WO2010133866A1

WO2010133866A1 - Side-channel pump with axial gas bearing

Info

Publication number: WO2010133866A1
Application number: PCT/GB2010/050801
Authority: WO
Inventors: Nigel Paul Schofield
Original assignee: Edwards Limited
Priority date: 2009-05-20
Filing date: 2010-05-18
Publication date: 2010-11-25
Also published as: US9334873B2; CN102428279A; US9127685B2; TW201109531A; JP5775513B2; US20120051887A1; JP5718906B2; EP2433012B1; CN102428280A; EP2433009A1; JP2012527569A; JP2012527568A; WO2010133868A1; JP5718907B2; US9086071B2; US20120051893A1; TW201111638A; WO2010133867A1; US20120057995A1; JP2012527570A

Abstract

The present invention provides a pump comprising a regenerative pumping mechanism. A rotor of the pump has a generally disc-shaped pump rotor mounted on an axial driveshaft for rotation relative to a stator, the pump rotor having rotor formations disposed in a surface and defining at least a portion of a flow path for pumping gas from an inlet to an outlet. The rotor formations are formed between the pump rotor and the stator of the pumping mechanism. The pump rotor and the stator comprise an axial gas bearing arrangement for controlling axial clearance between the rotor and the stator during pump operation. Thus, this configuration of pump provides a gas bearing disposed on the rotor which enables an improved control of axial clearance between the pump's rotor and stator components.

Description

SIDE-CHANNEL PUMP WITH AXIAL GAS BEARING

The present invention relates to a pump for pumping fluid media (gases or liquids). In particular, but not exclusively, the present invention relates to a vacuum pump configured as regenerative vacuum pump.

The present invention is described below with reference to vacuum pumps, although it is understood that the invention is not limited in any way to vacuum pumps and can equally apply to other types of pump, such as liquid pumps, gas compressors, or the like. Vacuum pumps which comprise a regenerative pumping mechanism are known hereto. Known regenerative pumping mechanisms comprise a plurality of annular arrays of rotor blades which are mounted on a rotor and extend axially from the rotor into respective annular channels formed in a stator. Rotation of the rotor causes the blades to travel along the channels forming a gas vortex which flows along a flow path between and inlet and an outlet of the pumping mechanism.

Examples of this type of vacuum pump are known in the art and specific variations of the pump are described in EP0568069 and EPl 170508. Regenerative pumping mechanisms described in these documents can comprise a rotor which is formed in a disc-like configuration with pump elements on either side of the rotor. The pumped gas follows a flow path arranged such that the gas flows along one side of the rotor from an inlet and is then transferred in a serial fashion to the other side of the rotor and thence onwards to an outlet.

The present invention provides an improved pump over conventional pumps. The present invention provides a pump comprising a regenerative pumping mechanism having a generally disc-shaped pump rotor mounted on an axial driveshaft for rotation relative to a stator, the pump rotor having rotor formations disposed in a surface and defining at least a portion of a flow path for pumping gas from an inlet to an outlet and being formed between the pump rotor and the stator of the pumping mechanism, the pump rotor and the stator comprising an axial gas bearing arranged to control axial clearance between the rotor and the stator during pump operation. Thus, this configuration of pump provides a gas bearing disposed on the rotor which enables and improved control of axial clearance between the pump's rotor and stator components. Alternatively, or in addition, the present invention provides a pump comprising a regenerative pumping mechanism which comprises a generally disc-shaped pump rotor mounted on an axial shaft for rotation relative to a stator, the pump rotor having first and second surfaces each having a series of shaped recesses formed in concentric circles thereon, and a stator channel formed in a surface of the stator which faces one of the pump rotor's first or second surfaces, wherein each of the concentric circles is aligned with a portion of a stator channel so as to form a section of a gas flow path extending between an inlet and an outlet of the pump, and the pump rotor divides the section of flow path into sub-sections such that gas can flow towards the outlet simultaneously along any sub-section. As a result, the gas being pumped flows in a parallel fashion along both surfaces of the rotor. Thus, this configuration can provide a pumping mechanism where gas pressures on either side of the rotor can be substantially equal or balanced.

