EP4724712A1

EP4724712A1 - Methods of manufacturing vacuum pump permanent magnetic bearing inserts, permanent magnetic bearing inserts and temporary masking shroud

Info

Publication number: EP4724712A1
Application number: EP24730405.8A
Authority: EP
Inventors: Emiliano LUCCHETTA; Daniel SUAREZ ARIAS; Eng Keen KHOR
Original assignee: Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2023-06-08
Filing date: 2024-05-17
Publication date: 2026-04-15
Also published as: WO2024252124A1; GB202308521D0; GB2630916A

Abstract

A method of manufacturing a permanent magnetic bearing insert for a vacuum pump comprising a rotor shaft defining a cavity configured to receive the magnetic bearing insert in an interference-fit configuration, the method comprising the steps of: a) providing one or more permanent magnet alloy rings of a rotor bearing half of a permanent magnetic bearing; b) masking one or more surfaces of the or each permanent magnet alloy ring which are configured to substantially interface with another surface of the permanent magnetic bearing insert and/or the rotor shaft cavity when the permanent magnetic bearing insert is received by the rotor shaft cavity; and c) applying a coating to the or each permanent magnet alloy ring; wherein step b) substantially prevents those masked surfaces from being coated during step c); and wherein step b) and step c) are carried out substantially externally from a vacuum pump.

