GB2632108A

GB2632108A - A scroll pump and thrust bearings for scroll pumps

Info

Publication number: GB2632108A
Application number: GB2311294.9A
Authority: GB
Inventors: Ernest Kinnaird Holbrook Alan
Original assignee: Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2023-07-24
Filing date: 2023-07-24
Publication date: 2025-01-29
Also published as: GB202311294D0; WO2025022081A1

Abstract

A thrust bearing assembly 5 for providing axial support for a first scroll member 20 with respect to a second scroll member 30 within a scroll pump 10; the thrust bearing assembly comprising at least one ball bearing 40; a first 22 and second 32 ball bearing cage for accommodating and constraining movement of the at least one ball bearing; wherein the first and second ball bearing cages comprise at least one aperture, the at least one aperture of the first ball bearing cage overlapping with the at least one aperture of the second ball bearing cage; wherein the at least one ball bearing is accommodated within the respective overlapping apertures of the ball bearing cages and at least one control aperture of the first ball bearing cage is configured to have a predetermined length in one dimension that is at least 2% greater than a predetermined length in a direction perpendicular to said one dimension, such that a clearance relative to a path traced by said corresponding ball bearing in said one dimension is greater than a clearance in said perpendicular direction. A scroll pump and method of mounting at least one thrust bearing is also claimed.

Description

A SCROLL PUMP AND THRUST BEARINGS FOR SCROLL PUMPS

FIELD OF THE INVENTION

The field of the invention relates to scroll pumps and thrust bearings for scroll 5 pumps.

BACKGROUND

A scroll pump comprises two interleaving scrolls or involutes which are mounted to exhibit relative orbital motion with respect to each other, thereby trapping and io pumping or compressing pockets of fluid between the scrolls. In some cases, one of the scrolls is fixed, while the other is mounted on a drive shaft with an eccentric journal 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, the two scrolls are mounted on parallel shafts and the relative motion is the same as if one were orbiting and the other stationary.

In order to reduce leakage across the involute walls, some scrolls have tip seals, while others may be non-contacting scrolls which are configured to have low axial clearances between the involute wall of one scroll and the radially extending base wall of the other. In the latter case it is particularly, important for performance that the axial clearance between the scroll members is carefully controlled and thrust bearings may be used to engage with one of the scrolls and keep the scroll in the correct axial position relative to the other scroll and achieve and maintain a desired low clearance gap.

SUMMARY

An aspect provides a thrust bearing assembly for providing axial support for a first scroll member with respect to a second scroll member within a scroll pump; 3o said thrust bearing assembly comprising: at least one ball bearing; a first and second ball bearing cage for accommodating and constraining movement of said at least one ball bearing; wherein said first and second ball bearing cages each -2 -comprise at least one aperture, the at least one aperture of said first ball bearing cage overlapping with said at least one aperture of said second ball bearing cage; wherein said at least one ball bearing is accommodated within the respective overlapping apertures of said ball bearing cages and at least one control aperture of said first ball bearing cage is configured to have a predetermined length in one dimension that is at least 2% greater than a predetermined length in a direction perpendicular to said one dimension, such that a clearance relative to a path traced by said corresponding ball bearing in said one dimension is greater than a clearance in said direction perpendicular to said one dimension.

Scroll pumps are configured such that the scroll members are mounted for relative orbital motion with respect to each other. The thrust bearing provides for accurate control of the axial distance between the scroll members while allowing them to exhibit the relative orbital motion. The one or more ball bearings of the thrust bearing assembly are retained in position within overlapping one or more apertures within two axially offset bearing cages. The two bearing cages are mounted to move with respective scroll members during pumping.

The relative orbital motion will cause the ball bearings to trace a circular path and the apertures are configured to retain the ball bearings within this path. The radius of the circular path is dependent upon the largest offset of the two centres of the scroll members. If the apertures are much larger than required to retain the ball bearing within the path, the orbiting scroll is unstable and generates noise, while if the apertures are too small the bearings cannot roll and are forced to slide which causes wear and early failure. The present invention provides in one of the cages at least one aperture that is elongated, so that one dimension has a predetermined length that is selected to provide a predetermined clearance to the outer surface of the rotating ball bearing and thereby provides control in 3o this dimension while having a larger length in a perpendicular dimension that provides a larger clearance and thereby increased freedom in this dimension allowing for misalignment. -3 -

In some embodiments, said at least one control aperture comprises a distorted circular aperture, with an elongated diameter in said perpendicular dimension.