Alternatively, or in addition, the present invention provides a regenerative pump rotor comprising a generally disc-shaped pump rotor mountable onto an axial shaft for rotation relative to a pump stator, the pump rotor having first and second surfaces each having a series of shaped recesses formed in concentric circles thereon and being configured to face a stator channel formed in a surface of a stator, wherein, during use each of the concentric circles is aligned with a portion of a stator channel so as to form a section of a gas flow path extending between an inlet and an outlet of a vacuum pump and the gas flow path is divided by the rotor such that gas can flow towards the outlet simultaneously along the first and second surfaces. Thus, this configuration can provide a pumping rotor mechanism where gas pressures on either side of the rotor can be substantially equal or balanced.

The axial gas bearing can comprises a rotor part on the pump rotor and a stator part on the stator. This configuration allows relatively easy manufacture of multiple pump parts on relatively few components.

The stator can comprise two stator portions located adjacent respective axial sides of the pump rotor, the rotor formations are disposed on each of the axial sides of the pump rotor, and the flow path is divided by the pump rotor into sub-flow paths so that gas can flow simultaneously along each axial side of the pump rotor to the outlet. In addition, the sub-flow paths can be arranged to be symmetrical about a radial centre line of the pump rotor. Additionally, first and second flow path sub-sections can be defined by first and second surfaces disposed on both sides of the pump rotor and first and second stator channels facing the respective one of pump rotor's first and second surfaces, respectively. Furthermore, a first flow path sub-section defined by the first stator channel and a second flow path sub-section defined by the second stator channel can be arranged to pump an equal volume of gas. Yet further, the first and second flow path sub-sections can be arranged to direct gas in the same radial direction, for example to direct gas from an inner radial position of the pump rotor to an outer radial position. This configuration provides a balanced pumping arrangement whereby pressure exerted by the pumped gases on either side of the rotor is substantially equal to one another. As a result, the axial clearance between the rotor and stator pump components can be maintained at a relatively small distance thereby reducing gas leakage between the rotor and stator, which in turn can improve pumping efficiency.

An axial gas bearing rotor component can be arranged to cooperate with gas bearing stator component for controlling the axial running clearance between the rotor and a pump's stator during a pump's operation. Furthermore, a portion of the axial gas bearing component is in the same plane as the first surface. The axial gas bearing can comprise rotor parts on each axial side of the pump rotor and which are co-operable with stator parts on respective stator portions so that gas that has been pumped along the flow paths can pass between the two parts on each axial side of the rotor. In other words, the exhaust gas can be used to supply at least a portion of the gas needed to operate the gas bearing. As a result, the pumped gases can be used to drive the axial gas bearing. The inlet of the regenerative pumping mechanism can be located at a radially inner portion of the pump and the outlet is located at a radially outer portion of the pump. Thus, the gas flow path is arranged such that gas being pumped flows from the inner portion of the mechanism to the outer portion of the mechanism. In addition, if the air bearing is located at a radial outer portion of the pump rotor and the stator proximate the outlet then the gases at higher Outlet pressures' can used to drive the bearing.

Furthermore, this arrangement can allow the axial running clearance between the pump rotor and stator to be in the order of either one of less than 40 μm, less than 30 μm less than 20 μm, or less than 15 μm. Indeed, the clearance can be approximately 8 μm. Such clearances are typically much smaller than those that can be achieved on conventional regenerative pump mechanism. As a result, pumped gas leakage between the rotor and stator can be minimised, thereby leading to a potential improvement in pump efficiency and/or throughput.