Description

METHODS OF MANUFACTURING VACUUM PUMP PERMANENT MAGNETIC BEARING INSERTS Field [001] The present invention relates to methods of manufacturing permanent magnetic bearing inserts for vacuum pumps, and methods of manufacturing vacuum pumps. The present invention further relates to permanent magnetic bearing inserts and to vacuum pumps comprising the same. Background [002] Vacuum pumps typically comprise a housing having an inlet and an outlet, wherein a stator is connected to the housing. Within the housing a rotor is disposed and rotatably supported by bearings. The rotor comprises at least one rotor element to interact with the stator in order to convey a gaseous medium from the inlet to the outlet. In the case of a turbomolecular vacuum pump, the stator typically includes a plurality of vanes interacting with a plurality of rotor vanes. [003] Generally, vacuum pumps use permanent magnetic bearings to support the rotor. [004] A permanent magnetic bearing typically comprises rotor and stator bearing halves each including one or more permanent magnets. Typically, the permanent magnets on each bearing half are arranged as one or more permanent magnet alloy rings. The bearing halves are placed in close proximity to one another and are generally configured to be in mutual repulsion, in use. The rotor is thereby contactlessly supported, avoiding any need for lubrication or grease. [005] There is, however, an ongoing need to provide permanent magnetic bearings with protective coatings to protect them from the often corrosive components of vacuum pump processes, while maintaining precise tolerances between the permanent magnets and the cavities on the rotor and stator configured to receive them. [006] The present invention aims to solve these and other problems with the prior art. Summary [007] Accordingly, in a first aspect, the present invention provides a method of manufacturing a permanent magnetic bearing insert for a vacuum pump comprising a rotor shaft defining a cavity configured to receive the magnetic bearing insert in an interference-fit configuration. [008] The method comprises the steps of: a) providing one or more permanent magnet alloy rings of a rotor bearing half of a permanent magnetic bearing; b) masking one or more surfaces of the or each permanent magnet alloy ring which are configured to substantially interface with another surface of the permanent magnetic bearing insert or the rotor shaft cavity when the permanent magnetic bearing insert is received by the rotor shaft cavity; and c) applying a coating to the or each permanent magnet alloy ring. [009] Step b) is carried out such that the masking substantially prevents those masked surfaces from being coated by the coating while a mask is in place. [010] Furthermore, step b) and step c) are carried out substantially externally from a vacuum pump. [011] Generally, the coating is a substantially corrosion-resistant coating, e.g. a coating which prevents or reduces corrosion of the permanent magnet alloy ring. In embodiments, the coating may be a coating other than a substantially corrosion- resistant coating, or a combination of coatings or coating layers. [012] It is known that vacuum pump process components are often corrosive to permanent magnetic bearings. Known protective coatings and methods of coating have therefore been developed with the aim of minimising or avoiding corrosion through, for example, hydrogen (H2) induced decrepitation, where hydrogen molecules contact a magnet (e.g. a neodymium or samarium cobalt magnet) and a chemical reaction therebetween produces hydrogen decrepitation on the magnet. Protective coatings may also be provided to protect permanent magnetic bearings from corrosion caused by the environment, i.e. atmospheric air, when the corresponding vacuum pump is not in use. [013] Known coating methods, however, are often detrimental to the need for precise tolerances between the permanent magnets and the corresponding cavity on the rotor shaft configured to receive them. In particular, permanent magnet alloy rings are usually press fit inside a rotor shaft cavity. Precise tolerances are thus necessary to minimise the risk of damage to the magnetic elements, which are typically brittle and may fracture under tensile stresses, while ensuring an effective interference fit between the permanent magnetic bearing insert and the rotor shaft cavity. An effective interference fit is one which minimises movement of the permanent magnetic bearing insert relative to the rotor shaft cavity as vacuum pump components undergo thermal expansion and contraction during use. [014] Protective coatings have previously been applied to magnetic elements prior to vacuum pump assembly, typically before the magnetic elements are magnetised for ease of handling and to minimise the risk of demagnetisation (coating processes often require high temperatures, which can cause demagnetisation). These existing methods, however, increase tolerances in the press fit between the rings and the rotor shaft cavity. Importantly, magnetic alloy rings are typically substantially entirely coated. Increasing tolerance on the fit increases the risk of the magnets being stressed beyond their limit and/or the magnets losing contact with the vacuum pump during use. [015] Other known coating methods, such as that described in EP2759726A1, describe coating permanent magnet alloy rings in situ, e.g. once they have already been inserted into a rotor shaft cavity, in order to selectively coat only those surfaces of the magnetic alloy rings which may come into contact with corrosive process gasses, in use. However, access to the permanent magnetic bearing is limited once it has been placed in a vacuum pump. This limits the coating processes available, and the accuracy with which the magnet alloy rings may be coated, due to the mechanical constraints of coating the permanent magnet alloy rings as they sit within the vacuum pump. For example, dipping or bathing processes would be difficult to achieve. [016] The present invention is particularly advantageous because it provides a method to selectively coat only those surfaces which may be exposed to potentially corrosive process gases, in use, while not coating those surfaces which are not configured to be exposed, e.g. surfaces which are best left uncoated. This is achieved while addressing the existing mechanical constraints of coating permanent magnets in situ. More specifically, because the permanent magnetic bearing insert is coated externally from a vacuum pump, a greater range of coating processes may be used, with improved accuracy. The permanent magnetic bearing insert may also be more straightforwardly examined, once coated, to ensure that all surfaces that should be coated are coated, and those surfaces which should not be coated are not. Thus, precise tolerances between the permanent magnetic bearing insert and the rotor shaft cavity which receives it may be achieved. [017] As used herein, the term “interference fit” refers to a fit between parts in which the external dimension of a first part is at least substantially the same as (or greater than) the internal dimension of a second part into which the first part is fitted. [018] In embodiments, the permanent magnetic bearing insert may comprise a sleeve which is configured to couple to an circumferential surface of the or each permanent magnet alloy ring. Typically, the or each permanent magnet alloy ring is coupled to the sleeve outside a vacuum pump and the permanent magnetic bearing insert (including the sleeve and each permanent magnet alloy ring) is subsequently inserted into the rotor shaft cavity. The sleeve is generally a radially outwardly extending sleeve which is configured to couple to an outer circumferential surface of the or each permanent magnet alloy ring. In embodiments, the sleeve may be a radially inwardly extending sleeve which is configured to couple to an inner circumferential surface of the or each permanent magnet alloy ring. [019] Generally, when the or each permanent magnet alloy ring is coupled to the sleeve, a first compressive force is applied to the permanent magnet alloy ring and, when the permanent magnetic bearing insert (including the permanent magnet alloy ring and sleeve) is later inserted into the rotor