Field

[001] The present invention relates to an improved stator blade assembly for a single stage of a vacuum pump. The present invention also relates to an improved vacuum pump comprising such a stator blade assembly, and a method of designing a stator blade array.

Background

[002] Vacuum systems comprise at least one pump stage. Multistage vacuum systems comprise two or more pump stages. Typically, a pump stage comprises at least one rotor configured to rotate relative to a stator assembly. The stator assembly may define a pump chamber of the pump stage. During operation, the rotor may rotate relative to the stator to displace fluid within the pump chamber.

[003] Typically, the rotor may comprise a plurality of annular arrays of rotor blades or lobes. The rotor blade arrays are generally axially spaced within the pump chamber. The rotor blades are substantially regularly spaced within each array, and extend radially outwards from a central hub and/or shaft. The stator blade assembly of the pump stage surrounds the rotor. As the rotor rotates, the rotor blades impact incoming gas molecules and transfer the mechanical energy of the blades into gas molecule momentum, that is directed from the pump inlet through the pump stage towards the pump outlet.

[004] Typically, the stator blade assembly comprises a nest of alternating stator blade arrays and spacing rings. The nest may also be referred to as a stator stack. The spacing rings axially separate each stator blade array from the adjacent rotor blade array. The spacing rings maintain the axial position of the stator blades within the pump stage to prevent clashing between the rotor and stator blades. To improve pumping efficiency of the vacuum system, the clearances between rotor blades and adjacent stator blades should be relatively small, whilst ensuring that the rotor blades and stator blades do not clash.

[005] Typically, each pump stage may comprise, for example, about 6 to about 9 stator blade arrays in the nest, along with the spacing rings arranged therebetween. Unavoidable manufacturing variations introduce a tolerance between each stator blade array and spacing ring that must be accounted for when designing the stator blade assembly. Due to the stacking of the tolerances of each interface between a stator blade array and the adjacent spacing ring, the tolerance chain of the nest may include, for example, 12 to 18 tolerances. The nest should be designed such that the sum of the tolerances in the tolerance chain is small enough to substantially prevent clashing between the rotor and stator blades during operation of the vacuum system. Accordingly, the maximum number of stator blade arrays in the nest may be determined by the tolerance chain.

[006] For some applications it is desirable to increase the number of stator blade arrays within the nest. However, increasing the number of stator blade arrays in the nest increases the tolerance stacking. To accommodate such increased tolerance stacking, internal pump clearances must be increased to accommodate this variation. However, this is undesirable as it may reduce the pumping efficiency.

[007] There is therefore an ongoing need for improved stator blade assemblies. In particular, there is a need for stator blade assemblies with tighter tolerances, and an increased number of stator blade arrays in the nest.

[008] The present invention address these and other problems with known stator blade assemblies.

Summary

[009] In a first aspect, the present invention provides a stator assembly for a single pump stage of a vacuum pump. The stator assembly comprises a plurality of substantially axially aligned stator arrays. The stator assembly further comprises a first sector comprising a first quotient of the stator blade arrays, and a second sector comprising a second quotient of the stator blade arrays. The first sector has a first tolerance stack, and the second sector has a second tolerance stack. The stator assembly is configured such that the first tolerance stack and the second tolerance stack are separate.

[010] For the purposes of the invention, a single pump stage is a pumping mechanism (e.g. the rotors and stators) housed within a single pump chamber of the vacuum pump. The single pump stage and pump chamber may be alone or form part of a multistage pump, however, it shall be understood that a single stator assembly as described herein would be for a single stage of said multistage pump. The vacuum pump may be a turbomolecular pump, a multistage roots pump, or a roots/claw pump. For example, the vacuum pump may be an EXT turbomolecular pump produced by Edwards Vacuum, or a FliPace turbomolecular pump as produced by Pfeiffer Vacuum.

[011] The stator assembly may be configured to substantially surround a rotor of the vacuum pump.

[012] Each stator blade array may comprise a substantially annular array of stator blades. The configuration of the stator blades will depend on the type of vacuum pump. For example, in a turbomolecular pump, the stator array may comprise a substantially annular array of radially extending, inclined stator blades. Preferably, each stator blade array may comprise from about 30 to about 40 stator blades. The skilled person will appreciate that the number of stator blades in each stator blade array depends on the size of the vacuum pump.

[013] The stator blades may be arranged to incline in an alternate axial direction to the rotor blades of the pump stage of the vacuum pump. Each stator blade array may comprise an annular outer rim portion that is coupled to the stator blades. Typically, each stator blade array is assembled from a plurality of stator blade array sections. For example, each stator blade array may comprise a pair of half-stator blade array sections which, in use, are arranged to provide a stator blade array.

[014] In some embodiments, one or more stator blade array may comprise an integrally formed spacing portion. The spacing portion may be configured to maintain the axial position of the stator blade array within the pump stage to prevent clashing between the rotor and stator blades during operation of the pump.

