Rotor assembly

Abstract

A rotor assembly for an electric motor includes a shaft having a rotational axis, a magnet having a first contact surface, and a coupling assembly coupled to the shaft and having a second contact surface having a sloped profile relative to a plane perpendicular to the rotational axis of the shaft. The coupling assembly couples the magnet to the shaft via an interference fit between the first contact surface of the magnet and the second contact surface of the coupling assembly.

Description

[0001] ROTOR ASSEMBLY

[0002] BACKGROUND

[0003] There is a general desire to improve components of electric motors, such as rotor assemblies, in a number of ways. For example, improvements may be desired in terms of reliability, operational lifetime, performance, efficiency and manufacturing cost.

[0004] SUMMARY

[0005] According to a first aspect, there is provided a rotor assembly for an electric motor, the rotor assembly comprising: a shaft having a rotational axis; a magnet having a first contact surface; and a coupling assembly coupled to the shaft and having a second contact surface having a sloped profile relative to a plane perpendicular to the rotational axis of the shaft, wherein the coupling assembly couples the magnet to the shaft via an interference fit between the first contact surface of the magnet and the second contact surface of the coupling assembly.

[0006] A known method of connecting a magnet to a shaft of a rotor assembly involves adhering the magnet to the shaft using an adhesive. In this known method, the magnet is tubular with an internal diameter that is slightly larger than an outer diameter of a cylindrical or tubular shaft. The magnet is mounted on the shaft, leaving a gap between the internal diameter of the magnet and the outer diameter of the shaft. The gap is filled by the adhesive (which may be referred to as a substrate or a filler material). The adhesive is then cured using a curing process which may involve the application of ultraviolet (UV) light, heat or an anaerobic process. The known manufacturing method is time consuming and it can be difficult to achieve a tolerable alignment between the magnet and the shaft.

[0007] The performance of the rotor assembly, and hence of an electric motor comprising the rotor assembly, depends on the performance of the substrate. For example, the strength of the substrate determines the amount of torque that can be transmitted to the shaft from the magnet. The temperature dependence of the mechanical properties of the substrate also contribute to the performance of the rotor assembly. Known substrate materials have highly temperature dependent mechanical properties, which can reduce rotor assembly performance. Furthermore, exposure of the substrate to high temperatures during the operational life of the rotor assembly can reduce the strength of the rotor assembly and may lead to reduced efficiency or premature failure of the rotor assembly. The substrate is typically exposed to temperature cycling over its lifetime. This can cause fatigue in the substrate, which must be monitored over time to reduce the risk of unexpected failures. In general, the use of an adhesive to attach the magnet to the shaft increases the bill of materials and manufacturing cost for a rotor assembly and also limits the overall performance of the electric motor comprising the rotor assembly.

[0008] In examples herein, a magnet is mounted relative to a shaft of a rotor assembly via an interference fit between a first contact surface of the magnet and a corresponding second contact surface of a coupling assembly. The second contact surface has a sloped profile relative to a plane perpendicular to the rotational axis of the shaft. The interference fit between the first and second contact surfaces allows the magnet to be mounted relative to the shaft using friction. The use of a sloped second contact surface for example provides a sufficiently secure coupling between the magnet and shaft. The magnet can thus be coupled to the shaft without using an adhesive, which can improve rotor assembly performance and / or reliability and reduce manufacturing cost, time and complexity. In particular, the interference fit may allow torque to be transmitted in an efficient manner from the magnet to the shaft, which can improve the effectiveness of the rotor assembly.

[0009] A sloped profile for example refers to a surface with a slope, which may be a slope overall, broadly, on average or in a general sense. For example, a sloped profile may be a generally rising or falling surface (such as an inclined surface) relative to a flat plane, e.g. the plane perpendicular to the rotational axis of the shaft, in the context of the first contact surface. A surface with a sloped profile may be a flat surface, which is at a nonperpendicular angle with respect to a flat plane. In other cases, though, a surface with a sloped profile may not be flat and may for example have a curved profile, which may be inwardly or outwardly curved (e.g. concave or convex), which may be angled with respect to a flat plane. A sloped profile may also include at least one protrusion provided the overall shape of the profile is sloped. For example, a surface with a sloped profile may include threads and / or teeth but nevertheless have a broadly sloping profile.

[0010] The interference fit between the first and second contact surfaces may urge a rotational axis of the magnet to tend towards the rotational axis of the shaft, in use. This for example provides a more straightforward method of aligning the magnet with the shaft than known methods. Moreover, the interference fit can continue to realign the magnet with the shaft over time, should the magnet and shaft become misaligned. Alignment between the magnet and the shaft may therefore be maintained over time, which can increase the lifetime of the rotor assembly and may allow the magnet to transmit torque to the magnet more efficiently than with a greater extent of misalignment the magnet and the rotor assembly.

[0011] The first contact surface may have a complementarily sloped profile relative to the second contact surface. This may increase the extent of contact between the first and second contact surfaces, which can improve the effectiveness with which the magnet is coupled to the shaft by the coupling assembly. A complementarily sloped profile may be an inverse of a given profile, which for example has the same or a similar general shape, but with a slope in an opposite direction so that the two profiles generally fit together.

[0012] The first contact surface of the magnet and the second contact surface of the coupling assembly may each extend circumferentially about the shaft. For example, the interference fit may be a circumferential interference fit about the shaft. First and second contact surfaces that extend circumferentially about the shaft can aid in evenly distributing the torque from the magnet to the shaft, which can further assist in maintaining alignment between the magnet and the shaft. It is to be appreciated that the first contact surface may extend along the entirety of an edge surface of the magnet, from an inner surface of the magnet facing generally towards the shaft to an outer surface of the magnet facing generally away from the shaft (e.g. along the entirety of an edge side of the magnet), or the first contact surface may be less than all of the edge surface, such as a portion of the edge surface extending partway between the inner surface and the outer surface.

[0013] The first contact surface may be a conical surface. A conical surface may be straightforward to manufacture and may be more effective at maintaining alignment between the magnet and the shaft than other surface shapes.

[0014] The coupling assembly may comprise a plurality of coupling assembly portions, each comprising a part of the second contact surface. Various different coupling assembly configurations may be enabled by having a plurality of coupling assembly portions. For example, the second contact surface may be discontinuous. This may allow a smaller coupling assembly to be used than otherwise, which may be more lightweight. Such a coupling assembly may nevertheless provide adequate coupling between the magnet and the shaft. In this way, the overall weight of the rotor assembly may be reduced, which can be beneficial in situations in which a lightweight electric motor is desired. To provide a discontinuous second contact surface, the plurality of coupling assembly portions may be spaced evenly about the rotational axis, which may improve alignment between the magnet and the shaft.

[0015] With a plurality of coupling assembly portions each comprising a part of the second contact surface, and the second contact surface being discontinuous, the first contact surface may be continuous. This may simplify the creation of the interference fit between the first and second contact surfaces. For example, if the first contact surface is symmetrical about the rotational axis, an interference fit between the first and second contact surfaces may be created with the coupling assembly portions rotated to various different angular positions about the rotational axis. The interference fit may be formed between the first contact surface and each part of the second contact surface, to attach the magnet more securely to the shaft than otherwise.

