Field

[001 ] The present invention relates to vacuum pumps and, in particular, to Siegbahn drag pumps, Siegbahn drag pump stages, and to Siegbahn drag pump rotors and rotor assemblies and their methods of assembly.

Background

[002] There has been a long-standing challenge in the field to provide a vacuum pump which is suitably sized, i.e. more compact and, ideally, portable, such that the scope of vacuum pump applications may be widened. It is hoped, for example, that if more compact vacuum pumps can be provided a portable mass spectrometry market may develop.

[003] However, due to the number of parts and functional complexity of existing vacuum pumps, and the pump speed, compression and efficiency characteristics typically required by their application, the production of ever-more compact vacuum pumps is not straightforward. As it stands, either larger sized vacuum pumps, more complex pumps, or several smaller sized vacuum pumps coupled together, for example a turbomolecular pump coupled to a diaphragm pump, are required in order to provide satisfactory vacuum pumping.

[004] Molecular drag pumps, for example Holweck, Gaede or Siegbahn drag pumps, operate on the general principle that momentum from one or more fast-moving rotors is transferred to gas molecules such that the gas molecules are ‘dragged’ from a lower pressure region to a higher pressure region.

[005] Typically, in a Siegbahn drag pump (or in a Siegbahn pump stage), a shaft rotates about a longitudinal axis and mounted thereon is a number of planar, disc-like rotors that extend, from a centre point through which the longitudinal axis passes, orthogonal to the longitudinal axis. Stator elements are arranged either side of each rotor, typically each stator element including an arrangement of spiral or helical channels configured to facilitate the pumping action of the Siegbahn drag pump upon rotation of the shaft. The spiral or helical channels of a stator element are arranged to force pumped gas towards an aperture on the stator. Apertures are often located sequentially towards the centre and then periphery of adjacent stators.

[006] Siegbahn pumps offer a relatively simple pump design compared to other vacuum pumps, but they also typically require more pumping stages and tighter clearances between rotor and stator elements than other types of vacuum pump, including Holweck and Gaede pumps, to provide equivalent pumping performance. More pumping stages require more power and thus more energy is consumed during their use, and a heavier pump with a larger footprint results. Generally, Siegbahn drag pumps have not been widely used, particularly as standalone pumps.

[007] The provision of a more compact vacuum pump which maintains necessary levels of pumping has therefore been challenging to realise.

[008] The present invention, at least in part, addresses these and other problems with the prior art.

Summary

[009] Accordingly, in a first aspect the present invention provides a Siegbahn drag pump comprising a shaft rotatable about a longitudinal axis thereof; two or more rotors mounted to the shaft and extending radially therefrom; and a stator forming at least one pump stage with each rotor. A rotor-facing surface of the stator at each pump stage includes a substantially spiral groove for conveying a gaseous medium. Each rotor is formed at least partially of a carbon fibre reinforced polymer.

[010] Each rotor may have a diameter which is from about 50 mm to about 90 mm.

[011 ] In embodiments, each rotor may be formed substantially, preferably entirely, of a carbon fibre reinforced polymer.

[012] As used herein, the term “carbon fibre reinforced polymer” refers to a composite material containing carbon fibres and a polymer, most often where carbon fibres are the primary structural component of the composite material. For example, each rotor may be formed at least partially, optionally entirely, of a resin filled or bonded carbon fibre sheet. For example, in embodiments, each rotor may be formed of a carbon fibre sheet which is impregnated with an epoxy resin. However, it is envisaged that the proportion of carbon fibre and polymer material may be substantially equal, or that the polymer material may form the primary structural component of the carbon fibre reinforced polymer rotor.

[013] The composite material may include a thermoset resin, such as an epoxy resin, and/or a thermoplastic resin.

[014] In embodiments, the carbon fibre reinforced polymer may comprise polyether ether ketone, most commonly known as PEEK. This configuration is particularly advantageous due to the favourable toughness, elasticity and tensile strength characteristics of PEEK.

[015] As used herein, the term “longitudinal axis” refers to the axis about which the shaft rotates, in use, and about which each rotor and the stator are typically arranged. The longitudinal axis typically passes through the centre of mass of the shaft and each rotor.

[016] Each rotor typically includes a central opening which receives the shaft. In embodiments, each rotor may comprise a boss for locating the rotor on the shaft. Alternatively, the shaft may include supporting means for locating and supporting each rotor during use of the pump.

[017] Typically, each rotor of the Siegbahn drag pump is a substantially planar rotor disc which, in use, is rotated under the action of the shaft driven by a pump motor.

[018] The configuration of the present invention is particularly advantageous because each rotor being comprised at least partially of a carbon fibre reinforced polymer, and preferably having a diameter in the range of from about 50 mm to about 90 mm, minimises the size and mass of each rotor. Thus, the shaft requires lower torque to provide sufficient rotation of each rotor, and each rotor has low inertia during use. [019] This configuration, in turn, enables the stator and other parts of the Siegbahn drag pump to be constructed from lighter-weight materials, such as polymers or cast metal, which can safely house each rotor without an increased risk of failure of the Siegbahn drag pump.

[020] A multi-stage Siegbahn drag pump may thus be provided within a lighterweight, and smaller footprint unit. A lighter-weight, multi-stage Siegbahn drag pump allows the drag pump to be used as a standalone vacuum pump, i.e. without a need for it to be coupled to additional pumps to meet desired pumping requirements.

