Transmission system for aircraft propulsion assembly with the ability to decouple two rotating elements by means of the force produced by a helical gear

Abstract

A decoupling transmission system (2) for an aircraft propulsion unit (1) comprises: a first and a second rotating element; a pinion (6) mounted for rotation about an axis (14); coupling means (40) for coupling the pinion with one of the rotating elements (3); helical meshing means (42) for coupling the pinion with the other rotating element (7); and an actuator (10). The actuator adopts, upon command: a coupling configuration in which it holds the pinion in a coupling position, in which the coupling means and the helical meshing means are engaged; and a disengagement configuration allowing an axial force (F1) induced by the helical meshing means to cause the pinion to move about its axis to a disengagement position, in which the coupling means or the helical meshing means are disengaged. Figure 2

Description

Title of the invention: Transmission system for an aircraft propulsion unit with the capacity to decouple two rotating elements by means of the force produced by a helical mesh Technical field

[0001] The present invention relates to the field of aircraft propulsion systems and more particularly concerns a transmission system with the capacity to decouple two rotating elements.

[0002] Such transmission systems aim in particular to allow efficient transmission of torque between a turbomachine and an electric machine, while allowing decoupling between them in the event of failure, or to prevent failure of the electric motor, or of the equipment, components or systems related to the latter, without requiring the complete shutdown of the turbomachine.

[0003] The main propulsion engines of aircraft generally drive non-propulsive electrical power generating machines, at a moderate speed, typically of the order of a few thousand revolutions per minute. These electrical machines are sometimes subject to failures, particularly of mechanical origin, with relatively high prevalence rates compared to the reliability required for main engines. One way to be able to continue using the propulsion engine is then to mechanically disconnect the electrical machine from the turbomachine.

[0004] With the development of hybrid thermal / electric propulsion systems, it is desirable to develop reliable, lightweight, inexpensive, and easily integrated mechanical decoupling systems, which are also adapted to the specificities of such systems.

[0005] Indeed, in propulsion units comprising a turbogenerator intended to generate propulsive electrical energy, the electrical machines have a power of an order of magnitude comparable to the power of the turbomachine (this nevertheless depending on the number of electrical machines driven by the turbomachine), whereas the non-propulsive electrical machine of a conventional propulsion unit is relatively “small” compared to the thermal engine.

[0006] Furthermore, the mechanical power received by an electric machine within such a propulsion assembly is considerably greater than that received by a non-propulsion electric machine of a conventional propulsion assembly.

[0007] Furthermore, the mass of such a machine is not negligible compared to that of the complete propulsion system, which results in a strong interest in optimizing this mass by the use of high-speed electrical machines (of the order of several tens of thousands of revolutions per minute) and / or by the use of permanent magnet machines, also called "PMG" from the Anglo-Saxon terminology "Permanent Magnet Generators", which have a high power mass density, but for which short-circuit cases cannot be treated simply by de-energizing the rotor.

[0008] Furthermore, transmission systems with decoupling capability also find application within “parallel” hybrid propulsion systems, where power can be selectively supplied by a heat engine and / or by an electric motor to a propulsion unit such as a propeller. When it is desired to use reversible electric machines, in particular to be able to perform functions such as recharging power batteries, it is then advantageous or even necessary to also have decoupling mechanisms between the electric motor-generator and the propulsion unit.

[0009] The invention is the result of technological research aimed at significantly improving the performance of aircraft and, in this sense, contributes to reducing their environmental impact and combating climate change. State of the prior art

[0010] A wide variety of disengageable coupling devices between two rotating elements, generally shafts, are known. These devices essentially consist of: - ramp devices, in which an actuator acts radially on a ramp secured to one of the rotating elements so as to cause an axial displacement of the rotating element causing decoupling between the latter and the other rotating element; - screw devices, which operate on a principle similar to the previous one, the ramp being replaced by a thread and the actuator being shaped to act on the thread and cause axial movement of the rotating element in the manner of a screw-nut effect; - ball devices, in which the connection between the rotating elements is achieved by means of balls capable of being disengaged from their housing to effect decoupling; - freewheel devices that can be disengaged on command; - devices with actuated frangible section, in which the rupture of a section is caused by contact with a friction member controlled for this purpose; - devices with a section that can be broken by thermal fusion; and - axial piston devices, in which a piston is moved axially to cause one of the rotating elements to mechanically decouple from the other rotating element.