Furthermore, surfaces of the pump's mechanism can be coated with a material that is harder than the material from which the component is made. For instance, at least one of the pump rotor surface having rotor formations disposed therein; a stator surface facing the pump rotor surface; or a surface of the pump rotor or stator comprising the axial gas bearing can be coated with such material. The coating material can be any one of a nickel PTFE matrix, anodised aluminium, a carbon-based material, or a combination thereof. What is more, the carbon-based material can be any one of Diamond- like material, or synthetic diamond material deposited by a chemical vapour deposition (CVD) process. Such hard coatings can be used to help protect the pump components from wear. Also, the coating can help prevent particulates entrained in the pumped gas stream from entering the clearance space between the pump rotor and stator. First and second surfaces of the pump rotor can be arranged parallel to one another. Also, advantageously the first and second surfaces can be arranged to have flat surfaces (that is planar surfaces) wherein the plane of the first surface is parallel to the plane of the second surface. Furthermore, a portion of the axial gas bearing component can be arranged to be in the same plane as either the first or second surface. As a result, the surfaces can be machined, lapped or polished to a relatively high degree of flatness. This can help maintaining a small axial clearance between the rotor and stator pump components.

Other preferred and/or optional aspects of the invention are described herein and defined in the accompanying claims. In order that the present invention may be well understood, an embodiment thereof, which is given by way of example only, will now be described with reference to the accompanying drawings, in which:

Figure 1 shows schematically a vacuum pump; Figure 2 is a plan view of a rotor of the vacuum pump shown in Figure 1 ;

Figure 3 is a plan view of a stator of the vacuum pump shown in Figure 1 ;

Figure 4 shows in more detail a rotor formation of the rotor shown in Figure 2; and

Figure 5 shows in more detail an alternative rotor formation. Referring to Figure 1, a vacuum pump 10 is shown which comprises a regenerative pumping mechanism 11. The vacuum pump has an inlet 13 for connection to an apparatus or chamber to be evacuated, and an outlet 15 which typically exhausts to atmosphere. The vacuum pump shown in Figure 1 further comprises a molecular drag pumping mechanism 90 disposed upstream of the regenerative mechanism and which is explained in more detail below.

The regenerative pumping mechanism comprises a generally disc-shaped rotor 12 mounted on an axial shaft 14 for rotation relative to a stator 16. The shaft is driven by a motor 18 and may rotate at speeds of between 10,000 rpm and 75,000 rpm and preferably at around 40,000 rpm. The rotor 12 has a plurality of rotor formations 20 for pumping gas along channels 22 in the stator along a flow path between an inlet 24 and an outlet 26 of the pumping mechanism when the rotor is rotated. The inlet and the outlet are shown in more detail in Figure 3. As explained in more detail below, the rotor formations are recesses formed in each of the planar axially facing surfaces of the rotor. The rotor 12 and the stator 16 comprise an axial gas bearing 28 for controlling axial clearance X between the rotor and the stator. A passive magnetic bearing 30 controls the radial position of the rotor 12 relative to the stator 16.

The axial gas bearing 28 comprises a rotor part 32 on the pump rotor and a stator part 34 on the stator. The bearing is located at a low vacuum, or atmospheric, part of the pumping mechanism proximate the outlet 26. The gas bearing is beneficial because it allows a small axial running clearance between rotor and stator which is necessary for reducing leakage of pumped gas from the channel and producing an efficient small pump. Typical axial clearances achievable in embodiments of the invention are less than 30 μm and even in the range of 5 - 15 μm.

Although an air bearing is able to produce small axial running clearances, air bearings are not well suited to carrying relatively heavy loads. Accordingly, in Figure 1, the stator 16 comprises two stator portions 36, 38 located adjacent respective axial sides 40, 42 of the rotor and the rotor comprises rotor formations 20 on each axial side thereof for pumping gas through channels 22 in respective stator portions 26, 28 along respective flow paths between inlets 24 and outlets 26. In this way, the flow path is split or divided by the rotor such that sub-flow paths are mirrored about an axial centre line of the rotor 12: the pumped gas flows in parallel along both sides of the rotor. Forces generated during pumping are generally balanced (i.e. there is no net loading exerted by the pumped gas) to such an extent that the air bearing 28 is able to resist the applied loading. In other words, the gas being pumped and compressed by the pumping mechanism will exert an axial load on the rotor and stator of the pumping mechanism. The arrangement described above results in a net axial load being applied to the rotor which is substantially equal to ON (Newtons) because the axial loads on either side of the rotor are typically equal and applied in opposite directions so as to cancel one another out.

