GB2621853A

GB2621853A - Rotor

Info

Publication number: GB2621853A
Application number: GB2212297.2A
Authority: GB
Inventors: Verma Mayank; Peters Benedikt
Original assignee: Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2022-08-24
Filing date: 2022-08-24
Publication date: 2024-02-28
Also published as: GB202212297D0

Abstract

The rotor 1 comprises a hub 2 configured for connection to a rotor shaft, and at least one rotor blade 3 coupled to the hub at a rotor blade root 4 and extending to a rotor blade tip 5. Each rotor blade comprises metallic skeletal support structure 6 that is monolithic with the hub and extends from the rotor blade root towards the rotor blade tip. Each rotor blade further comprises an external shell 7 extending from the rotor blade root to the rotor blade tip. The skeletal support structure and the external shell are substantially discrete, and each define at least 10% of the volume fraction of the rotor blade. The material of the shell has a lower density than the metallic material of the skeletal support structure. The present invention further discloses a vacuum pump or compressor comprising such a rotor, and methods for producing such rotors.

Description

ROTOR

Field

The present invention relates to an improved rotor for use in a vacuum system or compressor, to a vacuum system or compressor, and to a method of producing a rotor.

Background

Strength, stiffness, creep resistance, high-temperature stability and fatigue resistance are all highly desirable properties for rotors of vacuum pumps or compressors. During use, a rotor may be subjected to substantially continuous stresses due to their high rotational speed. For example, a rotor for a turbomolecular pump may rotate at up to about 90,000 RPM during operation. Also, the rotors may be maintained at elevated temperatures throughout use. Under such conditions, deformation via creep is more likely to occur. Thus, there is a desire to provide rotors for vacuum pumps or compressors having improved creep resistance.

However, creep resistance must also be balanced against other factors, such as the weight of the rotor. Some creep resistant materials may be relatively dense, and therefore undesirable for use in rotors. By increasing the weight of the rotor, the stresses that the rotor is exposed to when rotating will also increase. Furthermore, some creep resistant materials may be expensive in terms of raw materials and/or manufacturing costs. This may render them not economically viable for use in rotors.

Typically, rotors of the prior art may comprise a single material, such as aluminium. Aluminium is often selected due to having a relatively low cost, acceptable mechanical properties (e.g. density, corrosion resistance, tensile strength, fatigue resistance), and being relatively easy to machine. However, there is a desire for improved rotors, which may in turn improve pumping capabilities and/or reduce costs.

In use, the rotors may be mounted upon a rotor shaft. Providing an interference-fit between these components would be desirable from a manufacturing perspective. Often the rotor shaft will comprise a different material to that of the rotor. Accordingly, as the temperature of the respective components increases during use, the rotor and rotor shaft may expand at different rates. This may make it difficult to attach the rotor onto the rotor shaft.

Accordingly, there is a requirement for an improved rotor for a vacuum pump or compressor.

The present invention aims to solve, at least in part, these and other problems associated with the prior art.

Summary

Accordingly, in an aspect the present invention provides a rotor for use in a vacuum system or compressor. The rotor comprising a hub configured for connection to a rotor shaft, and at least one rotor blade coupled to the hub at a rotor blade root and extending to a rotor blade tip. Each rotor blade comprises a metallic skeletal support structure that is monolithic with the hub and extends from the rotor blade root towards the rotor blade tip, and an external shell extending from the rotor blade root to the rotor blade tip. The skeletal support structure and the external shell are substantially discrete, and each define at least 10% of the volume fraction of the rotor blade. The material of the shell has a lower density than the metallic material of the skeletal support structure.

Preferably, the rotor may comprise a plurality of rotor blades. The rotor blades may be regularly spaced about the hub. The hub may be substantially annular. The rotor blades may extend radially outwardly from the hub. The rotor blades may be integral with the hub.

The rotor may be for vacuum pump. By way of non-exhaustive example, the rotor may be for a turbomolecular pump, a Roots pump, a Northey (claw) pump, a scroll pump, or a Siegbahn pump. Preferably the rotor may be for a turbomolecular pump.

