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 at least one first gearing (34a, 34b) meshing with the ring gear (20) and two second gearing (35a-35d) meshing with the sun gear (10), and these gearing are offset relative to each other in the axial direction. Each of the first gear transmissions (34a, 34b) of the first plurality of planetary gears (31) has a first median diameter (D1), and each of the second gear transmissions (35a-35d) of the second plurality of planetary gears (32) has a second median diameter (D2) larger than the first median diameter (D1). The second gearing (35c, 35d) of the second plurality of planetary gears (32) is arranged in the axial direction between the second gearing (35a, 35b) of the first plurality of planetary gears (31).

Description

Technical Field

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

[0002] A gear is a mechanical component in which motion can be transmitted and, where applicable, converted between two meshing physical elements. Their meshing can be ensured by contact, friction or even magnetic fields, and contact meshing can usually be ensured using a gear transmission, 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 and vice versa, for example by means of a rack and pinion transmission, but in particular to convert a first rotary motion into a second rotary motion that differs from the first rotary motion in terms of rotational speed and / or direction of rotation. It is therefore particularly possible to use at least one gear or several gears together forming a gear train in a mechanical reducer, thereby converting a faster rotation of an input shaft into a slower rotation of an output shaft.

[0003] One particular area of ​​application of mechanical reducers is in turbines. This is because turbines, in particular gas turbine engines, can have relatively high rotational speeds, which it is advisable to reduce for practical use. Thus, in turbofans, the direct drive of the fan by the turbine can limit the maximum diameter of the fan and thus the bypass ratio of the turbojet, since even with moderate diameters, the rotational speeds of the fan and turbine can quickly reach transonic speeds at the tip of the fan airfoil. Therefore, provision has been made to insert a mechanical reducer between the turbine and the fan, so that the fan is driven in rotation at a much lower speed than the turbine driving it. For similar reasons, provision has also been made to insert mechanical reducers for other turbomachines. Thus, in the field of aeronautical propulsion, a mechanical reducer is usually added between the turbine and the propeller in a turboprop engine or between the turbine and the lift rotor in a turboshaft engine. The addition of mechanical reducers to turbines is also common in other fields, such as in the field of land or marine propulsion or power generation.

[0004] In several of these fields, in particular in the field of aerospace propulsion, it may be 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 a relatively small volume is a gear train having: a sun gear, a ring gear coaxially arranged around the sun gear, and a planet carrier with planetary gears 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, a mechanical reducer for a turbine including a gear train having such a configuration has been disclosed in European patent applications with publication numbers EP 3 726 031 A1 and EP 3 361 122 A1 and in French patent application with publication number FR 3 008 463 A1.

[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 volume of the gear train is limited, the interference between the planetary gears limits their number. In order to avoid this, provision has been made, in particular in the European patent application with publication number EP 1 113 193 A2, to have a first plurality of planetary gears and a second plurality of planetary gears, wherein the gearing with the largest diameter is axially offset relative to the gearing of the first plurality of planetary gears. However, in such a gear train, this requires the gearing of the two plurality of planetary gears to have different overhangs, which may lead to bending and torsional torques in the planetary gears and make balancing the gear train more difficult. Summary of the invention

[0006] A first aspect of the present disclosure relates to a gear train for a mechanical reducer of a turbine. The gear train includes: a sun gear having a central axis, a ring gear coaxially arranged around the sun gear, a planet carrier, and a first plurality of planetary gears and a second plurality of planetary gears, wherein each planetary gear is supported by the planet carrier. Each planetary gear may include: at least one first gear transmission device and two second gear transmission devices, which are offset relative to each other in an axial direction parallel to the central axis. Each of the first gear transmission devices can be meshed with the ring gear and have a first median diameter, and each of the second gear transmission devices can be meshed with the sun gear and have a second median diameter larger than the first median diameter. In order to allow the staggering of the second gear transmission devices, the second gear transmission devices of the second plurality of planetary gears can be arranged between the second gear transmission devices of the first plurality of planetary gears in the axial direction.

[0007] Thus, due to the staggering of the second gearing with a larger diameter, a high reduction ratio and a large number of planetary gears can be combined, thereby increasing the power transmission capacity of the gear train. However, this arrangement can also minimize the overhang, torsion and bending torques in each planetary gear.

