Field of the Invention [001 ] The present invention relates to a seal and, in particular, an interstage seal for a multistage turbomolecular pump. The invention further relates to a turbomolecular pump comprising such a seal and methods of assembling turbomolecular pumps. Background to the Invention

[002] Multistage turbomolecular pumps may be used for evacuating the stages of mass spectrometer. Referring to the schematic representation in figure 1 , such a pump may comprise a pump envelope (3) containing a cartridge (2) in which a plurality of pumping stages is located. Each pump stage may comprise a group of one or more stators (6) and corresponding rotors (23) supported on an impeller shaft (5) for rotation about an axis (A) for pumping fluid from a main pump inlet (4) to a main pump outlet (8). The main pump inlet (4) is connected for evacuating a high vacuum stage of the mass spectrometer. An inter-stage inlet (9) is provided between pumping stages and is connected for evacuating a lower vacuum stage of the mass spectrometer.

[003] Typically, one or more elastomeric O-rings (7) are used to provide a seal between each pumping stage. During assembly, the pump cartridge (2) may be slid axially into position within the pump envelope (3) by which action the O-rings (7) are compressed between the cartridge and the envelope. Typically, the seals will be rated to a pressure difference of at least 1 bar.

[004] A disadvantage of this arrangement is that significant difficulty may be experienced when assembling and dismantling multistage turbomolecular pumps. In some instances, once inserted the cartridge may be all but immovable, making servicing difficult and risking damage to the pump.

[005] The present invention addresses these and other problems with the prior art.

Summary of the Invention

[006] Accordingly, in a first aspect, the invention provides, a radially extending annular seal of substantially uniform cross-section for a multistage turbomolecular pump cartridge comprising a circumferential engagement surface configured to slidably engage a surface of the pump in a sealing relation.

[007] Preferably, wherein, when engaged, the friction between the seal and the pump surface is less than about 100 N, preferably less than about 50 N.

[008] Advantageously, the invention thereby provides a seal which enables the pump to be assembled and disassembled by hand and/or without risk of damage to the pump or injury to the servicing engineer. Preferably, the axial force required to slide the pump cartridge in to our out of the pump is less than 150 N, preferably less than 100 N, preferably less than 50 N, even when the longitudinal axis of the pump cartridge is vertical.

[009] Additionally, or alternatively, in a further aspect, at least a portion of the cross-section of the seal has an aspect ratio of at least about 1.2. The aspect ratio may for instance be greater than about 1.5, preferably greater than about 2, more preferably from about 1.5 to about 15, more preferably from about 2 to about 10, preferably less than about 5. [0010] For the purposes of the invention, the aspect ratio refers to ratio of the radial extension of the portion (i.e. the length of an imaginary chord running the full length of the identified portion in a radial direction) to the maximum thickness of the identified portion in an axial direction (i.e. the maximum thickness of the portion in a direction tangential to the radial direction).

[001 1 ] Typically, the portion will extend from the outermost circumference of the seal. Preferably the portion extends across a majority of the radial extension of the cross-section, more preferably the portion extends across the entire cross-section of the seal.

[0012] Typically, the seal may have a radiused and/or tapered cross-section extending from the circumferential engagement surface. Typically, the inner surface of the envelope and outer surface of the cartridge are substantially cylindrical and/or substantially concentrically aligned.

[0013] Advantageously, the seal may deflect axially when the pump cartridge is slid into the pump envelope. Typically, a circumferential portion of the seal may trail the remainder of the seal when sliding. Typically, the deflection during sliding, or once inserted, will be from about 0° to about 30°, more preferably to about 20°. The amount of deflection will affect the normal force applied by the seal on the pump envelope, and therefore the amount of friction therebetween.

[0014] Typically, the seal is configured such that when the pump is fully assembled the circumferential engagement surface exerts a normal force of less than about 100 N on the pump envelope, preferably less than about 50 N, preferably less than about 40 N, preferably from about 5 N to about 40 N, more preferably from about 10 N to about 30 N.