The skilled person will appreciate that the dimensions, number, and arrangement of the rotor blade(s) may vary according to the pump type. The rotor blade(s) may be lobe(s), disk(s), claw(s), or other features configured to displace fluid (i.e. gas) during operation of the pump.

The metallic skeletal support structure (i.e. the skeletal support structure or support structure) may be configured to strengthen and increase the stiffness of the rotor. This may particularly apply in regions exposed to the greatest stresses during use. For example, the rotor blade root may be exposed to greater stresses than the rotor blade tip during operation of the pump. Therefore, the skeletal support structure may be configured to stiffen the rotor blade root. The skeletal support structure may define a structure about which the shell is arranged. Preferably, the skeletal support structure may be substantially endo-skeletal, i.e. within the rotor blade. Although, in some embodiments, the skeletal support structure may define at least a portion of the outer surface of the rotor blade.

The skeletal support structure is monolithic with the hub, meaning that the skeletal support structure and the hub are a single, unitary component. The skeletal support structure extends from rotor blade root towards the rotor blade tip. The rotor blade root may be defined as the intersection between the rotor blade and the hub. The rotor blade tip may be defined as the portion of the rotor blade most distal from the rotor blade root. Typically, the skeletal support structure may extend generally radially outwardly from the hub towards the rotor blade tip.

The metallic skeletal support structure may comprise a material with a high creep resistance. For the purposes of the present invention, a high creep resistance may be defined as the material having a melting temperature at least three times higher than the highest operational temperature of the rotor in Kelvin. Preferably, the highest operational temperature may be from about 353 K to about 408 K. For example, the internal metallic lattice may comprise a titanium alloy, or a high-strength steel alloy.

The hub may be configured to couple the rotor to a rotor shaft of the vacuum pump or compressor when in use. The hub may comprise a metallic material. Preferably the hub may comprise the same material as the skeletal support structure.

The external shell (i.e. the shell) may be metallic and/or non-metallic. In embodiments, the shell may comprise a polymeric, ceramic, and/or composite material. Preferably, the shell may comprise a polymeric material. The shell may comprise a polymer resin. The shell may comprise a thermoplastic material. For example, the shell may comprise polyetherketoneketone (PEKK), polyetheretherketone (PEEK), polyamide-imide (PAI), polyamide (PA), polycarbonate (PC). In embodiments, the shell may comprise a metallic material, such as a magnesium alloy.

The shell may comprise a matrix material with reinforcing fibres embedded therein. Preferably, the reinforcing fibres may be carbon fibres, glass fibres, and/or polymer fibres. More preferably, the reinforcing fibres may be carbon fibres, The reinforcing fibres may be arranged to strengthen and/or increase stiffness in regions of the rotor that are exposed to higher stresses during use. In contrast, the shell may have a lower density, which may correspond to lower density and/or fewer reinforcing fibres, in regions of the rotor that are exposed to lower stresses during use. The reinforcing fibres may be positioned within the matrix in a configuration to increase creep resistance. The fibre direction and/or fibre density and/or fibre pattern may be selected to strengthen particular regions of the rotor. For example, the fibre density of reinforcing fibres may be greater towards a rotor blade root than towards a rotor blade tip.

The skeletal support structure may define at least 20%, or at least 30%, or at least 40%, or at least 50% of the volume fraction of the rotor blade. The shell may define at least 20%, preferably at least 30%, more preferably at least 40 %, for example at least 50% of the volume fraction of the rotor blade. In some embodiments, each rotor blade may consist of the skeletal support structure and the shell.

The skeletal support structure and the external shell are substantially discrete, i.e. not a homogeneous mixture.

The material of the shell has a lower density than the metallic material of the skeletal support structure. Specifically, the shell has a lower density than the metallic material of the skeletal support structure, excluding any 'voids' within the skeletal support structure. Preferably, the shell may have a density lower than about 2900 kgm-3.

