EP3924623A1

EP3924623A1 - Scroll pump

Info

Publication number: EP3924623A1
Application number: EP20707775.1A
Authority: EP
Inventors: Peter David Jones; Richard Glyn Horler; Peter Charles LAMB; Ian David Stones
Original assignee: Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2019-02-15
Filing date: 2020-02-13
Publication date: 2021-12-22
Anticipated expiration: 2040-02-13
Also published as: CN113396282A; GB2581387A; GB2581387B; EP3924623B1; CN113396282B; GB201902135D0; WO2020165590A1

Abstract

A pump comprising: two plates mounted parallel to and separated from each other; an outer member surrounding at least one flexible member arranged between and substantially perpendicular to said two plates. The flexible member has a spiral form and forms at least one side wall of a channel running from an inlet to an outlet of the pump. A driving member is configured to drive an inner portion of said flexible member such that said inner portion processes around a path within said outer member such that a distance between said inner portion and said outer member is reduced on one side of said inner portion and increased on the other side of said inner portion. The reduced distance between the inner portion and outer member causes a cross section of the at least one channel to be reduced and a constriction to be formed in the at least one channel, movement of the inner portion causing corresponding movement of the restriction such that a fluid is pushed along the at least one channel as the inner portion processes.

Description

SCROLL PUMP

FIELD OF THE INVENTION

The field of the invention relates to scroll pumps.

BACKGROUND

A traditional scroll pump uses two rigid matched spiral scrolls that mesh together. One of the scrolls has an orbital motion with respect to the other thereby trapping and pumping pockets of fluid between the involutes or scrolls. In some cases, one of the scrolls is fixed, while the other is mounted on a drive shaft with an eccentric centre such that it orbits eccentrically without rotating. Another method for producing the relative orbiting motion is by co-rotating the scrolls, in

synchronous motion, but with offset axes of rotation. Thus, in this case the two scrolls are mounted on parallel shafts and the relative motion is the same as if one were orbiting and the other stationary.

The efficiency of a scroll mechanism is undermined by gas flowing in the reverse direction through the clearances between the scrolls. In order to govern such leakage effectively, such a pump requires high precision in the form of the scrolls and in the control of their relative positions. These high tolerance requirements make such pumps costly to manufacture. The leakage problem is particularly acute between the tips of the spiral walls and the opposing plates and in order to reduce this tip seals are used on the end surfaces of the spiral walls that contact the opposing plate. These tip seals wear out over time and generate debris within the pump.

It would be desirable to form a scroll-type pump with improved lifetime and reduced costs.

SUMMARY

A first aspect provides a pump comprising: two plates mounted parallel to and separated from each other; an outer member surrounding at least one flexible member arranged between and substantially perpendicular to said two plates, said at least one flexible member having a spiral form, said at least one flexible member forming at least one side wall of a channel running from an inlet to an outlet of said pump; a driving member configured to drive an inner portion of said flexible member such that said inner portion processes around a path within said outer member such that a distance between said inner portion and said outer member is reduced on one side of said inner portion and increased on the other side of said inner portion; wherein said reduced distance causes a cross section of said at least one channel to be reduced and a constriction to be formed in said at least one channel, movement of said inner portion causing corresponding movement of said restriction such that a fluid is pushed along said at least one channel as said inner portion processes.

The inventors of the present invention recognised that were a flexible scroll to be used in a scroll pump, then the ability to distort the scroll in response to the centre following a rotating path could allow the two rigid scrolls of a scroll pump to be replaced by a single flexible scroll, distortion of the flexible scroll allowing the channel(s) formed by the flexible scroll to be correspondingly distorted. In effect the inner portion of the flexible scroll moving closer to the outer member causes different portions of the scroll between the inner portion and outer member to be squashed closer together, which leads to one or more constrictions within the one or more channels, which constriction(s) move with the moving inner portion and pump the fluid along the channel(s).

The use of a single flexible scroll reduces the tolerance requirements of the pump and potentially forms the basis for a low cost, long life pump.

In some embodiments the pump comprises a single flexible member, said single flexible member forming the side walls of said channel running from an inlet to an outlet. In this case processing of said inner portion causes a distance between adjacent wraps of said flexible member across a diameter of said spiral to become closer together on one side of said inner portion and further apart on an opposing side of said inner portion, movement of said inner portion causing said closer together wraps to move along a corresponding path and push a fluid along said channel from said inlet to said outlet.