[0011] The implementation of ramp devices and screw devices presents a risk of jolts which can limit their ability to operate at high speed, and these devices generally present a risk of premature wear. In addition, the radial size of the actuator of such devices is sometimes unacceptable.

[0012] Ball devices are generally characterized by irreversible operation, preventing any possibility of resetting.

[0013] Devices with frangible sections are destructive and therefore risk damaging nearby components. In addition, these devices cannot be tested.

[0014] Document EP3746669A1 provides an example of a device of the latter type, i.e. with axial piston, in which an annular hydraulic actuator is arranged around an intermediate pinion itself interposed axially between two shafts to be connected. The hydraulic actuator is configured to axially move the intermediate pinion so as to mechanically decouple the latter from one of the shafts, under the effect of fluid pressure.

[0015] Such a device, however, proves to be bulky and expensive due to a relatively large number of moving parts involved in its operation.

[0016] There is thus a need for transmission systems with decoupling capacity which are of limited mass and size.

[0017] There is also a need for transmission systems with decoupling capacity which are as far as possible suitable for coupling rotating elements intended for the transmission of propulsive power (typically having to transmit power levels of the order of several hundred kilowatts) and / or elements rotating at high speeds (typically of the order of several tens of thousands of revolutions per minute), and / or which are suitable for power transmission in both directions of rotation. This latter property is particularly used in the case of electrical machines for generating propulsive power which also act as starters for turbomachines.

[0018] Finally, there is a need for transmission systems with decoupling capacity having a reversible operating mode allowing them to be reset. Presentation of the invention

[0019] The invention aims to at least partially address these needs.

[0020] To this end, it proposes a transmission system with decoupling capacity for an aircraft propulsion unit, comprising: - a first rotating element and a second rotating element; - a coupling pinion mounted to rotate along an axis; - coupling means configured to mechanically couple the coupling pinion with one of the first and second rotating elements with the ability to move relative along the axis; - helical meshing means configured to mechanically couple the coupling pinion with the other of the first and second rotating elements; - an actuator configured to adopt, on command: • a coupling configuration in which an actuating member of the actuator axially biases the coupling pinion with a force sufficient to maintain the coupling pinion in a coupling position, in which the coupling means are engaged and the helical meshing means are engaged, and • a disengagement configuration allowing an axial force induced by the helical meshing means to cause a displacement of the coupling pinion along the axis into a disengagement position, in which the coupling means or the helical meshing means are disengaged.

[0021] The invention thus proposes a transmission system with decoupling capacity, offering advantages in terms of limitation of size and mass, and suitable for the applications mentioned above.

[0022] In preferred embodiments, the actuator is a single-acting cylinder.

[0023] In preferred embodiments, the system comprises a preload spring which axially stresses the actuating member in a direction going from the disengaged position to the coupling position

[0024] In preferred embodiments, the coupling means are straight teeth or straight splines or dogs.

[0025] In preferred embodiments, the helical meshing means are helical splines or helical teeth.

[0026] In preferred embodiments, the first rotating element comprises a shaft mounted for rotation about an axis coinciding with the axis of rotation of the coupling pinion.

[0027] In preferred embodiments, the second rotating element is a return pinion offset from the axis of rotation of the coupling pinion.

[0028] The invention also relates to a propulsion assembly for aircraft, comprising a propulsion member, an electric machine, a turbomachine and a system of transmission of the type described above, wherein the first rotating element of the transmission system is kinematically connected to a rotor of the electric machine while the second rotating element of the transmission system is kinematically connected to a rotor of the turbomachine, and wherein at least one of the electric machine and the turbomachine is configured to transmit propulsive power to the propulsion member.

