WO2013114093A2

WO2013114093A2 - Pump

Info

Publication number: WO2013114093A2
Application number: PCT/GB2013/050188
Authority: WO
Inventors: William Malcolm GRAY; Paul David Neller; Neil Turner; Jack TATTERSALL
Original assignee: Edwards Ltd
Priority date: 2012-01-30
Filing date: 2013-01-29
Publication date: 2013-08-08
Also published as: GB2498807A; US20150037187A1; CN104126053A; JP2015509165A; EP2809880A2; TW201341663A; KR20140119711A; WO2013114093A3; GB201201555D0

Abstract

The present invention relates to a multi-stage vacuum pump 10 which has a plurality of pumping stages 12, 14, 16, 18. The pump comprises a stator forming the pumping chambers 32, 34, 36, 38 of respective pumping stages and at least one shaft 40, 42 for supporting a plurality of rotors 44a, 44b; 46a, 46b; 48a, 48b; 50a, 50b for rotation in respective pumping chambers. The stator comprises: a stator envelope 20 which extends over an axial extent of a plurality of stator stages thereby circumscribing said at least one shaft and forming an internal profile of a plurality of stator stages: and a plurality of transverse walls 22, 24, 26, 28, 30 located on either axial side of the stator stages to form respective said pumping chambers. At least one of the transverse walls is an inter-stage 26, 28, 30 between stator stages and the stator envelope is configured to circumscribe the inter-stage for location of said inter-stage radially inward of the stator part.

Description

PUMP

The invention relates to a multi-stage vacuum pump and a stator of such a pump. A vacuum pump may be formed by positive displacement pumps such as roots or claw pumps, having one or more pumping stages connected in series. Multi-stage pumps are desirable because they involve less manufacturing cost and assembly time compared to multiple pumps in series.

Multi-stage roots or claw pumps are typically manufactured and assembled in one of two common forms, that is, as a stator stack or clamshell.

A pump 110 having a stator stack arrangement is shown in Figure 10. Pump 110 comprises a plurality of pumping stages, 112, 114, 116, 118. Each of the pumping stages comprises a rotor arrangement (not shown) and a stator arrangement 120, 122, 124, 126 for pumping fluid from an inlet to an outlet of each stage. The outlet of one pumping stage is in fluid communication with an inlet of the adjacent downstream stage so that the compression achieved by the pump is cumulative of each of the stages. Inter-stage arrangements 128, 130, 132 interpose adjacent pumping stages. The inter-stage arrangements separate the pumping chambers of adjacent pumping stages and convey fluid from the outlet of an upstream pumping stage to the inlet of a downstream pumping stage. Two head plates 134, 136 are located at each end of the pumping stack. The head plates separate the pumping chambers of the most upstream and most downstream pumping stages, respectively, from other components of the pump, such as gears and motor, and convey fluid into the inlet of the first pumping stage and from the outlet of the final pumping stage. Accordingly, the pump is manufactured from a plurality of discrete layers which are laminated together to form the pump. Lamination may suitably be achieved by one or more anchor rods which pass through apertures in each of the layers and fastened with fasteners such as bolts. Herein, the stator arrangements will be referred to as stator slices and the inter-stage arrangements simply as inter-stages.

A section through the pump 110 is shown in Figure 11. The rotor arrangement is shown in Figure 11 and comprises a plurality of rotor stages 138, 140, 142, 144 each comprising in this example a pair of co-operating rotors A, B. Only the rotors in the first rotor stage 138 are referenced A, B for the sake of not over-populating the drawing with reference numerals. The rotors A are supported for rotation by shaft 146 and the rotors B are supported for rotation by shaft 148. On rotation the rotor stages pump fluid from an inlet of the stage to an outlet of the stage such that fluid is pumped through the stages from the pump inlet (IN) to the pump outlet (OUT).

In assembly, the individual components of the pump are stacked together in order. A first head plate 134 and shafts 146, 148 are assembled. The stator stage 120 is positioned in location against the head plate 134 typically with dowels. One or both of the head plate and stator stage may comprise annular grooves which receive an O-ring for sealing the interface between the head plate and the stator stage. The rotors 112 are fitted on the shafts and may have a keyed arrangement for locating the rotors in the correct position. The inter-stage 128 is then fitted against the stator stage 120, again typically with the use of dowels and having a O-ring for sealing between the interface. The remainder of the pump stack is assembled in similar fashion and the stack may be clamped to resist movement in the axial direction between components. In some arrangements each inter-stage is integral with an adjacent stator stage and this integrated component is assembled similarly as described above. It will also be apparent that the pump may be assembled in a different order, for example, the head plates may be fitted last.

