WO2023275543A1

WO2023275543A1 - Screw-type vacuum pump

Info

Publication number: WO2023275543A1
Application number: PCT/GB2022/051668
Authority: WO
Inventors: Robert Desmond WILD; Neil Turner; Malcolm William Gray; Paul Milner; Mayank VERMA
Original assignee: Edwards Limited
Priority date: 2021-06-29
Filing date: 2022-06-29
Publication date: 2023-01-05
Also published as: GB2608379A; GB202109329D0; TW202311627A

Abstract

A screw rotor for a vacuum pump, the screw rotor having a first end and a second end, and being arranged to rotate about a longitudinally extending rotational axis thereof; a rotor wall having an external pump surface defining a substantially helical thread extending from said first end to said second ends; the screw rotor further comprising one or more internal cores, at least one said internal core having a density which is substantially different to that of the rotor wall; and the or each core being distributed such that a principal axis of inertia of the screw rotor and the rotational axis thereof are substantially coaxial when rotating about said rotational axis, in use.

Description

Screw-type vacuum pump

Field

[001] The present invention relates to vacuum pumps and, in particular, to vacuum pumps having a screw mechanism, screw rotors for screw-type vacuum pumps, and methods of designing and manufacturing the same.

Background

[002] Screw-type vacuum pumps are a type of dry, positive displacement, vacuum pump and are typically employed for industrial vacuum processes, such as vacuum coating, drying, plasma processes and LED manufacture. A screw pump may be utilised as a standalone pump, or as part of a pump combination in a vacuum system.

[003] In a screw-type vacuum pump, or compressor, typically a pair of opposed synchronously rotating screw rotors rotate at high speed, in opposing rotational directions, and mesh to force a fluid from a fluid inlet towards a fluid outlet, i.e. an exhaust. A small clearance is provided between opposing screw rotors and between each screw rotor and a stator periphery within a pump chamber. A screw-type vacuum pump is typically non-contacting and lubricant-free.

[004] A screw rotor for a screw pump typically comprises a central cone or cylinder around which a substantially helical thread, or profile, is arranged. It is in the void formed by the central cone and the thread that fluid flows in use as the opposing screw rotors synchronously rotate.

[005] A screw rotor may have a uniform pitch, i.e. a single pitch, or a non-uniform pitch and may have a uniform profile or a non-uniform profile along the longitudinal axis of the rotor. [006] A disadvantage of many existing screw pumps is that a screw rotor is typically inherently unbalanced because distribution of mass of the screw rotor is not uniform. Ordinarily, at a transverse cross section of the a rotor at a plane substantially perpendicular to the rotational axis, the thread of the screw will be arranged around only a portion of the circumference of the cone and not around the whole circumference of the cone due to the helical nature of the screw. Thus, the screw rotor is unbalanced due to the additional mass of the thread around only a portion of the circumference of the cone at an axial plane.

[007] Known screw rotors are substantially solid. In some examples, a screw rotor may comprise a through-bore configured to accommodate a rotor shaft which facilitates rotation of the screw rotor. However, a through-bore for a shaft is necessarily filled (by the shaft) in use. Nevertheless, if present, such a through-bore is substantially uniformly distributed about the axis of rotation of the screw pump rotor and thus does not contribute to the balance (or unbalance) of the screw rotor.

[008] Unbalance leads to forces being transmitted to the bearings on which a rotor is suspended when it is rotated. Such forces result in vibration of the machine containing the rotor, and whatever it is attached to or incorporated within, along with associated noise. It is almost always desirable to minimise the vibration in a machine.

[009] A rotor can have static unbalance. That is, the centre-of-mass is displaced from its axis of rotation. Even if the rotor possesses (or is corrected to have) static balance, the distribution of mass around and along the rotation axis can mean there is residual couple unbalance, so-called because the forces transmitted to the bearing locations are of equal magnitude but opposite direction and form a couple.

[010] It is known to correct the balance of a rotor by adding (removing) mass to (from) regions of the rotor. Static unbalance can be removed by adding corrections in any single location on the rotor. If a rotor has a combination of static and couple unbalance, it is possible to remove both in a single correction at a precisely defined location. For example, mass may be removed from external surfaces of the rotor, such as the profile surface or end faces of the rotor. A rotor may thus comprise a number of bores or similar. However, these features are defined by the outer surface of the rotor wall, and are thus outside and do not form part of the rotor per se.

[011 ] However, in practice it is almost impossible to guarantee that a correction in the precise location required will not interfere with the function of the rotor, so it is much more common to remove couple unbalance by adding two corrections at different planes along the axial length of the rotor that can be freely and conveniently chosen. For example, where mass is removed from an end face of a rotor, a further correction is typically required at a different plane to remove couple unbalance. For a screw pump rotor it is more convenient to correct the unbalance at planes beyond the two ends of the functional part of the screw profile with counter-balance masses. However, this means that portions of the rotor or rotor shaft must be set aside to accommodate the corrections.

[012] Correction of static and couple unbalance in this way eliminates dynamic forces transmitted to the bearing locations of the screw pump when it is rotated at relatively low speed. However, because the source of the unbalance (the rotor profile) and the correction do not lie at the same axial positions along the rotor, the transmitted forces are eliminated at the expense of introducing internal bending stresses in the rotor. The internal stresses are largest when the rotational frequency is high and/or the unbalance correction is large.

