Rotor assembly with a common elastically compressible element attached to several fixing screws 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 said sleeve (50) and is screwed into said second mounting hole (32). An elastically compressible element (90) is threaded onto the ends (51) of the N bushings so that the screw heads (41) compress the elastically compressible element (90) against the central part (12) and the bushings against the hub. See Figure 3 for abbreviations.

Description

Title of the invention: Rotor assembly with a common elastically compressible element for several fixing screws 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 being characterized in that the first N mounting holes are of a larger diameter than the corresponding second N mounting holes and The rotor assembly comprises an elastically compressible element, N bushings, and N fixing screws, each having a screw head, each bushing being received in a said first fixing hole such that one end of said bushing protrudes from the central part opposite a said second fixing hole, a said fixing screw extending through said bushing and being screwed into said second fixing hole such that the screw head of said fixing screw bears against the end of said bushing to compress said bushing against the hub, in which the elastically compressible element is threaded onto the ends of the N bushings, and is disposed between the screw heads of the N fixing screws and the central part such that the screw heads compress the elastically compressible element against the central part.

[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 element, and the electromagnetic torque 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 element tends 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 via the compressed elastic element to remain relatively constant, 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 element is simultaneously 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 element makes it possible to 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] 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.

[0011] According to a possible feature of the invention, the rotor comprises a fret circular prestressing to hold the permanent magnets in the housings.

[0012] 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 the compressible elastic element, 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 the elastically compressible element, the compression exerted by the fastening screws is distributed over a larger surface area of ​​the central part than in the absence of the elastically compressible element.Thus, the elastically compressible element reduces the risk of polymer fining, 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 element so that the polymer does not fine at the rotor assembly's maximum operating temperature.

[0013] According to a possible feature of the invention, the elastically compressible element comprises an elastomer block.

[0014] 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.

[0015] According to one possible feature of the invention, the elastically compressible element comprises a spring made of metallic material with a compression coil. 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.

[0016] According to one possible feature of the invention, the rotor assembly comprises a flat washer threaded onto the ends of the N bushings and disposed between the elastically compressible element and the central part, and the screw heads compress the elastically compressible element against the central part via the flat washer. The flat washer tends to prevent the peripheral portion of the elastically compressible element from sinking into the non-magnetic composite material and does not damage it. The flat washer also serves to determine the surface area of ​​the central part that is compressed by the elastically compressible element.

[0017] According to a possible feature of the invention, the flat washer is formed in one piece with the elastic washer.

[0018] 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.

[0019] 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.

[0020] The invention also relates to an axial flux electric machine comprising a rotor assembly as described above.

[0021] The invention also relates to a method for assembling a rotor assembly as described above.

[0022] According to the invention, the assembly process comprises the following steps: - a step of inserting the N sockets into the first N fixing holes; - a step of threading the elastically compressible element, and where applicable the flat washer, onto the N ends of the N sockets; - a step of screwing the N fixing screws into the N second fixing holes through the N bushings, the screwing step leading to the compression of the N bushings against the hub and to the compression of the elastically compressible element against the central part, where appropriate via the flat washer.

[0023] 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 the sockets is available to thread the elastically compressible element onto the sockets, while preventing the elastically compressible element from being pinched between the screw heads and the sockets. Brief description of the figures

[0024] 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:

[0025] [Fig. 1] represents a schematic cross-sectional view of an axial flux electrical machine,

[0026] [Fig.2] represents a perspective view of a rotor,

[0027] [Fig.3] represents a partial schematic cross-sectional view of a rotor assembly according to the invention,

[0028] [Fig.3A] represents a front view of an elastically compressible element of the rotor assembly according to the invention,

[0029] [Fig.3B] shows a cross-sectional view of the elastically compressible element,

[0030] [Fig.4] shows a cross-sectional view of a bushing of the rotor assembly according to the invention,

[0031] [Fig.5A] represents a partial schematic cross-sectional view of a rotor assembly according to the invention during assembly,

[0032] [Fig.5B] represents a partial schematic cross-sectional view of a rotor assembly according to the invention during assembly,

[0033] [Fig.5C] represents a partial schematic cross-sectional view of a rotor assembly according to the invention during assembly,

[0034] [Fig.5D] represents a partial schematic cross-sectional view of a rotor assembly according to the invention during assembly,

[0035] [Fig.5E] represents a partial schematic cross-sectional view of a rotor assembly according to the invention during assembly,

[0036] [Fig.6] illustrates an undesirable situation during the assembly of a rotor assembly according to the invention. Description of method(s) of implementation

[0037] In the figures, and unless otherwise specified, identical elements shall bear the same reference symbols.

