GB2440947A

GB2440947A - A stator blade made of at least two stacked sheets

Info

Publication number: GB2440947A
Application number: GB0616205A
Authority: GB
Inventors: Michael Simmonds
Original assignee: BOC Group Ltd; Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2006-08-16
Filing date: 2006-08-16
Publication date: 2008-02-20
Also published as: GB0616205D0

Abstract

A stator for a turbomolecular pump is made up of at least two curved sections 10, 12 comprising blades 14, 16, one curved section 10 being positioned on top of the other curved section 12 such that blades 14 of one section are alternately arranged with the blades 16 of the other section 12. The blades are equidistantly spaced and extend in the same direction, the blades of one section 12 being longer than the blades of the other section 10 so that when superimposed the blade tips are substantially co-planar. Each curved section comprises an inner and outer rim (see figs. 1 and 2) between which the stator blades are press machined. The blades of the lower section ideally go through apertures 18 in the upper section. The sections may be semi-circular, made from stainless steel and/or aluminium sheets, and bolted or spot welded together. Also disclosed is a method of assembling said stator.

Description

STATOR BLADE UNIT FOR A TURBOMOLECULAR PUMP

The present invention relates to a stator blade unit for a turbomolecular pump, a turbomolecular pump including such a stator blade unit, and to a method of assembling a stator blade unit for a turbomolecular pump.

A turbomolecular pump generally comprises a rotor having a plurality of axially spaced, annular arrays of inclined rotor blades. The blades are regularly spaced within each array, and extend radially outwards from a central shaft. A stator of the pump surrounds the rotor, and comprises annular arrays of inclined stator blades which alternate in an axial direction with the arrays of rotor blades. Each adjacent pair of arrays of rotor and stator blades forms a stage of the turbomolecular pump. As the rotor rotates, the rotor blades impact incoming gas molecules and transfer the mechanical * **.

energy of the blades into gas molecule momentum, that is directed from the.:* pump inlet through the stages towards the pump outlet. S.

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It is common for the rotor of a turbomolecular pump to be assembled as a single piece, with the blades integral with the shaft. In this case, during pump assembly the arrays of stator blades are progressively assembled between the arrays of rotor blades. In one known assembly technique, each array of stator blades is divided into two semi-annular units each comprising a semi-annular section in which the blades are supported radially by inner and outer nm portions. Starting towards the base of the rotor, the two sections which provide the first array of stator blades are radially inserted between the same two arrays of rotor blades so that the two sections form a continuous annular stator blade array. An annular spacer is then placed on the outer rim portion of the assembled stator blade array to axially separate that array from the next stator blade array to be assembled and prevent clashing between the 3D rotor and stator blades. The two sections for forming the next stator blade array to be assembled are then inserted between the arrays of rotor blades so that one side of the outer rim portions of these sections rest upon the spacer.

Another annular spacer is then placed on the other side of the outer rim portion of that assembled stator blade array. This process continues until all of the stator blade arrays have been assembled to form a stator stack surrounding the rotor. A casing is then assembled about the stator stack, the stator stack being radially centred by the inner wall of the casing.

The compression ratio of the pump is dependent, inter alia, upon the number of arrays of rotor and stator blades, the number of blades within each array, the angle of inclination of the blades, and the rotational speed of the shaft. In order to enhance the inlet capacity of the turbomolecular pump, the sizes of the blades of the inlet stage of the pump, that is, the stage closest to the pump inlet, are generally relatively large, with the sizes of the blades of the stages gradually decreasing from the pump inlet towards the pump outlet. In other words, the axial lengths of the arrays of rotor and stator blades gradually * decrease from the pump inlet towards the pump outlet.

Towards the pump outlet, where the axial lengths of the blades are relatively small, the semi-annular sections of the stator stack are generally formed from thin pieces of stainless steel or aluminium sheet material. The portions for the stator blades are defined by etching the sheet material, and the blades are folded from the sheet material to a predetermined inclination by press machining. Formation of the stator blade sections in this manner means that, within a single semi-annular section, no two blades may axially overlap.

Whilst this is not an issue when the required axial blade length is relatively small, towards the inlet of the pump, where the required axial blade length is relatively large, an alternative technique needs to be employed to manufacture the semi-annular blade sections so that adjacent blades may overlap. This can enable the number of stator blades within an array to be maintained throughout the stages of the pump.

One technique that is commonly used to manufacture the stator sections for at least the inlet stage of the pump is milling, in which the inclined stator blades and rim portions of the stator section are machined from a single piece of material. In comparison to press machining techniques, the milling process is relatively expensive; a milled stator section typically costs at least ten times as much as a machine pressed stator section.

It is an aim of at least the preferred embodiment of the present invention to provide a relatively cheap method of assembling a stator blade unit having axially overlapping stator blades.