The flexible scroll may be formed of a single spiral such that where the inner portion moves towards the outer member overlapping wraps of a flexible member are pushed towards each other. Movement of the inner portion may be such that all the wraps are pushed close together or contact each other such that a plurality of constrictions are formed and a plurality of discrete pockets of fluid are provided pumping fluid along the channel from the inlet to the outlet. Alternatively, it may be that only a subset of the flexible wraps become close enough to form a constriction to provide effective pumping and in this case the pump may still operate effectively but with fewer pockets.

Alternatively, the pump may be a multiple start pump with a plurality of inlets and a plurality of channels with a spiral form. The advantage of having multiple inlets is that the capacity of the pump is increased. In this regard the formation of the pumping mechanism from a flexible member allows a variety of designs of scrolls to be used with a single or multiple channels depending on the requirements of pumping capacity or pressure differential.

In some embodiments, said at least one flexible member is formed such that a resistance to lateral deformation is substantially uniform from said outer portion to said inner portion.

One potential issue with the flexible spiral member is that if it is formed as a uniform member then owing to its shape it is more easily distorted in its outer portions where it has a straighter form than it is in its inner portions where it follows a tighter arc. This can have consequences when the flexible member is distorted by the inner portion moving around a rotating offset path. In particular, the outer flexible members forming the outer channels may be pushed more closely together than the inner members leading to either the inner members not forming an effective constriction or to friction between the contacting outer members which can lead to higher power requirements for the motor driving the pump and potentially to damage of the flexible member. This can be addressed by varying the properties of the flexible member(s) along their length to compensate for the differences due to their changing curvature. Thus, the flexible member may be formed such that a resistance to lateral deformation is substantially uniform along its length, in this regard substantially uniform is taken to be that the variation in lateral deformation is within a 20% range. Thus, in effect the outer portions are made stiffer than the inner portions in a way that compensates for the less tightly curved form.

The variation in stiffness can be provided in a number of ways, in some embodiments, said at least one flexible member is configured such that a thickness of said flexible member decreases from said outer portion towards said inner portion.

Alternatively and/or additionally said at least one flexible member is formed of one or more materials structured such that said stiffness of said material decreases from said outer portion to said inner portion.

In some embodiments, said two plates are mounted on either side of said outer member.

The outer member and plates provide an outer envelope for the pumping of the fluid and in some embodiments the outer member may act as a supporting member for the plates.

In some embodiments, said outer member has a height dimension perpendicular to said plates that is greater than a corresponding dimension of said at least one flexible member. Where the plates are mounted on the outer member the height dimension of the outer member can be used to set the plate separation and to provide a simple yet accurate way of controlling the axial gap between the flexible member and the plates. In this regard, leakage in such a pump will occur both circumferentially between the constricted flexible members and also radially between the flexible member and the plates. The radial leakage occurs across the whole length of the flexible member. Conventionally this radial gap has been controlled with the use of tip seals on rigid scrolls which tip seals wear over time reducing the lifetime of the pump and causing debris within it. In the current case the use of an outer member which can be machined to dimensions which are closely aligned but different to that of the flexible member allows the axial clearances to be accurately controlled in a simple and inexpensive manner. It thereby allows tip seals to be dispensed with.

In some embodiments, said outer member has a height perpendicular to said plates that is the same as a corresponding dimension of said at least one flexible member, said plates being mounted on shims resting on said peripheral outer member.

Although, the clearance distances can be controlled by controlling the difference in height of the outer member and of the flexible member, they may alternatively be formed of the same height, indeed they may be formed from a single sheet and shims may be used between the outer member and the plates to provide the low clearances required.

In some embodiments, said plates are held apart by a distance that is greater than a height of said at least one flexible member, said distance being less than 100 microns more, preferably less than 50 microns.

As noted previously the clearances between the flexible member and the plates can be controlled by the difference in height of the outer member and the flexible member. This difference in height can be controlled to high tolerances such that the difference may be less than 100 microns and preferable less than 50 microns. In some cases it may be as low as 10 microns.

In some embodiments, said outer member and said at least one flexible member are formed from a single sheet of material, one or more spirals being cut out of said sheet to form said one or more channels.

The outer member and flexible member may be formed of a single sheet of material and this may be machined such that the flexible member has a different height than the outer member. Machining techniques are such that this can be done to high accuracy in a relatively straightforward manner allowing differences in heights of tens of microns to be accurately achieved. Alternatively, they may be machined to a single uniform height and shims may be used to provide the clearances between the flexible member and the plates.