[015] The first sector comprises a quotient of stator blade arrays. Said quotient of stator blade arrays comprises two or more stator blade arrays. Preferably, the first quotient comprises from about 6 to about 9 stator blade arrays. The second sector comprises a second quotient of stator blade arrays. Said second quotient of stator blade arrays comprises two or more stator blade arrays. Preferably, the second quotient comprises from about 6 to about 9 stator blade arrays. Typically, the first sector and second sector comprise an equal number of stator blade arrays. Although, equally, the first sector and second sector may comprise a different number of stator blade arrays. The first sector may not include stator blade arrays of the second sector and vice versa. [016] It will be understood by those skilled in the art that a tolerance stack or tolerance stack-up refers to the result of conventional analyses performed by engineers to account for the accumulated variations in specified tolerances and dimensions between connected parts in an assembly. Such variations may be in part due to machine limitations influencing the manufacturing accuracy that is achievable. Typically, parts are designed and manufactured to account for maximum and minimum variations in dimensions or clearances. Therefore, reducing the number of connected parts in an assembly may reduce the magnitude of potential variations, i.e. the tolerance stack. Having a large tolerance stack may adversely affect proper meshing and functioning of the overall assembly, especially considering service factors such as temperature and wear. [017] Specifically, in relation to stator assemblies, a minimum clearance is provided between each stator blade array of the stator assembly and an adjacent rotor of the vacuum pump when assembled. The minimum clearance should be maintained to substantially prevent clashing of the rotor and stator blade arrays. For example, the minimum clearance between each stator blade array and an adjacent rotor of the vacuum pump when assembled may be 1 .0 mm. Accordingly, the tolerance stack of the stator assembly may not exceed 1.0 mm without risking clashing of the rotor and stator arrays.

[018] In some embodiments, if the rotor assembly comprises a plurality of parts, the minimum clearance may account for the tolerance stack of the rotor assembly as well as that of the stator assembly. Accordingly, it is important to reduce the amount of the minimum clearance that is consumed by the tolerance stack of the stator assembly. Typically, this introduces a restriction on the number of stator blade arrays that a stator assembly may include.

[019] By providing the stator assembly with a first sector and a second sector, the tolerance stack of the stator assembly is divided into the first tolerance stack and the second tolerance stack. Therefore, according to the aforementioned example wherein the minimum clearance is 1 .0 mm, neither the first tolerance stack nor the second tolerance stack may exceed 1.0 mm. However, the theoretical combination of the first tolerance stack and second tolerance stack may exceed the minimum clearance, as the first and second tolerance stacks are separate and discrete.

[020] For the purposes of the invention, “a rotor clashing with a statof may be defined as direct contact between a rotor and a stator blade array. Typically, the rotor may be rotating when the clash occurs with the stator blade array. This may be caused by movement of the rotor in an axial direction during operation of the pump, for example due to a changing pressure gradient within the pump stage. During a clash, the friction and impact between the rotor and stator blade array may stop the rotation of the rotor. A rotor clashing with a stator blade array may cause damage to the rotor and/or stator blade array(s). A clash may require repair and/or replacement of one or more components of the pump.

[021] Typically, during operation of the vacuum pump, the rotor of a pump stage may rotate relative to the stator of said pump stage. Typically, the rotor may rotate at speeds of up to about 90000 RPM. The skilled person will appreciate that the rotation speed of the rotor during operation of the vacuum pump may vary according to a number of factors, including the type of vacuum pump, the system that is being evacuated by the vacuum pump, and the ultimate pressure desired. For the purposes of the present invention, “substantially preventing clashing of the rotor and stator blade arrays” may be defined as there being less than about 0.1 % chance of the rotor clashing with a stator blade array, preferably less than about 0.01 % chance of the rotor clashing with a stator blade array, during operation of the vacuum pump. Such clashing may occur operation of the vacuum pump may be when the clearances are at their extremes, for example when under a relatively high gas load which increase stress on the rotor and stator blades, during baking, or whilst pumping gases such as Argon or Xenon.

[022] Advantageously, the present invention may enable an increased number of stator blade arrays to be employed in a single pump stage, whilst substantially avoiding clashing of the rotor and stator blade arrays. By providing a stator assembly having first and second sectors, the number of stator blade arrays within the stator assembly may be doubled. For certain applications, having an increased number of stator blade arrays in the stator assembly may be beneficial.

[023] Typically, stator blade arrays vary in geometry along the length of the assembly, with stator blades progressively reducing in size and blade angle from one stator blade array to the next in the pumping direction, i.e. from an inlet towards an outlet of the pump stage. The compression ratio of the pump is dependent, inter alia, upon the number of arrays of rotor and stator blades, the number of blades within each array, the angle of inclination of the blades. In order to enhance the inlet capacity of a turbomolecular pump, the sizes of the blades of the stator blade arrays of the pump, that is, the stator blade array closest to the pump inlet, are generally relatively large, with the sizes of the blades of the stator blade arrays gradually decreasing from the pump inlet towards the pump outlet. In other words, the axial extent of the stator blade arrays gradually decrease from the pump inlet towards the pump outlet. Likewise, the angle of the blades tends to decrease from the pump inlet towards the pump outlet.

[024] Typically, the first sector further comprises one or more spacing rings. Preferably, the first sector further comprises a plurality of spacing rings. Each spacing ring may be positioned between a pair of adjacent stator blade arrays of the first sector. Additionally, or alternatively, the second sector may further comprise one or more spacing rings. Preferably, the second sector further comprises a plurality of spacing rings. Each spacing ring may be positioned between a pair of adjacent stator blade arrays of the second sector.

[025] Each spacing ring may be substantially annular, and configured to directly connect with a stator blade array when the stator assembly is constructed. The spacing rings may be configured to provide a clearance for rotor blades of a rotor array to rotate between adjacent stator blade arrays when the vacuum pump is in operation.

