System having mechanical connection with two mechanical torque transmission paths

Abstract

The invention relates to a system comprising a connection arrangement (20) having a first shaft (25) and a second shaft (30), the connection arrangement (20) comprising a flywheel (35) interposed between the first shaft (25) and the second shaft (30), the flywheel (35) being capable of transmitting engine torque only from the first shaft (25) to the second shaft (30) when motor mode is enabled. The connection device (20) comprises a coupler (40) provided with a fusible section (45), the connection device (20) comprising a dog clutch connector (50) capable of transmitting engine torque only from the coupler (40) to the first shaft (25) when the power generation mode is enabled.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of FR2310437 filed on September 29, 2023, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0002] The invention relates to a system having a mechanical connection with two mechanical torque transmission paths. For example, a vehicle may be provided with a mechanical device connected to a hybrid device having at least one electric machine connected to the mechanical device via the mechanical connection.

[0003] In particular, the mechanical device may include at least one rotor that is activated by a gearbox to contribute to the lift and / or propulsion of the aircraft. For example, the aircraft may include a gearbox that activates at least one main rotor to ensure at least part of the lift or propulsion or a rotor that helps control yaw movement.

[0004] The hybrid combustion / electric power plant may be connected to a mechanical device to enable its starting, and in particular may be connected to a gearbox according to the examples described above.

[0005] Such a hybrid machine comprises at least one heat engine. The expression "heat engine" refers to a turboshaft engine or in fact all internal combustion engines, in particular piston engines, which can be used in said power machine. The term "heat engine" is used in contrast to the term "electric motor" which describes a motor that consumes only electrical energy.

[0006] The hybrid power plant also includes at least one electric machine that can be operated in electric motor mode, converting electrical energy into mechanical energy to help start the gearbox, but can also be operated in generator mode, converting the extracted mechanical energy into electrical energy that can be fed into the aircraft's electrical grid. Background Art

[0007] Document FR3106570 describes a rotary-wing aircraft comprising at least one thermal engine and a backup electric motor.

[0008] Furthermore, the electric machine can be controlled to operate in electric motor mode or in generator mode, depending on the flight phase.The mechanical energy delivered to the mechanical device in electric motor mode may be different from the mechanical energy that can be extracted during application of generator mode.

[0009] In addition, the motor may have the risk of internal blocking. In order to solve this problem, the motor can be connected to the mechanical device by a mechanical connection device that can separate the mechanical device from the motor. The mechanical connection device can, for example, enable the motor to be movably separated from the mechanical device in the event of a blockage.

[0010] The document EP3693627B1 does not belong to the technical field of the present invention and does not provide a solution to the above problems. In fact, the document EP3693627B1 relates to an optional clutch device between a main drive system and a secondary drive system, wherein the main drive system has a gearbox and a main engine, such as a turboshaft engine. The secondary drive system has a secondary engine, such as a turboshaft engine. The optional clutch device includes an input gear fixed on an outer ring. The flywheel is between the outer ring and the inner ring constrained to rotate with the output gear. The optional clutch device includes a bypass assembly for constraining the outer ring and the inner ring to rotate in a coupling mode and separating the outer ring from the inner ring in rotation in a decoupling mode. The bypass assembly includes a member, which includes a spline engaged with the spline of the inner ring. The bypass assembly also includes an actuator for connecting the spline of the member to the spline of the outer ring as required. In the decoupling mode, the optional clutch device operates in a one-way torque transmission mode, transmitting torque from the input gear to the output gear. In the coupled mode, the selectable clutch device operates in a bidirectional torque transfer mode, transferring torque between the input gear and the output gear through the bypass assembly.

[0011] Document FR3122407 discloses a system for transmitting an engine torque from a primary shaft of an electric machine operating in motor mode to a secondary shaft or another engine torque from a secondary shaft to a primary shaft of an electric machine operating in generator mode. The system comprises two flywheels arranged in anti-phase.