[013] Moreover, materials forming vacuum pumps have some inherent flexibility and, in particular when a vacuum pump is in use, the rotors of the vacuum pump flex in response to the internal bending stresses. Distortion of the rotor changes its mass distribution and reintroduces unbalance and therefore vibration. In addition, distortion increases the risk of contact between opposing screw rotors or between a screw rotor and a pump stator. The need to take these distortions into account during rotor design results in greater clearances being provided. An inherently unbalanced rotor will need a greater correction, which results in higher internal stresses and greater distortion, and therefore must run with greater clearances. This inhibits performance. Conversely an inherently balanced rotor may run with less clearance and thus with improved performance. [014] Providing balance corrections sufficient to remove the static and couple unbalance without regard for the distortions induced by internal stresses is known as “low speed dynamic balancing”.

[015] The internal stresses in the rotor can be reduced by adding corrections at multiple (more than two) planes that are more closely aligned with the source of unbalance. Providing balance corrections selected to remove the static and couple unbalance and reduce distortions under conditions close to the normal operating conditions for a rotor is known as “high speed dynamic balancing” or “at-speed dynamic balancing”. For the screw pump rotor, it is impossible to provide corrections within the functional part of the rotor without impairing the performance of the machine. Adding material anywhere will result in clashing of the rotors or between the rotor and stator. Removing material from the profile will adversely affect the gas leakage and therefore reduce the performance of the vacuum pump. Alternatively sections of a rotor can be set aside for incorporating dedicated counter-balance features. Therefore, fewer wraps, i.e. turns, of a rotor can be fitted into a given rotor length which inhibits performance. If it were not necessary to set aside sections of a rotor for providing balancing features, the number of rotor wraps that could be fitted to a given rotor could be maximised, which would improve throughput to foot print ratio of a screw rotor

[016] There remain limitations in the design and use of screw pumps because existing practises for countering the unbalance of a screw rotor are not sufficiently effective.

[017] Acceptable balancing may be considered to be achieved when the level of vibration experienced or predicted is at an acceptable level. Tolerances may be guided by ISO 21940 relating to mechanical vibration and rotor balancing which inter alia provides a balance quality grade (G). A balance quality grade defines the limits of permitted residual unbalance.

[018] Accordingly, there is an ongoing need to address at least the problem of unbalance in screw rotors for screw pumps, and thus improve the performance of vacuum pumps. There is also a corresponding need to reduce the risk of contact between pump components, thereby improving the service life of these apparatus. [019] The present invention aims to address these and other problems with the prior art.

Summary

[020] Accordingly, in a first aspect, the present invention provides a screw rotor for a vacuum pump, the screw rotor having a first end and a second end, and being arranged to rotate about a longitudinally extending rotational axis thereof; a rotor wall having an external pump surface defining a substantially helical thread extending from said first end to said second end; the screw rotor further comprising one or more internal cores, at least one said internal core having a density which is substantially different to that of the rotor wall; and the or each core being distributed such that a principal axis of inertia of the screw rotor and the rotational axis thereof are substantially coaxial when rotating about said rotational axis, in use.

[021] As used herein, the term “rotor wall” refers to the part of the screw rotor comprising the external pump surface which defines a substantially helical thread.

[022] Advantageously, the configuration of the screw rotor of the present invention allows the balance of the rotor to be improved. More specifically, the screw rotor comprises one or more internal cores, at least one of which having a density which is substantially different to the density of the rotor wall, which are distributed such that couple unbalance and/or dynamic unbalance is minimised or eliminated because a principal axis of inertia of the screw rotor and the rotational axis thereof are substantially coaxial in use. Preferably, the screw rotor may be balanced without the need for external balancing means. For example, the screw rotor may be balanced without the need for mass to be added to or removed from the exterior surface of the rotor shaft and/or rotor respectively.

[023] In embodiments, the at least one said core having a density which is substantially different to that of the rotor wall refers to a density (i.e. the mass per unit volume) which is sufficiently different to that of the rotor wall in order that distribution of the or each core causes the principal axis of inertia of the screw rotor and the rotational axis thereof to be substantially coaxial when rotating about said rotational axis, in use, and preferably without the need for external balancing means.

[024] In embodiments, a said internal core may be a substantially gas-filled chamber defined by the rotor wall and may therefore have a density which is substantially different to the density of the rotor wall. For example, an air-filled core may have a density of approximately 1 .20 kg/m³ and the rotor wall may be comprised of steel at a density of approximately 7,850 kg/m³. Thus, the rotor wall and an internal core may have relative densities at a ratio of around 6,500 : 1 . Preferably, the ratio of the density of the rotor wall to the density of the internal core is greater than about 500:1 , preferably greater than about 1000:1 , preferably greater than about 5000:1 .

[025] In another example, the rotor wall may comprise steel at a density of approximately 7,850 kg/m³ and an internal core may comprise aluminium at a density of approximately 2,600 kg/m³. Thus, the rotor wall and an internal core may have relative densities at a ratio of around 3:1.

[026] For the avoidance of doubt, all measurements, including densities, provided herein are measured at 20°C and 1 atmosphere unless stated otherwise.

[027] The one or more internal cores of the screw rotor allows mass to be evenly distributed without the external pump surface of the rotor being modified. In particular, it is not necessary to set aside room within the pump for counter-balancing weights or to substantially add (remove) mass to (from) regions of the rotor or rotor shaft. Thus, the exterior surface of the rotor is not impaired and the number of rotor wraps that can be fitted to a given rotor may be maximised, which improves throughput to foot print ratio of the screw rotor. Therefore, performance of the screw rotor can be optimised because the requirement to add material to or remove material from the external pump surface, i.e. the exterior, of the rotor is minimised or avoided altogether. The screw rotor, and in particular the configuration of the or each core also removes existing limitations and increases the scope of design and configuration of screw rotors.