Advantageously, the present invention may provide a rotor having a lower second moment of inertia with respect to the rotational axis, in comparison to rotors of the prior art. Additionally, the metallic lattice may improve the strength and stiffness of the component, thereby improving the creep and fatigue resistance of the rotor. The combination of a metallic lattice structure and a lower-density non-metallic shell provides a desirable combination of low weight and creep resistance, due to the localised strengthening provided by the metallic lattice structure in high stress regions of the rotor. This enables a lower density, and lower strength material (i.e. the shell) to define the remainder of the rotor, without compromising the performance of the rotor in use.

Typically, the skeletal support structure may be a lattice. The lattice may comprise a repeating structure of struts and nodes. In some embodiments, the metallic lattice structure may comprise a substantially uniform repeating structure of struts and nodes.

Preferably, the struts may comprise substantially linear beams. The struts may have, for example, a substantially circular, elliptical, triangular, square, pentagonal, or hexagonal cross-sectional area. Preferably, the struts may have a substantially circular cross-sectional area. The struts may be substantially solid or may be hollow.

Typically, the struts intersect at nodes. Each node may have a plurality of struts intersecting thereat. Typically, the number of struts extending from each node will depend on the unit cell of the lattice in that region of the rotor and/or the position of the node within the lattice. By way of example, each node may have from about 2 to about 16 struts extending therefrom.

The smallest repeating structure of struts and nodes may define the unit cell of the lattice. The unit cell may have a variety of configurations depending on the required properties of the lattice. The lattice defining the skeletal support structure may comprise different unit cells in different regions of the skeletal support structure lattice.

Advantageously, the skeletal support structure being a lattice may provide relatively high strength and stiffness with relatively low weight, in comparison with rotors of the prior art.

The lattice may be a two-dimensional lattice wherein the unit cells of the lattice repeat in a substantially planar manner. Alternatively, the lattice may be a three-dimensional lattice.

The lattice may be an auxetic lattice. An auxetic lattice is a lattice having a negative Poisson's ratio. Therefore, if the auxetic lattice undergoes a positive strain in a longitudinal axis, the strain in a transverse axis will also be positive. This atypical property may enable improved mechanical characteristics (e.g. high stiffness and reduced weight), particularly when combined with the shell.

The auxetic lattice may have a re-entrant unit cell. This may be defined as the unit cell comprising generally inward-protruding struts. Some examples of auxetic lattice unit cells include re-entrant triangle, hexagonal honeycomb, star, rotating square, triangle, and tetrahedron.

Advantageously, the skeletal support structure being an auxetic lattice may provide improved creep resistance for the rotor. The auxetic lattice may beneficially disperse stresses, reducing stress localisation. Additionally, providing an auxetic lattice may increase the shear modulus of the lattice.

Typically, the lattice may have a non-uniform lattice density. Preferably, the lattice may have a greater lattice density towards the rotor blade root than towards a rotor blade tip. Differences in lattice density may correspond to differences in material and/or the unit cell of the lattice in specific regions.

The lattice density (p) may be a volume density. Preferably, the lattice density (pd) may be defined as the fraction of strut and node volume (Vs) in comparison to total nominal volume of the lattice (Vt). i.e. p = Vs / Vt The lattice density may also be calculated by measuring the fraction of the void volume (Vv). Void volume (Vv) may be defined as the voids of the lattice not occupied by struts and/or nodes. i.e p = 1 -(Vv / Vt) Typically, the lattice density towards the rotor blade root (pr) may be greater than the lattice density towards the rotor blade tip (pt). Therefore, the lattice may be a graded lattice. Preferably, the lattice density towards the rotor blade root (pr) may be from about 0.20 to about 0.95, more preferably from about 0.50 to about 0.80. Preferably, the lattice density towards the rotor blade tip (pt) may be from about 0.01 to about 0.30, more preferably from about 0.05 to about 0.20.