In some embodiments, the second ball bearing cage may have substantially circular apertures, while in other embodiments at least one aperture of the second ball bearing cage may also be elongated.

It may be preferable for the apertures of the second ball bearing cage to have a to substantially circular configuration and to be configured to have a diameter substantially that of the predetermined length so that a low clearance relative to the ball bearing path is provided. In this way the second ball bearing cage provides good ball control while the elongated aperture in the first bearing cage provides some freedom in one direction.

The elongated dimension may be at least 2%, preferably at least 5 or in some cases 10% larger than the predetermined length. The diameter of the apertures in the second ball bearing cage have a diameter that is nominally the same as the predetermined length and may be within 1% of the predetermined length, The second ball bearing cage may be configured to be mounted in a fixed relation to the second scroll member, which in some embodiments may be the fixed scroll member, while the first ball bearing cage may be configured to be mounted coupled to the first scroll member which may be the orbiting scroll.

In some embodiments, said ball bearing cages comprise a portion of a sector of a circle, and said one dimension comprises a radial direction.

The ball bearing cages may comprise a portion of a sector of the circle and the 3o one dimension that provides the degree of freedom may comprise the radial direction. -4 -

In some embodiments, said ball bearing cages comprise a portion of a sector of a circle and comprise a plurality of apertures at radially inner and radially outer positions, said at least one control aperture comprises one of said radially outer apertures of said first ball bearing cage.

It may be advantageous if there are a plurality of apertures and ball bearings in only a subset of the apertures are control apertures with the constrained clearance in the circumferential direction and a greater degree of freedom in a radial direction. In some cases, it may be one of the radially outer apertures, and io in some cases, it may be the aperture at one edge of the first bearing cage. In other embodiments, said first ball bearing cage comprises a plurality of control apertures including one at each edge of said radially outer apertures.

In still other embodiments, said plurality of control apertures comprise apertures at each end of said radially outer and radially inner apertures.

In some embodiments, said plurality of apertures of said first ball bearing cage, comprise at least one control aperture and at least one further aperture, said at least one further aperture comprising a circular aperture configured with a diameter that is greater than said predetermined length of said at least one control aperture.

It may be desirable for the apertures that are not providing the control in the first ball bearing cage to be enlarged to reduce dimensional conflict between apertures. This allows for misalignment which can arise from variations in machining and assembly. It can also be difficult to align several holes with sufficient accuracy to achieve low noise and low wear requirements. Thus, providing cages where only one or a subset of the apertures need to be accurately aligned may relieve this problem while still providing effective control 3o of the ball bearings. The apertures may be enlarged by having a diameter similar to the length of the elongated dimension that is at least 2 preferably 10% longer than the predetermined length. -5 -

The second bearing cage may have circular holes for the ball bearings all of which have the close predetermined clearance and this will maintain the circular motion of all the balls by this bearing cage which is generally a bearing cage in a fixed relation to the fixed scroll.

In some embodiments, each bearing cage comprises nine apertures, five in the radially outer line and four in a radially inner line.

io In some embodiments, said first ball bearing cage comprises a portion of a sector of a circle and said first ball bearing cage comprises at least one distortable member on radially inner and radially outer surfaces.

It may be desirable for the thrust bearing cage to have a distortable member on the radially inner and radially outer surfaces. This allows it to be mounted within a pocket in a way that it can slide allowing it to self align on initial start up of the pump. Appropriate selection of the elasticity of the distortable members and of the relative sizes of the cage and the pocket can enable the force retaining the cage in position in the pocket to be selected to be high enough to retain it in position during normal pumping, but low enough to enable it to slide where it is mis aligned during initial start up. Although the thrust bearing cage with the distortable member(s) is disclosed with respect to a thrust bearing cage having a distorted circular control aperture, it should be understood that it could be used with a thrust bearing cage without such a control aperture and the self aligning would function in the same way.

In some embodiments, said first ball bearing cage comprises a portion of a sector of a circle and said first ball bearing cage comprises at least one distortable member on radially extending side surfaces. -6 -

The cage may also have elastic distortable members on each side surface and these may provide further resistance to circumferential movement at either edge of the pocket.

