System for coupling between the input shaft and the sun gear of an epicyclic gear train
Patent Information
- Application Number
- EP2024719233
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2023-03-16
- Filing Date
- 2024-03-14
- Publication Date
- 2026-01-21
AI Technical Summary
Existing coupling systems for aircraft turbomachines face challenges in balancing flexibility to prevent overload on epicyclic gear train components while avoiding resonance, especially as turbomachines are miniaturized to reduce consumption and increase reduction ratio, leading to increased misalignment issues and potential damage.
A coupling system with primary and secondary flexible coupling members that allow longitudinal and radial movements, absorbing misalignments and movements to prevent overload on the input pinion, while ensuring the resonance frequency is outside the operating range, comprising flexible annular structures and longitudinal shaft seals to manage these movements effectively.
The system effectively limits loads on the input pinion, preventing excessive wear and ensuring proper alignment without inducing resonance, thus enhancing the durability and efficiency of the turbomachine components.
Smart Images
Figure FR2024050305_19092024_PF_FP_ABST
Abstract
Description
[0001] TITLE: COUPLING SYSTEM BETWEEN THE INPUT SHAFT AND THE SOLATOR OF AN EPICYLCOIDAL GEAR TRAIN
[0002] FIELD OF THE INVENTION
[0003] The present invention relates to couplings between the input pinions of mechanical reducers and the input shafts of such reducers. It relates more particularly to couplings intended for mechanical reducers of aircraft propulsion systems.
[0004] TECHNOLOGICAL BACKGROUND
[0005] There are various types of aircraft propulsion systems, including in particular dual-flow turbomachines, composed of a primary body of the gas turbine type and a fan driven by a turbine shaft of the primary body. New generations of these dual-flow turbomachines, particularly those with a high bypass ratio, generally include a mechanical reducer to drive the fan shaft. Usually, the reducer is intended to transform the relatively fast rotation speed of the turbine shaft into a slower rotation speed for the fan shaft.
[0006] Most often, the reducers used in such turbomachines are epicyclic geartrain reducers comprising a central pinion, called a sun gear, a ring gear coaxial with the sun gear, surrounding the latter, and pinions, called satellites, which are meshed between the sun gear and the ring gear and are held by a frame called a planet carrier. Conventionally, the sun gear is rotated by the turbine shaft and the fan shaft is rotated by the planet carrier (when the reducer is in epicyclic configuration, or "planetary" in English) or by the ring gear (when the reducer is in planetary configuration, or "star" in English).
[0007] For optimal operation of such a turbomachine, the various elements that are connected by a shaft must be perfectly aligned with each other and with said shaft. Excessive misalignment of a shaft relative to the elements to which it is connected can cause damage to the shaft itself, which may lead to breakage of the latter, or to the elements connected to the shaft which may wear quickly, in particular due to friction. However, since a certain degree of misalignment, which may be horizontal and / or vertical, and axial displacements at the level of a shaft are inevitable, it is known to position one or more flexible coupling devices on the shaft.Thus, it is known to position on the turbine shaft, between the upstream bearing and the reducer, flexible coupling devices intended to prevent misalignment between the upstream bearing and the reducer from causing damage or wear to the shaft, the low pressure turbine and / or the reducer, in particular the solar one.
[0008] These flexible coupling devices must meet opposing constraints. On the one hand, they must be flexible enough not to induce excessive overload at the level of the solar toothing. On the other hand, they must be stiff enough not to enter into resonance in the operating range of the turbomachine.
[0009] However, in order to reduce the consumption of such turbomachines, we seek to minimize the size of the reducers while maximizing their reduction ratio, which leads to a reduction in the load level that can be supported by solar energy. Under these conditions, it becomes increasingly difficult to size flexible coupling devices so that they can satisfy the aforementioned double constraint.
[0010] STATEMENT OF THE INVENTION
[0011] An objective of the invention is to enable a gas turbine shaft to be coupled to a reducer, in particular an epicyclic gear train, by means of a coupling which reduces the loads transmitted to the input pinion while having a resonant frequency which is outside the operating range of the gas turbine.