[021 ] The machining requirements of the Siegbahn pump are also simplified and the number of parts required for construction minimised, which also results in a lower production and maintenance burden, as well as simplified assembly and disassembly of the pump.

[022] Importantly, therefore, a vacuum pump having a reduced footprint and weight may be produced, allowing a desktop, or even a portable, pump to be provided. Additionally, less energy is required during use to produce satisfactory pumping performance.

[023] As described above, it has been a long-standing challenge with existing vacuum pump technology to provide a small, i.e. desktop and/or portable sized, vacuum pump with satisfactory pumping performance.

[024] In embodiments, the two or more rotors are mounted in series on the shaft. In other words, the two or more rotors may be arranged alongside one another along part of the length of the shaft. Typically, the two or more rotors are located towards an end of the shaft.

[025] The shaft and each rotor are typically balanced about the longitudinal axis of the shaft to minimise unbalance, be it static or couple unbalance.

[026] In embodiments, the Siegbahn drag pump may be configured to have a pump speed of around 70,000 RPM, or greater. In embodiments, the Siegbahn drag pump may be configured to produce a vacuum below 10’ ⁴ mbar. [027] In embodiments, each rotor may have a diameter which is around 70 mm.

[028] In embodiments, each rotor may have a substantially circular axial cross section.

[029] In embodiments, each rotor may have a substantially uniform thickness. Each rotor is therefore more balanced compared to a rotor having a substantially non- uniform thickness. This simplifies requirements in producing a rotor. For example, each rotor may be straightforwardly cut from a sheet of a carbon fibre reinforced polymer, e.g. by press-cutting, water-jet cutting or machining.

[030] In embodiments, a said rotor may have a thickness in the range of from around 0.5 mm to around 3 mm. In embodiments, a said rotor may have a thickness of about 1 mm, preferably 1 mm. In embodiments, each rotor may have a substantially similar thickness to all other rotors in the Siegbahn drag pump. For example, where the Siegbahn drag pump comprises only two rotors, the two rotors may have the same thickness as one another. In alternative embodiments, the Siegbahn drag pump may comprise more than two rotors. In embodiments, a first rotor may be of a different thickness to a second rotor.

[031 ] In embodiments, the Siegbahn drag pump may include balancing means. For example, the shaft and/or each rotor may have mass added or removed to improve the balance of the shaft and rotors. For example, each rotor may be filed or otherwise reduced in mass towards its periphery. However, the configuration of this aspect is advantageous because the rotors are inherently well balanced. In embodiments, the shaft may have mass added and/or removed to balance the shaft and rotors during use. For example, mass may be added or removed from an end of the shaft opposite the end towards which each rotor is mounted.

[032] In embodiments, the stator may include a pump inlet cover and a pump motor headplate. The pump inlet cover includes an inlet through which a gaseous medium is drawn into the pump during use. The inlet may be located substantially centrally at the pump inlet cover such that the longitudinal axis passes through the centre of the inlet. [033] In embodiments, the pump inlet cover and the pump motor headplate may each comprise a rotor-facing surface having a spiral groove for conveying a gaseous medium (which is drawn in via the inlet). Advantageously, therefore, the stator may be formed by these components of the pump to minimise the total number of parts of the pump and simplify assembly. By providing the pump inlet cover and pump motor headplate comprising rotor-facing surfaces having spiral grooves, the pump inlet cover and pump motor headplate can form part of a pump stage together with a rotor.

[034] In embodiments, the Siegbahn drag pump may comprise more than two rotors. For example, the Siegbahn drag pump may comprise three or four rotors. Preferably, in order to strike a balance between minimising the size of the Siegbahn pump and its pumping performance, and to improve the ease with which the pump may be assembled, the pump comprises two rotors.

[035] In embodiments, the stator may include at least one mid-stage stator element configured to separate adjacent rotors along the length of the shaft.

[036] In embodiments, the or each mid-stage stator element may include two opposing rotor-facing surfaces having spiral grooves for conveying a gaseous medium. In other words, a mid-stage stator element may form pump stages with two adjacent rotors.

[037] Therefore, multiple Siegbahn drag pump stages may be provided. For example, where the pump includes only two rotors and the stator comprises the pump inlet cover, the pump motor headplate and a mid-stage stator element, each having a rotorfacing surface with a spiral groove, four Siegbahn drag pump stages are provided in a compact and lightweight pump. The two rotors may each be utilised twice in series in this configuration.

[038] Thus, a multi-stage Siegbahn drag pump may be provided by the pump inlet cover, pump motor headplate and the or each mid-stage stator element each including rotor-facing surfaces having spiral grooves for conveying a gaseous medium. [039] In embodiments, the rotor-facing surface of the stator at each pump stage may form an aperture, and the spiral groove of the rotor-facing surface may be configured to direct a gaseous medium towards the aperture. Thus, in use of the pump, a gaseous medium is directed from the pump inlet through the one or more pump stages via the spiral groove of each rotor-facing surface of the stator, towards a pump outlet.

[040] In embodiments, an aperture may be formed at the periphery of each rotor, between the rotor and the stator. In other words, the pump may be configured such that a gaseous medium flows between a periphery of each rotor and the stator towards a subsequent pump stage or the pump outlet.

