This disclosure relates to a multi-stage vacuum pump. This disclosure also relates to a method of assembling the multi-stage vacuum pump.

BACKGROUND

Vacuum systems commonly utilise pumps in order to evacuate gases from the system. One type of vacuum pump used in such systems is a Roots vacuum pump.

A Roots vacuum pump generally includes two counter-rotating shafts with rotors mounted on each shaft. The rotors include a series of lobes and recesses defined between the lobes. The rotors are mounted such that a lobe of a rotor on one shaft cooperates with a corresponding recess of a rotor on the other shaft. As the shafts and rotors rotate, gas is trapped and compressed between the cooperating lobes and the recesses. The repeated trapping and compression of gas between the rotors can generate a pumping action that can be used to pump gas from an inlet on one side of the rotors to an outlet on the opposite side to evacuate gas from a system.

It is common for Roots vacuum pumps to feature a plurality of stages of cooperating rotors, with each stage being axially spaced apart along the shafts and separated by a stator structure that defines a series of chambers of decreasing volume that house the rotors of each stage. By having multiple stages of decreasing volume, progressive increases of gas compression can occur across the pump, allowing it to provide a higher degree vacuum for the system in an efficient manner.

Clearances are defined between the rotors and walls of the chambers in each stage of the vacuum pump. The clearances can differ in each stage depending on a variety of factors, such as rotor assembly positioning/calibration during assembly, manufacturing tolerances, and thermal expansion during operation.

When the vacuum pump is first assembled, the rotor assembly will be positioned to provide a set axial clearance between a rotor and a wall of one of the chambers. This is known as “setting off” the rotor assembly. The set axial clearance is known as the “set off distance”, which is the minimum axial distance present between any of the rotors and chamber walls in the pump after assembly of the pump. The chamber wall from which the rotor assembly is “set off’ is known as the “set face” or “set side wall” of the chamber.

The set off distance will define the total clearance available between the rotors and the chambers of the pump stages during operation. In particular, it will dictate the axial clearance available between the rotor and the opposing side wall of the chamber. This clearance typically allows for thermal expansion of the rotor assembly in the axial direction during operation of the pump, and so the opposing chamber wall to which it is defined may be known as the “expansion side wall”.

Typically, only one stage in a multi-stage vacuum pump will have a clearance equal to the set off distance. Manufacturing variability makes it virtually impossible to guarantee that more than one stage has the same clearance from the set side wall of the chambers. The designer of a multi-stage vacuum pump can choose which of its stages becomes the datum for the setting off process.

Historically, the set off distance between the rotors and chambers has been defined between the rotor and the chamber in the outlet stage (or low vacuum end) of the pump. It has also been the historical norm to position the outlet stage closer to the fixed side of the rotor assembly and the inlet stage closer to the expanding side of the rotor assembly.

It has been found that this can lead to increases in clearances (and the tolerances thereof) between the rotors and chambers in the inlet or high vacuum stages that can have negative implications on the performance and efficiency of the pump for particular vacuum pressure regimes.

Accordingly, there is a need to provide a multi-stage vacuum pump that enables improvements in performance and efficiency for different target vacuum pressure ranges.

Although the present disclosure is exemplified with regards to a multi-stage Roots vacuum pump, it is to be appreciated that this disclosure and its benefits are applicable and extend to any other suitable type of multi-stage vacuum pump that includes a plurality of rotors interacting within a plurality of chambers in a stator. Such types of pump include, amongst others, claw type vacuum pumps with two or more stages. In some such pumps, different types of rotors can be combined in the same pump, so the pump is a mixture of types. For example, some stages in the pump may be of a Roots type and other stages in the pump may be of a claw type. SUMMARY

In one aspect, the present invention provides a multi-stage vacuum pump comprising a stator assembly defining at least three chambers; a rotor assembly having at least three rotors housed in respective ones of the chambers to define at least three pump stages, a shaft rotatable about a central axis and on which the rotors are mounted, and a first bearing and a second bearing supporting the shaft for rotation relative to the stator assembly, wherein the at least three pump stages provide at least an inlet stage and an outlet stage; the first bearing is a moveable bearing that is axially moveable relative to the stator assembly to allow axial expansion of the rotor assembly during operation; the second bearing is a fixed bearing that is fixed in axial position relative to the stator assembly to react against the axial expansion of the rotor assembly during operation; the outlet stage is positioned closer to the fixed bearing than the inlet stage; and the inlet stage is positioned closer to the fixed bearing than at least one of the other pump stages.

This arrangement will place the inlet stage a closer axial distance to the fixed bearing than in historical designs, which permits tighter control of the total clearance in the inlet stage. This can reduce gas leakage and backflow in the inlet stage of the pump to provide improved pumping performance and efficiency therein. This results in more efficient pump operation at low (or “rough”) vacuum pressure ranges (e.g., above 10 mbar, such as between 50 to 1 ,000 mbar).

The inlet stage is the pump stage that includes the inlet for the pump and has the largest volume chamber of the pump stages. The outlet stage is the pump stage that includes the outlet for the pump and has the smallest volume chamber of the pump stages.

In embodiments, there may be any suitable number (i.e. , one or more) of intervening stages fluidly connected to the inlet and outlet stages. Such intervening stages will have chamber volumes between that of the inlet and outlet stage and will be fluidly connected in order of progressively decreasing chamber volume between the inlet and outlet stage. The intervening stage(s) are the “other pump stages”.

Whilst the outlet stage is positioned closer to the fixed bearing that the inlet stage, the inlet stage is positioned closer to the fixed bearing than at least one of the other pump stages. This permits a compromise of improved clearance tolerances and pumping efficiency between both the inlet and outlet stages. In this context, ‘closest’ relates to the relative distance between the stages. In general, the stages are arranged adjacent to each other along an axis (e.g., a rotor central axis). Therefore, this distance will commonly be defined by the axial distance between stages parallel to this axis.

In one embodiment, the inlet stage is positioned closer to the fixed bearing than the moveable bearing.

When the pump stages are provided in between axially opposed fixed and moveable bearings, this arrangement will allow the aforementioned advantages to be realised.

This distinguished from an alternative “cantilevered” pump arrangement, where the pump stages and the moveable bearing are positioned on opposite sides of the fixed bearing. It will be appreciated that in such an arrangement the inlet stage will be positioned closer to the fixed bearing than the movable bearing in all positions compared to the other pump stages.

The inlet stage may be position directly adjacent to the outlet stage such that there are no other pump stages positioned between them.

In this manner, when present, there will be no intervening stages positioned between (i.e. , axially separating) the inlet stage and the outlet stage.