[026] Typically, the stator assembly comprises a housing containing the first sector and the second sector. The housing may comprise a chamber dimensioned to receive the stator assembly. The chamber may be defined by a housing wall. Preferably, the housing wall may have a substantially annular cross-section. The housing may comprise a plurality of semi-annular sections to allow for easier construction of the stator assembly. The housing wall may include the inlet and outlet of the pump stage. When the vacuum pump is in use, the pump stage inlet may allow a fluid connection between the pump stage and a chamber to be evacuated. In use, the pump stage outlet may allow a fluid connection between the pump stage and a pump outlet and/or a further pump stage. [027] The first tolerance stack may be separated from the second tolerance stack by an abutment. Preferably, the abutment is located on the housing. The first tolerance stack may be separated from the second tolerance stack by an abutment arranged therebetween. The first sector and/or second sector may be configured to abut against the abutment. Preferably, the abutment may be arranged at an end of the first and/or second sector. The abutment may allow a compressive force to be maintained on the first and/or second sector when the stator assembly is assembled. Advantageously, this may enable an increased number of stator blade arrays in each of the first and second sectors, whilst allowing clashing between the rotor blades and stator blade arrays to be substantially prevented.

[028] Typically, the housing comprises a first housing portion. The first housing portion may contain the first sector. The first housing portion may have a substantially annular cross-section having a first internal diameter. The housing may further comprise a second housing portion. The second housing portion may contain the second sector. The second housing portion may have a substantially annular cross-section having a second internal diameter. Typically, the first internal diameter is less than the second internal diameter. The substantially annular cross- section may be a cross-section that is substantially perpendicular to a central axis of the stator assembly.

[029] The first internal diameter may be defined by the inner wall of the first housing portion. Preferably, the diameter of the first housing portion may be substantially uniform along the axial length of the first housing portion. The first internal diameter may be greater than the outermost diameter of the stator blade arrays of the first sector. The size of the first internal diameter depends on the dimensions of the quotient of stator blade arrays of the first sector, especially the outermost diameter. [030] The second internal diameter may be defined by the inner wall of the second housing portion. Preferably, the diameter of the second housing portion may be substantially uniform along the axial length of the second housing portion. The second internal diameter may be greater than the outermost diameter of the stator blade arrays of the second sector. The size of the second internal diameter depends on the dimensions of the quotient of stator blade arrays of the second sector, especially the outermost diameter.

[031] Preferably, the first internal diameter is less than the second internal diameter. Accordingly, the outermost diameter of the stator blade arrays of the first sector is less than the outermost diameter of the stator blade arrays of the second sector.

[032] Preferably, the mating arrangement between the first internal diameter and second internal diameter is stepped to provide the abutment against which the second sector abuts. Advantageously, this may allow the separation of the first tolerance stack from the second tolerance stack.

[033] In some embodiments, the abutment may comprise a flange projecting radially inwardly from the housing. The flange may project radially inwardly from the housing by such a distance that the innermost diameter of the flange is less than the outermost diameter of a stator blade array. Advantageously, the first sector and/or second sector may abut against the flange, such that the first tolerance stack and second tolerance stack are separate.

[034] The flange may be a separate part to the housing. In such embodiments, the flange may be directly connected to the housing by a fastener. The flange may be directly connected about its outermost edge to an inner wall of the housing. The fastener may comprise one or more dowel pins, plungers, wire rings, clips, or a combination thereof. Preferably, the flange is a spacing ring as defined hereinbefore. The flange may be a modified spacing ring configured to directly connect to the housing by a fastener. Advantageously, this may provide a relatively low cost, yet secure, mechanism by which the first tolerance stack and second tolerance stack may be separated. Additionally, the internal diameter of the inner wall of the housing may be substantially uniform along the axial length of the housing, simplifying manufacture of the housing.

[035] Alternatively, the flange and the housing may be integrally formed. Preferably, the flange and the housing may be integrally formed as a single unitary component.

[036] Typically, the first sector and/or the second sector comprises at least one spacing ring having an elongate member. The elongate member may extend axially along the length of the first sector or second sector. Further features of the elongate member will be described in reference to the first sector, but the skilled person will appreciate that they may be applied to the second, or other sectors.

[037] Preferably, the spacing ring may be positioned at an end of the first sector and the elongate member may extend axially along substantially the entire length of the first sector. The elongate member may be integrally formed with the spacing ring or may be affixed thereto.

[038] The elongate member may be configured when arranged within the stator assembly to abut against a surface. The surface may be a surface of the housing, an abutment, a preload element, or a spacing ring of an adjacent sector. The stator blade assemblies of the first sector may be positioned between the spacing ring comprising the elongate member and said surface. Advantageously, the elongate member abutting against the surface may separate the first tolerance stack from other tolerance stacks of the stator blade assembly.

[039] The elongate member may extend from the spacing ring in an axial direction. The elongate member may be a finger-like projection and/or have an annular or part-annular cross-section.

[040] In embodiments when the elongate member has an annular or part-annular cross-section, the stator blade arrays of the first sector may be arranged within an inner circumference of said elongate member. Advantageously, as the elongate member at least partially circumferentially surrounds the stator blade arrays, it may provide a thermal buffer and minimise thermal conductivity to the stator blade arrays. The outermost surface of the elongate member may define at least a portion of substantially circular cross-section; however, the skilled person will appreciate that other shapes of cross-section may be employed without departing from the invention, e.g. hexagonal or octagonal.

[041] The elongate member may be in the form of a finger-like member and/or part-annular member, or similar. In such embodiments, the housing may comprise a channel configured to receive the elongate member.

[042] Preferably, the first sector and the second sector each comprise a spacing ring comprising an elongate member. The elongate member of a first sector may abut against the spacing ring comprising an elongate member of the second sector. The stator blade assemblies of the first sector may be arranged between the spacing ring comprising an elongate member of the first sector and the spacing ring comprising an elongate member of the second sector. Accordingly, the first tolerance stack and second tolerance stack may be separate.