In some embodiments, the spiral may be a circular involute. In other

embodiments, it may be an elliptical involute or archimedes type spiral or it may have some other spiral shape.

In some embodiments, said driving member is configured to drive said inner portion around an offset circular orbit, such that said wraps move towards each other in phase with said offset.

The driving member may be configured to drive the inner portion around a circle that is offset from the centre of the outer member, that is it may be a circular path around the centre but it is not at the centre.

In some embodiments, said driving member comprises a motor and crankshaft.

In effect, the motor may have a crankshaft and the crankshaft may have an offset which provides the offset circular motion. ln some embodiments, said driving member is configured to drive said outer two overlapping flexible members to be between 10 and 200 microns of each other at a closest point.

The driving member and in particular the path of rotation that it is configured to follow can be selected such that the distance between the flexible members forming the channel can be controlled to provide an appropriate constriction. In this regard, it is advantageous if the distance between adjacent flexible members is small but finite such that they do not contact. Where a driving member with, for example a crankshaft is used, knowledge of the stiffness of the flexible members and the size of the axial offset of the crankshaft can be used to set the distance between adjacent flexible members and thus, the size of the constriction used in the pumping to a certain value. In some embodiments the distance between adjacent flexible members is set to be between 10 and 200 microns of each other at the closest point. This sets the constriction size and affects the efficiency of the pumping. In the case of a single uniform spiral then it will be the distance between the two outer wraps that is set to this value, the inner wraps being further apart than this. Where the flexible member has been selected such that its stiffness varies along its length then it may be that each of the overlapping flexible members along the radius where the inner portion is closest to the outer member are separated by a distance of between 10 and 200 microns of each other.

In other embodiments, said driving member comprises a rotating electromagnetic field generator mounted around an exterior of said outer member and a magnet mounted within said outer member and plates and configured to drive said inner portion of said at least one flexible member in response to said rotating

electromagnetic field.

An alternative to providing the driving member as a motor and a crankshaft is to provide the flexible member with a magnetic inner portion or attach it to a magnetic hub, such that an external rotating magnetic field drives this inner portion around a rotating path. In this regard, there may be less accuracy in the positional control of the inner portion using this driving mechanism, but it does have the advantage of not requiring moving parts to pass through the plates forming the pumping envelope and thus, there may be improved sealing for such a pump.

In some embodiments, said inlet is towards an outer edge of said at least one flexible member and said outlet is towards a centre of said at least one flexible member, while in other embodiments the inlet may be towards the centre with the outlets towards the outer edge. In this regard, depending on the direction of the spiral and the direction of rotation fluids may be pumped either from an outer edge towards the centre or from the centre towards the outer edge.

It should be noted that in some embodiments the pump may be a vacuum pump and in other embodiments it may be a compressor.

Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims.

Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which:

Figure 1 schematically shows the flexible spiral of a pump according to an embodiment;

Figure 2 schematically shows the movement of the pump of figure 1 ; Figure 3 shows a cross section through a pump according to an embodiment; and

Figure 4 shows the flexible scroll of a further embodiment.

DESCRIPTION OF THE EMBODIMENTS

Before discussing the embodiments in any more detail, first an overview will be provided.

Embodiments provide a scroll-type vacuum pump with a single flexible scroll, that may be formed of one or more flexible members each having a spiral form.

A traditional scroll pump requires two rigid matched spirals in the form of interleaving scrolls that mesh together. Embodiments employ a single flexible scroll, which in some embodiments also comprises a central hub driven in an orbiting motion attached to a central portion of the flexible scroll. The orbiting motion has a large enough orbit to push at least some of the adjacent flexible wraps or members along the constricted radius together in phase with the offset. In some embodiments, all of the flexible members or wraps are pushed together. In this regard flexible members are considered to be pushed together where they contact each other or are within 200 microns of each other.

A crescent-shaped inlet volume is swallowed at the inlet which in some

embodiments is located at the outer diameter, and compressed through one or more successive revolutions to an outlet which in some embodiments is towards the centre, in the same manner as a scroll pump.

Figure 1 schematically shows a pump according to an embodiment. In this pump, gas is compressed and moved by a single flexible spiral element. It has similar form constraints to a traditional rigid scroll: these tend to be circular involutes, but some other shapes (e.g .elliptical involute, Archimedes spiral) are also valid. The pump comprises a static outer circular part 10, an orbiting circular part 20, and a flexible spiral forming plural wraps 40. A wrap is a 360° portion of the spiral. In the example of Figure 1 the crank is at 3 o’clock and the orbiting inner part is close to the outer static part at this point 30. This results in the wraps within portion 30 being compressed together leading to reduced radial clearances between them. These reduced radial clearances acts as an obstacle to fluid flow and a pumping chamber is formed between the wraps of the involute that pushes the fluid around the spiral of the involute as the inner circular part rotates.

