Axial seal

Abstract

An axial seal 40 for an electric machine of a vehicle traction motor, the axial seal comprising an annular main body 46 which extends around a central axis 48; and at least one annular bead 50A, 50B which protrudes in an axial direction from an axial end surface 52A, 52B of the annular main body. The annular bead may comprise a first annular bead 50A which protrudes in a first axial direction from a first axial end surface 52A of the main body, and a second annular bead 50B which protrudes in a second axial direction from a second axial end surface 52B of the main body opposite to the first axial end surface. The annular bead may define a curved abutment surface. The curved abutment surface may be defined by a radius of curvature in the range of 0.1mm to 1mm. The annular main body may have a cross-sectional shape comprising trapezoidal portions (fig 6, 62A, 62B). The seal may have radial protrusions (fig 4, 70, 72) which protrudes in a radial direction from a radial side surface of the annular main body; the protrusion comprising an elongate rib which extends axially along the radial side surface. The radial protrusion may comprise outer (fig 4, 70) and inner (fig 4, 72) radial protrusions.

Description

TECHNICAL FIELD The present disclosure relates to an axial seal for an electric machine of a vehicle traction motor. Aspects of the invention relate to an axial seal, to a stator assembly, to an electric machine, and to a vehicle. BACKGROUND It is known to provide an axial seal which is configured to be positioned axially between two components of an electric machine of a vehicle traction motor to provide a seal therebetween. Typically, axial seals are formed of polymeric materials which allow compressions to be applied to axial ends of the seal’s crosssections. Such compression provides a contact pressure between the axial seal and the respective component, and thereby inhibits leakage of fluid past the axial seal. In one example, the axial seal is an annular seal which provides sealing around a circumference of the respective components. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide an axial seal for an electric machine of a vehicle traction motor, a stator assembly, an electric machine, and a vehicle as claimed in the appended claims. According to an aspect of the present disclosure, there is provided an axial seal. The axial seal comprises a main body which extends around a central axis and at least one bead which protrudes in an axial direction from an axial end surface of the main body. It will be understood that such a bead provides a smaller surface area than the portion of the main body from which the bead protrudes. Therefore, when the axial seal is compressed axially between two components of an electric machine, the bead provides a smaller contact area between the axial seal and the respective component, which facilitates increased contact pressure for a given compression force. This may facilitate use of the axial seal in higher pressure applications, and / or use of a lower compression force during assembly. In this context, the term “axial seal” will be understood to mean a seal which is configured to have compression applied to axial ends of the seal’s cross-sections. This contrasts with a radial seal which is configured to have compression applied to the inner and outer radial surfaces of the seal. The axial seal may be for an electric machine of a vehicle traction motor. In other words, the axial seal may be for an electric machine which is used to drive wheels of a hybrid or electric vehicle, rather than a supplementary electric machine for performing auxiliary functions, such as positioning vehicle seats. Optionally, the main body is annular. Optionally, each bead of the at least one bead is annular. The main body and / or at least one bead being annular allows sealing to be achieved about a circumference of a component (e.g., about an inner or outer circumference of an annular or disc-shaped component). Optionally, the at least one bead comprises a first bead which protrudes in a first axial direction from a first axial end of the main body, and a second bead which protrudes in a second axial direction from a second axial end of the main body opposite to the first axial end. Having such first and second beads facilitates increased contact pressure at both axial ends of the axial seal. Optionally, the first bead is annular. Optionally, the second bead is annular. Optionally, the at least one bead defines a curved abutment surface. Such a curved abutment surface allows for slight misalignment / imperfections in corresponding component surface in comparison to flat abutment surfaces. In addition, such a curved abutment surface further increases contact pressures and / or reduces assembly insertion force relative to a flat abutment surface. As used herein, the term “curved abutment surface” refers to the transverse cross-sectional shape of the abutment surface. Optionally, the curved abutment surface is defined by a radius of curvature in the range of 0.1 mm to 1 mm, optionally in the range of 0.2mm to 0.7mm, optionally in the range of 0.3mm to 0.5mm; optionally, in the range of 0.35mm to 0.45mm. The curved abutment surface being defined by such a radius of curvature facilitates increased contact pressure, whilst still being easily manufactured and allowing for slight misalignment / imperfections in the corresponding component surface. Optionally, the axial seal is symmetrical about a plane arranged transverse to the central axis. This facilitates assembly of the axial seal in either orientation within an electrical machine or other assembly. For example, the axial seal can be installed in a seat with either axial end of the axial seal facing the seat. Optionally, the main body comprises a cross-sectional shape which is elongated in the axial direction. In other words, an axial dimension of the cross-sectional shape is greater than a radial dimension of the cross-sectional shape. This reduces the amount of axial force required to achieve a given axial compression distance in comparison to seals which are elongated in the radial direction or of equal axial and radial dimensions. Optionally, the axial seal comprises a maximum axial dimension of less than 20mm, for example in the range of 5 to 15mm, in the range of 6 to 10 mm, or in the range of 7 to 9mm. In one example, the axial seal has a maximum axial dimension of around 8.5mm. It will be understood that the maximum axial dimension of the axial seal is the sum of the maximum