Compact gear train for turbine reducer

Abstract

The invention relates to a gear train comprising a sun gear (10) having a central axis (X), a ring gear (20) arranged coaxially around the sun gear (10), a planet carrier (30) and a first plurality of planet gears (31) and a second plurality of planet gears (32). Each planet gear (31, 32) is supported by the planet carrier (30) and comprises two first gearing means (34a, 34b) meshing with the ring gear (20), offset relative to each other in an axial direction parallel to the central axis (X). Each planetary gear (31) of the first plurality of planetary gears (31) comprises two second gearing means (35a, 35b) meshing with the sun gear (10), offset relative to each other in the axial direction.

Description

Technical Field

[0001] The present disclosure relates to the field of gear trains, and in particular to gear trains for turbomachine mechanical reducers. Background Art

[0002] A gear is a mechanical component that can transmit and possibly convert motion between two meshing solid elements. Their meshing can be ensured by contact, friction or even magnetic fields, and contact meshing can usually be ensured using gears, which can be, for example, spur-toothed or inclined, or even helical. Gears can be used to convert a rotary motion into a linear motion, or vice versa, for example using a rack, but in particular for converting a first rotary motion into a second rotary motion that differs from the first in speed and / or direction of rotation. Thus, in particular, at least one gear or a plurality of connected gears forming a gear train can be used in a mechanical reducer in order to convert a faster rotation of an input shaft into a slower rotation of an output shaft.

[0003] One particular area of ​​application for mechanical reducers is in turbines. More specifically, turbines (especially gas turbine engines) can have relatively high rotational speeds, which may need to be reduced for practical use. Therefore, in a turbofan engine, the direct driving of the fan by the turbine limits the maximum diameter of the fan and, therefore, the bypass ratio of the turbofan, since even at moderate diameters, the rotational speeds of the fan and turbine will quickly reach transonic speeds at the tip of the fan blades. Therefore, it has been proposed to insert a mechanical reducer between the turbine and the fan so that the rotational speed of the fan is much lower than the rotational speed of the turbine driving the fan. For similar reasons, it has also been proposed to use mechanical reducers in other turbomachines. Therefore, in the field of aviation propulsion devices, a mechanical reducer is usually added between the turbine and the propeller of a turboprop engine or between the turbine and the lift rotor of a turboshaft engine. The addition of mechanical reducers to turbines is also common in other fields, such as land or marine propulsion or power generation devices.

[0004] In many of these fields (particularly in the field of aerospace propulsion), it is advantageous to have a mechanical reducer that is particularly compact but provides a high reduction ratio. One type of gear train that provides a relatively high reduction ratio and is relatively reduced in size is a gear train having: a sun gear, a ring gear coaxially arranged around the sun gear, and a planetary gear with a planet carrier meshing with the sun gear and the ring gear, as disclosed, for example, in the French patent application with publication number FR 2 928 976 A1. In order to provide a particularly high reduction ratio, each planetary gear may have at least two gear transmission devices of different diameters, one meshing with the sun gear and the other meshing with the ring gear. For example, in the publication texts of European patent applications EP 3 726 031 A1 and EP 3 361 122 A1 and the French patent application with publication number FR 3 008 463 A1, a mechanical reducer for a turbine including a gear train having such a configuration has been disclosed.

[0005] In such a gear train, it may be advantageous to increase the number of planetary gears in order to distribute the torque transmitted by the gear train between them. However, when the radial dimension of the gear train is limited, interference between the planetary gears may limit their number. In order to avoid this, in particular in the publication of European patent application EP 1 113 193 A2, it is proposed to have a first set of planetary gears and a second set of planetary gears, wherein the gear transmission with the largest diameter is axially offset relative to the gear transmission of the first set of planetary gears. However, in such a gear train, this requires that the gear transmissions for the two sets of planetary gears have different cantilevers, which may cause bending and torsional moments in the planetary gears and make the balancing of the gear train more difficult. Summary of the invention