The rotor comprises at least one through-bore 25 shown in broken lines in Figure 1 for allowing the passage of gas therethrough from one axial side of the rotor to the other axial side of the rotor. The through-bore allows gas to be pumped along flow paths on each axial side of the rotor.

In order to control the axial clearance between the upper surface 40 of the rotor and stator portion 36 and the axial clearance between the lower surface 42 of the rotor and stator portion 38, the axial gas bearing 28 comprises rotor parts 44, 46 on each axial side of the rotor. The rotor parts 44, 46 are co-operable with stator parts 48, 50 on respective stator portions 36, 38 so that gas in the exhaust region feeds into the space between the bearing components and controls the axial clearances X between the rotor and both the stator portions. What is more, gases pumped along the flow paths can pass between the two parts 44, 48; 46 50 on each axial side of the rotor and form at least a portion of gas utilised in the bearing.

As shown in more detail in Figures 1 and 3, the inlets 24 are located at a radially inner portion of the pumping mechanism 11 and the outlets 26 are located at a radially outer portion of the pumping mechanism. The radially outer portion of the mechanism is at relatively higher pressure than the radially inner portion. Typically, the pump exhausts to atmosphere or relatively low vacuum. The gas bearing is located at the radial outer portion of the pumping mechanism at low vacuum since the gas bearing requires a sufficient amount of gas to support the rotor relative to the stator. In prior art regenerative mechanisms, the inlet is typically located at a radial outer portion and the outlet is located at a radially inner portion. However, when using a gas bearing it is preferable to locate the bearing at an outer radial portion of the rotor and the stator because it provides greater stability and can more accurately control the axial clearance X. Therefore, in the present embodiment, the inlet and outlet locations are interchanged so that the gas bearing is at an outer radial portion proximate the relatively high pressure outlet so that not only does it receive sufficient gas for operation but also it provides greater support and stability. An additional advantage to providing the outlet of the pumping mechanism at an outer radial portion is that particulates entrained in the gas flow are generally urged by centrifugal force towards the outlet and out of the pumping mechanism. The gas bearing will now be described in more detail with reference to Figures 2 and 3. Figure 2 shows a plan view of an upper axial side 40 of the rotor 12 and Figure 3 shows a plan view of stator portion 36.

In Figure 2, the rotor part 32 of the gas bearing is located at an outer radial portion of the rotor and comprises a plurality of bearing surfaces 52 distributed equally about the circumference of the rotor to provide a symmetrical bearing force on the rotor. The bearing surfaces are level with, or in the same plane as, the upper surface 40 of the rotor. Respective recessed portions 54 are located at the leading edges of bearing surfaces 52 with respect to a direction R of rotation (anti-clockwise in this example). In this example, the recessed portions 54 each comprise two recessed surfaces 56, 58 recessed by different depths from the bearing surface and decreasing in depth towards the bearing surface. The recessed surface 56 is relatively deep in the region of lmm from the upper surface 40 of the disc 12. The recessed surface 58 is relatively shallow in the region of 15 μm from the upper surface 40. The stator part 48 shown in Figure 3 comprises a planar circumferential bearing surface 60 which extends through a radial distance comparable to that of the rotor bearing surface 52. The bearing surface 60 is level with, or in the same plane as, the planar surface 69, 71 of the stator portions 36, 38. It will be appreciated that in an alternative arrangement the bearing surfaces 52 may be provided on the stator and the circumferential bearing surface 60 may be provided on the rotor.