[0008] In order to simplify the balancing of the gear train, in particular the two second gearing arrangements of each planetary gear can be arranged symmetrically with respect to a transverse plane perpendicular to the centre axis.

[0009] At least one of the planetary gears may comprise a single first gearing arrangement centred on the transverse plane. However, it is also conceivable that at least one of the planetary gears comprises two first gearing arrangements, which may then be arranged symmetrically with respect to the transverse plane.

[0010] The first gearing may be spur-toothed. To allow them to mesh correctly with the ring gear, the gears may have at least one corresponding spur-toothed gearing.

[0011] Alternatively, however, the first gearing may be helical. In this case, in order to balance the axial forces on the planet gears, each planet gear may comprise two of the first gearings inclined in opposite directions. Furthermore, in order to allow them to mesh correctly with the ring gear, this may also have a corresponding helical gearing. However, it is also conceivable to have a single first herringbone gearing on each planet gear and a corresponding gearing on the ring gear.

[0012] The second gear transmission device of the planet gear in the first plurality of planet gears and / or the second gear transmission device of the planet gear in the second plurality of planet gears can also be spur-toothed. In order to allow them to mesh correctly with the sun gear, the gear can also have a corresponding spur-toothed gear transmission device.

[0013] However, alternatively, the second gearing of the planet gears in the first plurality of planet gears and / or the second gearing of the planet gears in the second plurality of planet gears may also be spur geared, or may be helical geared. In this case, in order to balance the axial forces on the planet gears, the second gearing of each planet gear in the first plurality of planet gears and / or the second plurality of planet gears may be inclined in opposite directions. Furthermore, in order to allow them to mesh correctly with the sun gear, this may also have a corresponding helical gearing.

[0014] Each planet gear of the first and second pluralities of planet gears may be mounted to rotate freely about a respective axis using a corresponding bearing, in particular a ball bearing, although other types of bearings are also conceivable, in particular hydrostatic bearings.

[0015] Each of the first and second plurality of planetary gears comprises at least two parts rotationally attached to each other by complementary non-axisymmetric profiles. In particular, these non-axisymmetric profiles may be polygonal profiles with rounded corners, such as a three-leaf profile such as a P3G profile according to the OIN 32711 standard, which is particularly suitable for rotationally fixing parts that are not subject to axial sliding, although other non-axisymmetric profiles may also be envisaged, such as a four-leaf P4C profile or a spline profile according to the OIN 32712 standard. Each planetary gear may also comprise a retaining element such as a nut to axially retain the at least two parts together.

[0016] A second aspect of the present disclosure relates to a mechanical reducer, which includes a gear system according to the first aspect, at least one input shaft and an output shaft, wherein the input shaft is mechanically rotationally connected to one of the sun gear, the ring gear and the planetary carrier, and the output shaft is mechanically rotationally connected to another one of the sun gear, the ring gear and the planetary carrier.

[0017] 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, the at least one turbine can be mechanically rotationally coupled to at least one input shaft of the reducer, and the rotating device can be mechanically rotationally coupled to the output shaft. In particular, the rotating device can be a fan, thereby forming a turbofan with a speed reduction fan. However, it is also conceivable that the rotating device is a propulsion propeller or a lift fan, or even a drive wheel, a hydraulic transmission or a generator. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The subject matter of this overview and its advantages will be better understood by reading the following detailed description of an embodiment given by way of non-limiting example. This description refers to the accompanying drawings, in which:

[0019] [ Figure 1 ] Figure 1 is a schematic illustration of a turbine with a mechanical reducer.

[0020] [ Figure 2 ] Figure 2 A mechanical speed reducer according to a first embodiment is schematically illustrated.

[0021] [ Figure 3 ] Figure 3 yes Figure 2 Detailed view of the planetary carrier and lubrication system of a mechanical reducer.

[0022] [ Figure 4 ] Figure 4 A non-axisymmetric profile of the P3G type is schematically illustrated, which can be used to Figure 4 At least two individual parts of each planetary gear of the mechanical reducer are rotationally fixed to one another.

[0023] [ Figure 5 ] Figure 5 Schematically illustrates Figure 2 Orientation of the planetary gear transmission of a mechanical reducer.

[0024] [ Figure 6 ] Figure 6 The orientation of the gear transmission of the planetary gear of the mechanical reducer according to the second embodiment is schematically illustrated.

[0025] [ Figure 7 ] Figure 7 Illustrated Figure 6 The arrangement of the bearings of the planetary gears of a mechanical reducer.

