GB2579028A

GB2579028A - Molecular drag stage

Info

Publication number: GB2579028A
Application number: GB1818600.7A
Authority: GB
Inventors: James Patey Alexander; Glyn Horler Richard
Original assignee: Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2018-11-14
Filing date: 2018-11-14
Publication date: 2020-06-10
Also published as: WO2020099834A1; GB201818600D0; TW202106979A; EP3880969A1

Abstract

A molecular drag stage 14 with a housing 16 comprising an inlet 28 and an outlet 30, a rotor shaft 18 rotatably supported within the housing, an electric motor 22 wherein the motor is built by a motor stator 58 surrounding the rotor shaft in order to rotate the rotor shaft, where a small gap 60 is defined between the outer side 62 of the rotor shaft and the inner side 64 of the motor stator, and the outer side of the rotor shaft or the inner side of the motor stator comprises a thread 66 or a threaded groove in order to convey a gaseous medium from the inlet to the outlet. The molecular drag stage may be used in a turbomolecular pump, which may have multiple stages. The gap may be the main or only flow path for fluid to reach the outlet. The drag stage may be used in a vacuum pump.

Description

MOLECULAR DRAG STAGE

The present invention relates to a molecular drag stage, preferably for a turbo-molecular pump. The present invention further relates to a turbomolecular pump with such a molecular drag stage.

Known turbomolecular vacuum pumps comprise a housing forming an inlet and an outlet. In the housing a motor is arranged in order to rotate a rotor shaft. The turbomolecular vacuum pump comprises a turbomolecular stage where veins are interacting with a stator in order to convey a gaseous medium from the inlet towards the outlet. The turbomolecular stage is followed by a molecular drag stage such as a Holweck stage. Known molecular drag stages comprise a rotating cylinder close to a stator with a narrow gap in between wherein either the stator or the cylinder has a threaded groove or a thread.

In order to enhance the efficiency of the molecular drag stage, several stages are nested together, also to keep the dimension of the casing small. Thereby, in subsequent molecular drag stages the gaseous medium is pumped in opposite directions. An even number of stages complicates the design by leaving the gas within the pump mechanism, requiring additional channels or drillings to facilitate gas movement to the exhaust. Thus, usually only an odd number of molecular drag stages is possible such that the last molecular drag stage ends towards the outlet of the vacuum pump, thereby limiting the freedom of the design of the vacuum pump.

However, concepts are known comprising an even number of molecular drag stages wherein the gaseous medium is flowing through the gap between a motor stator and the rotor shaft towards the outlet. However, this arrangement causes a heat build-up within the motor and leads to frictional losses rendering this arrangement inefficient.

It is an object of the present invention to provide a molecular drag stage as well as a turbomolecular pump which is more efficient and provides a high degree of freedom in design.

The present technical problem is solved by a molecular drag stage in accordance to claim 1 as well as a turbomolecular pump in accordance to claim 11.

A molecular drag stage in accordance with the present invention comprises a housing with an inlet and an outlet. A rotor shaft is rotatably supported within the housing. The molecular drag stage further comprises an electric motor within the housing, wherein the motor is built by a motor stator surrounding the rotor shaft in order to rotate the rotor shaft. Thereby a small gap is defined between the outer side of the rotor shaft and the inner side of the motor stator, such that the rotor shaft and the motor stator are contact-free and the rotor shaft is rotatable within the motor stator. Thereby the outer side of the rotor shaft or the inner side of the motor stator comprises a thread or a threaded groove in order to convey a gaseous medium from the inlet to the outlet. Thus, one of the outer side of the rotor shaft or the inner side of the motor stator comprises a thread or a threaded groove while the opposite surface is smooth. Hence, the motor itself of the molecular drag stage is adapted to actively convey the gaseous medium towards the outlet. A heat build-up in the motor is prevented and the efficiency of the molecular drag stage is enhanced by utilizing the motor itself as active element for conveying the gaseous medium. The inner side of the motor stator together with the outer side of the rotor shaft thus is building an additional molecular drag stage. Thereby molecules of the gaseous medium are conveyed by a molecular drag process.

Preferably, at least one further pump stage built as molecular drag stage is implemented wherein the pump stage comprises a rotor element connected to the rotor shaft to interact with a non-rotating pump stator element to convey the gaseous medium from the inlet to the outlet. Thereby the rotor element may be built as cylindrical element surrounded by or surrounding the pump stator element. Thereby, either the rotor element or the pump stator element comprise a thread or threaded groove to build a molecular drag stage. A small gap is defined between the rotor element and the pump stator for a contact-less rotation of the rotor element, in order to convey a gaseous medium from the inlet towards the outlet.

