GB2592618A

GB2592618A - Turbine blades and methods of manufacture of turbine blades

Info

Publication number: GB2592618A
Application number: GB2003073.0A
Authority: GB
Inventors: David Stones Ian
Original assignee: Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2020-03-03
Filing date: 2020-03-03
Publication date: 2021-09-08
Also published as: GB202003073D0

Abstract

The angle and/or thickness of a turbine blade 32, or stator blade, for the rotor 30 of a turbomolecular pump varies step‐wise from root 48 to tip 50, eg each blade is multi-faceted having five integrally formed portions 34, 36, 38, 40 and 42 having flat leading and trailing surfaces with radially inner portions being thicker and more steeply angled relative to the plane of the disc than radially outer portions. The blades may be machined from a cylindrical metal blank using a slit cutter having a pair of adjustably spaced cutting discs 58, 60 by incrementally adjusting the spacing 62 and angle of inclination 66 of the cutting discs 58, 60 in successive, partial cutting operations in which the discs 58, 60 are advanced into and retracted from the blank.

Description

TURBINE BLADES AND METHODS OF MANUFACTURE OF TURBINE BLADES

Description:

This invention relates to turbine blades for turbomolecular pumps and methods of manufacturing turbine blades for turbomolecular pumps. It is understood that a turbomolecular pump is a type of vacuum pump that can be used to form and/or sustain a high vacuum environment (that is) typically an environment at 10-6 mbar or below).

A turbomolecular pump generally comprises a rotor and a stator, the rotor being mounted to rotate at a high velocity within the stator. The general construction and design considerations of turbomolecular pumps are well known and do not require detailed explanation here. It should be noted, however, that a turbomolecular pump differs from a conventional gas turbine because it operates at a molecular level, rather than in a viscous fluid (macroscopic) pressure regime -a turbomolecular pump operates typically at gas pressures of 10-6 mbar and below. In essence, the blades of the rotor and stator of a turbomolecular pump are designed to collide with gas molecules to move them in a preferred direction, usually substantially along the pump axis in a desired pumping direction. As such, the "aerodynamic" considerations that apply to the design of macroscopic turbine blades do not apply at the molecular level.

One area where the design considerations of turbomolecular pumps and conventional turbine designs differ, is in the angle of attack of the rotor blades. In a conventional turbine, the angle of attack decreases from root to tip so that the theoretical axial displacement of gas is substantially the same at all points along the blade given that the linear tangential velocity of the blade increases from root to tip. At the molecular level, the relationship between linear tangential velocity and the "angle of attack" on the theoretical axial displacement of gas molecules is less direct because it is dominated by elastic collision events between the blade surfaces and the pumped gas molecules, rather than by aerodynamic effects. As such, the blade angle of a turbomolecular pump can be constant from root to tip with less detriment to the pumping efficiency of the rotor. The blade velocity, angle, length and spacing do contribute significantly to the performance of the pump.

Another area where turbomolecular pumps differ from conventional gas turbines (atmospheric turbines) is that their rotors are often driven to rotate at very high speeds, often in excess of 60,000 revolutions per minute. This makes structural, balancing and resonance issues more dominant factors in the design of turbine blades than "aerodynamic" factors, which are usually more dominant in conventional turbine blade design.

When designing turbomolecular pump blades, therefore, a designer needs to be more acutely aware of the radial stresses that are imposed on the blades, particularly towards the blades' roots, and the modulus of the blades, which is critical where the blade tips are designed to run at radial and axial spacings typically of less than a millimetre.