shaft cavity, a further compressive force is applied to the permanent magnetic bearing insert. [020] This is advantageous, particularly where at least one of the permanent magnet alloy rings is a neodymium magnet or neodymium magnet alloy, because such a magnet or magnet alloy typically has a low coefficient of thermal expansion. Therefore, it is desirable to progressively apply a compressive force to the permanent magnet alloy ring because cooling the magnetic elements during a fitting process typically has a negligible thermal expansive effect and, depending on rotor shaft material, the maximum temperature that the magnetic elements can be exposed to before demagnetisation takes place is relatively low. Alternatively, or additionally, the rotor maximum heating temperature may limit the interference fit achievable. The present configuration is therefore advantageous because seating of the permanent magnet alloy rings in the rotor shaft cavity is more easily achieved with a reduced risk of demagnetisation of or damage to the at least one permanent magnetic alloy ring and with a sufficient interference fit. [021] Typically, the sleeve of the permanent magnetic bearing insert will be generally tubular. The sleeve may extend from a proximal end to a distal end. When in situ in the vacuum pump, the proximal end is typically located towards a low pressure, inlet, end of the rotor, whereas the distal end is typically located towards a relatively higher pressure, exhaust, section of the rotor. [022] In embodiments, the sleeve may include one or more radially inwardly extending annular flanges for retaining the permanent magnet alloy ring(s). In use, a permanent magnet alloy ring may directly abut against the radially inwardly extending annular flange. For example, the flange may be located towards the proximal, low pressure, end of the sleeve. Such a radially inwardly extending annular flange may be an integral part of the sleeve, or may be a distinct, typically annular, unit. In embodiments, the sleeve may include a plurality of radially inwardly extending annular flanges. In embodiments, a surface of one or more of the flanges may be coated during step c). [023] In embodiments, one end of the sleeve may include a substantially funnelled entrance. This may assist with forming an interference fit between the sleeve and the or each permanent magnet alloy ring. Typically, before the permanent magnet alloy rings are inserted, the funnelled entrance is such that the internal diameter of the sleeve decreases from a diameter greater than that of the permanent magnet alloy ring(s) to a diameter that is smaller than that of the permanent magnet alloy ring(s). Once in situ, the rings will typically be in an interference fit. [024] In embodiments, an end of the sleeve may include a tapered and/or stepped outer surface. A stepped outer surface may include one or more steps. This may assist with pushing the bearing insert into the rotor shaft cavity. Typically, before the bearing insert is inserted into the rotor shaft cavity, the tapered outer surface is such that the outer diameter of the sleeve increases from a diameter less than the inner diameter of the rotor shaft cavity to a diameter greater than the diameter of the rotor shaft cavity. This may assist with pushing the bearing insert into the rotor shaft cavity and/or forming an interference fit. [025] Typically, the rotor shaft cavity is located at or towards a distal, or lower pressure, end of the rotor shaft. Although, in embodiments, the rotor shaft cavity may be present at or towards the proximal, relatively higher pressure, end of the rotor shaft, or at or towards both ends of the rotor shaft. The rotor shaft cavity may be configured to slidably receive the bearing insert to achieve an intermate engagement about the circumference of the bearing insert. Typically, the rotor shaft cavity has a generally circular cross-section. The fit between the rotor shaft cavity and the bearing insert may be permanent or semi-permanent. [026] Typically, the bearing insert may comprise from about 1 to about 10 permanent magnet alloy rings, more preferably from about 2 to about 6 permanent magnet alloy rings, 2 to 4 permanent magnet alloy rings are particularly preferred. [027] In embodiments, step b) may include coupling the or each permanent magnet alloy ring to the sleeve and forming and interference fit therebetween. Therefore, the coupling of the sleeve to the or each permanent magnet alloy ring may occur prior to step c) and the sleeve may act to mask one or more surfaces of the or each permanent magnet alloy ring and/or aid seating the permanent magnet alloy ring in the rotor shaft cavity. In embodiments, where the coating is applied to the sleeve in step c), the method may include the step of removing at least part of the coating applied to the sleeve. For example, in embodiments coating may be applied to surface(s) of the sleeve and subsequently (partially or entirely) removed from those surfaces of the sleeve which are configured to interface with the rotor cavity, by machining. Thus, the permanent magnetic bearing insert can be suitably coated while ensuring that a suitable interference fit between the insert and the rotor cavity is achieved and maintained. [028] Coupling the or each permanent magnet alloy ring to the sleeve prior to step c) masks those surfaces of the permanent magnet alloy ring which will not be coated. [029] In use, the sleeve is generally in touching engagement with the rotor shaft cavity of the vacuum pump, and the or each permanent magnet alloy ring is coupled to the sleeve. The sleeve may therefore act as a buffer between the rotor shaft cavity and the or each permanent magnet alloy ring. This is particularly advantageous because the sleeve may also act to enhance a required interference fit configuration between the rotor shaft cavity and the permanent magnetic bearing insert. [030] Importantly, therefore, the sleeve may act to mask those surfaces which are not configured to be coated while also applying a compression force to the or each permanent magnet alloy ring outside of a vacuum pump. [031] In embodiments, step b) may consist substantially only of coupling the sleeve to the or each permanent magnet alloy ring. In other words, the sleeve may provide the only masking of surfaces of the or each permanent magnet alloy ring. [032] In embodiments, step b) may include applying a temporary masking shroud to the permanent magnetic bearing insert and removing the temporary masking shroud following step c). For example, step b) may include applying a separate masking shroud or similar to the permanent magnetic bearing insert together with coupling the sleeve to the or each permanent magnet alloy ring. For example, a temporary non- stick masking agent may be applied to one or more permanent magnet alloy ring surfaces and/or one or more surfaces of the sleeve to substantially prevent those surfaces from being coated during step c). [033] In embodiments, the temporary masking shroud may be applied to one or more surfaces of the sleeve which are configured to be uncoated during step c). In other words, step b) may include masking one or more surfaces of the permanent magnetic bearing insert, including the sleeve, which are configured to substantially interface with another surface of the permanent magnetic bearing insert or the rotor shaft cavity when the permanent magnetic bearing insert is received by the rotor shaft cavity. Therefore, in embodiments, masking in step b) may include a combination of the sleeve masking one or more surfaces of each permanent magnet alloy ring and a temporary shroud masking one or more surfaces of the sleeve and/or one or more of the permanent magnet alloy rings. [034] In embodiments, the temporary masking shroud may comprise a non-stick agent. For example, the temporary masking shroud may include a gel or similar which may be applied to one or more surfaces of the or each permanent magnet alloy ring during step b). [035] In embodiments, the temporary masking shroud may comprise a casing. For example, the casing may take the form of a shell, having one or more segments, which is provided for temporarily coupling to the permanent magnetic bearing insert prior to step c). In another example, the casing may include