding element is arranged between two rotor elements.

6. The multi-stage vacuum pump according claim 1, wherein, along the axial direction, said inner bearing element is arranged at least partially within a rotor element.

7. The multi-stage vacuum pump according to claim 1, wherein a rotor element is arranged between said inner bearing element and an outer bearing element.

8. The multi-stage vacuum pump according to claim 7, wherein said outer bearing element is arranged between two rotor elements.

9. The multi-stage vacuum pump according to claim 7, wherein said inner bearing element and/or said outer bearing element are configured as rolling bearings.

10. The multi-stage vacuum pump according to claim 9, wherein said rolling bearing is grease-lubricated.

11. The multi-stage vacuum pump according to claim 2, wherein the holding element defines a plurality of throughflow openings which are shaped as partial ring segments.

12. The multi-stage vacuum pump according to claim 7, wherein the rotor element is tightly connected to the shaft.

13. The multi-stage vacuum pump according to claim 8, wherein the outer bearing element is fixed via a holding element comprising throughflow openings.

14. The multi-stage vacuum pump according to claim 9, wherein the inner bearing element and/or the outer bearing element are roller bearings.

15. A multi-stage vacuum pump comprising: a pump housing which defines at least a first inlet and a second inlet and an outlet;a drive shaft;a first rotor element mounted to the drive shaft and configured to draw gas into the housing through the first inlet;a second rotor mounted to the drive shaft and configured to draw gas into the housing through the second inlet and to push the gas drawn into the housing through the first and second inlets out the outlet;a first bearing mounted to the shaft on an opposite side of the first rotor element from the first inlet to rotatably support the shaft in the housing;a second bearing mounted to the shaft on a side of the second rotor element facing the outlet to rotatably support the shaft in the housing; anda motor mounted around the shaft between the first and second bearings to rotate the shaft and the first and second rotor elements.