WO2018162925A1

WO2018162925A1 - Rotating machine and rotors for use therein

Info

Publication number: WO2018162925A1
Application number: PCT/GB2018/050609
Authority: WO
Inventors: Michael Henry North; Phillip North
Original assignee: Edwards Limited
Priority date: 2017-03-10
Filing date: 2018-03-09
Publication date: 2018-09-13
Also published as: GB2560375A; GB201703856D0; TW201837321A

Abstract

A rotating machine and rotor are disclosed. The rotating machine comprises: a stator; a rotor rotatable about an axis of rotation within the stator; at least one fixed supporting member comprising a shaft passing through the rotor and supporting the two bearings, the two bearings being mounted around the at least one fixed supporting member; wherein the rotor is rotatably mounted on an outer surface of the two bearings, and the two bearings are located towards opposing axial ends of the rotor.

Description

ROTATING MACHINE AND ROTORS FOR USE THEREIN

FIELD OF THE INVENTION

The invention relates to the field of rotating machines and rotors for use therein. BACKGROUND

The invention relates to the field of rotating machines and in preferred embodiments to pumps, and in particular to vacuum pumps. Rotating machines need to be carefully designed and manufactured in order for the moving parts to cooperate with each other accurately. Radial clearances, for example, can result in the moving parts of a rotating machine seizing when they are too small, while when they are too large they can result in poor performance. There is an increasing desire for pumps to become smaller. The smaller a pump is the faster it must rotate to pump the same amount of fluid in a given time. Faster rotation can lead to less stability in the moving parts which with small clearances can lead to problems. Furthermore, increased speeds can also lead to increased temperatures which can lead to issues due to uneven expansion of components and due to increased wear and tear.

Conventional pumps such as that shown in Figure 1 comprise a rotor 20 within a stator 1 0, the rotor being mounted for rotation within bearings 30. An increased speed of rotation of the rotor can lead to the rotor flexing and this can flexion can increase markedly when the resonant frequency of the rotor is neared.

It would be desirable to provide a rotating machine with an improved tolerance for increased speeds and temperatures.

SUMMARY

A first aspect of the present invention provides, a rotating machine comprising: a stator; a rotor rotatable about an axis of rotation within said stator; said rotor being rotatably mounted on two bearings, said two bearings being located towards opposing axial ends of said rotor; at least one fixed supporting member supporting at least one of said bearings, said at least one of said bearings being mounted around said at least one fixed supporting member, such that said rotor is rotatably mounted on an outer surface of said at least one of said bearings; wherein said at least one fixed supporting member comprises a shaft passing through said rotor, said two bearings being mounted on portions of said shaft located towards either end of said rotor.

The inventors of the present invention recognised that the resonant frequency and the stiffness of the rotor is related to the thickness or diameter of the shaft on which it is mounted; an Increase in the thickness of the shaft increasing both the resonant frequency and the stiffness. However, they also recognised that an increased thickness of shaft requires an increased bearing size and this leads to higher power consumption and increased heat generated in the rotating machine. The inventors addressed these competing issues in a simple yet elegant manner by mounting the rotor on the outer surface of at least one of the bearings as opposed to in the conventional way of mounting within the bearing inner surface. In this way bearings of a same size can support a substantially thicker and therefore stiffer shaft. In order to do this the at least one bearing is mounted around a fixed supporting member, and the rotor is mounted on the outer surface of the bearings allowing for the rotor to be supported on the outer bearing diameter rather than the inner bearing diameter. In this way for the same size of bearing an increased diameter rotor support can be used. The inner race of the bearing is supported by the fixed member and is therefore stationery while the outer race supports the rotating rotor and moves. This is different to the conventional bearings where it is the outer race that is stationery, the inner race moving with the rotating rotor shaft.

The rotor is mounted towards either axial end to increase the resistance of the rotor to lateral movements. In this regard the bearings are each located away from a central position, such that in some embodiments the rotor either does not extend beyond the bearings, or only extends beyond the bearings by a small amount of less than 5% of the axial length of the rotor. Furthermore the fixed supporting member is a shaft that is stationary and not rotatable and passes through the rotor. This provides a robust support for the rotor.