[0038] 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.

[0039] A rotor shaft 8 passes through the internal space and is supported by bearings, here by roller bearings.

[0040] 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.

[0041] The composite structure 11 comprises a central part 12 and branches 14 arranged angularly around the central part 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 part 12. The housings 16 comprise permanent magnets 18 which receive the axial magnetic flux generated by the stator windings 6, 7, which drives 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 ring 19 that comprises the rotor 5.

[0042] 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.

[0043] 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 with fixing screws 40, 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.

[0044] 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.

[0045] The hub 30 has second fixing holes 32 corresponding to the first fixing holes 22.

[0046] With reference to [Fig. 3], [Fig. 3A] and [Fig. 3B], the rotor assembly 10 further comprises a spring washer 90. The spring washer 90 comprises a central portion 91, which is flat, and an inclined peripheral portion 99 surrounding the central portion 91. The spring washer 90 is capable of elastically deforming to pass from a rest state represented in [Fig. 3B] to a compressed state represented in [Fig. 3], in response to a compressive force applied to the central portion 91, and is capable of returning to the rest state when the compressive force ceases to be applied to the central portion 91.

[0047] The central portion 91 has a through hole 98 for the passage of the rotor shaft 8, and through holes 92 arranged angularly around the through hole 98.

[0048] Fig. 3 being a partial cross-sectional view, only one first fixing hole 22, one second fixing hole 32, one through hole 92, 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 prestressed fret 19 are not shown in [Fig.3], and [Fig.3] is not necessarily indicative of the dimensions of the elements shown.

[0049] The description below refers to a first mounting hole 22, a second mounting hole 32, a through hole 92, and a mounting screw 40, but is also applicable to mounting holes 22, 32, through holes 92, 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 through holes 92, 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 portion 12 may optionally include additional mounting holes 29, without departing from the scope of the invention.

[0050] Referring to [Fig.3], the first fixing hole 22 is aligned with a second fixing hole 32. 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.

[0051] A fixing screw 40 extends through 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. Advantageously, with reference to [Fig.4], a second end 52 of the sleeve 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.

[0052] The screw head 41 of the fixing screw 40 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 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.

[0053] The first end 51 of the sleeve 50 is inserted into the through hole 92, so that the spring washer 90 is threaded onto the first end 51 of the sleeve 50. The central portion 91 of the spring washer 90 is positioned between the screw head 41 and the central part 12. The spring washer 90 is thus compressed by the screw head 41 against the central part 12.

[0054] With this arrangement, the composite material of the central part 12 is subjected to compression at Cl under the elastic washer 90. 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.

[0055] Preferably, a flat washer 95 is threaded onto the first end 51 of the sleeve 50 and is positioned between the spring washer 90 and the central portion 12. Thus, the spring washer 90 is compressed by the screw head 41 against the central portion 12 via the flat washer 95. The flat washer 95 tends to prevent the peripheral portion 99 of the spring washer 90 from penetrating the non-magnetic composite material and damaging it. The flat washer 95 also serves to determine the surface area of ​​the central portion 12 that is compressed by the spring washer 90.

[0056] It is understood that the spring washer 90 is simultaneously threaded onto the first N ends 51 of the N bushings 50. The spring washer 90 is therefore simultaneously compressed by the N screw heads 41 against the central part 12, where applicable via the flat washer 95 threaded simultaneously onto the first N ends 51 of the N bushings 50.

[0057] In some variants, the flat washer 95 can be formed in one piece with the elastic washer 90.