In an alternative arrangement, a stator component may comprise two of the previously mentioned stator components, for example parts 122 and 130 may be integral, provided such integrated components form no more than one inter-stage.

The axial spacings, or clearances, between the rotors and the inter-stages or head plates must be controlled accurately because otherwise pumped fluid may leak from a low vacuum region of the pumping chamber to a high vacuum region through the axial clearances. Whilst the components are machined accurately, there are inevitably variations in the component configurations which require tolerances to be imposed on pump design that potential increase axial clearances between the rotors and the inter-stage or head plate. The pump stack suffers from an accumulation of tolerances provided by each of the many interfaces between components. In the illustrated pump there are eight such interfaces. It will be seen therefore that relative location of the rotors and the inter- stages or head plates cannot be controlled accurately, which either leads to leakage of pumped fluid or contact between the rotors and inter-stages or head plates.

Variation in component sizes can result in excessive clearance or inadequate clearance leading to seizure. To eliminate the possibility of seizure, the nominal clearances will be increased, leading to an increased likelihood of excessive clearance and impaired vacuum performance. In turn this may result in a need for additional pumping stages, with the associated increase in complexity and cost.

Additionally, as each interface requires sealing, a large number of interfaces requires a large number of seals, each of which is potential source of leakage. The seals may not be assembled correctly; the O-rings degrade over time by chemical erosion; and imperfect sealing faces abutting the O-rings all contribute amongst other things to reduced sealing.

The requirement for O-rings increases the cost of the pump and adds additional machining for the O-ring grooves. The dowels and dowel holes also contribute to the cost of manufacture.

A modified stator stack arrangement is disclosed in EP0480629. This document discloses a stack of stator parts 16 which are joined together end of end. Inter-stages 17 are located radially inside respective stator parts. The outer perimeter of the inter-stages and the axial interface between stator parts are sealed with O-rings. This arrangement suffers from many of the disadvantages of the stator arrangement described in more detail above. The interfaces between stator parts require sealing and fastening together to prevent pumped fluid from escaping from the pump. There is also inevitably an accumulation of axial tolerances which restricts the ability to design an accurate pump.

An alternative pump arrangement comprises a so-called clamshell as illustrated in Figure 12. The pump 150 comprises two stator parts, or shells 152, 154. The stator part 154 is shown from the drawings perspective in more detail and the stator part 152 has a corresponding configuration. The stator part 154 comprises head plate portions 164, 166 and inter-stages 168, 170, 172, 174, 176 which together with part 152 form stator stages 155, 156, 157, 158, 159, 160. The head plates and the inter-stages have recesses 178, which when assembled receive two shafts on which the rotors are supported.

In assembly, the rotors and shafts (not shown) are brought together as shown by the arrows in Figure 12 and located in place by dowels, then sealed and clamped to form the pump. The interfaces 174 between the first and second stator parts are typically sealed with a gasket or sealant, which is inherently less resistant to leakage than the previously discussed O-rings.

The radial spacing or tolerances between the rotors and the stator stages is required to be tightly controlled so that the rotors may efficiently sweep the internal surface of the pumping chambers during rotation and resist the leakage of fluid past the rotors. In a clam shell stator, the radial tolerances are larger because the stator profile cannot be machined as easily or accurately as the bore of a stator stack. Additionally axial tolerances are required in case of potential misalignment between two stator halves.

The present invention provides an improved vacuum pump.

The present invention provides a multi-stage vacuum pump comprising: a stator forming a plurality of pumping chambers and at least one shaft for supporting a plurality of rotors for rotation in respective pumping chambers, the stator comprising: a one piece stator envelope enclosing a plurality of axially adjacent pumping chambers about the shaft and at least one inter-stage transverse wall located radially inwardly of the stator envelope and between axially adjacent pumping chambers.

The invention also provides a stator for the multi-stage pump.