It should be noted that conventionally the bearing selection is often driven by shaft diameter due to the rotordynamics and the requirement for a certain minimum stiffness and this can lead to the load capability of the bearings being significantly higher than necessary. By mounting the rotor on the outer surface of at least one of the bearings, the bearing selection can be driven much more strongly by the load capability of the bearing.

It should be noted that the rotor may be mounted on a bearing towards either end of the rotor, or on a plurality of bearings towards either axial end. The bearings may have a number of forms and may in some embodiments be rolling element bearings mounted within a housing.

In some embodiments, said at least one fixed supporting member comprises a cooling fluid inlet, a cooling fluid flow path and a cooling fluid outlet for the flow of cooling fluid.

One particularly advantageous feature of mounting the bearings on a fixed support and allowing the rotor to rotate about the bearings, is that this provides the opportunity for providing cooling fluid to the fixed support allowing the bearings and potentially other features of the rotating machine in their vicinity to be cooled. Applying a cooling fluid to a member that is fixed is a simple matter, while applying a cooling fluid to a rotating member is considerably more complex. Thus, mounting the bearings on a fixed member rather than on the rotating shaft of the rotor provides an opportunity to apply cooling fluid to this fixed member. The bearings will function better and have a longer lifetime if they are not allowed to get too hot. Overheating of bearings can lead to the internal clearances changing and to the metals becoming softer. Furthermore, the seals located in the vicinity of the bearings may also be cooled by the cooling fluid which again can lead to a longer lifetime and also to a more efficient seal. In this regard, it should be noted that the bearings generate significant heat during use and by cooling the supporting member of the bearing, this heat can be dissipated. The cooling fluid may be any fluid able to transport heat away but in some embodiments is water, water having a high heat capacity and being cheap, non-toxic and readily available.

In some embodiments, said fluid inlet and fluid outlet are at opposing axial ends of said shaft. In other embodiments the inlet and outlet are at the same end for cooling the whole shaft or there is an inlet and outlet at each end to cool each end of the shaft independently.

Having the fluid inlet and outlet at opposing axial ends of the shaft allows the cooling fluid to flow through the shaft and to transfer heat along the entire length of the shaft not just in the region of the bearings. This in turn provides a cooling effect on the rotor. In this regard, cooling the stator of a rotating machine is generally quite straightforward while cooling the rotor which is sealed within the machine is more difficult. Mounting the rotor on a fixed shaft and providing cooling fluid to go through that shaft is an efficient and convenient way of providing cooling to the rotor. In other embodiments the inlet and outlet may be at a same end, with a flow path passing along the shaft and back to the cooling fluid outlet which is located at the same end as the inlet.

In some embodiments, said shaft has a central portion between said bearing mounting portions, an outer circumference of said central portion being in proximity to an inner circumference of said rotor.

Where the shaft passes through the rotor, the outer circumference of the central portion of the shaft is in proximity to an inner circumference of the rotor allowing cooling of the shaft to have a cooling effect on the rotor due to heat transfer between the corresponding surfaces. In this regard, the distance between them should be large enough so that any flexing of the rotor does not cause the surfaces to contact while being low enough to improve heat transfer. In this regard, a distance of between 1 00 microns and 3mm may be envisaged. In some embodiments, said central portion of said shaft has a larger diameter than said bearing mounting portions.

It may be advantageous to increase the diameter of the shaft within the rotor away from the bearings. In this regard, a narrower shaft where the bearings are supported leads to smaller bearings and lower power requirements and less heating, however a wider shaft where the shaft is being cooled can lead to improved heat transfer and a lower temperature rotor. Thus, in some

embodiments it may be advantageous to increase the diameter of the shaft at the central portion when compared to the portion where the bearings are mounted.

In some embodiments, the outer surface of the shaft may be roughened such that heat transfer from the shaft surface is improved. In this regard, increasing the surface area increases heat transfer from this surface but may also lead to an increased distance between at least some of the surface and the rotor. This in turn will reduce heat transfer between the rotor and the shaft to some extent. Thus, the provision of small fins such as a spiral pattern on the outer surface of the shaft may improve heat transfer and may be advantageous.