A further aspect provides a scroll pump comprising: two scroll members comprising interleaving scrolls mounted for relative orbital motion with respect to each other; at least one thrust bearing assembly according to an aspect for axially supporting one scroll member with respect to the other scroll member; said first and second ball bearing cages being coupled to respective scroll members.

The first and second ball bearing cages may be mounted on or adjacent to the respective scroll members. They may be mounted in a constrained relation with respect to them, such that they move with the scroll members during normal pumping operation.

In this regard, the first bearing cage may be mounted within a pocket within the first scroll member which may be the orbiting scroll member such that it can slide circumferentially on initial start up of the pump and self align, while the second bearing cage may be mounted in a fixed relation to the fixed scroll.

In some embodiments, said scroll pump comprises three thrust bearing assemblies arranged at different angular positions of said scroll members.

The thrust bearings are configured to provide axial alignment and thus, three may be used to inhibit tilting of one scroll member with respect to the other.

In some embodiments, said first ball bearing cage of said at least one thrust bearing assembly is mounted within a pocket of said first scroll member, said 3o pocket being sized such that said ball bearing cage can move in a circumferential direction along said pocket. -7 -

In some embodiments, the scroll pump comprises a thrust bearing assembly where said pocket and said at least one distortable member on said radially inner or outer surfaces are configured such that said first ball bearing cage is held within said pocket with a force that is greater than a force on said first ball bearing cage due to gas load during pumping and less than a force on said first ball bearing cage during an initial start up of said pump.

The scroll pump may be configured such that the first bearing cage aligns on initial start-up of the pump and the force due to friction between the pocket and io the distortable members will then hold it in position.

In this regard, the balls will exert a frictional force on the cage if sliding then once in position will be able to roll and the frictional force will be reduced. By appropriate selection of the elasticity of the material and the size of the bearing cages and pockets along with a knowledge of the forces that will arise on the bearing cage due to operation of the pump. A suitable self-aligning mechanism can be configured.

A further aspect provides a method of mounting at least one thrust bearing, within a scroll pump comprising two scroll members comprising interleaving scrolls mounted for relative orbital motion with respect to each other, said method comprising: mounting said thrust bearing to said scroll pump such that a first thrust plate is mounted on a radially extending surface of a scroll member, a first ball bearing cage is within a recess on said radially extending surface and an adjustment pin is attached to the scroll pump housing and extends axially into a recess in a second thrust plate adjacent to a second ball bearing cage, the first and second ball bearing cage comprising overlapping apertures retaining ball bearings; adjusting said adjustment pin to provide a desired axial clearance between the two scroll members of the scroll pump. -8 -

In some embodiments, the method further comprises aligning said first ball bearing cage by initiating rotation of said scroll pump such that said first ball bearing cage slides in a circumferential direction in said recess.

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, io 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 shows a thrust bearing assembly within a portion of a scroll pump according to an embodiment; Figure 2 shows a bearing cage of a thrust bearing assembly according to an embodiment; Figure 3 shows the bearing cage of Figure 2 mounted within a pocket on an orbiting scroll; Figure 4 schematically shows possible locations of control apertures of bearing cages according to an embodiment; Figures 5A and 5B show the diameters and clearances of apertures of the bearing cage of an embodiment; and Figure 6 shows a method of mounting a thrust bearing assembly to a scroll pump according to an embodiment.

DESCRIPTION OF THE EMBODIMENTS

3o Before discussing the embodiments in any more detail, first an overview will be provided. -9 -

Scroll pumps may comprise thrust bearings to provide axial alignment between the two scroll members. These thrust bearings may also act as anti-rotational devices to resist relative rotation between the two scroll members.

Some scroll pumps have tip seals between the involute walls and the base wall of the opposing scroll to provide sealing between the pumping channels while other non-contacting scroll pumps may have no such tip seals and require very careful axial alignment with low clearances between the involute walls and the back wall of the opposing scroll. Thrust bearings provide for accurate axial alignment and io are particularly applicable to non-contacting scrolls.

Scroll pumps may use three adjustable thrust bearings with adjustable pins to position the thrust bearings and accurately set the scroll-to-scroll axial clearance. Bearing cages are used to contain the bearing balls and to control the orientation of the orbiting scroll. Embodiments provide a cage design that provides both of these functions and also accommodates typical m is-alignment that may occur due to manufacturing and assembly variation.