In order that the invention may be well understood, some embodiments thereof, which are given by way of example only, will now be described with reference to the drawings in which:

Figure 1 shows schematically a cross-section through a multi-stage vacuum pump; Figure 2 shows a radial cross-section taken along line II-II in Figure 1 ;

Figure 3 is a simplified view of the stator part shown in Figure 1;

Figures 4a, 4b, and 4c show a modified stator part together with rotors and transverse walls during assembly; and Figure 5 shows another modified stator part and transverse walls;

Figure 6a to 6d show fastening of a transverse wall to the stator part;

Figure 7 shows another was of fastening a transverse wall to the stator part;

Figure 8 shows an inter-stage transverse wall in more detail;

Figure 9A is a cross-section through a modified pump and Figure 9B is an enlarged view of a fixing arrangement;

Figure 10 shows a vacuum pump having a stator stack arrangement;

Figure 11 shows a cross-section taken through the vacuum pump shown in Figure

10; and

Figure 12 shows a perspective view of a clamshell type vacuum pump.

Referring to Figure 1, a multi-stage vacuum pump 10 is shown having a plurality of pumping stages 12, 14, 16, 18. In this example, four pumping stages are provided but there may be two, three or more than four pumping stages depending on requirements. The pump comprises a stator comprising components 20, 22, 24, 26, 28, 30 forming the pumping chambers 32, 34, 36, 38 of respective pumping stages. Figure 1 shows a roots or claw type pump in which there are two shafts 40, 42 that support a plurality of intermeshing rotor pairs 44a, 44b; 46a, 46b; 48a, 48b; 50a, 50b for rotation in respective pumping chambers 32, 34, 36, 38. Other types of pumping mechanisms fall within the scope of the present invention, such as a rotary vane pump having a single shaft supporting a single rotor in each pumping chamber.

As shown in Figure 1, the stator comprises a one-piece stator part, or envelope, 20 which circumscribes the axial shafts and forms an internal profile 52 of a plurality of stator stages. This arrangement is to be distinguished from the known stator stack arrangement in which a stator part circumscribes the shafts but defines the internal profile of one stator stage only. Likewise, the present arrangement is to be distinguished from the known clam-shell arrangement in which a stator part only partly extends around the shaft and has an internal profile which forms part of the stator stages of a plurality of pumping chambers.

Reference is also made to Figure 2 which shows a radial cross-section taken through the pump in Figure 1 along line II-II through pumping stage 16. It will be appreciated that the pumping stages 12, 14 and 18 have similar cross-sections. The stator part 20 forms the internal profile 52 of each of the four stator stages. In use, the internal profile 52 of the stator stages are swept by the rotors 44 to 50 during rotation. A small clearance between the radial extremity of the rotors and the internal profile 52 is maintained to allow for expansion during use. The outer profile 54 of the stator part 20 may be any suitable shape such as a block. Preferably however the outer profile is relatively thin to allow heat transfer away from the pumping chambers and in this case the outer profile 54 may be approximately the same shape as the internal profile.

Referring again to Figure 1, a plurality of transverse walls 22, 24, 26, 28, 30 are located on either axial side of the stator stages to form the pumping chambers 32, 34, 36, 38. Transverse walls 22, 24 are so-called head plates located at axial ends of the stator part. One of the head plates may if required be formed integrally with the stator part 20. The transverse walls 26, 28, 30 are so-called inter-stages as they are located between two adjacent stator stages. The stator part 20 is configured to circumscribe the transverse walls 26, 28, 30 for location of the transverse walls radially inward of the stator part. Fixing means 56 fix the inter-stages in location. The present invention relates to a multistage pump having two or more stages and in a pump having only two stages only one inter-stage need be located radially inwardly of the stator part 20. The stator part encloses the axis or axes of the pump and extends through 360°, unlike the previously discussed clamshell arrangement in which each stator part extends about the axis or axis only about 180°. Additionally, the stator part defines a plurality of stator stages which together with one or more inter-stages forms a plurality of pumping chambers. This arrangement is unlike the previously discussed stator stack arrangement in which a stator part encloses the axes of the pump and extends through 360° but each stator part defines only a single stator stage.

Accordingly, the stator part or enclosure has a longitudinally, or axially, extending internal cavity, that extends partially or fully through the enclosure. Figure 3 shows the cavity 64 of the stator part 20 without other parts of the pump for clarity. The locations 60 of the head plates and the locations 62 of the inter-stages are shown by broken lines. Formations 58 are shown at which the inter-stages can be fixed in location and are described in greater detail below. As shown in Figure 3, the cross-section through each of the stator stages is generally uniform and the cross-section from one stage to the next is also uniform. The inter-stages are generally therefore of similar size and shape.