Increased lattice density may correspond to increased strength of the lattice, and accordingly to increased creep resistance. Advantageously, thereby rotors of the invention may have increased creep resistance in the high-stress regions towards the rotor blade root, whilst maintaining low weight by having reduced lattice density towards the rotor blade tip.

Typically, the rotor may be for use in a fast-rotating vacuum system or compressor, preferably a turbomolecular vacuum pump. A fast-rotating vacuum system may be defined as a system in which the rotor is typically configured to rotate at speeds exceeding 24,000 RPM during operation.

Preferably, the shell may fill 'voids' in the skeletal support structure. Thereby, the rotor may be substantially solid (i.e. non-hollow).

Typically, the shell may be directly connected to the skeletal support structure. Preferably, the non-metallic shell may be moulded or cast about the metallic structure. In some embodiments, the shell may be connected to the skeletal support structure by an adhesive. For example, a resin glue may be present to affix the shell to the skeletal support structure.

The shell may be produced via an over-moulding technique. The skeletal support structure may be placed into a mould, and the shell may be injected into the mould about the skeletal support structure.

Advantageously, the shell being directly connected to the skeletal support structure may provide strong bonding between the parts. Additionally, the manufacturing costs may be reduced as additional fixing means may not be required.

In some embodiments, the rotor may consist of a hub, a skeletal support structure, and a shell.

Typically, the shell may comprise a polymeric and/or composite material and/or metallic. Preferably, the shell may comprise a polymer composite material. For example, the shell may comprise polyetherketoneketone (PEKK), polyetheretherketone (PEEK), polyamide-imide (PAI), polyamide (PA), polycarbonate (PC), and/or polyphthalamide (PPA), with reinforcing fibres as defined hereinbefore. Advantageously, this may enable relatively low-cost manufacturing techniques to be used, such as injection moulding. Furthermore, some polymers may out-perform certain metals (e.g. aluminium) for creep resistance to weight ratio.

Typically, the skeletal support structure and the hub may be additive manufactured. Additive manufacture (3D printing) involves the layer-by-layer building up of a unit of material from one or more precursor material sources. Typically, each individual layer of a component may have the same depth, although equally they may vary. The skeletal support structure and the hub may be additive manufactured from a single precursor material, or alternatively from a plurality of precursor materials.

The skilled person will appreciate that a variety of additive manufacturing techniques may be suitable for production of the component according to the present disclosure. By way of example, the additive manufacturing technique may comprise selective laser sintering, selective laser melting, direct metal laser sintering, binder jetting, electron beam melting, and/or directed energy deposition.

Advantageously, producing the skeletal support structure and the hub by additive manufacturing technique may enable complex skeletal support structure to be manufactured in a reproducible and cost-effective manner. Producing such complex skeletal support structures via a subtractive manufacturing method such as machining may be too time consuming, expensive, and/or wasteful to be economically viable. The strength to weight ratio of the skeletal support structure may also be improved as otherwise unfeasible skeletal support structures may be used.

Advantageously, the hub being additive manufactured may provide improved mechanical interlocking with a rotor shaft to which the hub may be connected during use. This improvement in mechanical interlocking may be, at least in part, due to the increased surface roughness of the additively manufactured components. The improved mechanical interlocking may allow for a press-fit connection between the rotor and the rotor shaft.

Typically, the surface of the skeletal support structure that is directly connected to the shell may have a surface roughness greater than about 3.0 Ra. Advantageously, this may improve the bonding between the skeletal support structure and the shell.

Typically, the skeletal support structure may extend at least from the rotor blade root to a point 25% between the rotor blade root and the rotor blade tip. Preferably, the metallic lattice structure may extend at least 50% of the distance between the rotor blade root and the rotor blade tip. For example, the skeletal support structure may extend at least 75% of the distance between the rotor blade root and the rotor blade tip. In some embodiments, the skeletal support structure may extend the entire distance between the rotor blade root and the rotor blade tip.