In existing thrust bearing designs, the cages are fixed to their adjacent thrust plates and the apertures or holes that control the balls' movement are round.

Testing has shown that if these holes are too large, the orbiting scroll is unstable and generates noise. If, however, the cage holes are too small the bearing balls are forced to slide which causes wear and early failure. However, it is difficult to have all nine holes in the conventional cage positioned with sufficient accuracy to achieve the low noise and low wear requirements.

Embodiments provide a thrust bearing assembly for a scroll pump with one of the bearing cages having at least one distorted circular aperture termed a control aperture with an enlarged dimension in the radial direction of the scroll member, 3o such that the circumferential travel of the ball bearing is constrained by this aperture in the usual way but there is some freedom in the radial positioning. In some embodiments, the apertures other than the control aperture(s) are circular -10 -but with diameters that are enlarged compared to the diameter of the control aperture in a direction perpendicular to the radial direction of the scroll member, and in some cases have a diameter that is substantially equal to the diameter of the control aperture in the radial direction of the scroll.

The thrust bearing assembly is mounted to the scroll pump such that it engages with one of the scroll members and with the housing to keep the scroll member in the correct axial position relative to the other scroll member. In order to allow for relative orbital motion of the two scrolls the thrust bearing assembly has two ball bearing cages configured to move with respective scrolls and comprising overlapping apertures to house ball bearings. The ball bearings roll on thrust plates and perform a circular motion in response to the relative orbital motion. It is desirable for the apertures to be sized such that the bearing can roll in a circle required by the orbital motion such that they do not slide over the thrust plate, but equally they should be constrained to remain within that circle. In some embodiments, the apertures of the fixed bearing cage are sized to accommodate the ball bearing to execute the circular motion with a small clearance of perhaps between 0.05 and 0.2 mm, while the orbiting bearing cage may have at least one control aperture that is sized to have a similar small clearance for the rotating motion in the circumferential direction of the scroll member, that is perpendicular to the radial direction, and an enlarged clearance of over 0.2mm perhaps up to 1mm in the radial direction. Thus, they may be non-circular with a smaller 'width' and a larger perpendicular length'. In some cases the orbiting bearing cage has some non-control apertures which are substantially circular apertures and have a diameter that provides increased clearance and may correspond to the length of the control apertures.

The bearing cage that is mounted on at least one of the scrolls, in some cases on the orbiting scroll, may be self-aligning. That is it may have elastic or deformable 3o protrusions on the sides that are configured to engage with the edge of the pocket in which it is mounted and to retain it in position during normal pumping operation, but be sufficiently flexible to allow it to move to an aligned position during the initial operation of the pump.

Figure 1 is a schematic illustration (not to scale) showing a cross-sectional view of a thrust bearing assembly 5 of a scroll pump 10.

The thrust bearing assembly 5 comprises a first thrust plate 25 mounted on the orbiting scroll 20, a second thrust plate 35 mounted in a fixed relation to the fixed scroll 30, a first ball bearing cage 22 mounted within a pocket 24 in the orbiting scroll and a second ball bearing cage 32 mounted in a fixed relation to the fixed scroll 30. The ball bearing cages 22, 32 have overlapping apertures that retain ball bearings 40. The thrust bearing has an adjustment pin 60 and provides axial support to the orbiting scroll 20 while also facilitating the orbiting of the orbiting scroll 20 during operation, as will be described in more detail below.

The first thrust plate 25 is fixed on one side to a back surface of the orbiting scroll 20 and on the other side to the first ball bearing cage 22. The first and second ball bearing cages are spaced apart from each other axially and comprise overlapping apertures that retain the ball bearings 40. The thrust plate 35 is fixed to the second ball bearing cage and is engaged with an end of the adjustment pin 60. Adjustment of the pin alters the axial distance between the fixed and orbiting scroll and allows this to be accurately set.

The first and second ball bearing cages 22, 32 each comprise a plurality of apertures within which the plurality of ball bearings 40 are located. Each aperture of the first ball bearing cage 22 at least partially overlaps with a corresponding hole of the second ball bearing cage 32 to form a plurality of aperture pairs. Each aperture pair houses a single ball bearing 40. The partial overlap of the apertures enables the first and second bearing cages to accommodate the orbiting motion of the orbiting scroll 20 during operation while constraining the movement of the ball bearings 40. This is illustrated further in Figure 5A.