In an alternative arrangement shown in Figure 4, the cross sections of each stage may be uniform but vary from one stage to the next. The stator stages have greater volume towards the right in the diagram which are typically the upstream pumping stages of the pump. Figure 4 shows schematically three manufacturing phases of the pump. In Figure 4a, the stator part 20 is shown together with two inter-stages 26, 28 in an unassembled condition. In a partially assembled condition in Figure 4b, a first rotor or rotors 44 are inserted through the internal cavity 64 and located in position on one or more shafts (not shown). A first inter-stage 26 is then inserted through the cavity and locked in position. Subsequently, a second rotor or rotors 46 are located in position on the shafts and the second inter-stage 28 is inserted through the cavity 64 and locked in position. In a fully assembled condition in Figure 4c, a third rotor or rotors 48 are locked in place on the shaft or shafts and head plates 22, 24 are fixed in position at axial ends of the stator part 20.

In a still further alternative, the axi al spacing A¹, A², A³, A⁴, A⁵ (Figure 5) between adjacent transverse walls may vary from one stator stage to the next according to pumping requirements. In one example, the inter-stages may be located at predefined formations of the stator part, as previously discussed. In this regard, the fixing means are configured for locating and fixing at least one and preferably all of the inter-stages at any selected location along the axial extent of the stator part. The inter-stages may be interference fitted inside the stator 20 and may have a radial outer perimeter which comprises a sealing material for sealing against the inner surface of the stator 20.

In another example as shown in Figure 5, the location of the inter-stages in the stator part is not predefined and can be selected in situ by the operative assembling the pump. In selecting the interstage location at the time of assembly, it is possible to eliminate from the clearance some of the allowance for manufacturing variation in the rotor and stator components, thus improving the machine' s performance without increasing manufacturing cost. In this regard, one or more inter-stage may be arranged to float between adjacent rotors. A floating inter-stage is not fixed to the stator and is free to move in the axial direction. Angular movement of the inter-stage is prevented by virtue of the complementary shape of the inter-stage and the inner surface of the stator. Axial movement is restrained by the axial ends of the rotors adjacent the floating inter-stage. Preferably, the inter-stage and the axial ends of the rotors are coated with a friction reducing material to reduce resistance to rotation of the rotors and to reduce frictional heating. This arrangement allows a further reduction in the total axial clearance required since the floating inter-stages will be located in position by the adjacent rotors rather than by fixing to the stator envelope and because much of the allowance for thermal expansion can be removed from the pump clearance.

Figure 6 shows in more detail an arrangement of the fixing means 56 shown in Figure 1. Figure 6a shows an enlarged section of portion VI shown in Figure 1 looking in a tangential direction, whilst Figure 6b shows the same portion looking in an axial direction. Figures 6c and 6d show a fastener 58 prior to fitting.

A fastener 58 comprises a fixing part 66 which in a first condition allows an interstage 26 to be inserted through the stator part 20 and in a second condition allows the inter-stage to be fixed in location. A head part 68 is operable for transferring the fixing part between first and second conditions. The fixing part comprises a partially arcuate flange preferably having a thickness which tapers. The stator part 20 has an undercut groove 70 formed in the internal profile 52 for receiving the fixing part in the second condition. The inter-stage 26 has a cavity 72 for receiving the fastener. The cavity opens radially outwardly to allow the arcuate flange to project from the inter-stage when the flange is in the second condition and opens axially to allow an operative to insert a tool into the head part for operation. The head part is shaped to receive a complementarily shaped tool for rotating the fixing part between conditions.

Preferably, at least three fasteners 58 are provided around the periphery of the inter-stage for fixing the inter-stage to the stator part.

In use, the inter-stage 26 is inserted through the stator part whilst the fixing part 66 is in the first condition. A lip 78 extending radially inwardly from the inner profile 52 of the stator part may be provided for locating the inter-stage. When at the correct location, the tool is used to rotate the flange of the or each fastener 58 so that it projects into the undercut groove 70 of the stator part 20. The thinnest part of the flange enters the groove first and continued rotation causes a thicker part of the flange to engage with axial faces 74, 76 of the groove for accurately locating and locking the inter-stage in position.