[0016] The first contact surface may be tapered away from the rotational axis. A width of the magnet parallel to the rotational axis of the shaft may thus decrease with increasing distance from the shaft. The width of the magnet may hence decrease away from the shaft, in a direction perpendicular to the rotational axis in the plane perpendicular to the rotational axis. The decreasing width of the magnet in this direction may facilitate the application of a stabilising force towards the shaft by the coupling assembly, due to the sloped interface between the coupling assembly and the magnet, so as to maintain alignment between the magnet and the shaft.

[0017] The coupling assembly may comprise at least one shaft contact surface configured to contact a surface of the shaft to couple the magnet to the shaft. For example, there may be a shaft interference fit between the shaft contact surface and the surface of the shaft. The coupling assembly may thus be coupled to the shaft via friction rather than using an adhesive, which can improve reliability and / or performance and reduce manufacturing cost, time and complexity.

[0018] The coupling assembly may comprise a force transmission element comprising the second contact surface and a fastener comprising the shaft contact surface, the force transmission element comprising a different material from the fastener. The force transmission element may transfer a clamping force from the fastener to the magnet. Using a different material for the force transmission element and the fastener may improve robustness. For example, the force transmission element may have a lower degree of hardness than the fastener, such as a lower elastic modulus than the fastener. This may reduce the risk of contact between the force transmission element and the magnet causing damage to the magnet, while providing a sufficiently hard fastener to provide for effective torque transfer to the shaft.

[0019] The fastener may comprise a first element comprising an unthreaded portion of the coupling assembly and a second element comprising a threaded portion of the coupling assembly, the first element disposed between the force transmission element and the second element, wherein the unthreaded portion of the coupling assembly contacts an unthreaded portion of the shaft and the threaded portion of the coupling assembly contacts a threaded portion of the shaft. A threaded connection between the coupling assembly and the shaft may provide an increased clamping force for clamping the coupling assembly to the shaft. This may also allow different materials to be used for the first element and the second element, to improve torque transmission and provide appropriate frictional forces for securely coupling the coupling assembly to the shaft.

[0020] In examples, a friction coefficient between the first element and the second element is lower than a friction coefficient between the force transmission element and the first element. For example, the friction coefficient between the first element and the second element may be around 0.05 and the friction coefficient between the force transmission element and the first element may be around 0.25. This for example causes a lower frictional force between the first element and the second element than between the force transmission element and the first element. A frictional force is for example a force resisting a relative sliding motion between two components. A lower friction coefficient between the first and second elements means that if any slippage is to occur, the slippage is between the first and second elements rather than between the first element and the force transmission element. This can reduce the risk of damage to the magnet, which may otherwise occur if the magnet slips relative to the coupling assembly. The friction coefficient between the first element and the second element may be lowered by introducing a lubricant or other material with a relatively low friction coefficient between the first and second elements, for example by applying a coating of a suitable material to the first and / or second elements. Conversely, the friction coefficient between the force transmission element and the first element may be increased by disposing a material with a relatively high friction coefficient between the force transmission element and the first element, for example by applying a coating of such a material to the force transmission element and / or the first element.

[0021] The coupling assembly may comprise a resilient member. A or the force transmission element of the coupling assembly, which may comprise the second contact surface, may be disposed between the resilient member and the magnet. For example, the resilient member may be a constant rate spring or a variable rate spring. The resilient member may apply an axial force to the force transmission element. The resilient member may assist in coupling the coupling assembly to the magnet, which may further aid in transferring torque from the magnet to the shaft. The coupling assembly may be a first coupling assembly, the rotor assembly may comprise a second coupling assembly, the magnet may have a first side and a second side opposite to the first side, the first side may comprise the first contact surface, the second side may comprise a further first contact surface, opposite to the first contact surface, and the second coupling assembly may comprise a further second contact surface in contact with the further first contact surface to couple the magnet to the shaft. In these examples, the magnet may be clamped along opposite surfaces at opposite sides of the magnet by the first and second coupling assemblies. The magnet may therefore be more securely coupled to the shaft than in other cases in which a coupling assembly couples to one side of the magnet and not to an opposite side of the magnet.

[0022] The first contact surface and / or the further first contact surface may be tapered towards the rotational axis such that the width of the magnet parallel to the rotational axis decreases with increasing distance from the shaft. Tapering of the first contact surface and the further first contact surface in this direction may assist the first and second coupling assemblies in maintaining alignment between the magnet and the shaft.

[0023] The first contact surface may be tapered away from the second end and / or the further first contact surface may be tapered away from the first end. This may aid in stabilising the magnet in alignment with the shaft.

[0024] The first coupling assembly may apply a first axial force to the first contact surface towards the second end and the second coupling assembly may apply a second axial force to the further first contact surface towards the first end. The first and / or second axial forces resist the torque induced by the magnet, in use, which can allow the magnet to transmit torque to the shaft in a more efficient manner.

[0025] The interference fit may be a first interference fit, and the second coupling assembly may couple the magnet to the shaft via a second interference fit between the further first contact surface of the magnet and the further second contact surface of the coupling assembly. Use of two interference fits at opposite sides of the magnet can further improve the effectiveness with which the magnet is coupled to the shaft, and thus can improve the efficiency of torque transmission to the shaft.

[0026] The further first contact surface may have a sloped profile relative to the plane and the further second contact surface may have a complementarily sloped profile relative to the further first contact surface. The sloped profile of the first contact surface may be an inverse of the sloped profile of the further first contact surface relative to the plane. This may provide for symmetry of the coupling between the magnet and the first and second coupling assemblies about the plane, which can further aid in securing the magnet to the shaft and / or reducing misalignment between the magnet and the shaft.

[0027] The coupling assembly and / or the shaft may be at least partly threaded. This provides an additional or alternative way in which the coupling assembly and the shaft can be coupled without the use of an adhesive, and may allow the coupling assembly, and hence the magnet, to be more securely connected to the shaft.

[0028] At least one of: a first surface roughness of the first contact surface may be greater than a further magnet surface roughness of at least one further surface of the magnet; and a second surface roughness of the second contact surface may be greater than a further coupling assembly surface roughness of at least one further surface of the coupling assembly. This may be achieved by applying a suitable roughening treatment to the first contact surface and / or the second contact surface, for example. A surface roughness of a given surface (such as the first surface roughness, the further magnet surface roughness, the second surface roughness and / or the further coupling assembly surface roughness) may be taken to correspond to a deviation from an intended or otherwise predefined form for that surface, such as an average deviation (which may be an arithmetic average) in a direction of a normal vector of that surface. Various techniques may be used to measure a surface roughness, such as a profilometer, which may be a contact-type profilometer or an optical profilometer, such as a white light interferometer or a laser scanning confocal microscope. The magnet may have teeth configured to engage with complementary teeth of the coupling assembly. The first contact surface may comprise respective ends of each of the teeth and the second contact surface may comprise respective ends of each of the complementary teeth. The engagement of teeth of the magnet with the complementary teeth of the coupling assembling may assist in transferring torque from the magnet to the shaft, via the coupling assembly. In such cases, the first contact surface may be a conical surface, which may improve centralisation of the magnet with respect to the shaft. In these examples, the first and second contact surfaces may each extend circumferentially about the shaft, and the interference fit may be a circumferential interference fit about the shaft, to further improve the alignment of the magnet with the shaft. In these examples, the alignment may therefore be aided by the circumferential interference fit, with the torque transfer assisted by the engagement of the teeth on the coupling assembly and magnet. The teeth of the magnet may extend in a direction substantially parallel to or parallel to the rotational axis of the shaft to aid in torque transfer, e.g. so that the teeth are positioned along the rotational axis of the shaft. The teeth of the magnet may each be disposed such that the engaging faces of the teeth are perpendicular to the rotational axis (but at different respective positions parallel to the rotational axis). Alternatively, the engaging faces of the teeth may be non-perpendicular to the rotational axis, e.g. in a similar manner to a helical gear. The complementary teeth of the coupling assembly may be disposed at a complementary position and angle to engage with the teeth of the magnet.