[0015] Additionally, or alternatively, when the pump is fully assembled, preferably less than about 100 N of axially applied force is required to overcome the friction between the seal and the pump surface, more preferably less than about 30 N, or more preferably less than about 20 N, preferably from about 1 N to about 10 N. [0016] Preferably, the axial force required to slide the pump cartridge in to our out of the pump is less than 150 N, preferably less than 100 N, preferably less than 50 N, when the longitudinal axis of the pump cartridge is vertical. [0017] Preferably, when the pump is fully assembled, the seal will be deflected towards a higher-pressure stage of the turbomolecular pump. Preferably the sliding direction will be such that the seal deflects towards the higher-pressure stage of the turbomolecular pump. [0018] Applicant has found that the high aspect ratio, blade-like seals of the invention provide an effective barrier to flow between the sections of a turbomolecular pump without the need for a large compressive force between the blade and the pump envelope. Accordingly, by employing a lower force of engagement between the seal and the envelope, the friction therebetween is also reduced, enabling the pump cartridge to be moved in and out of the pump envelope more easily.

[0019] Accordingly, in a further aspect, the present invention provides a radially extending seal for a turbomolecular pump cartridge. The seal may comprise a circumferential engagement surface configured to slidably engage a surface of the pump in a sealing relation. Typically, the seal is configured such that when the pump is fully assembled the circumferential engagement surface exerts a normal force of less than 100N on the pump envelope, preferably less than 50 N on the pump envelope, preferably less than about 40, preferably from about 5 N to about 40 N, more preferably from about 10 N to about 30 N.

[0020] As in other aspects of the invention, preferably less than about 100 N of axially applied force is required to overcome the friction between the seal and the surface, more preferably less than about 50 N, or more preferably less than about 10 N. [0021 ] Typically, the surface of the pump is an inner surface of the pump envelope. Typically, the seal is substantially annular. Preferably the seal has a substantially uniform cross-section. Typically, the seal is coupled to the pump cartridge, preferably substantially fixedly attached thereto.

[0022] The inventors have found that in lower pressure sections of a multistage turbomolecular pump the pressure differential across the seals is minimal and thus only a barrier to flow is required to prevent movement of gas between pump sections. Advantageously, by replacing a deeply compressed O-ring, with a seal according to the invention, friction between the cartridge and envelope may be reduced, allowing them to move more easily relative to one another.

[0023] In a further aspect, the invention provides a turbomolecular pump cartridge comprising a circumferential engagement surface configured to slidably engage with a surface of the pump and wherein the seal comprises a main body comprising a first material and a relatively lower friction second material at the engagement surface. The second material may be coated, co-moulded or otherwise fixed to the surface of the first material. The second material may full encapsulate the first material, or only cover a portion thereof, for instance a minor portion thereof immediately adjacent the circumferential engagement surface.

[0024] Typically, the pump surface is metallic, for instance an aluminium alloy, titanium alloy or stainless steel. Preferably the static coefficient of friction between the second material and the pump surface is less than about 0.5, preferably less than 0.25, more preferably from about 0.04 to about 0.2.

[0025] In embodiments, the first material will be more flexible and/or have a lower compressive strength than the second material. [0026] As in other aspects, the seal may be configured such that when the pump is fully assembled less than about 100 N of applied force is required to overcome the friction between the seal and the surface, more preferably less than about 50 N, or more preferably less than about 30 N, or more preferably less than about 20 N, preferably from about 1 N to about 10 N.

[0027] Preferably, the circumferential engagement surface exerts a normal force of less than 100 N on the surface of the pump, preferably less than 50 N, more preferably from about 5 N to about 40 N, more preferably from about 10 N to about 30 N.

[0028] In a further aspect, the invention provides a turbomolecular pump comprising a housing and a pump cartridge slidably receivable within the housing and comprising a flexible annular seal attached to the cartridge for circumferential frictional engagement with the pump housing to seal a first relatively low pressure portion of the pump from a second relatively high pressure portion of the pump characterised in that the friction between flexible annular seal and the housing is less than about 100 N, preferably less than about 50 N.