a substantially cylindrical tube configured to be placed around an outer circumferential surface of the or each permanent magnet alloy ring prior to step c). Thus, those surfaces of the or each permanent magnetic alloy ring which are configured to interface with another surface of the permanent magnetic bearing insert (either a permanent magnet alloy ring or the sleeve, if present) or the rotor shaft cavity when the permanent magnetic bearing insert is received in the rotor shaft cavity, in use, are not coated during step c). [036] In embodiments, step b) may include arranging two or more permanent magnet alloy rings with respect to one another in order to substantially prevent coating of one or more surface of at least one said permanent magnet alloy ring. For example, step b) may include stacking a plurality of permanent magnet alloy rings in order to mask surfaces of one or more of those rings prior to coating. [037] Typically, a said permanent magnet alloy ring has a substantially continuously annular configuration. Although, in embodiments, a said permanent magnet alloy ring may have a substantially discontinuous structure. For example, in embodiments, a said permanent magnet alloy ring may be in the form of a plurality of permanent alloy magnet alloy blocks which are arranged substantially circumferentially, e.g. into a substantially annular unit. [038] In embodiments, the coating may be a heat-curable electrical insulating coating or conformal coatings, and/or UV-curable and/or visible light-curable and/or moisture curable electrical insulating coating or conformal coatings. In embodiments the coating may be based on synthetic resins, like epoxy, acrylic, acrylic urethane, polyurethane, silicone. [039] In embodiments, the permanent magnetic bearing insert may be substantially unmagnetised during step c). For example, where the bearing insert comprises a single permanent magnet alloy ring, the bearing insert may be substantially unmagnetised during step c) and may be magnetised thereafter. This is advantageous because handling and coating of the bearing insert is more straightforward. [040] In embodiments, the method may further comprise the step of magnetising the or each permanent magnet alloy ring prior to step c). In embodiments, the method may comprise the or each permanent magnet alloy ring prior to step b). [041] Typically, where the permanent magnetic bearing insert includes the sleeve, the or each permanent magnet alloy ring is magnetised prior to being coupled to the sleeve, and prior to step c). Advantageously, the sleeve improves the ease with which the permanent magnetic bearing insert may be handled during a coating process. [042] In embodiments, for example where the permanent bearing insert includes the sleeve, the coating process may typically be carried out at temperatures below a demagnetisation threshold of the or each permanent magnet alloy ring, where demagnetisation and/or thermal ageing of the or each permanent magnet alloy ring may occur. For example, where a said permanent magnet alloy ring comprises neodymium (NdFeB), for the grades typically used demagnetisation may start occurring at temperatures above 120°C. [043] In embodiments, the coating may be a varnish, and the varnish may be one that is curable at a temperature below the demagnetisation threshold. [044] In embodiments, the coating may be a two-component coating. For example, a first and/or second component may comprise a nickel copper nickel coating (NiCuNi). [045] In embodiments, the coating may be applied to provide a coating thickness of between around 0.008 mm to around 0.5 mm, preferably around 0.02 mm to around 0.2 mm, preferably around 0.05 mm to around 0.2 mm, for example 0.1mm. For example, for certain epoxy coatings a thickness of between around 0.02 mm to around 0.03 mm may be provided. Thus, the permanent magnetic bearing insert may be provided with improved protection e.g. shock resistance to minimise the risk of damage to the permanent magnetic bearing insert during vacuum pump assembly. For example, the thickness of the coating may be increased to provide an additional protection and/or encapsulating effect for the or each permanent magnet alloy ring, to provide enhanced protection. In embodiments, different coating types may be provided as separate coating layers with differing intended effects, where at least one said layer may be configured to provide the coated surface(s) with shock resistance and another said layer may be configured to provide the coated surface(s) with protection against corrosion by an atmospheric and/or process gas. [046] In embodiments, the coating may be applied by brushing, dipping, painting, bathing, spraying, curing, vacuum pressure impregnation (VPI), chemical vapour deposition, physical vapour deposition, electrodeposition, or a combination thereof. For example, the permanent magnetic bearing insert may be dipped into a varnish bath during step c). Alternatively, or in addition, the coating may be applied by a spray or brush. In embodiments, the coating may be applied by a combination of coating processes. [047] In embodiments, the or each permanent magnet alloy ring may comprise a neodymium magnet or neodymium magnet alloy, such as NdFeB. NdFeB magnets may contain other elements beside Nd, Fe and B. For example, praseodymium may be added as a replacement for neodymium in variable quantities. Other elements may include one or more of Dy, Ga, Co, Al and Cu. In other embodiments, the magnets may comprise PrFeB magnets. [048] In embodiments, the or each permanent magnet alloy ring may comprise samarium cobalt. [049] In a further aspect, the present invention provides a permanent magnetic bearing insert of a rotor bearing half of a permanent magnetic bearing manufactured in accordance with any preceding aspect. [050] In a further aspect, the present invention provides a method of manufacturing a permanent magnetic bearing insert for a vacuum pump comprising a stator defining a seat configured to interface with the permanent magnetic bearing insert. The method comprises the steps of: a) providing a one or more permanent magnet alloy rings of a stator bearing half of a permanent magnetic bearing; b) masking surfaces of the or each permanent magnet alloy ring which are configured to substantially interface with another surface of the permanent magnetic bearing insert or the stator seat when the permanent magnetic bearing insert interfaces with the stator seat; and c) applying a coating to the or each permanent magnet alloy ring. [051] Thus, the method of coating described above may also be carried out with respect to a stator bearing half of a permanent magnetic bearing in addition to, or instead of, a rotor bearing half. For example, two permanent magnetic bearing inserts (one rotor half and one stator half) of a permanent magnetic bearing may each be coated in accordance with the methods described above. [052] Typically, where a vacuum pump includes two permanent magnetic bearing inserts, a rotor half and a stator half, both inserts may be received by the rotor cavity, with the rotor half insert being received by the rotor cavity in an interference fit configuration. [053] In embodiments, the stator seat may include a surface of the stator against which an inner circumferential surface of the or each permanent magnet alloy ring abuts when the permanent magnetic bearing insert is in situ. For example, the permanent magnetic bearing insert, which is typically substantially annular, may engage the stator in a sliding and/or interference fit configuration. Typically, the permanent magnetic bearing insert is configured such that it forms a sliding fit with the stator such that the axial position of the permanent magnetic bearing insert may be adjusted relative to the rotor bearing half of the permanent magnetic bearing. Selectively coating the permanent magnetic bearing insert of a stator half of the permanent magnetic bearing provides improved radial location of the bearing insert on the stator. [054] In embodiments, the permanent magnetic bearing insert may comprise a sleeve which is configured to couple to an circumferential surface of the or each permanent magnet alloy ring. Generally, the sleeve may be a radially inwardly extending sleeve which is configured to