-12 -During operation, to facilitate the orbiting of the orbiting scroll 20, the plurality of ball bearings 40 roll against the respective first and second thrust plates 25, 35 within the aperture pairs of the first and second ball bearing cages 22, 32 and trace a circular path constrained by the apertures that are sized to accommodate the circular path. During operation of the scroll pump, the first thrust plate 25 and first ball bearing cage 22, which are fixed to each other and to the orbiting scroll 20, move together with the orbiting scroll 20. Thus, during operation, the first thrust plate 25, first ball bearing cage 22 and orbiting scroll 20 all move together relative to the second thrust plate 35 and the second ball bearing cage 32 on the plurality of ball bearings 40.

In this embodiment, the end of the adjustment pin sits in a tapered recess in one side of the second thrust plate 35. The tapered recess has a generally conical shape in this embodiment and the end of the adjustment pin 60 comprises a rounded surface which is engaged with a surface of the second thrust plate 35.

In this way, the rounded surface of the end of the adjustment pin 60 and the surface defining the tapered recess together form a ball and socket joint, which enables the second thrust plate 35 and second ball bearing cage 32 to articulate/rotate on the first end of the adjusting pin 60. This means that the second thrust plate 35 is able to rotate or tilt relative to the adjustment pin to level itself with respect to the first thrust plate 25 (i.e. so that the sides of the first and second thrust plates 25 and 35 facing each other are parallel). This levelling advantageously tends to mean that all of the ball bearings 40 receive approximately the same amount of axial load and so will tend to wear at the same rate. This arrangement also provides friction in the interface between the pin and socket, which stabilises the thrust plate and impedes unwanted rotational oscillation. It should be noted that other interface arrangements between the adjustment pin and non-orbiting thrust plate are possible that also act to position the non-orbiting thrust plate and yet still allow rotation and levelling of the plate. 3o

Figure 2 shows a first bearing cage 22 of the thrust bearing assembly for mounting within a pocket on the orbiting scroll 20. Bearing cage 22 comprises -13 -nine apertures of which the outer two 27 on the radially outer row are control apertures. These apertures are smaller in a direction perpendicular to a radius of the scroll member, and provide a small clearance with respect to an outer surface of a ball bearing tracing a circular path, the clearance typically being 0.1mm and a larger clearance in a radial direction typically greater than 0.2mm perhaps about 0.5 mm. The freedom provided by the enlarged dimension in the radial direction allows for radial misalignment of the pin of the thrust bearing which can arise from variations in machining and assembly. The remaining non-control apertures 29 in the first bearing cage 22 are enlarged, and typically have a io diameter that is substantially equal to the increased radial length of the control apertures 27. This avoids or at least reduces dimensional conflict with the control apertures in the cage.

The fixed bearing cage 32 has circular apertures for the ball bearings all of which have a close clearance of typically 0.1mm with respect to the circular path of the ball bearing. The circular motion of the balls is constrained by the apertures of the fixed cage. The stability of the fixed scroll member and bearing cage 32 is maintained by the friction between the thrust pin 60 and the fixed thrust plate 35.

The bearing cages 22, 32 have compliant members 26 on their radially inner and outer surfaces which fit tightly into annular pockets 24 in the orbiting scroll, see Figure 3. The arrangement allows the cages to move in a circumferential direction along the annular pockets, enabling the cage to accommodate misalignment of the setting pins. This alignment takes place during the first orbit of the scroll. After this, there is no further movement and the friction between the cages and the pockets is sufficient to resist the movement of the bearing cage 22 on the orbiting scroll from the gas pressure within the scrolls during pumping operation. Further compliant pads 28 are provided on each end of the cages to preserve the final cage location (see Figure 2 and 3). 3o

Figure 4 shows alternative locations for control apertures for the first bearing cages. The centre 50 of the orbiting scroll and position of the crank pin 44 are -14 -shown, along with the position of the control apertures that are indicated by the arrows 28 that indicate control points. The left-hand figures shows control apertures at each end of the bearing cage, at the radially inner and radially outer rows. The central figures shows one control aperture at each end of the cage, in most cases this will be in the radially outer row. While the right-hand figure shows a single control aperture at one end of each cage, the control apertures in one of the cages is at a different end to the control aperture of the other cages.