axial dimension of the main body and the maximum axial dimension of the at least one bead. Optionally, the maximum axial dimension of the main body is in the range of 75 to 95% of the maximum axial dimension of the axial seal. In other words, the maximum axial dimension of the at least one bead is in the range of 5 to 25% of the maximum axial dimension of the axial seal. In embodiments having a first bead and a second bead, the maximum axial dimension of the at least one bead may be split equally between the first and second beads. In other words, the maximum axial dimension of the first bead may be equal to the maximum axial dimension of the second bead (e.g., each may be in the range of 2.5 to 12.5% of the maximum axial dimension of the axial seal). Alternatively, the maximum axial dimension of the first bead may differ from the maximum axial dimension of the second bead. Optionally, the main body comprises a maximum radial dimension of less than 10mm, for example in the range of 2 to 6mm, or in the range of 3 to 5mm. In one example, the main body has a maximum radial dimension of around 4mm. In some embodiments, the ratio of the maximum axial dimension of the axial seal to the maximum radial dimension of the main body may be in the range of 1.2:1 to 5:1, e.g., 1.5:1 to 4:1, e.g. 1.8:1 to 3:1. In some embodiments, the or each bead has a maximum radial width which is in the range of 30 to 70 %, optionally 40 to 60 %, optionally 45 to 55 %, of the maximum radial width of the respective axial end surface. Optionally, the main body has a cross-sectional shape which narrows in a radial direction towards one or both axial ends; optionally, wherein the cross-sectional shape gradually narrows in the radial direction towards one or both axial ends. This reduces the compression force required to achieve a given axial compression in comparison to shapes with a constant radial dimension between the first and second axial ends. Optionally, the main body has a cross-sectional shape comprising a first trapezoidal portion on a first axial side of the annular main body and a second trapezoidal portion on a second axial side of the annular main body, wherein the at least one bead protrudes from an outboard base of the first and / or second trapezoidal shape. This reduces the compression force required to achieve a given axial compression in comparison to shapes with a constant radial dimension between the first and second axial ends. Optionally, the cross-sectional shape comprises a rectangular central portion located between the first and second trapezoidal portions. This provides a more rigid central region which inhibits buckling and / or eversion of the axial seal. Optionally, the first and / or second trapezoidal portion comprises a curved transition surface, or fillet, between the outboard base and adjacent side surfaces. For example, the curved transition surface may be defined by a radius of curvature in the range of 0.2 to 0.6mm, e.g. 0.3 to 0.5mm, e.g. 0.45 to 0.55mm. Optionally, the axial seal comprises a curved transition surface, or fillet, between the at least one annular bead and the corresponding axial end of the annular main body. For example, the curved transition surface may be defined by a radius of curvature in the range of 0.2 to 0.6mm, e.g. 0.3 to 0.5mm, e.g. 0.45 to 0.55mm. Optionally, the main body has a generally octagonal cross-sectional shape. Optionally, the octagonal shape is elongated in the axial direction. Optionally, the axial seal further comprises at least one radial protrusion which protrudes in a radial direction from a radial side surface of the main body. During use, such a radial protrusion can brace against one or more side walls of a seat in which the axial seal is located, to inhibit buckling under compressive axial loading of the axial seal. In addition, such a radial protrusion can facilitate correct lateral positioning of the axial seal. Optionally, the radial protrusion comprises an elongate rib which extends axially along the radial side surface. The radial protrusion comprising an elongate rib which extends axially along the radial side surface can act to strengthen the axial seal and inhibit buckling and / or eversion of the axial seal. Optionally, the at least one radial protrusion comprises a plurality of radial protrusions. This may further reduce the likelihood of buckling and / or eversion under compressive axial loading, in comparison to a single radial protrusion. Optionally, the at least one radial protrusion comprises a plurality of outer radial protrusions which protrude from the outer radial surface of the annular main body, and a plurality of inner radial protrusions which protrude from the inner radial surface of the annular main body. This may further reduce the likelihood of buckling and / or eversion under compressive axial loading, in comparison to a single radial protrusion, or radial protrusions on only the inner or outer radial surface. In addition, having radial protrusions on both inner and outer radial surfaces allows the radial protrusions to brace against both sides of a seat in which the axial seal is located during use. This may also facilitate correct lateral positioning. Optionally, the number of outer radial protrusions is different to the number of inner radial protrusions. This provides a visual indication if the axial seal has been inadvertently everted. Optionally, the number of outer radial protrusions is greater than the number of inner radial protrusions. Optionally, the number of outer radial protrusions is twice the number of inner radial protrusions. Optionally, the axial seal comprises a plurality of radial protrusion groups each comprising two outer radial protrusions and an inner radial protrusion positioned circumferentially between the two outer radial protrusions. Such groups inhibit buckling in two opposite directions (e.g., radially inward and outward). Optionally, the radial protrusion groups are distributed circumferentially around the axial seal. This inhibits buckling in opposite directions (e.g., radially inward and outwards) about a circumference of the axial seal. The radial protrusion groups may be evenly distributed around the axial seal. Optionally, the circumferential spacing between adjacent radial protrusion groups is greater than the circumferential spacing within each radial protrusion group. This reduces amount of material required for the radial protrusions by having bigger spacing between different groups. Optionally, the axial seal comprises a diameter in the range of 50 mm