In some embodiments, the rotating machine comprises at least one further rotor rotatable about at least one further axis of rotation parallel to said axis of rotation, each of said at least one further rotor being mounted on two further bearings, at least one of said further bearings mounted around at least one further fixed supporting member; wherein each of said at least one further rotor is rotatably mounted on an outer surface of said at least one of said two further bearings, said two further bearings being located towards opposing axial ends of said at least one further rotor. Although embodiments of the invention are applicable to single rotor machines, they are particularly applicable to multiple such as dual rotor machines where two or more rotors rotate within a stator and where each rotor is mounted on bearings on fixed supporting members. Such dual or multiple rotor machines require particularly small clearances and therefore high tolerances and mounting at least one end of the rotors on the external surface of the bearings and thereby providing less flexion and more stiffness can be very

advantageous. In some embodiments, the rotor, fixed supporting members and bearings of the rotor and at least one further rotor correspond and have the same features.

In some embodiments, said rotor and said further rotor each comprise radial protrusions and are mounted such that said radial protrusions intermesh.

Many dual rotor or multiple rotor machines have inter-meshing radial

protrusions on their rotors which act to pump a fluid and these types of rotating machines are particularly applicable to embodiments of the invention. In some embodiments, the rotating machine further comprises a motor and gear means driven by said motor for driving said rotor, said motor being offset with respect to said rotor.

The rotor may be driven by a motor and this motor may be offset with respect to the rotor with gears being used to drive the rotor. Where this is a dual rotor machine, then the gears will drive both rotors.

In other embodiments, the motor may be directly coupled to the rotor; and where there are dual rotors then gears may be used to drive the second rotor.

In some embodiments, said rotor comprises axial protrusions at a defined nonzero radial position on either end of said rotor, said axial protrusions forming a hollow cylindrical shape, an internal circumference of said axial protrusions being mounted on an outer circumference of one of said bearings.

In order to mount the motor on the outer surface of the bearings, the rotor may have axial protrusions at a particular radial position, which axial protrusions will have a hollow cylindrical form and can be mounted around the bearings. These protrusions provide a mounting surface for supporting the rotor on the outer surfaces of the bearings.

In some embodiments, said gear means are configured to contact said axial protrusions to drive said rotor.

Where there are axial protrusions used to mount the rotor on the bearings, these may be driven by the gear means to rotate the rotor. In some embodiments, said bearings are mounted as a close clearance fit on said fixed supporting member and said rotor is mounted on said bearings as a light interference fit.

In order to provide bearings where the outer surface of the bearings support the rotating member, then it may be advantageous to mount them as a close clearance fit on the fixed internal supporting member and to mount the outer rotor as a light interference fit such that it can rotate freely. The close clearance fit allows axial movement of the bearing on the shaft due to thermal expansion. The light interference fit ensures that the outer race rotates with the rotor. The term 'light' indicates that some room is allowed for thermal expansions.

Although embodiments of the invention are applicable to a number of different rotating machines, they are particularly applicable to a vacuum pump where clearances are small and where it is therefore advantageous to reduce the flexibility of a rotor, particularly when it is rotating fast or at under high

temperature conditions.

The vacuum pump may be one of a number of different types of pumps including a dry pump such as a screw (single or double-ended) or roots pump or blower, a turbo pump or a rotary vane pump.

In some embodiments, said pump comprises one of a double ended pump comprising exhaust outlets at both axial ends, a single ended pump with an exhaust at one axial end and a pump with an inlet at opposing radial sides. A double ended pump can be advantageous as it helps to manage thermal loads as compression occurs in both directions and the mechanical loadings on the shaft are equal in either direction and on both bearings. However, the heat associated with such a pump can be high and such pumps tend to be more expensive. Thus, providing them with additional cooling facilities and also with a stiffer mounting means can be advantageous and worthwhile in such a pump.

A second aspect of the present invention provides a rotor for a pump, said rotor comprising an axial protrusion at a defined non-zero radial position on both axial ends of said rotor, each of said axial protrusions forming a hollow cylindrical shape and being configured to rotatably mount said rotor via bearings located within said hollow cylindrical shape wherein said rotor comprises a cylindrical hole passing through a centre of said rotor such that a shaft may pass through said rotor.

A rotor that comprises hollow cylindrical mounting protrusions at each end adapted for mounting on the outer surface of bearings allows the bearings to be mounted on fixed supports and allows the at least one protrusion which acts as the supporting shaft of the rotor to be wider than a supporting shaft that is mounted on the inner surface of the bearings as is the conventional case. An increased diameter in the supporting shaft of the rotor provides stiffness and stability and increases its resonant frequency. The rotor comprises axial protrusions at both ends, said protrusions forming hollow cylindrical shapes and being configured to rotatably mount said rotor via bearings located within said hollow cylindrical shape.