In a first aspect, the present invention provides a method of assembling a stator blade unit for a turbomolecular pump, the method comprising the steps of press machining stator blades from at least two, but preferably two, curved sections of sheet material, and bringing the curved sections together, one over the other, to form a stator blade unit comprising a circumferential array of * **.

equidistant, substantially co-planar stator blades, in which the blades of one.*:** curved section are circumferentially alternately arranged with the blades of the other curved section. * *. * * . *

By interleaving the stator blades of the two curved sections in this way, a stator blade unit may be formed with the same number and size of stator blades as a stator blade unit manufactured using a milling technique. Whilst the number of sections forming a stator blade unit is effectively at least doubled, the formation of the stator blade unit from sections formed using a press machining technique can enable the cost of the stator blade unit to be at least five times cheaper than one manufactured using a milling technique.

The stator blades of one curved section are preferably longer than the stator blades of the other curved section so that in the circumferential array the tips of the stator blades are substantially co-planar. Each curved section preferably comprises an inner rim and an outer rim between which the stator blades are press machined, the rims of one curved section contacting the rims of the other curved section when the curved sections are brought together.

Consequently, the difference in length between the blades of one curved section and the blades of the other curved section is, in the direction of thickness of the curved sections, preferably substantially equal to the thickness of the rims of the other curved section.

To facilitate the regular spacing of the stator blades in the array, at least one of the rims of each curved section preferably comprises a reference point1 with the reference points being located in different respective locations relative to the stator blades on the curved sections. Consequently, when these reference points are aligned as the curved sections are brought together, the stator blades in the array become regularly spaced. Such a reference point may comprise a mark, ridge, indent or other visible or tactile feature located on the curved sections, and may conveniently comprise an aperture, with the apertures being concentrically aligned as the curved sections are brought together to regularly space the stator blades in the array.

For example, each aperture (or other feature) may be located on the outer rim of the curved section, with the aperture of one curved section being angularly offset relative to the aperture of the other curved section by an angular spacing substantially equal to desired angular pitch of the stator blades in a circumferential array. A bolt, such as a pin, rivet or other connector, may be inserted through the apertures to join the curved sections together.

Alternatively, the curved sections may be spot welded together.

The curved sections are preferably semi-annular.

In a second aspect, the present invention provides a stator blade unit for a turbomolecular pump, the stator blade unit comprising a circumferential array of equidistant, substantially co-planar stator blades pressed machined from at least two curved sections of sheet material which are positioned one over the other so that stator blades of one curved section are circumferentially alternately arranged with stator blades of the other curved section.

Each curved section preferably comprises an inner rim and an outer rim between which the stator blades are press machined, the rims of one curved section contacting the rims of the other curved section. The stator blades of one curved section are preferably longer than the stator blades of the other curved section so that in the circumferential array the tips of the stator blades are substantially co-planar.

The present invention also provides a stator stack comprising at least one stator blade unit as aforementioned, and a turbomolecular pump comprising such as stator blade unit.

Features described above in relation to method aspects of the invention are equally applicable to apparatus aspects, and vice versa. S * S...

Preferred features of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

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Figure 1 illustrates a plan view of one curved section of a stator blade unit, Figure 1(a) illustrating the curved section before press machining and Figure 1(b) illustrating the curved section following press machining; :E;:; Figure 2 illustrates a plan view of the other curved section of a stator blade unit, Figure 1(a) illustrating the curved section before press machining and Figure 1(b) illustrating the curved section following press machining; Figure 3 illustrates a perspective view of the press machined curved section of Figure 1(b); Figure 4 illustrates a cross-sectional view along line A-A of Figure 3 of part of the curved section of Figure 3, and Figure 5 illustrates a similar view as Figure 1 with the two curved sections of the stator blade unit brought together.

A stator blade unit is formed from at least two curved sections of sheet material. In this example, the curved sections are semi-annular, so that two stator blade units are required to form an annular array of stator blades. The sheet material is preferably formed from one of stainless steel and aluminium.

With reference to Figures 1(a) and 2(a), the two curved sections 10, 12 are press machined to form stator blades. The two curved sections 10, 12 are of substantially the same size and width. Portions are cut from the first curved section 10, for example using an etching technique, to define a series of regularly spaced stator blades 14. Portions are similarly cut from the second curved section 12 to define a series of regularly spaced stator blades 16. As discussed in more detail below, the stator blades 16 of the second curved section 12 are slightly longer than the stator blades 14 of the first curved section 10, whilst the angular pitch of the stator blades 16 is substantially the same as that of the stator blades 14. ::::.

The portions for the stator blades 14, 16 are folded using a press machining technique to have a predetermined and regular inclination, as illustrated in Figures 1(b) and 2(b). The press-machining of the first curved section 10 creates apertures 18 between the stator blades 14, with the stator blades 14 being radially supported between an inner rim 22 and an outer rim 24 of the first curved section 10. Figure 3 illustrates a perspective view of part of the first curved section 10, and Figure 4 illustrates a cross-sectional view along part of line A-A of Figure 3, both of which illustrate the inclination of the stator blades 14 of the first curved section 10. The press-machining of the second curved section 12 creates apertures 20 between the stator blades 16, with the stator blades 16 being radially supported between an inner rim 26 and an outer rim 28 of the second curved section 12.