[0029] In preferred embodiments, the electric machine is configured to receive, from the first rotating element, propulsive power. Brief description of the drawings

[0030] The invention will be better understood, and other details, advantages and characteristics thereof will appear on reading the following description given by way of non-limiting example and with reference to the appended drawings in which:

[0031] [Fig-1] is a schematic axial sectional view of an aircraft propulsion unit comprising an electric machine, a turbomachine and a transmission system with decoupling capacity;

[0032] [Fig.2] is a partial schematic view in axial section of a system of decoupling transmission according to an embodiment of the invention, which may be part of the propulsion assembly of [Fig.l], and shown in a coupling configuration;

[0033] [Fig.3] is a view similar to [Fig.2], in which the transmission system is shown in a decoupled configuration;

[0034] [Fig.4] is a partial schematic view in axial section of an actuator belonging to such a transmission system, according to a variant;

[0035] [Fig.5] is a partial schematic view in axial section of a transmission system with decoupling capability according to another embodiment of the invention, which may be part of the propulsion assembly of [Fig.l], and shown in a decoupling configuration.

[0036] Throughout these figures, identical references may designate identical or similar elements. Detailed disclosure of preferred embodiments

[0037] [Fig.l] very schematically illustrates a propulsion assembly 1 for an aircraft, comprising a transmission system 2 with decoupling capacity. The latter generally comprises a first rotating element 3, a coupling pinion 6, a second rotating element 7, as well as an actuator 10. The transmission system 2 is intended to transmit a rotational movement between the rotating elements 3 and 7 by means of the coupling pinion 6.

[0038] In the embodiment illustrated in more detail in Figures 2 and 3, the first rotating element 3 comprises a shaft 4 and a coupling sleeve 8 integral with each other in rotation, while the second rotating element 7 is a return pinion 26.

[0039] The first rotating element 3 (in this case the shaft 4) is rotatably mounted relative to a stator 16, for example by means of one or more guide bearings 18 (one of which is visible in FIGS. 2 and 3), on a first side C1 of the transmission system. The coupling pinion 6 is rotatably mounted along an axis 14 relative to the stator 16, typically by means of one or more - for example two - guide bearings 20A, 20B, on a second side C2 of the transmission system.

[0040] The axis of rotation of the shaft 4 (or of the rotating element 3) is advantageously coincident with the axis 14 of rotation of the coupling pinion 6.

[0041] In the example illustrated, the second rotating element 7 (in this case the return pinion 26) is arranged offset relative to the axis 14. More precisely, the return pinion 26 is mounted to rotate along an axis 17 distinct from the axis 14.

[0042] According to the terminology used in the present description, the “axial” direction is, for each of the elements 3, 6 and 7, the direction of the axis of rotation 14, 17 of the element. In preferred embodiments, such a direction is parallel or coincident with the axis 14 for each of these elements. Thus, in the example illustrated, the axis 17 of the idler pinion 26 is parallel to the axis 14 of the shaft 4 and the coupling pinion 6. Alternatively, the axis 17 of the idler pinion and the axis 14 of the coupling pinion may be intersecting, provided that the means allowing the mutual coupling of these elements are configured appropriately.

[0043] In embodiments, the first rotating element 3 is kinematically connected to an electric machine rotor, and the second rotating element 7 is kinematically connected to a turbomachine rotor. By “kinematically connected” elements, it is generally understood in the context of the present description that the elements are integral in rotation or form a gear, or form a gear train by being mechanically connected via one or more pinions. The transmission system 2 is thus configured to transmit a rotational movement between the electric machine rotor and the turbomachine rotor.

[0044] In the example illustrated, the shaft 4 is thus secured to the rotor 22 of an electric machine 24, while the idler pinion 26 is engaged with a rotor shaft 28 of a turbine 29 of a turbomachine 30 ([Fig.l]). The idler pinion 26 thus ensures a transmission of rotational movement between the coupling pinion 6 and the rotor 28 of the turbine 29. In view of the above, it will be understood that the shaft 4 may, in other examples, be in direct or indirect mechanical engagement with the rotor 22 of the electric machine 24, that is to say form a gear or a gear train with the latter.

