Flanged rotor assembly for an axial flux electric machine

Abstract

The invention relates to a rotor assembly (10) for an axial flux electric machine (1), the rotor assembly (10) comprising a rotor (5) and a hub (30). A central portion (12) of the rotor (5) has N first mounting holes (22), and the hub (30) has N second mounting holes (32) corresponding to the first N mounting holes (22). A sleeve (50) is received in a first mounting hole (22) such that one end (51) of said sleeve (50) projects from the central portion (12) opposite a second mounting hole (32). A mounting screw (40) extends through a third mounting hole (82) of a flange (80) and through said sleeve (50), said mounting screw (40) being screwed into said second mounting hole (32). An elastically compressible element (60) is threaded onto the end (51) so that the flange (80) compresses said elastically compressible element (60) against the central part (12).Figure for the abbreviation: Figure 3.

Description

Title of the invention: Flanged rotor assembly for an axial flux electric machine. Technical field

[0001] The present invention relates to a rotor assembly for an axial flux electric machine. Technological background

[0002] An axial flux electric machine comprises at least one stator, and at least one rotor equipped with permanent magnets capable of receiving the axial magnetic flux generated by the stator.

[0003] Document WO 2024 / 022675 A1 relates to an axial flux electric machine in which the rotor comprises a non-magnetic, electrically non-conductive composite structure having housings arranged angularly around a central portion. The housings have permanent magnets. The central portion has a hole for a rotor shaft and is screwed to a hub fixed to the rotor shaft.

[0004] Such an electric machine is satisfactory due to its small size parallel to the axis of rotation of the rotor shaft. However, in several applications, for example as a traction or propulsion motor for a hybrid or hybrid-electric vehicle, the electric machine is not intended to be frequently replaced or maintained. Therefore, it is desirable for the electric machine to have a long service life. Summary of the invention

[0005] The invention relates to a rotor assembly for an axial flux electric machine.

[0006] According to the invention, the rotor assembly comprises: - a rotor, the rotor comprising a structure of non-magnetic, non-electrically conductive composite material which has a central part and housings arranged angularly around the central part, the housings having permanent magnets, and the central part having N initial fixing holes, N being an integer at least equal to 2; and - a hub having N second mounting holes corresponding to the N first mounting holes, The rotor assembly is characterized in that the first N mounting holes have a larger diameter than the second N mounting holes, and the rotor assembly comprises N bushings, N elastically compressible elements, N flat washers, N fixing screws each having a screw head, and a flange having N third fixing holes, each sleeve being received in a first fixing hole such that one end of said sleeve protrudes from the central part opposite a second fixing hole, said fixing screw extending through said third fixing hole and through said sleeve, said fixing screw being screwed into said second fixing hole such that the screw head of said fixing screw is bearing against the flange so that the flange compresses said sleeve against the hub, said elastically compressible element and said flat washer being threaded onto the end of said sleeve, and being disposed between the flange and the central part so that the flange compresses said elastically compressible element against the central part via the flat washer.

[0007] At the central part of the rotor, the non-magnetic composite material is subjected to compression by the fixing screws via the compressed elastic elements, and the electromagnetic torque—the compressive force—is transmitted to the hub by friction between the bushings and the non-magnetic composite material on the hub. These elements fix the central part of the rotor to the hub. However, the rotor assembly is likely to undergo significant temperature variations during the operation of the electric machine, while the non-magnetic composite material and the hub material are likely to have different coefficients of thermal expansion. By compressing more or less depending on the temperature of the rotor assembly, the elastically compressible elements tend to compensate for the differential expansion between the central part and the hub.This allows the compression of the non-magnetic composite material by the fixing screws to remain relatively constant via the compressed elastic elements, and thereby maintains the distribution of compressive forces transmitted to the hub in order to transmit the electromagnetic torque by friction of the bushings and the non-magnetic composite material on the hub, throughout the lifetime of the rotor assembly.

