WO2019086852A1

WO2019086852A1 - Stator blade unit for a turbomolecular pump

Info

Publication number: WO2019086852A1
Application number: PCT/GB2018/053130
Authority: WO
Inventors: Stephen Dowdeswell
Original assignee: Edwards Limited
Priority date: 2017-11-02
Filing date: 2018-10-30
Publication date: 2019-05-09
Also published as: GB201718206D0; GB2568066A

Abstract

The invention provides an additive manufactured stator blade unit for a turbomolecular pump comprising a plurality of stator stages, the stator blade unit comprising a longitudinally extending outer wall with a substantially arcuate cross-section and two or more integrally formed stator blade arrays extending radially inwardly therefrom, each stator blade array corresponding to a separate stator stage of the turbomolecular pump.

Description

STATOR BLADE UNIT FOR A TURBOMOLECULAR PUMP

Field of the Invention

[001 ] The present invention relates to a stator blade unit for a turbomolecular pump, methods for manufacturing the same, a turbomolecular pump including such a stator blade unit, and methods of assembling a turbomolecular pump.

Background

[002] 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. [003] 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 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 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.

[004] 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. [005] 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. Likewise, the angle of the blades tends to decrease from the pump inlet towards the pump outlet.

[006] 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 cutting the sheet material, and the blades are folded from the sheet material to a predetermined inclination either by cutting and pressing in a single step, or cutting and generating the profile in series of steps by press machining. Whilst pressing stators in this manner requires significant investment in tooling to manufacture the piece parts are relatively low cost. However, the nature of the process makes the pressed part more flexible and can leave significant residual stresses in the pressed part. Consequently, internal pump clearances must accommodate this variation. Moreover, formation of the stator blade sections in this manner means that, within a single semi-annular section, no two blades may axially overlap.

[007] 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.

[008] 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 alloy. 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.

[009] There is therefore an ongoing need for improved stator arrays. In particular, there is a need for stator arrays that are more straightforward to manufacture; allow for the provision of more complex geometries; that are more reliable; can be produced to narrower tolerances; and at a reduced cost compared to pressing and/or machining alloy blades.

[0010] The present invention address these and other problems with known stator blade units.

Summary of the Invention

[001 1 ] Accordingly, in a first aspect, the invention provides a stator blade unit for a turbomolecular pump comprising a plurality of stator stages. The stator blade unit comprises a longitudinally extending outer wall with a substantially arcuate cross- section and two or more integrally formed stator blade arrays extending radially inwardly therefrom, wherein each stator blade array corresponds to a separate stator stage of the turbomolecular pump wherein the substantially arcuate cross- section has a central angle of about 120° or less, preferably about 90° or less. Typically, the stator blade unit is metallic and may be additive manufactured using an a powder bed fusion method, such as direct metal laser sintering or direct metal laser melting, preferably direct metal laser melting.

[0012] Advantageously, the present invention significantly reduces the number of components in a stator stack, easing manufacture. Moreover, because the stator arrays are integrally formed with the outer wall, the invention reduces the axial tolerance stack, compared to conventional stator stacks with spacing rings, providing an increased running clearance and ultimately a more robust pump.

[0013] Axially overlapped stator blades within an array may be desirable, such as in the compressive stages nearer the pump outlet. Stator arrays useful in the invention may therefore comprise overlapping stators and/or be axially opaque: i.e. no spaces are visible between the stator blades when an array is viewed directly from above or below. [0014] Preferably, the outer wall has a cross-section comprising an arc at the outer wall's inner surface with a central angle of about 90° or less. By having a central angle of about 90° or less the stator blade units with stator stages comprising overlapping stator blades can be additive manufactured without the need for support structures on the blades, further easing manufacture, improving the tolerance stack and reducing the potential for geometric errors.

[0015] Typically, each stator array further comprises an integrally formed inner rim. The inner rim may improve the stiffness of the stator array and, in use, the running clearance between the stator and an adjacent rotor: improving pump robustness. [0016] The stator unit may comprise three, four, five or more stator arrays. Preferably, the unit may comprise stator arrays for all the stator stages of the turbomolecular pump. [0017] Three, four, or more stator blade units may be mated to form a plurality of substantially annular stator stages. Preferably, each unit is substantially quarter- annular and/or substantially identical.

