WO2005001294A1

WO2005001294A1 - Rotary impeller for a turbomolecular pump

Info

Publication number: WO2005001294A1
Application number: PCT/GB2004/002495
Authority: WO
Inventors: Nigel Paul Schofield; Emmanuel Uzoma Okoroafor; Michael Chung Kau Liu; Graeme Huntley
Original assignee: The Boc Group Plc
Priority date: 2003-06-25
Filing date: 2004-06-15
Publication date: 2005-01-06
Also published as: EP1636496A1; GB0314692D0; TW200506221A; JP2007516372A

Abstract

A rotary impeller for a turbomolecular pump comprises a hub and a plurality of blades attached to the hub. The blades are formed from strips of fibre or particle reinforced polymeric material.

Description

ROTARY IMPELLER FOR A TURBOMOLECULAR PUMP

This invention relates to a rotary impeller for a turbomolecular pump, such pumps being typically used to create vacuum environments for semiconductor processing.

A standard turbomolecular pump includes a rotary impeller comprising arrays of (normally) angled blades arranged for rotation at high speed, for example up to sixty thousand revolutions per minute, between alternately arranged arrays of stationary blades of a stator. Generally the blades of the stator are inclined in the opposite direction to those of the rotor.

In turbomolecular pumps, gas is typically received from a high vacuum chamber, compressed and delivered to a backing pump, for example a multi-stage rotary pump. The backing pump is required as turbomolecular pumps normally operates with an exhaust pressure up to about 10^"1 mbar and so the use of the backing pump can provide backing pressures in this region and deliver pumped gas to the atmosphere.

The pumping capacity of a turbomolecular pump is heavily dependent on the absolute speed that the blades are able to travel. This speed is limited by allowable stresses within the material of the impeller blades. Conventionally, the hub and blades of a rotary impeller of a turbomolecular pump are machined from a solid piece of lightweight metal, for example aluminium or one of its alloys.

The present invention proposes alternative materials and methods of manufacture for rotary impellers for turbomolecular pumps.

The present invention provides a rotary impeller for a turbomolecular pump, the impeller comprising a hub, a plurality of blades formed from a reinforced polymeric material, and means for attaching the blades to the hub. A number of advantages can be obtained from the use of reinforced polymeric materials in place of the metal materials conventionally used for the impeller blades. A first advantage is due to the lighter weight of the blades and superior strength to weight ratios in comparison to blades formed from metal. In view of this, a higher rotation speed can be achieved, and this can result in improved pump performance. A second advantage is that the impeller has a reduced rotational inertia in comparison to an impeller having metal blades. This can improve safety within the environment surrounding the pump during use, as in the event of an impeller seizure less energy is imparted to the pump stator.

In the preferred embodiments, the polymeric material is reinforced by one of fibres and particles. Advantageously, these fibres or particles are preferably directionally aligned to provide greatest strength in the region and/or direction of the highest load acting on the blade during use of the impeller. For example, a substantial proportion, for example 75 to 85%, of the fibres or particles are preferably aligned in a direction extending from the root to the tip of the blade.

The polymeric material may be reinforced by carbon fibres, fibres or particles formed from glass or a ceramic material, or synthetic fibres, for example para- aramid, or Kevlar™, fibres. The polymeric material may be a matrix or epoxy, such as a polyetheretherketone.

The blades are preferably in the form of lengths or strips of reinforced polymeric material attached to the hub of the impeller, using, for example, pins. These strips may be attached to the hub using a number of different techniques.

In one technique, each strip is bent to provide a pair of impeller blades, with the bend in the strip being located around a retaining pin fastened to the hub. Optionally, the strip (prior to bending) may be shaped to have two substantially equally sized and parallel aligned end portions bridged by a central portion, the central portion being angled to the parallel line of the two end portions providing a dog-leg bend in the strip. In such an arrangement, the strip is bent along the central portion to provide the pair of impeller blades.