[0006] A first aspect of the present disclosure relates to a gear train, comprising: a sun gear having a central axis, a ring gear coaxially arranged around the sun gear, a planet carrier, and a first plurality of planet gears and a second plurality of planet gears, wherein each planet gear is supported by the planet carrier. Each planet gear may include: two first gear transmission devices meshing with the ring gear. These first gear transmission devices of each planet gear may be arranged offset relative to each other along an axial direction parallel to the central axis. Each planet gear in the first plurality of planet gears may also include: two second gear transmission devices meshing with the sun gear, and each planet gear in the second plurality of planet gears may include: one or two second gear transmission devices meshing with the sun gear. Each of the second gear transmission devices of the planet gears in the first plurality of planet gears and the second plurality of planet gears may have a second median diameter larger than the first median diameter. The two second gear transmission devices of each planet gear in the first plurality of planet gears may be offset relative to each other along the axial direction. In order to enable the second gear transmission devices to be staggered, each second gear transmission device in the second plurality of planet gears may be arranged between the second gear transmission devices of the first plurality of planet gears along the axial direction.

[0007] Thus, by staggering the second gear transmission (i.e. the gear transmission with the largest diameter), a high reduction ratio and a large number of planetary gears can be combined to achieve a high power transmission capacity of the gear train. However, this arrangement also minimizes the cantilever and torsional and bending moments in each planetary gear.

[0008] In order to facilitate the balance of the gear train, in particular, the two first gear transmission devices of each planetary gear in the first plurality of planetary gears and the second plurality of planetary gears can be symmetrically arranged relative to a transverse plane perpendicular to the center axis, and the two second gear transmission devices of each planetary gear in the first plurality of planetary gears can also be symmetrically arranged relative to the transverse plane.

[0009] At least one of the second plurality of planetary gears may comprise a single second gear transmission device, the second gear transmission device being centered on the transverse plane. However, at least one of the second plurality of planetary gears may also comprise two second gear transmission devices, which may then be arranged symmetrically with respect to the transverse plane.

[0010] The first gearing of the planetary gears in the first plurality of planetary gears and the second plurality of planetary gears may be spur geared. In order to enable them to mesh correctly with the ring gear, they may also have corresponding spur gearing.

[0011] However, as an alternative, the first gearing of the planetary gears of the first plurality of planetary gears and the second plurality of planetary gears may be helical. In this case, in order to balance the axial forces on the planetary gears, the first gearing of each of the first plurality of planetary gears and the second plurality of planetary gears may be inclined in opposite directions. Furthermore, in order to enable them to mesh correctly with the ring gear, they may also have corresponding helical gearing.

[0012] The second gearing of the planetary gears in the first plurality of planetary gears may also be spur geared. In order to enable them to mesh correctly with the sun gear, they may also have spur gearing.

[0013] However, as an alternative, the second gearing of the planet gears of the first plurality of planet gears may also be helical. In this case, in order to balance the axial forces on the planet gears, the second gearing of each of the first plurality of planet gears may be inclined in opposite directions. Furthermore, in order to enable them to mesh correctly with the sun gear, they may also have corresponding helical gearing.

[0014] The second gearing of the planet gears in the second plurality of planet gears may also be spur geared. In order to enable them to mesh properly with the sun gear, there may also be one or two corresponding spur gearings. In addition, if the second gearing of the planet gears in the first plurality of planet gears and the second plurality of planet gears are all spur geared, the sun gear may include: a single spur gearing that meshes with the second gearing of the planet gears in the first plurality of planet gears and the second plurality of planet gears.

[0015] However, as an alternative, the second gearing of the planetary gears of the second plurality of planetary gears may also be helical. In this case, in order to balance the axial forces on the planetary gears, each planetary gear of the second plurality of planetary gears may include: two second gearings inclined in opposite directions. Furthermore, in order to enable them to mesh correctly with the sun gear, they may also include two corresponding helical gearings. However, each planetary gear of the second plurality of planetary gears may also include a single second herringbone gearing and / or the sun gear may also include a single herringbone gearing meshing with all the second gearings of the second plurality of planetary gears.

[0016] Each planet gear of the first and second plurality of planet gears may be mounted to rotate freely about an axis using a corresponding bearing, which may in particular be a rolling bearing, but other types of bearings, in particular hydrostatic bearings, are also possible.