[0012] To this end, the invention has as its first subject a coupling system comprising an input shaft extending along a longitudinal axis, an output shaft and a reduction gear coupling the input shaft to the output shaft, the reduction gear comprising an input pinion driven in rotation by the input shaft and at least partially surrounding the input shaft, in which the coupling system further comprises a device for coupling the input shaft to the input pinion, configured to allow longitudinal movement of the input shaft relative to the input pinion, the coupling device comprising two primary flexible coupling members arranged axially on either side of the input pinion, each primary flexible coupling member forming a connection between the input pinion and the input shaft, said connection allowing longitudinal translations and rotations about radial axes perpendicular to the longitudinal axis.
[0013] The coupling system according to the first object of the invention makes it possible to limit the impact of longitudinal displacements of the input shaft on the reducer. Indeed, during operation of the coupling system, the longitudinal displacements are at least partly absorbed by the coupling device and are therefore not fully transmitted to the input pinion. The loads supported by the input pinion are thus limited, which prevents excessive wear of the latter.
[0014] This primary coupling device makes it possible to decouple the management of longitudinal displacements from the management of misalignments. Thus, any other coupling members intended for the management of misalignments can be stiffened so that their resonance frequency is located outside the operating range of the propulsion system in which the coupling system is intended to be integrated. Thus, the coupling system does not risk entering into resonance during the operation of said propulsion system.
[0015] According to particular embodiments of the invention which can be taken alone or in combination:
[0016] - the connection between the input pinion and the input shaft may in particular be a connection allowing at most only longitudinal translations and rotations around axes perpendicular to the longitudinal axis;
[0017] - each primary flexible coupling member may comprise a flexible annular structure interposed between an inner ring connected to one of the input pinion and the input shaft and an outer ring connected to the other of the input pinion and the input shaft; thus, the input pinion can move radially and longitudinally relative to the input shaft 54 and these movements are transmitted to the rings connected to the input pinion, then to the flexible ring which absorbs at least part of these movements; the movements of the input pinion relative to the flexible annular structure and therefore to the input shaft are then limited, which avoids any overload on the input pinion and therefore avoids having to increase the size of the pinion;
[0018] - the flexible annular structure can, for example, be formed from a solid circular metal membrane pierced in its center; thus, each primary flexible coupling member allows good transmission of torque around the longitudinal axis and has good resistance over time;
[0019] - the inner ring is rigidly connected to one of the input gear and the input shaft, and the outer ring is rigidly connected to the other of the input gear and the input shaft; the coupling device thus has a relative flexibility offering a compromise between sufficient flexibility to avoid inducing excessive overloads on the input gear and limited flexibility to avoid entering into resonance in the operating range of the propulsion system in which the coupling system is intended to be placed;
[0020] - the input shaft may be formed of several sections, including a proximal section around which the input pinion is arranged, a distal section and an intermediate section interposed between the distal and proximal sections, the input shaft may further comprise a first and a second secondary flexible coupling members connecting the intermediate section respectively to the distal section and to the proximal section, each secondary flexible coupling member allowing longitudinal translations and rotations around axes perpendicular to the longitudinal axis; the secondary flexible coupling members make it possible to absorb part of the misalignments and / or displacements of the input shaft and are thus complementary to the primary flexible coupling members;the misalignments and / or displacements of the input shaft being compensated by all the flexible coupling members, the flexibility of each coupling member can be limited so as to prevent them from entering into resonance in the operating range of the propulsion system in which the coupling system is placed;
[0021] - each of the second secondary flexible coupling members only allows longitudinal translations and rotations around axes perpendicular to the longitudinal axis;
[0022] - the input shaft may be formed from a single piece; the secondary flexible coupling members are then, for example, one-piece coupling members and may be formed in particular by portions of enlarged diameter of the input shaft;
[0023] - each secondary flexible coupling member is a longitudinal shaft seal which may be a double-diaphragm or multi-diaphragm longitudinal shaft seal; and
[0024] - the reducer may be an epicyclic gear reducer comprising a sun gear constituting the input pinion, a crown surrounding the sun gear and a plurality of satellites, carried by a planet carrier, meshing with the sun gear and the crown gear, one of the crown gear and the planet carrier being fixed and the output shaft being driven in rotation by the other of the crown gear and the planet carrier.