Again, compared to historical designs, this can permit a compromise of improved clearance tolerances and pumping efficiency between both the inlet and outlet stages. It also means heat generated in the outlet stage during operation can be better distributed to the inlet stage that is relatively cooler during operation. This may advantageously reduce temperature gradients across the pump during operation.

Alternatively, in other embodiments, one or more of the other pump stages may be positioned axially between the inlet and outlet stages in a more traditional manner.

The inlet stage, the outlet stage and the other pump stages are thus positioned adjacent to each other in a non-sequential order, that is, not in order of decreasing volume. It will be understood, that although the inlet stage, the outlet stage and the other pump stages are positioned adjacent each other in a nonsequential order, they are nonetheless serially fluidly connected (e.g., via gas passages) in order of decreasing volume (i.e., in order to allow for normal compressive operation of the pump).

By placing the inlet stage out of normal sequential order and closer to the outlet stage in this manner, enhanced heat transfer between the outlet and inlet stages is permitted that can keep the working gases therein at the appropriate temperature during pump operation. This can prevent condensation and/or thermal decomposition/unwanted chemical reactions of the working gas. Moreover, this can be achieved without the need for additional heat transfer members or external devices

In another aspect, the present invention provides a multi-stage vacuum pump comprises a stator assembly defining at least three chambers; a rotor assembly having at least three rotors housed in respective ones of the chambers to define at least three pump stages, a shaft rotatable about a central axis and on which the rotors are mounted, and bearings supporting the shaft for rotation relative to the stator assembly, wherein the at least three pump stages are positioned adjacent to each other in a non-sequential order.

In yet another aspect, the present invention provides a multi-stage vacuum pump comprising an inlet stage defining a compression chamber of a largest volume; an outlet stage defining a compression chamber of a smallest volume; and at least two intervening stages that have compression chambers of incrementally decreasing volume between that of the inlet stage and the outlet stage; wherein a first of the intervening stages defines a compression chamber of a second largest volume and a second of the intervening stages defines a compression chamber of a second smallest volume; and wherein the inlet, outlet and intervening stages are positioned adjacent to each other in a non-sequential order such that the inlet stage has fewer than two other stages positioned between itself and the outlet stage.

In this manner, the inlet stage can be said to be one of two stages closest to the outlet stage.

In one embodiment, the inlet stage is positioned adjacent to the outlet stage such that there are no intervening stages positioned between them. In this manner, the inlet stage can be said to be directly adjacent to the outlet stage.

In this manner, the heating of working gas in the inlet stage due to heat transferred from the outlet stage can be sped up due to its closer proximity to the outlet stage.

In a further embodiment of either of the above, the outlet stage is positioned between and closest to both the inlet stage and the first intervening stage.

In this manner, the outlet stage is positioned directly adjacent to both the inlet stage and the first intervening stage and separates them from each other. This arrangement means both the inlet stage and the first intervening stage are in close proximity to the outlet stage for improved heat transfer thereto. This can speed up heating of working gas in both the inlet stage and the first intervening stage. Moreover, having the inlet stage and the first intervening stage either side of the outlet stage can help act as a heatsink for the outlet stage.

In further embodiments of any of the above, the first intervening stage is positioned adjacent the inlet stage such that there are no other stages positioned between them.

In this manner, the inlet stage and the first intervening stage are positioned directly adjacent to each other (i.e., paired together). Either one of the inlet stage or the first intervening stage can be directly adjacent to the outlet stage in different examples of this embodiment. Again, this embodiment places both the inlet stage and the first intervening stage in close proximity to the outlet stage which provides enhanced heat transfer thereto, and the paired inlet stage and first intervening stage can act as a heat sink for heat produced by the outlet stage.

In another embodiment, the first intervening stage is positioned between and closest to both the inlet stage and the outlet stage.

In this manner, the first intervening stage is positioned directly adjacent to both the inlet stage and outlet stage and separates them from each other.

As well as the heat transfer advantages discussed above, it is thought that by having the first intervening stage separating the inlet stage and outlet stage in this manner, the working gas pressure difference across the stages can be reduced (i.e., compared to the outlet stage and inlet stage being placed directly next to each other). This can help reduce (or make it easier to reduce) gas leakage between the stages.

In a further embodiment of any of the above, the second intervening stage is positioned adjacent the first intervening stage such that there are no other stages positioned between them.

In this manner, the second intervening stage is positioned directly adjacent the first intervening stage. This can help transfer heat generated in the second intervening stage to the first intervening stage.

In another embodiment, the second intervening stage is positioned adjacent the inlet stage such that there are no other stages positioned between them. In this manner, the second intervening stage is positioned directly adjacent the inlet stage. This can help transfer heat generated in the second intervening stage to the inlet stage.

In a further embodiment of any of the above, a third of the intervening stages defines a compression chamber of a third largest volume, and the second intervening stage is positioned adjacent the third intervening stage such that there are no other stages positioned between them.

In this manner, the second intervening stage is positioned directly adjacent the third intervening stage. This can help transfer heat generated in the second intervening stage to the third intervening stage.

As discussed above, although the stages are positioned non-sequentially they are still serially fluidly connected in order of decreasing volume, such that in this example, working gas passes through the stages in the order: inlet stage > first intervening stage > third intervening stage (>further intervening stages, if present) > second intervening stage > outlet stage.

The various placements of intervening stages relative to each other provided within the scope of this disclosure can help enhance heat transfer between stages that typically generate more heat during pump operation to those that typically generate less heat. This can help keep working gases at suitable temperatures across all the stages of the pump, as well as help balance temperatures (i.e. , reduce temperature gradients) across the pump.

Further embodiments can feature any suitable number of further intervening stages (e.g., fourth and/or fifth intervening stages) positioned adjacent to each other in various orders (e.g., either directly adjacent to each other or separated by one or more of the aforementioned intervening stages).

It will be appreciated that the total number of stages utilised in the vacuum pump will depend on the particular application and pumping requirements thereof.

In a further embodiment of any of the above, the multi-stage vacuum pump further comprises a stator assembly defining the compression chambers of each stage therein, a rotor assembly having rotors housed in respective ones of each of the compression chambers of each pump stage, and a plurality of gas passages serially fluidly connecting respective pairs of pump stages, wherein each connected pair of pump stages have compression chambers of incrementally decreasing volume. In this manner, the gas passages enable serial fluid communication of working gas between the pump stages and can be of any suitable form.

In a further embodiment of the above, the plurality of gas passages is defined within the stator assembly.