[0029] According to a second aspect, there is provided an electric motor comprising the rotor assembly of the first aspect, a stator assembly and an impeller mounted to the rotor assembly.

[0030] According to a third aspect, there is provided an appliance comprising the electric motor of the second aspect. The appliance may for example be a vacuum cleaner, an HVAC (heating, ventilation and air conditioning) unit or a haircare appliance, although other appliances or other machines comprising a rotor assembly or an electric motor in accordance with examples herein are envisaged. For example, the rotor assembly or an electric motor comprising the rotor assembly of examples herein may be used in a motor vehicle, a turbine or in a hand tool.

[0031] BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 is a schematic cross-sectional view of a rotor assembly according to first examples;

[0033] Figure 2 is a schematic cross-sectional view of a rotor assembly according to second examples;

[0034] Figure 3 is a schematic cross-sectional view of a portion of a rotor assembly according to third examples;

[0035] Figure 4 is a schematic cross-sectional view of a portion of a rotor assembly according to fourth examples;

[0036] Figure 5 is a schematic cross-sectional view of a portion of a rotor assembly according to fifth examples;

[0037] Figure 6 is a schematic cross-sectional view a portion of a rotor assembly according to sixth examples;

[0038] Figure 7 is a schematic cross-sectional view of a portion of a rotor assembly according to seventh examples;

[0039] Figure 8 is a schematic cross-sectional view of a rotor assembly according to eighth examples;

[0040] Figure 9 is a schematic side view of the rotor assembly of Figure 8;

[0041] Figure 10 is a schematic cross-sectional view of a rotor assembly according to ninth examples;

[0042] Figure 11 is a schematic side view of a rotor assembly according to tenth examples;

[0043] Figure 12 is a schematic exploded side view of the rotor assembly of Figure 11;

[0044] Figure 13 is a schematic perspective view of an electric motor; and

[0045] Figure 14 is a schematic cross-sectional view a portion of a rotor assembly according to eleventh examples.

[0046] DETAILED DESCRIPTION

[0047] A rotor assembly 102 according to first examples is shown schematically in Figure 1. The rotor assembly 102 comprises a shaft 104, a magnet 106, a first coupling assembly 108 and a second coupling assembly 110. The shaft 104 has a rotational axis 112 about which the shaft 104 rotates, in use. The rotational axis 112 of the shaft 104 coincides with a longitudinal axis of the shaft 104, along which the shaft 104 extends.

[0048] The shaft 104 is mounted through a through-hole 114 of the magnet 106, so that the magnet 106 surrounds the shaft 104. The through-hole 114 of the magnet 106 extends along a central axis of the magnet 106, which in Figure 1 aligns with the rotational axis 112 of the shaft 104. The magnet 106 has an inner surface 116 facing the shaft 104 and an outer surface 118 facing away from the shaft 104. The inner and outer surfaces 116, 118 are flat, planar surfaces that are each substantially parallel to the rotational axis 112 of the shaft 104. The through-hole 114 has a larger diameter than a diameter of the shaft 104 so that the inner surface 116 of the magnet 106 (defining the through-hole 114 of the magnet 106) is spaced apart from the shaft 104. The spacing between the inner surface 116 of the magnet 106 and the shaft 104 can reduce wear or damage to the magnet 106 that may otherwise occur if the magnet 106 comes into contact with the shaft 104, in use.

[0049] The first and second coupling assemblies 108, 110 couple the magnet 106 to the shaft 104 so that, when the shaft 104 rotates, the magnet 106 rotates along with the shaft 104 without substantially moving about a circumference of the shaft 104. The first and second coupling assemblies 108, 110 thus maintain an alignment between the magnet 106 and the shaft 104 over time, so that the rotational axis of the magnet 106 generally coincides with the rotational axis 112 of the shaft 104. The first coupling assembly 108 is disposed at a first end 120 of the magnet 106, which is opposite to a second end 122 of the magnet 106 at which the second coupling assembly 110 is disposed. The second coupling assembly 110 is a mirror image of the first coupling assembly 108 as reflected in a mirror plane lying along a central transverse plane 124 of the magnet 106, which is perpendicular to the rotational axis 110 of the shaft 104 and passes through a central point of the magnet 106.

[0050] The magnet 106 is coupled to the shaft 104 by a first interference fit between a first contact surface 126 of the first end 120 of the magnet 106 and a second contact surface relative to the central transverse plane 124. In Figure 1, the first contact surface 126 is a flat surface that slopes at a constant angle of about 17 degrees with respect to the central transverse plane 124, although this angle is merely an example and other angles are envisaged in other examples, such as angles between 5 and 30 degrees. The first contact surface 126 slopes in a direction away from the shaft 104 so that the first contact surface 126 is tapered away from the rotational axis 110. In this case, the first contact surface 126 is thus a conical surface, which is continuous in a direction circumferentially about the shaft 104. The first interference fit urges the magnet 106 to remain in alignment with the shaft 104 about the rotational axis 104, in use, so that the magnet 106 remains symmetrically disposed about the shaft 104.

[0051] The first contact surface 126 may be formed by casting the magnet 106 in a die with a conical shape or by extruding the material to form the magnet 106 with the first contact surface 126 having a conical shape. In other cases, though, the first contact surface 126 may instead be formed by first producing a cylindrical magnet and then machining an end of the cylindrical magnet at an angle to create the sloped profile of the first contact surface 126.

[0052] The second contact surface 128 of the first coupling assembly 108 also has a sloped profile relative to the central transverse plane 124. The first contact surface 126 has a complementarily sloped profile relative to the second contact surface 126. In Figure 1, the second contact surface 128, like the first contact surface 126, is a flat surface that slopes at a constant angle of about 17 degrees with respect to the central transverse plane 124. However, the second contact surface 128 slopes in the opposite direction to the first contact surface 126 so that the first and second contact surface 126, 128 conform to each other and fit snugly together. The second contact surface 128 in this case slopes in a direction towards the shaft 104 so that the second contact surface 128 is tapered towards the rotational axis 110, rather than away from the rotational axis 110. The second contact surface 128 also extends circumferentially about the shaft 104, and is continuous about the shaft 104. Whereas the first contact surface 126 is a convex conical surface relative to the shaft 104, the second contact surface 128 is a concave conical surface with the same shape as the first contact surface 126 but angled in the opposite direction.

[0053] It is to be appreciated that an angle at which the first and second contact surfaces 126, 128 slope relative to the central transverse plane may differ in other examples. In general, the angle at which the first and second contact surfaces 126, 128 slope may be selected to obtain a desired distribution of pressure along the interference fit between the first and second contact surfaces 126, 128 and / or a desired urging of the rotational axis of the magnet 106 towards the rotational axis 112 of the shaft 104, in use. A larger angle between the sloped profile of the first contact surface 126 and the central transverse axis 124 has been found to improve the maintenance of alignment between the magnet 106 and the shaft 104. In contrast, a smaller angle between the sloped profile of the first contact surface 126 and the central transverse axis 124 has been found to provide a more even distribution of stress over the second contact surface 128.