[0018] When part-annular stator arrays are arranged to form an annular stator stage, longitudinal "line of sight" joints may be located between the adjoining parts of the array. When a separate spacer is employed, the spacer may span the joint and prevent direct leakage therealong. However, when an integrally formed outer wall is employed an axial joint may be located along the full length of the outer wall, providing a potential leak path through the pump. Preferably, therefore the joint between adjacent outer walls is sealed to reduce leakage therealong. Typically, one or both of the adjacent outer walls will comprise tongue and groove features, or the like, to reduce leakage along the axial joint.

[0019] Typically, stator arrays vary in geometry along the length of the stack, with stator blades progressively reducing in size and angle from one array to the next from the inlet end towards the outlet end of the turbomolecular pump. Stator arrays nearer the inlet end of the stator unit may comprise larger, more space blades, whereas stator arrays nearer the outlet end may have smaller, over lapping blades. [0020] Typically, the stator blades of the invention have a thickness of less than about 1 mm, preferably from about 0.1 mm to about 0.75 mm.

[0021 ] Typically, the stator arrays are axially separated within the unit so as to provide, in use, a clearance of 2 mm or less between the stator blades of an array and the blades of an adjacent rotor, preferably the clearance is from about 0.5 mm to about 2 mm, a of about 1 mm is particularly preferred. [0022] Typically, the stator blade unit is manufactured using a powder bed fusion, such as selective laser melting. The stator blade unit will typically comprise a laser sintered material, preferably a laser sintered metal or alloy, preferably a laser sintered aluminium alloy. The skilled person will be able to select an appropriate additive manufacturing technique depending on the desired stator material and/or stator unit geometry and/or stator unit function. A preferred aluminium alloy is AL7075.

[0023] The skilled person may select a suitable material based upon the exact geometries and the prevailing conditions found within a specific turbomolecular pump. In particular, the material may be selected so that the stator blade may be used at an operating temperature of up to about 90°C, preferably from about 20°C to about 90°C.

[0024] Outgassing may be problematic and so is preferably kept to a level which does not deleteriously affect turbomolecular pump performance. Preferably the stator material has a total mass loss, collected volatile condensable materials, and water vapour release each of less than about 1 wt%, more preferably less than about 0.5 wt%, more preferably less than about 0.1 wt%.

[0025] Advantageously, it has been found that laser sintered aluminium stator units according to the invention have substantially the same outgassing properties as similar machined or pressed aluminium stator blade arrays.

[0026] In a second aspect, the invention provides a method of manufacturing a stator blade unit for a turbomolecular pump comprising the step of additive manufacturing a single unitary structure comprising an array of stator blades, wherein the array has a substantially arcuate cross-section with a central angle of about 120° or less, preferably less than about 90°. Typically, the stator blades extend radially inwardly from a longitudinally extending outer wall with a substantially arcuate cross-section. [0027] An integrally formed support structure may be used to support the outer wall during manufacture and then removed afterwards. Advantageously, no integrally formed support structure is required to support the stator blades during their additive manufacture.

[0028] The stator blade unit is typically metallic, preferably aluminium, and/or the additive manufacturing step comprises powder bed fusion, such as selective laser melting.

[0029] Typically, the unit comprises two or more longitudinally distinct stator blade arrays corresponding to separate stator stages of the turbomolecular pump. It will be appreciated that the arrays are spaced apart in order to accommodate a rotor blade array therebetween when in use. Preferably, the unitary structure comprises one stator array for every stator stage of the turbomolecular pump.

[0030] Typically, each stator array further comprises an integrally formed inner rim. Typically, the inner rim is partially annular and has substantially the same central angle as the remainder of the stator array. The inner rim may improve the stiffness of the stator array and, in use, the running clearance between the stator and an adjacent rotor: improving pump robustness.