[0008] Furthermore, since the ends of the bushings protrude from the central portion, and the elastically compressible elements are threaded onto these ends of the bushings, it is not necessary to impose very strict dimensional tolerances on the bushings and the central portion. Indeed, the elastically compressible elements compensate for any length variations that might exist between different bushings and / or local variations in the thickness of the central portion.

[0009] In summary, with the invention, the rotor assembly retains its mechanical properties throughout its lifetime while not requiring very strict dimensional tolerances on its components.

[0010] In addition to compressing the bushings and elastically compressible elements, the flange makes it easy to balance the rotor assembly by locally removing material from the flange to bring the center of inertia of the rotor assembly to or closer to the axis of rotation X of the rotor shaft.

[0011] According to one possible feature of the invention, the flange is made of a metal alloy such as steel. The flange may consist of several parts assembled together or be a single piece.

[0012] According to one possible feature of the invention, the hub is made of a metal alloy such as steel. The hub can be formed as a single piece with a rotor shaft of the axial flux electric machine, or it can be attached to this rotor shaft.

[0013] According to a possible feature of the invention, the rotor includes a circular pre-stressed ring to hold the permanent magnets in the housings.

[0014] According to one possible feature of the invention, the non-electrically conductive, non-magnetic composite material is a fiber-reinforced polymer. The polymer then constitutes the matrix of the non-magnetic composite material. The invention is particularly advantageous in this case. Indeed, there is a risk that the polymer will thin due to the compression exerted by the fastening screws in the absence of bushings and compressible elastic elements, which compromises the transmission of electromagnetic torque by friction of the non-magnetic composite material on the hub because of the reduction in compressive force resulting from the thinning. This risk increases with the operating time and temperature of the rotor assembly. However, with elastically compressible elements, the compression exerted by the fastening screws is distributed over a larger surface area of ​​the central part than in the absence of elastically compressible elements.Thus, elastically compressible elements reduce the risk of polymer creep, thereby increasing the lifespan of the rotor assembly. It is even possible to select the surface area of ​​the central portion compressed by the elastically compressible elements so that the polymer does not thin at the rotor assembly's maximum operating temperature.

[0015] According to a possible feature of the invention, the elastically compressible element comprises an elastomer block.

[0016] According to a possible feature of the invention, the elastically compressible element comprises an elastic washer made of elastic metallic material, in particular spring steel, of conical shape.

[0017] According to one possible feature of the invention, the elastically compressible element comprises a compression coil spring made of metallic material. An elastic washer is preferred in the context of the invention because it has a small axial footprint, that is to say a small footprint parallel to the axis of rotation X of the rotor shaft.

[0018] The rotor assembly includes a flat washer threaded onto the end of the sleeve and positioned between the elastically compressible element and the central portion. The screw head compresses the elastically compressible element against the central portion via the flat washer. The flat washer prevents the periphery of the elastically compressible element from penetrating and damaging the non-magnetic composite material. The flat washer also serves to determine the surface area of ​​the central portion that is compressed by the elastically compressible element.

[0019] According to one possible feature of the invention, the first N mounting holes are untapped. Thus, the bushings can be inserted into the first mounting holes without screwing.

[0020] According to one possible feature of the invention, the sleeve has a chamfer opposite said end of the sleeve. The chamfer tends to facilitate the insertion of the sleeve into the first fixing hole, and tends to reduce the risk of the sleeve catching on the walls of the first fixing hole and thus damaging the composite material of the central part.

[0021] According to a possible feature of the invention, in a plane orthogonal to the axis of rotation X of the rotor assembly, the flange has a circular outer contour.

[0022] Thus, the rotor assembly is better balanced in rotation around the X axis.

[0023] According to a possible feature of the invention, the flange is one piece.

[0024] This allows the flange to be manufactured at a moderate cost. Furthermore, the balancing of The rotor assembly is further simplified because: - the flange can be machined without significant difficulty and without fear of having to discard an expensive part, unlike the case where one would machine the central part of the rotor or the hub, for example; - the balance is not affected by the relative positions of the different parts that would make up the flange.