[0054] The magnet 106 is further coupled to the shaft 104 by a second interference fit between a further first contact surface 130 of the second end 122 of the magnet 106 and a further second contact surface 132 of the second coupling assembly 110. The further first contact surface 130 is opposite to the first contact surface 126 and angled in the opposite direction to the first contact surface 126. Similarly, the further second contact surface 132 is opposite to the second contact surface 128 and angled in the opposite direction to the second contact surface 128. Due to the equal but opposite angles of the first contact surface 126 and the further first contact surface 130, the magnet 106 decreases in size, in a direction parallel to the rotational axis 110, at a constant rate from the inner surface 116 to the outer surface 118 of the magnet 106. Conversely, the first and second coupling assemblies 108, 110 each increase in size, in the direction parallel to the rotational axis 110, at a constant rate outwardly from the shaft 104. The second interference fit further assists in urging the rotational axis of the magnet 106 to coincide with the rotational axis 112 with the shaft 104, over time.

[0055] The strength of the first interference fit may be enhanced by increasing friction between the first contact surface 126 and the second contact surface 128. For example, a roughness of the first contact surface 126 and / or the second contact surface 128 may be increased via a suitable chemical or physical treatment process and / or a high friction material or substrate may be deposited, coated or otherwise applied to the first contact surface 126 and / or the second contact surface 128, which may include a material such as a rubber membrane. Suitable techniques for increasing friction between two surfaces, such as the first contact surface 126 and the second contact surface 128, include electroless nickel plating (ENP), electrospark deposition (ESD) and titanium aluminium nitride (TiAlN) physical vapour deposition (PVD). Alternatively or additionally, the first and / or second contact surface 126, 128 may be treated with a laser, may undergo chemical etching and / or may undergo physical etching to increase surface roughness.

[0056] If the first contact surface 126 and / or the second contact surface 128 has undergone treatment to increase surface roughness, for example, a first surface roughness of the first contact surface 126 may be greater than a further magnet surface of at least one further surface of the magnet 106, such as the inner and outer surfaces 116, 118, and / or a second surface roughness of the second contact surface 128 may be greater than a further coupling assembly surface roughness of at least one further surface of the first coupling assembly 108, such as a surface of the first coupling assembly 108 opposite to the second contact surface 128, facing away from the magnet 106.

[0057] The strength of the second interference fit between the further first contact surface 130 and the further second contact surface 132 may be additionally or alternatively be enhanced using at least one of the above-discussed techniques for increasing the strength of the first interference fit (although it is to be appreciated that at least one different technique may be used for the first and second interference fits, or the same technique(s) may be used for the first and second interference fits).

[0058] The first and further first contact surfaces 126, 130 can be considered to have sloped profiles which are the inverse of each other. The second and further second contact surfaces 128, 132 can also be considered to have sloped profiles which are the inverse of each other (and which are complementary, and the inverse of, the first and further first contact surfaces 126, 130 respectively).

[0059] The magnet 106 is compressed between the first and second ends 120, 122 by the first and second coupling assemblies 108, 110. Thus, the first coupling assembly 108 applies a first axial force to the first contact surface 126 towards the second end 122 and the second coupling assembly 110 applies a second axial force to the further first contact surface 130 towards the first end 120. The first and second axial forces are parallel to the rotational axis 110, but in opposite directions to each other. The first and second axial forces generate a frictional force between the magnet 106 and the first and second coupling assemblies 108, 110 so as to couple the magnet 106 to the first and second coupling assemblies 108, 110 via friction.

[0060] Although not shown in Figure 1, it is to be appreciated that the coupling assembly 108 is itself coupled to the shaft 104, for example via one of the mechanisms shown in Figures 3 to 7 (discussed further below).

[0061] In Figure 1, the first contact surface 126 and the further first contact surface 130 extend along the first and second sides 120, 122 of the magnet 106 respectively, from the inner surface 114 to the outer surface 116. The second contact surface 128 and the further second contact surface 132 similarly extend along sides of the first and second coupling assemblies 108, 110 respectively so that the first interference fit is along an entirety of the first side 120 of the magnet 106 and the first coupling assembly 108 and the second interaction fit is along an entirety of the second side 122 of the magnet 106 and the second coupling assembly 110. The width of the magnet 106 is the same as the width of each of the first and second coupling assemblies 108, 110, perpendicular to the rotational axis 110 and in a direction parallel to the central transverse plane 124. The magnet 106 and the first and second coupling assemblies 108, 110 are aligned with each other and the shaft 104 such that a central longitudinal axis of each of the magnet 106 and the first and second coupling assemblies 108, 110 is aligned with the rotational axis 110 of the shaft 104, and such that the magnet 106 and first and second coupling assemblies 108, 110 together form a continuous, smooth surface facing away from the shaft 104 and an opposing continuous, smooth surface facing towards the shaft 104.

[0062] Figure 2 shows a rotor assembly 202 according to a second example. The rotor assembly 202 of Figure 2 is the same as the rotor assembly 102 of Figure 1, except for the extent of the first and second coupling assemblies 208, 210 relative to the magnet 206. Features of Figure 2 that correspond to features of Figure 1 are labelled with the same reference numerals incremented by 100; corresponding descriptions are to be taken to apply.

[0063] In Figure 2, the first contact surface 226 of the magnet 206 extends along part of the first end 220 of the magnet 206 (from the outer surface 218 to a position partway between the inner surface 216 and the outer surface 218 in a direction parallel to the central transverse axis 224 of the magnet 206), rather than along the entirety of the first end 220 of the magnet 206. Similarly, the further first contact surface 230 of the magnet 206 extends along part of the second end 220 of the magnet 206 (from the outer surface 218 to the position partway between the inner surface 216 and the outer surface 218 in the direction parallel to the central transverse axis 224 of the magnet 206). In Figure 2, the length of the first contact surface 226 and the further first contact surface 230 is the same in the direction parallel to the central transverse axis 224 of the magnet 206, but need not be in other examples. Hence, in Figure 2, the first and second ends 220, 222 of the magnet 206 each comprise respective surfaces, from the inner surface 216 to the position partway between the inner surface 216 and the outer surface 218 in the direction parallel to the central transverse axis 224 of the magnet 206, that are uncontacted by the first and second coupling assemblies 208, 210, and are opposite to each other.

[0064] However, the second contact surface 228 and the further second contact surface 232 of the first and second coupling assemblies 208, 210 each extend along an entirety of a respective side of the first and second coupling assemblies 208, 210 facing the first and second ends 220, 222 of the magnet 206, respectively, as the first and second coupling assemblies 208, 210 are smaller in size than the magnet 206 in the direction parallel to the central transverse axis 224 of the magnet 206. The sides of the first and second coupling assemblies 208, 210 facing away from the shaft 204 are aligned with the outer side 218 of the magnet 206 to form a continuous, smooth surface facing away from the shaft 204.

[0065] Despite the first and second interference fits being over less than all of the first and second ends 220, 222 of the magnet 206, respectively, in this example, the magnet 206 is nevertheless securely coupled to the shaft 204 by the first and second coupling assemblies 208, 210.

