WO2005047705A1

WO2005047705A1 - Vacuum screw pump

Info

Publication number: WO2005047705A1
Application number: PCT/GB2004/004654
Authority: WO
Inventors: Emmanuel Uzoma Okoroafor; Nigel Paul Schofield
Original assignee: The Boc Group Plc
Priority date: 2003-11-10
Filing date: 2004-11-04
Publication date: 2005-05-26
Also published as: GB0326235D0; TW200521329A

Abstract

A screw-shaped rotor for a vacuum pump comprises an array of mutually spaced bands of abrasive material mounted about the thread of the rotor. During rotor rotation, any contact between the rotor and the stator causes the bands to abrade a series of grooves in the pump stator, both preventing pump seizure and resulting in a close fitting seal between the rotor and stator.

Description

VACUUM PUMP

This invention relates to a vacuum pump, and in particular to a rotor for a screw pump.

Screw pumps are potentially attractive since they can be manufactured with few working components and they have an ability to pump from a high vacuum environment at the inlet down to atmospheric pressure at the outlet. Screw pumps usually comprise two spaced parallel shafts each carrying externally threaded rotors, the shafts being mounted in a pump body such that the threads of the rotors intermesh. Close tolerances between the rotor threads at the points of intermeshing and with the internal surface of the pump body, which acts as a stator, causes volumes of gas being pumped between an inlet and an outlet to be trapped between the threads of the rotors and the internal surface and thereby urged through the pump as the rotors rotate. Minimising the running clearance between the rotors and the stator can reduce pump power consumption but can, however, increase the likelihood of the rotors and the stator contacting, for example, due to thermal expansion of the rotors as the pump warms up during use, which can result in pump seizure.

In order to reduce the risk of seizure, one solution is to apply PTFE or other "soft" coating on either the rotors or the stator. In the event of contact between the rotors and the stator, the coating is worn away whilst still permitting the rotors to rotate relative to the stator. However, there are a number of problems associated with this solution. Firstly, many of the available coatings are susceptible to chemical attack from a variety of gases, such as fluorine, which may be pumped when the pump is used in a semiconductor application. Secondly, the coating can be gradually eroded by particles contained in the pumped gases. Finally, instead of wearing away when the rotors and stator contact, the coating may simply rub, generating heat in the rotors (which are typically formed from cast iron), which can increase the amount of thermal expansion of the rotors and thus increase the risk of pump seizure. ln at least its preferred embodiment, the present invention seeks to solve these and other problems.

In a first aspect the present invention provides a screw-shaped rotor for a vacuum pump, the rotor comprising an array of mutually spaced bands of abrasive material located on the thread of the rotor, the bands extending about and generally radially outwards from the thread of the rotor.

During use of the pump, the abrasive bands may contact the pump stator, for example, due to thermal expansion of the rotor and cause a series of grooves to be abraded in the stator. As well as significantly reducing the risk of pump seizure, this can result in a close fitting seal between the rotor and stator during use, reducing pump power consumption. By arranging the abrasive material in bands extending about the rotor, abrasion of the stator, and the level of torque required to abrade the stator, can be minimised.

Preferably, the bands are mounted about planes substantially orthogonal to the longitudinal axis of the rotor. Each band may comprise a plurality of abrasive particles or projections mounted about a respective plane substantially orthogonal to the longitudinal axis of the rotor. Alternatively, each band may comprise a ridge of abrasive material.

Preferably, the bands have a width in the longitudinal direction of the rotor in the range from 0.5 to 3mm, more preferably in the range from 1 to 2mm. The bands preferably extend outwards from the surface of the rotor by a distance in the range from 0.01 to 0.15mm, typically in the range from 0.03 to 0.1mm.

Preferably, the bands are spaced apart along the thread in the range from one sixth to one half of the pitch of the thread, more preferably around one quarter to one third of the pitch of the thread. The bands may be fixedly secured to, or integrally formed with, the thread of the rotor. Preferably, the bands are deposited on the surface of the thread. A coating technique may be used to deposit the bands. For example, the rotor may be selectively masked and material sprayed through apertures in the mask to form the bands. The mask may comprise a plurality of pinholes, with the rotor being rotated during spraying so as to cause a similar number of bands or ridges to be formed over the surface of the rotor. Alternatively, the rotor may be coated with material, which is subsequently selectively removed, for example, through abrasion, to leave only the bands on the thread.

The bands may be formed from particles or ridges of ceramic material, such as aluminium oxide or chromium oxide. The use of such materials is advantageous as they are resistant to corrosion by pumped gases such as fluorine, and to damage or wear from particulates contained in the pumped gases.

In another aspect, the present invention provides a vacuum pump comprising at least one rotor as aforementioned. In the preferred embodiment, the rotor is rotatable relative to a stator, whereby, in use, as the rotor is rotated, contact between the rotor and the stator can cause the bands to wear a plurality of grooves in the stator. Thus, the present invention also provides a vacuum pump comprising a stator, at least one screw shaped rotor rotatable relative to the stator, the rotor comprising an array of mutually spaced projections formed from material harder than the stator and mounted along and about the thread of the rotor and extending generally radially outwards therefrom whereby, as the rotor is rotated in use, any contact between the rotor and the stator causes a plurality of grooves to be abraded in the stator.