[0017] The second median diameter may be greater than a third median diameter, the third median diameter being a median diameter of a gearing of a sun gear meshing with the second gearing of each of the first and second pluralities of planet gears.

[0018] A second aspect of the present disclosure relates to a mechanical reducer, comprising: a gear system according to the first aspect, at least one input shaft, and an output shaft, wherein the input shaft is mechanically connected to one of the sun gear, the ring gear and the planetary carrier during rotation, and the output shaft is mechanically connected to another of the sun gear, the ring gear and the planetary carrier during rotation.

[0019] A third aspect of the present disclosure relates to a turbomachine comprising: a mechanical reducer according to the second aspect, at least one turbine and a rotating device, and the use of the mechanical reducer according to the second aspect in such a turbomachine. In the turbomachine, at least one turbine can be mechanically coupled to at least one input shaft of the reducer in rotation, and the rotating device can be mechanically coupled to the output shaft in rotation. In particular, the rotating device can be a fan so as to form a turbofan including a fan with a reducer. However, the rotating device can also be a propulsion propeller or a lift rotor, or even a propulsion wheel, a hydraulic transmission or a generator. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The objects and advantages of the present disclosure will be better understood after reading the following detailed description of embodiments presented by way of non-limiting examples. This description refers to the accompanying drawings, in which:

[0021] [ Figure 1 ] Figure 1 is a schematic representation of a turbine with a mechanical reducer.

[0022] [ Figure 2 ] Figure 2 A mechanical reducer according to a first embodiment is schematically shown.

[0023] [ Figure 3 ] Figure 3 yes Figure 2 Detailed view of the planetary carrier and lubrication system of a mechanical reducer.

[0024] [ Figure 4 ] Figure 4 Schematically shows Figure 2 Orientation of the planetary gear transmission of a mechanical reducer.

[0025] [ Figure 5 ] Figure 5 The orientation of the gearing of the planetary gears of the mechanical reducer according to the second embodiment is schematically shown.

[0026] [ Figure 6 ] Figure 6 Shows Figure 5 The arrangement of the bearings of the planetary gears of a mechanical reducer.

[0027] [ Figure 7 ] Figure 7 A mechanical reducer according to a third embodiment is schematically shown.

[0028] [ Figure 8 ] Figure 8 Schematically shows Figure 7 Orientation of the planetary gear transmission of a mechanical reducer.

[0029] [ Fig. 9 ] Fig. 9 A mechanical reducer according to a fourth embodiment is schematically shown.

[0030] [ Fig.10 ] Fig.10 A turbine with a mechanical reducer according to a fifth embodiment is schematically shown.

[0031] [ Fig.11 ] Fig.11 A mechanical reducer according to a fifth embodiment is schematically shown. DETAILED DESCRIPTION

[0032] like Figure 1 As shown, the turbine 1 may take the form of a turbofan, which includes a fan S, a low-pressure compressor 1a, a high-pressure compressor 1b, a combustion chamber 1c (which may be annular), a high-pressure turbine 1d, a low-pressure turbine 1e and an exhaust nozzle 1h in a conventional manner. The high-pressure compressor 1b and the high-pressure turbine 1d may be mechanically connected in rotation by a high-pressure shaft 2, thereby forming a high-pressure (HP) body together. The low-pressure compressor 1a and the low-pressure turbine 1e may be mechanically connected in rotation by a low-pressure shaft 3, thereby forming a low-pressure (LP) body together.

[0033] In order to be driven, the fan S can be constrained to rotate with the fan shaft 4, which is mechanically connected to the low-pressure shaft 3 in rotation via a mechanical reducer 6. For example, the mechanical reducer 6 can be a planetary reducer, an epicyclic reducer, or a differential reducer. Therefore, through the mechanical reducer 6, the fan S can be driven at a speed lower than the speed of the low-pressure shaft 3, the low-pressure compressor 1a, and the low-pressure turbine 1e, so that the fan S has a larger diameter and provides a higher bypass ratio.