[043] Typically, the first sector and/or the second sector comprises a preload element for biasing the stator blade arrays in an axial direction. Preferably, each sector comprises a preload element. The preload element may comprise a wave washer, a stator, and/or an O-ring. The stator could be, for example, milled, printed, or moulded. The O-ring could be, for example, a flexible rubber O-ring.

[044] Typically, the preload element is configured to compress the stator blade arrays, and optionally the spacing rings, of a sector into engagement. The preload element may be arranged at a first end of a sector. The preload element may be positioned between and engage a stator blade array or spacing ring of the sector, and a surface. The surface may be an abutment at the first end of the sector. At a second end of said sector, a stator blade array or spacing ring of the sector may engage an abutment. When in use, the compressive force provided by the preload element at the first end on the stator blade arrays and the reaction force provided by the abutment at the second end may bias the stator blade arrays, and optionally the spacing rings, of the sector into engagement.

[045] Preferably, each sector comprises a separate preload element. The preload elements of each sector may be identical, or alternatively, the preload elements of each sector may differ. The preload element for each sector may be selected according to the requirements of the sector, the type of vacuum pump, and/or the number and configuration of stator blade arrays in the sector. Ideally the compressive force produced by the preload element should be sufficient to maintain a compressive force on the sector to substantially prevent separation of stator blade arrays thereof. Any such separation may lead to clashing of the rotor and a stator blade array during operation of the vacuum pump.

[046] In an example embodiment, the first sector is arranged towards a pump inlet, and the second sector is arranged towards a pump outlet. The preload element of the second sector may be arranged at the end of the sector closest to the pump outlet. Thereby, the stator blade arrays and optionally the spacing rings of the second sector are compressed against an abutment positioned between the first and second sectors.

[047] The requirements of the first and second sectors during operation of the pump may differ. This may be, for example, to account for different pressures between the sectors due to pressure gradient within pump stage during operation. Furthermore, the complexity of stator blade arrays at different positions of the pump may require different materials.

[048] Towards the pump outlet, where the axial lengths of the blades may be relatively small, the semi-annular sections of the stator assembly are generally formed from pieces of stainless steel or aluminium sheet material. The portions for the stator blades may be defined by cutting the sheet material. Then the blades may be folded from the sheet material to a predetermined inclination either by cutting and pressing in a single step, or cutting and generating the profile in series of steps by press machining. Whilst pressing stators in this manner requires significant investment in tooling to manufacture the piece parts are relatively low cost. However, the nature of the process makes the pressed part more flexible and can leave significant residual stresses in the pressed part. Consequently, internal pump clearances must be increased to accommodate this variation. Moreover, formation of the stator blade sections in this manner means that, within a single semi-annular section, no two blades may axially overlap.

[049] One or more stator blade arrays of the first sector may comprise a first material and one or more stator blade arrays of the second sector may comprise a second material. The first material may be different to the second material. [050] The first material may comprise a metallic, polymeric, or composite material.

Preferably, the first material may comprise a metallic material. For example, the first material may comprise a stainless-steel alloy.

[051] The second material may comprise a metallic, polymeric, or composite material. Preferably, the second material may comprise a metallic material. For example, the second material may comprise an aluminium alloy.

[052] In some embodiments, such as certain ultra-high vacuum (UHV) vacuum pump systems, a system bake out may be performed to remove water vapour from the system. This may involve heating the vacuum pump to temperatures of up to about 100 °C, as well as heating the chamber being evacuated by the vacuum pump to temperatures of 100 °C to about 400 °C, preferably from about 200 °C to about 300 °C. This heating may result in an increase in the temperature of the components of the pump stage. Particularly, heat from the bake out may enter the pump stage through the pump inlet. This may cause thermal expansion of the components and may reduce the efficiency of the vacuum pump. Operation of the vacuum pump at increased temperatures, when the rotor blades are rotating and under stress, may result in creep of the rotor blades and lead to clashing of the rotor blades with the pump housing. Furthermore, increased temperatures may result in degradation of the oil lubrication present in bearings of the vacuum pump. [053] To overcome these problems, a first and/or second material may be provided with a relatively low thermal conductivity. Advantageously, this may act as a thermal buffer to reduce thermal conductivity within the pump stage. Thereby, the temperature creep of the rotor blades of the pump stage may be reduced, and the efficiency of the vacuum system may be maintained. Additionally, this may reduce degradation of the oil lubrication enabling the bearings.

[054] In a preferred embodiment, one or more stator blade array towards a pump inlet of the pump stage may comprise a first material having a relatively low thermal conductivity. Most preferably, one or more stator blade array closest to the pump inlet may comprise said first material. One or more stator blade array towards a pump outlet may comprise a second material having a relatively high thermal conductivity. For the purposes of the invention, a relatively high thermal conductivity may be greater than about 40 Wm ¹ K ¹ , preferably greater than about 100 Wm ¹ K ¹ , most preferably greater than about 200 Wm ¹ K ¹ . A relatively low thermal conductivity may be less than about 40 Wrrr ¹ K ^_1 , preferably less than about 10 Wrrr ¹ K- ¹ .

[055] In such an arrangement, the stator blade array(s) comprising the first material may act as a thermal buffer to reduce thermal conductivity to the remainder of the stator blade assembly, including the stator blade array(s) comprising the second material. Advantageously, this may reduce the thermal expansion of the components of the pump stage and improve the pumping efficiency. This may also enable the operator to perform the bake out at a higher temperature, and therefore achieve a lower pressure quicker upon initiation of pumping. [056] Additionally, providing one or more stator blade assemblies comprising such a second material may enable increased thermal conductivity of heat energy generated during operation of the vacuum system, i.e. due to rotation of the rotor. [057] In embodiments comprising third, fourth, fifth, or more sectors, one or more stator blade arrays of each sector may comprise a first material and/or a second material, and/or a further material. In an example embodiment, the stator assembly comprises a first sector, second sector, and third sector. One or more stator blade arrays of the first sector comprise a first material, one or more stator blade arrays of the second sector comprise a second material, and one or more stator blade arrays of the third sector comprise a third material. The first, second and third materials may be different. Specifically, the second material may have a lower thermal conductivity than either the first material or the second material. Advantageously, the reduced thermal conductivity of the second material may limit the thermal conductivity between the first and third sectors. This may provide a thermal break between the first sector and third sector.