In use, the deeper recessed surfaces 56 together with bearing surface 60 of the stator trap ambient air or gas exhausted through outlet 26. Rotation of the rotor causes the trapped gas to be urged between stepped surface 58 and stator surface 60 thereby increasing in pressure as it is compressed by the shallower depth of the intermediate pocket. The step between the deeper pocket and the bearing surface allows a more gradual increase in pressure and therefore promotes the flow of gas between the bearing surface 52 and stator surface 60. Gas is subsequently urged between bearing surface 52 and stator surface 60 further increasing in pressure as the gas is compressed. The axial clearance X is controlled by the distance between bearing surface 52 and stator surface 60 where the relatively high pressure gas supports the rotor and resists axial movement relative to the stator. That is, the bearing arrangements on both axial sides of the rotor together resist movement in both axial directions. Typically, the axial clearance between bearing surface 52 and stator surface 60 is between 10 and 30 μm and preferably 15 μm. The leading edges 62 between the bearing surface 52 and recessed portion 54 are angled with respect to a radial direction (shown in broken lines) so that particulates along the flow path or paths are directed downstream towards the pump outlet 15 by the leading edges 62 during use by the action of centrifugal force. In this example, the angle is approximately 30° although other angles may be adopted as required. Similarly, the intersections 64 between the recessed surfaces 56, 58 are angled with respect to the radial direction also so that particulates along the flow paths are directed towards the outlet. The angle of the intersections 64 and the leading edges 62 are preferably the same so that gas travelling over the surface 58 or the bearing surface 52 travels approximately the same distance at an inner radial location and an outer radial location so that pressure is generally equal across the surfaces. There is a small difference between such angles as the tangential speed of the rotor is greater at an outer radial location than at an inner radial location of the surfaces. The air bearing surfaces may be made from a ceramic or coated with a ceramic since such materials provide a relatively flat and low friction surface suitable for gas bearings. When operation of the rotor is commenced the rotor and the stator are initially in contact and rub until the speed reaches about 1000 rpm. Once the rotor builds sufficient speed the gas bearing supports the rotor away from the stator. Preferably therefore, the surfaces of the gas bearing are very smooth or self- lubricating.

The relative radial positioning of the rotor and the stator is controlled by a passive magnetic bearing 30 shown in Figure 1. In an alternative arrangement a ball bearing may be adopted. However, a magnetic bearing provides a dry bearing which is preferred for many vacuum pump applications. Further, in a small pump of this kind which is configured to be run at relatively high speeds, the combination of a gas bearing and a magnetic bearing provides a contact free bearing arrangement with relatively little resistance to rotation. Additionally, the gas bearing resists relative movement of the magnetic bearing elements in the axial direction. A back-up bearing may be provided (not shown) in case of failure of the magnetic bearing. The regenerative pumping mechanism of the present embodiment will now be described in more detail with reference to Figures 2 to 5.

The planar surfaces 40, 42 of the rotor are closely adjacent and parallel to the planar surfaces 69, 71 of the stator portions 36, 38. The rotor formations 20 of the rotor 12 are formed by a series of shaped recesses (or buckets) arranged in concentric circles 66, or annular arrays, in the planar surfaces 40, 42 of the rotor. In the present embodiment, the formations are formed in both surfaces 40 and 42, although in other arrangements, the rotor recesses may be provided in only one axial side of the rotor. In Figure 2, seven concentric circles of recesses 20 are shown, however, greater or fewer numbers can be provided depending on requirements. A plurality of generally circumferential channels 68 are formed in planar surface 69 of the first stator portion 36 and aligned with the concentric circles 66 formed in one face 40 of the rotor. A second plurality of generally circumferential channels 68 are formed in planar surface 71 of the second stator portion 38 and aligned with the concentric circles 66 formed in the other face 42 of the rotor. It will be noted that only three channels 68 are shown in Figure 3 for simplicity although a stator adapted for use with the rotor shown in Figure 2 would comprise seven channels aligned with each of the seven concentric circles 66.

The planar surfaces 40, 69 of the rotor and the stator on the one axial side and the planar surfaces 42, 71 on the other axial side are each separated by an axial running clearance X. As the running clearance is small, leakage of gas from the recesses and channels 68 is resisted so that a gas flow path 70 is formed on each side of the rotor from an inlet 24 to an outlet 26 of the pumping mechanism. Accordingly, when the rotor is rotated the shaped recesses generate a gas vortex which flows along the flow path. The stator channels 68 are circumferential throughout most of their extent but comprise a generally straight section 72 for directing gas from one channel to a radially outer channel. Thus, these straight sections are analogous with the so-called "stripper" sections found on conventional regenerative pumps which also act to transfer gas from one pump channel to the next. The shaped recesses 20 pass over the planar surface 69 of the rotor as shown by the broken lines in Figure 3.