to 500 mm, optionally in the range of 100 mm to 300 mm, optionally in the range of 130 mm to 230 mm. Such a diameter may be particularly suitable for providing sealing in an electrical machine for driving one or more wheels of a vehicle. Optionally, the axial seal is formed of an elastomeric material, such as natural or synthetic rubber. The axial seal may be a resilient axial seal. Optionally, the axial seal is formed of fluorosilicone material (e.g., a compound comprising fluorine and silicon and optionally other atoms such as oxygen and carbon in its molecular structure). Optionally, the axial seal is formed of a material having a shore A hardness in the range of 40 to 80, optionally in the range of 40 to 60, optionally in the range of 45 to 55. A further aspect of the present disclosure provides a stator assembly for an electric machine of a vehicle traction motor, the stator assembly comprising an axial seal as disclosed herein. Such a stator assembly benefits from the advantages of the axial seal outlined above. Optionally, the stator assembly comprises a stator core and a cooling manifold positioned against an axial end of the stator core, wherein the axial seal is compressed axially between the cooling manifold and the axial end of the stator core. A further aspect of the present disclosure provides an electric machine for driving one or more wheels of a vehicle. The electric machine comprises an axial seal as disclosed herein, and / or a stator assembly as disclosed herein. Such an electrical machine benefits from the advantages of the axial seal outlined above. Optionally, the electric machine further comprises a cooling fluid recirculation circuit configured to supply cooling fluid to the cooling manifold. A further aspect of the present disclosure provides a vehicle comprising an electric machine as disclosed herein. Such a vehicle benefits from the advantages of the axial seal outlined above. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a vehicle according to an embodiment; Figure 2 shows a longitudinal cross-sectional view of an electric machine comprising a stator assembly according to an embodiment; Figure 3 shows a transverse cross-sectional view of the electrical machine of Figure 2; Figure 4 shows a perspective view of an axial seal according to an embodiment; Figure 5 shows a cross-sectional view of the axial seal of Figure 4, through a plane passing through a central axis of the seal; Figure 6 shows an enlarged view of the cross-sectional shape of the axial seal of Figures 4 and 5; Figures 7A and 7B show enlarged perspective views of outer and inner radial protrusions of the axial seal of Figures 4 to 6; and Figure 8 shows a cross-sectional view of an axial seal according to a further embodiment. DETAILED DESCRIPTION Examples of the present disclosure relate to an axial seal. In particular, examples of the present invention relate to an axial seal for an electric machine of a vehicle traction motor. Such an electric machine may be of a synchronous type or asynchronous type, for example a permanent magnet synchronous motor. Nonlimiting examples will now be described with reference to accompanying Figures 1 to 8, where the figures illustrate an axial seal 40, a stator assembly 10, an electric machine 100, an electric drive unit (EDU) 160 and a vehicle 200. Figure 1 illustrates a vehicle 200 having a first electric machine 100-1 which is part of a vehicle traction motor for driving one or more front wheels of the vehicle 200. The vehicle 200 also has a second electric machine 100-2 which is part of a vehicle traction motor for driving one or more rear wheels of the vehicle 200. In other embodiments the vehicle 200 may comprise only a single electric machine 100, which is part of a vehicle traction motor which is arranged or configured to drive one or more front wheels of the vehicle 200 and / or one or more rear wheels of the vehicle 200. At a vehicle axle the electric machine 100 may be arranged to drive both wheels, either directly or through other transmission components. In other arrangements there may be more than one electric machine 100 arranged to provide torque to a vehicle axle, for example, to provide torque vectoring functionality for the vehicle 200. Other arrangements may have one electric machine 100 arranged or configured to drive each wheel of the vehicle 200. The electric machine 100 comprised in the vehicle 200 may have a stator assembly 10 with an axial seal 40 as described herein. For example, the electric machine 100 comprised in the vehicle 200 may be the electrical machine of Figure 2, described below. As illustrated schematically on Figure 1, the electric machine(s) 100 may be part of an electric drive unit (EDU) 160. For example, the EDU may include transmission components, lubrication and cooling components, and / or power electronics, in addition to the electric machine 100. In the vehicle 200 of Figure 1, the first electric machine 100-1 is part of a first EDU 160-1 for driving front wheels of the vehicle 200, and the second electric machine 100-2 is part of a second EDU 160-2 fordriving rear wheels of the vehicle 200. Referring to Figures 2 and 3, the electric machine 100 of the vehicle 200 is illustrated. The electric machine includes a stator assembly 10 which has an annular stator core 12. The stator core 12 has a cylindrical inner channel 14, which defines a central stator axis 16. The cylindrical inner channel 14 extends in a 6 direction parallel to the central stator axis 16 from a first end 18 of the stator core 12 to a second end 20 of the stator core 12. The stator core 12 has a plurality of winding slots 22 extending radially to support electrical stator windings 150. For simplicity, the electrical stator windings 150 are illustrated in Figure 3 in only one winding slot 22. A plurality of stator teeth 24 are provided between the winding slots 22. In other words, the stator teeth 24 are interspersed between the winding slots 22 in a circumferential direction about the stator core 12. Both the plurality of winding slots 22 and the plurality of stator teeth 24 extend from the first end 18 to the second end 20 of the stator core 12. The electric machine 100 also includes a rotor 112. The rotor 112 is configured to fit within the cylindrical inner channel 14 of the stator core 12 with a small air gap 26 therebetween. The outside surface of the rotor 112 provides a surface concentric with a circumference of the cylindrical inner channel 14, such that as the rotor 112 rotates within the cylindrical inner channel 14 