In this manner, the gas passages are defined within the body of the stator assembly, which can keep the pump compact. The gas passages can be formed in the body of stator assembly in any suitable manner, such as by machining (e.g., drilling) or casting them into the stator assembly, or by additively manufacturing the stator assembly with them.

In an alternative embodiment, the gas passages are provided by pipes (or ducts) that connect to the stator assembly and extend externally therefrom. This may simplify the design of the stator assembly at the expense of compactness and increased assembly time.

In a further embodiment of any of the above, the rotor assembly includes a shaft rotatable about a central axis on which the rotors are mounted, the shaft having axially opposed ends each supported by a respective one of a first and second bearing. The rotor can be driven to rotate to operate the pump in any suitable manner, e.g., by a motor.

In a further embodiment of any of the above, at least one pump stage includes rotors of a Roots type and/or a claw type. In some examples, all the pump stages have rotors of a Roots and/or claw type. In other examples, the pump has a mixture of Roots and claw type stages.

Preferably the chamber of each pump stage defines a set side wall closest to the fixed bearing and an expansion side wall opposite the set side wall, and the rotor of each pump stage is disposed between the set side wall and the expansion side wall, and a set axial clearance for the rotor assembly is defined between the set side wall and the rotor of the inlet stage.

The set axial clearance (i.e. , “set off distance”) is defined during assembly and is the minimum axial clearance set for the rotor assembly (i.e., the minimum axial clearance between a chamber set side wall and rotor in the pump). It dictates the amount of clearance left between the rotor and the expansion side wall of the chamber available during operation. This clearance is necessary to allow for thermal expansion of the rotor assembly during operation, but if it is too large it may promote gas leakage/backflow that can cause inefficiencies in the pump stage. Setting this axial clearance at the inlet stage can improve the tolerance of the expansion clearance therein compared to historical designs. This means excess clearance in the stage (beyond what is required to account for thermal expansion) can be reduced to reduce gas leakage/backflow effects in the inlet stage. This can further improve the pump efficiency in the inlet stage and operation at low (or “rough”) vacuum pressure ranges (e.g., above 10 mbar, such as between 50 to 1,000 mbar).

Preferably the set axial clearance (i.e., “set off distance”) is between 7 to 1,000 pm, for example, between 30 to 150 pm. In one particular example, it is between 30 to 80 pm.

These values are set axial clearance ranges that have generally been found to be suitable across different vacuum pump applications and sizes.

Preferably, the fixed bearing is movable between different fixed axial positions relative to the stator assembly. The fixed bearing may include a stop that is fixedly attached to the stator assembly and moveable to the different fixed axial positions, for example, by being threadably engaged with the stator assembly.

In this manner, the fixed bearing and/or stop can be conveniently used to vary the set axial clearance during assembly of the pump.

Alternatively, the fixed bearing can remain fixed in one position and other means, such as shims, can be used to move rotor assembly to different fixed axial positions during assembly.

Preferably, the moveable bearing is slideably engaged with the stator assembly, and axial movement of the moveable bearing is opposed by a biasing force.

The biasing force will resist the movement of the rotor assembly to help absorb the movement from thermal growth thereof.

The biasing force can be provided in any suitable manner. For example, by a biasing member attached between the moveable bearing and a fixed stop positioned a predetermined axial distance away. The biasing member can be any suitable member for generating an appropriate biasing force, such as a spring or resilient member.

Preferably, the shaft defines axially opposed ends of the rotor assembly, and each opposed end is supported by a respective one of the fixed and moveable bearings. In this manner, the fixed and moveable bearings are axially outboard of the pump stages, and thus define opposing extremities of the pump. The pump stages will be positioned between the opposing ends.

In alternative embodiments, the fixed and moveable bearings can be provided at any suitable position along the shaft. For example, as mentioned above, the pump stages may be “cantilevered” from the fixed bearing. In such an arrangement the pump stages and the moveable bearing are positioned on opposite axial sides of the fixed bearing.

It is to be understood that within the scope of this disclosure, the multi-stage vacuum pump can be a Roots pump, or any other suitable type of pump, such as claw pump, or a mixture of such types (i.e., combining pump stages of different types in the same pump).

As will be appreciated by the skilled person, vacuum pumps according to the present disclosure include two of the rotor assemblies adjacent each other, with respective rotors of each rotor assembly interacting within the stator chambers to provide the pump stages.

Features described above in relation to one aspect of the invention are equally applicable to each of the other aspects of the invention.

BRIEF DESCRIPTION OF DRAWINGS

One or more non-limiting embodiments according to the present disclosure will now be described, with reference to the accompanying figures in which:

Figure 1 shows a cross-sectional view of a known multi-stage vacuum pump;

Figure 2 shows a cross-sectional view of one example of a multi-stage vacuum pump;

Figure 3A shows a graph comparing simulations of pump inlet pressure vs pumping speed for the vacuum pumps of Figures 1 and 2;

Figure 3B shows a graph comparing simulations of pump inlet pressure vs pumping efficiency for the vacuum pumps of Figures 1 and 2;

Figure 4 shows a cross-sectional view of another example of a multi-stage vacuum pump;

Figure 5 shows yet another example of a multi-stage vacuum pump;

Figure 6 shows a schematic illustration of another example of a multi-stage vacuum pump; Figure 7A shows a schematic illustration of yet another example of a multistage vacuum pump;

Figure 7B shows a schematic illustration of yet another example of a multistage vacuum pump;

Figure 7C shows a schematic illustration of yet another example of a multistage vacuum pump; and

Figure 7D shows a schematic illustration of yet another example of a multistage vacuum pump.

DETAILED DESCRIPTION

With reference to Figure 1, a known multi-stage vacuum pump 100 is shown in cross-section. The vacuum pump 100 includes a stator assembly 102 and a rotor assembly 110.

The rotor assembly 110 comprises a series of rotors 112a-112f that are mounted to a shaft 114 that extends axially along a central axis X-X. The rotors 112a- 112f are spaced axially apart along the shaft 114 and protrude radially therefrom.

The rotors 112a-112f may be integrally formed with the shaft 114 (e.g., via casting) or may be formed and connected thereto separately (e.g., via welding). Moreover, although a single shaft 114 is shown, separate sections of shaft 114 may be provided and connected together instead. Such separate sections may include one or more rotors 112a-112f attached thereto.

Each rotor 112a-112f is housed within a respective chamber 104a-104f defined in the stator assembly 102. Each respective rotor 112a-112f and chamber 104a-104f combination defines a stage 100a-100f of the vacuum pump 100. In particular, an inlet stage 100a and an outlet stage 10Of fluidly connected by a plurality of intervening stages 100b-100e.