[0025] According to a possible feature of the invention, the flange has N grooves, each groove extending between two adjacent third fixing holes, the grooves opening onto a first face of the flange on which the screw heads are supported, so that the flange has N thinned portions between the N grooves and a second face of the flange opposite the first face.

[0026] The grooves tend to lighten the flange. The thinned portions are advantageously able to deform when the fixing screws are tightened, which makes it possible to compensate for the height differences between bushings due to dimensional tolerances on the bushings, and thereby ensure that the flange is supported on all the bushings.

[0027] The invention also relates to an axial flux electric machine comprising a rotor assembly as described above.

[0028] The invention also relates to a method for assembling a rotor assembly as described above.

[0029] According to the invention, the assembly method comprises the following steps: - a step of inserting the N sockets into the first N fixing holes; - a step of threading the N elastically compressible elements and the N flat washers onto the respective ends of the N sockets; - a step of screwing the N fixing screws into the N second fixing holes through the N third fixing holes and through the N bushings, the screwing step causing the flange to compress the N bushings against the hub and to compress the N elastically compressible elements against the central part via the N flat washers.

[0030] According to a possible feature of the invention: - before the screwing stage, the N sockets are only partially inserted into the first N fixing holes; and - the screwing step completes the insertion of the N sockets into the first N fixing holes. This ensures that sufficient length of bushings is available to thread elastically compressible elements onto the bushings, while preventing elastically compressible elements from being pinched between the flange and the bushings.

[0031] According to a possible feature of the invention, the assembly method further includes a balancing step of the rotor assembly, the balancing step including locally removing material from the flange, for example by machining, in particular by machining one or more blind holes in the flange. Brief description of the figures

[0032] The following description, with reference to the accompanying drawings, given by way of non-limiting examples, will clearly explain what the invention consists of and how it can be implemented. In the accompanying figures:

[0033] [Fig-1] represents a schematic cross-sectional view of an electrical flux machine axial,

[0034] [Fig.2] represents a perspective view of a rotor,

[0035] [Fig.3] represents a partial schematic cross-sectional view of a rotor assembly according to the invention,

[0036] [Fig.4] represents a cross-sectional view of a bushing of the rotor assembly according to the invention,

[0037] [Fig.5A] represents a partial schematic cross-sectional view of a rotor assembly according to the invention during assembly,

[0038] [Fig.5B] represents a partial schematic cross-sectional view of a rotor assembly according to the invention during assembly,

[0039] [Fig.5C] represents a partial schematic cross-sectional view of a rotor assembly according to the invention during assembly,

[0040] [Fig.5D] represents a partial schematic cross-sectional view of a rotor assembly according to the invention during assembly,

[0041] [Fig.5E] represents a partial schematic cross-sectional view of a rotor assembly according to the invention during assembly,

[0042] [Fig.6] illustrates an undesirable situation during the assembly of a rotor assembly according to the invention,

[0043] [Fig.7] represents a top view of the flange of a rotor assembly according to the invention,

[0044] [Fig. 8] shows a partial cross-sectional view of the installed flange with the screws tightened. Description of embodiment(s)

[0045] In the figures, and unless otherwise specified, identical elements shall bear the same reference symbols.

[0046] Referring to [Fig. 1], an axial flux electric machine 1 comprises an external protective housing 2 in the form of a first yoke 3 attached to a second complementary yoke 4, said yokes 3, 4 thus assembled delimiting an internal space in which a rotor 5 and two stators 6, 7 are housed. The rotor 5 is in a central position in the housing 2 and the stators 6, 7 are positioned on either side of the rotor 5.

[0047] A rotor shaft 8 passes through the internal space and is supported by bearings, here by roller bearings.

[0048] Referring to [Fig. 2], the rotor 5 comprises a composite structure 11 made of a non-magnetic, electrically non-conductive composite material. The non-magnetic composite material is, for example, a fiber-reinforced polymer. The polymer is, for example, a polyepoxide or polyether ketone (PEEK). The fibers are, for example, glass fibers.