[0045] Furthermore, the propulsion unit 1 illustrated further comprises a propulsion member 32, such as a propeller or a fan, the drive of which relies on energy supplied by at least one of the electric machine 24 and the turbomachine 30 (where appropriate, by the rotor 28 of the turbine 29), to ensure the propulsion of an aircraft. The energy supplied by the electric machine 24 may be electrical or mechanical energy, depending on the type of architecture of the propulsion unit and depending on the role played by the electric machine within this propulsion unit.

[0046] The transmission system 2 is in particular intended to constitute a mechanical propulsion power transmission system within a hybrid aircraft propulsion assembly.

[0047] The propulsion unit 1 illustrated is for example a series hybrid propulsion unit, in which the propulsion member 32 is mechanically engaged with an electric motor 33 supplied with electrical energy by a distribution unit 34, itself supplied with electrical energy in parallel by a power battery 35, on the one hand, and by the electric machine 24 on the other hand. The latter is thus configured to operate as a generator of propulsive electrical energy by converting mechanical energy received from the turbomachine 30 and by supplying the resulting electrical energy to the distribution unit 34. The latter typically comprises DC / AC and AC / DC conversion means. The distribution unit 34 thus also makes it possible to recharge the power battery 35 with surplus electrical power in certain operating phases.In such a case, a transmission system with decoupling capacity similar to the transmission system 2, which will be described in detail below, can, as a variant or in a complementary manner, provide a disengageable mechanical coupling between a shaft of the propulsion member 32 and the rotor of the electric motor 33.

[0048] In other examples, the propulsion assembly 1 may be of the parallel hybrid type, that is to say comprising a transmission system, for example of the type comprising a gear train reducer, configured to mechanically couple to the propulsion member the electric machine, operating selectively as a motor or generator, and the turbomachine, in parallel. In this case, the electric machine is supplied with electrical energy by a power battery when it operates as a motor and recharges said battery when it operates as a generator driven by the turbomachine, via the transmission system.

[0049] The remainder of this description is made with reference to Figures 2 and 3 and with constant reference to [Fig.l] when necessary.

[0050] To enable the transmission of a rotational movement between the rotating elements 3 and 7 by means of the coupling pinion 6, the transmission system 2 comprises coupling means 40, configured to mechanically couple the coupling pinion 6 with one of the first and second rotating elements 3 and 7, for example with the first rotating element 3, and helical meshing means 42 for mechanically coupling the coupling pinion 6 with the other of the first and second rotating elements 3 and 7, for example with the second rotating element 7. These helical meshing means 42 are configured to induce, in operation, an axial force F1 tending to move the coupling pinion 6 along the axis 14 in a direction going from a coupling position ([Fig.2]) towards a disengagement position ([Fig.3]), that is to say in the direction of the second side C2 in the illustrated example.

[0051] Generally speaking, the coupling position is a position in which the coupling means 40 are engaged and the helical meshing means 42 are also engaged, so that the first and second rotating elements 3 and 7 are kinematically connected, while the disengaging position is a position in which the coupling pinion 6 is offset along its axis of rotation, relative to the coupling position, so that: either the coupling means 40 are out of engagement (this is the case in the embodiment of Figures 2 and 3), or the helical meshing means 42 are out of engagement (this is the case in alternative embodiments such as that of [Fig. 5] which will be described below), whereby the kinematic connection between the first and second rotating elements 3 and 7 is broken.It should be noted that the axial force Fl varies increasingly with the speed of rotation of the helical meshing means 42.

[0052] In the embodiment of Figures 2 and 3, the helical meshing means 42 are therefore configured to remain engaged even when the coupling pinion 6 is in the disengaged position. Thus, re-coupling can be envisaged without requiring the helical meshing means 42 to be re-engaged.