[041 ] In embodiments, the or each mid-stage stator element may comprise an aperture towards its centre, i.e. proximal to the shaft. In embodiments, the spiral groove of a first rotor-facing surface of the mid-stage stator element directs a gaseous medium towards the aperture, and a second rotor-facing surface of the mid-stage stator element opposing the first rotor-facing surface directs the gaseous medium away from the aperture. Thus, the gaseous medium may be directed towards the aperture and then away from it between adjacent stages of the Siegbahn drag pump.

[042] Adjacent pump stages of the Siegbahn drag pump may also be referred to as ‘passes’. For example, a gaseous medium may flow from the pump inlet towards a first aperture formed at the periphery a first rotor during a first ‘pass’, and the gaseous medium may subsequently flow from the first aperture towards a second aperture located towards the centre of a mid-stage stator element during a second ‘pass’.

[043] In embodiments, the pump inlet cover and the pump motor headplate may be configured to serve as a retainer for the or each mid-stage stator element. The or each mid-stage stator element may thus be held in place by the pump inlet cover and pump motor headplate, minimising the number of parts required to produce the Siegbahn drag pump. This configuration also improves the ease with which the Siegbahn drag pump may be assembled.

[044] In embodiments, the or each mid-stage stator element may be comprised of a pair of substantially semi-annular connecting parts. The or each mid-stage stator element may thus be formed as a split, double-sided stator element that can be straightforwardly arranged about the periphery of the shaft and between adjacent rotors. Providing the or each mid-stage stator element as a pair of substantially semicircular connecting parts simplifies the manufacture and assembly of the Siegbahn drag pump.

[045] In embodiments, the two parts of the or each mid-stage stator element may be held in place by the pump inlet cover and the pump motor headplate. In embodiments, one or more seals, for example an O-ring, may be located about the circumference of the or each mid-stage stator element.

[046] In embodiments, the Siegbahn drag pump may be devoid of any other midstage stator element retaining means. In other words, no other retaining means may be required in order for the pair of mid-stage stator element parts to be connected to one another, or to retain the or each mid-stage stator element at their location within the pump.

[047] In embodiments, the or each mid-stage stator element may comprise an extension at its periphery which is configured to abut at least one of the pump inlet cover and/or pump motor headplate. For example, the extension may extend substantially longitudinally from a periphery of the mid-stage stator element.

[048] In embodiments, the or each mid-stage stator element, or the two connecting parts thereof, may have a substantially T-shaped axial cross-section. In other words, the or each mid-stage stator element may include two longitudinal projections at its periphery which are configured to abut the pump inlet cover and pump motor headplate, or alternatively one of the pump inlet cover and pump motor headplate and an adjacent mid-stage stator element, or alternatively two adjacent mid-stage stator elements (if there more than one mid-stage stator element is provided), such that the or each mid-stage stator element is held in place. The or each mid-stage stator element is therefore held securely between the pump inlet cover and pump motor headplate and the risk of failure, including rupture of the drag pump, is thereby reduced. In embodiments, a seal may be located on at least one said longitudinal projection on the or each mid-stage stator element. [049] In embodiments, at least one said rotor includes a boss for locating the rotor on the shaft. In embodiments, each of the rotors includes a boss for located the rotor on the shaft. This configuration is advantageous because no additional support means are required for mounting each rotor on the shaft.

[050] In embodiments, the shaft may include at least one shoulder element corresponding to each rotor, wherein each shoulder element is configured to support the corresponding rotor on the shaft. In embodiments, each rotor may be mounted to a corresponding shoulder element. Each shoulder element provides a platform on which the corresponding rotor may be mounted to the shaft. Rotational force applied by a pump motor to the shaft may therefore be more effectively transferred to each rotor. Each shoulder element also reduces the risk of damage to the corresponding rotor, and thus the pump as a whole, during use because the corresponding rotor is more securely mounted to the shaft. In embodiments, each shoulder element may be integrally formed with the shaft.

[051] In embodiments, each rotor may be bonded directly to the corresponding shoulder element of the shaft. For example, each rotor may be irreversibly fused to the corresponding shoulder element such that the shaft and each rotor are substantially unitary. Alternatively, each rotor may be reversibly mounted or bonded to the corresponding shoulder element. In embodiments, each rotor may be fixed to the corresponding shoulder element by means of a fixing element. In embodiments, each shoulder element may include a sleeve or bracket for receiving the corresponding rotor.

[052] In embodiments, the Siegbahn drag pump comprises two or more rotors and the shaft may include two or more corresponding shoulder elements, wherein the at least two said shoulder elements may be located on opposing stator-facing surfaces of adjacent rotors. In other words, adjacent shoulder elements may face one another such that their rotors may be received on the shaft from opposite ends of the shaft without removal of the shoulder elements. When the pump comprises two rotors, the shoulder elements are located between the rotors. Assembly (and disassembly) of the Siegbahn drag pump is thus more straightforward. [053] In embodiments, a said shoulder element may have a diameter which is approximately 10% to 30% of the diameter of the corresponding rotor, preferably 10 to 20% of the diameter of the corresponding rotor. For example, a said rotor may have a diameter of around 70 mm, and the corresponding shoulder element may have a diameter of around 15 mm.