[0012] Document DE102015214266 relates to a mechanical drivetrain for a motor vehicle, which is used to drive the drivetrain using a heat engine and an electric motor. A torque transmission device connects the heat engine and a drive shaft of the drivetrain, to which the electric motor is connected via a dedicated drive device (e.g. via a belt). A dual-mass flywheel is interposed between the heat engine and the torque transmission device. A starting clutch is interposed between the torque transmission device and the electric motor and the drivetrain. The torque transmission device comprises a disconnect clutch, which is provided with two coupling components that can be connected to each other via a forward connector and a flywheel.

[0013] Document US9447734 discloses an aircraft provided with at least one rotor driven by a gearbox, at least one heat engine and at least one electric motor operating in an "electric motor" mode and in an "electricity generation" mode. Each electric motor is mechanically connected to the gearbox via a first connection device and to the gas generator of the heat engine via a second connection device. The electric motor can be mechanically connected to a dedicated input of the gearbox via the first connection device and to the gas generator via the second connection device through the same shaft. Alternatively, the electric motor can be between the free turbine of the heat engine and the gearbox. The first connection device can include a clutch or a lockable flywheel.

[0014] Document DE102022209300 describes a drive system for an electric vehicle, which includes two transmissions and a dog clutch with a flywheel function. The dog clutch includes an inner element with a pulling claw on the end face and an outer element that surrounds the inner element at least partially in the radial direction and has a pushing claw on the end face. The pulling claw and the pushing claw have a wedge-shaped structure that is inclined in opposite directions, and the inner element and the outer element can be moved axially toward a coupling element with a toothed ring gear through their corresponding claws. The inner element transmits torque in a first direction, while the outer element transmits torque in an opposite second direction. If the two elements are inserted simultaneously, torque can be transmitted in two directions like a conventional dog clutch.

[0015] Document US2022 / 136594 describes a powertrain for a motor vehicle, comprising one or more electric motors, a one-way clutch fixedly mounted on a first rotating shaft, a dog clutch mounted to be able to slide on the first rotating shaft and designed to rotate with the first rotating shaft, a clutch ring located between the one-way clutch and the dog clutch, and a second rotating shaft constrained to rotate with the clutch ring. The clutch ring is designed to transmit rotational motion from the second rotating shaft to the first rotating shaft through the one-way clutch and the dog clutch. Summary of the invention

[0016] The object of the present invention is therefore to propose an innovative mechanical connection designed to limit the risk of blocking of the mechanical device in the event of a blockage of the electric motor and to limit the risk of fire in the event of a short circuit of the electric motor.

[0017] The present invention therefore relates to a transmission system provided with a mechanical device connected to a power device, the power device comprising an electric motor capable of operating in a motor mode for starting the mechanical device and in a power generation mode started by the mechanical device, the electric motor being movably connected to the mechanical device via a two-way connection device, the connection device comprising a first shaft movably connected to the electric motor and a second shaft movably connected to the mechanical device, the second shaft being capable of rotating around a rotation axis in a first rotation direction, the connection device comprising a flywheel between the first shaft and the second shaft, which is used to transmit engine torque only from the first shaft to the second shaft when the motor mode is enabled.

[0018] The bidirectional connection comprises a coupler provided with a fusible section, the coupler being constrained to rotate with the second shaft about the rotation axis, the connection comprising a dog clutch connector capable of transmitting engine torque only from the coupler to the first shaft when the generating mode is enabled.

[0019] The engine torque transmitted through the dog clutch connector when the generator mode is enabled may be referred to as generator torque to distinguish it from the engine torque transmitted through the flywheel when the motor mode is enabled.

[0020] The dog clutch connector and flywheel can thus provide two different mechanical torque transfer paths in two different operating modes.