[028] The improvement in balance is achieved through the configuration of a substantially internal part of the screw rotor, and in particular by the density of at least one core compared to the rotor wall, and the distribution of the or each core. The configuration of the one or more cores acts as an internal counterbalance to the external thread of the screw rotor so that vibrations, internal stresses or other undesirable forces are minimised or eliminated. Each internal core is inside the rotor and is not therefore defined by the rotor wall at an external surface and thus outside the rotor, in contrast to the known balancing techniques described above. Shaft distortion at high speed may be minimised or eliminated. The serviceable life of pump components, such as bearings, may also be improved.

[029] The skilled person will be familiar with methods for determining whether a screw rotor, or a set of screw rotors, is sufficiently balanced within a given balance specification. For example, determining whether the screw rotor is balanced, i.e. whether a principal axis of inertia of the screw rotor and the rotational axis thereof are substantially coaxial, may be achieved with the use of a computer program. For example, a computer program may comprise a computer-implemented simulation for simulating the rotation of a screw rotor at a predetermined working rotational speed. The skilled person will be familiar with suitable computer programs which are configured to provide inertia tensor data including principal axis directions, in order to determine whether a screw rotor, or a set of screw rotors, is acceptably balanced. Thus the balance (or lack thereof) of a subsequently formed screw rotor may be determined prior to manufacture.

[030] Additionally or alternatively, a balancing machine, for example a twin plane horizontal balancer, may be used to test whether / confirm that a screw rotor is sufficiently balanced. For example, a screw rotor may be run at a planned operational rotation speed, e.g. 5,000 RPM to determine whether the screw rotor is sufficiently balanced or whether any couple unbalance exists.

[031] In embodiments, the principal axis of inertia of the screw rotor and rotational axis thereof may be substantially coaxial, and the screw rotor may thus be considered acceptably balanced, when the screw rotor has a balance quality grade of at least G 6.3, and more preferably at least G 2.5, in view of ISO 21940. [032] In embodiments, the or each internal core may be distributed such that the centre of mass of any transverse cross section of the screw rotor in a plane substantially perpendicular to the rotational axis of the rotor, may be substantially coincident with the rotational axis of the rotor.

[033] In embodiments, the screw rotor may be a single pitch screw rotor, a variable pitch screw rotor or a stepped pitch screw rotor. In embodiments, the screw rotor may have a variable pitch, as described in EP1960671.

[034] In embodiments, the or each core is configured such that the centre of mass of the screw rotor is located substantially on the rotational axis of the screw rotor. In embodiments, the rotor may comprise one or more cores which are distributed along at least part of the length of the rotor and are distributed such that the principal axis of inertia of the screw rotor and the rotational axis thereof are substantially coaxial when rotating about said rotational axis, in use. In embodiments, a core may extend along substantially the entire length of the rotor. In embodiments, a core may extend along only part of the length of the rotor. In embodiments, the centre of mass of any transverse cross section of the rotor in a plane substantially perpendicular to the rotational axis is located substantially on the rotational axis of the screw rotor.

[035] In embodiments, the screw rotor may comprise a plurality of internal cores.

[036] In embodiments, the screw rotor may comprise two or more internal cores; a region of each said core having a density which is substantially different to the density of the rotor wall. The or each core may thus be arranged so that mass of the screw rotor is distributed about the screw rotor to substantially statically balance the rotor, and substantially dynamically balance the rotor, in use without the requirement for a separate counter balance weight being coupled to the rotor wall, and without modifying the rotor wall. Because the or each core is internal the material need not be the same as that which forms the rotor wall. In embodiments, a said core may have a density which is substantially greater than or substantially lower than that of the rotor wall. In embodiments, the screw rotor comprises more than one core, and at least one said core has a density which is substantially similar to or substantially the same as the rotor wall. [037] In embodiments, the screw rotor may comprise three or more cores.

[038] In embodiments, at least one said internal core comprises a region having a density which is substantially lower than the density of the rotor wall. For example, the rotor wall may be formed of a material such as an iron alloy, for example stainless steel, and a core may formed of a material which is substantially less dense than the rotor wall, for example aluminium. In embodiments, a core may be a void, i.e. a substantially gas-filled space, which inherently has a lower density than the rotor wall and which is substantially internal to the screw rotor.

[039] In embodiments, the screw rotor may comprise a first core and a second internal core, at least a region of the first core having a density which is substantially lower than the density of at least a region of the second core.

[040] In embodiments, the screw rotor may comprise three cores, wherein at least a region of two said cores have a density which is substantially less than that of the rotor wall.

[041] In embodiments, at least one said internal core has a substantially asymmetrical axial cross section. In embodiments, the screw rotor comprises a plurality of internal cores and two or more said cores have a substantially asymmetrical axial cross section. In embodiments, each core has a substantially asymmetrical axial cross section.

[042] In embodiments, at least one said internal core comprises an internal chamber, the internal chamber being distributed such that the principal axis of inertia of the screw rotor and the rotational axis thereof are substantially coaxial when rotating about said axis, in use.

[043] In embodiments, an internal chamber substantially defines a void, i.e. a substantially empty or gas-filled space. In embodiments, an internal chamber substantially defines a void within which a lattice or foam is located. [044] In embodiments, the screw rotor comprises a single core, the core comprising an internal chamber. Thus, the internal chamber has a lower density than the rotor wall and is distributed such that the principal axis of inertia of the screw rotor and the rotational axis thereof are substantially coaxial, in use.

[045] In embodiments, the screw rotor may be substantially hollow. In other words, the internal volume of the screw rotor may substantially comprise one or more internal cores formed as internal chambers.