[0026] [ Figure 8 ] Figure 8 A mechanical speed reducer according to a third embodiment is schematically illustrated.

[0027] [ Fig. 9 ] Fig. 9 Schematically illustrates Figure 8 Orientation of the planetary gear transmission of a mechanical reducer.

[0028] [ Fig.10 ] Fig.10 A mechanical speed reducer according to a fourth embodiment is schematically illustrated.

[0029] [ Fig.11 ] Fig.11 A turbomachine having a mechanical reducer according to a fifth embodiment is schematically illustrated.

[0030] [ Fig.12 ] Fig.12 A mechanical speed reducer according to a fifth embodiment is schematically illustrated. DETAILED DESCRIPTION

[0031] like Figure 1As illustrated in FIG. 1 , the turbine 1 may be in the form of a turbofan, which generally 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. The high-pressure compressor 1b and the high-pressure turbine 1d may be mechanically connected in rotation via a high-pressure shaft 2, thereby forming a high-pressure (HP) valve core therewith. The low-pressure compressor 1a and the low-pressure turbine 1e may be mechanically connected in rotation via a low-pressure shaft 3, thereby forming a low-pressure (LP) valve core therewith.

[0032] In order to be driven, the fan S can be rotationally fixed to the fan shaft 4, and the fan shaft is mechanically rotationally connected to the low-pressure shaft 3 through a mechanical reducer 6. For example, the mechanical reducer 6 can be planetary, epicyclic or differential. Due to the mechanical reducer 6, the fan S can be driven at a lower speed than the low-pressure shaft 3, the low-pressure compressor 1a and the low-pressure turbine 1e, thereby allowing the fan S to have a larger diameter and thus provide a higher bypass ratio.

[0033] like Figure 1 As illustrated in the figure, the reducer 6 can be positioned in the front part of the turbine 1, in the upstream direction relative to the low-pressure valve core and the high-pressure valve core. However, an alternative arrangement can be envisaged: the reducer 6 is arranged in the rear part of the turbine, or even in the middle part. As illustrated in the figure, the fixed structure forming the engine casing or stator 5 and capable of including an upstream part 5a and a downstream part 5b can be arranged in a manner to form a chamber E around the reducer 6. As illustrated in the figure, the chamber E can be closed upstream by a seal flush with the bearing allowing the fan shaft 4 to pass through, and closed downstream by a seal flush with the low-pressure shaft 3 passing through.

[0034] according to Figure 2 In the first embodiment illustrated in the figure, the mechanical reducer 6 can be a planetary reducer with a gear train, which includes: a sun gear 10, which is rotationally fixed to the low-pressure shaft 3; a ring gear 20, which is coaxially arranged around the sun gear 10, has the same central axis X as the gear, and is rotationally fixed to the fan shaft 4; and a planet carrier 30, which is fixed to the housing 5 and is thus 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 performed, for example, by splines.

[0035] The gear train of the mechanical reducer 6 may further 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 may be mounted to rotate freely around a corresponding axis Y using a corresponding bearing 33, in particular, the axis Y may be substantially parallel to the central axis X. Each planetary gear 31, 32 may include: two first gear transmissions 34a, 34b having a first median diameter D1, which mesh with two corresponding gear transmissions 20a, 20b in the ring gear 20. The first gear transmissions 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 transmissions 34a, 34b may have substantially the same width and be symmetrically arranged relative to the transverse symmetry plane A.

[0036] 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 having 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 planetary gear 31 of the first set of planetary 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 planetary 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 first gear transmission devices 34a, 34b are larger than the axial offset L1 between the first gear transmission devices 34a, 34b so as to be axially arranged outside these first gear transmission devices 34a, 34b.