[0049] The composite structure 11 comprises a central portion 12 and branches 14 arranged angularly around the central portion 12. Two adjacent branches 14 define a housing 16. Thus, the composite structure 11 comprises a plurality of housings 16 arranged angularly around the central portion 12. The housings 16 comprise permanent magnets 18 that receive the axial magnetic flux generated by the stator windings 6, 7, which causes the rotor 5 to rotate about the axis of rotation of the rotor shaft 8. The permanent magnets 18 are held in the housings 16, advantageously by a circular prestressed fret 19 which comprises the rotor 5.

[0050] Still referring to [Fig.2], the central part 12 has a through hole 28 which allows the passage of the rotor shaft 8, and the first fixing holes 22 arranged angularly around the through hole 28.

[0051] Referring now to [Fig. 3], the central part 12 of the rotor 5 is fixed to a hub 30. More specifically, the central part 12 is fixed to the hub 30 by screwing it in place using fixing screws 40 and a flange 80, as will be detailed below. The rotor 5 is thus fixed to the hub 30. The rotor 5 and the hub 30 thus fixed to each other form a rotor assembly 10, which can be assembled separately from the rest of the electrical machine 1.

[0052] The hub 30 is here made of a metal alloy such as steel. The hub 30 can be formed as a single unit with the rotor shaft 8 as shown in [Fig. 3], or it can be attached to the rotor shaft 8. The straight dashed line at the bottom of [Fig. 3] indicates the axis of rotation X of the rotor shaft 8.

[0053] The hub 30 has second fixing holes 32 corresponding to the first fixing holes 22.

[0054] With reference to [Fig.3] and [Fig.7], the flange 80 has a through hole 88 which allows the passage of the rotor shaft 8, and third fixing holes 82 arranged angularly around the through hole 88. The reference R on [Fig.7] designates the distance between the center of the through hole 88 and the respective centers of the third fixing holes 82.

[0055] Fig. 3 being a partial cross-sectional view, only one first fixing hole 22, Only one second fixing hole 32, one third fixing hole 82, and one fixing screw 40 are shown. It is further specified that the arms 14, the housings 16, the permanent magnets 18, and the circular pre-stressed collar 19 are not shown in [Fig. 3], and that [Fig. 3] is not necessarily indicative of the dimensions of the elements shown.

[0056] The description below refers to a first mounting hole 22, a second mounting hole 32, a third mounting hole 82, and a mounting screw 40, but is also applicable to mounting holes 22, 32, 82, and mounting screws 40 that are not shown in [Fig. 3]. Generally, the rotor assembly 10 comprises N first mounting holes 22, N second mounting holes 32, N third mounting holes 82, and N mounting screws 40, where N is an integer at least equal to 2, i.e., N > 2. The first N mounting holes 22 are preferably arranged angularly at regular intervals around the through hole 28. In the example shown in [Fig. 2], N = 10. The central part 12 may possibly include additional fixing holes 29, without going outside the scope of the invention.

[0057] Referring to [Fig.3], the first fixing hole 22 is aligned with a second fixing hole 32 and a third fixing hole 82. A sleeve 50 is received in the first fixing hole 22, so that a first end 51 (cf. [Fig.4]) of the sleeve 50 is projecting out from the central part 12 opposite the second fixing hole 32.

[0058] A fixing screw 40 extends through the third fixing hole 82 and the sleeve 50 and is screwed into the second fixing hole 32. For this purpose, the first fixing hole 22 has a larger diameter than the second fixing hole 32, the sleeve 50 is hollow and has an internal diameter greater than the diameter of the second fixing hole 32, and the second fixing hole 32 is tapped. Conversely, the first fixing hole 22 is advantageously untapped, so that the sleeve 50 can be inserted into the first fixing hole 22 without screwing. Similarly, the third fixing hole 82 is advantageously untapped, so that the fixing screw 40 can be inserted into the third fixing hole 82 without screwing. Advantageously, with reference to [Fig.4], a second end 52 of the socket 50 opposite the first end 51 has a chamfer 59.The chamfer 59 tends to facilitate the insertion of the sleeve 50 into the first fixing hole 22, and tends to reduce the risk of the sleeve 50 catching in the walls of the first fixing hole 22 and thus damaging the composite material of the central part 12.