[0058] In certain embodiments, other types of elastically compressible elements can be used in addition to or as an alternative to the spring washer 90, for example, an elastomer block or a compression coil spring also threaded simultaneously onto the first N ends 51 of the N bushings 50. The spring washer 90 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 90 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.

[0059] We now describe a method for assembling a rotor assembly 10 according to the invention.

[0060] 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 washer 90, if present, of the flat washer 95 onto the first ends 51 of the sockets 50; - a step of screwing the fixing screws 40 into the second fixing holes 32 of the hub 30 through the bushings 50, the screwing step leading to the compression of the bushings 50 against the hub 30 and to the compression of the elastic washer 90 against the central part 12 via the flat washer 95. We thus arrive at the rotor assembly 10 represented on the [Fig.3].

[0061] 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 initial tightening of the fixing screws 40, and Figures 5D and 5E show the subsequent tightening of the fixing screws 40. Tightening the fixing screws 40 brings the screw heads 41 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). The tightening of the fixing screws 40 continues until the bushings 50 are compressed against the hub 30, as shown in Figure 3. Simultaneously, tightening the fixing screws 40 brings the screw heads 41 into contact with the spring washer 90 (see Figure 5E), which is then progressively compressed by the screw heads 41 against the central portion 12 via the flat washer 95.

[0062] 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 washer 90 to be threaded onto the bushings 50, while preventing the spring washer 90 from being pinched between the screw heads 41 and the end surfaces 51C, as indicated by the reference mark Q in [Fig. 6]. Such pinching of the spring washer 90 is undesirable since it partially, or even completely, prevents the spring washer 90 from compressing as shown in [Fig. 3].

Claims

Demands

1. A rotor assembly (10) for an axial flux electric machine (1), the rotor assembly 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 having permanent magnets (18), and the central part (12) having N first mounting holes (22), N being an integer at least equal to 2; and - a hub (30) having N second mounting holes (32) corresponding to the first N mounting holes (22), the rotor assembly (10) being characterized in that the first N mounting holes (22) are of a larger diameter than the corresponding N second mounting holes (32) and the rotor assembly comprises an elastically compressible element (90), N bushings (50), and N mounting screws (40) each having a screw head (41),Each socket (50) is received in a first mounting hole (22) such that one end (51) of the socket (50) protrudes from the central portion (12) opposite a second mounting hole (32). A mounting screw (40) extends through the socket (50) and is screwed into the second mounting hole (32) such that the screw head (41) of the mounting screw (40) bears against the end (51) of the socket (50) to compress the socket (50) against the hub (30). The elastically compressible element (90) is threaded onto the ends (51) of the N sockets (50) and is positioned between the screw heads (41) of the N mounting screws (40) and the central portion (12) such that the screw heads (41) compress the elastically compressible (90) against the central part (12).

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 (90) comprises an elastomer block.

4. Rotor assembly (10) according to any one of claims 1 to 3, wherein the elastically compressible element (90) comprises an elastic washer made of elastic metallic material, in particular spring steel.

5. Rotor assembly (10) according to any one of claims 1 to 4, wherein the rotor assembly (10) comprises a flat washer (95) threaded onto the ends (51) of the N bushings (50) and disposed between the elastically compressible element (90) and the central part (12), and the screw heads (41) compress the elastically compressible element (90) against the central part (12) via the flat washer (95).

6. Rotor assembly (10) according to claim 5 taken in combination with claim 4, wherein the flat washer (95) is formed in one piece with the spring washer.

7. Rotor assembly (10) according to any one of claims 1 to 6, wherein the first N mounting holes (22) are untapped.

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

9. A method for 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 elastically compressible element (90) onto the N ends (51) of the N bushings (50); - a step of screwing the N fixing screws (40) into the second N fixing holes (32) through the N bushings (50), the screwing step resulting in the compression of the N bushings (50) against the hub (30) and the compression of the elastically compressible element (90) against the central part (12).

10. 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 - the screwing step completes by inserting the N sockets (50) into the first N fixing holes (22).

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