[046] In embodiments, the first and second ends of the screw rotor may be attachable to a drive unit of a vacuum pump. In embodiments, the screw rotor may comprise end caps at the first and second ends thereof for attachment to a drive unit of a vacuum pump. In other words, the screw rotor may be arranged to rotate, in use, in the absence of an internal rotor shaft.

[047] In embodiments, a core may comprise an internal chamber arranged as a through-bore for accommodating a shaft, in use, the through-bore being distributed such that the principal axis of inertia of the screw rotor and the rotational axis thereof are substantially coaxial when rotating about said axis, in use. In embodiments, the through-bore may have a substantially asymmetrical axial cross section.

[048] In embodiments, an internal core may be arranged such that the rotational axis of the screw rotor substantially passes through the said core. In embodiments, an internal core may be substantially entirely axially bounded by the rotor wall. In embodiments, an internal core may extend continuously along substantially the entire length of the rotor.

[049] In embodiments, an internal chamber may be arranged such that the rotational axis of the screw rotor substantially passes through the chamber. In embodiments, an internal chamber may be relatively centrally disposed within the screw rotor.

[050] In embodiments, an internal chamber may be substantially entirely axially bounded by the rotor wall. In other words, the internal chamber may be closed or substantially closed. The internal chamber may be substantially closed, except for openings at the first end and second end of the rotor.

[051] In embodiments, an internal chamber is arranged to allow a fluid to flow therethrough. Thus, a gas or other fluid may be passed into or through the internal chamber to, for example, control the temperature of the screw rotor. This may be particularly advantageous because, whilst reduction of high temperatures in existing vacuum pumps may be achieved by gas injection (e.g. nitrogen), large quantities of gas are required for adequate reduction which is expensive and may cause cold spots leading to condensation and/or corrosion of pump components. Providing an internal chamber through which a fluid may be passed allows, for example, a coolant to be passed therethrough whilst avoiding the described cold spot issues.

[052] In embodiments, an internal chamber may extend along substantially the entire length of the rotor.

[053] In embodiments, the screw rotor may comprise a plurality of internal chambers, and the internal chambers may be fluidly connected to one another or, alternatively, may be fluidly sealed from one another, or the screw rotor may have a combination of fluidly connected and fluidly sealed chambers.

[054] In embodiments, the external pump surface of the rotor wall is substantially continuous. In other words, the external pump surface does not comprise any exterior concavities. Thus, the performance of the screw rotor is improved because clearances between the screw rotor and another pump component may be minimised across substantially the entire exterior surface of the screw rotor. In embodiments, the external rotor surface and/or rotor shaft may be devoid of counter balancing means, such as an exterior concavity or counterbalance weight.

[055] In embodiments, the rotor wall may comprise at least a first portion which is relatively more thick, and a second portion which is relatively less thick. In other words, the thickness of the rotor wall may vary about the perimeter thereof in order that the mass of the screw rotor may be distributed such that the principal axis of inertia of the screw rotor and the rotational axis thereof are substantially coaxial, in use. In embodiments, the screw rotor comprises one or more cores comprising an internal chamber, the or each internal chamber being configured such that the rotor wall varies in thickness about the perimeter thereof.

[056] In embodiments, at a transverse cross section in a plane substantially perpendicular to the rotational axis, the rotor wall has a portion substantially defining a crest of the helical profile (i.e. a ‘crest region’) and a portion substantially defining a root of the helical profile (i.e. a ‘root region’). In embodiments, the rotor wall may have a substantially greater thickness at the root region than at the crest region. The greater thickness of the rotor wall and thus the mass of the rotor wall at the root region therefore acts to counterbalance the mass of the rotor wall at the crest region. In embodiments, the rotor may comprise a single internal core arranged as an internal chamber which is distributed such that the principal axis of inertia of the screw rotor and the rotational axis thereof are substantially coaxial when rotating about said rotational axis, in use.

[057] In embodiments, the rotor wall may have a substantially uniform thickness. In embodiments, the rotor wall may have a substantially greater density at the root region than at the crest region.

[058] In embodiments, the screw rotor comprises a core comprising an internal chamber and the rotor wall has a portion of relatively greater thickness and a portion of relatively lesser thickness, the portion of relatively lesser thickness substantially corresponding to the portion of the rotor wall which is furthest from the rotational axis.

[059] In embodiments, at least one said internal core may substantially define an internal thread, the internal thread being arranged to be substantially coincident with the helical thread defined by the external pump surface. In these embodiments, the at least one core may comprise an internal chamber. In embodiments, the internal thread is formed as part of the rotor wall. For example, a core may comprise an internal chamber which is configured to define an internal thread on an internal surface of the rotor wall. Thus an internal thread may be disposed internally to counter the external helical thread of the external pump surface. [060] In embodiments, a said core comprises a distinct body which is not unitary with the rotor wall and which substantially defines an internal thread. In embodiments, a said core may comprise a body which is integrated with the rotor wall.

[061] In embodiments, the internal thread may extend along substantially the entire length of the rotor. In embodiments, the internal thread may extend between the first and second ends of the screw rotor. Thus, a counterbalancing mass may be located substantially along the entire length of the screw rotor.

[062] In embodiments, the screw rotor may comprise a rotor wall having an external pump surface defining a substantially helical thread extending between said first and second ends; a first core comprising an internal chamber and a second core; an internal wall substantially separating the first and second cores. In embodiments, the second core may comprise an internal chamber. In embodiments, the screw rotor may comprise a third core, the third core comprising a mass which is distributed to substantially dynamically balance the screw rotor in use. In embodiments, the rotor screw may comprise a number of supporting struts which extend between the internal wall separating the first and second cores and the rotor wall.