[0059] The screw head 41 of the fixing screw 40 bears against the flange 80, which itself bears against the first end 51 of the sleeve 50, more precisely against a flat end surface 51C (see [Fig. 4]) formed on the first end 51. The sleeve 50 is thus compressed by the screw head 41, via the flange 80, against the hub 30. More precisely, a flat end surface 52C (see [Fig. 4]) formed on the second end 52 is held in contact with the hub 30.

[0060] In addition, an elastic washer 60 is threaded onto the first end 51 of the sleeve 50, and is disposed between the flange 80 and the central part 12. The elastic washer 60 is thus compressed by the screw head 41, via the flange 80, against the central part 12.

[0061] With this arrangement, the composite material of the central part 12 is subjected to compression at Cl under the elastic washer 60. The compressive force is transmitted at C2 to the hub 30. The electromagnetic torque is transmitted by friction of the sleeve 50 and the composite material of the central part 12 at C2 on the hub 30.

[0062] Preferably, a flat washer 65 is threaded onto the first end 51 of the sleeve 50 and is positioned between the spring washer 60 and the central part 12. Thus, the spring washer 60 is compressed by the screw head 41 against the central part 12 via The flat washer 65 and the flange 80. The flat washer 65 prevents the periphery of the spring washer 60 from sinking into the non-magnetic composite material and damaging it. The flat washer 65 also serves to determine the surface area of ​​the central portion 12 that is compressed by the spring washer 60.

[0063] In some variants, the flat washer 65 can be formed in one piece with the elastic washer 60.

[0064] In certain embodiments, other types of elastically compressible elements may be used in addition to or as an alternative to the spring washer 60, for example, an elastomer block or a compression coil spring. The spring washer 60 is preferred, however, because it has a small axial footprint, i.e., a small footprint parallel to the axis of rotation X of the rotor shaft 8. The spring washer 60 is made of a high-yield-strength metallic material whose mechanical properties do not vary within the operating temperature range of the rotor 5. An elastomer can lose its elastic and stiffness properties if the temperature rises.

[0065] We now describe a method for assembling a rotor assembly 10 according to the invention.

[0066] The assembly process comprises: - a step of inserting the sockets 50 into the first fixing holes 22 of the central part 12; - a step of threading the elastic washers 60 and, if present, the flat washers 65 onto the first ends 51 of the bushings 50; - a step of screwing the fixing screws 40 into the second fixing holes 32 of the hub 30 through the bushings 50 and the third fixing holes 82, the screwing step causing the flange 80 to compress the bushings 50 against the hub 30 and to compress the elastic washers 60 against the central part 12 via the flat washers 65. This leads to the rotor assembly 10 shown in [Fig.3].

[0067] According to an advantageous embodiment of this assembly method: - before the screwing step, the sockets 50 are only partially inserted into the first fixing holes 22, as shown in Figures 5A and 5B; and - the screwing step completes the insertion of the sockets 50 into the first fixing holes 22, as shown in Figures 5C, 5D, and 5E. Figure 5C shows the rotor assembly 10 before the fixing screws 40 are tightened, and Figures 5D and 5E show the tightening of the fixing screws 40. Tightening the fixing screws 40 brings the flange 80 into contact with the bushings 50 (see Figure 5D), and then gradually inserts the bushings 50 into the first fixing holes 22 (see Figure 5E). Tightening the fixing screws 40 continues until the bushings 50 are compressed by the flange 80 against the hub 30, as shown in the [Fig.3]. At the same time, tightening the fixing screws 40 brings the flange 80 into contact with the spring washers 60 (cf. [Fig.5E]), which are progressively compressed by the flange 80 against the central part 12 via the flat washers 65.