Turbomolecular pump rotors have, for a long time, been fabricated using different techniques including CNC milling machines, which can cut the blade and rotor profiles from a single disc or ingot of metal. Modern CNC milling machines can be programmed to cut a cylindrical ingot to the complicated shape of a unitary, single-or multi-stage rotor comprising a central axle and a number of axially spaced apart rotor discs. Given the flexibility of the CNC milling process, it is not uncommon for the rotor blades to be cut to have a three-dimensional profile, that is, a curved and variable cross-section from root to tip. In most cases, three-dimensional rotor blades have profiles in which the blades' roots are thicker and more steeply angled than at their tips, which optimises the mass distribution of the blades and can help to reduce root stresses enabling larger blade diameters and improved performance for a given rotational frequency and material stress limit. Unfortunately, however, the fabrication of three-dimensional blade profiles using CNC milling techniques can be time-consuming and expensive for certain turbomolecular optimised blade geometries. This is especially true for very low blade angles and tight blade pitches (their spacings in the circumferential direction) where CNC cutting tools reach their limits of length-to-diameter ratio resulting in undesirably slow feed rates and long machining times. To overcome this, it is also common to fabricate rotors and rotor blades using manufacturing techniques, such as a slit-cutting process. Slit-cutting is an alternative to CNC milling in the manufacture of turbine blades and is a process in which turbine blades are formed by cutting into the edge of a flat disc of metal (also known as a "blank"). The slitting machine comprises a pair of parallel and spaced-apart cutting discs, which can be advanced into the edge of the blank to form the individual blades: the blade thickness being determined by the spacing of the cutting discs; the blade angle being determined by the angle of the cutting discs relative to the axis of the blank; and the root diameter being determined by the depth of the cut into the edge of the blank and relates to the radial portion of the blade's root. The use of a slitting machine to form turbine blades from a disc-shaped blank is much quicker than an equivalent CNC milling process, although the blade profile is restricted to parallel-sided blades haying a constant thickness and angle. This method also enables much smaller blade angles and tighter circumferential blade spacing when compared to CNC milling techniques due to the increased rigidity of the cutting discs compared to a milling cutter.

Whilst a three-dimensional rotor or blade can be designed for optimum operational efficiency, and slit-cut blades are generally quicker and less expensive to manufacture, there exists a need for a rotor or blade providing increased freedom in design of the blade at low costs and fast manufacturing times. In addition, a need exists for a rotor or blade that has reduced material stress and higher performance. This invention aims to provide a solution to one or more of the problems highlighted above and to provide an improved alternative rotor blade and method of manufacturing the same.

A first aspect of the invention provides a method of manufacturing a turbine blade using a slit-cutting tool comprising the steps of: adjusting the spacing and inclination of the slit-cutting tool's cutting discs to a first spacing and a first inclination and advancing the cutting discs into the edge of a blade blank by a first distance, wherein the slit-cutting tool's cutting discs are adjusted to a first spacing and a first; retracting the cutting discs; and advancing the cutting discs into the edge of a blade blank by a second distance, wherein the slit-cutting tool's cutting discs are adjusted to a second spacing and a second inclination. Therein, first spacing and second spacing as well as first inclination and second inclination are different. Further, also first distance and second distance might be different. Therein, cutting the blade blank is performed using a single pair of cutting disks, wherein the inclination and spacing are adjusted accordingly. Alternatively, different pairs of cutting discs are used wherein each have a certain and preferably fixed inclination and spacing relating at least to the first and second spacing as well as the first and second inclination, respectively.

The method of manufacture advantageously enables a multi-faceted vacuum pump turbine blade to be formed using a slit-cutting technique. Therein, increased performance can be achieved and/or lower stress while reducing the costs of manufacturing.

The steps of retracting and adjusting the spacing and inclination of the cutting discs, followed by advancing the cutting discs into the edge of a blank can be repeated any number of times to form turbine blades having any desired number of integrally formed blade portions.

The first spacing is preferably greater than the second spacing. The first inclination is preferably steeper than the second inclination. The first distance is preferably greater than the second distance. Alternatively, second spacing is preferably greater than the first spacing. The second inclination is preferably steeper than the first inclination. The second distance is preferably greater than the first distance.

The spacings and inclinations of the cutting discs of the slitting tool in each cutting step may be selected to preserve a common leading, trailing or leading and trailing edge of the blade so formed.

The cutting discs may be advanced into the edge of the blank along a straight trajectory that intersects the longitudinal axis of the blank, to form radially extending blades. Alternatively, the cutting discs may be advanced into the edge of the blank along a straight trajectory that is offset, or at an angle, with respect to a radial line intersecting the longitudinal axis of the blank, to form swept turbine blades.

The process of cutting individual blades from the blank may be carried out simultaneously, that is to say, using more than one slit-cutting tool, or sequentially/consecutively. In the latter case, the blank will need to be indexed about its longitudinal axis at intervals to form the individual blades of the rotor disc.