[0034] like Figure 1As shown, the reducer 6 can be positioned in the front part of the turbine 1, upstream relative to the low-pressure body and the high-pressure body. However, an alternative arrangement can also be adopted to arrange the reducer 6 in the rear part of the turbine, or even in the middle part. The fixed structure (which forms the engine casing or stator 5 and can include an upstream part 5a and a downstream part 5b as shown in the figure) can be designed to form a shell E around the reducer 6. As shown in the figure, the shell E can be closed upstream by a seal at the bearing through which the fan shaft 4 passes, and closed downstream by a seal at the passage of the low-pressure shaft 3.

[0035] according to Figure 2 In the first embodiment shown, the mechanical reducer 6 can be a planetary reducer with a gear train, the gear train comprising: a sun gear 10, which is constrained to rotate with the low-pressure shaft 3; a ring gear 20, which is coaxially arranged around the sun gear 10, has the same central axis X, and is constrained to rotate with the fan shaft 4; and a planet carrier 30, which is integrally formed with the housing 5 and is therefore fixed. The common central axis X of the sun gear 10 and the ring gear 20 can be aligned with the rotation axis of the high-pressure shaft 2 and the low-pressure shaft 3. Therefore, the rotational connection between the sun gear 10 and the low-pressure shaft 3 and / or between the ring gear 20 and the fan shaft 4 can be achieved, for example, by means of splines.

[0036] The gear train of the mechanical reducer 6 may also include: two planetary gear assemblies 31, 32, which are carried by the planet carrier 30 and distributed around the central axis X, for example, distributed around the central axis X at substantially the same angular intervals. Each planetary gear 31, 32 can be mounted to rotate freely around the corresponding axis Y using a corresponding bearing 33, and the axis Y can in particular be substantially parallel to the central axis X. Each planetary gear 31, 32 may include: two first gear transmission devices 34a, 34b, which have a first median diameter D1 and mesh with two corresponding gear transmission devices 20a, 20b in the ring gear 20. The first gear transmission devices 34a, 34b of each planetary gear 31, 32 may be offset relative to each other in the direction of the axis Y, with an axial offset L1. In particular, in order to ensure a good balance of forces on the planetary gears 31, 32, the first gear transmission devices 34a, 34b may have substantially the same width and be symmetrically arranged relative to the transverse symmetry plane A.

[0037] The planet gears 31 in the first set of planet gears 31 and the planet gears 32 in the second set of planet gears 32 may be alternately arranged around the sun gear 10. In addition to the two first gear transmission devices 34a, 34b, each of the planet gears 31 in the first set of planet gears 31 may also include: two second gear transmission devices 35a, 35b, which have a second median diameter D2, meshing with the corresponding gear transmission devices 10a, 10b of the sun gear 10. The gear transmission devices 10a, 10b of the sun gear 10 may have a third median diameter D3 smaller than the second median diameter D2, and the gear transmission devices 20a, 20b of the ring gear 20 may have a fourth median diameter D4 larger than the first median diameter D1, so as to obtain a reduction ratio between the rotation speed of the sun gear 10 and the rotation speed of the ring gear 20. The second median diameter D2 may be larger than the first median diameter D1, so as to obtain a higher reduction ratio between the rotation speed of the sun gear 10 and the rotation speed of the ring gear 20. The second gear transmissions 35a, 35b of each planet gear 31 of the first set of planet gears 31 may also be offset relative to each other in the direction of the axis Y. In particular, in order to ensure a good balance of forces on the planet gears 31, the second gear transmissions 35a, 35b may also have substantially the same width and be arranged symmetrically relative to the transverse symmetry plane A, with an axial offset L between them. 21 The axial offset L1 between the first gear transmission devices 34a and 34b is smaller than that between the first gear transmission devices 34a and 34b, and is thus disposed between these first gear transmission devices 34a and 34b.