In an alternative fixing arrangement shown in Figure 7, a closed bore 80 is formed at an oblique angle in the internal profile 52 of the stator part 20. A through bore 82 is formed in the inter-stage extending from an axial end face obliquely through to a radial periphery of the inter-stage. The fastener is inserted through aligned bores 80, 82 to fix the inter-stage in location. One or both of the bores 80, 82 may be threaded to receive a threaded fastener 58. The fastening of the fastener in the threaded bore may be arranged to expand the inter-stage to a small extent against the inner wall of the stator to improve sealing.

In a still further arrangement, the inter-stages may be interference fitted to fix them in position in the stator envelope. In this case, the stator envelope need not be provided with fixing formations and may have a smooth inner surface. Alternatively, the inner surface may be provided with annular lips for locating the inter-stages in position prior to fixing. In assembly, an inter-stage is made of a material which undergoes thermal expansion, such as a metal or metal alloy. Prior to insertion in the stator envelope, an inter-stage is cooled by any suitable means so that it contracts. Preferably, it is contracted so that its outer profile just fits within the stator envelope and therefore can be inserted along the envelope until it abuts an annular lip. The inter-stage is then allowed to warm under ambient temperature conditions so that it undergoes thermal expansion and is interference fitted in position. In an alternative, the stator envelope may be heated so that it undergoes thermal expansion to allow the inter-stage to be inserted and then allowed to cool to produce the interference fit.

An exemplary inter-stage transverse wall 26 is shown in Figure 8 for a claw type pump. The external profile 84 of the inter-stage is shaped to correspond with the internal profile 52 of the stator part so that the inter-stage can pass in an axial direction through the internal cavity 64 of the stator part during assembly. The inter-stage 26 comprises two bores 86, 88 for receiving shafts 40, 42 (shown in Figure 1). One of the bores 88 is configured to provide an outlet from an upstream pumping chamber to an inlet of a downstream pumping chamber. The profile 84 in this example is suited for a roots or claw type pumping arrangement in which the rotors rotate generally in respective pumping chamber portions and left and right hand lobes of the inter-stage as shown in the Figure are configured complementarily with the respective pumping chamber portions. In an alternative arrangement, particularly if the rotor and inter-stage sub-assembly is assembled prior to insertion in the stator envelope the inter-stages may be formed by two parts in order to be fitted to rotor and drive shaft.

A modification of the Figure 1 embodiment is shown in Figures 9A and 9B. Like features of the two embodiments will be given like reference numerals and the description of Figure 9 will omit any aspects already covered above.

In Figure 9, the way in which the inter-stages are fastened to the one-piece stator part is different from the Figure 1 arrangement. In more detail, a one-piece stator component 90 comprises a plurality of through bores 94 aligned with the inter-stages 91, 92, 93. A fastener 95 is configured for extending partially through each of the through bores and engaging with a closed bore 96 of an inter-stage. The through bores 94 have countersunk shoulders 97 for locating the fasteners in the radial direction. Figure 9A shows six such fastening arrangements and Figure 9B shows an enlarged view of one such arrangement as marked by the circle IXB in Figure 9A.

The pump shown in Figure 9A may be assembled by first assembling a rotor and inter-stage sub-assembly. The sub-assembly may then be inserted into the stator component 90. When in place the sub-assembly is fastened in position by fastening each of the inter-stages 91, 92, 93 to stator component 90 with fasteners 95. After fastening the through bore 94 is preferably closed with a closure member 98 and sealed with sealant. In this way, the rotors can be fixed relative to the shafts 40, 42 prior to insertion in the stator envelope 90 and therefore the angular alignment of the multiple rotors can be more accurately controlled. In a further modification, the rotors may be manufactured integrally with the shafts. However, if this is the case, each of the inter-stages must be made from at least two components which can be assembled together in between the rotors. Whilst the modified arrangements may be more susceptible to leakage than the Figure 1 arrangement they benefit from increased rotor alignment. Depending on the particular pumping requirement, accurate rotor alignment may be desirable even if leakage may increase.

In a modification of the Figure 9 arrangement, the fixings may be similar to those described in relation to Figure 6. The fastener with arcuate flange may be located in the stator envelope and accessible through a bore by in the envelope for rotating the fastener with a tool so that it engages and locks the inter-stage in position.