The first and second curved sections 10, 12 are then brought together by inserting the stator blades 16 of the second curved section 12 through the apertures 18 of the first curved section 10 until the inner and outer rims 22, 24 of the first curved section 10 overlay the inner and outer rims 26, 28 respectively of the second curved section 12. The curved sections 10, 12 are then angularly aligned so that an array of equidistant, substantially co-planar stator bladesl4, 16 is formed, in which the stator blades 14 of the first curved section 10 are circumferentially alternately arranged with the stator blades 16 of the second curved section 12, as illustrated in Figure 5.

To facilitate the angular alignment of the curved sections 10, 12, an aperture 30, 32 may be formed in each curved section 10, 12, for example during the press machining of the stator blades 14, 16. As illustrated in Figures 1 and 2, each aperture 30, 32 may be located on the outer rim 24, 28 of its curved s.., section 10, 12. Alternatively, each aperture may be located on the inner rim of its curved section. The aperture 30 of the first curved section 10 is angularly offset relative to the aperture 32 of the second curved section 12 by an angular spacing substantially equal to desired angular pitch of the stator:.:.

blades 14, 16 in the circumferential array. Consequently, when the apertures 30, 32 are brought into alignment, the stator blades 14, 16 have a regular pitch in the circumferential array. As indicated in Figure 5, the curved " sections 10, 12 may then be joined together by a bolt 40 passing through the aligned apertures 30, 32. Alternatively, the curved sections 10, 12 may be spot welded together. Other co-operating features such as ribs or other raised features may be formed on one or both of the rims of each curved section 10, 12 to facilitate the radial alignment of the curved sections 10, 12.

With reference also to Figure 5, in the circumferential array the tips of the stator blades 14, 16 are preferably co-planar. In view of this, the stator blades 16 of the second curved section 12 are preferably longer than the stator blades 14 of the first curved section 10 so that the axial distance h between the tip of a stator blade 14, 16 and the upper surface of the rims 22, 24 of the first curved section 10 is constant about the array, To achieve this, the difference in length between the stator blades 14, 16 is substantially equal to (t.sin6), where t is the thickness of the first curved section 10 and 0 is the angle of inclination of the stator blades 14, 16. * * * S.. * . . * *

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Claims

CLAIMS

1. A method of assembling a stator blade unit for a turbomolecular pump, the method comprising the steps of press machining stator blades from at least two curved sections of sheet material, and bringing the curved sections together, one over the other, to form a stator blade unit comprising a circumferential array of equidistant, substantially co-planar stator blades, in which the blades of one curved section are circumferentially alternately arranged with the blades of the other curved section, wherein each curved section comprises an inner rim and an outer rim between which the stator blades are press machined, the rims of one curved section contacting the rims of the other curved section when the curved sections are brought together, the stator blades of one curved section being longer than the stator blades of the *a..

other curved section so that when the curved sections are brought together the tips of the stator blades are substantially co-planar.

2. A method according to Claim 1, wherein at least one of the rims of each curved section comprises a reference point, the reference points being aligned as the curved sections are brought together to regularly space the stator blades in the array.

3. A method according to Claim 2, wherein each reference point comprises an aperture, the apertures being concentrically aligned as the curved sections are brought together.

4. A method according to Claim 3, wherein a bolt is inserted through the apertures to join the curved sections together.

5. A method according to Claim 1 or Claim 2, wherein the curved sections are spot welded together.

6. A method according to any preceding claim, wherein the curved sections are substantially semi-annular.

7. A method according to any preceding claim, wherein the sheet material is formed from one of stainless steel and aluminium.

8. A stator blade unit for a turbomolecular pump, the stator blade unit comprising a circumferential array of equidistant, substantially co-planar stator blades pressed machined from at least two curved sections of sheet material which are positioned one over the other so that stator blades of one curved section are circumferentially alternately arranged with stator blades of the other curved section, wherein each curved section comprises an inner rim and an outer rim between which the stator blades are press machined, the rims of one curved section S...

contacting the rims of the other curved section, the stator blades of one curved section being longer than the stator blades of the other curved section so that in the circumferential array the tips of the stator blades are substantially co-planar. * S**

9. A stator blade unit according to Claim 8, wherein at least one of the S...

rims of each curved section comprises a reference point, the reference * points being aligned within the stator blade unit.

10. A stator blade unit according to Claim 9, wherein each reference point comprises an aperture, the apertures being concentrically aligned within the stator blade unit.

11. A stator blade unit according to Claim I 0, wherein the curved sections are connected together by a bolt passing through the apertures.

12. A stator blade unit according to Claim 8 or Claim 9, wherein the curved sections are spot welded together. -Il-

13. A stator blade unit according to any of Claims 8 to 12, wherein the curved sections are substantially semi-annular.

14. A stator blade unit according to any of Claims 8 to 13, wherein the sheet material is formed from one of stainless steel and aluminium.

15. A stator stack comprising at least one stator blade unit according to any of Claims 8 to 14.

16. A turbomolecular pump comprising at least one stator blade unit according to any of Claims 8 to 14. *s..

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