Figure 5A schematically shows six thrust bearings arranged at different angular positions around the scroll members. In this Figure the thrust bearings each comprise a ball bearing 40 constrained by circular and oval apertures (37, 27) in bearing cages associated with fixed and orbiting scroll members (30, 20) respectively. In this figure ball bearings 40 follow a circular path and the diameter of the apertures 37 in the fixed bearing cage are slightly larger than the path followed by the outer surface of the ball bearing at the axial location of the fixed bearing cage, such that there is a small clearance between the ball bearing path and the aperture, this may typically be between 0.05 and 0.2 mm. These apertures constrain the ball bearings to remain within the path.

The control aperture 27 of the orbiting cage has an elongated diameter in the radial direction and this provides an increased clearance between the path of the outer surface of the ball bearing 40 and the aperture 27. This clearance may be greater than 0.2 mm and may be up to imm. The clearance between the smaller diameter of control aperture 27 and the path of the outer surface of ball bearing 40 may be similar to that of the fixed aperture that is between 0.05 and 0.2mm. This arrangement constrains the movement of the balls 40, and in this way, the orbiting scroll member, in the direction of the crank offset 42, that is perpendicular to the radial direction. The elongated diameter of control aperture 27 in the radial direction provides a larger clearance in the radial direction and thereby increased 3o freedom in this dimension allowing for misalignment. Each of the six balls will at some point of operation provide rotational control allowing orbital motion, while resisting rotational motion.

-15 -Figure 5B shows the dimensions of the crank offset 42 Rc and the ball bearing 40 Db, and how these can be used to determine the dimensions of the apertures and how these relate to the clearances between the path of the ball and the aperture.

So if the diameter of the aperture of cage 1 is Dal and that of cage 2 is Dal and the diameter of the ball bearing is Db at the axial location of the cages, and the crank offset is Rc, then cage clearance C (clearance between aperture and to outside surface of ball bearing tracing a circular path) can be determined to be: Cage clearance 'C' = Da1/2 -Rc + Da2/2 -Db If Dai = Da2= Da then C = Da -Rc -Db For example: Let Da = 14.1 mm, let Rc = 4 mm and let Db = 10 mm Then C = 14.1 -4 -10 = 0A mm In other words where the diameters of the apertures in the cages are the same, the clearance equals the diameter minus the crank offset and the diameter of the ball, so the diameter of the aperture should be set to be equal to the crank offset plus the diameter of the ball plus whatever clearance is desired.

Figure 6 schematically shows a flow diagram illustrating steps in a method for mounting a thrust bearing according to an embodiment to a scroll pump. The method comprises the steps of: S10 mounting a thrust bearing to a scroll pump by: at step S20 mounting a first thrust plate on a radially extending surface of a scroll member and at step S30 mounting a first ball bearing cage within a recess 3o in the radially extending surface. At step S40 an adjustment pin is attached to the scroll pump housing, the adjustment pin extending axially into a recess in a second thrust plate adjacent to a second ball bearing cage. The first and second ball bearing cages are mounted axially offset with respect to each other and comprise overlapping apertures that constrain the ball bearings. At step S50 the -16 -adjustment pin is adjusted to provide a desired axial clearance between the two scroll members of the scroll pump. In some embodiments this is done by adjusting the pin until the scroll members contact each other and then undoing the pin by a desired amount that provides the required axial clearance. At step S60 the first bearing cage is aligned by initiating rotation of the scroll pump such that the first bearing cage slides circumferentially in the recess into its aligned position.

It should be noted that although the method describes the mounting of one axial io thrust bearing, there may be three axial thrust bearings and in which case the alignment by adjusting the pins will be done in a coordinated manner to provide the desired axial clearance at different positions on the scroll member and avoid or at least inhibit any tilt in the scroll member.

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.