couple to an inner circumferential surface of the or each permanent magnet alloy ring. [055] In embodiments, the stator half may be substantially slidably received on the stator so that the axial position of the stator bearing half can be altered. For example, in embodiments where the insert includes a sleeve, the sleeve may be slidably engaged with the stator. Alternatively, the stator may include a stator support to which the stator bearing half is attached and the stator support may itself be adjustable such that the axial position of the stator bearing half may be altered. [056] Typically, a said permanent magnetic bearing comprises a rotor bearing half which, in use, sits outside a stator bearing half thereof. In other words, the stator bearing half has a smaller outer diameter than the inner diameter of the rotor bearing half and the stator bearing half resides within the boundary defined by the inner diameter of the rotor bearing half, with a void therebetween. Although, in embodiments, this may not be necessarily the case and the rotor half of the permanent magnetic bearing may, for example, comprise the innermost portion thereof, whereas the stator half of the permanent magnetic bearing may comprise the outermost portion thereof. In such cases, the sleeve in the stator insert may be located outside the permanent magnet alloy rings and the sleeve in the rotor insert may be located inside the permanent magnet alloy rings. [057] In a further aspect, the present invention provides a permanent magnetic bearing insert for a stator bearing half of a permanent magnetic bearing manufactured in accordance with the preceding aspect. [058] In a further aspect, the present invention provides a method of manufacturing a vacuum pump, preferably a turbomolecular pump, the vacuum pump comprising a rotor shaft configured to have one or more rotor blades coupled thereto and defining a cavity for receiving a magnetic bearing insert for a rotor half of a permanent magnetic bearing and/or a stator defining a seat for seating a magnetic bearing insert for a stator half of the permanent magnetic bearing. The method comprises the steps of: i. carrying out the method of any of any preceding aspect; and ii. inserting a said permanent magnetic bearing insert for the rotor half into the rotor shaft cavity of the vacuum pump in an interference-fit configuration and/or onto the stator seat of the vacuum pump. [059] By selectively coating the permanent magnetic bearing insert prior to inserting it into the rotor shaft cavity or stator cavity of the vacuum pump, a greater range of coating processes may be utilised, and improved coating of the permanent magnetic bearing insert may be achieved, as described above. [060] Typically, the or each permanent magnetic alloy ring is magnetised prior to step ii, usually during step i. Although, in embodiments, the method may comprise the step of magnetising the or each permanent magnet alloy ring of the insert after step ii. [061] In a further aspect, the present invention provides a vacuum pump manufactured in accordance with any preceding aspect. [062] In a further aspect, the present invention provides a permanent magnetic bearing insert for a vacuum pump, the vacuum pump including a rotor shaft having one or more rotor blades coupled thereto and the rotor shaft defining a cavity configured to receive the magnetic bearing insert in an interference-fit configuration. [063] The permanent magnetic bearing insert comprises one or more permanent magnet alloy rings of a rotor bearing half of a permanent magnetic bearing. [064] The permanent magnetic bearing insert includes a coating. The coating is arranged such that one or more surfaces of the permanent magnet alloy ring which are configured to substantially interface with another surface of the permanent magnetic bearing insert or the rotor shaft cavity when the permanent magnetic bearing insert is received by the rotor shaft cavity are substantially uncoated. [065] Advantageously, the permanent magnetic bearing insert is coated prior to being inserted into the rotor shaft cavity. [066] Generally, the coating is a substantially corrosion-resistant coating, i.e. a coating which prevents or reduces corrosion of the permanent magnet alloy ring. [067] In embodiments, the permanent magnetic bearing insert may comprise a sleeve coupled to the or each permanent magnet alloy ring substantially around a circumferential surface thereof, e.g. an outer circumference of the or each permanent magnet alloy ring. The sleeve protects the or each permanent magnet alloy ring and increases a compression force for an improved interference fit between the permanent magnetic bearing insert and a rotor cavity of the vacuum pump. The sleeve is generally a radially outwardly extending sleeve but, in embodiments, the sleeve may be a radially inwardly extending sleeve. [068] In embodiments, the or each permanent magnet alloy ring may be coupled to the sleeve prior to the permanent magnetic bearing insert being coated. Thus, the sleeve may act to mask surfaces of the or each permanent magnet alloy ring which are to be substantially uncoated following coating of the permanent magnetic bearing insert. [069] In embodiments, the or each permanent magnet alloy ring may comprise a neodymium magnet or neodymium magnet alloy, preferably Nd₂Fe₁₄B. In embodiments, the or each permanent magnet alloy ring may comprise samarium cobalt, preferably SmCo5 or Sm2Co17. Neodymium magnet(s) are particularly preferred. Typically, where a plurality of magnet alloy rings are present, each comprise substantially the same material. [070] In embodiments, the coating may be a heat-curable electrical insulating coating or conformal coatings and/or UV-curable and/or visible light-curable and/or moisture curable electrical insulating coating or conformal coatings. In embodiments the coating may be based on synthetic resins, like epoxy, acrylic, acrylic or acrylated urethane, polyurethane, silicone. [071] In embodiments, the coating may have a coating thickness of between around 0.005 mm to around 0.20 mm, preferably around 0.01 mm to around 0.15 mm, for example 0.05mm, approximately 0.05mm or approximately 0.13 for example. For example, for certain epoxy coatings a thickness of between around 0.02 mm to around 0.03 mm may be provided. [072] In embodiments, the coating may be applied by dipping, painting, bathing, spraying, UV curing, moisture curing, vacuum pressure impregnation (VPI), chemical vapour deposition, physical vapour deposition, electrodeposition, or a combination thereof. [073] In a further aspect, the present invention provides a vacuum pump, preferably a turbomolecular pump, comprising a rotor shaft configured to have one or more rotor blades coupled thereto and the rotor shaft defining a cavity containing, typically in an interference-fit configuration, a permanent magnetic bearing insert according to any preceding aspect. [074] In a further aspect, the present invention provides a permanent magnetic bearing insert for a vacuum pump, the vacuum pump including a stator defining a seat configured to interface with the magnetic bearing insert in an interference-fit configuration. [075] The permanent magnetic bearing insert comprises one or more permanent magnet alloy rings of a stator bearing half of a permanent magnetic bearing. [076] The permanent magnetic bearing insert includes a coating. Generally, the coating is a substantially corrosion-resistant coating, i.e. a coating which prevents or reduces corrosion of the permanent magnet alloy ring. The coating is arranged such that one or more surfaces of the permanent magnetic bearing insert which are configured to substantially interface with another surface of the permanent magnetic bearing insert or the stator seat when the permanent magnetic bearing insert is received by the stator seat are substantially uncoated. [077] Advantageously, the permanent magnetic bearing insert is coated prior to interfacing with the stator seat. [078] In embodiments, the permanent magnetic bearing insert may comprise a sleeve which is configured to couple to an circumferential surface of the or each permanent magnet alloy ring. Generally, the sleeve may be a radially inwardly extending sleeve which is configured to couple to an inner