[058] Preferably, the first material is a metallic material. For example, the first material is a stainless-steel alloy. The second material may be a metallic, polymeric, or composite material. Preferably, the second material may is an aluminium alloy. The third material may be a metallic, polymeric, or composite material. Preferably, the third material may be an aluminium alloy. Such an arrangement may provide the thermal buffer as described previously. [059] Typically, the first sector comprises at least 3 stator blade arrays.

Preferably, the first sector comprises at least 6 stator blade arrays, more preferably at least 9 stator blade arrays. In some embodiments, the first sector comprises a spacing ring positioned between each adjacent pair of stator blade arrays. [060] Additionally, or alternatively, the second sector may comprise at least 3 stator blade arrays. Preferably, the second sector comprises at least 6 stator blade arrays, more preferably at least 9 stator blade arrays. In some embodiments, the second sector comprises a spacing ring positioned between each adjacent pair of stator blade arrays.

[061] Advantageously, the present invention enables an increased number of stator blade arrays within a single pump stage due to improved tolerance control between adjacent stator blade arrays and rotor blade arrays of the pump stage. This is beneficial for certain applications, as the provision of additional stator blade arrays may enable increased pump compression by the pump stage during operation. A single pump stage with an increased number of stator blade arrays may achieve comparable compression performance to a multi-stage system of the prior art. Therefore, comparable compression performance may be achievable by a smaller vacuum pump. Beneficially, a smaller vacuum pump is more versatile as it may be used for applications where space is a limiting factor. [062] In embodiments wherein the stator blade assembly comprises third, fourth, fifth, sixth, or more sectors, the stator blade assembly may comprise a plurality of methods to divide adjacent sectors.

[063] For example, in an embodiment comprising first, second, and third sectors, wherein the first sector is arranged closest to the pump inlet, the first sector may comprise a first spacing ring having an elongate member. Preferably, the first spacing ring may be positioned at an upstream end of the first sector. The elongate member may be in the form of a substantially tubular member extending in a downstream axial direction from the spacing ring such that the stator blade arrays of the first sector are arranged in a radially extending direction within said substantially tubular member. The second sector may comprise a second spacing ring having an elongate member. Preferably, the second spacing ring may be positioned at an upstream end of the second sector. The elongate member may comprise a substantially tubular member extending in a downstream axial direction from the spacing ring such that the stator blade arrays of the second sector are arranged in radially extending direction within said substantially tubular member. The substantially tubular member of the first spacing ring may abut against the second spacing ring to divide the first and second sectors.

[064] The first and second spacing rings may comprise a material having a relatively low thermal conductivity, such as a stainless-steel alloy. Advantageously, the first and second spacing rings may provide a thermal buffer to reduce thermal conductivity to the sectors of the pump stage as the first and second sectors are adjacent the pump inlet, through which a majority of heat ingress may occur during a system bake out.

[065] The first and second sectors may be arranged within a first housing portion, having a substantially annular cross-section with a first internal diameter. The third sector may be arranged within a second housing portion, having a substantially annular cross-section with a second internal diameter. The first internal diameter is less than the second internal diameter. Thus, the outermost diameter of the stator blade arrays of the first and second sectors are less than the outermost diameter of the stator blade arrays of the third sector. The mating arrangement between the first internal diameter and the second internal diameter may be stepped to provide an abutment against which the third sector abuts to divide the second and third sectors. Preferably, the third sector may comprise a material with a relatively high thermal conductivity, such as aluminium or an alloy thereof.

[066] Alternatively, in embodiments wherein the stator blade assembly comprises third, fourth, fifth, sixth, or more sectors, the stator blade assembly may comprise the same method to divide adjacent sectors. For example, the first sector and second sector may be divided by a flange, and the second sector and third sector may be divided by a flange.

[067] In a further aspect, the present invention provides a vacuum pump comprising a pump stage including the stator assembly according to any preceding aspect or embodiment. Preferably, the vacuum pump is a turbomolecular pump or a sliced multistage roots pump. For example, the vacuum pump may be an EXT pump as produced by Edwards Vacuum, or a HiPace turbomolecular pump as produced by Pfeiffer Vacuum.

[068] Typically, the vacuum pump may further comprise a second pump stage. Preferably, the second pump stage comprises a second stator assembly according to any preceding aspect or embodiment.

[069] In a further aspect, the present invention provides a method of designing a stator assembly for a single pump stage of a vacuum pump. The method comprises the steps of: a) determining a minimum clearance required between each stator blade array of the stator assembly and an adjacent rotor of the vacuum pump when assembled to substantially prevent clashing therebetween; b) determining the minimum number of stator blade arrays required for the stator assembly; and c) dividing the stator assembly into a plurality of sectors, each sector having a separate tolerance stack, such that the tolerance stack of each sector does not exceed the minimum clearance required to substantially prevent clashing; and d) optionally fabricating a stator assembly according to the design.