[0025] The second subject of the invention is an aircraft propulsion system comprising a gas turbine, a fan and a coupling system according to the first subject, the input shaft being driven in rotation by a turbine rotor of the gas turbine and the output shaft being movable in rotation together with a fan rotor.
[0026] A third object of the invention is an aircraft comprising the propulsion system according to the second object. BRIEF DESCRIPTION OF THE FIGURES
[0027] Other characteristics and advantages of the invention will appear on reading the description which follows, given solely by way of example and made with reference to the appended drawings, in which:
[0028] [Fig. 1] Figure 1 is a schematic top view of an aircraft according to one embodiment of the invention;
[0029] [Fig. 2] Figure 2 is a schematic partial sectional view of a propulsion system of the aircraft of Figure 1;
[0030] [Fig. 3] Figure 3 is a schematic view of a reducer of a coupling system of the propulsion system of Figure 2;
[0031] [Fig. 4] Figure 4 is a partial schematic perspective view of a coupling system of the propulsion system of Figure 2 according to a first embodiment of the invention;
[0032] [Fig. 5] Figure 5 is a partial schematic and sectional view of the coupling system of Figure 4;
[0033] [Fig. 6] Figure 6 is a partial schematic and sectional view of a coupling system of the propulsion system of Figure 2 according to a second embodiment of the invention;
[0034] [Fig. 7] Figure 7 is a partial schematic and sectional view of a coupling system of the propulsion system of Figure 2 according to a third embodiment of the invention.
[0035] DETAILED DESCRIPTION OF AN EXAMPLE OF IMPLEMENTATION
[0036] The aircraft 1 shown in Figure 1 includes propulsion systems 10 for propelling it.
[0037] In the example shown, the aircraft 1 is an airplane. This comprises, in a conventional manner, a fuselage 4, a tailplane 6 and two wings 8. The propulsion systems 10 are here two in number and are each housed under a respective wing 8. As a variant (not shown), the propulsion systems 10 are arranged along the fuselage 4, for example near the tailplane 6. As a further variant (also not shown), the aircraft 1 comprises a single propulsion system 10 or at least three propulsion systems 10.
[0038] One of the propulsion systems 10 is shown in Figure 2.
[0039] In Figure 2, the propulsion system 1 has a main direction extending along a longitudinal axis X. The propulsion system 1 comprises a fan section 2 and a primary body 3, often called a “gas generator”. The fan section 2 comprises a fan 22 and a fan casing 12. The fan 22 comprises a fan rotor 9. The fan casing 12 surrounds the fan rotor 9. The fan rotor 9 is rotatably mounted relative to the fan casing 12.
[0040] The fan rotor 9 comprises a fan hub 13 and fan blades 14 extending radially from the hub 13. The fan blades 1 may be fixed relative to the fan hub 13 or have a variable pitch. In the latter case, each of the fan blades 14 is pivotally mounted relative to the fan hub 13 along a pitch axis and is connected to a pitch change mechanism mounted in the propulsion system 1. The pitch change mechanism makes it possible to adjust the pitch angle of the fan blades 14 according to the flight phases.
[0041] In this example, the fan section 2 also includes a fan stator 16 fixedly mounted on the fan casing 12. The fan stator 16 includes fixed vanes 17 generally referred to as “outlet guide vanes” (or “OGV”).
[0042] Alternatively, the outlet blades 17 could have a variable pitch. If necessary, and similarly to the fan blades 14 of the fan rotor 9, the root of the outlet blades 17 is pivotally mounted along a pitch axis and is connected to a pitch change mechanism (not shown), the pitch being adjusted according to the flight phases by the pitch change mechanism.