According to a second aspect of the invention there is provided a turbomolecular pump rotor blade in which the angle or thickness. Alternatively, angle and thickness of the blade together varies step-wise from root to tip.

Advantageously, by varying the angle and/or thickness of the blade in a step-wise manner (as opposed to in a continuous or infinitesimal manner as in a three-dimensional blade), the blades can be formed using relatively simple manufacturing techniques, such as with a slitting machine, but still achieve improved profiles leading to higher performance compared to blades of constant thickness and angle.

The angle and/or thickness of the blade varies step-wise, but may vary incrementally from root to tip, that may be in equal steps or increments. Alternatively, steps or increments may vary between at least two steps. Thus, sufficient freedom of design is achieved.

Each blade thus is multi-faceted, that is to say, formed from a number of integrally formed blade portions, each blade portion having a different angle and/or thickness compared to its neighbours to achieve the step-wise or incremental variation in angle and/or thickness of the blade from root to tip.

Preferably, the angle of a portion of the blade towards the root of the blade is steeper than the angle of a portion of the blade towards the tip of the blade. Such an arrangement may lead to improved pumping efficiency, cancel out the variation in linear tangential velocity along the blade and reduce blade root stresses during use.

Additionally or alternatively, the thickness of a blade portion at, or towards, the root of the blade is greater than the thickness of a portion of the blade towards the tip of the blade. Such an arrangement may improve or optimise the material or mass distribution of the blade such that the portions thereof carrying relatively higher forces have larger cross-sections than those carrying relatively lower forces with the effect of reducing stress in the blade.

Each blade may comprise two or more blade portions, each blade portion having a different angle, thickness or angle and thickness to that of an adjacent blade portion. The blade or blades may be integrally formed with a hub that can be affixed to an axle of a rotor, or integrally formed with a rotor axle.

A third aspect of the invention provides a turbomolecular vacuum pump rotor comprising a plurality of turbine blades, each blade having an angle and/or thickness that varies step-wise (or incrementally) from root to tip.

The rotor comprises a plurality of blades, which may be arranged in one or more groups around a central axle or hub. Each group of turbine blades may be coplanar, to form an individual pumping stage, and a number of pumping stages may be provided at axially spaced-apart locations on a common axle or hub. The blades may be integrally formed with an axle, or the blades of each pumping stage may be integrally formed with a hub that can be affixed to an axle.

The blades of the rotor may extend radially outwardly from the axle, where provided, or may be arranged in a swept configuration.

A fourth aspect of the invention provides a turbomolecular vacuum pump comprising a rotor mounted for rotation relative to a stator, the rotor comprising a plurality of turbine blades having a angle, thickness or angle and thickness that varies step-wise (or incrementally) from root to tip.

Preferred embodiments of the invention shall now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a perspective view of a known turbine blade disc; Figure 2 is a perspective view of a turbine blade disc in accordance with the invention; Figure 3 is a perspective view of a turbine blade in accordance with the invention; Figure 4 is a schematic view of a slitting operation in accordance with the invention for forming a turbine blade, such as that shown in Figure 3; Figure 5 is a schematic end view of the turbine blade of Figure 3; and Figures 6 to 9 show a sequence of manufacturing steps for manufacturing a turbine blade as shown in Figure 3 using the apparatus and method of Figure 4.

In Figure 1, a known turbine blade disc 10 for a turbomolecular vacuum pump is formed from a circular disc of metal (a blank) 30 and has a central aperture 12 for receiving a spindle or axle (not shown) to which the blade disc 10 is assembled onto the axle (not shown) by a known process such as shrink-fitting by heat treatment process, and about which the blade disc 10 rotates, in use. It will be noted that surrounding the central aperture 12 there is provided an integrally formed circumferential rib 14, which serves to provide spacing between adjacent blade discs when multiple discs are stacked together. It may also provide reinforcement to the blade disc 10 and to increase the contact area of the interior surface 16 of the central aperture 12 with the axle (not shown). In certain circumstances, the central aperture 12 can be omitted, for example where the blade disc 10 is formed integrally with the axle (not shown).

In the illustrated example, the blade disc 10 has sixteen radially extending blades 18 each having a constant thickness 20 and angle 22, although any number of blades 18 may be provided.