[0037] In this first embodiment, in addition to the two first gear transmission devices 34a, 34b, each planet gear 32 in the second set of planet gears 36 may also include: 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 balance of forces on the planet gears 31, the second gear transmission devices 35c, 35d of each planet gear 32 in the second set of planet gears 32 may also have substantially the same width and be symmetrically arranged relative to the transverse symmetry plane A. 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 22 It can also be larger than the axial offset L1 between the first gear transmission devices 34a, 34b, so that the first gear transmission devices 34a, 34b are also arranged between the second gear transmission devices 35c, 35d of the second plurality of planetary gears 32; however, it can be smaller than the axial offset L2 between the second gear transmission devices 35a, 35b in each planetary gear 31 in the first group of planetary gears 31, so that the second gear transmission devices 35c, 35d of the planetary gear 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 gear 31 in the first group of planetary gears 31. Therefore, an axial offset ΔL can be obtained between the second gear transmission devices 35a, 35b of the first group of planetary gears 31 and the second gear transmission devices 35c, 35d of the second group of planetary gears 32, which is equal to the axial offset L 21 With L 22 This allows the axes Y of the two groups of planetary gears 31, 32 to be closer without collision between the second gear transmission devices 35a to 35d of adjacent planetary gears 31, 32, thereby distributing the mechanical load between a larger number of planetary gears 31 and 32 with a smaller radial volume.

[0038] like Figure 3As illustrated in detail in , each bearing 33 (which may be a ball bearing or a hydrostatic bearing, for example) may be mounted on the shaft 30b of the planetary carrier 30 so that: each planetary gear 31 or 32 is internally supported by a corresponding bearing 33 around one of the shafts 30b. In this case, all shafts 30b of the planetary carrier 30 may be positioned relative to each other using one or more structural frames 30a of the planetary carrier 30. For reasons of operation, installation, manufacturing, maintenance control and / or replacement, the shaft 30b and frame 30a of the planetary gear 30 may be separated into several parts. In order to lubricate the reducer 6, this may include: a lubricant distributor 40 having an injector 41 for lubricating the gears and a plurality of arms 42 which open to a supply inlet 44 of the bearing 33 along the shaft 30b of the planetary gear through a buffer chamber 43 to lubricate these bearings. Thus, the lubricant may be distributed along the shaft 30b of the planetary gear. Figure 3 The arrows illustrate the path for lubrication of bearings and gears.

[0039] Furthermore, in order to simplify the assembly of the gear train, each planet gear 31, 32 may comprise at least two (preferably three) parts 31a, 31b, 31c, which are rotationally attached to each other, for example with assembled complementary non-axisymmetric profiles 34, in particular, the profile may be a P3G three-leaf profile in accordance with the OIN 32711 standard, such as Figure 4 The three parts 31a, 31b, 31c can be axially retained by a nut 35.

[0040] In this first embodiment, the gear transmission may be a helical gear transmission. Figure 5 As illustrated in , the two first gear transmission devices 34a, 34b of each planetary gear 31, 32 can be tilted in opposite directions (especially symmetrically tilted) to obtain a balance of axial forces in each planetary gear 31, 32. Therefore, in order to obtain correct meshing, the corresponding gear transmission devices 20a, 20b in the ring gear 20 can have the same inclination. Similarly, the two second gear transmission devices 35a to 35d of each planetary gear 31, 32 can also be tilted in opposite directions (especially symmetrically tilted) to obtain a balance of axial forces in each planetary gear 31, 32. Therefore, in order to obtain correct meshing, the corresponding gear transmission devices 10a to 10d on the sun gear 10 can also have the same inclination.

[0041] However, it is also conceivable that at least some of all of these gear transmissions have spur teeth. Figure 6In the second embodiment illustrated in , the gear transmission device group 34a, 34b and 35a to 35d of each planet gear 31, 32 can be a spur gear transmission device, just like the corresponding gear transmission devices on the sun gear 10 and the ring gear 20. In this case, the four corresponding gear transmission devices on the sun gear 10 can be replaced by a single straight-through gear transmission device 10a, which has a sufficient width to mesh with the second gear transmission device 35a to 35d of each planet gear 31, 32. It is also conceivable that, as shown in the first embodiment, the planet gears 31 and 32 are supported at their axial ends by external bearings 33. In this second embodiment, the other elements of the reducer 6 can be the same or at least equivalent to the elements of the first embodiment, and can also be arranged in the same or similar manner. Therefore, they are Figure 6 and Figure 7 The same reference numerals as in the previous figures are used.

[0042] Since the two first gear transmission devices in each planetary gear are adjacent, it is also conceivable that Figure 8 In the third embodiment illustrated in FIG. 1 , they are replaced by a single first gear transmission 34, which can be embodied as a single component centered on the transverse symmetry plane A. In this case, the first gear transmission 34 in each planetary gear 32 can be a herringbone gear transmission, such as Figure 8 However, alternatively, it may be a spur gear transmission, as shown in the second embodiment. Figure 8 As illustrated in FIG. 1 , the first gear transmission device 34 of the planetary gear 31 of the third embodiment can be meshed with a single corresponding gear transmission device 20a located on the ring gear 20. In the third embodiment, the other elements of the reducer 6 can also be the same or at least equivalent to the elements in the previous embodiments, and can also be arranged in the same or similar manner. Therefore, they are Figure 8 and Fig. 9 The same reference numerals as in the previous figures are used.