[0031 ] Axially overlapped stator blades within an array may be desirable, such as in the compressive stages nearer the pump outlet. Stator arrays according to the invention may therefore comprise overlapping stators and/or be axially opaque: i.e. no spaces are visible between the stator blades when an array is viewed directly from above or below. A stator unit may comprise a stator array(s) with overlapping stator blades and/or a stator array(s) that are axially opaque stator and/or a stator array(s) with non-overlapping stator blades. The skilled person will be able to select a combination of stator blade arrays depending upon the intended use of the turbomolecular pump for which the stator blade unit is designed. [0032] Preferably, the outer wall has a cross-section comprising an arc at the outer wall's inner surface with a central angle of about 90° or less. By having a central angle of about 90° or less the stator blade units with stator stages comprising overlapping stator blades can be additive manufactured without the need for support structures on the blades, further easing manufacture, improving the tolerance stack-up and reducing the potential for geometric errors.

[0033] The invention further provides a stator blade unit manufactured according to or obtainable by the above described method, and a stator blade stack comprising at least one stator blade unit manufactured according to the method.

[0034] In a further aspect, the invention provides, substantially annular stator blade stack for a turbomolecular pump, the stator blade stack comprising an outer wall with a substantially annular cross-section and a plurality of longitudinally distinct stator blade arrays extending radially inwardly therefrom, wherein the stator blade stack comprises three or more, preferably four or more, longitudinally extending segments each of which comprises a portion of each of the stator blade arrays and is an additive manufactured unitary structure. Preferably, the longitudinally extending segments are substantially identical.

[0035] The invention further provides a turbomolecular pump comprising at least one stator blade unit or stack according to, or manufactured according to, other aspects of the invention. [0036] The invention further provides a method of assembling a turbomolecular pump, the pump comprising a stator stack comprising two or more annular stator arrays each comprising an array of machined or pressed metallic stator blades, the method comprising the steps of removing the two or more annular stator arrays from the stator stack and replacing the removed annular stator arrays with a stator stack according to the invention. [0037] For the avoidance of doubt, all aspects and embodiments described hereinbefore may be combined mutatis mutandis.

Brief Description of the Figures: [0038] Preferred features of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 shows a stator stack according to the invention. Fig. 2 shows a stator unit according to the invention.

Detailed Description of the Invention

[0039] The present invention provides an additive manufactured metallic stator blade unit (1 ) for use with a turbomolecular pump (not shown).

[0040] As illustrated in Fig. 1 , the stator unit (1 ) may comprise an outer wall (2) and two or more integrally formed stator blade arrays (3, 4, 5, 6, 7, 8) extending radially inwardly therefrom. Each array (3, 4, 5, 6, 7, 8) corresponds to a stator stage of the turbomolecular pump. In the illustrated example, the stator unit (1 ) comprises selective laser melted AL7075, available from LPW, and consists of a single piece of material. Advantageously, this may reduce the axial tolerance stack: increasing clearances between stators and adjacent rotors, and thereby improving the robustness of the turbomolecular pump. [0041 ] Additionally, the total number of parts in the stator stack of the turbomolecular pump may be reduced significantly. Known stator stacks, such as found on Edwards Vacuums™ nEXT80™ may include 17 or more individual components, including multiple semi-annular stator arrays, and spacers. In contrast, by employing a stator stack comprising stator units according to the invention, the number of components may be reduced to as few as three or four. This may provide a cheaper and more robust pump.

[0042] The illustrated cross-section of the outer wall (2) is substantially arcuate with a central angle Θ of about 90°. For the purpose of the invention, the central angle is the angle subtended by the arc of circumference of the inner (stator-side) surface of the outer wall when viewed in cross-section, the angle's vertex being at the centre of an imaginary circle including said arc of circumference. [0043] The illustrated example has a central angle Θ of about 90°; however, higher and lower central angles can be employed. Preferred central angles may be less than or equal to about 120°, less than or equal to about 90°, from about 30° to about 90°, from about 45° to about 90°, or from about 60° to about 90°. Typically, the central angle will be selected such that during additive manufacturing no support structure is required for the blades. A preferred central angle may thereby be selected for a preferred blade geometry.