[054] In embodiments, each rotor may include a reinforcing collar mounted thereto. Each reinforcing collar may be located towards the centre of the corresponding rotor proximal to the shaft. Each reinforcing collar may be substantially annular and may include a central aperture which receives the shaft, wherein the central aperture of the collar is substantially similar in size to the central opening of the corresponding rotor. A reinforcing collar aids location of the rotor on the shaft and increases the structural integrity of the associated rotor, thus reducing the risk of failure of the Siegbahn drag pump.

[055] In embodiments, each reinforcing collar mounted to a said rotor may be located on a surface of the rotor opposite the surface which is mounted to a shoulder element of the shaft. In other words, a said rotor may be located, or ‘sandwiched’, between a shoulder element of the shaft and a reinforcing collar.

[056] In embodiments, each reinforcing collar may have a diameter which is approximately 10% to 30% of the diameter of the corresponding rotor, preferably 10 to 20% of the diameter of the corresponding rotor. For example, a said rotor may have a diameter of around 70 mm, and the corresponding reinforcing collar may have a diameter of around 15 mm.

[057] In embodiments, each reinforcing collar may be formed at least partially of a carbon fibre reinforced polymer. In embodiments, each reinforcing collar may be formed substantially, preferably entirely, of a carbon fibre reinforced polymer. In embodiments, each reinforcing collar may be formed of substantially the same material as the corresponding rotor.

[058] In embodiments, each reinforcing collar may be formed partially or entirely of steel. [059] In embodiments, each rotor may be treated with material configured to reduce outgassing during use of the Siegbahn drag pump when the pump is in use.

[060] As used herein, the term ‘outgassing’ refers to the release or escape of gas from the surface of a solid component of the Siegbahn drag pump as a vacuum is generated during use.

[061 ] In embodiments, the stator may be treated with material configured to reduce outgassing when the pump is in use.

[062] Treatment of each rotor and/or the stator with a material configured to reduce outgassing may include impregnating and/or coating each rotor and/or the stator with said material.

[063] In embodiments, each rotor may be coated with a metallic coating, for example an aluminium and/or nickel based coating. Such a metal coating improves the outgassing characteristics of the Siegbahn drag pump during its use.

[064] In embodiments, the shaft is in the form of a cantilever. In embodiments, the shaft has a free end and a retained end, and each rotor is located towards the free end of the shaft. In embodiments, the retained end of the shaft is located towards a motor of the Siegbahn drag pump. Thus, a cleaner inlet vacuum is produced. In embodiments, the shaft may be supported by a number of bearings towards the retained end of the shaft. Each rotor may also be supported by these bearings and no other structure.

[065] In a further aspect the present invention provides a rotor assembly for a Siegbahn drag pump, the rotor assembly comprising a shaft rotatable about a longitudinal axis thereof; and two or more rotors mounted to the shaft and extending radially therefrom, wherein each rotor is formed at least partially, preferably substantially, of a carbon fibre reinforced polymer; and wherein the shaft is in the form of a cantilever; and wherein each rotor is located towards one end of the shaft.

[066] In embodiments, the free end of the shaft may be substantially hollow. In embodiments, the entire shaft may be substantially hollow, i.e. the shaft may have a cavity which extends therethrough substantially (although possibly not entirely) between the free end and the retained end thereof. These configurations reduce the weight of the shaft and thus further reduce the weight of the Siegbahn drag pump. Moreover, the cavity of the shaft at the free end thereof may be configured to receive a turbo-impeller, turbomolecular pump stage or similar to enhance the pumping performance of the Siegbahn drag pump.

[067] In embodiments, one or more parts of the Siegbahn drag pump may be formed by injection moulding. In embodiments, one or more parts of the Siegbahn pump may be formed by an additive manufacturing process, for example by three-dimensional printing.

[068] In embodiments, the stator may be formed of a polymer material. In embodiments, the stator may be formed of a metal.

[069] In a further aspect, the invention provides a method of assembly of the Siegbahn drag pump according to any suitable preceding aspect, the method comprising the steps of: mounting each rotor, and optionally each corresponding reinforcing collar, to the shaft, each rotor being located towards the free end thereof; connecting the restrained end of the shaft to a pump motor; and connecting the one or more mid-stage stator elements and the pump inlet cover to the pump motor headplate.

[070] In embodiments, the Siegbahn drag pump may comprise two adjacent rotors and the shaft includes two corresponding adjacent shoulder elements that are located between the adjacent rotors on opposing stator-facing surfaces thereof; and the method further comprises the steps of: sliding a first rotor onto the shaft from the free end thereof; and sliding a second rotor onto the shaft from the opposite end thereof.

[071 ] In a further aspect, the invention provides a Siegbahn drag pump shaft including two adjacent rotors and two corresponding shoulder elements configured to support the rotors; wherein the shoulder elements are located between the adjacent rotors on opposing stator-facing surfaces thereof. [072] In other words, adjacent shoulder elements may face one another such that their corresponding rotors may be received on the shaft from opposite ends of the shaft without removal of the shoulder elements. Assembly (and disassembly) of the Siegbahn drag pump shaft is thus more straightforward.