In a turbine of, say, a turbomolecular pump there will generally be provided a number of spaced apart blade discs 10 mounted for rotation in unison on a common axle (not shown), each blade disc 10 forming a separate "pumping stage" of the turbomolecular pump. Axially interposed between each pumping stage, there will be a stator (not shown) comprising an annulus (not shown) having radially inwardly projecting blades (not shown) of substantially opposite angle to the blades 18 of the adjacent pumping stages.

It will be noted that each blade 18 of the known blade disc 10 comprises a leading surface 24 (the lower surface in Figure 1) and a trailing surface 26 (the upper surface in Figure 1), which are both monoplanar (flat) and inclined 22 with respect to a longitudinal axis of the axle (not shown). Blade discs 10 of this known type can be readily manufactured using a known slitting process in which a pair of spaced-apart cutting blades are advanced radially inwardly into the edge of a blank, the waste of the cutting process forming the gaps between adjacent blades, and the remaining material forming the blades 18 themselves.

In a known slit-cutting process, the spacing of the cutting blades determines the blades' thickness 20 and the angle of the cutting blades relative to the normal of the blade disc 10 determines the blades' angle 22. The process of "slit cutting" is relatively quick, inexpensive and reproducible, thereby enabling turbine blade discs 10 of the type shown in Figure 1 to be manufactured rapidly and at low cost.

In Figure 2, a blade disc 30 in accordance with the invention is shown, which is similar to the known blade disc 10 of Figure 1, except for the geometry of the blades 32 themselves. It will be noted that each blade 32 is made up of a number of (in this case, five) integrally formed blade portions 34, 36, 38, 40, 42, which each have parallel and monoplanar leading 44 and trailing 46 surfaces. The thickness of the radially inwardly located blade portions 34, 36 (that is to say, the blade portions nearer to the root 48 of each blade 32), is greater than the thickness of the radially outwardly located blade portions 40,42 (that is to say, the blade portions located at, or nearer to) the tip 50 of each blade 32. It will also be noted that the angle of the radially inwardly located blade portions 34, 36 relative to the plane of the disc is steeper (or that the angle 22 of the radially inwardly located blade portions 34,36 relative to the axis of the pump is shallower) than the angle of the radially outwardly located blade portions 40, 42. As such, each blade 32 is a multi-faceted. The main advantage of making the blade portions 34 towards the root 48 of each blade 32 thicker than those located at or towards the tip 50 of each blade 32 is that the radially inwardly located blade portions carry higher tensile loads, in use, and the provision of additional material towards the root 48 of each blade 32 enables the tensile stress to be reduced. Furthermore, by providing blade portions of reduced thickness at, or towards, the tips 50 of each blade enables the mass of the blade disc 30 to be reduced considerably, enables the mass distribution to be optimised and removes excess material where the tensile loads are lower and also reduces the inertia of the rotor 30.

The multi-faceted, pseudo-three-dimensional, surface of the blades 32 of a blade disc 30 according to the invention can, nevertheless, be manufactured via a similar, but novel, slit-cutting process to that used to manufacture the known slit-cut blade disc 10 of Figure 1.

Figure 3 is a closer view of an individual turbine blade 32 in accordance with the invention. The blade 32 of Figure 3 has three integrally formed blade portions 34, 36, 38 that extend radially outwardly from a central hub portion 52 with which it is integrally formed. As in the blade disc of Figure 2, the radially inwardly located blade portion 34 has a greater thickness and steeper angle (relative to the plane of the disc) than the radially outwardly located blade portions 36, 38. Each blade portion 34, 36, 38 has an inclined, flat or monoplanar leading surface 24 and flat or monoplanar trailing surface 26 that is parallel to, and spaced-apart from the leading surface 24.

As can be seen most clearly from Figure 5, which is an end view of Figure 3 along arrow 54 of Figure 3, each blade portion 34, 36, 38 has a different thickness 20 and angle 22 to its neighbour. Turbine blades 32 and turbine blade discs 30 in accordance with the invention can be manufactured using a slit cutting tool as shown schematically in Figure 4. In Figure 4 it will be seen that the blade disc 30 starts out as a nominally cylindrical metal blank 30 that is mounted to be rotationally indexed about its longitudinal axis 56 to correspond to the pitch (that is, the angular blade spacings), for example, in 22.5° increments for a sixteen-bladed blade disc 30. The slitting tool itself comprises a pair of parallel and spaced apart cutting discs 58, 60 which spacing 62 can be adjusted such that the gap between the discs 58, 60 corresponds to desired blade portion thicknesses 20. The cutting discs 58, 60 are mounted and driven to rotate about a common axis 64, which axis 64 can be inclined with respect to the longitudinal axis 56 of the blade disc blank 30 through an angle 66 that corresponds to a desired angle 22 of the individual blade portions 34, 36, 38.