In some embodiments, the skeletal support structure may not be present towards the rotor blade tip or may have a reduced density towards the rotor blade tip. In embodiments wherein the skeletal support structure is a lattice, the lattice may have a reduced lattice density towards the rotor blade tip. Advantageously, the skeletal support structure may strengthen the rotor towards the rotor blade root, which may experience relatively high stresses during operation. The skeletal support structure may not be present in the regions of the rotor that experience relatively low stresses during operation, such as the rotor blade tip. This may beneficially reduce the weight of the rotor whilst maintaining strength where required.

Typically, the hub may comprise a binding layer on a radially inwardly facing surface configured to contact the rotor shaft in use. Preferably, said binding layer may be configured to enable an interference-fit connection between the rotor and the rotor shaft. Providing such an interference-fit connection between a rotor and a rotor shaft had previously been challenging due to differences in thermal expansion therebetween across the operational temperature range. However, providing a binding layer may improve the coupling between the hub and the rotor shaft during use.

In a further aspect, the present invention provides a vacuum system or compressor comprising a rotor shaft and at least one rotor according to any embodiment of the preceding aspect. The vacuum system may be a turbomolecular pump, a Roots pump, a Northey (claw) pump, a scroll pump, or a Siegbahn pump. Preferably the vacuum system may be a turbomolecular pump. For example, the vacuum system may be an nEXT300 as produced by Edwards TM.

The turbomolecular pump may comprise a rotor according to the present invention having a plurality of axially spaced, annular arrays of inclined rotor blades. The blades may be regularly spaced within each array and extend radially outwards from a central hub. A stator of the pump may surround the rotor. The stator of the pump may comprise annular arrays of inclined stator blades which alternate in an axial direction with the arrays of rotor blades. Each adjacent pair of arrays of rotor and stator blades may form a stage of the turbomolecular pump. As the rotor rotates during operation, the rotor blades may impact incoming gas molecules and transfer the mechanical energy of the blades into gas molecule momentum that is directed from a pump inlet through the stages towards a pump outlet.

Preferably, the at least one rotor may be press-fit onto the rotor shaft. More preferably, a plurality of rotors may have a press-fit connection with the rotor shaft. Additionally, or alternatively, at least one rotor may be fused or bolted to the rotor shaft.

In a further aspect, the present invention provides a method for producing a rotor according to any embodiment of the first aspect. The method comprises the steps of: a) Providing the hub and the skeletal support structure; and b) Moulding or casting the shell about the skeletal support structure.

Preferably, the hub and skeletal support structure may be provided by an additive manufacturing technique. The additive manufacturing technique may comprise selective laser sintering, selective laser melting, direct metal laser sintering, binder jetting, electron beam melting, and/or directed energy deposition.

In embodiments wherein the shell is moulded about the skeletal support structure, the moulding may comprise injection moulding, casting, or die casting. The skeletal support structure may be placed within a mould, and the shell may be injected into the mould such that it fills the mould about the skeletal support structure. Preferably, the shell may comprise a polymeric or fibre reinforced polymeric material or a metallic material.

In embodiments wherein the shell is cast about the skeletal support structure, the casting may comprise die casting or investment casting. For example, the shell may comprise a magnesium alloy that is die cast about the skeletal support structure.

Advantageously, this may provide a relatively low cost and repeatable process for the production of rotors having a reduced weight and material usage in comparison to rotors of the prior art.

The skeletal support structure and the hub are provided as a single unitary component, preferably by additive manufacturing.

The method may further comprise the step of performing a finishing step on the skeletal support structure and/or the rotor.

Preferably the finishing step comprises a machining step. In some embodiments, following the production of the skeletal support structure, a machining and/or abrasive finishing step will be performed to improve the surface consistency of the metallic lattice structure. Additionally, or alternatively, following the step of moulding the shell about the skeletal support structure, a machining step may be performed on the part or all of the external surface of the rotor.

Additionally, or alternatively, the finishing step may comprise a surface engineering step. For example, the surface engineering step may comprise a blasting, peening, barrel finishing, electrochemical surface modification, or etching.