[073] In a further aspect, the invention provides a method of assembly of the Siegbahn drag pump shaft according to the immediately preceding aspect, comprising the steps of: sliding a first rotor onto the shaft from a free end thereof; sliding a second rotor onto the shaft from the opposite end thereof; and mounting the first and second rotors to their corresponding shoulder elements.

[074] In a further aspect, the invention provides a method of assembly of a rotor assembly for a Siegbahn drag pump, comprising the steps of: sliding a first rotor onto the shaft from a free end thereof; sliding a second rotor onto the shaft from the opposite end thereof; and mounting the first and second rotors towards the free end of the shaft, each rotor being formed at least partially, preferably substantially, of a carbon fibre reinforced polymer.

[075] In a further aspect, the invention provides a Siegbahn drag pump stage formed between a stator-facing surface of a rotor and an opposing rotor-facing surface of a stator; wherein the rotor-facing surface of the stator comprises a spiral groove configured to convey a gaseous medium. The stator-facing surface of the rotor is formed at least partially of a carbon fibre reinforced polymer and the stator-facing surface of the rotor preferably has a diameter which is from about 50 mm to about 90 mm.

[076] In embodiments, the Siegbahn drag pump stage may be the only pump stage in a Siegbahn drag pump, or one of multiple Siegbahn drag pump stages in a Siegbahn drag pump, or one of multiple pump stages in a vacuum pump comprising the Siegbahn drag pump stage and another type of pump stage, for example a turbomolecular pump stage.

[077] In embodiments, the stator-facing surface of the rotor is formed substantially, preferably entirely, of a carbon fibre reinforced polymer. [078] In embodiments, the stator-facing surface of the rotor and/or the rotor-facing surface of the stator may comprise an aperture, and the spiral groove of the rotorfacing surface may be configured to direct a gaseous medium towards the aperture.

[079] In embodiments, the aperture may be located towards a periphery of the statorfacing surface of the rotor between the rotor and the rotor-facing surface of the stator. In embodiments, the aperture may be located towards the centre of the rotor-facing surface of the stator.

[080] In embodiments, the rotor-facing surface of the stator may be formed of a polymer. In embodiments, the rotor-facing surface of the stator may be formed of metal.

[081 ] In embodiments, one or more parts of the Siegbahn drag pump stage may be formed by injection moulding. In embodiments, one or more parts of the Siegbahn drag pump stage may be formed by an additive manufacturing process, for example by three-dimensional printing.

[082] In a further aspect, the invention provides a Siegbahn drag pump stator arrangement comprising a pump inlet cover, a pump motor headplate, and at least one mid-stage stator element. Each of the pump inlet cover, pump motor headplate and the or each mid-stage stator element includes one or more rotor-facing surfaces having spiral grooves for conveying a gaseous medium. The pump inlet cover and pump motor headplate are configured to serve as a retainer for the or each mid-stage stator element.

[083] In embodiments, the Siegbahn drag pump stator arrangement may comprise a single mid-stage stator element, or a plurality of mid-stage stator elements.

[084] In embodiments, the or each mid-stage stator element may comprise an extension at its periphery which is configured to abut at least one of the pump inlet cover and/or pump motor headplate. For example, the extension may extend substantially longitudinally from a periphery of the mid-stage stator element.

[085] In embodiments, the or each mid-stage stator element may have a substantially T-shaped axial cross-section. In other words, the or each mid-stage stator element may include two longitudinal projections at its periphery which are configured to abut each of the pump inlet cover and pump motor headplate such that the mid-stage stator element is held in place therebetween. This risk of failure of a Siegbahn drag pump including the mid-stage stator element is thereby reduced because the mid-stage stator element can abut a pump inlet cover and/or pump motor headplate of the drag pump.

[086] In embodiments, one or more parts of the Siegbahn drag pump stator arrangement may be formed by injection moulding. In embodiments, one or more parts of the Siegbahn drag pump stator arrangement may be formed by an additive manufacturing process, for example by three-dimensional printing.

[087] In a further aspect, the invention provides a mid-stage stator element for a Siegbahn drag pump, the mid-stage stator element comprising at least one surface including a spiral groove configured to convey a gaseous medium; and wherein the mid-stage stator element has a substantially T-shaped axial cross section.

[088] In a further aspect, the invention provides a Siegbahn drag pump rotor formed at least partially of a carbon fibre reinforced polymer and having a diameter which is from around 50mm to around 90mm.

[089] In embodiments, the rotor may be substantially planar, and may have a substantially circular axial cross section.

[090] In embodiments, the rotor may be formed substantially, preferably entirely, of a carbon fibre reinforced polymer.

[091 ] In embodiments, the rotor may have a diameter which is around 70 mm.

[092] In embodiments, the rotor may have a substantially uniform thickness.

[093] In embodiments, the rotor may have a thickness of from about 0.5 mm to about 3 mm. In embodiments, the rotor may have a thickness of about 1 mm, preferably 1 mm. [094] In embodiments, the rotor may be cut from a sheet of carbon fibre reinforced polymer, e.g. by press-cutting, water-jet cutting or machining.

[095] In embodiments, the rotor may include a reinforcing collar. In embodiments, the reinforcing collar may be approximately 10% to 30% of the size of the corresponding rotor. In embodiments, the reinforcing collar may be formed substantially, preferably entirely of a carbon fibre reinforced polymer. In embodiments, the reinforcing collar may be formed of steel.