In a further aspect, the present invention provides a method of manufacturing a screw-shaped rotor for a vacuum pump, the method comprising forming from abrasive material an array of mutually spaced bands on the thread of the rotor, the bands extending about and generally radially outwards from the thread of the rotor. Preferred features of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 illustrates a side view of a screw-shaped rotor;

Figure 2 is a cross-sectional view of part of a thread of the rotor of Figure 1 ;

Figure 3 is a cross-sectional view of part of a vacuum pump including the rotor of Figure 1 ; and

Figure 4 is a cross-sectional view of a groove formed in the wall of the stator of the pump of Figure 3.

With reference to Figures 1 and 2, the rotor 10 is attached to a shaft 12 and is adapted for rotation about its main, longitudinal axis 14. The rotor 10 has a continuous helical vane, or thread, 16 on its outer surface. An array of bands 18 of abrasive material is mounted about and along the thread 16. Each band 18 lies in a respective plane orthogonal to the axis 14.

In the preferred embodiment, the bands 18 have a width in the direction of the axis 14 in the range from 1 to 2mm, and extend radially outwards from the surface of the thread 16 by a distance in the range from 0.03 to 0.1mm, typically around 50μm. The bands 18 are spaced apart along the axis 14 by around one third of the pitch length of the thread 16, and due to the helical form of the thread 16, generally extend less than 360°, typically about 180°, around the rotor 10.

The bands 18 may be fixedly secured to, or integrally formed with, the thread 16 of the rotor 10. In this embodiment, the bands 18 are formed from ceramic material deposited on the surface of a thread 16 by any convenient technique. For example, a spray coating technique may be used to deposit the bands on the thread. A mask may be employed for the selective deposition of the ceramic material, or alternatively the threads may be coated with ceramic material which is subsequently selectively removed, for example, by grinding, to leave only the bands 18 on the thread 16.

The bands 18 are preferably formed from material, such as aluminium oxide or chromium oxide, which is resistant to corrosion by pumped gases such as fluorine, and to damage or wear from particulates contained in the pumped gases. Each band may comprise a respective ridge of abrasive material, as shown in Figure 2, or a respective plurality of abrasive particles or projections.

With reference to Figure 3, an embodiment of a screw vacuum pump includes a pair of such rotors 10 each attached to a respective shaft 12. Typically, an electric motor drives one shaft, with a gear arrangement linking that shaft with the other to drive the shafts at the same speed of rotation but in an opposite directions. The threads 16 of the rotors 10 intermesh at the pump centre and, in use of the pump, have close tolerances with the internal walls 20 of the pump (generally formed from cast iron), which provide a stator for the pump.

In use, the pump is first conditioned before use in, say, a semiconductor application. The motor is switched on so that gas enters the pump inlet, is pumped by the rotating rotors down the screw threads, and leaves the pump via a pump outlet. Heat transferred to the rotors during rotation causes the rotors to expand, bringing the bands 18 into contact with the surface of the stator 20 of the pump. As the bands are formed from material that is harder than the stator 20 of the pump, the bands 18 abrade grooves 22 in the stator 20, as shown in Figure 4. Once the abrasion has been completed (when the pump has reached a steady operating state) the pump is roughed out to remove any abraded material. The pump is then ready for use.

As well as preventing pump seizure, the abrasion of these grooves results in a close fitting seal between the rotor and stator during use, reducing pump power consumption. Consequently, wider initial tolerance between the rotors and the stator can be specified, reducing manufacturing costs.

In summary, a screw-shaped rotor for a vacuum pump comprises an array of mutually spaced ridges or bands of abrasive material mounted along and about the thread of the rotor. During rotor rotation, any contact between the rotor and the stator causes the bands to abrade a series of grooves in the pump stator, both preventing pump seizure and resulting in a close fitting seal between the rotor and stator.

Claims

1. A screw-shaped rotor for a vacuum pump, the rotor comprising an array of mutually spaced bands of abrasive material located on the thread of the rotor, the bands extending about and generally radially outwards from the thread of the rotor.

2. A rotor according to Claim 1 , wherein the bands extend about mutually spaced planes substantially orthogonal to the longitudinal axis of the rotor.

3. A rotor according to Claim 1 or Claim 2, wherein each band comprises a ridge of abrasive material.

4. A rotor according to any preceding claim, wherein the bands have a width in the longitudinal direction of the rotor in the range from 0.5 to 3mm.

5. A rotor according to any preceding claim, wherein the bands project from the thread of the rotor a distance in the range from 0.01 to 0.15mm.

6. A rotor according to any preceding claim, wherein the bands are spaced apart along the thread in the range from one sixth to one half of the pitch of the thread.

7. A rotor according to any preceding claim, wherein the bands are formed from ceramic material.

8. A rotor according to Claim 7, wherein the bands are formed from one of aluminium oxide or chromium oxide.

9. A vacuum pump comprising at least one rotor according to any preceding claim.

10. A vacuum pump according to Claim 9, wherein the rotor is rotatable relative to a stator, whereby, in use, as the rotor is rotated, any contact between the rotor and the stator can cause a plurality of grooves to be abraded in the stator.

11. A vacuum pump comprising a stator, at least one screw shaped rotor rotatable relative to the stator, the rotor comprising an array of mutually spaced projections formed from material harder than the stator and mounted along and about the thread of the rotor and extending generally radially outwards therefrom whereby, as the rotor is rotated in use, any contact between the rotor and the stator causes a plurality of grooves to be abraded in the stator.

12. A method of manufacturing a screw-shaped rotor for a vacuum pump, the method comprising forming from abrasive material an array of mutually spaced bands on the thread of the rotor, the bands extending about and generally radially outwards from the thread of the rotor.