Although only one rotor assembly 110 is shown in the sectional view of Figure 1, it will be understood by the skilled person that a second cooperating rotor assembly is positioned adjacent the rotor assembly 110 within the stator assembly 102 and will interact therewith to provide the vacuum pump 100. The depicted example is a Roots vacuum pump, and so the rotors 112a-112f will include a series of interacting lobes and recesses (not shown) as discussed above. Nonetheless, as discussed above, in other examples within the scope of this disclosure, the rotors 112a-112f and/or rotor assembly 110 may include different geometries (e.g., claw type etc.) depending on the type of multi-stage vacuum pump that is implemented.

The chambers 104a-104f are arranged serially between an inlet 106 and an outlet 108. The inlet 106 and outlet 108 can be formed by openings or ports defined through the stator assembly 102.

As is known, the chambers 104a-104f are in fluid communication with each other via passages (not shown) defined through the stator assembly 102. The passages may be defined in any suitable manner, such as by being drilled through the stator assembly 102, cast into the stator assembly 102 or provided by ducts or pipes extending between chambers 104a-104f. This provides a path for a gas flow G to pass from the inlet 106 (or high vacuum side of the pump 100) to the outlet 108 (or low vacuum side of the pump 100) via each stage 100a-100f of the vacuum pump 100.

As will be appreciated, this configuration provides a multi-stage vacuum pump 100 that can evacuate a system or space by ingesting gas at the inlet 106 and progressively compressing the gas through the pump stages 100a-100f (by rotating the shaft 114 and rotors 112a-112f within the chambers 104a-104f) before the gas is exhausted at the outlet 108. The rotor assembly 110 is rotated during operation by any suitable means, for example, shaft 114 can be operatively connected to a motor (not shown).

The chambers 104a-104f progressively decrease in volume as gas is progressively compressed to a greater extent in each subsequent stage 100a-100f. Accordingly, a larger volume chamber provides a so-called ‘higher vacuum’ stage than that of an adjacent chamber of smaller volume downstream thereof, which is thus a so-called ‘lower vacuum’ stage.

The shaft 114 is supported for rotation by first and second bearings 120, 130. The first bearing 120 is disposed adjacent and axially outboard of the inlet 106 relative to central axis X-X. The second bearing 130 is disposed adjacent and axially outboard of the outlet 108 relative to central axis X-X.

The bearings 120, 130 are operatively connected to the rotor assembly 110 such that movement of the rotor assembly 110 during operation (e.g., axial movement or growth) is transferred to the bearings 120, 130. This operative connection can be provided in any suitable manner, e.g., via a tube or sleeve (not shown) around the shaft 114 rigidly connecting the bearings 120, 130 to the rotor assembly 110 and/or a firm fit being provided between the shaft 114 and inner bearing races.

In the depicted example, the bearings 120, 130 are ball bearings, and opposing axial ends 114a, 114b of the shaft 114 pass through respective apertures defined by inner races of the bearings 120, 130, whilst outer races of the bearings 120, 130 are connected to the stator assembly 102.

The first bearing 120 is a moveable bearing 120 that is axially moveable relative to the stator assembly 102 to allow axial expansion of the rotor assembly 110 during operation.

In the depicted embodiment, the moveable bearing 120 is supported against a first stop 122 attached to the stator assembly 102. In the depicted embodiment, a biasing member 124 is attached between the bearing 120 and the stop 122, and provides a biasing force that opposes movement of the first bearing 120. Accordingly, the axial movement of bearing 120 and rotor assembly is absorbed and resisted by the biasing member 124. The biasing force may be provided by any suitable biasing member 124, such as a spring or other resilient member.

In contrast to the first bearing 120, the second bearing 130 is a fixed bearing 130 that is fixed in axial position relative to the stator assembly 102 to react against the axial expansion of the rotor assembly 110 during operation.

In the depicted embodiment, the fixed bearing 130 is supported directly against (i.e. , in direct contact with) a second stop 132 that is fixedly attached to the stator assembly 102. Accordingly, the axial movement of bearing 130 is restricted to a fixed axial position limited by the stop 132.

The fixed axial position can be varied by the stop 132 being moved axially and locked back in place. In the depicted example, this is achieved by the stop 132 being threadably engaged with the stator assembly 102, such that rotation of the stop 132 advances the stop 132 a desired amount in the axial direction to provide different fixed axial positions for the bearing 130. In this manner, the stop 132 may be provided in the form of a threaded nut.

Nonetheless, within the scope of this disclosure any other suitable configuration and/or mechanism for achieving the provision of different fixed positions for the rotor assembly 110 can be used, for example, by positioning shims between the fixed bearing 130 and the rotor assembly 110.

Although one particular example of providing a moveable bearing 120 and fixed bearing 130 have been described, it is to be understood that any other suitable method of providing bearings 120, 130 with such functionality is envisaged within the scope of this disclosure. This includes alternative arrangements to those depicted, in which the pumping stages are “cantilevered” from the fixed bearing (i.e. the pump stages and the moveable bearing are on opposite sides of the fixed bearing).

Each chamber 104a-104f defines a set side wall (or face) 105a-105f closest to the fixed bearing 130 and an expansion side wall (or face) 107a-107f opposite the set side wall (or face) 105a-105f. The expansion side wall 107a-107f is so called because it is the wall of the chambers 104a-104f towards which the rotor assembly 110 will expand owing to thermal effects during operation. In this way, each rotor 112a-112f is disposed in its respective chamber 104a-104f axially between the set and expansion side walls 105a-105f, 107a- 107f thereof.

Clearances Ca-Cf are defined axially between the set side walls 105a-105f of the chambers 104a-104f and the rotors 112a-112f therein. The size of the set side clearances Ca-Cf will also define the amount of clearance left on the opposing expansion side of the chambers 104a-104f (i.e., the clearance between the expansion side walls 107a-107f and the rotors 112a-112f). The expansion side clearances are necessary to allow thermal expansion of the rotor assembly 110 during operation without the rotors 112a-112f making contact with the chamber walls/stator assembly 102.

When the pump 100 is assembled, the rotor assembly 110 is positioned and secured within the stator assembly 102. In one example, a series of portions of the stator assembly 102 may be slid over the rotor assembly 110 and stacked on top of each other to surround the rotors 112a-112f, and then secured together. In another example, the stator assembly 102 may be defined by two stator clamshell halves that are positioned around the rotors 112a-112f and secured together.

In the depicted example, during assembly, the rotor assembly 110 is positioned such that the rotor 112f is stacked or placed in contact against the set side wall 105f of the outlet chamber 104f. The stator and rotor assemblies 102, 110 are designed so that each of the other rotors 112a-112e have some (small) clearance left between them and their corresponding set side walls 105a-105e to facilitate assembly.