[096] In a further aspect, the invention provides the Siegbahn drag pump rotor according to any suitable preceding aspect, wherein the rotor is formed by the process of cutting a sheet of carbon fibre reinforced polymer. In embodiments, the process of cutting the sheet of carbon fibre reinforced polymer includes press-cutting, water-jet cutting and/or machining.

[097] In a further aspect, the invention provides the Siegbahn drag pump rotor according to any suitable preceding aspect, wherein the rotor is formed by an additive manufacturing process.

[098] In a further aspect, the invention provides a vacuum pump comprising a Siegbahn drag pump shaft, a Siegbahn drag pump stage, a Siegbahn drag pump stator arrangement, a mid-stage stator element, a Siegbahn drag pump rotor and/or a Siegbahn drag pump rotor assembly according to any preceding aspect.

[099] For the avoidance of doubt, all aspects and embodiments may be combined mutatis mutandis.

Description of the Figures

[100] The invention will now be described with reference to the following figures, which are intended to be non-limiting.

[101 ] Figure 1 shows a longitudinal cross section of a Siegbahn drag pump.

[102] Figure 2 shows a perspective cross sectional view of a Siegbahn drag pump. [103] Figure 3 shows a cutaway, part exploded view of a shaft and two rotors of a Siegbahn drag pump.

[104] Figure 4 shows an exploded view of part of a Siegbahn drag pump.

[105] Figure 5 shows a perspective view of a pump motor headplate of a Siegbahn drag pump.

Detailed Description

[106] Referring to Figure 1 , a Siegbahn drag pump 10 comprises a shaft 12 which is rotatable, in use, about a longitudinal axis (illustrated in Figure 1 by dashed line ‘A’), two rotors 14, 16 which are mounted to the shaft 12, and a stator 18 which surrounds and houses the rotors 14, 16 and part of the shaft 12.

[107] The rotors 14, 16 are disc-shaped, annular rotors which include an aperture (seen in more detail in Figure 4) for receiving the shaft 12.

[108] Together with the two rotors 14, 16, the stator 18 provides four pump stages, or passes. The drag pump is thus a 4-stage Siegbahn drag pump, wherein each rotor is exploited twice in series i.e. both surfaces of each rotor are utilised during use as a gaseous medium flows from a pump inlet 20 to a pump outlet 22.

[109] The stator 18 comprises a pump inlet cover 24, a pump motor headplate 26 and a mid-stage stator element 28 which is located between the two rotors 14, 16 along the length of the shaft 12.

[110] Each of the pump inlet cover 24, the pump motor headplate 26 and the midstage stator element 28 comprise rotor-facing surfaces including a substantially spiral groove, such as 30. As discussed in greater detail below with reference to Figure 5, each spiral groove is in the form of a multi-start spiral. The mid-stage stator element 28 includes two opposing rotor-facing surfaces such that the mid-stage stator element 28 forms a pump stage with each of the two rotors 14, 16. The groove at each rotorfacing surface of the stator 18 is configured to direct the gaseous medium from the pump inlet 20 towards the pump outlet 22. [111 ] In use, the shaft 12 is rotated by a pump motor (not shown) such that the rotors 14, 16 rotate between the stator parts. The pump motor is housed within housing 32. A number of bearings, such as 34, support and facilitate movement of the shaft 12.

[112] Moreover, the shaft 12 is in the form of a cantilever. In other words, the shaft 12 is supported at one end and not at the other. As shown more clearly in Figure 4, the shaft has a free end 36 and a retained end (not shown). The free end 36 of the shaft 12 is free of bearings or other supporting means so that the gaseous medium flowing into the pump 10 via the pump inlet 20 is not impeded.

[113] Referring back to Figure 1 , each rotor 14, 16 is formed of a carbon fibre reinforced polymer, e.g. a carbon fibre sheet impregnated with epoxy resin, and has a diameter of around 70 mm. Furthermore, each rotor 14, 16 is substantially uniform in thickness, of around 1 mm, and has substantially the same dimensions as the other rotor.

[114] Because the rotors 14, 16 are formed of a carbon fibre reinforced polymer, the stator 18 and other parts of the Siegbahn drag pump 10 can be formed of lighterweight materials than is conventionally possible. For example, the stator may be formed of a polymer or of a cast metal. The portability of the Siegbahn drag pump 10 is thus improved.

[115] The Siegbahn drag pump 10 is configured to have a pumping speed of around 70,000 RPM and to produce a vacuum below 10’ ⁴ mbar.

[116] As described, the stator 18 is formed of a pump inlet cover 24, a pump motor headplate 26 and a mid-stage stator element 28, each having a rotor-facing surface including a spiral groove. Each rotor-facing surface forms an aperture towards which the spiral groove of that rotor-facing surface directs a gaseous medium during use.

[117] Referring now to Figure 2, at a first pump stage a first rotor-facing surface, belonging to the pump inlet cover 24 includes a spiral groove configured to direct a gaseous medium towards the periphery of the first rotor 14. An aperture is formed between the pump inlet cover 24 and the periphery of the first rotor 14 so that the gaseous medium can flow from the first pump stage into a second pump stage formed between the first rotor 14 and the mid-stage stator element 28, as illustrated by the dashed arrows in Figure 2.