of the stator core 12, a consistent air gap 26 is maintained between the rotor 112 and the stator core 12. The electric machine 100 may have forty-eight winding slots 22 and eight rotor poles. Other combinations of winding slot numbers and rotor pole numbers are useful. As shown in Figures 2 and 3, the electric machine 100 has a housing 102 surrounding the stator assembly 10. In some embodiments the housing 102 is a cylindrical housing (as illustrated in Figure 3), though it will be understood that the stator core 12 may have a non-circular cross section, in particular where the outer form of the stator core 12 is oblate or has projections thereon, such that the housing 102 may be noncircular. The stator core 12 has a plurality of cooling passageways 28 which extend in a longitudinal direction (i.e., in a direction parallel to the central stator axis 16) through the stator core 12 away from the first end 18 of the stator core 12. In particular, the cooling passageways 28 extend from the first end 18 to the second end 20 of the stator core 12. The cooling passageways 28 are distributed about a circumference of the stator core 12. The electric machine 100 has a first cooling manifold 30A which at least partly covers the first end 18 of the stator core 12. The first cooling manifold 30A defines a first chamber 32A in fluid communication with the cooling passageways 28. The first cooling manifold 30A also defines a manifold inlet 34A for connecting the first chamber 32Ato a cooling fluid source. In this way, cooling fluid may pass through the manifold inlet 34A, into the first chamber 32A and then through the cooling passageways 28. In the arrangement of Figure 2, the manifold inlet 34A is defined by a radially outer edge of the first cooling manifold 30A, which is open to the housing 102 so that cooling fluid can enter the first chamber 32A through a first opening 104 in the housing 102. In other words, a radially outer edge 36A of the first chamber 32A is defined by the housing 102. The electric machine 100 also has a second cooling manifold 30B which at least partly covers the second end 20 of the stator core 12. The second cooling manifold 30B defines a second chamber 32B in fluid communication with the cooling passageways 28 and a manifold outlet 34B for expelling cooling fluid from the second chamber 32B. In this way, cooling fluid may pass through the manifold inlet 34A into the first chamber 32A of the first cooling manifold 30A, through the cooling passageways 28 to the second chamber 32B and then out of the manifold outlet 34B. In the arrangement of Figure 2, the manifold outlet 34B is defined by a radially outer edge of the second cooling manifold 30B, which is open to the housing 102 so that cooling fluid can be expelled from the second chamber 32B through a second opening 106 in the housing 102. In other words, a radially outer edge 36B of the second chamber 32B is defined by the housing 102. It will be understood that, although the schematic arrows on Figure 2 show a flow of cooling fluid through the first opening 104 in the housing, through the first cooling manifold 30A, along the cooling passageways 28, into the second cooling manifold 30B and then out of the second opening 106 in the housing 102, the flow of cooling fluid could be reversed. In other words, cooling fluid could instead flow through the second opening 106 in the housing 102, through the second cooling manifold 30B, along the cooling passageways 28, into the first cooling manifold 30A and then out of the first opening 104 in the housing 102. In such a configuration, the manifold inlet 34A of the first cooling manifold 30A would instead be a manifold outlet, and similarly the manifold outlet 34B of the second cooling manifold 30B would instead be a manifold inlet. In the illustrated arrangement, the first and second cooling manifolds 30A, 30B are annular. In this way, the first and second chambers 32A, 32B are also annular. The first and second cooling manifolds 30A, 30B can be considered as part of the stator assembly 10, along with the stator core 12. The electric machine 100 of Figure 2 also has a cooling fluid recirculation system 120 which is configured to supply cooling fluid to the manifold inlet 34A of the first cooling manifold 30A in a cooling fluid recirculation circuit 122 (i.e., via the first opening 104 in the housing 102). In the illustrated arrangement, the stator core 12 is defined by a stator lamination stack 44. In other words, the stator core 12 is formed of a plurality of layers. The layers are stacked one atop the other to form the stator lamination stack 44, which extends from the first end 18 of the stator core 12 to the second end 20 of the stator core 12. The layers may be stamped from sheet material (e.g., sheet metal), or formed via any other suitable process. In this way, a stator core 12 having a complex cross-sectional shape can be formed. In some arrangements the cooling passageways 28 are coated on their inner surfaces with a plastic material to provide sealing such that coolant is prevented, or resisted, from passing between the laminations (i.e. layers 44A, 44B) of the stator lamination stack 44 under the temperatures and pressures described herein. There are different methods for producing this arrangement. The plastic may be applied to the inner surface of the cooling passageway 28 by flowing the thermoplastic through the cooling passageway 28, or by inserting a formed thermoplastic tube into the cooling passageway 28 and optionally applying pressure to the tube to blow mould the tube to the shape of the cooling passageway 28. The thermoplastic may have a complex cross-sectional shape either internally and / or externally. The external surface of the thermoplastic tube may have a simple rounded shape and the inner surface of the cooling passageway 28 may be similarly rounded. Once the plastic coating is in place the thickness of the coating of the inner surface of the cooling passageways 28 is in the region of 0.5mm, but other thicknesses are useful. The thermal conductivity of the plastic material is in the range 1 to 40 watts / mK. The functioning of electrical machines 100 and electric drive units (EDUs) 160 is known, and so will not be described here in more detail. In the arrangement of Figure 2, a first axial seal 40A is located between the first end 18 of the stator core 12 and the first cooling manifold 30A to seal a radially inner edge 38A of the first chamber 32A. In particular, the first axial seal 40A is an annular seal. The first axial seal 40A is