[0038] In this first embodiment, each planet gear 32 in the second set of planet gears 36 may also include, in addition to the two first gear transmission devices 34a, 34b, two second gear transmission devices 35c, 35d having the same second median diameter D2. Like the second gear transmission devices 35a, 35b of the first set of planet gears 31, the second gear transmission devices 35c, 35d of the second set of planet gears 32 may also mesh with the corresponding gear transmission devices 10c, 10d of the sun gear 10. The second gear transmission devices 35c, 35d of each planet gear 32 in the second set of planet gears 32 may also be offset relative to each other in the direction of the axis Y. In particular, in order to ensure a good balancing force on the planet gear 31, the second gear transmission devices 35c, 35d of each planet gear 32 of the second set of planet gears 32 may also have the same width and be symmetrically arranged relative to the transverse symmetry plane A. However, the axial offset L between the two second gear transmission devices 35c, 35d of each planet gear 32 in the second set of planet gears 32 is less than 1. 22 It can be smaller than not only the axial offset L1 between the first gear transmission devices 34a and 34b, but also the axial offset L2 between the second gear transmission devices 35a and 35b of each planetary gear 31 in the first set of planetary gears 31.21 , so that: the second gear transmission devices 35c, 35d of the planetary gears 32 in the second group of planetary gears 32 are arranged in the axial direction between the second gear transmission devices 35a, 35b of the planetary gears 31 in the first group of planetary gears. Therefore, an axial offset ΔL can be obtained between the second gear transmission devices 35a, 35b of the planetary gears 31 of the first group and the second gear transmission devices 35c, 35d of the planetary gears 32 of the second group, which is equal to half of the difference between the axial offsets L1 and L2, which makes the axes Y of the planetary gears 31, 32 of the two assemblies close without collision between the second gear transmission devices 35a-35d of the adjacent planetary gears 31, 32, and thus the mechanical load can be distributed between a larger number of planetary gears 31, 32 with limited radial dimensions.

[0039] To facilitate the installation of the mechanical reducer 6 , the ring gear 20 may comprise two parts, each integrally formed with one of the gear transmissions 20 a , 20 b , which may be fixed to each other by means of a flange 20 c which may be substantially aligned with the transverse plane of symmetry A.

[0040] like Figure 3 As shown in detail in , each bearing 33 (for example, it can be a rolling bearing or a hydrostatic bearing) can be mounted on the axis 30b of the planet carrier 30 so that: each planet gear 31 or 32 is internally supported by a corresponding bearing 33 around one of the axes 30b. In this case, one or more structural chassis 30a of the planet carrier 30 can be used to position all the axes 30b of the planet carrier 30 relative to each other. For reasons of operation, assembly, manufacturing, control, maintenance and / or replacement, the axis 30b and the chassis 30a of the planet carrier 30 can be separated into several parts. In order to provide lubrication for the reducer 6, the reducer 6 may include: a lubricating fluid distributor 40, which has an injector 41 and an arm 42 for lubricating the gears, which opens to a supply port 44 of the bearing 33 through a buffer chamber 43 along the axis 30b of the planet carrier to lubricate the bearing. Therefore, the lubricating fluid can follow Figure 3 Follow the path shown by the arrows in order to lubricate the bearings and gears.

[0041] In this first embodiment, the gear transmission may be a helical gear transmission. Figure 4As shown, the two first gear transmissions 34a, 34b of each planet gear 31, 32 can be tilted in opposite directions, in particular, symmetrically tilted in opposite directions, so as to obtain a balance of axial forces in each planet gear 31, 32. Therefore, in order to obtain correct meshing, the corresponding gear transmissions 20a, 20b in the ring gear 20 can have the same inclination. Similarly, the two second gear transmissions 35a-35d of each planet gear 31, 32 can also be tilted in opposite directions, in particular, symmetrically tilted in opposite directions, so as to obtain a balance of axial forces in each planet gear 31, 32. Therefore, in order to obtain correct meshing, the corresponding gear transmissions 10a-10d in the sun gear 10 can have the same inclination.