[070] The minimum clearance between each stator blade array of the stator assembly and an adjacent rotor of the vacuum pump when assembled is maintained to substantially prevent clashing of the rotor and stator blade arrays. For the purposes of the invention, “a rotor clashing with a stator” may be defined as direct contact between a rotor and a stator blade array. Typically, the rotor may be rotating when the clash occurs with the stator blade array. This may be caused by movement of the rotor in an axial direction during operation of the pump, for example due to a changing pressure gradient within the pump stage. During a clash, the friction and impact between the rotor and stator blade array may cease rotation of the rotor. [071] A rotor clashing with a stator blade array may cause damage to the rotor and/or stator blade array(s). A clash may require repair and/or replacement of one or more components of the pump. Such repair and/or replacement of components of the pump would require machine downtime during which the pump would be inoperable. Accordingly, preventing such clashing is essential for enabling the pump to be operated without issue.

[072] The determination of the minimum number of stator blade arrays required for the stator assembly depends on the specific application of the vacuum pump when in use. Typically, for each stator blade array in the pump stage there is a corresponding rotor blade array. The number of stator blade arrays in the pump stage may be the same as the number of rotor blade arrays ±1.

[073] For certain applications, it may be advantageous to have a relatively large number of stator blade arrays, and corresponding rotor blade arrays, in a single stage of the vacuum pump. Typically, a single stage of a vacuum pump may comprise from about 6 to about 9 stator blade arrays. It may be beneficial to have greater than about 12 stator blade arrays, preferably greater than about 15 stator blade arrays in a single stage of the vacuum pump. This may enable the vacuum pump to achieve lower pumping pressures without requiring additional pump stages. Advantageously, this may allow improved pumping performance whilst retaining a relatively compact size of vacuum pump.

[074] Each sector of the stator assembly has a separate tolerance stack. The tolerance stack of each sector includes the accumulated variations in specified tolerances and dimensions of the connected parts in the stator assembly. The stator assembly, and thus each sector, may comprise a plurality of stator blade arrays, and optionally, spacing rings. The tolerance stack of each sector must not exceed the minimum clearance required to substantially prevent clashing of the rotor and stator blade arrays. [075] There may be multiple viable options for dividing the stator into a plurality of sectors, whilst ensuring that the tolerance stack of each sector does not exceed the minimum clearance required to substantially prevent clashing. Accordingly, the step of dividing the stator assembly into a plurality of sectors may involve determining the optimum number of sectors to divide the stator assembly into, and where such divisions should be within the stator blade array. Each sector may include an equal number of stator blade arrays, or alternatively, the number of stator blade arrays in a first sector may differ from the number of stator blade arrays in a second sector. The stator assembly may comprise two, three, four, five, or more sectors.

[076] The method may further comprise the step of selecting an arrangement for separating the sectors of the stator assembly according to the requirements of the stator assembly during operation of the vacuum pump.

[077] The arrangement for separating the sectors of the stator assembly may include any method, feature or configuration as described hereinbefore, or a combination thereof. The specific method(s) may be determined according to the requirements of the pump. Considerations may include, for example, the pump pressure required, the size of the vacuum pump, whether the pump is a single stage or multistage pump, and/or whether other pumps connected.

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

Brief Description of Figures

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

[080] Figure 1 shows a cross-sectional schematic view of a pump stage according to the prior art. [081] Figure 2 shows a cross-sectional schematic view of an embodiment of a pump stage according to the present invention. [082] Figure 3 shows a cross-sectional schematic view of an alternative embodiment of a pump stage according to the present invention.

[083] Figure 4 shows a cross-sectional schematic view of an alternative embodiment of a pump stage according to the present invention.

[084] Figure 5 shows a cross-sectional schematic view of an alternative embodiment of a pump stage according to the present invention.

Detailed Description

[085] Figure 1 illustrates a cross-sectional schematic view of a pump stage (1) according to the prior art.

[086] The pump stage (1 ) comprises a rotor assembly (2). The rotor assembly (2) comprises a plurality of rotor blade arrays (3). The rotor blade arrays (3) are arranged on a rotor shaft (4). The rotor blade arrays (3), and rotor shaft (4) are axially aligned with the central axis (X) of the pump stage (1 ).

[087] The pump stage (1) further comprises a stator assembly (5). The stator assembly (5) comprises a plurality of alternating stator blade arrays (6) and spacing rings (7). The stator blade arrays (6) are supported at predetermined intervals by the spacing rings (7). The spacing rings (7) support the stator blade arrays (6) at outer circumferences thereof. Each stator blade array (6) comprises an array of regularly spaced stator blades. [088] The rotor blade arrays (3) and rotor shaft (4) are arranged for rotation about the central axis (X) relative to the stator assembly. The stator assembly (5) is contained within a housing (8). [089] The stator assembly (5) further comprises at least one preload element (9).

The preload element (9) is positioned between a stator blade array (6) and the housing (8). The preload element (9) provides a compressive pressure on the stator assembly (5). The preload element (9) maintains the position of the stator assembly (5) within the housing (8).

[090] Due to unavoidable manufacturing variations, a tolerance is present at the interface between each stator blade array (6) and spacing ring (7). In the illustrated embodiment, the stator assembly (5) comprises six stator blade arrays (6) and five spacing rings (7). Therefore, the total tolerance of the stator assembly (5) includes ten tolerances.

[091] To avoid clashing of the stator blade arrays (6) with rotor blade arrays (3), the total tolerance of the stator assembly (5) must be less than the clearance between each stator blade array (6) and rotor blade array (3). This imparts a limitation on the total number of stator blade arrays (6) that can be present in the stator assembly (5).

[092] Figure 2 illustrates a cross-sectional schematic view of an embodiment of a pump stage (10) according to the present invention.