located in a first annular recess 42A in the first cooling manifold 30A, which locates the first axial seal 40A in the correct position. Similarly, a second axial seal 40B is located between the second end 20 of the stator core 12 and the second cooling manifold 30B to seal a radially inner edge 38B of the second chamber 32B. In particular, the second axial seal 40B is an annular seal. The second axial seal 40B is located in a second annular recess 42B in the second cooling manifold 30B, which locates the second axial seal 40B in the correct position. Similarly, a third axial seal 40C is provided at an opposite side of the second cooling manifold 30B to the second axial seal 40B. The third axial seal 40C is configured to form a seal between the second cooling manifold 30B and an axial end of the housing 102. The third axial seal 40C is located in a third annular recess 42C in the second cooling manifold 30B, which locates the third axial seal 40C in the correct position. In the arrangement of Figure 2, a radial seal 41 is located between the radially outer edge 36A of the first cooling manifold 30A and a radial inner surface of the housing 102. In particular, the radial seal 41 is an annular seal (e.g., an O-ring). The radial seal 41 is located in a fourth annular recess 43 in the radially outer edge 36A of the first cooling manifold 30A, which locates the radial seal 41 in the correct position. In this context, the term “axial seal” will be understood to mean a seal which is configured to have compression applied to axial ends of the seal’s cross-section. Conversely, the term “radial seal” will be understood to mean a seal which is configured to have compression applied to the inner and outer radial surfaces of the seal. Often, axial seals have a cross-section which is elongate in the axial direction, although this is not always the case. Referring now to Figures 4 to 7B, an axial seal is indicated at 40. The axial seal 40 illustrated in these Figures may be used as the first, second and / or third seals 40A, 40B, 40C in the electric machine 100 of Figures 2 and 3, described above. The axial seal 40 includes an annular main body 46 which extends around a central axis 48. It will be understood that when the axial seal 40 is installed in a stator assembly 10 as in Figures 2 and 3 above, the central axis 48 of the axial seal 40 may be coaxial with the central stator axis 16 described above. In the illustrated embodiment, the axial seal 40 forms a circular annulus. However, it will be understood that other annular shapes may also be applicable, for example in embodiments in which the stator core and / or a groove within which the axial seal is located has a non-circular shape. The axial seal 40 also includes at least one annular bead 50A, 50B which protrudes in an axial direction (i.e., in a direction parallel to the central axis 48) from an axial end surface 52A, 52B of the annular main body 46. In this context, the term “axial end surface” 52A, 52B will be understood to mean a surface of the main body 46 which extends generally in a direction perpendicular to the central axis 48. In other words, when the axial seal 40 is compressed axially between two components, each axial end surface 52A, 52B faces the respective component. It will be understood that such an annular bead 50A, 50B provides a smaller surface area than the portion of the main body 46 from which the annular bead 50A, 50B protrudes (i.e. the respective axial end surface 52A, 52B). For example, in the illustrated embodiment the annular bead 50A, 50B has a maximum radial width which is approximately half of the maximum radial width of the respective axial end surface 52A, 52B. Therefore, when the axial seal 40 is compressed axially between two components, the annular bead 50A, 50B provides a smaller contact area between the axial seal 40 and the respective component, which facilitates increased contact pressure for a given compression force. This may facilitate use of the axial seal 40 in higher pressure applications, and / or use of a lower compression force during assembly. The axial seal 40 illustrated in Figures 4 to 7B includes a first annular bead 50A which protrudes in a first axial direction from a first axial end 52A of the main body 46, and a second annular bead 50B which protrudes in a second axial direction from a second axial end 52B of the main body 46 opposite to the first axial end 52A. This facilitates increased contact pressure at both the first and second axial ends 52A, 52B of the axial seal 40. In an alternative embodiment, illustrated in Figure 8, the axial seal 40 has the first annular bead 50A, but the second annular bead 50B is omitted. In particular, the second axial end 52B of the main body 46 has an approximately flat surface 53. It will be understood that, apart from this difference, the axial seal 40 of Figure 8 may be the same as the axial seal 40 of Figures 4 to 7B. In other words, the details of the axial seal 40 described below may apply to the axial seal 40 of Figure 8, as well as the axial seal 40 of Figures 4 to 7B. In the illustrated embodiments, each annular bead 50A, 50B defines a curved abutment surface 54. Such a curved abutment surface 54 allows for slight misalignment / imperfections in the corresponding component surface in comparison to flat abutment surfaces. In addition, such a curved abutment surface 54 further increases contact pressures and / or reduces assembly insertion force relative to a flat abutment surface. In the illustrated embodiments, the curved abutment surface 54 is defined by a radius of curvature of approximately 0.4mm. This facilitates increased contact pressure, whilst still being easily manufactured and allowing for slight misalignment / imperfections in the corresponding component surface. In other embodiments, the curved abutment surface 54 may be defined by any other suitable radius of curvature. For example, the curved abutment surface 54 may be defined by a radius of curvature in the range of 0.1mm to 1mm. In alternative embodiments, the curved abutment surface 54 may be replaced by an approximately triangular abutment surface (i.e., having two angled sides converging towards a point). The axial seal 40 of Figures 4 to 7B is symmetrical about a plane P arranged transverse to the central axis 48. This facilitates assembly of the axial seal 40 in either orientation within an electrical machine or other assembly. For example, the axial seal 40 can be installed in a seat (e.g., one of the annular recesses 42A, 42B, 42C described above) with either