[063] In embodiments, the or each core is configured such that a principal axis of inertia of one or more imaginary longitudinal segments of the screw rotor and the rotational axis of the screw rotor are substantially coaxial when rotating about said rotational axis, in use. Each imaginary longitudinal segment may be defined by two spaced apart transverse planes which are substantially parallel to one another and substantially perpendicular to the rotational axis of the screw rotor. In embodiments, a first transverse plane may substantially correspond to the first end of the screw rotor, and a second transverse plane may substantially correspond to the second end of the screw rotor. In other words, an imaginary longitudinal segment may comprise substantially the entire screw rotor.

[064] In embodiments, the screw rotor may comprise two, three, four or more imaginary longitudinal segments, each of which having a principal axis of inertia which is substantially coincident with the rotational axis of the screw rotor. Thus, in embodiments, the screw rotor may have a plurality of imaginary longitudinal segments which are each themselves independently statically and dynamically balanced. This is advantageous because, if each imaginary longitudinal segment is in itself balanced in accordance with the broadest aspect of the present invention, potential stresses on the screw rotor through rotation thereof are minimised. In embodiments, the plurality of imaginary longitudinal segments may together span substantially the entire length of the screw rotor. In other words, the screw rotor may be devoid of imaginary longitudinal segments which are not in themselves statically and dynamically balanced. In embodiments, the screw rotor may comprise several, for example five or more, imaginary longitudinal segments which each have a principal axis of inertia which is substantially coincident with the rotational axis of the screw rotor. In embodiments, a said imaginary longitudinal segment may have a length of between around 0.05mm and 100mm.

[065] In embodiments, at least one said core may have a substantially latticed structure. In embodiments, a core may be formed substantially of a foam having a substantially cellular structure.

[066] In embodiments, at least one said core may comprise a substantially latticed structure having a plurality of supporting struts. The supporting struts improve the structural strength of the screw rotor. In embodiments, the supporting struts may be configured to act as a thermal bridge between the rotor wall and a core of the screw rotor. Thus, heat energy may be transferred from the rotor wall towards a core to more effectively control the temperature of the screw rotor during use. In embodiments, the or each core may be configured to transfer heat energy from the rotor wall towards a shaft.

[067] In embodiments, the screw rotor may be formed at least partially by an additive manufacturing process. Therefore, the or each core and thus the mass of the screw rotor may be distributed by the building up of contiguous layers of material as the screw rotor is formed. The configuration of the or each core may be defined at each layer of the additively manufactured screw rotor such that, at each layer, the screw rotor is substantially balanced and will be substantially dynamically balanced when rotating about the rotational axis thereof, in use. In embodiments, the screw rotor is substantially formed, preferably entirely formed, by an additive manufacturing process. In embodiments, the screw rotor may be formed substantially by a three-dimensional (3D) printing process.

[068] In embodiments, the or each core may be formed at least partially by an additive manufacturing process.

[069] In embodiments, the or each core may be formed separately, and/or from a different material, to the rotor wall. In embodiments, at least a portion of a said core may be formed of a substantially porous material.

[070] In embodiments, a said core may be formed substantially entirely of a single material, wherein the density of the core is varied across an axial cross-section of the core.

[071] In embodiments, the screw rotor may be formed of one or more materials selected from the group comprising: iron, steel, aluminium, copper and/or one or more polymers. In embodiments, the rotor wall is formed substantially of stainless steel, and at least one core is formed of a substantially thermally conductive material. In embodiments, at least one core is formed substantially of aluminium.

[072] In a further aspect, the present invention provides a screw rotor for a vacuum pump, the screw rotor having a first end and a second end, and being arranged to rotate about a longitudinally extending rotational axis thereof; a rotor wall having an external pump surface defining a substantially helical thread extending from said first end to said second end; the screw rotor further comprising one or more internal cores, at least one said core having a density which is substantially different to that of the rotor wall. In embodiments, said or each at least one core is a gas-filled void.

[073] Preferably, the or each at least one core is distributed within the screw rotor such that a principal axis of inertia of the screw rotor and rotational axis thereof may be substantially coaxial, and the screw rotor may thus be considered acceptably balanced, when the screw rotor has a balance quality grade of at least G 6.3, and more preferably at least G 2.5, in view of ISO 21940. [074] In a further aspect, the present invention provides a screw rotor for a vacuum pump, the screw rotor having a first end and a second end, and being arranged to rotate about a rotational axis thereof; a rotor wall having an external pump surface defining a substantially helical thread extending between said first and second ends; the screw rotor further comprising one or more cores, at least one said core having a density which is substantially different to that of the rotor wall, and the or each core being distributed such that a principal axis of inertia of one or more imaginary longitudinal segments of the screw rotor and the rotational axis thereof are substantially coaxial when rotating about said rotational axis, in use.

[075] Each imaginary longitudinal segment of the screw rotor may be defined by two spaced apart transverse planes which are substantially parallel to one another and substantially perpendicular to said rotational axis.

[076] In embodiments, the or each core may be distributed such that the centre of mass of any transverse cross section of the screw rotor in a plane substantially perpendicular to said rotational axis is located substantially on the rotational axis of the screw rotor.

[077] In embodiments, the screw rotor may comprise a first core comprising an internal chamber and having a density substantially less than that of the rotor wall; the core being substantially asymmetrical.

[078] In a further aspect, the present invention provides a vacuum pump comprising one or more, preferably two or more, screw rotors in accordance with any preceding aspect.