[0053] To enable the coupling pinion 6 to be moved on command, the actuator 10 is configured to adopt, on command, a coupling configuration and a disengaging configuration. In the coupling configuration ([Fig. 2]), the actuator 10 axially biases the coupling pinion 6 with a force F2 sufficient to maintain the coupling pinion 6 in its coupling position, despite the axial force F1, whereby the coupling pinion 6 is able to transmit a rotational movement between the first rotating element 3 and the second rotating element 7. In the disengaging configuration ([Fig. 3]), the actuator 10 allows the axial force F1 induced by the helical meshing means 42 to exceed the force F2 (which may be zero in this case) and thus causes the coupling pinion 6 to move along its axis of rotation into its disengaging position, whereby the first rotating element 3 and the second rotating element 7 are then free to move in rotation relative to each other around their respective axes of rotation 14 and 17.

[0054] In the illustrated embodiment, the coupling means 40 are arranged to mechanically couple the coupling pinion 6 to the coupling sleeve 8, and for this purpose comprise straight external splines 40A formed on an external surface 8C of the coupling sleeve 8 and straight internal splines 40B formed on an internal surface 6B of the coupling pinion 6. The relative axial displacement between the coupling pinion 6 and the coupling sleeve 8 is thus made possible without inducing any significant axial force on these elements.

[0055] In addition, the helical meshing means 42 advantageously comprise an external toothing 42A, formed on an external surface 6C of the coupling pinion 6, and a toothing 42B formed on the return pinion 26, engaged with the external toothing 42A of the pinion 6 when the latter is in its coupling position. These teeth 42A and 42B have conjugate helical conformations.

[0056] As explained above, the actuator 10 is intended to axially urge the coupling pinion 6, on command, in order to maintain the latter in its coupling position. The control of the actuator 10 is for example triggered by a control unit 36 ​​receiving signals representative of the state of the propulsion assembly emanating from one or more sensors provided for this purpose.

[0057] The actuator 10 is for example a single-acting hydraulic or pneumatic cylinder, configured to be able to deploy an actuating member 43 in the direction going from the disengaged position towards the coupling position of the pinion 6, that is to say in the direction of the first side C1 in the example illustrated.

[0058] At least a portion of the actuating member 43 extends into a bore 6A of the coupling pinion 6, from one end of the latter, for example located on the second side C2. Furthermore, a rolling bearing 44 is interposed between the aforementioned portion of the actuating member 43 and an internal surface 6B of the coupling pinion 6 so as to axially secure the actuating member 43 and the coupling pinion 6 while allowing free rotation of the coupling pinion 6 relative to the actuating member 43.

[0059] More precisely, the actuator 10 comprises a cylinder body 110 secured to the stator 16 and delimiting a cylinder chamber 112 into which a conduit 114 suitable for fluid circulation opens. This conduit is for example connected to a pump 116 capable of pressurizing a fluid in the conduit 114, and therefore, in the cylinder chamber 112. The pump 116, and / or a valve 117 provided on the conduit 114, is operationally connected to the control unit 36 ​​to be controlled by the latter and thus allow the actuator 10 to be controlled. The conduit 114 and the pump 116 are for example part of a lubrication circuit of the turbomachine, in which case the fluid considered is a lubricating fluid such as oil.

[0060] On the first side C1, or more generally on the side of the coupling position, the cylinder chamber 112 is delimited by a piston 120 axially movable and rigidly connected to the actuating member 43. The actuating member 43, in the general form of a rod, extends beyond the piston 120 in the direction of the first side C1 into the bore 6A of the coupling pinion 6. The piston 120 is for example provided, on its periphery, with a compressible seal 123 in contact with the inner surface of the cylinder body 110 to ensure the sealing of the cylinder chamber 112. The pressure of the fluid contained in the cylinder chamber 112, induced by the pump 116, thus results in a force FP applied to the piston 120, in the direction of the first side C1, or more generally in the direction going from the disengaged position to the coupling position.