[0021] When the motor mode is enabled, the first shaft is started by the electric motor. The connection device transmits the engine torque from the first shaft to the second shaft through the flywheel due to the size of the dog clutch connector. The dog clutch connector is inactive in this case. Therefore, if the electric motor is blocked, this will not lead to a blocking of the mechanical device due to the flywheel and the fusible link. If the electric motor is blocked, the flywheel will be desynchronized and cause a change in the force transmission, stressing the fusible link. If no action is taken, the fusible link will eventually rupture and separate the electric motor from the mechanical device.

[0022] When the generator mode is enabled, the second shaft is started by the mechanical device. The connection device transmits the engine torque from the second shaft to the first shaft through the dog clutch connector. The flywheel is then desynchronized. If the motor is blocked, the fusible section can be ruptured so as not to hinder the operation of the mechanical device.

[0023] This separation of the mechanical torque transmission path allows the fusible section to be arranged inside the coupler. In fact, the separation of the mechanical torque transmission path enables accurate and easy determination of the size of the fusible section, in particular in the case where the power transmitted through the connection device when the generating mode is activated is less than the power transmitted in the connection device when the motor mode is activated. For example, the connection device can transmit a mechanical power of about 250-300 kilowatts when the motor mode is activated, and a mechanical power of about 30-50 kilowatts when the generating mode is activated.

[0024] Such a connection arrangement may also be compact and / or relatively lightweight and / or may be relatively easily arranged within existing architecture.

[0025] The transmission system comprises, for example, one or more of the following features considered individually or in combination.

[0026] The coupler may include an intermediate shaft provided with a fusible section and constrained to rotate with the second shaft about the rotation axis, the intermediate shaft being connected to the first shaft by a dog clutch connector having at least a non-zero rotational clearance in a rotational direction about the rotation axis.

[0027] The clearance prevents the intermediate shaft from being driven by the first shaft when the motor mode is activated, thereby disconnecting the power transmission path in the connection.

[0028] According to a possibility compatible with the aforementioned possibility, the dog clutch connector may include a plurality of first teeth constrained to rotate together with the first shaft around the rotation axis and a plurality of second teeth constrained to rotate together with the intermediate shaft around the rotation axis, each second tooth being arranged on an arc of a circle centered on the rotation axis between the upstream first tooth and the downstream first tooth and being separated from the upstream first tooth by a gap, the upstream first tooth being located upstream of the downstream first tooth from the perspective of an observer during rotation in the rotation direction around the rotation axis of the first shaft.

[0029] For example, the gap may be angularly defined at an angle between 45° and 60°.

[0030] The coupler is thus simple and compact.

[0031] According to a possibility compatible with the preceding possibility, the intermediate shaft may comprise a first spline engaging with a second spline of the second shaft.

[0032] According to a possibility compatible with the preceding one, the connection device may comprise an elastic return system between the intermediate shaft and the second shaft, the elastic return system urging the coupler into a predetermined coupling position to allow the dog clutch connector to transmit engine torque only from the coupler to the first shaft.

[0033] For example, the return system tends to bring a particular first tooth into contact with a second tooth.

[0034] According to a possibility compatible with the aforementioned possibility, the connection device may include an actuator configured to translate the coupler from a predetermined coupled position allowing the dog clutch connector to transmit engine torque from the coupler to the first shaft to a predetermined decoupled position deactivating the dog clutch connector.

[0035] The actuator may be, for example, a hydraulic, pneumatic or electronic actuator. The actuator may be, for example, a jack. The actuator prevents the transmission of engine torque from the second shaft to the first shaft for safety reasons, in particular in the event of a short circuit of the motor when the generator mode is activated. The actuator may also be used to check the operation of the connection device during a test before use.

[0036] The actuator can be controlled by the controller, for example, to move the coupler to the uncoupling position in the event of a blockage or short circuit in the motor. To this end, the system may include at least one sensor device for detecting this condition, such as a current sensor and / or a rotation speed sensor.