[0042] However, at least some of all of these gear transmissions may also be spur gear transmissions. Figure 5 In the second embodiment shown, all the gear transmissions 34a, 34b and 35a-35d of each planet gear 31, 32 can be spur gear transmissions, just like the corresponding gear transmissions on the sun gear 10 and in the ring gear 20. In this case, the four corresponding gear transmissions on the sun gear 10 can be replaced by a single straight-through gear transmission 10a, which has a sufficient width to mesh with the second gear transmission 35a-35d of each planet gear 31, 32. The planet gears 31 and 32 can also be supported at their axial ends by external bearings 33, such as Figure 6 As shown, instead of being supported by internal bearings as shown in the first embodiment. In this second embodiment, the other elements of the reducer 6 can be the same or at least equivalent to the elements in the first embodiment, and are also arranged in the same or similar manner. Figure 5 and Figure 6 The same reference numerals as in the previous figures are used.

[0043] Since the two second gear transmission devices of each planetary gear of the second group are adjacent, Figure 7 In the third embodiment shown, they can also be replaced by a single second gear transmission device 35c, which can be made in a single piece, centered on the transverse symmetry plane A, and whose width can be substantially equal to the sum of the widths of the second gear transmission devices 35a, 35b of the first set of planetary gears 31. In this case, the second gear transmission device 35c of each planetary gear 32 can be a herringbone shape, such as Figure 8 However, as an alternative, it can be a spur gear transmission, as in the second embodiment. Figure 7As shown, the second gear transmission device 35c of the planetary gear 31 of the third embodiment can be meshed with a single corresponding gear transmission device 10c on the sun gear 10. In the third embodiment, the other elements of the reducer 6 can also be the same or at least equivalent to the elements of the previous embodiment, and can also be arranged in the same or similar manner. Figure 7 and Figure 8 The same reference numerals as in the previous figures are used.

[0044] Although, in these three first embodiments, the gear train is a planetary gear train in which the planet carrier 30 is fixed, the mechanical reducer 6 may also include an epicyclic gear train in which the ring gear 20 is integrally formed with the housing 5 and is thus fixed, and the planet carrier 30 is integrally formed with the rotating fan shaft 4, as Fig. 9 In the fourth embodiment, the other elements of the reducer 6 may also be the same or at least equivalent to those of the previous embodiment, and may also be arranged in the same or similar manner. Fig. 9 The same reference numerals as in the previous figures are used.

[0045] Furthermore, in particular for a turbine 1 having a fan S with counter-rotating blades S1, S2, the gear train may also be a differential gear train. Fig.10 and Fig.11 As shown, the sun gear 10 can be constrained to rotate together with the low-pressure shaft 3, the ring gear 20 of the first fan shaft 4a, and the planetary gear carrier 30 of the second counter-rotating fan shaft 4b. In addition, in this fifth embodiment, the other elements of the reducer 6 can also be the same or at least equivalent to the previous embodiments, and can also be arranged in the same or similar manner. Therefore, they are Fig.10 and Fig.11 The same reference numerals as in the previous figures are used.

[0046] Although the invention has been described with reference to specific embodiments, it is obvious that modifications and variations may be made to these examples without departing from the general scope of the invention as defined in the claims. In particular, individual features of the various illustrated and / or described embodiments (such as the spur gear transmission and / or the external bearing of the planetary gear of the second embodiment and the general architecture of the following embodiments) may be combined in further embodiments. Therefore, the present description and the drawings are to be regarded as illustrative rather than restrictive.

Claims

1. A gear train for a mechanical reducer (6) of a turbine (1), the gear train comprising: a sun gear (10) having a central axis (X), a ring gear (20) coaxially disposed around the sun gear (10), a planet carrier (30), and a first plurality of planetary gears (31) and a second plurality of planetary gears (32), wherein each planetary gear (31, 32) is supported by the planet carrier (30) and comprises two first gear transmissions (34a, 34b), the two first gear transmissions (34a, 34b) meshing with the ring gear (20), offset relative to each other in an axial direction parallel to the central axis (X), Each planet gear (31) of the first plurality of planet gears (31) comprises two second gear transmissions (35a, 35b) meshing with the sun gear (10) and offset relative to each other in the axial direction, Each of the second plurality of planetary gears (32) includes one or two second gear transmission devices (35c, 35d) meshing with the sun gear (10), each of the first plurality of planetary gears (31) and the first plurality of planetary gears (34a, 34b) having a first median diameter (D1), and each of the second gear transmission devices (35a-35d) of the first plurality of planetary gears (31) and the second plurality of planetary gears (32) having a second median diameter (D2) larger than the first median diameter (D1), The gear train is characterized in that each second gear transmission device (35c, 35d) of the second plurality of planetary gears (32) is arranged between the second gear transmission devices (35a, 35b) of the first plurality of planetary gears (31) along the axial direction.