The cutting discs 58,60 can be advanced towards and into the edge of the blade disc blank along a straight trajectory, as indicated by arrow 68, to cut an individual blade portion. It will be appreciated that if the cutting discs 58, 60 are advanced 68 such that their axis of rotation 64 intersects the longitudinal axis 56 of the blade disc blank 30 at its geometric centre, that radially extending blade portions will be formed. However, by advancing the cutting discs 58, 60 along a trajectory that does not meet this criterion will result in a swept, or offset, blade configuration.

Turbine blades 32 or blade discs 30 in accordance with the invention can be manufactured using the slit-cutting tool of Figure 4 by incrementally adjusting the cutting disc spacing 62 and angle of inclination 66 in successive, partial cutting operations as shown sequentially in Figures 6 to 9.

Figure 6 shows one possible first stage of the manufacturing process in which an individual turbine blade 32 is roughed out by advancing the cutting discs 58, 60 into the blade disc blank 30 vertically, that is to say with the axis 64 of the cutting blades being perpendicular to the axis 56 of the blade disc blank 30. Of course, the first cut may be made at a non-vertical angle, and this will usually be the case, in particular where it is desired to overlap the leading edge of one blade with the trailing edge of another. The first cut (or roughing cut, if used) removes the majority of waste material from the blade disc blank 30 surrounding each blade 32 and defines a leading edge 70 and a trailing edge 72, which will be preserved in the subsequent manufacturing stages. The depth of the first cut also determines the nominal length 74 of the blade 32 and the nominal hub diameter 76, i. e. the root diameter.

In the next stage of the process, as shown in Figure 7, the cutting discs 58, 60 are fully retracted and re-spaced such that the gap between them 62 corresponds to the intended blade thickness 20 of a first blade portion, in this case, the radially inwardly located blade portion 34. In addition, the cutting discs 58, 60 are canted such that their axis of rotation 64 is inclined to match the intended angle 22 of the innermost blade portion 34. (Of course, if the first cut is an angled cut, then the second cut will determine the shape of the second blade portion). The cutting discs 58, 60 are then advanced into the edge of the blank 30 to remove excess material 78 to form the flat leading 24 and trailing 26 surfaces of the first blade portion 34. The cutting discs 58, 60 are then fully retracted before being re-spaced 62 and re-inclined 66 to match the desired blade thickness 20 and angle 22 of a second blade portion 36.

In the next stage of the manufacturing process, shown in Figure 8, the cutting discs 58,60 are advanced into the blade 32 by a shorter distance 80. In so doing, further material 78 is removed from the blade 32 to form the flat leading 24 and trailing 26 surfaces of the second blade portion 36.

This process is then repeated any desired number of times, as shown in Figure 9, to form subsequent blade portions 38, 40,42 and so on. It will be noted that the leading and trailing edges 70, 72 of the blade 32 are preserved to prevent "over machining" of these parts of the blade 32 which could lead to undesired blade geometry and pump performance or weaken it metallurgically. However, this need not necessarily be the case, for example where the blade portions are designed to have different chord lengths in addition to, or instead of, differing thicknesses and angles.

It will be appreciated that a number of blade portions can be formed by this process and the differences between the thickness, angle and cord length of adjacent blade portions can be as large or small as is required by the design constraints of the turbine itself. However, unlike a truly three-dimensional blade profile, each blade portion has flat or monoplanar leading and trailing surfaces 24, 26, which enables the individual blades to be readily manufactured using a slit-cutting process, as opposed to a three-dimensional blade, which conventionally needs to be made by a complex CNC milling process.

In view of the above, it will be appreciated that a wide range of simple and complex blade geometries can be formed in a relatively inexpensive and quick fashion using the method of the invention, enabling higher efficiency and more sophisticated turbines to be implemented at a lower price point. Further, lower stress is achieved.