In alternative embodiments, each blade is formed from a respective strip, with the attachment means being located at or towards one end of the strip. Various means for attachment will no doubt occur to the skilled addressee. Some general examples will now be described.

In one example, a pin passes through a hole in one end of the strip. This end of the strip may be thickened or otherwise reinforced in the region surrounding the hole. For example, each blade may comprise a pair of strengthening plates sandwiching one end of the strip, with the pin passing through holes formed in these plates. These holes may be machined at an angle, for example at an acute angle to a normal to the surface of the blade, corresponding to the angle of tilt the blade is desired to have to the plane of rotation of the impeller. Thus, if a pin is inserted orthogonally to the plane of rotation of the hub, the blade is secured in a position at an angle to the plane.

It is also possible to locate multiple blades in the same blade-row with a single locking pin, which will result in a more compact rotor design. Alternatively, or additionally, the pin may attach a plurality of blades in different rows to the hub.

In another example, an end of each blade may be fashioned to have a dovetail or similar shape. The dovetail is desirably configured to fit snugly into a similarly shaped slot provided in the hub. The dovetail may be provided from built up layers of the blade material. Alternatively, a dovetail of different material may be bonded to the blade. Where the dovetail portion is provided from a separate material, the separate material may be bonded or fastened to the end of the blade.

In another alternative, generally flat blades may be bonded or otherwise fastened into pre-machined slots in the hub. Any of the blades previously described may further comprise a coating (for example a metal) selected to be non-reactive with chemicals passing through the pump in use.

For the purposes of exemplification, some embodiments of the invention will now be further described with reference to the Figures in which;

Figure 1 (a) illustrates a plan view of a first embodiment of a strip for forming blades for a rotary impeller for a turbomolecular pump, and Figure 1 (b) shows the strip of Figure 1 (a) bent to form the blades;

Figure 2 illustrates a plan view of a second embodiment of a strip for forming blades for a rotary impeller;

Figure 3 illustrates a technique for attaching the blades of Figure 1 (b) to the hub of the impeller;

Figure 4 illustrates a perspective view of a third embodiment of a blade for a rotary impeller;

Figure 5 illustrates a perspective view of a fourth embodiment of a blade for a rotary impeller;

Figure 6 illustrates a technique for securing a fifth blade embodiment to the hub of the impeller; and

Figure 7 illustrates an embodiment a wherein individual pins are each used to secure more than one blade to the hub of the impeller.

Figures 1 to 7 illustrate various techniques for providing an impeller having blades formed from fibre or particle reinforced polymeric material attached to a hub of the impeller. In the illustrated embodiments, each of the blades is formed from a strip of reinforced polymeric material attached to the hub.

In the strips, a significant proportion of the particles or continuous fibres of the blades are directionally aligned to follow a line from root to tip of each of the two blades. This gives rise to tensile loads in the direction of alignment of the fibres and fibres can safely carry these loads at the required operational speeds of a turbomolecular pump.

Figure 1 illustrates a strip 10 of fibre or particle reinforced polymeric material used for forming adjacent blades 12, 14 of a rotary impeller of a turbomolecular pump. In this embodiment, the rectangular strip 10 is folded about the dashed line 16 shown in Figure 1 (a) to provide a substantially U-shaped, blade pair arrangement 18 shown in Figure 1 (b).

Figure 2 shows a broadly similar embodiment to Figure 1. In this embodiment, the strip 20 is not rectangular, but includes a dog-leg bend 22 in its centre portion. Similar to the first embodiment, the strip 20 is folded about the dashed line 24 across the dog-leg bend 22 to provide the substantially U-shaped, blade pair arrangement. This arrangement is advantageous over that shown in Figure 1 (b) in that the blade pair arrangement formed from the strip 20 has a reduced height compared to the arrangement of Figure 1 (b) formed from the rectangular strip 10.