[118] At the second pump stage, a first rotor-facing surface of the mid-stage stator element 28 includes a spiral groove configured to direct the gaseous medium towards an aperture 40 formed at a central region of the mid-stage stator element 28. The aperture 40 allows the gaseous medium to flow from the second pump stage to a third pump stage formed between the mid-stage stator element 28 and the second rotor 16.

[119] At the third pump stage, a second rotor-facing surface of the mid-stage stator element 28 includes a spiral groove configured to direct the gaseous medium towards an aperture formed between the mid-stage stator element 28 and the periphery of the second rotor 16. The aperture formed between the mid-stage stator element 28 and the periphery of the second rotor 16 allows the gaseous medium to flow from the third pump stage to a fourth pump stage formed between the second rotor 16 and the pump motor headplate 26.

[120] At the fourth pump stage, a rotor-facing surface of the pump motor headplate 26 includes a spiral groove configured to direct the gaseous medium towards the pump outlet 22, which is formed in the rotor-facing surface of the pump motor headplate 26.

[121 ] The rotors 14, 16 are both mounted, in series, to the shaft 12 and rotate synchronously such that the gaseous medium is directed from the pump inlet 20 towards the pump outlet 22, sequentially though the first, second, third and fourth pump stages of the Siegbahn drag pump 10.

[122] Were the Siegbahn drag pump 10 to comprise additional rotors and mid-stage stator elements, an analogous sequential configuration of apertures located towards the periphery of each rotor and central region of each mid-stage element would be provided to facilitate the flow of a gaseous medium from the pump inlet 20 towards the pump outlet 22.

[123] Referring to both Figures 1 and 2, the mid-stage stator element 28 is comprised of a pair of substantially semi-annular connecting parts (only one part, i.e. half of the mid-stage stator element, is shown in Figures 1 and 2). The mid-stage stator element 28 can therefore be straightforwardly brought together around the shaft 12 during assembly of the pump 10.

[124] The pump inlet cover 24 and pump motor headplate 26 are configured to serve as a retainer for the mid-stage stator element 28. In other words, the mid-stage stator element 28 is housed and each of the semi-annular connecting parts is held in place by the pump inlet cover 24 and the pump motor headplate 28.

[125] More specifically, the mid-stage stator element 28 comprises a projection at its periphery which is configured to abut the pump inlet cover 24 and the pump motor headplate 26 in order that the mid-stage stator element 28 is retained therebetween.

[126] In the embodiment of Figure 1 , the mid-stage stator element 28 includes a projection 42 which protrudes longitudinally towards the pump motor headplate 26. The projection 42 is configured to be held between the pump inlet cover 24 and the pump motor headplate 26 in order to retain the mid-stage stator element. Each connecting part of the mid-stage stator element 28 thus has a substantially L-shaped axial cross-section.

[127] In the embodiment of Figure 2, the mid-stage stator element 28 includes two projections 42, 44, a first projection 42 which protrudes longitudinally towards the pump motor headplate 26, and a second projection 44 which protrudes in the opposite direction towards the pump inlet cover 24. The first projection 42 abuts both the pump inlet cover 24 and the pump motor headplate 26 and the second projection 44 abuts the pump inlet cover 24. Each connecting part of the mid-stage stator element 28 thus has a substantially T-shaped axial cross-section.

[128] Several O-rings 38, which are seen more clearly in Figure 4, bound the midstage stator element 28 and function as a seal between the mid-stage stator element 28 and the pump inlet cover 24.

[129] Referring to Figure 3, the shaft 12 includes two shoulder elements 46, 48 which correspond to the first and second rotors 14, 16. The shoulder elements 46, 48 are configured to support the corresponding rotor 14, 16 on the shaft 12. Typically, the rotors 14, 16 are mounted directly and irreversibly to the shoulder elements 46, 48 but, in alternative embodiments, the rotors 14, 16 could be reversibly mounted to the shoulder elements 46, 48. The shoulder elements 46, 48, which are approximately 15% of the diameter of the corresponding rotor 14, 16, provide a surface through which a rotational force applied to the shaft 12 may be effectively transferred to the rotors 14, 16.

[130] The shoulder elements 46, 48 are adjacent one another on the shaft 12, and are mounted to opposing stator-facing surfaces of the adjacent rotors 14, 16 such that the shoulder elements 46, 48 are located between the two rotors 14, 16. This configuration allows the rotors 14, 16 to be received from opposite ends of the shaft, thereby improving assembly of the Siegbahn drag pump, as described in more detail below.

[131 ] Still referring to Figure 3, each rotor 14, 16 also includes a reinforcing collar 50, 52 mounted thereto. Each reinforcing collar 50, 52 is formed of a carbon fibre reinforced polymer, but it is envisaged that in alternative embodiments each reinforcing collar 50, 52 may be formed of steel or another metal. Each reinforcing collar 50, 52 need not be formed of the same material as its corresponding rotor, but in the embodiment of Figure 3 each rotor and its corresponding reinforcing collar is formed of substantially the same material.

[132] In the embodiment of Figure 3, each reinforcing collar 50, 52 is a separately formed unit which is mounted to its corresponding rotor. In alternative embodiments, each rotor 14, 16 may be formed with a region of greater thickness than the remainder of the rotor towards a central region thereof. In other words, each rotor may be provided without a separate reinforcing collar.