[0068] By providing that the bushings 50 are only partially inserted into the first mounting holes 22 before the tightening step of the mounting screws 40, sufficient length of the bushings 50 is ensured to allow the spring washers 60 to be threaded onto the bushings 50, while preventing the spring washers 60 from being pinched between the flange 80 and the end surfaces 51C, as indicated by the reference mark Q in [Fig. 6]. Such pinching of the spring washers 60 is undesirable since it partially, or even completely, prevents the spring washers 60 from compressing as shown in [Fig. 3].

[0069] Referring to [Fig. 7], in a plane orthogonal to the X-axis, the flange 80 preferably has a circular outer contour E centered on the through hole 88, in order to better balance the rotor assembly 10 rotating about the X-axis. Alternatively, the outer contour E could be non-circular, for example square or rectangular. In this case, again in order to better balance the rotor assembly 10 rotating about the X-axis, the outer contour E is preferably symmetrical with respect to at least one axis of symmetry S passing through the center of the through hole 88.

[0070] The assembly method described above advantageously includes a balancing step of the rotor assembly 10, the balancing step comprising locally removing material from the flange 80, for example by machining, in particular by machining one or more blind holes 89 (see [Fig.7]) in the flange 80. It is clear that the flange 80 makes it easy to balance the rotor assembly 10, simply by locally removing material from the flange 80, to bring the center of inertia of the rotor assembly 10 on or closer to the X axis.

[0071] The flange 80 is advantageously a single-piece unit, meaning that the flange 80 is made of a single piece instead of several parts assembled together. For example, the flange 80 is machined from a blank in a metal alloy such as steel. This allows the flange 80 to be manufactured at a moderate cost. Furthermore, balancing the rotor assembly 10 is further facilitated because: - the flange 80 can be machined without significant difficulty and without fear of having to discard an expensive part, unlike the case where one would machine the central part 12 of the rotor 5 or the hub 30, for example; - the balance is not affected by the relative positions of the different parts that would make up flange 80.

[0072] Referring to [Fig. 7], the one-piece flange 80 advantageously comprises N grooves 85. Each groove 85 extends between two adjacent third mounting holes 82. The grooves 85 extend radially, that is to say, the grooves 85 are straight lines along an axis passing through the center of the passage hole 88. In the example shown in [Fig.7], the grooves 85 open into the passage hole 88 and onto the outer contour E. Alternatively, the grooves 85 may not open into the passage hole 88 and / or onto the outer contour E.

[0073] Figure 8 is a partial cross-sectional view of the rotor assembly 10 and shows three slots 85 together with two adjacent third mounting holes 82. The slots 85 open onto the face 81 of the flange 80 against which the screw heads 41 bear. Between the opposite face 83 of the flange 80 and the slots 85, there are thinned portions 86 of the flange 80.

[0074] The thinned portions 86 have a thickness, parallel to the X-axis, that is sufficiently small to allow them to deform during the screwing step. For example, this thickness is approximately 1 mm and the width is 4 to 5 mm. However, the exact dimensions are determined either experimentally or by calculation using the finite element method, for example, with the requirement that the difference in compressive force in the bushings 50, between the most compressed bushing and the least compressed bushing, be less than 10%.

[0075] Due to dimensional tolerances on the bushings 50 and the fact that the flange 80 is a single piece, the flange 80 might only have three contact points with the three bushings 50 that protrude most from the central part 12 (isostaticity problem). These three bushings 50 would then be excessively loaded in compression, while the other bushings 50 would be insufficiently loaded in compression. This would lead to an undesirable situation where the portion of the electromagnetic torque that should be transmitted to the hub 30 by the friction of the N bushings 50 on the hub 30 would only be transmitted by three bushings 50. These three bushings 50 would not be able to transmit by friction the portion of the torque that should be transmitted by the bushings to the hub 30. They would slip and bear against the screws 40.The screws 40 would then be subjected to shear stress, which is not recommended by best practices in mechanical design and would affect the proper functioning of the rotor assembly 10. However, the deformation of the tapered portions 86 (reference FP in [Fig. 8]) during the tightening stage compensates for the height differences between the bushings 50 and thus ensures that the flange 80 bears against all the bushings 50. It should be noted that in [Fig. 8], the height difference between the two bushings 50 and the deformation of the tapered portion 86 in FP have been greatly exaggerated for illustrative purposes. Since the tapered portions 86 are not intended to transmit compressive forces, it is irrelevant whether the tapered portions 86 are deformed elastically or plastically.