[0043] Although in these three first embodiments the gear train is a planetary gear train in which the planet carrier 30 is fixed, it is also conceivable that the mechanical reducer 6 comprises an epicyclic gear train in which the ring gear 20 is fixed to the housing 5 and thus fixed, and the planet carrier 30 is rotationally fixed to the fan shaft 4, such as Fig. 9 In the fourth embodiment, the other elements of the reducer 6 may also be the same or at least equivalent to the elements of the previous embodiment, and may also be arranged in the same or similar manner. Fig.10 The same reference numerals as in the previous figures are used.

[0044] Furthermore, it is also conceivable, in particular for a turbine 1 having a fan S with counter-rotating blades S1, S2, that the gear train is a differential gear train. Fig.11 and Fig.12 As illustrated in FIG. 1 , the sun gear 10 can then be rotationally fixed to the low-pressure shaft 3, the ring gear 20 of the first fan shaft 4a, and the planet 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 elements of the previous embodiments, and can also be arranged in the same or similar manner. Therefore, they are Fig.11 and Fig.12 The same reference numerals as in the previous figures are used.

[0045] 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 different embodiments illustrated / mentioned (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 into other embodiments. Therefore, the present description and the drawings must be regarded as illustrative and not restrictive.

Claims

1. A gear train for a mechanical reducer (6) of a turbine (1), the gear train comprising: a sun gear (10), the sun gear having a central axis (X), a ring gear (20) coaxially arranged 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 at least one first gear transmission (34; 34a, 34b) and two second gear transmissions (35a-35d), the gear transmissions being offset relative to each other in an axial direction parallel to the central axis (X), Each of the first gear transmission devices (34; 34a, 34b) meshes with the ring gear (20) and has a first median diameter (D1), and Each of the second gear transmission devices (35a-35d) is meshed with the sun gear (10) and has a second median diameter (D2) larger than the first median diameter (D1), and the gear system is characterized in that the second gear transmission devices (35c, 35d) of the second plurality of planetary gears (32) are 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 second gear transmission devices (35a-35d) of each planetary gear (31, 32) are arranged symmetrically with respect to a transverse plane (A) perpendicular to the central axis (X).

3. The gear train according to any one of claims 1 or 2, wherein: At least one of the planetary gears (31, 32) comprises a single first gear transmission (34) centered on the transverse plane (A).

4. The gear train according to any one of claims 1 to 3, wherein: At least one of the planetary gears (31, 32) comprises two first gear transmissions (34a, 34b), which are 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 (34; 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 device (34; 34a, 34b), the second gear transmission device (35a, 35b) of the planetary gear (31) in the first plurality of planetary gears (31) and / or the second gear transmission device (35c, 35d) of the planetary gear (32) in the second plurality of planetary gears (32) are helical-toothed.

7. The gear train according to claim 6, wherein: Two of the first gear transmission devices (34a, 34b) of each planetary gear (31, 32), the second gear transmission device (35a, 35b) of each planetary gear (31) of the first plurality of planetary gears (31) and / or the second gear transmission device (35c, 35d) of each planetary gear (32) of 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: Each planet gear (31, 32) of the first plurality of planet gears (31) and the second plurality of planet gears (32) comprises at least two parts (31a, 31b, 31c) rotationally attached to each other by complementary non-axisymmetric profiles (34).

10. A mechanical reducer (6) for a turbine (1), comprising: According to a gear train as claimed in any one of the preceding claims, at least one input shaft (3) and an output shaft (4), the at least one input shaft being mechanically rotationally connected to one of the sun gear (10), the ring gear (20) and the planet carrier (30), and the output shaft being mechanically rotationally connected 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) for a turbomachine (1), at least one turbine (1e) and a rotating device according to claim 10, wherein the at least one turbine (1e) is mechanically rotationally coupled to at least one input shaft (3) of the mechanical reducer, and the rotating device is mechanically rotationally coupled to the output shaft (4).

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

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