of the first and second axial ends 52A, 52B of the axial seal 40 facing the seat. In the alternative embodiment of Figure 8, the axial seal 40 is not symmetrical about such a plane P, because of the omission of the second annular bead 50B. In the illustrated embodiments, the annular main body 46 has a cross-sectional shape which is elongated in the axial direction (i.e. in a direction parallel to the central axis 48). In other words, a maximum axial dimension 56 of the cross-sectional shape is greater than a maximum radial dimension 58 of the cross-sectional shape. This reduces the amount of axial force required to achieve a given axial compression distance in comparison to seals which are elongated in the radial direction or of equal axial and radial dimensions. In the illustrated embodiments, the maximum axial dimension of the axial seal 40 is approximately 8.5mm. In other embodiments, the maximum axial dimension of the axial seal 40 may be of any other suitable value (e.g., less than 20mm, for example in the range of 5 to 15mm, in the range of 6 to 10 mm, or in the range of 7 to 9mm). It will be understood that the maximum axial dimension ofthe axial seal 40 is the sum of the maximum axial dimension 56 ofthe main body, the maximum axial dimension 57A ofthe first annular bead 50A, and the maximum axial dimension 57B ofthe second annular bead 50B. In the illustrated embodiment, the maximum axial dimension 56 ofthe main body 46 is approximately 85% of the maximum axial dimension of the axial seal 40. In other words, the sum of the maximum axial dimensions 57A, 57B ofthe first and second annular beads 50A, 50B is approximately 15% ofthe maximum axial dimension ofthe axial seal 40. In other embodiments, the maximum axial dimension 56 ofthe main body 46 may be of a different size relative to the overall axial dimension ofthe axial seal 40. For example, the maximum axial dimension 56 ofthe main body 46 may be in the range of 75 to 95% ofthe maximum axial dimension ofthe axial seal. In the illustrated embodiment, the first and second annular beads 50A, 50B are ofthe same size and shape, so that the maximum axial dimension 57A, 57B of each annular bead 50A, 50B is approximately 7.5% of the maximum axial dimension of the axial seal 40. In other embodiments, the first and second annular beads 50A, 50B may be of different size and shape, so that the respective maximum axial dimensions 57A, 57B are different. The maximum radial dimension 58 ofthe main body 46 is approximately 4mm. In other embodiments, the maximum radial dimension 58 may be of any other suitable value (e.g., less than 10mm, for example in the range of 2 to 6mm, or in the range of 3 to 5mm). In the illustrated embodiment, the maximum axial dimension ofthe axial seal 40 is approximately double the maximum radial dimension 58 ofthe main body 46. In some embodiments, the ratio of the maximum axial dimension of the axial seal to the maximum radial dimension 58 of the main body 46 may be in the range of 1.2:1 to 5:1, e.g., 1.5:1 to 4:1, e.g. 1.8:1 to 3:1. In the illustrated embodiments, the annular main body 46 has a cross-sectional shape which narrows in a radial direction (i.e. in a direction perpendicular to the central axis 48) towards both axial ends 52A, 52B. In other words, a radial dimension of the cross-sectional shape reduces from the maximum radial dimension 58 towards a smaller radial dimension at the first and second axial ends 52A, 52B. In other embodiments, the main body 46 may have a cross-sectional shape which narrows in the radial direction towards only one of the axial ends 52A, 52B. Such a narrowing towards one or both axial ends 52A, 52B reduces the compression force required to achieve a given axial compression in comparison to shapes with a constant radial dimension between the first and second axial ends 52A, 52B. In the illustrated embodiments, the cross-sectional shape of the main body 46 gradually narrows in the radial direction towards the axial ends 52A, 52B (e.g., as opposed to narrowing suddenly at a step change). In the illustrated embodiment, the gradual narrowing is provided by taper surfaces 60. The taper surfaces 60 define approximately flat surfaces which are arranged at an oblique angle to the central axis 48. In alternative embodiments, the taper surfaces 60 are curved, rather than flat. In the illustrated embodiments, the cross-sectional shape of the main body 46 has a first trapezoidal portion 62A on a first axial side of the annular main body 46 and a second trapezoidal portion 62B on a second axial side of the annular main body 46. The annular beads 50A, 50B each protrude from an outboard base of the respective first or second trapezoidal portion 62A, 62B. In other words, the outboard base defines the respective axial end 52A, 52B of the main body 46. In the illustrated embodiments, the first and second trapezoidal portions have curved transition surfaces 66, or fillets, between the outboard base (i.e., the respective axial end surface 52A, 52B) and adjacent side surfaces (i.e., the taper surfaces 60). The curved transition surfaces 66 may each be defined by a radius of curvature in the range of 0.2 to 0.6mm, e.g. 0.3 to 0.5mm, e.g. 0.45 to 0.55mm. In the illustrated embodiments, the cross-sectional shape of the main body 46 also includes a rectangular central portion 64 located between the first and second trapezoidal portions 62A, 62B. This provides a more rigid central region of the axial seal 40 which inhibits buckling and / or eversion of the axial seal 40. In other words, the annular main body 46 has a generally octagonal cross-sectional shape. The octagonal cross-sectional shape is elongated in the axial direction (i.e., in a direction parallel to the central axis 48). In the illustrated embodiment, the axial seal 40 has curved transition surfaces 68, or fillets, between each annular bead 50A, 50B and the corresponding axial end 52A, 52B of the annular main body 46. For example, the curved transition surfaces may each be defined by a radius of curvature in the range of 0.2 to 0.6mm, e.g. 0.3 to 0.5mm, e.g. 0.45 to 0.55mm. As illustrated in Figures 4, 7A and 7B, the axial seal 40 has at least one radial protrusion 70, 72 which protrudes in a radial direction (i.e., in a direction perpendicular to the central axis 48) from a radial side surface 74, 76 of the annular main body 46. During use, such a radial protrusion 70, 72 can brace against one or more side walls of a seat in which the axial seal 40 is located (e.g., in one of the annular recesses 42A, 42B, 42C described