The rotor is hollow comprising a cylindrical space passing through a centre of said rotor such that a shaft may pass through said rotor.

Where the rotor has a hollow centre, then a shaft may pass through the centre. This reduces the material required for manufacturing the rotor and reduces its weight. Furthermore, where cooling fluids are used it provides a potential passage for cooling fluids to flow through the rotor and provide effective cooling along the length of the rotor.

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

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

BRIEF DESCRIPTION OF THE DRAWINGS

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

Figure 1 shows a pump according to the prior art;

Figure 2 shows a pump according to an embodiment;

Figure 3 shows a Roots blower design with an inlet at the top and bottom according to an embodiment;

Figure 4 shows a Roots blower pump driven by an offset motor coupled to the pump by gears according to an embodiment;

Figure 5 shows a directly coupled motor driving a pump as shown in Figure 6; and

Figure 6 shows a pump with improved cooling due to an increased diameter support shaft according to an embodiment.

DESCRIPTION OF THE EMBODIMENTS

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

As noted previously a small diameter shaft on a rotating machine can provide a rotor with a relatively low resonant frequency which can compromise running speeds. As rotating machines such as pumps or compressors become smaller they require higher running speeds to move the same amount of fluid. As speeds increase, the natural frequency of the rotor may be approached which leads to instabilities, . In particular, flexure of the rotor(s) result in loss of the necessary clearances between fixed and moving parts. Any movement of the rotor is particularly disadvantageous for machines with small clearances.

Increasing the diameter of the shaft may improve its stiffness and increase its natural frequency, however it also increases the size of the bearings on which the shaft is mounted which leads to an increase in heat generated and power required. This has been addressed by providing a rotor that is mounted on an outer surface of bearings causing the outer race to rotate with the rotor and the inner race to be fixed. In this way an increased mounting diameter for the rotor is provided without a corresponding increase in the size of the bearings.

In order to provide a rotor that can be mounted in this way, one with supporting stub shafts or protrusions on either end, each having a hollow cylindrical shape is used. Such a rotor is mounted on the outer surface of the bearings, the cylindrical stub shafts surrounding them and allowing rotation.

An additional benefit of mounting the rotor in this way is that cooling fluids may be used to cool the fixed supporting members and hence the bearings and other proximate components such as seals. Furthermore, providing a fixed supporting member as a shaft extending through the rotor allows any cooling fluid to flow along the entire shaft and have a cooling effect on the whole rotor. Rotors within a rotating device, in particular within a vacuum pump are hard to cool and providing cooling in this way can be advantageous.

Figure 2 shows a stator 1 0 with water cooling 1 2, the stator surrounding two pump rotors 20 and 22 which are inter-meshing pump rotors and which are themselves mounted on bearings 30. Pump rotors 20 and 22 are mounted via respective axial projections 21 and 23 which have a hollow cylindrical form and project from an outer axial edge of the pump rotor. These axial projections 21 , 23 are supported on the outer surfaces of bearings 30 and rotate around these bearings. Bearings 30 are mounted on a fixed shaft 40 with a cooling fluid inlet 41 and outlet 42. This cooling fluid allows the bearings to be cooled and helps protect them from overheating. In this embodiment there are shaft seals 50 between the stator 1 0 and the bearings 30 which act to stop or at least impede oil around the bearings from leaking into the pump main body. In some embodiments, in order to avoid an extra heat load on the rotors, the shaft seals 50 can

incorporate piston rings rather than lip seals. In other embodiments the seal may be in the form of a mechanical face seal between the rotor and headplate rather than a radial lip seal on the outside of the rotor. In such a case the headplate would have a slightly different shape to that shown to allow the mechanical face seal to be installed.

Figure 2 also shows gears 60 which are used to drive the rotors via the axial projections 21 and 23 causing them to rotate on bearings 30.