[093] The pump stage (10) comprises a rotor assembly (11 ). The rotor assembly (11) comprises a plurality of rotor blade arrays (12). The rotor blade arrays (12) are arranged on a rotor shaft (13). The rotor blade arrays (12), and rotor shaft (13) are axially aligned with the central axis (Y) of the pump stage (10). The rotor blade arrays (12) and rotor shaft (13) are arranged for rotation about the central axis (Y) relative to the stator assembly. [094] The pump stage (10) further comprises a stator assembly. The stator assembly comprises a first sector (14) and a second sector (15). The first sector

(14) and second sector (15) are contained within a housing (16). The housing (16) comprises a first housing portion (17). The first sector (14) is contained within the first housing portion (17). The housing (16) comprises a second housing portion

(18). The second sector (15) is contained within the second housing portion (18).

[095] The first sector (14) comprises a plurality of alternating stator blade arrays

(19) and spacing rings (20). The stator blade arrays (19) are supported at predetermined intervals by the spacing rings (20). The spacing rings (20) support the stator blade arrays (19) at outer circumferences thereof. Each stator blade array (19) comprises an array of regularly spaced stator blades.

[096] The second sector (15) comprises a plurality of alternating stator blade arrays (21) and spacing rings (22). The stator blade arrays (21) are supported at predetermined intervals by the spacing rings (22). The spacing rings (22) support the stator blade arrays (21) at outer circumferences thereof. Each stator blade array (21 ) comprises an array of regularly spaced stator blades. [097] In this embodiment, the first housing portion (17) has a substantially annular cross section having a first diameter (A). The second housing portion (18) has a substantially annular cross section having a second diameter (B). The first diameter (A) is less than the second diameter (B). The first diameter (A) and second diameter (B) are substantially perpendicular to the central axis (Y).

[098] Between the first housing portion (17) and the second housing portion (18) is an abutment (23). The abutment (23) comprises a step in the housing (16). The abutment (23) provides the transition between the first diameter (A) and the second diameter (B). The mating arrangement between the first sector (14) and the second sector (15) is stepped to correspond with the abutment (23). The second sector

(15) is abutted against the abutment (23). Thereby, the tolerance stack of the first sector (14) and the tolerance stack of the second sector (15) are separated by the abutment.

[099] The first sector (14) further comprises at least one preload element (24). The preload element (24) is positioned between a stator blade array (19) and the first housing portion (17). The preload element (24) is configured to compress the stator blade arrays (19) and spacing rings (20) of the first sector (14) into engagement. [100] The second sector (15) further comprises at least one preload element (25).

The preload element (25) is positioned between a stator blade array (21) and the second housing portion (18). The preload element (25) is configured to compress the stator blade arrays (21) and spacing rings (22) of the second sector (15) into engagement.

[101] Figure 3 illustrates a cross-sectional schematic view of an alternative embodiment of a pump stage according to the present invention.

[102] The pump stage (26) comprises a rotor assembly (27). The rotor assembly (27) comprises a plurality of rotor blade arrays (28). The rotor blade arrays (28) are arranged on a rotor shaft (29). The rotor blade arrays (28), and rotor shaft (29) are axially aligned with the central axis (Z) of the pump stage (26). The rotor blade arrays (28) and rotor shaft (29) are arranged for rotation about the central axis (Z) relative to the stator assembly.

[103] The pump stage (26) further comprises a stator assembly. The stator assembly comprises a first sector (30) and a second sector (31). The first sector (30) and second sector (31) are contained within a housing (32). The housing (32) comprises a first housing portion (38). The first sector (30) is contained within the first housing portion (38). The housing (32) comprises a second housing portion (39). The second sector (31) is contained within the second housing portion (39). [104] The first housing portion (38) and second housing portion (38) are connected via a gasket (40). The gasket (40) is substantially annular and extends about the substantially annular first housing portion (37) and substantially annular second housing portion (38).

[105] The first sector (30) comprises a plurality of alternating stator blade arrays (33) and spacing rings (34). The stator blade arrays (33) are supported at predetermined intervals by the spacing rings (34). The spacing rings (34) support the stator blade arrays (33) at outer circumferences thereof. Each stator blade array (33) comprises an array of regularly spaced stator blades.

[106] The second sector (31) comprises a plurality of alternating stator blade arrays (35) and spacing rings (36). The stator blade arrays (35) are supported at predetermined intervals by the spacing rings (36). The spacing rings (36) support the stator blade arrays (35) at outer circumferences thereof. Each stator blade array (35) comprises an array of regularly spaced stator blades.

[107] The first sector (30) and second sector (31 ) are separated by an abutment. In this embodiment, the abutment comprises a flange (39). The flange (39) projects radially inwardly from the housing (32). The flange (39) and second housing portion (38) are integrally formed as a single unitary component. The flange (39) projects radially inwardly by such a distance that the innermost diameter of the flange (39) is less than the outermost diameter of a stator blade array (33,35). [108] The first sector (30) and second sector (31) abut against the flange (39), such that the first tolerance stack of the first sector (30) and second tolerance stack of the second sector (31 ) are separate.

[109] The first sector (30) further comprises at least one preload element (41). The preload element (41) is positioned between a stator blade array (33) and the first housing portion (37). The preload element (41) is configured to compress the stator blade arrays (33) and spacing rings (34) of the first sector (30) into engagement.