Preferably, at least two and preferably four further pump stages are implemented wherein more preferably neighboring pump stages share a common rotor element and convey the gaseous medium in opposite directions. Thus, the outer surface of the rotor element may belong to a first pump stage wherein the inner surface of the same rotor element may belong to the neighboring pump stage, in order to achieve a compact design of the molecular drag stage.

Preferably, the gap between the outer side of the rotor shaft and the inner side of the motor stator defines an entrance through which the gaseous medium enters into the gap and an exit through which the gaseous medium leaves the gap, wherein the entrance is in fluid communication with an exit of the last pump stage and the exit of the gap is preferably in direct fluid communication with the outlet. Thus, in the molecular drag stage, the gaseous medium is first flowing to one or further pump stages built by respective rotor elements and pump stator elements, and after this the gaseous medium is conveyed through the gap formed by the electric motor towards the outlet. Thus, it is possible to implement an even number of pump stages without disadvantages since the active gap of the electric motor is utilized for conveying the gaseous medium through the outlet.

Preferably, at the entrance of the gap a pressure is below 30mbar and more preferably below 10-2 mbar. At the exit of the gap preferably a pressure is equal to the pressure of the environment or below. More preferably the pressure at the exit is between 100mbar and 0.02mbar.

Preferably, the main flow of the gaseous medium is directed through the gap. Thus, the gap of the electric motor is not utilized as bypass or the like.

Preferably, no purge gas is conveyed through the gap.

Preferably, the exit of the last pump stage is connected with the outlet only via the gap. Thus, gas leaving the last pump stage can only reach the outlet if flowing through the active gap of the electric motor.

Preferably, the pump stages are built as Holweck stages.

Preferably, the motor stator is built as a laminated stator core comprising a plurality of sheet elements. Thereby, the plurality of sheet elements is arranged within the motor stator, or in other words, the motor stator surrounds the plurality of sheet elements connected to the rotor shaft. In particular the sheet elements of the motor stator are made of steel.

Preferably, directly adjacent sheet elements of the motor stator differ at their inner side from each other in at least one structural feature, such as a protrusion or indentation that is displaced from one sheet element to the next, wherein along the length of the gap the thread or threaded groove is formed by the sum of the individual displaced structural features. Thus, by the angular displacement of the structural feature of directly neighboring sheet elements, the thread or threaded groove of the gap is built in a step-wise manner. Thereby, the active gap of the electric motor is easy to manufacture. Since the motor stator of the electric motor usually comprises sheet elements in any way, these sheet elements can be utilized to build the thread or threaded groove in order to form the active gap of the electric motor.

Further, the present invention relates to a vacuum pump comprising a molecular drag stage as described above. Preferably, the molecular pump is a turbomolecular pump comprising a turbomolecular pump stage. The turbomolecular pump stage comprises veins connected to the rotor shaft interacting with stator veins in order to convey a gaseous medium from the inlet towards the outlet. Thereby, preferably the turbomolecular pump stage is arranged upstream of the -3 -molecular drag stage in accordance with the general flow of the gaseous medium within the vacuum pump. Preferably, the vacuum pumpe is able to produce a vacuum of 10-2 to 10-12 mbar.

In the following, the invention is described with respect to a specific embodiment with reference to the accompanying drawings.

Fig. 1 shows a schematic drawing of a vacuum pump in accordance with the present invention, and Fig. 2 shows a detailed view of the motor stator in accordance to the present invention.

The vacuum pump 10 of Fig. 1 is built as turbomolecular vacuum pump comprising a turbomolecular pump stage 12 and a molecular drag stage 14. The vacuum pump 10 comprises a housing 16, wherein in the housing 16 a rotor shaft 18 is rotatably supported by, for example, ball bearings 20. The rotor shaft 18 is rotated by an electric motor 22.

In the turbomolecular pump stage 12, the rotor shaft 18 is connected with rotor elements built as veins 24, interacting with stator veins 26 in order to convey a gaseous medium from an inlet 28 towards an outlet 30. The gaseous medium is conveyed through the turbomolecular pump stage 12 and enters the molecular drag stage 14. In the molecular drag stage 14, the rotor shaft 18 is connected to a rotor element 32 with a first cylinder 34 and a second cylinder 36. Further, a threaded first stator 38 is implemented, wherein the thread of the first stator 38 faces the outer surface of the first cylinder 34 such that the gaseous medium is pumped through the first pump stage 40 from an entrance 42 of the molecular drag stage 14 to a first turning point 44. The inner surface of the first cylinder 34 is facing a second threaded pump stator 46 forming a second pump stage 48, pumping the gaseous medium from the first turning point 44 to a second turning point 50. From the second turning point 50, the gaseous medium is pumped through a third pump stage 52 and a fourth pump stage 54, which are equal or similarly built as the first pump stage 40 or the second pump stage 48. Thus, the gas reaches a last turning point 56 which lies within the vacuum pump 10.