2. The gear train according to claim 1, wherein: The two first gear transmission devices (34a, 34b) of each planetary gear (31, 32) in the first plurality of planetary gears (31) and the second plurality of planetary gears (32) are symmetrically arranged with respect to a transverse plane (A) perpendicular to the central axis (X), and the two second gear transmission devices (35a, 35b) of each planetary gear (31) in the first plurality of planetary gears (31) are also symmetrically arranged with respect to the transverse plane (A).

3. The gear train according to claim 2, wherein: At least one planet gear (32) of the second plurality of planet gears (32) comprises a single second gear transmission (35c) centered about the transverse plane (A).

4. The gear train according to any one of claims 2 or 3, wherein: At least one planet gear (32) of the second plurality of planet gears (32) comprises two second gear transmission devices (35c, 35d) arranged symmetrically with respect to the transverse plane (A).

5. The gear train according to any one of claims 1 to 4, wherein: The first gear transmission devices (34a, 34b) of the planetary gears (31, 32) in the first plurality of planetary gears (31) and the second plurality of planetary gears (32), the second gear transmission devices (35a, 35b) of the planetary gears (31) in the first plurality of planetary gears (31) and / or the second gear transmission devices (35c, 35d) of the planetary gears (32) in the second plurality of planetary gears (32) are straight-toothed.

6. The gear train according to any one of claims 1 to 5, wherein: The first gear transmission devices (34a, 34b) of the planetary gears (31, 32) in the first plurality of planetary gears (31) and the second plurality of planetary gears (32), the second gear transmission devices (35a, 35b) of the planetary gears (31) of the first plurality of planetary gears (31) and / or the second gear transmission devices (35c, 35d) of the planetary gears (32) of the second plurality of planetary gears (32) are helical gears.

7. The gear train according to claim 6, wherein: The first gear transmission device (34a, 34b) of each planetary gear (31, 32) in the first plurality of planetary gears (31) and the second plurality of planetary gears (32), the second gear transmission device (35a, 35b) of each planetary gear (31) in the first plurality of planetary gears (31) and / or the two second gear transmission devices (35c, 35d) of each planetary gear (32) in the second plurality of planetary gears (32) are inclined in opposite directions.

8. A gear train according to any one of the preceding claims, wherein: Each planet gear (31, 32) of the first plurality of planet gears (31) and the second plurality of planet gears (32) is mounted to rotate freely about a respective axis (Y) using a corresponding bearing (33).

9. A gear train according to any one of the preceding claims, wherein: The second median diameter (D2) is larger than the third median diameter (D3), and the third median diameter is the median diameter of the gear transmission device (10a, 10b) of the sun gear (10), and the gear transmission device (10a, 10b) of the sun gear (10) is meshed with the second gear transmission device (35c, 35d) of each planetary gear (31, 32) in the first plurality of planetary gears (31) and the second plurality of planetary gears (32).

10. A mechanical reducer (6) for a turbine (1), comprising: A gear train according to any one of the preceding claims; At least one input shaft (3) which is mechanically coupled in rotation to one of the sun gear (10), the ring gear (20) and the planet carrier (30); and an output shaft (4) which is mechanically coupled in rotation to the other of the sun gear (10), the ring gear (20) and the planet carrier (30).

11. A turbine (1), comprising: A mechanical reducer (6), at least one turbine (1e) and a rotating device according to claim 10, wherein the at least one turbine (1e) is mechanically coupled in rotation to the at least one input shaft (3) of the mechanical reducer (6), and the rotating device is mechanically coupled in rotation to the output shaft (4).

12. The turbine (1) according to claim 11, wherein: The rotating device is a fan (S).

Images