[0037] According to a possibility compatible with the aforementioned possibility, the power plant may comprise at least one heat engine kinematically connected to the mechanical device.

[0038] The invention also relates to an aircraft comprising the transmission system.

[0039] For example, the mechanical device comprises a gearbox which activates at least one lift and / or thrust rotor. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The invention and its advantages appear in more detail in the context of the following description of embodiments given by way of example and with reference to the accompanying drawings, in which:

[0041] Figure 1 is a schematic diagram showing a connection device provided with an actuator according to the present invention;

[0042] Figure 2 yes Figure 1 3D view of the dog clutch connector;

[0043] Figure 3 is a schematic diagram showing a connection device according to the invention, for example arranged in a transmission system such as that arranged in an aircraft;

[0044] Figure 4 is a schematic diagram showing a connection device according to the invention provided with an actuator in a coupled position; and

[0045] Figure 5 is a schematic diagram showing a connection device according to the invention provided with an actuator in a decoupled position. DETAILED DESCRIPTION

[0046] Elements that appear in more than one view have the same reference numeral in each view.

[0047] Figure 1 A bidirectional connection 20 is shown. The connection 20 comprises a first shaft 25 and a second shaft 30 rotatable about the same axis of rotation AXROT. The term "shaft" denotes a mechanical member transmitting power and rotational moment in the form of torque, which member can comprise one or more components.

[0048] The connection device 20 may be provided in any system requiring bidirectional transmission of power between two components.

[0049] According to the example described, the first shaft 25 can be movably connected to the electric motor 9 of the transmission system 3 .

[0050] For example, the first shaft 25 may thus be constrained to rotate about the axis of rotation AXROT together with the first gear wheel 26 connected by a mechanical link to the motor 9. Figure 1 In the illustrated embodiment, the electric motor 9 comprises a transmission shaft 10 fixed to an input / output gear 11 engaged with a first gear 26 .

[0051] According to another aspect, the first shaft 25 may be carried by the structure 200 .

[0052] The electric machine 9 is configured to operate in a motor mode MODMOT for starting the first shaft 25 and in a generator mode MODGEN for starting via the first shaft 25 .

[0053] The motor 9 is controlled in a conventional manner, for example, by a computer or a human-machine interface. The transmission system may include at least one sensing device (sensor) that emits a signal carrying information related to the operation of the motor 9. For example, the system includes a current sensing device that measures the current in the motor 9, a speed sensing device that measures the rotational speed of internal components of the motor 9, and so on.

[0054] Furthermore, the second shaft 30 may be kinematically linked / connected to the mechanism 4 of the transmission system 3 in a conventional manner.

[0055] According to another aspect, the connection device 20 is provided with a flywheel 35 between the first shaft 25 and the second shaft 30. The flywheel 35 is configured to transmit the engine torque from the first shaft 25 to the second shaft 30 only when the first shaft rotates around the rotation axis AXROT in a direction called "rotation direction" when the motor mode is activated. The flywheel is then synchronized. When the motor mode is activated, the first shaft 25 thus represents the driving shaft that rotates the second shaft 30 forming the driven shaft through the flywheel 35.

[0056] The flywheel 35 may be a conventional flywheel having a ramp or a pawl, for example. For example, the flywheel 35 may include an outer ring provided with a ramp and constrained to rotate around the rotation axis AXROT together with the first shaft 25, an inner ring constrained to rotate around the rotation axis AXROT together with the second shaft 30, and a rolling member provided between the outer ring and the inner ring.

[0057] Thus, when the flywheel 35 is synchronized, the first shaft 25 rotates the second shaft 30 in the rotational direction about the rotation axis AXROT.

[0058] However, if the second shaft 30 rotates faster in the rotational direction than the first shaft 25 , the flywheel 35 is desynchronized.