In order to tune the expansion side clearance that is left between the rotors 112a-112f and the chambers 104a-104f for operation, the rotor 112f is then moved apart from the set side wall 105f to provide a set axial clearance (i.e., a desired minimum clearance) Cf between the rotor 112f and the wall 105f. In the depicted embodiment, this is achieved by stop 132 being moved axially.

This procedure of moving the rotor assembly 110 relative to the stator assembly 102 is known as “setting off”. Accordingly, in Figure 1, the rotor assembly 110 is said to be “set-off” relative to the outlet stage 10Of of the pump 100 with the wall 105f providing a so-called “set face” and the set axial clearance Cf providing a “set off distance”.

Once the pump is set in this manner, each of the other clearances Ca-Ce will be larger than Cf. This variation in clearance can be particularly pronounced for clearances Ca-Cb in stages 100a-100b compared to clearances Ce-Cf in stages 100e-100f.

These increased clearances can provide a larger cross-sectional area for the back-flow of gas in a pump stage. Back-flow gases are generated due to higher pressure gas leaking upstream across a pump stage (e.g., from a pump stage outlet to pump stage inlet) and/or to an adjacent pump stage containing lower pressure gas (e.g., see gas leakage path L). The accumulation of such back-flow gas in a particular stage can negatively affect the throughput of gas being compressed through that stage and thus negatively affect the performance and efficiency of the pump 100.

It has been found that setting the rotor assembly 110 off the set side wall 105f as in Figure 1 causes the pump 100 to act more efficiently when it is being driven to maintain higher vacuum levels in a system (e.g., less than 10 mbar, such as between 1.0 to 0.1 mbar) and less efficiently when it is being driven to maintain lower vacuum levels in a system (e.g., above 10 mbar, such as between 50 to 1,000 mbar). This is thought to be due to the setting off procedure providing tighter clearance control in the downstream-most stages (e.g., stages 100e-100f) reducing back-flow accumulation therein and improving their gas throughput compared to the stages upstream thereof (e.g., stages 100a- 100b).

Although these lower vacuum pressure range inefficiencies may be acceptable for a pump 100 that is primarily intended to operate to maintain higher vacuum pressures, it does not make the pump 100 that useful for applications that require the maintenance of lower (or “rougher”) vacuum pressure ranges (e.g., 50 to 1,000 mbar or more specifically 100 to 500 mbar). As will be appreciated by the skilled person, there are numerous examples of such applications, which can include conveying, lifting, steel degassing, medical applications and chemical/pharmaceutical manufacturing.

It would therefore be advantageous if pump 100 could be adapted to improve its efficiency and performance at these lower vacuum pressure ranges to enhance is usage in such applications.

With reference to Figure 2, a multi-stage vacuum pump 200 in accordance with one embodiment of the present disclosure is shown in cross-section. As explained below, the pump 200 is adapted to improve its low vacuum pressure range efficiency and performance compared to pump 100.

The pump 200 is assembled in the same manner and includes many of the same features as pump 100, and these same features are denoted with the same numerals as in Figure 1. Unless otherwise stated below, the same method steps are implemented and the same features function and interact with each other as discussed above with regard to Figure 1 and so will not be repeated.

In contrast to pump 100, in pump 200 the fixed bearing 130 is disposed adjacent and axially outboard of the inlet 106 rather than the outlet 108, and the moveable bearing 120 is disposed adjacent and axially outboard of the outlet 108 instead of the inlet 106. In this manner, the inlet stage is positioned closer to the fixed bearing 130 than the moveable bearing 120, and in particular, is now positioned closer to the fixed bearing 130 than any of the other pump stages 100b- 100f.

In further contrast to pump 100, in pump 200 the rotor assembly 110 is setoff against the set side wall 105a of the inlet stage 100a (i.e., the pump stage that includes the inlet 106 of the pump 100 and the largest volume chamber 104a) instead of the set side wall 105f of the outlet stage 10Of.

Accordingly, the set axial clearance is defined by the clearance Ca between the set side wall 105a and the rotor 112a. The clearances Cb-Cf therefore tend to become less tightly controlled progressively downstream in the pump 200.

Due to the rearrangement of the inlet stage 100a closer to the fixed bearing 130 and the setting off of the rotor assembly 110 relative to the set side wall 105a of the inlet stage 100a, the expansion clearances in the upstream stages (e.g., stages 100a-100b) are more tightly controlled than those in the downstream stages (e.g., stages 100e-100f). Accordingly, the backflow accumulation in these upstream stages is reduced during operation, and thus the throughput, efficiency and performance thereof is improved compared to that of pump 100 at higher pressures.

The set axial clearance (i.e. , set off distance) Ca can be varied to suit the particular pump application and operating requirements. In the depicted example, the clearance Ca is set to a minimum of 50 pm; however, in other examples it may be set to between 30 to 80 pm, or 30 to 150 pm, or more widely between 7 to 1 ,000 pm (e.g., depending on the application and pump size).

Although the depicted pump 200 shows the inlet stage 100a being positioned closer to the fixed bearing 130 than any of the other pump stages 100b- 10Of (as will be better appreciated from Figure 5 discussed below) improved clearance control and low vacuum pressure performance can be realised as long as the inlet stage 100a is positioned closer to the fixed bearing 130 than at least one of the other pump stages 100b-100f present in the pump 200.

Moreover, although the feature of moving the inlet stage 100a closer to the fixed bearing 130 compared to pump 100 has been combined with the feature of setting off the rotor assembly 110 in the inlet stage 100a in pump 200, the two provide different ways of improving clearance control and low vacuum pressure performance that can be provided independently of each other. Thus, in addition to being combined in the same pump 200, in other embodiments (and as discussed further below in relation to Figure 4), only one need be employed in a given pump to achieve the aforementioned advantages.

Figures 3A and 3B show the different operating characteristics for the pumps 100 and 200 taken from computer simulations thereof (as well as pump 300 discussed below in relation to Figure 4). The X-axis in both Figures 3A and 3B refers to a logarithmic scale of the vacuum pressure at the pump inlet 106. This corresponds to the vacuum pressure for the system that the pump is operating to achieve/maintain. In Figure 3A, the Y-axis refers to the relative pumping speed or flow rate of gas passing through the pumps (in m ³ /hr). In Figure 3B, the Y-axis refers to the relative amount of power used per unit gas flow rate (in W/(m ³ /hr), which can be used to indicate a relative efficiency of the pump (i.e., a lower value is more efficient).