[0043] The primary body 3 comprises a compressor section 18, a combustion chamber 6 and a turbine section 19.
[0044] Compressor section 18 includes a low pressure compressor 4 and a high pressure compressor 5.
[0045] The turbine section 19 comprises a high pressure turbine 7 and a low pressure turbine 8.
[0046] The propulsion system 1 comprises a low pressure shaft 54 connecting the low pressure turbine 4 to the low pressure compressor 8, the low pressure shaft 54 being rotatably mounted relative to a casing 26 of the primary body 3 around the longitudinal axis X.
[0047] When the propulsion system 1 is in operation, the low pressure turbine 8 drives the low pressure compressor 4 in rotation via the low pressure shaft 54.
[0048] The propulsion system 1 further comprises a high pressure shaft 10 connecting the high pressure turbine 7 to the high pressure compressor 5, the high pressure shaft 10 being rotatably mounted relative to the casing 26 around the longitudinal axis X. The high pressure shaft 10 is coaxial with the low pressure shaft 54 and extends around the low pressure shaft 54.
[0049] When the propulsion system 1 is in operation, the high-pressure turbine 7 rotates the low-pressure compressor 4 via the low-pressure shaft 54. When the propulsion system is in operation, an air flow F entering the propulsion system 1 passes through the fan 22 and is then divided between a primary air flow F1 and a secondary air flow F2, which circulate from upstream to downstream in the propulsion system 1.
[0050] The secondary air flow F2, also called the "bypass air flow", flows in a secondary vein, around the primary body 3. The secondary air flow F2 allows the periphery of the primary body 3 to be cooled and is used to generate the majority of the thrust provided by the propulsion system 1.
[0051] The primary air flow F1 flows in a primary vein 29 inside the primary body 3, passing successively through the compressor section 18 (low pressure compressor 4 and high pressure compressor 5), the combustion chamber 6 where it is mixed with fuel to serve as an oxidizer, and the turbine section 19 (high pressure turbine 7 and low pressure turbine 8).
[0052] Furthermore, the turbomachine 10 comprises a coupling system 50 by which the low pressure turbine 8 drives the fan 22 in rotation around a longitudinal axis X.
[0053] The coupling system 50 comprises a reducer 52 for coupling an input shaft, which is the low pressure shaft 54, with an output shaft 56 which is the shaft driving the fan 22.
[0054] With reference to Figure 3, the reducer 52 comprises an input pinion 58 which is driven in rotation by the input shaft 54 and which at least partially surrounds the latter.
[0055] Preferably, the reducer 52 is an epicyclic gear reducer comprising a sun gear constituting the input pinion 58, a crown 53 surrounding the sun gear 58 and a plurality of satellites 53', carried by a planet carrier, meshing with the sun gear 58 and the crown. One of the crown and the planet carrier is fixed and the output shaft 56 is rotated by the other of the crown and the planet carrier. In the example shown, the crown 53 is fixed and the output shaft 56 is therefore driven by the planet carrier. In a variant not shown, the planet carrier is fixed and the output shaft 56 is then driven by the crown 53.
[0056] With reference to Figures 4 to 7, the coupling system 50 further comprises a primary coupling device 60 of the input shaft 54 to the input pinion 58.
[0057] This coupling device 60 is configured to allow longitudinal and / or radial displacement of the input pinion 58 relative to the input shaft 54. Thus, the coupling device 60 can absorb at least part of the unavoidable axial and / or radial displacements of the input pinion 58 relative to the input shaft 54 during operation of the propulsion system 10. The loads supported by the input pinion 58 due to its displacements relative to the input shaft 54 are thus limited, which prevents excessive wear of the input pinion. The input pinion 58 can thus remain correctly aligned with the other pinions of the reducer without creating a load and / or wear on these gears.
[0058] The coupling device 60 comprises two primary flexible coupling members 62a and 62b arranged on either side of the input pinion 58. In particular, a first primary flexible coupling member 62a is arranged upstream of the input pinion 58 and a second primary flexible coupling member 62b is arranged downstream of the input pinion 58.