[0061] Furthermore, a preload spring 64 may also be interposed between the piston 120 and a bottom of the cylinder chamber 112 so as to urge the piston 120, and therefore the actuating member 43, in the direction of the first side CL. Such a preload spring 64 permanently induces a force FR applied to the piston 120, in the direction of the first side Cl, or more generally in the direction going from the disengaged position to the coupling position. This force FR is therefore added, where appropriate, to the force FP resulting from the pressure of the fluid within the cylinder chamber 112, to define the force F2 applied to the coupling pinion 6 by the actuator 10.

[0062] The rolling bearing 44 is for example held axially in position relative to the coupling pinion 6 by being sandwiched between a shoulder 66 formed in the bore 6A of the coupling pinion 6, on the first side C1 relative to the bearing 44, and a nut 68 screwed into a thread formed on the internal surface 6B of the coupling pinion 6, on the second side C2 relative to the bearing 44. The rolling bearing 44 can thus be assembled to the coupling pinion 6 by bringing the bearing 44 to its final position in the bore 6A, from the opening of the latter located on the second side C2, and then screwing the nut 68 until an adequate tightening of an outer ring of the bearing 44 is obtained.

[0063] In addition, the rolling bearing 44 is for example held axially in position relative to the actuating member 43 by being sandwiched between a shoulder 124 of the actuating member 43, located on the second side C2 relative to the bearing 44, and a nut 126, located on the first side C1 relative to the bearing 44. The actuating member 43 can thus be assembled to the bearing 44 by inserting the member 43 through an inner ring of the bearing 44, in the direction of the first side C1, then by mounting the nut 126 on the member 43 from the first side C1.

[0064] Furthermore, the rotational connection of the coupling sleeve 8 with the shaft 4 is for example ensured by straight grooves 46.

[0065] In operation, as long as the transmission of a rotational movement between the rotating elements 3 and 7 is desired, the actuator 10 is controlled so as to maintain the coupling pinion 6 in its coupling position ([Fig.2]) by means of the force F2.

[0066] In the example illustrated, the pump 116 is thus controlled to maintain the fluid present in the cylinder chamber 112 under pressure so that the resulting force FP is applied to the piston 120.

[0067] Incidentally, the preload spring 64 contributes to maintaining the coupling pinion 6 in its coupling position by adding the force FR to the force FP. This is particularly advantageous at low speeds, in embodiments where the power of the pump 116 depends on the operating speed of the turbomachine, for example in cases where the pump 116 is driven by a rotor of the turbomachine. At low speeds, the axial force F1 induced by the helical meshing means 42 is in fact lower than at high speeds, so that the force FR resulting from the action of the preload spring 64 can then be sufficient to compensate for the axial force F1 induced by the helical meshing means 42.

[0068] A decoupling of the rotating elements 3 and 7 is implemented by controlling the actuator 10 so that it reduces the force F2 applied by the latter to the coupling pinion 6 or so that it ceases to apply such a force F2 to the coupling pinion, so that the coupling pinion 6 moves into its disengaged position ([Fig.3]) under the effect of the axial force F1 induced by the helical meshing means 42, the latter axial force then no longer being compensated by the force F2. In the disengaged position, the coupling means 40 are no longer engaged, so that the rotating elements 3 and 7 are mechanically decoupled.

[0069] Alternatively, the coupling means 40 may be dogs, in particular “curvic” coupling means, also called trapezoidal tooth coupling means or “curvic coupling” in English. These dogs may be formed on respective ends, facing each other, of the coupling pinion 6 and the first rotating element 3.

[0070] Alternatively, the actuator 10 may be an electromechanical linear actuator, for example a solenoid actuator. With reference to [Fig. 4], such an actuator comprises a solenoid 50, electrically controlled by the control unit 36, and fixedly housed in a housing 52 secured to the stator 16. Within the housing 52, the solenoid 50 delimits a central space 54 within which an end portion 43B, for example in the form of a rod, of the actuating member 43 extends.

[0071] Furthermore, in embodiments of the invention, the actuating member may not extend within the bore 6A of the coupling pinion 6 but, on the contrary, be generally in the form of a ring, surrounding a part end of the pinion 6, and connected to the latter by a rolling bearing in a configuration similar to that described above but inverted. Such an actuator may, for example, here again, be a hydraulic cylinder or an electromechanical actuator.