With reference to Figure 3A, it can be seen that pump 200 has improved flow rate performance at lower vacuum pressure ranges 10 to 1 ,000 mbar compared to pump 100. With reference to Figure 3B, it can also be seen that pump 200 provides a particular improvement in efficiency at lower vacuum pressure ranges 50 to 500 mbar compared to pump 100.

As discussed above, this makes pump 200 advantageously suited to a variety of vacuum pump applications for which pump 100 was not. This can lead to improved performance and cost benefits by utilising pump 200 in such applications.

With reference to Figure 4, a multi-stage vacuum pump 300 in accordance with another embodiment of the present disclosure is shown in cross-section.

Again, the pump 300 is assembled in the same manner and includes many of the same features as pump 100. These same features are denoted with the same numerals as in Figure 1. Unless otherwise stated below, the same method steps are implemented and the same features function and interact with each other as discussed above with regard to Figure 1 and so will not be repeated.

In contrast to pump 200, pump 300 provides the same configuration of bearings 120, 130 as pump 100; however, the rotor assembly 100 is instead set off against the set side wall 105a of the inlet stage 100a. In this manner, the set axial clearance Ca is provided in the inlet stage 100a.

Although setting off the rotor assembly 110 relative to the wall 105a in this manner improves the control of clearances in the upstream stages (e.g., stages 100a-100b) in the pump 300 compared to pump 100, it does not do so to the same extent as in pump 200. This is because the inlet stage 100a is in closer proximity to the moveable bearing 120 and so total control of the clearances therein are more limited. This does, however, also mean that pump 300 does not reduce control of the clearances in the downstream stages (e.g., stages 100e-100f) as much as pump 200 either.

As shown by comparison in Figures 3A and 3B, this results in pump 300 providing a ‘half-way house’, by providing a milder improvement in lower vacuum pressure range performance, but with less of a negative impact on higher vacuum pressure range performance compared to pump 200. Although the performance improvement is lower, it will be appreciated that it is realised with fewer structural modifications to the pump 100 design, which can have cost benefits.

With reference to Figure 5, a multi-stage vacuum pump 400 in accordance with another embodiment of the present disclosure is shown in cross-section.

Again, the pump 400 is assembled in the same manner and includes many of the same features as pump 100. These same features are denoted with the same numerals as in Figure 1. Unless otherwise stated below, the same method steps are implemented and the same features function and interact with each other as discussed above with regard to Figure 1 , and so will not be repeated.

Pump 400 has the same configuration as pump 100 except that the inlet stage 100a has been rearranged to be axially closer (i.e., closer in positional stage order) to the outlet stage 10Of. In the depicted example, the inlet stage 100a has been rearranged to be positioned directly adjacent to the outlet stage 10Of such that it is the stage closest to the outlet stage 10Of (i.e., with no other pump stages 100b- 100e positioned between them). This also results in the outlet stage 10Of being positioned closer to the fixed bearing 130 than the inlet stage 100a, but it will be appreciated that the inlet stage 100a is still positioned closer to the fixed bearing 130 than (at least one of) the other pump stages 100b-100e.

In this manner, the positional stage order for the pump 400 from left to right in Figure 5 is 100b, 100c, 100d, 100e, 100a, 10Of. However, the gas flow G through the pump 400 still follows the path from the inlet 106 to the outlet 108 via each stage 100a-100f in series. In other words, stages 100a-f are all still serially fluidly connected.

In the depicted embodiment, the inlet stage 100a is positioned axially inboard of the outlet stage 10Of (i.e., between stages 100e and 10Of); however, in another example it could be positioned axially outboard (to the right in Figure 5) of the outlet stage 10Of. In other examples, the outlet stage 10Of may be repositioned relative to the inlet stage 100a instead.

It will be appreciated that the rearrangement of stage 100a will necessitate a longer and/or more complicated gas flow path (shown schematically in a dotted line) between the outlet of stage 100a and the inlet of stage 100b, as well as between the outlet of stage 100e and the inlet of stage 10Of. This path can be provided in any suitable manner, such as by drilling or casting an appropriate fluid passage in the stator assembly 102 or providing ducting or piping between the stages.

The rotor assembly 110 is set off the set side wall 105a of the inlet stage 100a to provide the set axial clearance Ca. However, owing to the outlet stage 10Of being axially closer to the inlet stage 100a, the control of clearances in the outlet stage 10Of is improved compared to that of pumps 200, 300. This can provide a reduction in back flow accumulation for both the lower and higher vacuum stages 100a, 10Of to provide the pump 400 with more optimal performance and efficiency across both lower and higher vacuum ranges. Although this improvement in performance and efficiency across a broad range of vacuum pressures is advantageous for many applications, it will need to be balanced with the increases in costs and manufacture time associated with providing the aforementioned more complicated gas flow paths between stages.

Although inlet stage 100a and outlet stage 10Of have been placed directly adjacent each other in the depicted embodiment, it should be understood that the same advantages (albeit to a lesser extent) can be achieved by positioning inlet stage 100a in a different position that is closer to the fixed bearing 130. For example, by placing inlet stage 100a axially in between other intervening stages (e.g., stages 100d and 100e). In another example embodiment, the rotor assembly 110 could be set off from the set side wall 105f of the outlet stage 10Of instead, and an improvement in low vacuum pressure performance of pump 400 would still be found owing to the closer proximity of the inlet stage 100a to the fixed bearing 130 compared to historical designs.

Accordingly, it should be understood a variety of rearrangements of the inlet stage 100a relative to the outlet stage 10Of or vice versa can be made within the scope of this disclosure.

Although the depicted pumps illustrate six pump stages, it should be understood the present disclosure and its advantages apply to a multi-stage vacuum pumps with any number of stages of three and over. In other words, pumps having at least an inlet stage, an outlet stage and one intervening stage. In further examples, the pumps can feature one or more intervening stages fluidly connected between the inlet and outlet stages.

Returning to vacuum pump of Figure 1 , during pump operation, as gas is compressed in the stages 100a-100f, heat is generated. As discussed in more detail below, this heat can be distributed around the pump 100 to maintain the stages 100a-100f at particular temperatures to prevent unwanted condensation or decomposition of the working gas therein during operation.

The pump 100 is typically intended to maintain a system at a ‘high’ vacuum pressure in the order of 0.01 mbar, and compresses any gas to be exhausted to around atmospheric pressure (i.e. , in the order of 1000 mbar or 1 bar). In other words, during normal ‘high’ vacuum operation of the pump 100, the pressure of gas sucked in the inlet 106 (high vacuum side) is around 0.01 mbar, whilst the pressure of gas being exhausted at the outlet 108 (low vacuum side) is around 1,000 mbar. Accordingly, there is a pressure factor difference across the pump 100 of in the order of about 10 ⁵ (i.e. , the gas pressure at the inlet 106 is about 100,000 times lower than the gas pressure at the outlet 108). In addition, the volume of the chambers 104a-104f across the pump 100 only differ by a factor of between 2-20 (i.e., the inlet stage chamber 104a is at most 20 times the volume of the outlet stage chamber 104f).