[0029] In all aspects, the seal may be attached to a turbomolecular pump cartridge, preferably a cartridge of a multistage turbomolecular pump. Typically, pump cartridge comprises the impeller including rotors mounted on a rotor shaft for rotation about the axis of the shaft, stationary stators between the rotors and stator spacers, and optionally an outer housing. In embodiments, the seal may be located in a circumferential groove in an outside surface of the pump cartridge, for instance a pre-existing O-ring groove in the cartridge housing. Alternatively, the seal may be retained between stator spacer rings.

[0030] The invention further provides a turbomolecular pump cartridge comprising at least one seal according to any preceding claim, and/or a turbomolecular pump comprising at least one such cartridge. [0031 ] In a further aspect the present invention provides a turbomolecular pump comprising a housing, a pump cartridge slidably receivable within the housing, and a flexible annular seal attached to the cartridge and circumferential ly engaged with the pump housing to seal a first relatively low pressure portion of the pump from a second relatively high pressure portion of the pump characterised in that the seal will be breached by a pressure difference between the first and second portion of greater than 10E-2 mbar. Preferably, the pressure difference between the first relatively low pressure portion of the pump and the second relatively high pressure portion of the pump is less than 10E-2 mbar, preferably less than 10 E-3 mbar.

[0032] The seal may be any seal disclosed in an earlier aspect or embodiment of the invention.

[0033] In a further aspect, the present invention provides a turbomolecular pump comprising a housing, a pump cartridge slidably received within the housing, and a flexible annular seal attached to either the cartridge or the housing and in positive engagement with the other of the cartridge or the pump housing characterised in that the normal component of the force of engagement between the seal and the one or other of the housing or cartridge is less than 100 N, preferably less than about 40 N, preferably from about 5 N to about 40 N, more preferably from about 10 N to about 30 N. [0034] Likewise, the seal may be any seal disclosed in an earlier aspect or embodiment of the invention.

[0035] Typically, the seals according to the invention may comprise an elastomer. Particular preferred elastomers may be selected from the group consisting of fluoroelastomers, perfluoroelastomers and flurosilicones.

[0036] The elastomers may additionally include one or more from the group consisting antistatics, antioxidants, mould release agents, flameproofing agents, lubricants, colorants, flow enhancers, fillers, including nanofillers, light stabilizers and ultraviolet light absorbers, pigments, anti-weathering agents and plasticisers. [0037] Preferably the elastomer has a Shore A hardness from about 20 to about 80, more preferably from about 60 to about 70.

[0038] The skilled person may select a suitable material based upon the exact geometries and the prevailing conditions found within a specific turbomolecular pump. In particular, the seal material may be selected so that the seal may be used at an operating temperature of from about 0 °C to about 1 10 °C, more typically from about 0 °C to about 75 °C. [0039] In the multi-material, low friction seals of the invention, the second material may be selected from the group consisting of polyethylene, polypropylene, polyvinylchloride, polyethylene terephthalate and fluoropolymers, and derivatives and copolymers thereof. Fluoropolymers, such as polytetrafluoroethylene (PTFE) or fluorinated ethylene propylene (FEP), are particularly preferred.

[0040] In the multi-material seals, the first material may be selected from the elastomers identified above and/or from expanded elastomers and foams, including expanded ethylene propylene diene monomer or fluoro elastomer. [0041 ] Typically, the second material is less hard than the first material. Preferably the second material has a Shore A hardness of between about 20 and about 40. Preferably the first material has a Shore A hardness of from about 40 to about 70.

[0042] Preferably, the elastomer is selected such that outgassing is kept to a level which does not deleteriously affect turbomolecular pump performance. Preferably the polymer has a total mass loss, collected volatile condensable materials, and water vapour release each of less than about 1 wt%, more preferably less than about 0.5 wt%, more preferably less than about 0.1 wt%. [0043] The invention further provides a method of manufacturing a turbomolecular pump comprising providing a turbomolecular pump and installing one or more seals according to other aspects of the invention. The method may optionally include the steps of removing an O-ring seal and replacing the O-ring seal with a seal according to the invention.