[0072] Furthermore, [Fig. 5] illustrates another embodiment, in which it is the helical meshing means 42 which are configured to be out of engagement when the coupling pinion 6 is in its disengaged position. In such a case, the coupling means 40, for example consisting of straight splines, can remain engaged. Thus, the coupling pinion 6 remains engaged with the first rotating element 3 even in its disengaged position, by means of the coupling means 40, while decoupling takes place between the coupling pinion 6 and the second rotating element 7.

[0073] In other embodiments not illustrated in the figures, the helical meshing means 42 may be arranged to mechanically couple the coupling pinion 6 to the first rotating element 3, if applicable to the coupling sleeve 8, rather than to the second rotating element 7. For this purpose, the helical meshing means 42 are for example helical splines. In such a case, the coupling means 40 are for their part arranged to mechanically couple the coupling pinion 6 to the second rotating element 7. For this purpose, the coupling means 40 are for example straight teeth.

[0074] Generally speaking, the coupling sleeve 8 has the advantage of facilitating the assembly of the transmission system 2 while allowing an extended interface zone between the first rotating element 3 and the coupling pinion 6 in cases where these two elements are coupled to each other by means of splines.

[0075] In embodiments of the invention, the coupling sleeve 8 may nevertheless be omitted, in particular in the case mentioned above where the coupling means 40 are dogs directly coupling the shaft 4 to the coupling pinion 6.

Claims

Claims

1. Transmission system (2) with decoupling capacity for an aircraft propulsion unit (1), comprising: • a first rotating element (3) and a second rotating element (7); • a coupling pinion (6) mounted for rotation about an axis (14); • coupling means (40) configured to mechanically couple the coupling pinion (6) with one (3) of the first and second rotating elements (3, 7) with the ability to move relative about the axis (14); • helical meshing means (42) configured to mechanically couple the coupling pinion (6) with the other (7) of the first and second rotating elements (3, 7);• an actuator (10) configured to adopt, on command: • a coupling configuration in which an actuating member (43) of the actuator (10) axially biases the coupling pinion (6) with a force (F2) sufficient to maintain the coupling pinion (6) in a coupling position, in which the coupling means (40) are engaged and the helical meshing means (42) are engaged, and • a disengagement configuration allowing an axial force (F1) induced by the helical meshing means (42) to cause a displacement of the coupling pinion (6) along the axis (14) into a disengagement position, in which the coupling means (40) or the helical meshing means (42) are disengaged.;

2. A transmission system according to claim 1, wherein the actuator (10) is a single-acting cylinder.

3. A transmission system according to claim 1 or 2, comprising a preload spring (64) which axially biases the actuating member (43) in a direction from the disengaged position to the coupled position.

4. A transmission system according to any one of claims 1 to 3, wherein the coupling means (40) are straight teeth or straight splines or dogs.

5. A transmission system according to any one of claims 1 to 4, wherein the helical meshing means (42) are helical splines or helical teeth.

6. Transmission system according to any one of claims 1 to 5, in which the first rotating element (3) comprises a shaft (4) mounted to rotate about an axis coincident with the axis (14) of rotation of the coupling pinion (6).

7. Transmission system according to any one of claims 1 to 6, in which the second rotating element (7) is a return pinion (26) offset relative to the axis (14) of rotation of the coupling pinion (6).

8. Propulsion assembly (1) for an aircraft, comprising a propulsion member (32), an electric machine (24), a turbomachine (30) and a transmission system (2) according to any one of claims 1 to 7, in which the first rotating element (3) of the transmission system is kinematically connected to a rotor (22) of the electric machine (24) while the second rotating element (7) of the transmission system is kinematically connected to a rotor (28) of the turbomachine (30), and in which at least one of the electric machine (24) and the turbomachine (30) is configured to transmit propulsive power to the propulsion member (32).

9. A propulsion assembly according to claim 8, wherein the electric machine (24) is configured to receive, from the first rotating element (3), propulsive power.

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