Owing to the high pressure difference between the high vacuum side and low vacuum side of the pump 100 in this operating mode, the lower volume, downstream stages (e.g., stages 100e-100f) do significantly more compressive work than the larger volume, upstream stages (e.g., stages 100a-100b). Accordingly, they generate significantly more heat.

The heat generated can advantageously keep the gas in these lower volume stages at a high enough temperature to avoid the gas condensing during pump operation. However, the higher volume stages may not heat up enough to do so. It is therefore necessary to provide additional heat to these stages during such operations to avoid gas condensation.

Moreover, although this normal ‘high’ vacuum level operation generates additional heat in the lower volume stages, the pump 100 can be used in several different operating regimes. For example, on start-up or during lower vacuum operation, the gas pressures at the inlet 106 are higher, and so the compressive work and heat generated in the larger volume stages (e.g., 100a-100b) may be greater than the smaller volume stages (e.g., 100e-100f) that remain relatively cool. In such scenarios, it will therefore be necessary to transmit heat from the hotter, larger volume stages to the cooler, lower volume stages to avoid negative condensation effects.

As will be understood by the skilled person, one such example of this kind of mixed pump operation is in silicon wafer production and processing. For example, when depositing on a silicon wafer in a vacuum chamber, a relatively higher vacuum level is maintained and a relative low amount of working gas is put through the pump 100. However, once the wafer has been removed from the chamber, it can be necessary to clean the chamber. For the cleaning operation, a much higher amount of working gas is put through the pump 100, which will provide a much higher amount of compressive work through the larger volume stages.

In order to improve the speed of heat transfer between stages during pump operation (beyond what is available from the natural conduction of heat through the stator structure alone), it is known to provide pump 100 with heat transfer members. The heat transfer members are attached to the pump 100 and extend along the stator assembly 102 between different stages to conduct heat from the hotter stages towards the cooler stages during operation. The heat transfer members have typically been implemented as blocks of conductive material (e.g., aluminium blocks), but could take other suitable forms (e.g., heat pipes).

Although these heat transfer members can be effective at transferring heat from hotter stages to warm cooler stages during pump operation, it has nevertheless been found that they have certain limitations. For example, there is a limit to the amount heat that can be transferred and where it can be directed, and they can add unwanted bulk and costs to the pump design.

In addition to heating cooler stages, depending on the working gas and application, the hotter stages may generate so much heat that they require cooling to keep the temperature of the working gas low enough to prevent thermal decomposition or reaction.

If this issue is known for a particular application, then it is known to provide the stages in question with additional cooling features. These additional cooling feature can include cooling fins extending from the stator assembly 102 and/or an external cooler directing cooling fluid onto the stator assembly 102. It would also be advantageous if such additional cooling features could be minimised or removed by providing improved heat transfer away from hotter stages during operation.

With reference to Figure 6, an example pump stage arrangement in a multistage vacuum pump 500 in accordance with the present disclosure is shown schematically.

It is to be understood that the structural and functional features of pump 400 discussed above apply equally to pump 500, which differs from pump 400 discussed above in that there are seven pump stages 100a-100g, and the pump stages 100a-100g have been rearranged and the gas passages defined between them have been reconfigured accordingly.

It has been found that appropriate repositioning of the stages 100a-100g (as will be discussed further below) removes the need for the heat transfer members discussed above and improves the heat transfer characteristics for pump 500 in comparison to pump 100. It may also reduce the need for active cooling of stages 100a-100g.

As shown in Figure 6, the stages 100a-100g have been positionally arranged such that they are not disposed sequentially in series of decreasing chamber volume. The black line arrows represent gas passages connecting the stages 100a-100g. Accordingly, although the stages 100a-100g have been rearranged non-sequentially in their positioning, the fluid flow still proceeds sequentially there through from inlet stage 100a to outlet stage 100g (i.e., from first stage 100a to seventh stage 100g).

The stator assembly 102 is generally made of a thermally conductive material (e.g., a metallic material, such as aluminium, cast iron or stainless steel). Therefore, the heat generated in one stage chamber will be conducted to the stage chambers adjacent thereto. By positioning stages that are known to generate more heat in a particular operating regime next (or relatively close) to stages that are known to generate less heat during that operating regime, the heat will more effectively be conducted away from the stages that generate more heat to those that generate less heat. In this manner, the stages that generate less heat for themselves can be heated to an appropriate temperature to avoid working gas condensation, without using additional external means or heat transfer members.

It has also been found that by rearranging the stages 100a-100g in this manner, lower heat generating stages can be grouped appropriately to provide effective heat sinks for the higher heat generating stages, such that enough heat can be transferred therefrom to keep them cool enough to avoid thermal decomposition or unwanted chemical reactions for working gases therein. This can eliminate or at least reduce the reliance on additional cooling features to achieve this.

As shown in Figure 6, one example for pump 500 is to define the stage chambers in the stator assembly 102 such that the stages are non-sequentially positioned in the following order (from left to right): 5f ^h stage 100e - 4 ^th stage 100d - 6 ^th stage 100f- 3 ^rd stage 100c - 1 ^st stage 100a - 2 ^nd stage 100b - 7 ^th stage 100g.

In this example, the pump 500 is configured to operate to maintain high vacuum levels/with a low through-put of working gas. Accordingly, the smallest volume stage 100g (i.e., the 7 ^th or outlet stage 100g) that has the highest work of compression (and thus generates the most heat) at this operating condition is closest to (i.e., most adjacent) the two largest volume stages 100a, 100b (i.e., the 1 ^st or inlet stage 100a and the 2 ^nd stage 100b) that have the lowest work of compression (and thus generate the least heat) at this operating condition. This provides enhanced heat transfer to both stages 100a, 100b to prevent working gases from condensing therein, whilst also providing a sufficient heat sink to absorb enough heat to reduce the temperature of stage 100g sufficiently to avoid working gas undergoing thermal decomposition or unwanted chemical reactions.

Although the difference in relative heat generation is less pronounced in stages 100b-100f, by placing the 3 ^rd largest volume stage 100c directly next to the second smallest volume stage in 6 ^th stage 10Of and placing the 4 ^th largest volume stage 100d directly next to the third smallest volume stage in 5 ^th stage 100e, these stages will also provide a balance of heat transfer between them to keep them in the appropriate temperature window.