Figure 3 shows in section an example of how the previously discussed blade pair arrangements are attached to an impeller hub 26. The hub 26 is provided with a circumferential recess 28. Passing through the walls 30 bounding the recess 28 are holes or apertures 32 configured to receive a securing pin 34, about which the bend 36 in the blade pair arrangement 18 extends. The apertures 32 and pin 34 are so arranged in relation to the recess 28 and blade pair arrangement 18 that the blade pair arrangement 18 is securely pinned against an inner circumferential surface 38 of the hub 26. Figures 4 to 6 illustrate embodiments in which a blade 40, 50, 60 is formed from a single strip of material.

In the embodiment illustrated in Figure 4, a pair of reinforcing plates 42 are bonded or otherwise attached to one end of the blade 40. Two through holes 44 are drilled through the reinforced portion of the blade 40 at a slight angle to the plane of the widest surface of the blade 40. This angle corresponds to the desired angle of tilt of the blade 40 to the plane of rotation of the impeller. The through holes 44 are suitably shaped to receive locating pins 46 which, in use, would pass through apertures in the hub, similar to the pin 34 shown in Figure 3.

The plates 42 may be metallic, for example high grade aluminium or titanium, but other materials may also be employed for this purpose. The plate material is selected to withstand reaction forces of the locating pins 46 on the blade 40 and other tensile and compressive loads congregating in the reinforced portion of the blade 40 when the impeller is run at operational speeds required for a turbomolecular pump. As an alternative to the reinforcing plates, the blade 40 may simply be thickened in the region of the through holes 44.

Optionally a plurality of blade rows may be arranged such that a single locating pin 46 can be used to locate a plurality of parallel aligned blades in different rows.

In the embodiment illustrated in Figure 5, a pair of keying plates 48 are bonded or otherwise attached to one end of the blade 50. For example, during manufacture of the blade 50, the keying plates 48 may be located within a mould tool in which the blade 50 is cured.

The keying plates 48 serve a dual purpose of reinforcing the blade 50 at its root and providing a keying means for attaching the blade 50 into a hub. In the illustrated embodiment, the keying plates 48 are substantially dovetail-shaped, but these plates 48 may take any other shape that, desirably, includes a widening portion near the end of the blade. The material of the keying plates 48 and their dimensions are selected to withstand tensile and compressive loads congregating in the reinforced portion of the blade 50 when the impeller is run at operational speeds required for a turbomolecular pump. For example, the keying plates 48 may be formed from a metallic material, for example high grade aluminium or titanium, or from non-metallic material, for example, layers of the same material from which the blade 50 is formed. So that any centrifugal load may be spread evenly across the keying plates 48, these plates 48 are designed to be substantially mirror images of each other.

To attach the blades 50 to the hub, the hub is provided with machined key-holes each configured to receive the dovetailed end of a respective blade 50. The blades 50 may be friction fitted into these key-holes and/or bonded in position. Bonding may be through chemical means (for example, an adhesive) or physical means (for example, welding).

In the embodiment shown in Figure 6, the end of the rectangular blade 60 is simply fitted into a slot 62 formed in the hub 64. Adjacent the root of the blade 60 and to either side of the slot 62, the hub 64 is cut away, as indicated at 66, to provide for a reduction in dilation of the slot 62 should the hub 64 thermally expand during operation of the turbomolecular pump at normal operational speeds, and also to reduce the overall weight of the impeller.

Figure 7 shows a development of the embodiment of Figure 4. A plurality of blades 70 are arranged in parallel alignment about a hub 71 , and such that adjacent blades overlap in a vertical plane. A plurality of pins 72 extend vertically through holes 74 in the hub 71 , with each pin passing through two blades. Thus, each blade is secured at two ends and each pin secures two blades, thereby improving the security of the blades.

Claims

C LAI MS

1. A rotary impeller for a turbomolecular pump, the impeller comprising a hub, a plurality of blades formed from a reinforced polymeric material, and means for attaching the blades to the hub.