[0066] Figures 3 to 7 are schematic cross-sectional views of respective portions of rotor assemblies 302 to 702 according to third to seventh examples. Features of Figures 3 to 7 that correspond to features of Figure 1 are labelled with the same reference numerals incremented by 200 to 600, respectively; corresponding descriptions are to be taken to apply.

[0067] The portion of the rotor assembly 302 illustrated in Figure 3 comprises a portion of a shaft 304, a portion of a magnet 306 (corresponding to the first end 102, 202 of Figures 1 and 2) and a portion of a coupling assembly 308 (corresponding to the first coupling assembly 102, 202 of Figures 1 and 2) coupling the magnet 306 to the shaft 304. The portion of the rotor assembly 302 is shown in an orientation perpendicular to the orientation of the rotor assemblies 102, 202 of Figures 1 and 2. Figures 4 to 7 illustrate similar portions of different respective rotor assemblies 402 to 702 as the portion of the rotor assembly 302 shown in Figure 3. It is to be appreciated that the rotor assemblies 302 to 702 of Figures 3 to 7 may include another coupling assembly, which may be a mirror image of the illustrated coupling assembly 308 to 708, as in Figures 1 and 2, or of a different form or construction to the illustrated coupling assembly 308 to 708, disposed at an opposite end of the magnet 306 to 706 to that shown in Figures 3 to 7. The opposite end of the magnet 306 to 706 to that shown in Figures 3 to 7 may similarly have a shape that is a mirror image of the illustrated portion of the magnet 306 to 706, as in Figures 1 and 2, or may have a different shape. In Figure 3, the magnet 306 comprises a first magnet portion 334 with the same shape as the magnet 106, 206 of Figures 1 and 2. The first magnet portion 334 has a first contact surface 326, which is sloped relative to a plane perpendicular to the rotational axis 312 of the shaft 304, similarly to the first contact surface 126, 226 of Figures 1 and 2. The magnet 306 also comprises a second magnet portion 336 with a first additional contact surface 338. The first additional contact surface 338 is substantially perpendicular to the rotational axis 312 of the shaft 304 rather than being sloped.

[0068] The coupling assembly 308 similarly comprises a first coupling assembly portion 340 with the same shape as the first coupling assembly 108, 208 of Figures 1 and 2. The first coupling assembly portion 340 comprises a second contact surface 328, which is sloped relative to the plane perpendicular to the rotational axis 312 of the shaft 304. The first contact surface 326 is sloped complementarily to the second contact surface 328, similarly to the first contact surface 126, 226 of Figures 1 and 3, and forms an interference fit with the second contact surface 328. The coupling assembly 308 also comprises a second coupling assembly portion 342 with a second additional contact surface 344. The second additional contact surface 344 has a complementary shape to the first additional contact surface 338 and hence in this case is also substantially perpendicular to the rotational axis 312. The first and second additional contact surfaces 338, 344 contact each other in Figure 3 to further assist in coupling the magnet 306 to the coupling assembly 308. However, in other examples, the first and second additional contact surfaces need not contact each other, or may partly contact each other.

[0069] The second contact surface 328 and the second additional contact surface 344 of the coupling assembly 308 are disposed along a side of the coupling assembly 308 facing, and in contact with, the magnet 306, along a side of the magnet 306 (which may be referred to as an end of the magnet 306). The coupling assembly 308 also comprises a shaft contact surface 346 which contacts a surface 348 of the shaft 304 to couple the coupling assembly 308 to the shaft 304. The shaft contact surface 346 is arranged perpendicularly to the second additional contact surface 344, and is the closest surface of the coupling assembly 308 to the shaft 304. The shaft contact surface 346 is a surface of a protrusion 348 of the coupling assembly 308, which protrudes from the second coupling assembly portion 342 towards, and into contact with, the shaft 304 so as to create a gap between the magnet 306 and the shaft 304 while coupling the magnet 306 to the shaft 304.

[0070] In Figure 3, there is a shaft interference fit between the shaft contact surface 346 and the surface 348 of the shaft 304 to assist in coupling the magnet 306 to the shaft 304, via the coupling assembly 308. The shaft contact surface 346 and the surface 348 of the shaft 304 in this case are each generally smooth surfaces. However, one or both of the shaft contact surface 346 and the surface 348 of the shaft 304 may be configured or treated to increase friction therebetween so as to increase the security of the shaft interference fit. For example, a roughness of the shaft contact surface 346 and / or the surface 348 of the shaft 304 may be increased via a suitable chemical or physical treatment process and / or a high friction material or substrate may be deposited, coated or otherwise applied to the shaft contact surface 346 and / or the surface 348 of the shaft 304, which may include a material such as a rubber membrane. For example, any of the techniques discussed above to increase the strength of the first interference fit of Figure 1 may also be used to increase the strength of the shaft interference fit.

[0071] Figure 4 shows a portion of a similar rotor assembly 402 to the rotor assembly 302 of Figure 3. The magnet 406 of Figure 4 is the same as the magnet 306 of Figure 3. However, the coupling assembly 408 of Figure 4 comprises a force transmission element 450 comprising the second contact surface 428 and a fastener 452 comprising the shaft contact surface 446. The force transmission element 450 has the same shape as the first coupling assembly portion 340 of Figure 3 and forms an interference fit between the second contact surface 428 of the force transmission element 450 and the first contact surface 426 of the magnet 406.

[0072] The fastener 452 has a cross-sectional shape of a rectangle with a rectangular cut-out portion in a corner furthest from the shaft 404 and facing the force transmission element 450. A corner of the force transmission element 450 closest to the shaft 404 and facing the fastener 452 is received within the cut-out of the fastener 452 so that the comer of the force transmission element 450 contacts the cut-out of the fastener 452 to couple the force transmission element 450 to the fastener 452.

[0073] In Figure 4, the force transmission element 450 and the fastener 452 comprise different materials than each other. Specifically, the force transmission element 450 comprises a softer material than the fastener 452. For example, the force transmission element 450 may be non-metallic, such as a plastic, whereas the fastener 452 may be or comprise a metal.

[0074] Figure 5 shows a portion of a similar rotor assembly 502 to the rotor assembly 402 of Figure 4. In Figures 3 and 4, the shaft contact surfaces 346, 446 and the surfaces 348, 448 of the shaft 304, 404 are unthreaded surfaces. The rotor assembly 502 of Figure 5 is the same as the rotor assembly 402 of Figure 4 except that the shaft contact surface 546 and the surface 548 of the shaft 504 are each threaded surfaces. In Figure 5, the fastener 552 comprises a metal, to improve coupling between the coupling assembly 508 and the shaft 504, and the force transmission element 550 is non-metallic. Metal to metal contact (such as between the magnet 506 and a metallic component) may create high stress points which can cause cracking in the magnet 506. However, disposing the force transmission element 550 between the fastener 552 and the magnet 506, as in Figure 5, avoids contact between the metallic fastener 552 and the magnet 506, and reduces the risk of the fastener 552 damaging the magnet 506.

[0075] Figure 6 shows a portion of a similar rotor assembly 602 to the rotor assembly 402 of Figure 4. However, the fastener 652 of the coupling assembly 608 of Figure 6 comprises a first element 654 and a second element 656. The first element 654 is disposed between the force transmission element 650 of the coupling assembly 608 and the second element 656. The first element 654 and the second element 656 each contact the shaft 604 to couple the coupling assembly 608, and thereby the magnet 606, to the shaft 604.