Thus, in this embodiment, the pump rotors 20, 22 are supported on a stationary water cooled shaft 40 via bearings 30. The inner surface of the bearings are mounted as a close clearance fit on the shaft , while the outer surface of the bearings has a light interference fit with the rotor that it supports. This enables the rotors 20. 22 and axial protrusions 21 , 23 to be of a larger diameter than a conventional rotor shaft as the bearings are located inside the supporting shaft rather than on the outer surface of the shaft as is the case in the prior art, (see figure 1 ). Using larger diameter shafts increases the natural frequency of the rotating mass compared with a conventional assembly.

It should be noted that on machines with two or more rotors, careful selection of materials will allow the thermal expansions of the headplate, rotors and stator to be matched so that the radial clearances between the rotor and stator can be maintained under different operating conditions which have differing heat load conditions. For example in some cases the stator may be iron or steel and the housing aluminium and in such a case care should be taken such that where the housing is maintained at 1 00 °C and the stator at 200 °C, then the shaft and stator bore will remain aligned with each other. In practice, the aluminium housing will also hold the oil for lubricating the bearings and gears and will need to be maintained at around 80 °C as a maximum temperature. In Figure 2 water-cooled bearing supports are used to cool oil exiting the bearings.

In this embodiment, the bearings are a light interference fit in the rotor and have a close clearance fit on the bearing supports. The bearing pre-load is applied to the inner race on the support shaft using a spring which may be a wave or a coil spring.

Figure 2 shows a double-ended pump design which is designed to have the exhaust at both ends. Such a design helps manage the thermal loads on the rotor and stator and the axial loads on the bearings. As can be seen the stationary bearing support members 40 are in the form of a shaft with the cooling fluid flowing into an inlet 41 and out of an outlet 42 at the opposite axial end of the shaft. In this way, the cooling fluid not only provides a cooling effect to the bearing but also provides some cooling to the rotor. The rotor is mounted on the bearings and is shaped such that there is a narrow gap between the inner diameter of the main central body of the rotor and the stationary shaft. The narrow gap allows for some flexing of the rotor and/or shaft without the rotor contacting the shaft and seizing the pump while also providing for thermal transfer between the two bodies.

Figure 3 shows a Roots blower design with an inlet at the top and the bottom (not shown). Roots blower pumps can heat up significantly during use which can lead to excessive bearing temperatures. Thus, providing them with additional cooling can be very advantageous. In this embodiment, as in the embodiment of Figure 2, the bearings 30 are mounted on a through shaft 40 which provides cooling not only to the bearings and the seals 50 around the bearings, but also to the rotor 20, 22 itself. Figure 4 shows a Roots blower pump driven by an offset motor 70. The offset motor 70 drives the rotors 20, 22 of the pump via gears 60.

Figure 5 shows an alternative embodiment where the motor 70 is directly coupled to one of the rotors 22 and gearing is used to drive the other rotor 20. One effect of this is that the shaft 40 through the directly driven rotor 22 is extended and this leads to it being less well supported with the possibility of greater flexion. Figure 6 shows a pump where the shaft 40 for mounting bearings 30 has a smaller diameter around the bearing mounting portion than it does at its central portion. Providing an increased diameter at the central portion allows for increase heat capacity and fluid flow and increases the possibility of heat transfer. Furthermore, the heat capacity of the rotors 20, 22 being cooled is decreased due to the increased internal diameter. In some embodiments, there are small fins on the outer surface of the shafts 40 which improve the thermal transfer from the outer surface. In this regard, the fins are there to increase the surface area, but should not be too large as this would increase the distance between the rotor and the main body of the shaft. This particular embodiment is a double-ended pump which due to its increased costs and high efficiency is particularly applicable to this design.

Although embodiments of the invention are applicable to any type of rotating machines, they are particularly applicable to hot running semi-conductor vacuum pumps where stator surface temperatures often exceed 150°C.

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

Stator 1 0

Cooling 1 2

Rotors 20, 22

Rotor stub shaft 21 , 23 Bearing 30

Bearing support member 40 Cooling fluid inlet 41 Cooling fluid outlet 42 Shaft seal 50

Gears 60

Motor 70

Claims

1 . A rotating machine comprising:

a stator;

a rotor rotatable about an axis of rotation within said stator;

said rotor being rotatably mounted on two bearings, said two bearings being located towards opposing axial ends of said rotor;

at least one fixed supporting member supporting said two bearings, said bearings being mounted around said at least one fixed supporting member, such that said rotor is rotatably mounted on an outer surface of said at least one of said bearings; wherein

said at least one fixed supporting member comprises a shaft passing through said rotor, said two bearings being mounted on portions of said shaft located towards either end of said rotor.