[0044] It will be appreciated that all aspects and embodiments may be combined mutatis mutandis.

Brief Description of the Figures

[0045] Preferred features of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 provides a schematic of a prior art O-ring in situ in a turbomolecular pump.

Fig. 2 provides a schematic of a seal according to the invention in situ.

Fig. 3 provides a schematic of a seal according to the invention in situ. Fig. 4a shows a cross-section of a seal according to the invention. Fig. 4b shows a cross-section of a seal according to the invention.

Fig. 5a provides a schematic of a seal according to the invention in situ. Fig. 5b shows a cross-section of a seal according to the invention.

Fig. 6a provides a schematic of a seal according to the invention in situ. Fig. 6b shows a cross-section of a seal according to the invention.

Detailed Description [0046] The present invention provides a radially extending seal for a turbomolecular pump cartridge. The seal may comprise a circumferential engagement surface configured to slidably engage a surface of the pump in a sealing relation.

[0047] As illustrated in Fig. 2, in an example, the seals (1 ) are located in grooves (12) in an outer surface of the turbomolecular pump cartridge (2) previously occupied by O-rings (not shown). In the illustrated example, the seal (1 ) extends radially from the base of the groove (12). The seals (1 ) are annular and of constant cross-section, and are made from Viton™ available from Dupont™. The cross- section of the seal reveals a relatively thick base portion adjacent the base of the groove (12), a tapered shoulder, and a narrower neck portion with a tapered end meeting at a circumferential engagement surface (13). As shown, the circumferential engagement surface (13) lightly engages (< 100 N) the inner surface (14) of the pump envelope (3) to provide a sealing relation therewith. As illustrated, the portion of the cross-section including the neck portion, tapered end and circumferential engagement surface, extends further radially than its maximum thickness. Accordingly, when the pump cartridge (2) is slid in or out of the pump envelope (3) the seal (1 ) may deform axially about its neck portion. The seal (1 ) enables the pump cartridge (2) to slide easily within the pump envelope (3). For a 5 kg pump cartridge being pulled out of a vertically aligned envelope, the axial force required to do so may preferably not exceed 150 N.

[0048] For the purpose of the invention "axial", "axially" and "axial direction" refer to a direction parallel to the axis "A" of the turbomolecular pump. The direction will typically be normal to the radial extension of the seal and parallel to the inner surface of the pump envelope and outer surface of the pump cartridge. Friction refers to static friction. [0049] In other examples, the neck portion and/or tapered end may have a greater aspect ratio than illustrated in Fig. 2. Such an increase in aspect ratio increases the ease of seal deflection when the pump cartridge is inserted into the pump envelope, further reducing the friction between the pump envelope and the seal, and in turn further easing insertion. Aspect ratios from about 2 to about 10 have been found to be very effective. [0050] The illustrated example in Fig. 2 includes two seals (1 ). Depending on the number of side inlets (9) in a particular multistage turbomolecular pump, any number of seals (1 ) according to the invention may be employed, preferably wherever the pressure differential is low, for instance less than 10E-2 mbar, or less than 10E-3 mbar.

[0051 ] Multistage turbomolecular pumps suitable for use in the invention include EXT200/200H™ available from Edwards Vacuums™.

[0052] Fig. 3 illustrates an alternative example of the invention. In this example, the seals (10b) are sandwiched between the stator spacer rings (1 1 ). As illustrated in Fig. 4b, the exemplified seals (10b) extend in a radial direction (r) further than they do in an axial direction (a), with an aspect ratio of approximately 1 .8. The seals have a radiused outer circumference (15), which in use provides the circumferential engagement surface. Fig. 3 shows the turbomolecular pump fully assembled with the pump cartridge (2) slid into its "in use" position within the pump envelope (3). In this arrangement, the radially aligned midpoint of the seal (1 ) is substantially tangentially aligned with the inner surface (14) of the pump envelope (3). Typically, the normal force exerted on inner surface (14) of the pump envelope (3) is less than 50 N. Sliding the pump cartridge (2) longitudinally within the pump envelope (3) will cause the seal (10b) to deform in a longitudinal direction about its radius. Accordingly, the pump cartridge (2) may be easily removed and/or inserted into the turbomolecular pump.