[0059] The two primary flexible coupling members 62a and 62b together provide a sliding finger ball joint type connection between the input pinion 58 and the input shaft 54, said connection allowing longitudinal translations and rotations around radial axes perpendicular to the longitudinal axis.
[0060] In this way, at least part of the longitudinal displacements are absorbed thanks to the flexibility of the primary flexible coupling members 62a and 62b and are not transmitted to the input pinion 58 which therefore undergoes less load than in the systems of the prior art. In particular, the two primary flexible coupling members 62a and 62b deform under the effect of the displacements of the pinion 58 relative to the input shaft 54 so that the input pinion 58 undergoes less load and therefore less risk of wear.
[0061] The two flexible coupling members 62a, 62b are in particular positioned around the input shaft 54 near the input pinion 58, in particular inside the reducer 52. In particular, as shown in FIG. 3, the reducer comprises an enclosure 52' in which the input pinion 58, the satellites 53' and the crown 53 are housed. The enclosure 52' comprises an inlet 55 through which the input shaft 54 passes and an outlet 57 through which the output shaft 56 emerges. The two flexible coupling members 62a, 62b are located between the inlet 55 and the outlet 57.
[0062] As shown in Figure 5, each primary flexible coupling member 62a, b comprises a flexible annular structure 66a, b interposed between an inner ring 70a, b and an outer ring 64a, b. The inner ring 70a, 70b is here connected to the input gear 58 and the outer ring 64a, 64b is here connected to the input shaft 54. Alternatively, the inner ring is connected to the input shaft 54 and the outer ring is connected to the input gear 58.
[0063] The flexible annular structure 66a, b, by its flexibility, makes it possible to limit the transmission of the movements of the input pinion 58 to the input shaft 54. Indeed, the longitudinal and / or radial movements of the input pinion 58 which are transmitted to the inner ring 70a, b are absorbed, at least partially, by the flexible annular structure 66a, b and the transmission of these movements to the outer ring 64a, b, and therefore to the input shaft, is thus limited, or even avoided. Each annular structure 66a, b is in particular a flexible annular membrane, in particular a solid circular metal membrane pierced in its center, extending between the inner ring 70a, b and the outer ring 64a, b, said membrane having flexibility along the longitudinal axis X. Each annular structure 66a, b thus allows good transmission of the torque between the inner ring 70a, b and outer ring 64a, b and has good resistance over time.
[0064] Each inner ring 70a, b is rigidly connected to the input gear 58. For example, each inner ring 70a, b tightly surrounds the input gear 58 and is fixed thereto for example by means of screws.
[0065] Furthermore, each outer ring 64a, b is connected to the input shaft 54 by an annular disc 68a, b.
[0066] Each outer ring 64a, b allows an empty space 67a, b to be maintained between the flexible annular membrane 66a, b and the annular disc 68a, b, which contributes to the flexibility of the primary flexible coupling members 62a, b. The empty spaces 67a and 67b are connected by an empty zone 71 which allows the input shaft 54 and the input pinion 58 not to be in contact with each other. Thus, the drive of the input pinion 58 by the input shaft 54 is only done via the primary flexible coupling members 62a and 62b.
[0067] Here, the coupling system 50 further comprises a secondary flexible coupling device 80 comprising at least one secondary flexible coupling member 82 (Fig. 5) or 82a, 82b (Fig. 6). The input shaft 54 is then formed of several sections, including a proximal section 74 around which the input pinion 58 is arranged, these sections being connected by the secondary flexible coupling member 82, 82a, 82b. The or each secondary flexible coupling member 82, 82a, 82b forms, between two consecutive sections of the input shaft 54, a connection which allows longitudinal translations and rotations around radial axes perpendicular to the longitudinal axis X.
[0068] For this purpose, the or each secondary flexible coupling member 82, 82a and 82b, also called fleeter, is typically formed by a longitudinal shaft seal, in particular a double membrane seal. The or each secondary flexible coupling member may also be formed by another type of longitudinal shaft seal such as for example a longitudinal shaft seal with multiple membranes, or a coupling seal with metal membranes.