2. A rotating machine according to claim 1 , said at least one fixed supporting member comprising a cooling fluid inlet, a cooling fluid flow path and a cooling fluid outlet for the flow of cooling fluid.

3. A rotating machine according to claim 2 wherein said fluid inlet and fluid outlet are at opposing axial ends of said shaft.

4. A rotating machine according to claim 2, wherein said fluid inlet and fluid outlet are at a same axial end of said shaft.

5. A rotating machine according to any preceding claim, wherein said two bearings are mounted around said shaft and said rotor is rotatably mounted on an outer surface of said two bearings.

6. A rotating machine according to any preceding claim, wherein said shaft has a central portion between said bearing mounting portions, an outer circumference of said central portion being in proximity to an inner

circumference of said rotor.

7. A rotating machine according to claim 6, wherein said central portion of said shaft has a larger diameter than said portions on which said bearings are mounted.

8. A rotating machine according to any preceding claim, comprising at least one further rotor rotatable about a further axis of rotation parallel to said axis of rotation, each of said at least one further rotor being mounted on two further bearings, at least one of each of said two further bearings being mounted around at least one further fixed supporting member; wherein each of said at least one further rotor is rotatably mounted on an outer surface at least one of said two further bearings, said two further bearings being located towards opposing axial ends of said at least one further rotor.

9. A rotating machine according to claim 8, wherein said further rotor, said at least one further fixed supporting member and said two further bearings have corresponding features to said respective rotor, said at least one fixed supporting member and said two bearings.

10. A rotating machine according to claim 8 or 9, wherein said rotor and said further rotor each comprise radial protrusions and are mounted such that said radial protrusions intermesh.

1 1 . A rotating machine according to any preceding claim, further comprising a motor and gear means driven by said motor for driving said rotor, said motor being offset with respect to said rotor.

12. A rotating machine according to claim 1 1 , when dependent on any one of claims 8 to 1 0, wherein said motor is operable to drive said rotor and said further rotor via said gear means.

13. A rotating machine according to any preceding claim, further comprising a motor directly coupled to said rotor.

14. A rotating machine according to claim 1 3, when dependent on any one of claims 8 to 1 0, and further comprising gear means driven by said motor, wherein said gear means is operable to drive said further rotor.

15. A rotating machine according to any preceding claim, wherein said rotor comprises axial protrusions at a defined non-zero radial position on either end of said rotor, said axial protrusions forming a hollow cylindrical shape, an internal circumference of said axial protrusions being mounted on an outer circumference of one of said bearings.

16. A rotating machine according to claim 1 5 when dependent on claim 1 2, 13 or 14, wherein said gear means are configured to contact said axial protrusions to drive said rotor.

17. A rotating machine according to any preceding claim, wherein said bearings are mounted as a close clearance fit on said fixed supporting member and said rotor is mounted on said bearings as a light interference fit.

18. A rotating machine according to any preceding claim, wherein said rotating machine comprises a vacuum pump.

19. A rotating machine according to claim 1 8, wherein said vacuum pump comprises one of a dry pump such as a screw or Roots pump or blower, a turbo pump or a rotary vane pump..

20. A rotating machine according to claim 1 8 or 1 9, wherein said pump comprises one of a double ended pump comprising exhaust outlets at both axial ends, a single ended pump with an exhaust at one axial end and a pump with an inlet at opposing radial sides.

21 . A rotor for a pump, said rotor comprising axial protrusions at a defined non-zero radial position on both axial ends of said rotor, said axial protrusions each forming a hollow cylindrical shape and being configured to rotatably mount said rotor via bearings located within said hollow cylindrical shape; wherein said rotor comprises a cylindrical hole passing through a centre of said rotor such that a shaft may pass through said rotor.

22. A rotor according to claim 21 , wherein said rotor comprises said axial protrusions at a defined non-zero radial position at both axial ends of said rotor, said axial protrusions forming hollow cylindrical shapes and being configured to rotatably mount said rotor via bearings located within said hollow cylindrical shapes.

23. A rotor according to any one of claims 21 or 22, and further comprising bearings mounted within each of said axial protrusions.