circumferential surface of the or each permanent magnet alloy ring. [079] In embodiments, the stator half may be substantially slidably received on the stator so that the axial position of the stator bearing half can be altered. For example, in embodiments where the insert includes a sleeve, the sleeve may be slidably engaged with the stator. Alternatively, the stator may include a stator support to which the stator bearing half is attached and the stator support may itself be adjustable such that the axial position of the stator bearing half may be altered. [080] In a further aspect, the present invention provides a vacuum pump, preferably a turbomolecular pump, comprising a stator defining a seat interfacing with a permanent magnetic bearing insert according to any preceding aspect. In embodiments, the stator seat and the permanent magnetic bearing insert may be in a substantially interference-fit configuration. [081] In a further aspect, the present invention provides a temporary masking shroud for a permanent magnetic bearing insert for a rotor or stator bearing half of a permanent magnetic bearing of a vacuum pump, the permanent magnetic bearing insert comprising one or more permanent magnet alloy rings to be partially, e.g. selectively, coated. [082] The temporary masking shroud comprises one or more compartments for receiving the permanent magnetic bearing insert. The or each compartment is configured such that, when the permanent magnetic bearing insert is received therein, one or more surfaces of the or each permanent magnet alloy ring are substantially exposed for coating while one or more other surfaces of the or each permanent magnet alloy ring are substantially prevented from being coated. [083] For example, those surfaces which are substantially prevented from being coated when the permanent magnetic bearing insert is received in the temporary masking shroud may be those surfaces which are configured to interface another surface of the permanent magnetic bearing insert or a rotor shaft cavity or stator seat of the vacuum pump, when the permanent magnetic bearing insert is received therein. Those surfaces which are substantially exposed for coating may be those surfaces of the permanent magnet alloy ring which are configured to be exposed to process gases within the vacuum pump when the permanent magnetic bearing insert is received therein, during use. [084] In embodiments, the one or more compartments of the temporary masking shroud may be configured to receive the permanent magnetic bearing insert in a substantially interference fit configuration. [085] In embodiments, the permanent magnetic bearing insert may include a sleeve which is configured to couple to an circumferential surface of the or each permanent magnet alloy ring; and the temporary masking shroud may be configured to mask one or more surfaces of the sleeve. Generally, the sleeve is a radially outwardly extending sleeve which is configured to couple to an outer circumferential surface of the or each permanent magnet alloy ring. In embodiments, the sleeve may be a radially inwardly extending sleeve which is configured to couple to an inner circumferential surface of the or each permanent manet alloy ring. [086] In embodiments, the temporary masking shroud may include a non-stick, e.g. fluid, agent and/or a casing. For example, the temporary masking shroud may include a removable gel or the like. [087] For the avoidance of doubt, features of aspects and embodiments described herein may be combined, and still fall within the scope of the present invention. Brief Description of Figures [088] Preferred features of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: [089] Figure 1 illustrates a cross sectional view of a permanent magnetic bearing insert according to the present invention, located in a turbomolecular vacuum pump. [090] Figure 2 illustrates a cross sectional view of a permanent magnetic bearing insert according to the present invention, located in a turbomolecular vacuum pump. [091] Figure 3 illustrates a cross sectional view of a permanent magnetic bearing insert according to the present invention, located in a turbomolecular vacuum pump. [092] Figure 4 illustrates a cross sectional view of the permanent magnetic bearing insert of Figure 3. [093] Figure 5 illustrates a cross sectional view of another permanent magnetic bearing insert of according to the present invention. Detailed Description [094] Figure 1 shows a permanent magnetic bearing insert 10 of a rotor bearing half 12 of a permanent magnetic bearing 16 within a turbomolecular vacuum pump 100. [095] The vacuum pump 100 comprises a housing having an inlet and an outlet. A stator 103 forms part of or is connected to the housing and a rotor 104 is disposed and rotatably supported by bearings. The rotor comprises a plurality of rotor vanes (as shown in Figure 3) which are configured to interact with a plurality of stator vanes, in use, in order to convey a gaseous medium from the inlet to the outlet. [096] At least one of the bearings is a permanent magnetic bearing 16 having a rotor bearing half 12 and a stator bearing half 14. The other bearing may be a further permanent magnetic bearing, or may be a mechanical, e.g. rolling element bearing. [097] The rotor and stator bearing halves 12, 14 each comprise one or more permanent magnets in the form of permanent magnet alloy rings 18. In Figure 1, the rotor and stator bearing halves 12, 14 each include three vertically adjacently arranged permanent magnet alloy rings 18. In use, the bearing halves 12, 14 are placed in close proximity to one another and are configured to be in mutual repulsion in order that the rotor is substantially contactlessly supported by the permanent magnetic bearing 16. [098] In Figure 1, the rotor bearing half 12 is in the form of a permanent magnetic bearing insert 12. The rotor shaft defines a cavity 28 configured to receive the permanent magnetic bearing insert in an interference-fit configuration. [099] The permanent magnetic bearing insert 12 comprises three permanent magnet alloy rings 18. Generally, the permanent magnet alloy rings 18 have a substantially continuously annular configuration. [100] The permanent magnetic bearing insert 12 comprises a substantially corrosion- resistant coating. The substantially corrosion-resistant coating is configured to prevent or at least reduce corrosion of the permanent magnet alloy rings 18. [101] The substance of the substantially corrosion-resistant coating may be dependent on the material of the permanent magnet alloy rings 18 and/or one or more process gases of the vacuum pump 100. The coating may be selected in order to protect the magnet alloy rings from process gases and/or chemicals that could damage the integrity or performance of the permanent magnetic bearing insert. For example, a coating may be selected to protect the permanent magnet alloy rings against hydrogen (or other) decrepitation, as discussed above. For example, the substantially corrosion-resistant coating is may be a chemical or galvanic metal deposition. [102] More specifically, the coating is arranged such that one or more surfaces of the permanent magnetic bearing insert 12 which are configured to substantially interface with another surface of the permanent magnetic bearing insert or the rotor shaft cavity when the permanent magnetic bearing insert 12 is received by the rotor shaft cavity are substantially uncoated. In the embodiment of Figure 1, it is primarily the outer circumferential surface of each permanent magnet alloy ring which is uncoated. Thus, those surfaces which would otherwise be totally exposed to process gases within the vacuum pump when the permanent magnetic bearing insert is received therein are coated with the substantially corrosion-resistant coating. In the embodiment of Figure 1, it is primarily the inner circumferential surface of each permanent magnet alloy ring and, preferably, also the radial faces which are coated. [103] Importantly, the permanent magnetic bearing insert 12 is coated prior to being inserted into the rotor shaft cavity. The permanent magnetic bearing insert is coated outside of a vacuum pump, and surfaces to be selectively uncoated are masked by a permanent or temporary masking component. Individual permanent magnet alloy