[0053] Again, two seals (10b) are illustrated although more or less may be employed in particular turbomolecular pump. The seals (10b) are annular and of constant cross-section, and are made from Viton™ available from Dupont™. [0054] Fig. 4a shows a cross-section of an alternative seal (10a) according to the invention. In this instance the seal has a generally circular cross-section with a tapered, blade-like section extending to the circumferential engagement surface at the blade's radiused tip (16). At least a portion of the blade-like section has an aspect ratio greater than 1 .5. In use, the blade section (17) may deform as the seal (10a) slides along the inner surface of the pump housing (not shown). The seal material may be relatively flexible because in use it will only need to contain a pressure difference of less than 10E-2 mbar, for instance 10E-3 mbar. [0055] Fig. 5a and 5b illustrate an alternative example in which a compressible core (18) of expanded elastomer is coated (19) with fluorinated ethylene propylene (FEP). The expanded elastomer may be elthylene propylene diene monomer such as Nordel™ available from DuPont™. Suitable FEP may be Teflon™ 100 available from DuPont™. Again, in the illustrated embodiment the seals (20) of the invention have been located in grooves (22) previously occupied by an elastomeric O-ring. Advantageously, because the FEP provides a low friction surface, the force of engagement between the seal (20) and the pump envelope (3) may be greater whilst still maintaining ease of sliding. The illustrated seals (20) have substantially square cross-sections; however, the skilled person will appreciate that other shapes may be employed without departing from the invention, e.g. circular or blade-like. Preferably, the multi-material seal (20) allows easy insertion and extraction without risk of trapping or twisting of the seal.

[0056] Advantageously, the outer material further provides improved chemical resistance and/or reduced outgassing, whereas the inner material may be selected to provide the desired mechanical properties.

[0057] Figure 6a and 6b illustrate a still further alternative example in which a multi- material seal (24) comprising a relatively rigid ring (23) is over-moulded with a relatively flexible annular body (21 ). The example the ring (23) comprises PTFE and the annular body (21 ) comprises FKM fluoroelastomer. Suitably the FKM fluoroelastomer may be Viton™ available from Dupont™. Suitably the PTFE may be Teflon™ available from Dupont™. In the illustrated embodiment, a portion of the annular FKM fluoroelastomer body (21 ) is sandwiched between the stator spacer rings (1 1 ). [0058] The rigid PTFE ring (20) provides a low friction circumferential engagement surface which in use sealably engages the inner surface (14) of the pump envelope (4). Again, because the PTFE provides a low friction coating, the force of engagement may be increased whilst still maintaining ease of sliding. In this example the FKM fluoroelastomer body (21 ) allows the seal to deform during sliding assisting removal thereof.

[0059] It will be appreciated that various modifications may be made to the embodiments shown without departing from the spirit and scope of the invention as defined by the accompanying claims as interpreted under patent law.

Key

1 . Seal

2. Pump Cartridge

3. Pump Envelope

4. Main Pump Inlet

5. Impeller Shaft

6. Stator(S)

7. O-Rings

8. Main Pump Outlet

9. Inter-Stage Inlet

10. A) Seal

B) Seal

1 1 . Stator Spacer Rings

12. O-Ring Groove

13. Circumferential Engagement Surface

14. Inner Surface Of Pump Envelope

15. Seal Outer Circumference

16. Blade Seal Tip

17. Blade Section of Seal

18. Compressible Core of Multi-Material Seal

19. Coating on Multi-Material Seal

20. Seal

21 . Flexible Annular Body

22. O-ring Groove

23. Rigid Ring

24. Multi-material Seal

25. Rotors