[0076] The first element 654 has a first shaft contact surface 658 which contacts a first surface 660 of the shaft 604 and the second element 656 has a second shaft contact surface 662 which contacts a second surface 664 of the shaft 604. A side of the first element 654, perpendicular to the shaft 604, is disposed along and in contact with a side of the second element 656 facing the side of the first element 654 and also perpendicular to the shaft 604. Hence, the first shaft contact surface 658 adjoins the second shaft contact surface 662 to form a continuous shaft contact surface of the coupling assembly 608. The first surface 660 adjoins the second surface 664 to form a continuous surface of the shaft 604 in contact with the shaft contact surface of the coupling assembly 608.

[0077] The first shaft contact surface 658 and the first surface 660 of the shaft 604 are each unthreaded, and therefore correspond to unthreaded portions of the coupling assembly 608 and the shaft 604, respectively. The second shaft contact surface 662 and the second surface 664 of the shaft 604 are each threaded, and therefore correspond to threaded portions of the coupling assembly 608 and the shaft 604, respectively. The threads of the second shaft contact surface 662 mate with corresponding threads of the second surface 664 of the shaft 604 to securely couple the second element 656 to the shaft 604.

[0078] With the second element 656 securely coupled to the shaft 604 via a threaded connection, the risk of the magnet 606 slipping relative to the force transmission element 650, in use, may be mitigated by having a lower friction coefficient between the first element 654 and the second element 656 than between the force transmission element 650 and the magnet 606. The friction coefficient between the first and second elements 654, 656 may be lowered relative to that between the force transmission element 650 and the magnet 606 by, for example, depositing, coating or otherwise applying materials or substrates with a relatively low friction coefficient between the sides of the first element 654 and the second element 656 that contact each other and / or disposing grease or oil between the first and second elements 654, 656, so that slipping would occur between the first and second elements 654, 656 rather than between the force transmission element 650 and the magnet 606, should slipping occur. The friction coefficient between two elements, such as between the first and second elements 654, 656, may be reduced by various techniques including: applying a diamond-like carbon (DLC) coating, applying a thin dense chromium (TDC) and XADC chromium coating (available from Armoloy of Western PA, Inc., 1231 Rodi Road, Turtle Creek, PA 15145, USA) and / or applying a molybdenum disulphide (M0S2) coating to at least one of the two elements.

[0079] Figure 7 shows a portion of a similar rotor assembly 702 to the rotor assembly 402 of Figure 4. However, the rotor assembly 702 of Figure 7 additionally comprises a spring 766, which is an example of a resilient member. The spring 766 applies an axial force to the force transmission element 752 and hence to the magnet 706, via the force transmission element 752. The axial force applied by the spring 766 is parallel to the rotational axis 712 of the shaft 704, to clamp the force transmission element 752 to the magnet 706 or to assist in clamping the force transmission element 752 to the magnet 706. This can improve torque transmission from the magnet 706 to the shaft 704, in use. In further examples, though, the coupling assembly may comprise at least one further component, such as the fasteners 552, 652 of Figures 5 and 6, disposed between a resilient member such as the spring 766 and the force transmission element 752.

[0080] Figures 8 and 9 show a rotor assembly 802 according to an eighth example. The rotor assembly 802 of Figures 8 and 9 is the same as the rotor assembly 402 shown in part in Figure 4, except that the force transmission element 850 of Figures 8 and 9 has a more elongate shape along the rotational axis 810 of the shaft 804 than the force transmission element 450 of Figure 4 and the fastener 852 of Figures 8 and 9 has a greater extent along the rotational axis 810 of the shaft 804 than the fastener 452 of Figure 4. Features of Figures 8 and 9 that correspond to features of Figure 4 are labelled with the same reference numeral incremented by 400; corresponding descriptions are to be taken to apply.

[0081] The rotor assembly 802 of Figures 8 and 9 comprises a first coupling assembly 808, which is the same as the coupling assembly 408 of Figure 4 except for the shape of the force transmission element 850 and the fastener 852, discussed above. As can be seen in Figure 9, the first coupling assembly 808 (comprising the force transmission element 850 and the fastener 852) and the magnet 806 each extend circumferentially about the shaft 804, with the shaft 804 mounted, and extending, through a through-hole of the first coupling assembly 808 and the magnet 806 which is aligned with the rotational axis 812 of the shaft 804. The magnet 806 thus comprises a first contact surface 826 with a sloped profile which forms an interference fit with a second contact surface 828 of the force transmission element 850, which has a complementarily sloped profile.

[0082] The rotor assembly 802 of Figures 8 and 9 comprises a second coupling assembly 810, comprising a force transmission element 868 and a fastener 870, which also extends circumferentially about the shaft 804. The second coupling assembly 810 is the same as the first coupling assembly 808 but is arranged at an opposite side of the magnet 806 to the first coupling assembly 808 and in a mirror-image orientation to the first coupling assembly 808 with respect to a mirror plane perpendicular to the rotational axis 812 of the shaft 804 and comprising a central transverse axis 824 of the magnet 806. Hence, the force transmission element 868 is disposed between the fastener 870 and the magnet 806 and comprises a further second contact surface 872 forming an interference fit with a sloped further first contact surface 830 of the magnet 806 facing the second coupling assembly 810.

[0083] Figure 10 shows a rotor assembly 902 according to a ninth example. The rotor assembly 902 of Figure 10 is the same as the rotor assembly 902 of Figures 8 and 9 except for the shape of the further first and second contact surfaces 930, 972. Features of Figure 10 that correspond to features of Figures 8 and 9 are labelled with the same reference numeral incremented by 100; corresponding descriptions are to be taken to apply.

[0084] In Figure 10, the magnet 906 has one conical end (comprising the first contact surface 926, towards an upper end of Figure 10) and a straight end, opposite to the conical end (comprising the further first contact surface 930, towards a lower end of Figure 10). The further first contact surface 930 is thus a flat surface that lies in a plane parallel to the central transverse axis 924 of the magnet 906 (perpendicular to the rotational axis 912 of the shaft 904).

[0085] The first contact surface 926 of the magnet 906 forms an interference fit with the second contact surface 928 of the force transmission element 950 of the first coupling assembly 908, to couple the magnet 906 to the first coupling assembly 908. The second contact surface 928 has an inverse conical shape, which is complementary to the conical shape of the first contact surface 926. The force transmission element 950 of the first coupling assembly 908 is disposed between the magnet 906 and the fastener 952 of the first coupling assembly 908, which couples the first coupling assembly 908 to the shaft 904.

[0086] The further first contact surface 930 of the magnet 906 contacts the further second contact surface 972 of the force transmission element 968 of the second coupling assembly 910, to assist in transferring force from the magnet 906 to the second coupling assembly 910. The contact between the further first contact surface 930 and the further second contact surface 972 also aids in supporting the magnet 906 in position and maintaining alignment between the magnet 906 and the shaft 904, in use.

[0087] The further second contact surface 972 of the force transmission element 968 of the second coupling assembly 910 is a flat surface lying in a parallel to the central transverse axis 924 of the magnet 906. The force transmission element 968 of the second coupling assembly 910 is disposed between the magnet 906 and the fastener 970 of the second coupling assembly 910, which couples the second coupling assembly 910 to the shaft 904.