[133] Each reinforcing collar 50, 52 is annular and is located towards the centre of its corresponding rotor 14, 16 proximal to the shaft. Each reinforcing collar is provided with a central aperture, such as 54, to receive the shaft 12. The central aperture 54 is similar in size to a central opening 56 of its corresponding rotor 14. Each reinforcing collar also has a diameter which is approximately 15% of the diameter of its corresponding rotor. [134] Each reinforcing collar 50, 52 is mounted to the corresponding rotor 14, 16 on a surface of the rotor opposite the surface which is mounted to the corresponding shoulder element 46, 48. In other words, the rotor 14, 16 is arranged between its corresponding shoulder element 46, 48 and its corresponding reinforcing collar 50, 52.

[135] The rotors 14, 16 and/or the stator 18 is coated with a material configured to reduce outgassing when the pump is in use. In the embodiments of the Figures, the rotors 14, 16 and the stator 18 are coated with an aluminium and/or nickel based coating.

[136] Referring back to Figure 1 , it can be seen that a portion of the shaft 12 towards the free end thereof is hollow. More specifically, a cavity 58 extends from the free end of the shaft 12 along its length to a point beyond the first and second rotors 14, 16. The configuration of the hollow portion of the shaft may be utilised the balance the shaft 12 and rotors 14, 16.

[137] In certain embodiments, a turbo-impeller or similar (not shown) may be partially received within the cavity 58.

[138] As illustrated most clearly in Figure 3, the configuration of the Siegbahn drag pump allows its assembly (and disassembly) to be simplified. In particular, the configuration of the shaft 12 including two adjacent shoulder elements 46, 48 allows the two rotors 14, 16 to be placed onto the shaft 12 from opposite ends thereof until they abut and can be mounted to their corresponding shoulder element. Additionally, with reference to Figures 1 and 2, the configuration of the mid-stage stator element 28 (that it is formed of a pair of substantially semi-annular parts) allows the mid-stage stator element 28 to be straightforwardly assembled around the shaft 12 once the shaft is in position in the pump 10 with the rotors 14, 16 mounted thereto.

[139] Furthermore, the modular nature of the stator 18, being comprised of the pump motor headplate 26, the pump inlet cover 24 and the mid-stage stator element 28, allows straightforward assembly together with the shaft 12 and rotors 14, 16. For example, the shaft 12 including the rotors 14, 16 may be slid into a central aperture of the pump motor headplate 28 until the shaft 12 is connected to the pump motor (not shown). The mid-stage stator element 28, in its two semi-annular parts, may subsequently be fit around the shaft 12 between the rotors 14, 16 and their respective shoulder elements 46, 48. Lastly, the pump inlet cover 24 may be placed over the shaft 12, rotors 14, 16 and mid-stage stator element 28 until it abuts and connects to the pump motor headplate 26 in order to house the same. Because the shaft 12 is in the form of a cantilever, no securing of the shaft to the pump inlet cover 24 is required.

[140] Moreover, as shown clearly in Figure 4, the modular nature of the shaft 18 and rotors 14, 16, and of the stator 18 comprising the pump inlet cover 24, the pump motor headplate 26 and the mid-stage stator element 28 means that modules may be provided separately for improving the ease with which parts of the Siegbahn drag pump may be replaced over time.

[141 ] Figure 5 shows the pump motor headplate 26 of the Siegbahn drag pump 10, and in particular the rotor-facing surface 60 of the pump motor headplate 26. The rotorfacing surface 60 includes a spiral groove 62 which is configured to convey a gaseous medium during use of the Siegbahn drag pump. More specifically, the spiral groove 62 is in the form of a multi-start spiral. In other words, the rotor-facing surface 60 includes a plurality of distinct, i.e. discontinuous, curved grooves which together form a spiral arrangement.

[142] The distinct curved grooves of the rotor-facing surface 60 each have a central- most end, such as 64, and a peripheral-most end, such as 66. The central-most ends 64 of each groove are closer to one another on the rotor-facing surface 60 than the peripheral-most ends 66 of the grooves. The rotor-facing surface 60 further comprises an aperture 68 towards and through which a gaseous medium is urged during use. The spiral groove 62 of the rotor-facing surface 60 is thus arranged to direct a gaseous medium towards the aperture 68 as the adjacent rotor (not shown) rotates, in use, and ‘drags’ gas molecules around a pathway defined by the rotor-facing surface 60. Reference Key

10 Siegbahn drag pump

12 Shaft

14 First Rotor

16 Second Rotor

18 Stator

20 Pump inlet

22 Pump outlet

24 Pump inlet cover

26 Pump motor headplate

28 Mid-stage stator element

30 Spiral groove

32 Housing

34 Bearing

36 Free end of shaft

38 0-ring

40 Aperture of mid-stage stator element

42 First projection

44 Second projection

46 First shoulder element

48 Second shoulder element

50 First reinforcing collar

52 Second reinforcing collar

54 Central aperture of reinforcing collar

56 Central opening of rotor

58 Cavity of shaft

60 Rotor-facing surface of pump motor headplate

62 Spiral groove of rotor-facing surface of pump motor headplate

64 Central-most end of curved groove

66 Peripheral-most end of curved groove

68 Aperture of rotor-facing surface of pump motor headplate