2. An impeller according to Claim 1 , wherein the polymeric material is reinforced by one of fibres and particles.

3. An impeller according to Claim 2, wherein the fibres or particles are directionally aligned to provide greatest strength in the region and/or direction of the highest load acting on the blade during use of the impeller.

4. An impeller according to Claim 3, wherein a substantial proportion of the fibres or particles are aligned in a direction extending from the root to the tip of the blade.

5. An impeller according to Claim 4, wherein the substantial proportion is within the range from 75% to 85% of the fibres or particles within the blade.

6. An impeller according to any of Claims 2 to 5, wherein the polymeric material is reinforced by carbon fibres.

7. An impeller according to any of Claims 2 to 5, wherein the polymeric material is reinforced by fibres or particles formed from glass or a ceramic material.

8. An impeller according to any of Claims 2 to 5, wherein the polymeric material is reinforced with synthetic fibres.

9. An impeller according to Claim 8, wherein the synthetic fibres comprise para-aramid fibres.

10. An impeller according to any preceding claim, wherein the polymeric material is an epoxy.

11. An impeller according to Claim 10, wherein the polymeric material is a polyetheretherketone.

12. An impeller according to any preceding claim, wherein the attachment means is located at or towards one end of each blade.

13. An impeller according to any preceding claim, wherein each blade comprises a strip of reinforced polymeric material.

14. An impeller according to Claim 13, wherein adjacent pairs of blades are formed from a single strip of reinforced polymeric material.

15. An impeller according to Claim 14, wherein the strip is bent to define the adjacent blades.

16. An impeller according to Claim 15, wherein the unbent strip is shaped to have two substantially equally sized and parallel aligned end portions bridged by a central portion, the central portion being angled to the two end portions to provide a dog-leg bend in the strip, the strip being bent along the central portion.

17. An impeller according to any of Claims 13 to 16, wherein the attachment means comprises a pin for attaching the strip to the hub.

18. An impeller according to Claim 17 when dependent upon Claim 15 or Claim 16, wherein the bend in the strip is located about the pin.

19. An impeller according to Claim 17, wherein the pin passes through a hole in one end of the strip.

20. An impeller according to Claim 19, wherein said one end of the strip is locally thickened or otherwise reinforced.

21. An impeller according to Claim 20, wherein each blade comprises a pair of strengthening plates sandwiching said one end of the strip, the pin passing through holes formed in the strengthening plates.

22. An impeller according to any of Claims 19 to 21 , wherein the longitudinal axis of the pin is at an acute angle to a normal to the surface of the blade.

23. An impeller according to any of Claims 17 to 22, comprising a plurality of rows of blades aligned in parallel, the pin attaching a plurality of blades in different rows to the hub.

24. An impeller according to Claim 12, wherein the attachment means comprises a plurality of slots formed in the hub, each for receiving a respective end of a blade.

25. An impeller according to Claim 24, wherein said end of each blade has a dovetail or similar shape, the slots being configured to received the shaped ends of the blades.

26. An impeller according to Claim 25, wherein the dovetail or similar shape is provided from members attached to said end of each blade.

27. An impeller according to Claim 26, wherein the dovetail or similar shape is provided by metallic plates attached to the blade.

28. An impeller according to Claim 27, wherein the plates are formed from aluminium or an alloy thereof.

29. An impeller according to any of Claims 24 to 28, wherein said ends of the blades are bonded or otherwise secured into the slots.

30. An impeller according to any of Claims 24 to 29, wherein the hub is cut away adjacent the slots.

31. An impeller according to any preceding claim, wherein the reinforced polymeric material is selected to have mechanical properties able to withstand stresses experienced by the blade at rotational tip speeds of the order of the speed of sound.

32. An impeller according to any preceding claim, wherein the blades are provided with a coating selected to be non-reactive with chemicals passing through the pump during use.

33. An impeller according to Claim 32, wherein the coating is metallic.

34. A rotary impeller for a turbomolecular pump, the impeller comprising a hub, and a plurality of blades formed from strips of reinforced polymeric material attached to the hub.