In a known regenerative type pumping mechanism, the rotor formations are typically blades which extend out of the plane of a rotor surface and overlap with a plane of a stator surface. The blades are arranged in concentric circles which project into channels in the stator aligned with the concentric circles of the rotor. On rotation of such a prior art rotor, the blades generate a gas vortex compressing the gas along a flow path. There is a radial clearance between the blades or blade supporting member of the rotor and the channels which controls seepage of gas from the flow path. Operation of the pump causes the parts of the pump to increase in temperature however the rotor typically increases in temperature more than the temperature increase of the stator. The increase in temperature causes expansion of the rotor and the stator most significantly in the radial direction. As the rotor expands to a different extent to that of the stator, the radial clearance between the rotor blades or blade supporting member and the stator must be sufficiently large to accommodate the differential expansion rates so that the rotor blades or blade supporting members do not come into contact with the stator. Inevitably therefore, the radial clearance is relatively large and allows leakage of gas from the flow path.

In the present embodiment, the axial running clearance X between planar surfaces 40, 69 and 42, 71 of the rotor and the stator controls sealing of the flow path (i.e. between successive circles, or wraps, of the flow path). This arrangement is shown more clearly in Figure 1 in which three wraps are shown. Leakage of gas from a high pressure channel at a radially outer portion of the mechanism to a lower pressure channel radially inward therefrom is resisted because the axial clearance is small, preferably less than 50 μm, more preferably in the range of 10 μm to 30 μm, and most preferably about 15 μm. In the present arrangement, the gas bearing is able to provide sufficiently small axial running clearance so that seepage from the flow path is acceptably small. Moreover, there is no overlap between the rotor and the stator in the axial direction. Accordingly, any differential expansion in the radial direction between the rotor and the stator can be readily accommodated without increased seepage because expansion in the radial direction does not affect the axial clearance X between the rotor and the stator. Differential radial expansion may cause a small misalignment between the channels of the stator and the concentric circles of the rotor but such a misalignment does not significantly affect pumping. A further advantage of providing recesses in the rotor surface, rather than blades extending axially from the surface, is that recesses are more readily manufactured, for example by milling or casting. What is more, the rotor and stator surfaces can be machined, lapped or polished to a flat surface with a relatively high degree of surface flatness and to a high tolerance level. This allows the relative surfaces of the rotor and stator to pass within close distances during pump operation without clashing.

The recesses formed in the rotor will now be described in more detail with reference to Figures 4 and 5, which show respectively first and second examples of the recesses. Figure 4a shows a section taken through a circle 66 of rotor recesses 20 along central line C shown in Figure 4b. Figure 4b shows a plan view of the circle 66 of the rotor. The recesses are shaped so that in use they impart momentum to gas in a flow direction of the gas vortex along the flow path 70. That is, the recesses interact with gas along the flow path 70 to generate and maintain a gas vortex in the flow path. In addition to creating and maintaining the vortex the interaction of the recesses with the gas compresses the gas increasing vorticity or the rate at which the gas spins along the flow path.

As shown in Figure 4, a recess 20 is formed generally by an asymmetric cut in one of the planar surfaces 40 of the rotor 12. The recess has a leading portion 72 and a trailing portion 74 with respect to a direction of rotation R. The leading portion is formed by gradually increasing a depth D of the recess from an angled leading edge 76. In this regard, the leading edge 76 is angled at about 30° (+/- 10°) to the planar surface 40. The trailing portion is formed by a relatively steep decrease in depth D to a trailing edge 78. The trailing portion is at approximately right angles to the leading portion and at an angle of about 60° (+/- 10°) with the planar surface 40. The trailing portion 76 forms a curved surface which turns through about 180° with respect to direction R and approximates generally to a changing direction of flow of gas in the vortex. The ratio of distance along central line C between point 'a' and point 'b' and the width of the recess perpendicularly to the central line ' C ' is about 0.7:1.