The electric motor 22 comprises a motor stator 58, wherein a gap 60 is defined between the motor stator 58 and the outer surface 62 of the rotor shaft 18. Thereby the inner surface 64 of the motor stator 58 comprises a thread 66, thereby actively pumping the gaseous medium from the last turning point 56 forming the entrance of the gap 60 towards the outlet 30. Thus, the gaseous medium entering the molecular drag stage 14 through the entrance 42 can only reach the outlet 30 through the gap 60. Thereby the gaseous medium is actively pumped in the gap 60 by the thread 66 in order to enhance the efficiency of the molecular drag stage 14, and to avoid frictional losses as well as heat-up within the electric motor 22. Alternatively, the outer surface 62 of the rotor shaft 18 may comprise a thread or threaded groove. In this case, the inner surface 64 of the motor stator 58 may be built as planar surface.

The motor stator 58 is built by a plurality of sheet elements 68 preferably made of steel. Thereby, the sheet elements 68 may comprise a structural feature 70 which can be built as protrusion or indentation. Fig. 2 shows the structural feature 70 as protrusion. Thereby the structural feature 70 is displaced from one sheet element 68 to the next or neighboring by a small angular amount, thereby forming the thread 66 in a step-wise manner by the structural features 70. Of course, one sheet element 68 may comprise more than one structural feature 70. Thus, the thread or threaded groove in the active gap 60 of the electric motor 22 is easy to manufacture such that manufacturing costs of the motor stator 58 of the electric motor 22 can be reduced.

Claims

CLAIMS1. Molecular drag stage (14), preferably for a turbomolecular pump (10), with a housing (16) comprising an inlet (28) and an outlet (30), a rotor shaft (18) rotatably supported within the housing (16), a electric motor (22), wherein the motor (22) is built by a motor stator (58) surrounding the rotor shaft (18) in order to rotate the rotor shaft (18), wherein a small gap (60) is defined between the outer side (62) of the rotor shaft (18) and the inner side (64) of the motor stator (58), wherein the outer side (62) of the rotor shaft (18) or the inner side (64) of the motor stator (58) comprises a thread (66) or a threaded groove in order to convey a gaseous medium from the inlet (28) to the outlet (30).
2. Molecular drag stage in accordance with claim 1, characterized by at least one further pump stage (40), wherein the pump stage (40) comprises a rotor element (34) connected to the rotor shaft (18) to interact with a non-rotating pump stator element (38) to convey the gaseous medium from the inlet (28) to the outlet (30).
3. Molecular drag stage in accordance with claim 2, characterized by at least two and preferably four further pump stages (40, 48, 52, 54), wherein more preferably neighboring pump stages share a common rotor element (34, 36) and convey the gaseous medium in opposite directions.
4. Molecular drag stage in accordance with claim 2 or 3, characterized in that the gap (60) defines an entrance through which the gaseous medium enters into the gap (60) and an exit through which the gaseous medium leaves the gap (60), wherein the entrance is in fluid communication with -s -an exit (56) of the last pump stage (54) and the exit is preferably in direct fluid communication with the outlet (30).
5. Molecular drag stage in accordance with any of claims 1 to 4, characterized in that the main flow of the gaseous medium is directed through the gap (60).
6. Molecular drag stage in accordance with any of claims 1 to 5, characterized in that no purge gas is conveyed through the gap (60).
7. Molecular drag stage in accordance with any of claims 2 to 6, characterized in that the exit (56) of the last pump stage (54) is connected with the outlet (30) only via the gap (60).
8. Molecular drag stage in accordance with any of claims 2 to 7, characterized in that the pump stages (40, 48, 52, 54) are built as Holweck stages.
9. Molecular drag stage in accordance with any of claims 1 to 8, characterized in that the motor stator (58) is built as laminated stator core comprising a plurality of sheet elements (68) wherein the sheet elements (68) are preferably made of steel.
10. Molecular drag stage in accordance with claim 9, characterized in that directly adjacent sheet elements (68) differ at their inner side from each other such that at least one feature (70) is displaced from one sheet element (68) to the next, wherein along the length of the gap (60) the thread (66) is formed by the sum of the individual displaced features (70).
11. Vacuum pump (10), in particular a turbomolecular vacuum pump, comprising a molecular drag stage (14) in accordance with any of claims 1 to 10.