[079] In embodiments, the vacuum pump may comprise two or more substantially aligned counter-rotatable screw rotors.

[080] In embodiments, each screw rotor of the vacuum pump may comprise a first end and a second end, the screw rotor being arranged to rotate about a rotational axis thereof. Each screw rotor may comprise a rotor wall having an external pump surface defining a substantially helical thread extending between said first and second ends. Each screw rotor may further comprise one or more internal cores. An internal core may have a density which is substantially different to that of the rotor wall. The or each core may be distributed such that the principal axis of inertia of the screw rotor and the rotational axis thereof are substantially coaxial when rotating about said rotational axis, in use.

[081] In embodiments, a said core may comprise an internal chamber. In embodiments, the or each core may comprise an internal chamber. In embodiments, a screw rotor may be substantially hollow. The internal chamber of each screw rotor may be configured to contribute to the balancing of the respective screw rotor about the rotational axis thereof.

[082] The or each core of each screw rotor is distributed such that the principal axis of inertia of the screw rotor and the rotational axis thereof are substantially coaxial when rotating about said rotational axis, in use, without the external pump surface, i.e. the exterior surface, of the rotor being modified. Therefore, performance of the screw rotor can be optimised because material does not need to be added to, or machined or otherwise removed from the outer rotor surface. The configuration of the or each core acts as an internal counter balance to the external thread of the respective screw rotor so that vibrations or other undesirable forces are minimised or eliminated. Thus, the risk of pump seizure is reduced or eliminated.

[083] The present invention also provides a greater scope of design for vacuum pumps and, in particular, vacuum pumps having a screw mechanism.

[084] In embodiments, the vacuum pump may comprise one or more screw rotors according to the present invention, and one or more screw rotors not falling within the scope of the present invention. In other words, a vacuum pump may comprise several screw rotors, not all of which are in accordance with the present invention.

[085] In embodiments, the vacuum pump may comprise single pitch screw rotors, variable pitch screw rotors or stepped screw rotors. In embodiments, the vacuum pump may comprise screw rotors having a constant variable pitch, as described in EP1960671. [086] In embodiments, the vacuum pump may comprise a number of rotor shafts about which each said screw rotor is disposed and arranged to rotate in use. In embodiments, the vacuum pump may comprise a number of end caps which are connectable to the first and second ends of each screw rotor and arranged to facilitate the rotation of the screw rotors in use.

[087] In embodiments, the rotor wall of each screw rotor has a substantially continuous external pump surface. In other words, the external pump surface of each rotor may be devoid of any cavities. The clearance between surfaces of aligned screw rotors may therefore be minimised across substantially the entire external surface of each screw rotor. Thus, the performance of the vacuum pump may be improved.

[088] In embodiments, a said core may comprise an internal chamber. The internal chamber may be arranged to allow a fluid to flow therethrough. Thus, a gas or other fluid may be passed into or through the internal chamber of each screw rotor to, for example, control the temperature of the screw rotor and/or vacuum pump. In embodiments, the vacuum pump comprises means for introducing a fluid into the internal chamber of each screw rotor. In embodiments, each screw rotor is configured to allow the passage of a fluid between the first and second ends thereof.

[089] In embodiments, the or each core of a screw rotor may be configured such that one or more transverse cross sections in a plane substantially perpendicular to said rotational axis have a centre of mass which is located substantially on the rotational axis of the screw rotor.

[090] In embodiments, the screw rotors may be formed at least partially by an additive manufacturing process. Therefore, mass of each screw rotor may be distributed by the building up of contiguous layers of material as the screw rotor is formed. In embodiments, each screw rotor may be formed substantially by a three-dimensional (3D) printing process. [091] In embodiments, each screw rotor may have a substantially latticed configuration. In embodiments, a core may be formed substantially of a foam having a substantially cellular structure.

[092] In embodiments, each screw rotor may comprise a core having a substantially latticed structure having a plurality of supporting struts. The supporting struts may improve the structural strength of the screw rotor. In embodiments, the supporting struts may be configured to act as a thermal bridge between the rotor wall and a core of the screw rotor. Thus, heat energy may be transferred from the rotor wall towards a core to more effectively control the temperature of the screw rotor during use. In embodiments, the or each core may be configured to transfer heat energy from the rotor wall towards a shaft.

[093] The features of any or all of the above aspects may be used in a vacuum pump or a compressor.

[094] In a further aspect, the present invention provides a method of manufacturing a screw rotor for a vacuum pump according to any preceding aspect. In embodiments, the method comprises the step of at least partially forming the screw rotor using an additive manufacturing process.

[095] In embodiments, the method comprises the step of at least partially forming the screw rotor using a three dimensional (3D) printing process.

[096] In embodiments, the method may comprise varying the thickness and/or density of the rotor wall about the perimeter thereof such that the principal axis of inertia of the screw rotor and the rotational axis thereof are substantially coaxial when rotating about said rotational axis, in use.

[097] In a further aspect, the present invention provides a method of designing a screw rotor for a vacuum pump, comprising the steps of:

• determining a required configuration of a rotor wall of a screw rotor, the external pump surface of said rotor wall defining a substantially helical thread; • providing a digital model of said screw rotor including allocating a material to the rotor wall;

• dividing the digital model of said screw rotor into a number of imaginary longitudinal segments;

• creating one or more internal cores in each said imaginary longitudinal segment, allocating at least one said core a density which is substantially different to that of the rotor wall material, wherein the or each core is distributed such that a principal axis of inertia of the or each imaginary longitudinal segment and the rotational axis thereof are substantially coaxial when rotating about said rotational axis, in use.