[0088] Figures 11 and 12 show a rotor assembly 1002 according to tenth examples. The rotor assembly 1002 of Figures 11 and 12 is the same as the rotor assembly 902 of Figures 8 and 9 except that the first and second coupling assemblies 1008, 1010 of Figures 11 and 12 comprise a plurality of portions, respectively, so that the second contact surface 1028 and the further second contact surface 1072 are each discontinuous, and the fastener 1052 is adapted to retain the portions of the first and second coupling assemblies 1008, 1010. Features of Figures 11 and 12 that correspond to features of Figures 8 and 9 are labelled with the same reference numeral incremented by 200; corresponding descriptions are to be taken to apply.

[0089] In Figures 11 and 12, the force transmission element 1050 is in the form of four portions 1050a-1050d, which may each be considered to be a coupling assembly portion, each comprising a respective part of the second contact surface 1028. The portions 1050a- 1050d are generally elongate along the rotational axis 1012 of the shaft 1004, and are separate portions 1050a-1050d that are disconnected from each other. The portions 1050a-1050d are spaced evenly about the rotational axis 1012 to aid in maintaining alignment between the magnet 1006 and the shaft 1004, in use.

[0090] A first end of each of the portions 1050a-1050d facing the magnet 1006 has a sloped profile which is the same shape as the sloped profile of the force transmission element 850 of Figures 8 and 9, except that the portions 1050a-1050d form a discontinuous second contact surface 1028 about the shaft 1004 rather than the continuous second contact surface 828 formed by the force transmission element 850 of Figures 8 and 9. The force transmission element 1050 may thus be considered to be segmented into segmented conical parts, each comprising a respective part of a conical surface (the second contact surface 828).

[0091] The magnet 1006 has a conical first contact surface 1026, which is continuous about the shaft 1004. The first contact surface 1026 forms a first interference fit with the discontinuous second contact surface 1028, where the second contact surface 1028 contacts the first contact surface 1026 (which is where the portions 1050a-1050d extend towards, and into contact with, the magnet 1006). There are thus portions of the first contact surface 1026 that are uncontacted by the force transmission element 1050, as can be seen in Figure 11.

[0092] The fastener 1052 of Figures 11 and 12 is similar to the fastener 852 of Figures 8 and 9 except that the fastener 1052 includes four cut-out portions (two of which are shown in Figure 12 and labelled 1076a and 1076b), which are each shaped and sized to receive a second end of a respective portion 1050a-1050d of the force transmission element 1050, opposite to the first end of the respective portion 1050a-1050d that faces the magnet 106. The cut-out portions 1076a, 1076b receiving the portions 1050a-1050d of the force transmission element 1050 aids in coupling the force transmission element 1050 to the fastener 1052 and thus assists in transferring torque from the magnet 1006 to the shaft 1004, in use. As in Figures 8 and 9, the second coupling assembly 1010 of Figures 11 and 12 is identical to the first coupling assembly 1008 of Figures 11 and 12 but disposed at an opposite side of the magnet 1006 to the first coupling assembly 1008 and in a mirrorimage orientation to the first coupling assembly 1008 with respect to a mirror plane perpendicular to the rotational axis 1012 of the shaft 1004 and comprising a central transverse axis of the magnet 1006 (which is the same as the central transverse axis 824 shown in Figure 8).

[0093] The second coupling assembly 1010 thus also comprises a force transmission element 1068 disposed between the fastener 1070 and the magnet 1006. The force transmission element 1068 is in the form of four separate portions 1068a-1068d, each comprising a respective part of the further second contact surface 1072, and each disconnected from each other and spaced evenly about the shaft 1004. The further second contact surface 1072 is thus discontinuous. The further second contact surface 1072 forms a second interference fit with a sloped further first contact surface 1030 of the magnet 1006 facing the second coupling assembly 1010. The further first contact surface 1030 is a continuous conical surface. The second interference fit is thus where the portions 1068a-1068d of the force transmission element 1068 of the second coupling assembly 1010 contact the further first contact surface 1030.

[0094] The fastener 1070 of the second coupling assembly 1010 comprises four cut-out portions 1080a-1080d, which each receive a second end of a respective portion 1068a- 1068d of the force transmission element 1068 of the second coupling assembly 1010 opposite a first end of the respective portion 1068a-1068d comprising a respective part of the further second contact surface 1072.

[0095] An electric motor such as the electric motor 1 of Figure 13 may comprise any of the rotor assemblies described herein. In Figure 13, the electric motor 1 has a rotor assembly 2 comprising a magnet (not shown) coupled to a shaft 4 of the electric motor 1 by a coupling assembly in accordance with examples herein (not shown). The electric motor 1 further comprises a stator assembly 6, an impeller 8, and a housing 10. The rotor assembly 2 is positioned relative to the stator assembly 6 such that a portion of the shaft 4 and the magnet of the rotor assembly 2 sit inside an internal channel of the stator assembly 6, which is generally aligned with a rotational axis of the shaft 4. The impeller 8 is mounted to the shaft 4 upstream of the stator assembly 6. The housing houses internal components of the electric motor 1, including the rotor assembly 2, the stator assembly 6, the shaft 4 and the impeller 8.

[0096] In use, a voltage is applied to windings of a magnet of the stator assembly 6 such that the stator assembly 6 generates a magnetic field. The magnetic field interacts with the magnet of the rotor assembly 2, and with appropriate control of the voltage applied to the windings to vary the magnetic field produced by the stator assembly 6, the magnet of the rotor assembly 2, and hence the shaft 4, is caused to rotate within the stator assembly 6. Rotation of the shaft 4 causes rotation of the impeller 8, and rotation of the impeller 8 generates an airflow.

[0097] The electric motor 1 of Figure 13 can thus be used to generate an airflow. For example, the electric motor 1 can be comprised by a vacuum cleaner, an HVAC unit or a haircare appliance to generate an airflow, in use. In other examples, an electric motor comprising a rotor assembly according to examples herein may be comprised by a different appliance and / or used for a different purpose than airflow generation. In such cases, the electric motor may not comprise an impeller.

[0098] Figure 14 is a schematic cross-sectional view of a portion of a rotor assembly 1102 according to eleventh examples. Features of Figure 14 that correspond to features of Figure 3 are labelled with the same reference numerals incremented by 800; corresponding descriptions are to be taken to apply.

[0099] In the first to tenth examples, the second contact surface of the magnet has a sloped profile relative to a plane perpendicular to the rotational axis of the shaft and the first contact surface has a complementarily sloped profile relative to the second contact surface. However, in the example of Figure 14, the magnet 1106 of the rotor assembly 1102 has a straight side 1182 facing the coupling assembly 1108, which straight side 1182 is perpendicular to the rotational axis 1112 of the shaft 1104. The coupling assembly 1108 has a sloped side 1184, with a sloped profile, facing the straight side of the magnet 1106. The coupling assembly 1108 in this example has a conical shape, which forms the sloped side 1184 shown in Figure 14.

[0100] The straight side 1182 of the magnet 1106 comprises the first contact surface 1126, which forms an interference fit with the second contact surface 1128 of the sloped side 1184 of the coupling assembly 1108. The first contact surface 1126 corresponds to an edge of the magnet 1106 at which the straight side 1182 meets an outer surface 1118 of the magnet 1106 facing away from the shaft 1104. The interference fit is thus along a line at which the first contact surface 1126 contacts the sloped second contact surface 1128 of the coupling assembly 1108.