In accordance with the discussed embodiments, the rotors and inter-stages can be alternatively assembled within the stator. When an inter-stage is positioned within the stator 20 it can then be locked in position. This arrangement means that there is a reduced requirement for sealing since pumped fluid is always maintained within the stator envelope. The arrangement can be contrasted with the known designs in which the stator parts must not only be fastened together, typically with bolts, but seals must be provided to prevent fluid escaping from the pump between stator parts. In the present arrangement, such seals and fasteners are not required and therefore the stator body may be made thinner since it does not have to accommodate seals or fasteners. A thinner stator is more suitable for dissipating heat from the pumping chambers. Further cooling means, such as jackets may be located closer to the source of thermal increase. Since heat can readily be dissipated, the thermal characteristics of inter-stages is less important so that the interstage material can be primarily selected for other characteristics such as anti-corrosion.

The reduced functional and mechanical requirements of the inter-stages means that the choice of materials from which they may be made is increased such that more exotic materials can be considered. Less material also means more expensive materials can be considered such as Nickel enriched iron, stainless steel, PTFE, composites, or Ceramics.

Since there are fewer parts connected together in axial sequence there are fewer required axial tolerances at interfaces and therefore the pump as a whole may be configured more accurately.

The internal longitudinal cavity of the stator 20 may be manufactured by machining relatively easily and accurately.

Modifications to the above described embodiments are possible whilst still falling within the scope of the claims. For example, the stator 20 is a one piece component defining each of the stator stages. However, certain advantages of the invention may still be gained by adopting two stator parts for example whereby each stator part has an internal profile which defines more than one stator stage. Accordingly, there will again be fewer axial interfaces between stator parts.

Claims

1. A multi-stage vacuum pump comprising: a stator forming a plurality of pumping chambers and at least one shaft for supporting a plurality of rotors for rotation in respective pumping chambers, the stator comprising: a one piece stator envelope enclosing a plurality of axially adjacent pumping chambers about the shaft and at least one inter-stage transverse wall located radially inwardly of the stator envelope and between axially adjacent pumping chambers.

2. A multi-stage vacuum pump as claimed in claim 1, wherein the stator envelope has an axially extending internal bore formed therethrough which forms the profiles of the respective pumping chambers.

3. A multi-stage vacuum pump as claimed in claim 1 or 2, wherein a radial cross- section of the internal bore at each pumping chamber is generally uniform in the axial direction.

4. A multi-stage vacuum pump as claimed in claim 1 or 2, wherein the radial cross- section of at least one pumping chamber is different from that of at least one other pumping chamber.

5. A multi-stage vacuum pump as claimed in any of the preceding claims, wherein at least one end of the stator envelope is open to allow insertion of said at least one inter-stage wall along said internal bore for location between two pumping chambers.

6. A multi-stage vacuum pump as claimed in any of the preceding claims, wherein the internal bore comprises a formation for locating an inter-stage wall between two pumping chambers.

7. A multi-stage vacuum pump as claimed in claim 6, wherein the formation comprises a stepped surface extending radially inwardly into the internal cavity against which the transverse wall abuts when correctly located between the pumping chambers.

8. A multi-stage vacuum pump as claimed in claim 6, wherein the formation comprises at least one closed bore for receiving a fastener for fixing the transverse wall in location.

9. A multi-stage vacuum pump as claimed in any of the preceding claims, wherein said at least one inter-stage wall comprises a bore through which a fastener can extend for fixing the inter-stage to the stator envelope.

10. A multi-stage vacuum pump as claimed in any of the preceding claims, wherein the inter-stage is configured for selective fixing at more than one location in the stator envelope.

11. A multi-stage vacuum pump as claimed in any of the preceding claims, comprising a one or more fasteners for fastening one or more of said inter-stage walls to the internal profile of the stator envelope.

12. A multi-stage vacuum pump as claimed in claim 11, wherein the fastener is shaped such that in a first condition thereof the inter-stage can pass through the internal bore of the stator envelope and in a second condition thereof the fastener fastens the inter-stage in location to the stator envelope.

13. A multi-stage vacuum pump as claimed in any of the preceding claims, wherein at least one of an axial facing surface of the inter-stage wall or axial facing surface of an adjacent rotor is coated with a friction reducing material.

14. A multi-stage vacuum pump as claimed in any of the preceding claims, wherein the inter-stage walls are not fixed at predetermined locations within the stator envelope and are allowed to float between rotors.

15. A stator configured for the multi-stage vacuum pump as claimed in any of the preceding claims.