[098] In embodiments, the method or one or more steps thereof is carried out using a computer. Also provided is a method of manufacturing a screw rotor comprising the steps of fabricating a rotor designed according to the aforementioned method, preferably by an additive manufacturing process, such as laser sintering.

Brief Description of Figures

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

[100] Figure 1 illustrates a perspective cross-sectional view of an imaginary longitudinal segment of a screw rotor according to the present invention.

[101] Figure 2 shows a perspective view of an imaginary longitudinal segment of a screw rotor according to the present invention.

[102] Figure 3 shows a perspective view of an imaginary longitudinal segment of a screw rotor according to the present invention.

[103] Figure 4 shows an axial cross-section of a screw rotor according to the present invention. [104] Figure 5 shows a further axial cross-section of a screw rotor according to the present invention.

[105] Figure 6 shows a further axial cross-section of a screw rotor according to the present invention.

Detailed Description

[106] Figure 1 shows a longitudinal segment of a screw rotor referenced generally as 1 . It will be appreciated by the skilled person that Figure 1 shows only a portion of a screw rotor and the length and overall size of a screw rotor will depend on its specific use and the size of the vacuum pump in which it is installed.

[107] The screw rotor comprises a first end 2 and a second end 3. The first and second ends 2,3 are arranged to be connectable to a corresponding part of a vacuum pump arranged to drive rotation of one or more screw rotors. For example, a vacuum pump may comprise a shaft about which a screw rotor 1 is located. Alternatively, a vacuum pump may comprise end caps or plates which are arranged to be connected to the first and second ends 2,3 and which facilitate rotation of the screw rotor 1 in use.

[108] The screw rotor 1 comprises a rotor wall 4 which extends from the first 2 to the second 3 end, and which has an external pump surface 5 defining a substantially helical thread.

[109] The external pump surface 5 defines the helical profile of the rotor wall 4. In use, the screw rotor 1 cooperates with one or more other screw rotors and stators to force a fluid from a fluid inlet of a vacuum pump towards a fluid outlet, i.e. exhaust. It is in the void of the helical profile that fluid flows in use.

[110] The helical thread defined by the external pump surface 5 of the rotor wall 4, in part at least, dictates the balance of the screw rotor 1 . Hypothetically, were the screw rotor to be solid and of substantially uniform density, rotor 1 would be unbalanced due to the helical thread defined by the external pump surface 5, i.e. the non-uniform distribution of mass of the rotor. [111] However, the rotor wall 4 further comprises an internal core 7 comprising an internal chamber. The core 7 is bounded by an internal surface 6 of the rotor wall 4. The rotor wall 4 thus forms a skin or shell which bounds the core 7. The screw rotor 1 is substantially hollow. The core 7 thus has a lower density than the rotor wall 4, and the core 7 is distributed such that a principal axis of inertia of the screw rotor and the rotational axis thereof are substantially coaxial when rotating about said rotational axis, in use. The axial cross section of the core 7 is asymmetrical.

[112] The centre of mass of the screw rotor is located substantially on the rotational axis thereof.

[113] At the longitudinal segment of the screw rotor shown at Figure 1 , the core comprising an internal chamber 7 is configured so that mass is distributed within the screw rotor 1 to dynamically balance the rotor about the rotational axis.

[114] More specifically, the rotor wall 4 has a greater thickness at a root region 9 than at a crest region 8 in order to substantially dynamically balance the rotor 1 . The internal chamber extends from a first end of the longitudinal segment to the second end of the longitudinal segment in the embodiment of Figure 1. Throughout the length of the screw rotor 1 , the internal chamber is arranged so that the rotor wall 4 has a greater thickness at the root region 9 of each turn of the helical thread than at the crest region 8 at the same turn. The centre of mass of the screw rotor is thus located substantially at the rotational axis.

[115] It is envisaged that, in other embodiments, mass is distributed by varying the volume and/or density of the rotor wall 4, by varying the materials from which the rotor wall 4 is formed, and/or by providing a distinct mass within the internal chamber which is distributed such that the rotor 1 is substantially dynamically balanced.

[116] As shown in the embodiment of Figure 6, which is described in greater detail below, the screw rotor 1 may comprise means for connecting the rotor wall 4 to a shaft in order that drive from the shaft is transferred to the screw rotor 1 during use. Connecting means may comprise a number of supporting struts which are arranged to extend between the rotor wall 4 and a shaft.

[117] In the embodiments of Figures 1 to 5, the core comprises an internal chamber 7 which is configured such that the internal surface 6 forms an internal thread which is substantially coincident with the helical thread defined by the external pump surface 5. The internal thread is a unitary part of the rotor wall 4 and is defined by the configuration of the core 7.

[118] The screw rotor 1 is arranged such that the rotational axis of the rotor 1 passes through the internal chamber 7. The internal chamber is entirely bounded by the rotor wall 4.

[119] As described, the internal chamber 7 extends between the first 2 and second 3 ends of the rotor 1 .The screw rotor 1 is arranged to allow a fluid to flow therethrough. For example, a coolant may be passed through the internal chamber to control the temperature of the screw rotor 1 in a more consistent and uniform manner.

[120] As discussed, and referring to Figure 2, the screw rotor 1 is arranged to rotate about a rotational axis thereof. The rotational axis is generally referenced by line ‘A. Figure 2 shows a longitudinal segment of the screw rotor 1 . The helical thread defined by the external pump surface 5 is illustrated in Figure 2, as is the varying thickness of the rotor wall 4 to balance the rotor 1.