[0101] The first contact surface 1126 need not be precisely aligned with a particular point on the sloped side 1184 of the coupling assembly 1108 to form an interference fit as the sloped side 1184 may provide a relatively large area for the first contact surface 1126 to contact. The second contact surface 1128 may therefore be taken to correspond to a region of the sloped side 1184 of the coupling assembly 1108 that may be contacted by the first contact surface 1126 to couple the magnet 1106 to the coupling assembly 1108. For example, the magnet 1106 may move slightly during use of the rotor assembly 1102, which may cause the first contact surface 1126 of the magnet 1106 to move slightly with respect to the sloped side 1184 of the coupling assembly 1108. However, the first contact surface 1126 may nevertheless remain in contact with a portion of the sloped side 1184 (which may vary over time). In such cases, the second contact surface 1128 may include or otherwise correspond to the portions of the sloped side 1184 which are contacted (or may be contacted) by the first contact surface 126, over time. The sloped second contact surface 1128 may limit movement of the magnet 1106 away from the shaft 1104, in use, so as to maintain alignment between the magnet 1106 and the shaft 1104.

[0102] In Figure 14, the coupling assembly 1108 is coupled to the shaft 1104 via an interference fit between a shaft contact surface 1146 of the coupling assembly 1108 and a surface 1148 of the shaft 1104. In other examples, though, a magnet with the same form as the magnet 1106 of Figure 14 may be coupled to a shaft via a coupling assembly of a different form than that of Figure 14, such as any of the coupling assemblies of other examples described herein, e.g. those shown in Figures 3 to 12.

[0103] It is to be appreciated that the rotor assembly 1102 of Figure 14 may include another coupling assembly, which may be a mirror image of the illustrated coupling assembly 1108, as in Figures 1 and 2, or of a different form or construction to the illustrated coupling assembly 1108, disposed at an opposite end of the magnet 1106 to that shown in Figure 14. The opposite end of the magnet 1106 to that shown in Figure 14 may similarly have a shape that is a mirror image of the illustrated portion of the magnet 1106, as in Figures 1 and 2, or may have a different shape. For example, the magnet 1106 may have a straight side 1182 at one end, and a sloped side, such as a conical side, at an opposite end.

[0104] Further examples are envisaged. In examples above, a magnet is coupled to a shaft via first and second coupling assemblies arranged at opposite ends of the magnet. However, in other cases, a magnet may be coupled to a shaft in a similar manner but via a single coupling assembly (such as by one of the first or second coupling assemblies) or via more than two coupling assemblies. In addition or alternatively, there may be a single interference fit between a shaft and one of a plurality of coupling assemblies.

[0105] Various different magnets, of various different grades, may be used as the magnet of examples herein, such as sintered or bonded magnets.

[0106] It is to be appreciated that features of any of the coupling assemblies described herein may be combined.

Claims

CLAIMS1. A rotor assembly for an electric motor, the rotor assembly comprising: a shaft having a rotational axis; a magnet having a first contact surface; and a coupling assembly coupled to the shaft and having a second contact surface having a sloped profile relative to a plane perpendicular to the rotational axis of the shaft, wherein the coupling assembly couples the magnet to the shaft via an interference fit between the first contact surface of the magnet and the second contact surface of the coupling assembly.

2. The rotor assembly of claim 1, wherein the first contact surface has a complementarily sloped profile relative to the second contact surface.

3. The rotor assembly of claim 1 or claim 2, wherein the first contact surface and the second contact surface each extend circumferentially about the shaft.

4. The rotor assembly of any preceding claim, wherein the first contact surface is a conical surface.

5. The rotor assembly of any preceding claim, wherein the coupling assembly comprises a plurality of coupling assembly portions, each comprising a part of the second contact surface.

6. The rotor assembly of claim 5, wherein the second contact surface is discontinuous.

7. The rotor assembly of claim 6, wherein the first contact surface is continuous.

8. The rotor assembly of any preceding claim, wherein the first contact surface is tapered away from the rotational axis such that a width of the magnet parallel to the rotational axis decreases with increasing distance from the shaft.

9. The rotor assembly of any preceding claim, wherein the coupling assembly comprises at least one shaft contact surface configured to contact a surface of the shaft to couple the magnet to the shaft.

10. The rotor assembly of claim 9, comprising a shaft interference fit between the shaft contact surface and the surface of the shaft.

11. The rotor assembly of claim 9 or claim 10, wherein the coupling assembly comprises a force transmission element comprising the second contact surface and a fastener comprising the shaft contact surface, the force transmission element comprising a different material from the fastener.

12. The rotor assembly of claim 11, wherein the fastener comprises a first element comprising an unthreaded portion of the coupling assembly and a second element comprising a threaded portion of the coupling assembly, the first element disposed between the force transmission element and the second element, wherein the unthreaded portion of the coupling assembly contacts an unthreaded portion of the shaft and the threaded portion of the coupling assembly contacts a threaded portion of the shaft.

13. The rotor assembly of claim 12, wherein a friction coefficient between the first element and the second element is lower than a friction coefficient between the force transmission element and the first element.

14. The rotor assembly of any preceding claim, wherein the coupling assembly comprises a resilient member, and a or the force transmission element of the coupling assembly, comprising the second contact surface, is disposed between the resilient member and the magnet.

15. The rotor assembly of any preceding claim, wherein the coupling assembly is a first coupling assembly, the rotor assembly comprises a second coupling assembly, the magnet has a first end and a second end opposite to the first end, the first end comprises the first contact surface, the second end comprises a further first contact surface, opposite to the first contact surface, and the second coupling assembly comprises a further second contact surface in contact with the further first contact surface to couple the magnet to the shaft.

16. The rotor assembly of claim 15, wherein the first contact surface and / or the further first contact surface is tapered away from the rotational axis such that a or the width of the magnet parallel to the rotational axis decreases with increasing distance from the shaft.

17. The rotor assembly of claim 15 or claim 16, wherein the first coupling assembly applies a first axial force to the first contact surface towards the second end and the second coupling assembly applies a second axial force to the further first contact surface towards the first end.

18. The rotor assembly of any one of claims 15 to 17, wherein the interference fit is a first interference fit, and the second coupling assembly couples the magnet to the shaft via a second interference fit between the further first contact surface of the magnet and the further second contact surface of the coupling assembly.

19. The rotor assembly of claim any one of claims 14 to 17, wherein the further first contact surface has a sloped profile relative to the plane and the further second contact surface has a complementarily sloped profile relative to the further first contact surface.

20. The rotor assembly of claim 19, wherein the sloped profile of the first contact surface is an inverse of the sloped profile of the further first contact surface relative to the plane.

21. The rotor assembly of any preceding claim, wherein the coupling assembly and / or the shaft are at least partly threaded.

22. The rotor assembly of any preceding claim, wherein at least one of: a first surface roughness of the first contact surface is greater than a further magnet surface roughness of at least one further surface of the magnet; and a second surface roughness of the second contact surface is greater than a further coupling assembly surface roughness of at least one further surface of the coupling assembly.

23. An electric motor comprising the rotor assembly of any preceding claim, a stator assembly and an impeller mounted to the rotor assembly.

24. An appliance comprising the electric motor of claim 23, wherein optionally the appliance is any one of a vacuum cleaner, an HVAC unit, and a haircare appliance.

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