rings may be masked and selectively coated separately (in this case, the single magnet may be substantially unmagnetised during coating), or a plurality of permanent magnet alloy rings may be masked and coated concurrently. [104] The permanent magnetic bearing insert 12 is coated by: a) providing each of the permanent magnet alloy rings 18; b) masking those surfaces which are configured to substantially interface with another surface of the permanent magnetic bearing insert 12 and/or the rotor shaft cavity when the permanent magnetic bearing insert 12 is received therein; and c) applying the substantially corrosion-resistant coating to the permanent magnet alloy rings so that the unmasked surfaces are substantially coated. [105] Selectively coating the surfaces of the permanent magnet alloy rings 18 improves the interference-fit configuration of the permanent bearing insert 12 in the rotor shaft cavity 28. In particular, because those surfaces which are configured to interface another surface of the permanent bearing insert 12 or the rotor shaft cavity 28 are uncoated, the tolerance on the radial outer diameter of the permanent magnet alloy rings is not unnecessarily increased. Thus, tolerances between the permanent magnetic bearing insert 12 and the rotor shaft cavity are more precise, and the risk of damage to the permanent magnet alloy rings 18 or unseating of the permanent magnet alloy rings 18 is minimised. [106] In Figures 2 and 3, the permanent magnetic bearing insert 12 also comprises a radially outwardly extending sleeve 20. The sleeve 20 is coupled to an outer circumferential surface of the permanent magnet alloy rings 18 and, when the permanent magnetic bearing insert 12 is received in the rotor shaft cavity, is in touching engagement with a radially outwardly facing wall of the rotor shaft cavity. [107] Where the permanent magnetic bearing insert 12 includes a sleeve 20, masking of surfaces to be selectively uncoated may occur by attaching the permanent magnet alloy rings 18 to the sleeve 20. Thus, those surfaces of the permanent magnet alloy rings 18 which interface with the sleeve 20 will not be coated. It is envisaged that the stator bearing half 14 may also comprise a sleeve (which is not present in Figures 2 and 3). [108] When the permanent magnet alloy rings 18 are coupled to the sleeve 20, a first compressive force is applied to the permanent magnet alloy rings 18. When the permanent magnetic bearing insert 12 (including the permanent magnet alloy rings 18 and sleeve 20) is subsequently inserted in the rotor shaft cavity 28, a second compressive force is applied to the permanent magnetic bearing insert 12 and thus the permanent magnet alloy rings 18. [109] An outer surface of the permanent magnetic bearing insert 12 engages with an inward facing wall of the rotor cavity to provide an interference fit. [110] Referring to Figures 4 and 5, prior to insertion of the bearing insert 12 into the rotor cavity 28, surfaces of the permanent magnet alloy rings 18 including those surfaces 24 which are configured to be exposed to vacuum pump process gases are coated where those surfaces are unmasked. Where those surfaces are masked in step b), those surfaces are not coated. For example, axial surfaces of each permanent magnet alloy ring 18 may not be masked and are thus coated, while masked radial surfaces may be uncoated. [111] In the embodiments of Figures 4 and 5 each of the permanent magnet alloy rings 18 has substantially chamfered outer circumferential surface, although in embodiments, each of the permanent magnet alloy rings 18 may be substantially unchamfered. Thus, when the permanent magnet alloy rings 18 are coupled to the sleeve 20 at their outer circumferential surface, a pocket, such as 26, may exist between the chamfered portions of adjacent permanent magnet alloy rings 18 and the sleeve 20. As the chamfered portions may come into contact with process gases of the vacuum pump 100, they may be coated. For example, openings between adjacent surfaces of the permanent magnetic bearing insert 12 may, in practise, allow coating to reach the chamfered portions of the permanent magnet alloy rings 18, e.g. by capillary action, or those openings may themselves be coated and sealed to prevent process gases from reaching the coated or uncoated chamfered surfaces. The degree of opening between adjacent permanent magnet alloy rings will depend at least partially on the assembly process and the forces between the permanent magnet alloy rings. Alternatively, as the chamfered portions and the pocket 26 they define may be substantially sealed from the external environment when the permanent magnet alloy rings 18 are coupled to the sleeve 20, they may remain uncoated when the permanent magnetic bearing insert 18 is coated prior to insertion in the vacuum pump 100. [112] Preferably, where the permanent magnetic bearing insert 12 includes a sleeve 20, the permanent magnet alloy rings 18 are coupled to the sleeve 20 during the masking step b), i.e. prior to coating step c). This way, the sleeve 20 can act as a mask for masking those surfaces of the permanent magnet alloy rings 18 which are to remain uncoated. [113] Still referring to Figure 5, the permanent magnetic bearing insert 12 includes three permanent magnet alloy rings 18a-18c and a sleeve 20. The coating is arranged such that an internal circumferential surface of each permanent magnet alloy ring 18 is coated. A radial surface of one of the permanent magnet alloy rings, ring 18c, is also coated as that radial surface is otherwise exposed, in use. Surfaces of the sleeve 20 which are adjacent to but do not interface with the permanent magnet alloy rings 18 are also coated, and the coating is configured to substantially prevent process gases and chemicals from penetrating between the sleeve 20 and alloy rings 18, for example towards or into the pockets 26 where damage, e.g. hydrogen decrepitation, could otherwise occur. Where the coating coats the circumferential surface of the sleeve 20 opposite the permanent magnet alloy rings during the coating step, this coated surface may be subsequently machined in order to remove unneeded coating, so that a suitable interference fit between the insert 12 and the rotor cavity 28 is achieved and maintained. The coating from one or more radial surfaces of the sleeve may also be removed. [114] The sleeve 20 of the permanent magnetic bearing insert 12 of Figures 4 and 5 is generally tubular. In each case, the sleeve 20 comprises a radially inwardly extending annular flange for retaining the permanent magnet alloy rings. The sleeve 20 of Figure 5 additionally comprises a tapered portion 30 at an opposing end of the generally tubular structure. [115] Alternatively, or in addition, a temporary masking shroud, e.g. in the form of a casing or removable agent, is applied to one or more surfaces which are to remain uncoated. Where a temporary masking shroud is applied, the temporary masking shroud is removed prior to the permanent magnetic bearing insert 12 being inserted into the rotor shaft cavity. [116] During the coating step c), the substantially corrosion-resistant coating is applied via one or more of a multitude of coating processes. Because the permanent magnetic bearing insert 12 is coated externally from a vacuum pump and prior to being inserted into a vacuum pump, a broader range of coating processes are usable due to there being fewer mechanical and spatial constraints on the coating process. For example, the substantially corrosion-resistant coating may be provided by a vacuum pressure impregnation (VPI) process or by electrodeposition. [117] The coating method may be part of a method of manufacturing the vacuum pump 100, the method of manufacturing a vacuum pump comprising the steps of manufacturing the permanent magnetic bearing insert 12 as described above and subsequently inserting the permanent magnetic bearing insert 12 into the rotor shaft cavity of the vacuum pump 100 in an interference-fit configuration. [118] Generally, each of the permanent magnet alloy rings 18 comprises a neodymium magnet or neodymium magnet alloy. [119] Generally, the sleeve 20 is formed from metal, for example stainless steel alloy or an aluminium alloy. Although, the sleeve 20 may be formed of a composite material.