above). This inhibits buckling under compressive axial loading of the axial seal 40. In addition, such a radial protrusion 70, 72 can facilitate correct lateral positioning of the axial seal 40. In the illustrated embodiment, each radial protrusion 70, 72 is an elongate rib which extends axially (i.e., in a direction parallel to the central axis 48) along the respective radial side surface 74, 76. Such an elongate rib can act to strengthen the axial seal 40 and inhibit buckling and / or eversion of the axial seal 40. In alternative embodiments, each radial protrusion 70, 72 may be of any other suitable configuration (e.g., an approximately circular projection, rather than an elongate rib). In the illustrated embodiment, there are a plurality of radial protrusions 70, 72. This may further reduce the likelihood of buckling and / or eversion under compressive axial loading, in comparison to a single radial protrusion 70, 72. In more detail, the axial seal 40 has a plurality of outer radial protrusions 70 which protrude from the outer radial surface 74 of the annular main body 46, and a plurality of inner radial protrusions 72 which protrude from the inner radial surface 76 of the annular main body 46. This may further reduce the likelihood of buckling and / or eversion under compressive axial loading, in comparison to a single radial protrusion 70, 72, or radial protrusions 70, 72 on only the inner or outer radial surface 74, 76. In addition, having radial protrusions 70, 72 on both inner and outer radial surfaces 74, 76 allows the radial protrusions 70, 72 to brace against both sides of a seat in which the axial seal 40 is located during use (e.g., in one of the annular recesses 42A, 42B, 42C described above). This may also facilitate correct lateral positioning of the axial seal 40. In the illustrated embodiment, the number of outer radial protrusions 70 is different to the number of inner radial protrusions 72. This provides a visual indication if the axial seal 40 has been inadvertently everted. In particular, the number of outer radial protrusions 70 is greater than the number of inner radial protrusions 72. In more detail, the number of outer radial protrusions 70 is twice the number of inner radial protrusions 72. In alternative embodiments, the number of outer and inner radial protrusions 70, 72 may be equal, or the number of inner radial protrusions 72 may be greater than the number of outer radial protrusions 70. As best illustrated in Figures 4, 7A and 7B, the axial seal 40 has a plurality of radial protrusion groups 78. Each radial protrusion group 78 has two outer radial protrusions 70 and an inner radial protrusion 72 positioned circumferentially between the two outer radial protrusions 72. Such groups 78 inhibit buckling in two opposite directions (e.g., radially inward and outward). In the illustrated embodiment, the radial protrusion groups 78 are distributed circumferentially around the axial seal 40. This inhibits buckling in opposite directions (e.g., radially inward and outwards) about a circumference of the axial seal 40. In the illustrated embodiment, the radial protrusion groups 78 are evenly distributed around the axial seal 40. In the illustrated embodiment, the circumferential spacing between adjacent radial protrusion groups 78 is greater than the circumferential spacing between radial protrusions 70, 72 within each radial protrusion group 78. This reduces the amount of material required for the radial protrusions 70, 72 by having bigger spacing between different groups 78. In alternative embodiments, the outer radial protrusions 70 are evenly distributed around the circumference of the axial seal 40 (i.e., instead of being grouped in pairs around an inner radial protrusion 72). The axial seal 40 has a diameter D, as illustrated in Figure 4. In some embodiments, the diameter D is in the range of 50 mm to 500 mm (e.g., in the range of 100 mm to 300 mm, or in the range of 130 mm to 230 mm). Such a diameter may be particularly suitable for providing sealing in an electric machine 100 for a vehicle traction motor (e.g., in the stator assembly 10 described above). The axial seal 40 may be a resilient axial seal. For example, the axial seal 40 may be formed of an elastomeric material, such as natural or synthetic rubber. In some embodiments, the axial seal 40 is formed of fluorosilicone material (e.g., a compound comprising fluorine and silicon and optionally other atoms such as oxygen and carbon in its molecular structure). In some embodiments, the axial seal 40 is formed of a material having a shore A hardness in the range of 40 to 80, e.g., in the range of 40 to 60, e.g., in the range of 45 to 55, e.g. approximately 50. Although it has been described how the axial seals 40 of Figures 4 to 8 could be used in the stator assembly 10 of the electric machine 100 of Figures 2 and 3, it will be understood that such an axial seal 40 could be used in other applications where there is a need to seal between two components pushed axially together. For example, the axial seal 40 could be used in another part of the electrical machine 100 (e.g., between different axial sections of the rotor 112), or in another part of an electric drive unit 160 (e.g., between an electric machine section and a transmission section of the electric drive unit 160). Alternatively, the axial seal 40 could be used in other applications not relating to electric machines, electric drive units, or vehicles. It will also be appreciated that, although the axial seal 40 is annular in the illustrated embodiment, in other embodiments the axial seal 40 may not be annular. For example, in applications where there is a need to provide a linear, rather than annular seal, the axial seal may have a linear main body 46. In such embodiments, one or both of the first and / or second beads 50A, 50B may be present. In this case, the central axis of the axial seal would correspond to its longitudinal axis, and the beads 50A, 50B would protrude from the main body in a direction perpendicular to the longitudinal axis of the main body 46, on side surfaces which are configured to face the respective components which the axial seal 40 is compressed between. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application. It should also be noted that whilst the appended claims set out particular combinations of features described above, the scope of the present disclosure is not limited to the particular combinations hereafter claimed, but instead extends to encompass any combination of features herein disclosed.