[0044] Unless otherwise stated, for the purposes of the invention, axial and longitudinal refer to a direction substantially parallel to the rotational axis (A) of the rotor of the turbomolecular pump (not shown). In use, annular stator arrays will typically be substantially concentrically aligned with said axis. Radial and radially refer to directions substantially normal to said axis.

[0045] As illustrated in Fig. 2, four such stator blade units or segments (1 , 9, 10, 1 1 ) may be combined to provide the substantially annular stator stack (12) of a turbomolecular pump. Each stator stage (13, 14, 15, 16, 17, 18) in the stator stack (12) includes a substantially annular array (19) of stator blades (20), each with a substantially annular inner rim (21 ) integrally formed therewith. The outer wall (2, 22, 23, 24) acts as the stator spacer.

[0046] As better illustrated in Fig. 1 , the stator blade geometry may vary between stages, with larger blades (20) and a more open configuration towards the upper, inlet end of the unit, and smaller and more overlapped blades (25) towards the lower, outlet end of the unit. Additive manufacturing allows the formation of geometries not achievable with pressing or machining. For instance, overlapping stator blades and/or optically opaque stator arrays may be included in the multi- staged additive manufactured stator blade units (1 ).

[0047] The outer wall (2, 22, 23, 24) typically has a radial thickness of less than about 2 mm, preferably from about 1 mm to about 1 .5 mm. 0.75 mm is an example. Advantageously, the outer wall may include a formation, such as a groove (26), for receiving an elastomeric O-ring (not shown). The O-ring, when employed, may assist in sealing the stator stack (12) within the pump envelope and thereby reduce leakage form high pressure to low pressure regions of the turbomolecular pump.

[0048] A seal (not shown) may also be provided along the axial joint (27, 28) between two adjacent stator blade units (9, 10, 1 1 ) to reduce leakage along the joint.

[0049] In the illustrated example, the unit (1 , 9, 10, 1 1 ) is additive manufactured as a single piece of material. Typically, the outer wall (2, 22, 23, 24) is laid down on an integrally formed support structure (not shown) made from the same material, with the stator blades, and inner rim, laid down as the unit is built up. Advantageously, no additional integrally formed support structure is needed for the stator blades themselves. It has been found that if the central angle is 90° or less then overlapping stator blades can support themselves even at the extremities of the outer wall's arc. Once the unit has been built up, the support structure integrally formed with the outer wall may be machined off prior to use.

[0050] The illustrated stator unit may be manufactured using powder bed fusion, which includes electron beam melting, selective heat sintering, selective laser melting and selective laser sintering, including direct metal laser sintering. Selective laser melting is particularly preferred. [0051 ] Powder bed fusion methods use either a laser or electron beam to melt and fuse material powder together. Powder bed fusion processes typically involve the spreading of the powder material over previous layers using a roller or a blade. The component is built up layer by layer by coating, melting/sintering, and recoating.

[0052] Selective laser melting may be used with a variety of alloys. Since the components are built layer by layer, it is possible to design geometries, internal features and passages that cannot be cast or otherwise machined, including multistage stator units of the invention with overlapping stators and/or an integrally formed inner rim.

[0053] Typically, the selective laser melting process begins with a 3D CAD model being created. A technician works with this 3D model to properly orient the geometry and add support structures as appropriate.

[0054] Once the 3D model is complete, it is digitally sliced into the layers the machine will build and downloaded to the selective laser melting machine (e.g. SLM250HL). [0055] Typically, the selective laser melting machine uses a high-powered laser, for instance a 200W Yb fibreoptic laser. Inside the build chamber area, there is a material dispensing platform and a build platform along with a blade used to move new powder over the build platform. The technology melts metal powder to form a solid part using the high-powered laser beam. Parts are built up layer by layer, typically using layers of about 20 pm in thickness.

