EP3880969A1

EP3880969A1 - Motor as molecular drag stage

Info

Publication number: EP3880969A1
Application number: EP19804785.4A
Authority: EP
Inventors: Alexander James PATEY; Richard Glyn Horler
Original assignee: Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2018-11-14
Filing date: 2019-11-07
Publication date: 2021-09-22
Also published as: TW202106979A; GB2579028A; WO2020099834A1; GB201818600D0

Abstract

A 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). Thereby, a small gap (60) is defined be- tween the outer side (62) of the rotor shaft (18) and the inner side (64) of the motor stator (58). 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).

Description

MOTOR AS MOLECULAR DRAG STAGE

The present invention relates to a molecular drag stage, preferably for a turbo- molecular pum p. The present invention further relates to a turbomolecular pum p with such a m olecular drag stage.

Known turbom olecular vacuum pum ps com prise a housing form ing an inlet and an outlet. I n the housing a m otor is arranged in order to rotate a rotor shaft. The turbomolecular vacuum pum p comprises a turbom olecular 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 m olecular drag stage such as a Holweck stage. Known m olecular drag stages com prise 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.

I n order to enhance the efficiency of the m olecular drag stage, several stages are nested together, also to keep the dim ension of the casing sm all. Thereby, in subsequent molecular drag stages the gaseous m edium is pum ped in opposite directions. An even number of stages com plicates the design by leaving the gas within the pum p m echanism , requiring additional channels or drillings to facili tate gas movem ent to the exhaust. Thus, usually only an odd num ber of m olec ular drag stages is possible such that the last molecular drag stage ends towards the outlet of the vacuum pum p, thereby lim iting the freedom of the design of the vacuum pump.

However, concepts are known com prising an even num ber of m olecular drag stages wherein the gaseous m edium 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 arrangem ent inefficient. It is an object of the present invention to provide a molecular drag stage as well as a turbom olecular pump which is m ore efficient and provides a high degree of freedom in design.

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

A m olecular 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 m olecular drag stage further com prises an electric motor within the housing, wherein the m otor is built by a m otor 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 m otor 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 m otor stator com prises a thread or a threaded groove in order to convey a gaseous m edium from the inlet to the outlet. Thus, one of the outer side of the rotor shaft or the inner side of the m otor stator com prises a thread or a threaded groove while the opposite surface is sm ooth. Hence, the motor itself of the molecular drag stage is adapted to actively convey the gaseous m edium towards the outlet. A heat build-up in the m otor is pre vented and the efficiency of the molecular drag stage is enhanced by utilizing the motor itself as active elem ent for conveying the gaseous medium . The in nerm ost side of the motor stator together with the outer side of the rotor shaft thus is building an additional molecular drag stage. Thereby m olecules of the gaseous medium are conveyed by a molecular drag process.

Preferably, at least one further pum p stage built as m olecular drag stage is im plem ented wherein the pum p stage comprises a rotor elem ent connected to the rotor shaft to interact with a non-rotating pump stator elem ent to convey the gaseous m edium from the inlet to the outlet. Thereby the rotor elem ent m ay be built as cylindrical elem ent surrounded by or surrounding the pum p stator element. Thereby, either the rotor element or the pump stator element comprise a thread or threaded groove to build a m olecular drag stage. A sm all gap is defined between the rotor element and the pum p stator for a contact-less rota tion of the rotor element, in order to convey a gaseous m edium from the inlet towards the outlet.

Preferably, at least two and preferably four further pum p stages are imple mented wherein m ore preferably neighboring pum p stages share a com mon rotor element and convey the gaseous medium in opposite directions. Thus, the outer surface of the rotor element may belong to a first pum p stage wherein the inner surface of the sam e rotor elem ent m ay belong to the neighboring pum p 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 m otor 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 com m unication with an exit of the last pum p stage and the exit of the gap is preferably in direct fluid com m unication with the outlet. Thus, in the molecular drag stage, the gaseous m edium is first flowing to one or further pum p stages built by respective rotor elem ents and pum p stator elem ents, and after this the gaseous medium is conveyed through the gap formed by the electric motor towards the outlet. Thus, it is possible to im plement an even number of pum p stages without disadvantages since the active gap of the electric m otor is utilized for conveying the gaseous m edium through the outlet.

Preferably, at the entrance of the gap a pressure is below 30m bar and more preferably below 10 ² m bar . 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 l OOm bar and 0.02m bar.

Preferably, the main flow of the gaseous m edium is directed through the gap. Thus, the gap of the electric m otor is not utilized as bypass or the like. Preferably, no purge gas is conveyed through the gap.

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

Preferably, the pump stages are built as Holweck stages.