The invention is not restricted to details of the foregoing embodiments, which are merely exemplary of the invention. In particular, the blades could be manufactured to rotate in either direction, that is, either clockwise or anticlockwise, any number of blade portions may be provided on each blade, each blade disc may comprise any number of individual blades, the blade discs may be formed integrally with the axle, for example in a one-piece, "cut ingot" type turbine arrangement, and so on. Furthermore, the description above is limited to turbomolecular pump rotor or turbine blades, but the present invention could equally be applied to stator blade elements of a turbomolecular vacuum pump, or the like.

Claims

11 CLAIMS: 1. A method of manufacturing a turbine blade using a slit-cutting tool comprising the steps of: advancing cutting discs into the edge of a blade blank by a first distance, wherein the slit-cutting tool's cutting discs are adjusted to a first spacing and a first inclination of the cutting discs; retracting the cutting discs; and advancing cutting discs into the edge of a blade blank by a second distance different to the first distance, wherein the slit-cutting tool's cutting discs are adjusted to a second spacing and a second inclination different to the first spacing and inclination.

2. A method according to claim 1, wherein the steps of retracting and adjusting the spacing and inclination of the cutting discs, followed by advancing the cutting discs into the edge of a blank are repeated any plurality of times.

3. A method as claimed in claim 1 or claim 2, wherein the first spacing is greater than the second spacing.

4. A method as claimed in any of claims 1 to 3, wherein the first inclination is steeper than the second inclination.

5. A method as claimed in any of claims 1 to 4, wherein the first distance is greater than the second distance.

6. A method as claimed in any of claims 1 to 5, wherein the spacings and inclinations of the cutting discs of the slitting tool in each cutting step are selected to preserve a common leading, trailing or leading and trailing edge of the blade to be formed.

7. A method as claimed in any of claims 1 to 6, wherein the cutting discs are advanced into the edge of the blank along a straight trajectory that intersects the longitudinal axis of the blank.

8. A method as claimed in any of claims 1 to 6, wherein the cutting discs are advanced into the edge of the blank along a straight trajectory that is offset, or at an angle, with respect to a radial line intersecting the longitudinal axis of the blank.

9. A method as claimed in any of claims 1 to 8, wherein a plurality of blades are cut from the blank in simultaneously.

10. A method as claimed in any of claims 1 to 8, wherein a plurality of blades are cut from the blank sequentially.

11. A method as claimed in any of claims 1 to 10, further comprising the step of indexing the blank about its longitudinal axis at intervals.

12. A turbine blade for the rotor of a turbomolecular pump in which the angle and/or thickness of the blade varies step-wise from root to tip.

13. A turbine blade as claimed in claim 1 or claim 2, wherein the angle and/or thickness of the blade varies incrementally from root to tip.

14. A turbine blade as claimed in any of claims 1, 2 or 3, wherein each blade is formed from a number of integrally formed blade portions, each blade portion having a different angle and/or thickness compared to a neighbouring blade portion.

15. A turbine blade as claimed in claim 4, wherein the angle of a portion of the blade towards the root of the blade is steeper than the angle of a portion of the blade towards the tip of the blade.

16. A turbine blade as claimed in claim 4 or claim 5, wherein the thickness of a blade portion towards the root of the blade is greater than the thickness of a portion of the blade towards the tip of the blade.

17. A turbomolecular pump rotor comprising a plurality of turbine blades as claimed in any of claims 1 to 16.

18. A rotor according to claim 17, comprising a plurality of blades arranged in one or more groups around a central axle or hub.

19. A rotor according to claim 18, wherein the blades of each group of turbine blades are coplanar.

20. A rotor according to claim 19, comprising a plurality of groups of turbine blades, the groups being provided at axially spaced-apart locations on a common axle or hub.

21. A rotor according to any of claims 17 to 20, wherein the blades extend radially outwardly from an axle or hub.

22. A rotor according to any of claims 17 to 20, wherein the blades extend outwardly from an axle or hub in a swept configuration.

23. A turbomolecular pump comprising a rotor mounted for rotation relative to a stator, the rotor comprising a plurality of turbine blades according to any of claims 17 to 22.