Claims

Demands

1. Rotor assembly (10) for an axial flux electric machine (1), the rotor assembly (10) comprising: - a rotor (5), the rotor comprising a structure (11) of electrically non-conductive non-magnetic composite material which has a central part (12) and housings (16) arranged angularly around the central part (12), the housings (16) having permanent magnets (18), and the central part (12) having N first fixing holes (22), N being an integer at least equal to 2; and - a hub (30) having N second fixing holes (32) corresponding to the N first fixing holes (22), the rotor assembly (10) being characterized in that the N first fixing holes (22) are of a larger diameter than the N second fixing holes (32) and the rotor assembly has N bushings (50), N elastically compressible elements (60), N flat washers (65), N fixing screws (40) each having a screw head (41),and a flange (80) having N third fixing holes (82), each sleeve (50) being received in a first fixing hole (22) such that one end (51) of said sleeve (50) protrudes from the central part (12) opposite a second fixing hole (32), said fixing screw (40) extending through said third fixing hole (82) and through said sleeve (50), said fixing screw (40) being screwed into said second fixing hole (32) such that the screw head (41) of said fixing screw (40) bears against the flange (80) so that the flange (80) compresses said sleeve (50) against the hub (30), said elastically compressible element (60) and said flat washer (65) being threaded onto the end (51) of said sleeve (50),and being arranged between the flange (80) and the central part (12) such that the flange (80) compresses said elastically compressible element (60) against the central part (12) via the flat washer (65).

2. Rotor assembly (10) according to claim 1, wherein the non-magnetic, non-electrically conductive composite material is a fiber-reinforced polymer.

3. Rotor assembly (10) according to any one of claims 1 to 2, wherein the elastically compressible element (60) comprises a conical elastic washer made of elastic metallic material, in particular spring steel, or a compression coil spring made of metallic material.

4. Rotor assembly (10) according to any one of claims 1 to 3, wherein, in a plane orthogonal to an axis of rotation (X) of the rotor assembly (10), the flange (80) has a circular outer contour (E).

5. Rotor assembly (10) according to any one of claims 1 to 4, wherein the flange (80) is monobloc.

6. Rotor assembly (10) according to claim 5, wherein the flange (80) has N grooves (85), each groove (85) extending between two adjacent third fixing holes (82), the grooves (85) opening onto a first face (81) of the flange (80) on which the screw heads (41) bear, so that the flange (80) has N thinned portions (86) between the N grooves (85) and a second face (83) of the flange (80) opposite the first face (81).

7. Axial flux electric machine (1) comprising a rotor assembly (10) according to any one of claims 1 to 6.

8. Method of assembling a rotor assembly (10) according to any one of claims 1 to 7, the assembly method being characterized in that it comprises the following steps: - a step of inserting the N bushings (50) into the first N fixing holes (22); - a step of threading the N elastically compressible elements (60) and the N flat washers (65) onto the respective ends (51) of the N bushings (50); - a screwing step of the N fixing screws (40) into the N second fixing holes (32) through the N third fixing holes (82) and through the N bushings (50), the screwing step causing the flange (80) to compress the N bushings (50) against the hub (30) and to compress the N elastically compressible elements (60) against the central part (12) via the N flat washers (65).

9. An assembly method according to claim 9, wherein: - before the screwing step, the N sockets (50) are only partially inserted into the first N fixing holes (22); and

10. - the screwing step completes by inserting the N sockets (50) into the first N fixing holes (22). Assembly method according to any one of claims 8 to 9, wherein the assembly method further comprises a balancing step of the rotor assembly (10), the balancing step comprising locally removing material from the flange (80).

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