[0059] In order to rotate the first shaft 25 together with the second shaft 30 around the rotation axis AXROT and always in the rotation direction when the power generation mode is enabled, the connection device 20 includes a coupler 40. The term "coupler" refers to a device configured to transmit engine torque only from the second shaft 30 to the first shaft 25 when the power generation mode is enabled, which is different from the flywheel 35.

[0060] To this end, the connection device 20 has a dog clutch connector 50 which is configured to transmit the engine torque only from the coupling 40 to the first shaft 25 when the power generation mode is activated.

[0061] Furthermore, the coupler is provided with a fusible section 45 for separating the first shaft 25 from the second shaft 30 in the event of a blockage of the first shaft 25 .

[0062] According to one possibility, the coupler 40 has an intermediate shaft 55 provided with a fusible section 45. The fusible section 45 is formed by locally reducing the thickness of the intermediate shaft 55, for example.

[0063] The intermediate shaft 55 can be constrained to rotate together with the second shaft 30 about the axis of rotation AXROT. According to the example shown, the intermediate shaft 55 comprises first splines 56 engaging with the second splines 31 of the second shaft 30 .

[0064] Furthermore, the purpose of the dog clutch connector 50 is to constrain the intermediate shaft 55 and the first shaft 25 to rotate about the axis of rotation AXROT only when the generating mode is enabled and not in the event of a fault. Figure 2 The dog clutch connector 50 has at least a non-zero functional clearance 95 when rotating about the rotation axis AXROT in the first rotation direction 96 so that, at least in the absence of a fault, the first shaft 25 and the second shaft 30 cannot be constrained from rotating when the motor mode is enabled.

[0065] In particular, the dog clutch connector 50 includes a plurality of first teeth 60 constrained to rotate about the rotation axis AXROT with the first shaft 25. The first teeth can be arranged on a circle centered on the rotation axis AXROT, for example, equally spaced apart. Each first tooth 60 can extend from one end of the first shaft 25 parallel to the rotation axis AXROT.

[0066] The dog clutch connector 50 further includes a plurality of second teeth 65 constrained to rotate with the intermediate shaft 55 about the rotation axis AXROT. The second teeth 65 may be arranged on the circle centered on the rotation axis AXROT, for example, equally spaced apart. Each second tooth 65 may extend radially from the intermediate shaft 55 or from a disk 66 fixed to the intermediate shaft 55 relative to the rotation axis AXROT.

[0067] Each second tooth 65 is also arranged on an arc of a circle centered on the rotation axis AXROT between the upstream first tooth 601 and the downstream first tooth 602. The expressions "upstream tooth" and "downstream tooth" are to be understood from the perspective of an observer 97 when the first shaft 25 rotates in the rotation direction 96 about the rotation axis AXROT.

[0068] Each second tooth 65 is therefore spaced apart from the corresponding upstream first tooth 601 by a gap 95 sufficient to prevent the second tooth 65 from meshing with the associated upstream first tooth 601 when the motor mode is activated.

[0069] In addition, refer again Figure 1, the connection device 20 may comprise an elastic return system 70 between the intermediate shaft 55 and the second shaft 30 for positioning the first tooth 60 and the second tooth 65 on the same circle centered on the axis of rotation AXROT, when necessary.

[0070] The elastic return system 70 is therefore configured to urge the coupler 40 into the predetermined coupled position POSCOUP to allow the dog clutch connector 50 to transmit engine torque only from the coupler 40 to the first shaft 25 .

[0071] The elastic return system 70 may include, for example, a spring 71 or the like.

[0072] according to Figure 1 In the example shown, the spring 71 may include a ring that contacts or is fastened to the inner disk 57 fixed to the intermediate shaft 55 and a ring that contacts or is fastened to the connecting disk 73. The connecting disk 73 may be constrained to move in translation parallel to the axis of rotation AXROT together with the second shaft 30. For example, the connecting disk 73 is formed by a shoulder of a locking pin 72 passing through the spring 71. The locking pin 72 may be provided with a fastening end 75 that passes through the fastening disk 32 fixed to the second shaft 30. The nut 74 may be tightened on the fastening end 75 to compress the fastening disk 32.