[0056] As discussed previously, support structures may be removed using post- build machining. The stators of the invention may require a support structure for the outer wall of the stator unit; however, advantageously, no support structure may be required for the stator blades during manufacture. This reduces the number of manufacturing steps and may also allow the manufacture of more complex blade geometries, including axially opaque stator arrays and/or stator array with integrally - in formed inner rims. Typically, no heat treatment of the stator blade unit is required before use.

[0057] It will be appreciated that various modifications may be made to the embodiments shown without departing from the spirit and scope of the invention as defined by the accompanying claims as interpreted under patent law.

Reference Numeral Key:

1 Stator Blade Unit

2 Outer Wall

3-8 Stator Blade Arrays

9-1 1 Stator Blade Units

12 Annular Stator Stack

13- 18 Annular Stator Stages

19 Annular Array Of Stator Blades

20 Stator Blade

21 Inner Rim

22-24 Outer Walls

25 Overlapped Blades

26 Groove for Receiving O-ring

27-28 Axial Joint Between Adjacent Stator Blade Units

Claims

1 . A stator blade unit for a turbomolecular pump comprising a plurality of stator stages, the stator blade unit comprising a longitudinally extending outer wall with a substantially arcuate cross-section and two or more integrally formed stator blade arrays extending radially inwardly therefrom, each stator blade array corresponding to a separate stator stage of the turbomolecular pump, wherein the substantially arcuate cross-section has a central angle of about 120° or less, preferably about 90° or less.

2. The stator blade unit according to claim 1 wherein the stator blade unit is metallic.

3. The stator blade unit according to claim 2 wherein the stator blade unit comprises aluminium.

4. The stator blade unit according to any preceding claim wherein the stator blade unit is additive manufactured using selective laser melting.

5. The stator blade unit according to any preceding claim wherein the stator blade unit is in the form of a single unitary structure.

6. The stator blade unit according to any preceding claim wherein the stator blade unit comprises an array for every stator stage of the turbomolecular pump.

7. The stator blade unit according to any preceding claim wherein the array of blades does not require a support structure during its manufacture.

8. The stator blade unit according to any preceding claim wherein three or more stator blade units may be mated to form two or more substantially annular stator stages.

9. The stator blade unit according to any preceding claim wherein the substantially arcuate cross-section has a central angle of 90° or less, and where one or more of the stator arrays comprises stator blades that axially overlap.

10. A method of manufacturing a stator blade unit for a turbomolecular pump comprising the step of additive manufacturing a single unitary structure comprising an array of stator blades, wherein the stator blade unit has a substantially arcuate cross-section with a central angle of about 120° or less, preferably 90° or less.

1 1 . The method according to claim 10 wherein the array comprises an integrally formed outer wall and/or inner rim.

12. The method according to any one of claims 10 or 1 1 wherein the unit comprises two or more longitudinally spaced stator blade arrays corresponding to separate stator stages of the turbomolecular pump.

13. The method according to any one of claims 10 or 1 1 or 12 wherein the stator blade unit is metallic, preferably wherein the stator unit comprises aluminium.

14. The method according to any one claims 10 to 13 wherein the stator unit is additive manufactured using selective laser melting.

15. A stator blade unit manufactured according to or obtainable by the method of any one of claims 10 to 14.

16. A stator blade stack comprising two or more stator blade units according to any one of claims 1 to 8 or 15.

17. A substantially annular stator blade stack comprising a longitudinally extending outer wall with a substantially annular cross-section and a plurality of longitudinally distinct stator blade arrays extending radially inwardly therefrom, wherein the stack comprises three or more, preferably substantially identical, longitudinally extending segments, each of which comprises a portion of each of the stator blade arrays and is an additive manufactured unitary structure.

18. A turbomolecular pump comprising at least one stator blade stack according to any one of claims 16 or 17.

19. A method of assembling a turbomolecular pump, the pump comprising a stator stack comprising at least one annular stator array comprising an array of machined or pressed metallic stator blades, the method comprising the steps of removing the one or more annular stator arrays from the stator stack and replacing the one or more removed annular stator arrays with one or more annular stator arrays comprising a stator unit according to any one of claims 1 to 8 or 15.