[110] The second sector (31 ) further comprises at least one preload element (42). The preload element (42) is positioned between a stator blade array (35) and the second housing portion (38). The preload element (42) is configured to compress the stator blade arrays (35) and spacing rings (36) of the second sector (31) into engagement. [111] Figure 4 illustrates a cross-sectional schematic view of an alternative embodiment of a pump stage according to the present invention. Figures 3 and 4 have many corresponding features, which will not be repeated and for which the same reference numerals will be used. [112] The flange (43) is a separate part to the housing (32). The flange (43) is directly connected to the housing (32) by a fastener (44). The flange (44) is directly connected about its outermost edge to an inner wall of the housing (32). The fastener (44) comprises a plurality of dowel pins. [113] The flange (43) provides an abutment against which the first sector (30) and second sector (31) are arranged to abut. The first tolerance stack and second tolerance stack are therefore separate.

[114] Figure 5 illustrates a cross-sectional view of an alternative embodiment of a pump stage according to the present invention. Figures 3, 4 and 5 have many corresponding features, which will not be repeated and for which the same reference numerals will be used. Specifically, the details of the first sector (30) are the same as those of Figures 3 and 4. [115] The second sector (31) comprises a plurality of alternating stator blade arrays (35) and spacing rings (36). The stator blade arrays (35) are supported at predetermined intervals by the spacing rings (36). The spacing rings (36) support the stator blade arrays (35) at outer circumferences thereof. Each stator blade array (35) comprises an array of regularly spaced stator blades.

[116] The second sector (31) further comprises a spacing ring (45) having an elongate member (46). The spacing ring (45) is positioned at an end of the second sector (31) adjacent to the first sector (30). The elongate member (46) extends axially along the length of the second sector (31). The elongate member (46) is integrally formed with the spacing ring (45). [117] The elongate member (46) is a substantially tubular member extending in an axial direction from the spacing ring (45) along the length of the second sector (31 ). The stator blade arrays (35) and spacing rings (36) of the second sector (31 ) are arranged radially within said substantially tubular member. [118] The pump stage (26) further comprises a third sector (47). The third sector

(47) comprises a plurality of alternating stator blade arrays (48) and spacing rings (49). The stator blade arrays (48) are supported at predetermined intervals by the spacing rings (49). The spacing rings (49) support the stator blade arrays (48) at outer circumferences thereof. Each stator blade array (48) comprises an array of regularly spaced stator blades.

[119] The third sector (47) further comprises a spacing ring (50) having an elongate member (51). The spacing ring (50) is positioned at an end of the third sector (47) adjacent to the second sector (31 ). The elongate member (51 ) extends axially along the length of the third sector (47). The elongate member (51) is integrally formed with the spacing ring (50).

[120] The elongate member (51) is a substantially tubular member extending in an axial direction from the spacing ring (50) along the length of the third sector (47). The stator blade arrays (48) and spacing rings (49) of the third sector (47) are arranged radially within said substantially tubular member. [121] The first sector (30) is located within a first housing portion (52). The first housing portion (52) has a substantially annular cross section having a first diameter (C). The second (31) and third (47) sectors are located within a second housing portion (53). The second housing portion (53) has a substantially annular cross section having a second diameter (D). The first diameter (C) is less than the second diameter (D). The first diameter (C) and second diameter (D) are substantially perpendicular to the central axis (Y).

[122] A stator blade array (35) of the first sector (30) abuts against the spacing ring (45) having an elongate member (46) of the second sector (31 ).

[123] Between the first housing portion (52) and the second housing portion (53) is an abutment (54). The abutment (54) comprises a step in the housing (32). The abutment (54) provides the transition between the first diameter (C) and the second diameter (D). The mating arrangement between the first sector (30) and the second sector (31) is stepped to correspond with the abutment (54). The second sector (31 ) is abutted against the abutment (54). Specifically, the spacing ring (45) having an elongate member (46) of the second sector (31 ) is abutted against the abutment (54). Thereby, the tolerance stack of the first sector (30) and the tolerance stack of the second sector (31 ) are separated by the abutment (54).

[124] The elongate member (46) of the spacing ring (45) of the second sector (31 ) abuts against the spacing ring (50) having an elongate member (51) of the third sector (31 ). This provides the mating arrangement between the second sector (31 ) and the third sector (47), such that the tolerance stack of the second sector (31) and the tolerance stack of the third sector (47) are separated.

[125] The third sector (47) further comprises a preload element (55). The preload element (55) is positioned between a spacing ring (49) and housing (32). The preload element (55) is configured to compress the stator blade arrays (35,48), spacing rings (36,49) and spacing rings (45,50) of the second (31) and third (47) sectors into engagement. [126] For the avoidance of doubt, features of any aspects or embodiments recited herein may be combined mutatis mutandis. 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.

Reference Key

1. Pump stage (prior art)

2. Rotor stack (prior art)

3. Rotor blade array (prior art)

4. Rotor shaft (prior art)

5. Stator assembly (prior art)

6. Stator blade array (prior art)

7. Spacing rings (prior art)

8. Housing (prior art)

9. Preload element (prior art)

10. Pump stage

11. Rotor assembly

12. Rotor blade array

13. Rotor shaft

14. First sector

15. Second sector

16. Housing

17. First housing portion

18. Second housing portion

19. Stator blade array

20. Spacing rings

21. Stator blade array

22. Spacing rings

23. Abutment

24. Preload element

25. Preload element

26. Pump stage

27. Rotor assembly

28. Rotor blade array

29. Rotor shaft

30. First sector 31. Second sector

32. Housing

33. Stator blade array

34. Spacing ring 35. Stator blade array

36. Spacing ring

37. First housing portion

38. Second housing portion

39. Flange 40. Gasket

41. Preload element

42. Preload element

43. Flange

44. Fastener 45. Spacing ring

46. Elongate member

47. Third sector

48. Stator blade array

49. Spacing ring 50. Spacing ring

51. Elongate member

52. First housing portion

53. Second housing portion

54. Abutment 55. Preload element