[0069] In a first embodiment, shown in Figures 4 and 5, the secondary flexible coupling device 80 comprises a single secondary flexible coupling member 82 interposed between the proximal section 74 and a distal section 78 of the input shaft 54. The annular disc 68a, b of each primary flexible coupling member 62a, b is then made of a flexible annular membrane similar to that forming the annular structures 66a, b. The primary flexible coupling members 62a, 62b then form, in cooperation with the secondary flexible coupling member 82, a relatively homokinetic coupling connection between the input pinion 58 and the distal section 78 of the input shaft 54 which makes it possible to absorb at least part of the misalignments of the input pinion 58 which are therefore not fully transmitted to the distal section 78.
[0070] In a second and a third embodiment, shown in Figures 6 and 7, the secondary flexible coupling device 80 comprises several secondary flexible coupling members 82a, 82a. The input shaft 54 then comprises, in addition to the distal 78 and proximal 74 sections, an intermediate section 76 interposed between said distal 78 and proximal 74 sections. This intermediate section 76 is connected to the distal section 78 by a first 82a of the secondary flexible coupling members and to the proximal section 74 by a second 82b of the secondary flexible coupling members.
[0071] Furthermore, each outer ring 64a, b of the primary flexible coupling members 62a, 62b is rigidly connected to the input shaft. For this purpose, each disc 68a, b is rigid, and in particular has a rigidity greater than that of each flexible annular membrane 66a, b. The coupling device 60 thus has a relative flexibility offering a compromise between sufficient flexibility avoiding inducing excessive overloads on the input pinion 58 but limited flexibility to avoid entering into resonance in the operating range of the propulsion system 10.
[0072] The secondary flexible coupling members 82a and 82b thus form a relatively homokinetic coupling connection between the proximal section 74 and the distal section 78 of the input shaft 54, which makes it possible to absorb a portion of the misalignments and / or displacements due to the input pinion 58 which are therefore not fully transmitted to the distal section 78. The distal section 78 thus receives only a relatively small portion of the misalignments and / or displacements of the input pinion 58. The primary 60 and secondary 80 flexible coupling devices are thus complementary. Advantageously, the misalignments and / or displacements of the input pinion 58 which may impact the input shaft 54 are compensated for by all of the flexible coupling members 62a, 62b, 82a and 82b. Furthermore, the input pinion then undergoes fewer loads.Thus, the flexibility of each coupling member 62a, 62b, 82a and 82b can be limited so as to prevent them from entering into resonance in the operating range of the propulsion system in which the coupling system 50 is placed.
[0073] In the first and second embodiments, shown in Figures 4 to 6, the sections 74, 76, 78, the primary flexible coupling members 62a, 62b and the secondary flexible coupling members 82, 82a and 82b are formed from separate parts assembled, for example welded, brazed and / or screwed, to each other. In the third embodiment, shown in Figure 7, the input shaft 54 is formed from a single part. The secondary flexible coupling members 82a and 82b are then one-piece coupling members which can be formed by enlarged portions of the input shaft 54. They are typically machined from the mass of the input shaft 54. According to this third embodiment, the input shaft 54 is continuous and does not have any interruption between the proximal 74, intermediate 76 and distal 78 sections. This variant makes it possible to simplify the production of the input shaft 54.
[0074] Furthermore, in the third embodiment, the primary coupling device 60 comprises a sleeve (or bushing) 85, separate from the input shaft 54, carrying the primary coupling members 62a, 62b. The discs 68a, b are integral with this sleeve 85, which is attached around the input shaft 54, in particular around the proximal section 74. The sleeve 85 is here made integral in rotation around the axis X, relative to the input shaft 54, by splines 86 for transmitting the torque. The sleeve 85 is for example force-mounted around the input shaft 54. In this case, it is preferably provided, as shown, an annular shoulder 88 formed on the input shaft 54 and oriented upstream, for the abutment of the sleeve 85 during its force-mounted installation.