[0073] According to another aspect, the connection device 20 may include an actuator 80 configured to move the coupler 40 translationally parallel to the axis of rotation AXROT from the coupling position POSCOUP to a predetermined decoupling position POSDECOUP deactivating the dog clutch connector 50 and vice versa. In the decoupling position POSDECOUP, the first tooth 60 and the second tooth 65 are no longer located on the same circle centered on the axis of rotation AXROT.

[0074] The actuator 80 may be controlled by a controller 82, which may include a human-machine interface and / or an autonomous computer that may be linked to the aforementioned sensing devices for monitoring the system. For example, the computer is configured to send a signal to the actuator for placing the dog clutch connector 50 in the decoupled position POSDECOUP in the event of a blockage or short circuit of the motor or when the motor mode is enabled.

[0075] The actuator 80 shown is a hydraulic actuator including a conventional hydraulic network which is not shown here and is within the capabilities of those skilled in the art. Alternatively, the actuator 50 may be an electric or pneumatic jack, for example.

[0076] Figure 3-5 The operation of a connection device 20 according to the invention and an example of how it may be arranged in a transmission system 3 are shown.

[0077] The transmission system 3 is provided with a mechanical device 4 connected to a power device 6 .

[0078] The power plant 6 includes an electric machine 9 which is operable in a motor mode for starting the mechanical device 4 and in a generator mode started by the mechanical device 4 .

[0079] The motor 9 is kinematically connected to the mechanical device 4 via a connection device 20. In particular, a first shaft 25 is kinematically connected to the motor 9 and a second shaft 30 is kinematically connected to the mechanical device 4.

[0080] The power plant 6 may comprise at least one heat engine 7 connected to the mechanical device 4 in a conventional manner, possibly via a flywheel 8 .

[0081] The transmission system 3 may be arranged in the aircraft 1. In particular, the mechanical device 4 may comprise a gearbox 5 actuating at least one lift and / or thrust rotor 2. The gearbox 5 may then be connected to the second shaft 30 or the heat engine 7.

[0082] Figure 3 In particular, the connection device 20 is shown without an actuator.

[0083] When the motor mode is enabled, the electric machine 9 rotates the first shaft 25 about the axis of rotation AXROT. The connection device 20 transmits mechanical power only through the flywheel 35 from the first shaft 25 to the second shaft 30 rotating in the direction of rotation 96, as shown by the dashed arrow. The gap 95 prevents the transmission of power through the coupler 40. In addition, the second shaft 30 helps to start the gearbox 5 with or without the cooperation of the heat engine 7. In the event of a blockage of the electric machine 9, the fusible section can rupture and separate the electric machine 9 from the mechanical device.

[0084] When the generating mode is enabled, the second shaft 30 is rotated by the mechanical device 4. According to the example shown, the gearbox 5 is activated by the thermal engine 7 and rotates the second shaft 30 about the axis of rotation AXROT. The connection device 20 transmits mechanical power from the second shaft 30 to the first shaft 25 only through the coupling 40, as indicated by the solid arrow. The first shaft 25 then rotates the input / output shaft 10 of the electric machine 9.

[0085] In the event of a blockage of the motor 9, the fusible section 45 can break, thereby not blocking the mechanical device 4. The use of a different power transmission path used in motor mode allows the fusible section 45 to be installed.

[0086] Figure 4 and 5 The embodiment shows a device provided with an actuator 80. Figure 3 A variation of the connecting device 20 is provided.

[0087] refer to Figure 4 , the actuator 80 can position the coupler 40 in the coupling position POSCOUP. The connecting device 20 is then connected to the Figure 3Operate in the same way.