In use, the rotor is rotated in direction 'R' and gas enters the recess at point 'a' of the leading edge 76. At point 'a' the flow direction of the vortex is generally parallel to both the curved surface 74 and the leading portion (about 30°). An arrow in Figure 4b indicates the flow direction "Air flow into blade cavity". The angle of the curved trailing portion 74 and the angle of the leading portion 72 increases the amount of gas which enters the recess as it is complimentary with the flow direction of gas in the vortex. Gas in the recess is directed around the curved trailing portion 74. It will be seen from the plan view in Figure 4b that the gas is turned through approximately 90 - 180° so that when the gas flows out of the recess it is flowing in a generally right-angular or opposite direction to when it entered the recess. Moreover the gas is turned more quickly as it approaches the exit point 'b' of the trailing portion thereby imparting momentum to the gas and compressing gas along the flow path 70. The leading portion 72 gradually increases in depth as the gas flows along the trailing portion 74 until it reaches the deepest part of the recess at point 'd'.

A second example of the recesses is shown in Figure 5. Figure 5 a shows a plan view of the recesses. Figure 5b shows a section taken along a central line C of the rotor and the stator. Figure 5 c shows a section through a recess and channel taken along a line perpendicular to central line C. Unlike the recess shown in Figure 4, the recess shown in Figure 5 is symmetrical.

The recess 20 is formed generally by a symmetric cut in one of the planar surfaces 40, 42 of the rotor 12. The recess has a leading portion 78 and a trailing portion 80. The leading portion is formed by gradually increasing a depth of the recess from an angled leading edge 82. In this regard, the leading portion is angled at about 30° (+/- 10°) to the planar surface 40. The trailing portion 80 is formed by relatively steep decrease in depth to a trailing edge 84. The leading portion transfers smoothly by a curved surface into the trailing portion. The trailing portion 76 forms a curved surface which turns through about 180° and approximates generally to a changing direction of flow of gas in the vortex. The leading edge 82 is at right angles to the central line C. In use, the rotor is rotated in direction 'R' and gas enters the recess at the leading edge 76. The flow direction of the vortex is into the recess at an angle which approximates to 30° and generally parallel to central line C. An arrow in Figure 4b indicates the flow direction "gas in". The angle of the curved trailing portion is generally aligned with the flow direction at the inlet. Gas in the recess is directed around the curved trailing portion 80. It will be seen from the plan view in Figure 4b that the gas is turned through approximately 180° so that when the gas flows out of the recess it is flowing in a generally opposite direction to when it entered the recess thereby imparting momentum to the gas and compressing gas along the flow path 70. Figure 5 c shows a flow direction of the gas vortex within the conduit formed by the recesses 20 and the stator channels 68.

A coating on either the rotor and/or stator surfaces can assist with reducing wear. During the pump's start phase, as the rotor spins-up and reaches operation speed, the surfaces of the rotor and stator are likely to contact and rub against one another. This rubbing occurs whilst the rotor is rotating at a speed below a threshold level when the axial air bearing is not operating. Above this threshold, the air bearing provides sufficient "lift" to separate the rotor and stator components. By providing a hardened and/or self- lubricating coating the amount of wear can be controlled or limited. Furthermore, a coating can assist with preventing particles entrained in the pumped gas stream from entering the clearance gap between the rotor and stator. This is perceived as a particular problem due to the relatively small gap between the rotor and stator components. If dust particles, or the like, of a certain diameter or size are able to get into this gap they could act as an abrasive subjecting the pump components to excessive wear. In a worst case scenario the pump could seize. Many suitable coatings are envisaged, but the coating material can be any one of a nickel PTFE matrix, anodised aluminium, a carbon-based material, or a combination thereof. What is more, the carbon-based material can be any one of Diamond- like material (DLM), or synthetic diamond material deposited by a chemical vapour deposition (CVD) process. It is not necessary for the coating to be of the same material on both the rotor stator - different coating can be chosen to take advantage of each coating's properties. For instance, the stator component could be coated with a self-lubricating coating, whilst the rotor is coated with diamond-like material.