-17 -

REFERENCE SIGNS

thrust bearing assembly scroll pump orbiting scroll 22 first bearing cage 24 recess/pocket first thrust plate 26 compliant members 27 control aperture io 28 control points 29 aperture fixed scroll 32 second bearing cage second thrust plate 37 aperture ball bearing 42 crank offset 44 crank pin shaft centre 60 pin

Claims

-18 -CLAIMS1. A thrust bearing assembly for providing axial support for a first scroll member with respect to a second scroll member within a scroll pump; said thrust bearing assembly comprising: at least one ball bearing; a first and second ball bearing cage for accommodating and constraining movement of said at least one ball bearing; wherein said first and second ball bearing cages each comprise at least one io aperture, the at least one aperture of said first ball bearing cage overlapping with said at least one aperture of said second ball bearing cage; wherein said at least one ball bearing is accommodated within the respective overlapping apertures of said ball bearing cages and at least one control aperture of said first ball bearing cage is configured to have a predetermined length in one dimension that is at least 2% greater than a predetermined length in a direction perpendicular to said one dimension, such that a clearance relative to a path traced by said corresponding ball bearing in said one dimension is greater than a clearance in said perpendicular direction.
2. A thrust bearing assembly according to claim 1, wherein said at least one control aperture comprises a distorted circular aperture, with an elongated diameter in said one dimension.
3. A thrust bearing assembly according to claim 1 or 2, wherein said at least one aperture of said second ball bearing cage is substantially circular.
4. A thrust bearing assembly according to any preceding claim, wherein said ball bearing cages comprise a portion of a sector of a circle and said one dimension comprises a radial direction.
5. A thrust bearing assembly according to any preceding claim, wherein said ball bearing cages comprise a portion of a sector of a circle and comprise a -19 -plurality of apertures at radially inner and radially outer positions, said at least one control aperture comprises one of said radially outer apertures of said first ball bearing cage.
6. A thrust bearing assembly according to claim 5, wherein said first ball bearing cage comprises a plurality of control apertures at each end of said radially outer apertures.
7. A thrust bearing assembly according to claim 6, wherein said plurality of io control apertures comprise apertures at each end of said radially outer and radially inner apertures.
8. A thrust bearing assembly according to any one of claims 5 to 7, wherein said plurality of apertures of said first ball bearing cage, comprise at least one control aperture and at least one further aperture, said at least one further aperture comprising circular apertures configured with a diameter that is greater than a smaller dimension of said at least one control aperture.
9. A thrust bearing assembly according to any preceding claim, wherein said first ball bearing cage comprises a portion of a sector of a circle and said first ball bearing cage comprises at least one distortable member on radially inner and radially outer surfaces.
10. A thrust bearing assembly according to any preceding claim, wherein said first ball bearing cage comprises a portion of a sector of a circle and said first ball bearing cage comprises at least one distortable member on radially extending side surfaces.
11. A scroll pump comprising: 3o two scroll members comprising interleaving scrolls mounted for relative orbital motion with respect to each other; -20 -at least one thrust bearing assembly according to any preceding claim for axially supporting one scroll member with respect to the other scroll member; said first and second ball bearing cages being coupled to respective scroll members.
12. A scroll pump according to claim 11, said scroll pump comprising three thrust bearing assemblies arranged at different angular positions of said scroll members.io
13. A scroll pump according to claim 11 or 12, wherein said first ball bearing cage of said at least one thrust bearing assembly is mounted within a pocket of said first scroll member, said pocket being sized such that said ball bearing cage can move in a circumferential direction along said pocket.
14. A scroll pump according to any one of claims 11 to 13, comprising at least one thrust bearing assembly according to 9 or claim 10 when dependent on claim 10, wherein said pocket and said at least one distortable member on said radially inner or outer surfaces are configured such that said first bearing cage is held within said pocket with a force that is greater than a force on said first bearing cage due to gas load during pumping and less than a force on said first bearing cage during an initial start up of said pump.
15. A method of mounting at least one thrust bearing according to any one of claims 1 to 10, within a scroll pump comprising two scroll members comprising interleaving scrolls mounted for relative orbital motion with respect to each other, said method comprising: mounting said thrust bearing to said scroll pump such that a first thrust plate is mounted on a radially extending surface of a scroll member, a first ball bearing cage is within a recess on said radially extending surface and an 3o adjustment pin is attached to the scroll pump housing and extends axially into a recess in a second thrust plate adjacent to a second ball bearing cage, the first -21 -and second ball bearing cage comprising overlapping apertures retaining ball bearings; adjusting said adjustment pin to provide a desired axial clearance between the two scroll members of the scroll pump.
16. A method according to claim 15, comprising a further step of aligning said first ball bearing cage by initiating rotation of said scroll pump such that said first ball bearing cage slides in a circumferential direction in said recess.