Additionally, or alternatively, the finishing step may comprise a coating step. A coating may be applied to the skeletal support structure and/or the rotor. The coating may, for example, comprise anodisation, carbo-nitration, surface-carburisation, wet plasma coating, chemical vapour deposition (CVD), physical vapour deposition (PVD), atomic layer deposition (ALD), or an organic coating. The skilled person will appreciate that this list of coatings is non-limiting.

In a further aspect, the present invention provides a computer-readable medium storing data which defines both a digital representation of the skeletal support structure and the hub of a rotor of the first aspect of the invention, and operating instructions adapted to control an additive manufacturing (AM) device to fabricate the skeletal support structure and the hub using the digital representation of the skeletal support structure and the hub when said data is relayed to the additive manufacturing device. The additive manufacturing device that the instructions are adapted to control may be a device used in any AM process as described hereinbefore.

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

Brief Description of Figures

Preferred features of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a pad of a rotor in accordance with an embodiment of the present invention; Figure 2 shows a cross-sectional view of a part of the rotor of Figure 1; Figures 3A-B show an example of an auxetic lattice, and the corresponding unit cell.

Detailed Description

Figure 1 shows a view of part of a rotor (1) in accordance with an embodiment of the present invention. The rotor (1) is for use in a vacuum system or compressor (not shown). The rotor (1) comprises a hub (2). The hub (2) is substantially annular. The hub (2) is configured for connection to a rotor shaft (not shown). The rotor (1) further comprises a plurality of rotor blades (3). Each rotor blade (3) has a rotor blade root (4) and a rotor blade tip (5). Each rotor blade (3) is coupled to the hub (2) at the rotor blade root (4).

The rotor blades (3) are regularly spaced about the hub (2). The rotor blades (3) are integral with the hub (2), as a single, unitary component. The rotor blades (3) extend radially outwardly from the hub (2). In this embodiment, the rotor (1) is for a turbomolecular pump. The rotor blades (3) are inclined. The rotor blades (3) are arranged such that there is an overlap of the rotor blades (3) at their respective rotor blade roots (4). All of the rotor blades (3) have substantially identical dimensions.

Figure 2 shows a cross-sectional view of a pad of the rotor (1) of Figure 1. The hub (2) of the rotor (1) is a substantially solid metallic component. The rotor blade (3) comprises a metallic skeletal support structure (6) proximal the rotor blade root (4). The skeletal support structure (6) is a lattice. The rotor blade (3) further comprises an external shell (7) extending from the rotor blade root (4) to the rotor blade tip (5). The material of the shell (7) has a lower density than the metallic skeletal support structure (6).

The skeletal support structure (6) is monolithic with the hub (2), i.e. the skeletal support structure (6) and the hub (2) are a single, continuous and unitary component. The skeletal support structure (6) and the external shell (7) are substantially discrete. The skeletal support structure (6) and the shell (7) each define at least 10% of the volume fraction of the rotor blade (3).

The lattice of the skeletal support structure (6) comprises a repeating structure of struts and nodes. The cross-sectional view of Figure 2 passes through a plurality of nodes (8) of the lattice. The skeletal support structure (6) and the hub (2) are additively manufactured, preferably as a single component.

The shell (7) defines the outer surface of the rotor blade (3). The shell (7) is directly connected to the skeletal support structure (6). The shell (7) is moulded about the skeletal support structure (6). The shell (7) surrounds and encloses the skeletal support structure (6). The shell (7) fills any "voids" in the metallic lattice structure (6). In this embodiment, the rotor (1) is substantially solid (i.e. non-hollow). The shell (7) may comprise a polymeric and/or composite material.

Figure 3A shows an example of an auxetic lattice (9). The metallic lattice structure (6) may be an auxetic lattice. Although this representation is in two-dimensions, the skilled person will appreciate that three-dimensional auxetic lattices are also available and may be used to provide the skeletal support structure (6).