[0075] Advantageously, the primary coupling device 60 of the input shaft 54 to the input pinion 58 makes it possible to size the secondary flexible coupling members, in particular the members 82, 82a and 82b, so that their resonance frequency is located outside the operating range of the propulsion system 10. Indeed, the secondary flexible coupling members make it possible to absorb part of the misalignments and / or displacements of the input pinion and are thus complementary to the primary flexible coupling members.The misalignments and / or displacements of the input pinion 58 of the reducer relative to the input shaft 54 are thus compensated by all of the flexible coupling members 62a, 62b, 82, 82a and 82b and the flexibility of each coupling member 62a, 62b, 82, 82a and 82b can then be limited so as to prevent them from entering into resonance in the operating range of the propulsion system in which the coupling system is placed.
Claims
CLAIMS 1. Coupling system (50) comprising an input shaft (54) extending along a longitudinal axis (X), an output shaft (56) and a reduction gear (52) coupling the input shaft (54) to the output shaft (56), the reduction gear (52) comprising an input pinion (58) driven in rotation by the input shaft (54) and at least partially surrounding the input shaft (54), in which the coupling system (50) further comprises a primary coupling device (60) of the input shaft (54) to the input pinion (58), configured to allow longitudinal movement of the input shaft (54) relative to the input pinion (58), the primary coupling device (60) comprising two primary flexible coupling members (62a, b) arranged axially on either side of the input pinion (58), each primary flexible coupling member (62a, b) forming a connection between the input pinion (58) and the input shaft (54),said connection allowing longitudinal translations and rotations around radial axes perpendicular to the longitudinal axis., 2. Coupling system according to claim 1, in which the connection between the input pinion (58) and the input shaft (54) is a connection allowing at most only longitudinal translations and rotations around axes perpendicular to the longitudinal axis.
3. A coupling system according to claim 1 or 2, wherein each primary flexible coupling member (62a, b) comprises a flexible annular structure (66a, b) interposed between an inner ring (70a, b) connected to one of the input gear (58) and the input shaft (54) and an outer ring (64a, b) connected to the other of the input gear (58) and the input shaft (54).
4. A coupling system according to claim 3, wherein the inner ring (70a, b) is rigidly connected to one of the input gear and the input shaft, and the outer ring (64a, b) is rigidly connected to the other of the input gear and the input shaft.
5. Coupling system according to any one of claims 1 to 4, in which the input shaft (54) is formed of several sections, including a proximal section (74) around which the input pinion (58) is arranged, a distal section (78) and an intermediate section (76) interposed between the distal (78) and proximal (74) sections, the input shaft (54) further comprising a secondary flexible coupling device (80) comprising a first and a second flexible coupling members secondary (82a, 82b) connecting the intermediate section (76) respectively to the distal section (78) and to the proximal section (74), each secondary flexible coupling member (82a, 82b) allowing longitudinal translations and rotations around axes perpendicular to the longitudinal axis and in particular only allowing said translations and rotations.
6. A coupling system according to claim 5, wherein the input shaft (54) is formed in one piece.
7. Coupling system according to claim 5, wherein each of the secondary flexible coupling members (82, 82a, 82b) is selected from a longitudinal shaft seal, which may be a double-membrane or multi-membrane longitudinal shaft seal.
8. Coupling system according to any one of the preceding claims, in which the reducer (52) is an epicyclic gear reducer comprising a sun gear constituting the input pinion (58), a crown surrounding the sun gear (58) and a plurality of satellites, carried by a planet carrier, engaged with the sun gear (58) and the crown gear, one of the crown gear and the planet carrier being fixed and the output shaft (56) being driven in rotation by the other of the crown gear and the planet carrier.
9. A propulsion system (10) for an aircraft comprising a gas turbine, a fan (12) and a coupling system (50) according to any one of claims 1 to 8, the input shaft (54) being driven in rotation by a turbine rotor of the gas turbine (38) and the output shaft (56) being rotatable together with a rotor of the fan.
10. Aircraft comprising at least one propulsion system (10) according to claim 9.