[0088] When commanded to do so by controller 82, actuator 80 may position coupler 40 in a Figure 5 Decoupling position POSDECOUP is shown. The coupler 40 is moved in translation parallel to the axis of rotation AXROT to deactivate the dog clutch connector 50. The elastic return system 70 is compressed. The power generation mode is then deactivated for safety reasons.

[0089] Of course, the present invention can have multiple variations in its implementation. Although multiple embodiments are described above, it should be easily understood that it is impossible to determine all feasible embodiments in detail. Of course, any device described can be replaced with an equivalent device without exceeding the scope of the present invention and claims.

Claims

1. A transmission system (3) provided with a mechanical device (4) connected to a power device (6), the power device (6) comprising an electric motor (9) capable of operating in a motor mode for starting the mechanical device (4) and in a generator mode started by the mechanical device (4), the electric motor (9) being movably connected to the mechanical device (4) via a two-way connection device (20), the connection device (20) comprising a first shaft (25) movably connected to the electric motor (9) and a second shaft (30) movably connected to the mechanical device (4), the second shaft (30) being rotatable in a first rotational direction (96) about a rotation axis (AXROT), the connection device (20) comprising a flywheel (35) between the first shaft (25) and the second shaft (30), for transmitting engine torque only from the first shaft (25) to the second shaft (30) when the motor mode is activated, The bidirectional connection device (20) comprises a coupler (40) provided with a fusible section (45), the coupler (40) being constrained to rotate together with the second shaft (30) around the rotation axis (AXROT), and the connection device (20) comprises a dog clutch connector (50) capable of transmitting engine torque only from the coupler (40) to the first shaft (25) when the power generation mode is enabled.

2. The transmission system according to claim 1, wherein: The coupler (40) comprises an intermediate shaft (55) provided with the fusible section (45) and constrained to rotate together with the second shaft (30) about the rotation axis (AXROT), the intermediate shaft (55) being connected to the first shaft (25) via the dog clutch connector (50), the dog clutch connector (50) having at least a non-zero rotational clearance (95) about the rotation axis (AXROT) in the rotation direction (96).

3. The transmission system according to claim 2, wherein: The dog clutch connector (50) includes a plurality of first teeth (60) constrained to rotate together with the first shaft (25) around the rotation axis (AXROT) and a plurality of second teeth (65) constrained to rotate together with the intermediate shaft (55) around the rotation axis (AXROT), each of the second teeth (65) being arranged between an upstream first tooth (601) and a downstream first tooth (602) on an arc of a circle centered on the rotation axis (AXROT) and separated from the upstream first tooth (601) by the gap (95), the upstream first tooth (601) being located upstream of the downstream first tooth (602) from the perspective of an observer (97) during rotation around the rotation axis (AXROT) of the first shaft (25) in the rotation direction.

4. The transmission system according to claim 2, wherein: The intermediate shaft (55) includes a first spline (36) which engages with a second spline (56) of the second shaft (30).

5. The transmission system according to claim 2, comprising an elastic return system (70) between the intermediate shaft (55) and the second shaft (30), the elastic return system (70) urging the coupler (40) into a predetermined coupling position (POSCOUP) to allow the dog clutch connector (50) to transmit engine torque only from the coupler (40) to the first shaft (25).

6. The transmission system according to claim 1, wherein: The connection device (20) includes an actuator (80) configured to translate the coupler (40) from a predetermined coupled position (POSCOUP) that allows the dog clutch connector (50) to transfer engine torque from the coupler (40) to the first shaft (25) to a predetermined decoupled position (POSDECOUP) that deactivates the dog clutch connector (50).

7. The transmission system according to claim 1, wherein: The power plant (6) comprises at least one heat engine (7) which is movably connected to the mechanical device (4).

8. An aircraft (1), wherein: The aircraft (1) comprises a transmission system (3) according to claim 1.

9. The aircraft according to claim 8, wherein: The mechanical device (4) comprises a gearbox (5) which activates at least one lift and / or thrust rotor (2).

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