The auxetic lattice (9) is a lattice having a negative Poisson's ratio. Accordingly, if the auxetic lattice (9) undergoes a positive strain in a longitudinal axis (y), the resultant strain in a transverse axis (x) will also be positive. The auxetic lattice (9) comprises a repeating structure of struts (10) and nodes (11). In this embodiment, the auxetic lattice (9) has a re-entrant unit cell. The struts (10) comprise substantially linear beams. The struts (10) intersect at nodes (11). Each node (11) has a plurality of struts (10) intersecting thereat. If such an auxetic lattice (9) is used in a rotor according to the present invention.

In the finished rotor, the "voids" within the auxetic lattice (9), i.e. the volume not filled by a strut (10) or node (11), may be filled by shell.

Figure 3B shows the unit cell of the auxetic lattice (9) of Figure 3A. The unit cell defines the smallest repeating structure of struts (10) and nodes (11) within the auxetic lattice (9). In this embodiment, the unit cell is a re-entrant unit cell.

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 Rotor 2 Hub 3 Rotor blade 4 Rotor blade root Rotor blade tip 6 Skeletal support structure 7 Shell 8 Node 9. Auxetic lattice 10. Strut 11. Node

Claims

Claims 1 A rotor for use in a vacuum system or compressor, the rotor comprising: a hub configured for connection to a rotor shaft, and at least one rotor blade coupled to the hub at a rotor blade root and extending to a rotor blade tip; wherein each rotor blade comprises a metallic skeletal support structure that is monolithic with the hub and extends from the rotor blade root towards the rotor blade tip, and an external shell extending from the rotor blade root to the rotor blade tip; wherein the skeletal support structure and the external shell are substantially discrete, and each define at least 10% of the volume fraction of the rotor blade; and wherein the material of the shell has a lower density than the metallic material of the skeletal support structure.
2 The rotor according to claim 1, wherein the skeletal support structure is a lattice
3. The rotor according to claim 2, wherein the lattice has a non-uniform lattice density, preferably having a greater lattice density towards the rotor blade root than towards a rotor blade tip.
4. The rotor according to any preceding claim, wherein the rotor is for use in a fast-rotating vacuum system or compressor, preferably a turbomolecular vacuum pump.
5. The rotor according to any preceding claim, wherein the rotor blade is substantially non-hollow.
6 The rotor according to any preceding claim, wherein the shell is directly connected to the skeletal support structure, preferably wherein the shell is moulded or cast about the skeletal support structure.
7. The rotor according to any preceding claim, wherein the shell comprises a polymeric and/or composite and/or metallic material.
8. The rotor according to any preceding claim, wherein the skeletal support structure and the hub are additive manufactured.
9 The rotor according to claim 8, wherein the skeletal support structure and the hub are produced by selective laser sintering, binder jetting, electron beam melting, and/or directed energy deposition.
10. The rotor according to any preceding claim, wherein the surface of the skeletal support structure that is directly connected to the shell has a surface roughness greater than 3.0 Ra.
11. The rotor according to any preceding claim, wherein the skeletal support structure extends at least from the rotor blade root to a point 25% between the rotor blade root and the rotor blade tip, preferably at least 50% of the distance between the rotor blade root and the rotor blade tip.
12.A vacuum system or compressor comprising a rotor shaft and at least one rotor according to any of claims 1 to 11 mounted thereon, preferably wherein the vacuum system is a turbomolecular pump.
13. The vacuum system or compressor according to claim 12, wherein at least one rotor has an interference-fit, fused or bolted connection with the rotor shaft.
14. A method for producing a rotor according to any of claims 1 to 11, comprising the steps of: c) Providing the hub and the skeletal support structure, preferably by an additive manufacturing technique; d) Moulding or casting the shell about the skeletal support structure.
15. The method according to claim 14, further comprising the step of performing a finishing step on the skeletal support structure and/or the rotor, preferably wherein said finishing step comprises a machining step and/or a surface modification step and/or a surface engineering step and/or a coating step.