The position of the 1 ^st and 2 ^nd stages 100a, 100b may be switched around, such that 1 ^st stage 100a is positioned directly next to 7 ^th stage 100g (i.e., inbetween stages 100b and 100g). In such an example, the positional order of stages (from left to right) would be: 5 ^th stage 100e - 4 ^th stage 100d - 6 ^th stage 100f- 3 ^rd stage 100c - 2 ^nd stage 100b - 1 ^st stage 100a - 7 ^th stage 100g.

Although pump 400 is depicted with six stages and pump 500 is depicted with seven stages, the non-sequential arrangement of pump stages can be usefully applied to a pump with a different number of pump stages.

Examples of such multi-stage vacuum pumps 600, 700, 800, 900 are shown schematically in Figures 7A, 7B, 7C and 7D. The pumps 600, 700, 800, 900 generally include an inlet stage 100a defining a compression chamber of largest volume and an outlet stage 100d defining a compression chamber of smallest volume with at least two intervening stages 100b, 100c that have compression chambers of incrementally decreasing volume between that of the inlet stage 100a and the outlet stage 100d. For the avoidance of doubt, the compression chamber of stage 100b has a greater volume than that of stage 100c. In this manner, the intervening stage 100b has a compression chamber of the second largest volume and the intervening stage 100c has a compression chamber of the third largest (or second smallest) volume.

The stages 100a-100d are fluidly connected serially/sequentially in order of decreasing volume (as shown by the black arrow lines representing gas passages between the stages 100a-100d). However, the stages 100a-100d are not positioned in this serial/sequential order (i.e., from left to right in Figures 7A-7D). In other words, despite the stages 100a-100d being sequentially fluidly connected to each other they are non-sequentially positioned adjacent each other.

As shown in Figures 7A-7D, the inlet stage 100a is positioned such that it has fewer than two other stages 100b, 100c positioned between itself and the outlet stage 100d. In other words, the inlet stage 100a is positioned such that it is one of the two stages that are closest to (i.e. , most adjacent) the outlet stage 100d and, at most, has only one intervening stage 100b positioned there between.

In this manner, the inlet stage 100a is either positioned directly adjacent the outlet stage 100d with no intervening stages there between as in Figures 7 A, 7C and 7D, or is positioned with only one intervening stage 100b there between as in Figure 7B. It is thought that by having the inlet and outlet stages 100a, 100d positioned in this manner (i.e., within two adjacent stages of each other) the advantageous heat transfer characteristics between the two discussed above can be realised.

Moreover, it can also be preferable for the intervening stage positioned directly adjacent the inlet stage 100a to be the intervening stage 100b with a compression chamber of the second largest volume. As shown in Figures 7A and 7B, this can be accomplished by positioning stage 100b either in between the inlet stage 100a and the outlet stage 100d and directly adjacent thereto or directly next to the inlet stage 100a when the inlet stage 100a is directly next to the outlet stage 100d. Such configurations can advantageously help provide a larger heat sink to aid cooling of the outlet stage 100d.

Alternatively, as shown in Figure 7C, similar heat transfer and sink advantages can be achieved by the intervening stage 100b being instead placed outboard and directly next to the outlet stage 100d, with the inlet stage 100a inboard thereof.

In another advantageous example as shown in Figure 7D, the inlet stage 100a can be placed outboard of the outlet stage 100d with the intervening stage 100b inboard thereof.

It is therefore also generally to be understood that the advantages of the present disclosure can be found without the outlet stage 100d needing to be an outermost stage (i.e., right-most in Figures 7A-7C) of the pump.

Although two intervening stages 100b, 100c are shown in Figures 7A-7D, any suitable number of intervening stages of two or more may be provided. For example, a suitable vacuum pump may include at least five stages, with three or more intervening stages in addition to the inlet and outlet stages.

As discussed above with reference to Figure 6 and shown in Figures 7A and 7D, it is also advantageous for the intervening stages to be positionally interleaved with each other in such a way as to promote heat transfer between lower and higher compressive work intervening stages. This can be achieved generally by placing an intervening stage of lower volume between intervening stages of higher volume and vice versa. For example, when referring to Figure 6, stage 10Of is placed between and directly adjacent to the stages 100c, 100d. In the example shown in Figures 7 A and 7D, the second largest volume stage 100b is positioned directly adjacent the second smallest volume stage 100c. Alternatively, intervening stages of lower compressive work (i.e. , incrementally higher compression chamber volumes) can be suitably paired together and placed directly adjacent a higher compressive work intervening stage (i.e., of lower compression chamber volume) to act as heat sinks for the higher compressive work intervening stage and share in the heat transferred therefrom.

When comparing Figures 5, 6 and 7A-7D to Figure 1, the pumps 400, 500, 600, 700, 800, 900 require generally more complicated and longer gas passages (represented schematically as black arrows) to fluidly connect the pump stages than in pump 100. Nonetheless, these gas passages can still be provided by any suitable conventional means, such as by drilling passages or casting them into the stator assembly 102 or providing pipes of ducts that connect to the stator assembly 102 and extend externally from and around the stator assembly 102.

It is thought that the benefits of repositioning the pump stages in pumps 400, 500, 600, 700, 800, 900 discussed above justifies the extra design and manufacture steps necessary to provide appropriate gas passages between the stages. In addition, the improvements to the amount and control of heat transfer provided across the pump improves its operating characteristics for a wide range of applications. This is again thought to outweigh the potential costs of redesigning the gas passages. Moreover, these improvements may also avoid the use of additional heat transfer members, which may help reduce additional costs and keeps the pump more compact. It may also eliminate or at least reduce the need for additional cooling features to be added to or used with the pump that would also add cost and bulk to pump designs. LIST OF REFERENCE NUMERALS

A list of reference numerals used in the accompanying Figures 1 to 7D is provided for ease of reference:

100 Multi-stage vacuum pump

100a-100g Pump stages

102 Stator assembly

104a-104f Chambers

105a-105f Set side chamber walls

106 Inlet

107a-107f Expansion side chamber walls

108 Outlet

110 Rotor assembly

112a-112f Rotors

114 Shaft

114a, 114b Opposing shaft ends

120 First (‘moveable’) bearing

122 Stop

124 Biasing member

130 Second (‘fixed’) bearing

132 Stop

200 Pump

300 Pump

400 Pump

500 Pump

600 Pump

700 Pump

800 Pump

900 Pump

Ca-Cf Clearances

G Gas flow path

L Gas leakage path

X-X Shaft central axis