Reference Key 10 Permanent Magnetic Bearing Insert 12 Rotor Bearing Half 14 Stator Bearing Half 16 Permanent Magnetic Bearing 18(a-c) Permanent Magnet Alloy Ring 20 Sleeve 24 Radial Surface 26 Pocket 28 Rotor Cavity 30 Tapered Portion 100 Vacuum Pump 103 Stator 104 Rotor

Claims

Claims 1. A method of manufacturing a permanent magnetic bearing insert for a vacuum pump comprising a rotor shaft defining a cavity configured to receive the magnetic bearing insert in an interference-fit configuration, the method comprising the steps of: a) providing one or more permanent magnet alloy rings of a rotor bearing half of a permanent magnetic bearing; b) masking one or more surfaces of the or each permanent magnet alloy ring which are configured to substantially interface with another surface of the permanent magnetic bearing insert and/or the rotor shaft cavity when the permanent magnetic bearing insert is received by the rotor shaft cavity; and c) applying a coating to the or each permanent magnet alloy ring; wherein step b) substantially prevents those masked surfaces from being coated during step c); and wherein step b) and step c) are carried out substantially externally from a vacuum pump.

2. The method of claim 1, wherein the permanent magnetic bearing insert comprises a sleeve which is configured to couple to a circumferential surface of the or each permanent magnet alloy ring; and wherein step b) includes coupling the or each permanent magnet alloy ring to the sleeve and forming an interference fit therebetween; optionally, wherein step c) includes applying a coating to one or more surfaces of the sleeve.

3. The method of claim 1 or claim 2, wherein step b) includes applying a temporary masking shroud to the permanent magnetic bearing insert; and wherein the method further comprises the step of removing the temporary masking shroud after step c); optionally, wherein the temporary masking shroud comprises a non-stick agent and/or a casing.

4. A method of manufacturing a permanent magnetic bearing insert for a vacuum pump comprising a stator defining a seat configured to interface with the magnetic bearing insert, the method comprising the steps of: a) providing one or more permanent magnet alloy rings of a stator bearing half of a permanent magnetic bearing; b) masking one or more surfaces of the or each permanent magnet alloy ring which are configured to substantially interface with another surface of the permanent magnetic bearing insert and/or the stator seat when the permanent magnetic bearing insert interfaces with the stator seat; and c) applying a coating to the or each permanent magnet alloy ring; wherein step b) substantially prevents those masked surfaces from being coated during step c); and wherein step b) and step c) are carried out substantially externally from a vacuum pump. 5. The method of any preceding claim, wherein the coating is applied to provide a coating thickness of between around 0.008 mm to around 0.

5 mm, optionally of between around 0.02 mm to around 0.2 mm.

6. The method of any preceding claim, wherein the coating is a heat-curable and/or UV-curable and/or visible light-curable and/or moisture curable coating; optionally, wherein the coating is applied by brushing, dipping, painting, bathing, spraying, UV curing, moisture curing, vacuum pressure impregnation, chemical vapour deposition, physical vapour deposition, electrodeposition, or a combination thereof.

7. A method of manufacturing a vacuum pump, preferably a turbomolecular pump, the vacuum pump comprising a rotor shaft configured to have one or more rotor blades coupled thereto and defining a cavity for receiving a magnetic bearing insert for a rotor half of a permanent magnetic bearing and a stator defining a seat for seating a magnetic bearing insert for a stator half of the permanent magnetic bearing, the method comprising the steps of: i. carrying out the method of any of claims 1 to 6; and ii. inserting the permanent magnetic bearing insert for the rotor half into the rotor shaft cavity of the vacuum pump in an interference- fit configuration, and/or seating the permanent magnetic bearing insert for the stator half on the stator seat.

8. A permanent magnetic bearing insert manufactured in accordance with any of claims 1 to 6, or a vacuum pump, preferably a turbomolecular pump, manufactured in accordance with claim 7.

9. A permanent magnetic bearing insert for a vacuum pump, the vacuum pump including a rotor shaft having one or more rotor blades coupled thereto and the rotor shaft defining a cavity configured to receive the magnetic bearing insert in an interference-fit configuration; wherein the permanent magnetic bearing insert comprises one or more permanent magnet alloy rings of a rotor bearing half of a permanent magnetic bearing; wherein the permanent magnetic bearing insert includes a coating; wherein the coating is arranged such that one or more surfaces of the permanent magnet alloy ring which are configured to substantially interface with another surface of the permanent magnetic bearing insert or the rotor shaft cavity when the permanent magnetic bearing insert is received by the rotor shaft cavity are substantially uncoated; and wherein the permanent magnetic bearing insert is coated prior to being inserted into the rotor shaft cavity.

10. The permanent magnetic bearing insert according to claim 9, wherein the permanent magnetic bearing insert comprises a sleeve coupled to the or each permanent magnet alloy ring substantially around a circumferential surface thereof; and wherein the or each permanent magnet alloy ring is coupled to the sleeve prior to the permanent magnetic bearing insert being coated.

11. A permanent magnetic bearing insert for a vacuum pump, the vacuum pump including a stator defining a seat configured to interface with the magnetic bearing insert; wherein the permanent magnetic bearing insert comprises one or more permanent magnet alloy rings of a stator bearing half of a permanent magnetic bearing; wherein the permanent magnetic bearing insert includes a coating; wherein the coating is arranged such that one or more surfaces of the permanent magnet alloy ring which are configured to substantially interface with another surface of the permanent magnetic bearing insert or the stator seat when the permanent magnetic bearing insert interfaces with the stator seat are substantially uncoated; and wherein the permanent magnetic bearing insert is coated prior to interfacing with the stator seat.

12. A vacuum pump, preferably a turbomolecular pump, comprising a rotor shaft configured to have one or more rotor blades coupled thereto and the rotor shaft defining a cavity, and a stator defining a seat; wherein the rotor shaft cavity contains, in an interference-fit configuration, a permanent magnetic bearing insert according to claim 9 or claim 10; and/or wherein the stator seat interfaces with a permanent magnetic bearing insert according to claim 11.

13. The permanent magnetic bearing insert according to any of claims 9 to 11, or the vacuum pump according to claim 12, wherein the or each permanent magnet alloy ring comprises a neodymium magnet or neodymium magnet alloy.

14. The permanent magnetic bearing insert according to any of claims 9 to 11 or 13 or the vacuum pump according to claim 12 or claim 13, wherein the coating is a heat-curable and/or UV-curable and/or visible light-curable and/or moisture curable coating; optionally, wherein the coating has a coating thickness of between around 0.008 mm to around 0.5 mm, optionally of between around 0.02 mm to around 0.2 mm; further optionally, wherein the coating is a multi-component coating.

15. A temporary masking shroud for a permanent magnetic bearing insert for a rotor bearing half of a permanent magnetic bearing of a vacuum pump, the permanent magnetic bearing insert comprising one or more permanent magnet alloy rings to be partially coated; wherein the temporary masking shroud comprises one or more compartments for receiving the permanent magnetic bearing insert; and wherein the or each compartment is configured such that, when the permanent magnetic bearing insert is received therein, one or more surfaces of the or each permanent magnet alloy ring are substantially exposed for coating while one or more other surfaces of the or each permanent magnet alloy ring are substantially prevented from being coated.