In the embodiment shown in Figure 1 , the regenerative pumping mechanism 11 is in series with an up-stream molecular drag pumping mechanism 90. The molecular drag pumping mechanism 90 in this embodiment comprises a Siegbahn pumping mechanism which comprises a generally disc-shaped rotor 92 mounted on the axial shaft 14 for rotation relative to the stator. The stator is formed by stator portions 94, 96 located on each axial side of the rotor disc 92. Each stator portion comprises a plurality of walls 98 extending towards the rotor disc and defining a plurality of spiral channels 100. As the gas bearing 28 supports the rotor of the regenerative pumping mechanism and the regenerative pumping mechanism and the Siegbahn pumping mechanism are both mounted to shaft 14, the gas bearing provides axial support to the rotor of the Siegbahn mechanism. In use, a flow path through the Siegbahn mechanism is shown by arrows which passes radially outwardly over a first or upper axial side of the rotor and radially inwardly along a second or lower axial side of the rotor.

The radial location of the rotor relative to the stator is controlled by the bearing 30, which is a passive magnetic bearing. As indicated above, the bearing arrangements are both non-contact dry bearings which are particularly suitable for dry pumping environments.

The combination of the regenerative pumping mechanism 11 and the Siegbahn pumping mechanism provides a vacuum pump that is capable of pumping 10 cubic metres per hour and yet is relatively smaller than existing pumps.

Alternative embodiments of the present invention will be envisaged by the skilled without departing from the scope of the claimed invention. For instance, the through-bore 25 can comprise a series of bores disposed through the rotor. Further bores can be disposed at relatively outer radial positions to provide additional means by which gas pressure can be balanced on either side of the rotor. Alternatively, cross-feed channels can be provided in the stator to allow gas on one side of the rotor to flow to another side of the rotor if a pressure differential exists across the rotor.

Claims

1. A pump for pumping fluid, comprising a regenerative pumping mechanism comprising a generally disc-shaped pump rotor mounted on an axial shaft for rotation relative to a stator, the pump rotor having rotor formations disposed in a surface and defining at least a portion of a flow path formed between the pump rotor and the stator for pumping gas from an inlet to an outlet of the pumping mechanism, the pump rotor and the stator comprising an axial gas bearing arranged to control axial clearance between the rotor and the stator during pump operation.

2. A pump as claimed in claim 1, wherein said axial gas bearing comprises a rotor part on the pump rotor and a stator part on the stator.

3. A pump as claimed in claim 1 or 2, wherein the stator comprises two stator portions located adjacent respective axial sides of the pump rotor, the rotor formations are disposed on each of the axial sides of the pump rotor, and the flow path is divided by the pump rotor into sub-flow paths so that gas can flow simultaneously along each axial side of the pump rotor to the outlet.

4. A pump as claimed in claim 1 or 3, wherein said sub-flow paths are mirrored about a radial centre line of the pump rotor.

5. A pump as claimed in claim 1 or 2, wherein said axial gas bearing comprises rotor parts on each axial side of the pump rotor co-operable with stator parts on respective stator portions so that gas that has been pumped along the flow paths can pass between the two parts on each axial side of the rotor.

6. A pump as claimed in any one of the preceding claims, wherein the inlet of the regenerative pumping mechanism is located at a radially inner portion of the pump and the outlet is located at a radially outer portion of the pump.

7. A pump as claimed in any preceding claim, wherein said air bearing is located at a radial outer portion of the pump rotor and the stator proximate the outlet.

8. A pump as claimed in claim 1, wherein, during use, the axial running clearance between the pump rotor and stator is either one of less than 30 μm, less than 20 μm, or approximately 8 μm.

9. A pump as claimed in claim 8, wherein the rotor part of the axial gas bearing comprises said plurality of bearing surfaces and the stator part comprises a generally flat surface.

10. A pump according to claim 1, wherein at least one of; the pump rotor surface having rotor formations disposed therein; a stator surface facing the pump rotor surface; or a surface of the pump rotor or stator comprising the axial gas bearing; are coated with a material that is harder than pump rotor or stator material.

11. A pump according to claim 10, wherein the material is any one of a nickel PTFE matrix, anodised aluminium, a carbon-based material, or a combination thereof.

12. A pump according to claim 11, where the carbon-based material is any one of Diamond-like material, or synthetic diamond deposited by chemical vapour deposition.