Preferably, the m otor stator is built as a lam inated stator core com prising a plurality of sheet elements. Thereby, the plurality of sheet elements is arranged within the m otor stator, or in other words, the m otor stator surrounds the plu rality of sheet elem ents connected to the rotor shaft. I n particular the sheet elements of the motor stator are m ade of steel.

Preferably, directly adjacent sheet elements of the m otor 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 elem ent to the next, wherein along the length of the gap the thread or threaded groove is form ed by the sum of the individual displaced structural features. Thus, by the angular displace ment of the structural feature of directly neighboring sheet elem ents, the thread or threaded groove of the gap is built in a step-wise m anner. Thereby, the active gap of the electric motor is easy to m anufacture. Since the motor stator of the electric m otor usually com prises sheet elem ents in any way, these sheet ele ments can be utilized to build the thread or threaded groove in order to form the active gap of the electric m otor.

Further, the present invention relates to a vacuum pump com prising a m olecular drag stage as described above. Preferably, the m olecular pump is a turbomo- lecular pum p com prising a turbom olecular pum p stage. The turbom olecular pump stage com prises veins connected to the rotor shaft interacting with stator veins in order to convey a gaseous m edium from the inlet towards the outlet. Thereby, preferably the turbom olecular pump stage is arranged upstream of the molecular drag stage in accordance with the general flow of the gaseous m e dium within the vacuum pum p. Preferably, the vacuum pum p is able to produce a vacuum of 1 0 ² to 10 ¹² m bar.

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

Fig. 1 shows a schem atic drawing of a vacuum pum p in accordance with the present invention, and

Fig. 2 shows a detailed view of the m otor stator in accordance to the present invention.

The vacuum pump 10 of Fig. 1 is built as turbom olecular vacuum pum p com prising a turbomolecular pump stage 1 2 and a m olecular drag stage 14. The vacuum pum p 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.

I n the turbom olecular 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 m edium from an inlet 28 towards an outlet 30. The gaseous m edium is conveyed through the turbom olecular pum p stage 12 and enters the molecular drag stage 14. I n the molecular drag stage 14, the rotor shaft 1 8 is connected to a rotor element 32 with a first cylinder 34 and a second cylinder 36. Further, a threaded first stator 38 is implem ented, wherein the thread of the first stator 38 faces the outer surface of the first cylinder 34 such that the gaseous medium is pum ped through the first pum p stage 40 from an entrance 42 of the m olecular drag stage 14 to a first turning point 44. The inner surface of the first cylinder 34 is facing a second threaded pum p stator 46 form ing a second pum p stage 48, pum ping the gaseous m edium from the first turning point 44 to a second turning point 50. From the second turning point 50, the gaseous medium is pum ped through a third pum p stage 52 and a fourth pump stage 54, which are equal or sim ilarly built as the first pum p stage 40 or the second pum p stage 48. Thus, the gas reaches a last turning point 56 which lies within the vacuum pum p 1 0.

The electric m otor 22 com prises 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 m otor stator 58 comprises a thread 66, thereby actively pumping the gaseous m edium from the last turning point 56 form ing the entrance of the gap 60 towards the outlet 30. Thus, the gaseous medium entering the m olecular drag stage 14 through the entrance 42 can only reach the outlet 30 through the gap 60. Thereby the gaseous medium is actively pum ped 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 com prise a thread or threaded groove. I n 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 m ade of steel. Thereby, the sheet elem ents 68 may comprise a structural feature 70 which can be built as protrusion or indentation. Fig. 2 shows the structural fea ture 70 as protrusion . Thereby the structural feature 70 is displaced from one sheet elem ent 68 to the next or neighboring by a small angular amount, thereby form ing the thread 66 in a step-wise m anner by the structural features 70. Of course, one sheet elem ent 68 m ay com prise m ore than one structural feature 70. Thus, the thread or threaded groove in the active gap 60 of the electric motor 22 is easy to m anufacture such that m anufacturing costs of the m otor stator 58 of the electric motor 22 can be reduced.

Claims

CLAI MS

1. 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 en ters into the gap (60) and an exit through which the gaseous medium leaves the gap (60), wherein the entrance is in fluid communication with an exit (56) of the last pum p stage (54) and the exit is preferably in direct fluid com m unication 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 pum p 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 m otor stator (58) is built as lam inated stator core com prising a plurality of sheet elements (68) wherein the sheet elem ents (68) are pref erably m ade of steel.

10. Molecular drag stage in accordance with claim 9, characterized in that di rectly adjacent sheet elements (68) differ at their inner side from each other such that at least one feature (70) is displaced from one sheet ele ment (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) .

1 1 . Vacuum pump ( 10) , in particular a turbomolecular vacuum pum p, com prising a molecular drag stage ( 14) in accordance with any of claim s 1 to 1 0.