In this example all of the wraps are pushed close together. Were the spiral member to be formed uniformly, then it would be easier to compress the outer wraps together than the inner wraps, such that the outer wraps would be compressed together with some force before the inner wraps become close to each other. This can lead to wear on the flexible member and increased power required to rotate the driving member.

This potential problem can be addressed in a number of ways. In some embodiments, the drive member and crank are configured so that the outer involutes move very close to each other and the inner involutes remain remote from each other. This reduces the number of active wraps of the pump, but does allow reduced wear and can still provide effective pumping. In effect the distance of the gas path through the pump is reduced and as compression happens along the path the total amount of compression is reduced too.

In other embodiments, the flexible member is configured so that the forces required to move all of the wraps of the spiral are similar, and the distance between the involutes on compression is substantially uniform. This provides for an increased number of active wraps within the pump while allowing reduced wear. This can be achieved by the use of flexible members which have reduced stiffness towards the centre where they form a lower radius arc. This reduced stiffness may be achieved by reduced thickness of the flexible member or by a change in the material or structure of the material forming the flexible member. The latter is readily achieved using techniques such as 3D printing.

In the embodiments of figures 1 and 2 the flexible element has circular outer and inner parts connected by a single spiral. The central circular hub is driven in a circular orbit by a crankshaft connected to a motor. The outer circular ring is fixed, so the displacement of each part of the spiral varies with radial position (see figures 1 and 2).

The crank is offset so that the displaced centre pushes all the spiral wraps together in phase with the offset, with minimal radial clearance to combat back- leakage. A clearance is desirable to reduce friction, but some contact may be tolerated if a polymer spiral is used. The combination of radial offset and orbiting motion pushes gas along the pump channel in a manner similar to peristaltic pumps.

Very small axial clearances are possible if the flexible element is machined from a sheet and the moving portion is milled deeper than the static outer enclosing ring, which can then act as a spacer for end plates (see figure 3).

Figure 3 shows a vacuum enclosure that is formed by plates 50 sandwiching an inner flexible spiral with circular inner and outer rings formed from a sheet 60.

The outer ring member forms the support for the plates.50 and provides a sealed enclosure 70. The difference in dimensions of the flexible spiral and outer member machined into the sheet set the small running clearances 80 which allow the inner portion of the flexible spiral to slide freely while keeping axial clearances and leakage low.

The challenges of dynamic stability and component stresses are addressed by selecting the geometry and material of the flexible element appropriately. Stiff polymers (e.g. polyamides) may have more suitable characteristics than softer materials, but over-moulding to modify properties on a metal spring may be possible and may make the use of softer materials appropriate. The polymer wall could be reinforced with longitudinal fibres to resist gas pressure distortion (although this should be carefully managed as the increase in stiffness may have a detrimental effect on power requirements); or a moulded/extruded scroll form could be used where the wall has some internal hollow structure to increase buckling resistance. Dynamic behaviour and stress may dictate shaft rotational speed.

The spirals near the centre are stiffer than the outer portions because the curvature is tighter. If the radial wall thickness is constant and the material is uniform, stresses will be higher near the centre. However, it may be possible to vary wall thickness to modify stiffness.

In summary, a traditional scroll pump requires accurate control of the relative rotational positions of the scrolls (‘timing’), usually achieved using bellows or multiple cranks. The pump of embodiments may not need any such timing mechanism.

By reducing parts counts and component complexity, this design may produce a very inexpensive pump, albeit there may be a reduction in pumping performance due to the relative lack of precision. Inlet capacity and the number of wraps may be limited by the stiffness of the flexible element, stresses and dynamic stability. In some embodiments, stages may be stacked in parallel or series to

compensate for this and increase capacity. Series stages would allow different geometry (e.g. the exhaust stage may be tuned for small outlet volume, to reduce power); stacked stages could be out of phase for balance, and could have different geometry and crank offsets.

In some embodiments, the scroll deformation is driven by a rotating

electromagnetic field rather than a mechanical shaft and crank. A permanent magnet positioned within the hub of the flexible element would be attracted to a rotating electromagnetic field on the exterior of the scroll form, operating in a similar manner to an electric motor. An embodiment of the design with a magnetic drive could provide further significant part consolidation and design simplification, reducing cost further.