Claims

1. An axial seal for an electric machine of a vehicle traction motor, the axial seal comprising: an annular main body which extends around a central axis; andat least one annular bead which protrudes in an axial direction from an axial end surface of the annular main body.

2. The axial seal of claim 1, wherein the at least one annular bead comprises a first annular bead which protrudes in a first axial direction from a first axial end surface of the main body, and a second annular bead which protrudes in a second axial direction from a second axial end surface of the main body opposite to the first axial end surface.

3. The axial seal of claim 1 or 2, wherein the at least one annular bead defines a curved abutment surface.4 The axial seal of claim 3, wherein the curved abutment surface is defined by a radius of curvature in the range of 0.1mm to 1 mm, optionally in the range of 0.2mm to 0.7mm, optionally in the range of 0.3mm to 0.5mm; optionally, in the range of 0.35mm to 0.45mm.

5. The axial seal of any preceding claim, wherein the axial seal is symmetrical about a plane arranged transverse to the central axis.

6. The axial seal of any preceding claim, wherein the annular main body comprises a cross-sectional shape which is elongated in the axial direction.

7. The axial seal of any preceding claim, wherein the annular main body has a cross-sectional shape which narrows in a radial direction towards one or both axial ends; optionally, wherein the cross-sectional shape gradually narrows in the radial direction towards one or both axial ends.

8. The axial seal of any preceding claim, wherein the annular main body has a cross-sectional shape comprising a first trapezoidal portion on a first axial side of the annular main body and a second trapezoidal portion on a second axial side of the annular main body, wherein the at least one annular bead protrudes from an outboard base of the first and / or second trapezoidal shape.

9. The axial seal of any preceding claim, further comprising at least one radial protrusion which protrudes in a radial direction from a radial side surface of the annular main body; optionally, wherein the radial protrusion comprises an elongate rib which extends axially along the radial side surface.

10. The axial seal of claim 9, wherein the at least one radial protrusion comprises a plurality of outer radial protrusions which protrude from the outer radial surface of the annular main body, and a plurality of inner radial protrusions which protrude from the inner radial surface of the annular main body.

11. The axial seal of claim 10, wherein the number of outer radial protrusions is different to the number of inner radial protrusions.

12. A stator assembly for an electric machine of a vehicle traction motor, the stator assembly 5 comprising the axial seal of any preceding claim.

13. The stator assembly of claim 12, wherein the stator assembly comprises a stator core and a cooling manifold positioned against an axial end of the stator core, wherein the axial seal is compressed axially between the cooling manifold and the axial end of the stator core.1014. An electric machine for driving one or more wheels of a vehicle, the electric machine comprising: the axial seal of any preceding claim; and / or the stator assembly of claim 12 or 13; optionally, further comprising a cooling fluid recirculation circuit configured to supply cooling fluid to the cooling manifold.15 15. A vehicle comprising the electric machine of claim 14.

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