[121] Figure 3 shows a front-on perspective view of a screw rotor 1 according to the present invention. The exterior of the screw rotor 1 , i.e. the external pump surface 5, may be substantially conventional in form so that one or more screw rotors according to the present invention may be retrofitted to a screw-type vacuum pump. The external pump surface 5 is a substantially continuous skin which is devoid of any cavities or machined holes, or any counter balancing weights which are coupled to the rotor wall.

[122] Referring to Figures 4 and 5, there are shown cross-sectional views of screw rotors 1 according to the present invention. It can be clearly seen that at an axial cross- section of the screw rotor 1 that the core 7 is substantially asymmetrical, and the rotor wall 4 varies in thickness.

[123] In the embodiments of Figures 4 and 5, the rotor wall 4 is substantially unitary in its form and does not comprise a distinct counter balancing body. In other words, the additional mass at the rotor inner surface 6 at a root region 9 of the rotor 1 is part of the rotor wall 4 and not a separate body, and it is the configuration of the internal chamber which contributes and dictates the dynamic balance of the rotor 1. However, it is envisaged that an additional core may be present within the internal chamber 7 to provide an internal counter balance.

[124] In the embodiments of Figures 4 and 5, the core 7 and the thicker portion of the rotor wall 4, at any given transverse cross section in a plane substantially perpendicular to the rotational axis, are arranged such that the centre of mass of that section coincides with the axis of rotation.

[125] Figure 4 shows a transverse cross section having a more pronounced crest region 8 than the transverse cross section of Figure 5. The root region 8 at Figure 4 has a greater thickness than the root region 8 at Figure 5.

[126] Figure 6 shows a further embodiment of a screw rotor. The screw rotor 1 comprises a rotor wall 4 which has a substantially uniform thickness. The screw rotor 1 of Figure 6 has more than one core. The screw rotor 1 comprises a first core 7a, a second core 7b and a third core 7c. The first core 7a is formed substantially as an internal chamber and comprises a plurality of supporting struts 10. An internal wall 11 separates the first 7a and second 7b cores. The supporting struts 10 extend between the internal wall 11 and the rotor wall 4. The first core 7a is asymmetrical.

[127] The supporting struts 10 may be arranged as a thermal bridge between the rotor wall 4 and the interior of the rotor 1.

[128] The second core 7b is also formed substantially as an internal chamber. The second core 7b is arranged to house a shaft 12, in use, and is arranged to transfer drive from the shaft to the screw rotor 1. [129] The third core 7c has a density which is substantially greater than the densities of the first 7a and second 7b cores. The third core 7c is located substantially opposite a portion of the rotor wall 4 having the greatest mass at the transverse cross section illustrated by Figure 6. The cores 7a, 7b, 7c are each distributed such that a principal axis of inertia of the screw rotor 1 and the rotational axis thereof are substantially coaxial when rotating about the rotational axis of the rotor, in use

Reference Key

1 Screw rotor

2 First end

3 Second end

4 Rotor wall

5 External pump surface

6 Internal surface

7 Internal core

8 Crest region

9 Root region

10 Supporting struts

11 Internal wall

12 Shaft

Claims

1. A screw rotor for a vacuum pump, the screw rotor having a first end and a second end, and being arranged to rotate about a longitudinally extending rotational axis thereof; a rotor wall having an external pump surface defining a substantially helical thread extending from said first end to said second end; the screw rotor further comprising one or more internal cores, at least one said internal core having a density which is substantially different to that of the rotor wall; and the or each core being distributed such that a principal axis of inertia of the screw rotor and the rotational axis thereof are substantially coaxial when rotating about said rotational axis, in use.

2. A screw rotor according to claim 1 , comprising two or more cores; a region of each said core having a density which is substantially different to the density of the rotor wall.

3. A screw rotor according to claim 1 or claim 2, at least one said core comprising a region having a density which is substantially lower than the density of the rotor wall.

4. A screw rotor according to any preceding claim, at least one said core having a substantially asymmetrical axial cross-section.

5. A screw rotor according to any preceding claim, at least one said core comprising an internal chamber.

6. A screw rotor according to any preceding claim, comprising a first core and a second core; at least a region of the first core having a density which is substantially lower than the density of at least a region of the second core.

7. A screw rotor according to any preceding claim, the rotor wall comprising at least a first portion which is relatively more thick; and a second portion which is relatively less thick.

8. A screw rotor according to any preceding claim, at least one said core substantially defining an internal thread; the internal thread being arranged to be substantially coincident with the helical thread defined by the external pump surface.

9. A screw rotor according to any preceding claim, wherein the or each core is configured such that a principal axis of inertia of each of one or more imaginary longitudinal segments of the screw rotor and the rotational axis of the screw rotor are substantially coaxial when rotating about said rotational axis, in use.

10. A screw rotor according to any preceding claim, at least one said core having a substantially latticed structure.

11.A screw rotor according to any preceding claim, the screw rotor being formed at least partially by an additive manufacturing process.

12. A screw rotor according to any preceding claim, comprising a first core comprising an internal chamber and having a density substantially less than that of the rotor wall; the core being substantially asymmetrical.

13. A vacuum pump comprising one or more screw rotors in accordance with any preceding claim.

14. A method of designing a screw rotor for a vacuum pump according to any of claims 1 to 12, comprising the step of distributing the or each core such that, in use, a principal axis of inertia of the screw rotor and the rotational axis thereof are substantially coaxial when rotating about said rotational axis.

15. A method of manufacturing a screw rotor designed according to claim 14, comprising the step of at least partially fabricating a screw rotor so designed using an additive manufacturing process.