Control of the orbit radius (and thus the radial clearances between wraps) would be more challenging with a magnetic drive, but in embodiments the magnetic drive may be configured to provide a radial force to drive the various flexible members or wraps of the involute into contact. The flexible element would have to endure this sliding contact, but this is potentially an embodiment that would provide better sealing against back-leakage.

Servicing a flexible scroll design may be straightforward with a simple

replacement of the flexible element to restore performance back to original settings. A design of the flexible scroll with integrated axial clearance offset, as shown in figure 3, would facilitate serviceability.

Figure 4 shows an alternative multi-start embodiment, where multiple inlets 90 and multiple spiral flexible members 95 provide a scroll pump with increased capacity. In this case the channels are formed between adjacent flexible members and multiple paths between multiple inlets and a single outlet are provided.

Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents. REFERENCE SIGNS

10 outer member

20 orbiting inner portion

30 radial clearances

40 spiral wraps

50 plates

60 sheet forming flexible and outer member

70 seal between plates and outer member to provide pumping envelope 80 running clearances

90 inlets

95 flexible spiral members

Claims

1. A pump comprising:

two plates mounted parallel to and separated from each other;

an outer member surrounding at least one flexible member arranged between and substantially perpendicular to said two plates, said at least one flexible member having a spiral form, said at least one flexible member forming at least one side wall of a channel running from an inlet to an outlet of said pump; a driving member configured to drive an inner portion of said flexible member such that said inner portion processes around a path within said outer member such that a distance between said inner portion and said outer member is reduced on one side of said inner portion and increased on the other side of said inner portion; wherein

said reduced distance causes a cross section of said at least one channel to be reduced and a constriction to be formed in said at least one channel, movement of said inner portion causing corresponding movement of said restriction such that a fluid is pushed along said at least one channel as said inner portion processes.

2. A pump according to claim 1 , comprising a single flexible member, said single flexible member forming the side walls of said channel running from an inlet to an outlet.

3. A pump according to claim 1 , comprising a plurality of flexible members having spiral forms and a corresponding plurality of inlets, said plurality of flexible members being interleaved with each other and forming the side walls of a plurality of channels running from said plurality of inlets to said outlet.

4. A pump according to any preceding claim, wherein said at least one flexible member is formed such that a resistance to lateral deformation is substantially uniform from said outer portion to said inner portion.

5. A pump according to any preceding claim, wherein said at least one flexible member is configured such that a thickness of said flexible member decreases from said outer portion towards said inner portion.

6. A pump according to any preceding claim, wherein said at least one flexible member is formed of one or more materials structured such that said stiffness of said material decreases from said outer portion to said inner portion.

7. A pump according to any preceding claim, said two plates are mounted on either side of said outer member.

8. A pump according to claim 7, wherein said outer member has a height dimension perpendicular to said plates that is greater than a corresponding dimension of said at least one flexible member.

9. A pump according to claim 7, wherein said outer member has a height perpendicular to said plates that is the same as a corresponding dimension of said at least one flexible member, said plates being mounted on shims resting on said peripheral outer member.

10. A pump according to claim 8 or 9, wherein said plates are held apart by a distance that is greater than a height of said at least one flexible member, said distance being less than 100 microns more, preferably less than 50 microns.

11. A pump according to any one of claims 7 to 10 wherein said outer member and said at least one flexible member are formed from a single sheet of material, one or more spirals being cut out of said sheet to form said one or more channels.

12. A pump according to any preceding claim, wherein said spiral is a circular spiral.

13. A pump according to any preceding claim, wherein said driving member is configured to drive said inner portion around an offset circular orbit, such that said wraps move towards each other in phase with said offset.

14. A pump according to any preceding claim wherein said driving member comprises a motor and crankshaft.

15. A pump according to claim 14, wherein said driving member is configured to drive said outer two overlapping flexible members to be between 10 and 200 microns of each other at a closest point.

16. A pump according to any one of claims 1 to 14, wherein said driving member comprises a rotating electromagnetic field generator mounted around an exterior of said outer member and a magnet mounted within said outer member and plates and configured to drive said inner portion of said at least one flexible member in response to said rotating electromagnetic field.

17. A pump according to any preceding claim, wherein said inlet is towards an outer edge of said at least one flexible member and said outlet is towards a centre of said at least one flexible member.