POWER TRANSMISSION SYSTEM WITH IMPROVED OIL RECOVERY CHANNEL

Description

Technical field of the invention

[0001] The field of the invention is that of aircraft reduction gear power transmission systems, and more particularly that of oil recovery gutters equipping a reduction gear of such a power transmission system. Technical background

[0002] The state of the art includes in particular documents FR-A1-3.084.427, US-B1-6.223.616, FR-A1-3.052.522 and applications FR-A1-3.084.407, FR-A1-3.081.513, FR-A1-3.095.243, FR-A1-3.095.243, and FR-A1 -2,987,402.

[0003] Some turbomachines such as turbojets equipped with a fan, or turboprops with a single propeller or a pair of counter-rotating propellers are equipped with a power transmission system essentially comprising a differential speed reducer with epicyclic gear train or a speed reducer with planetary gear train (designated by the English acronym "RGB" for " Reduction Gear Box”). Differential gearboxes with epicyclic gear train, i.e. in which all the components are mobile, are applied to turboprops with double propellers. Planetary gearboxes, i.e. in which the planet carrier is fixed and the ring gear is mobile, are applied to single-propeller turboprops or to double-flow turbojets equipped with a fan.

[0004] In the latter case, generally speaking, a power transmission shaft drives the fan shaft of the turbofan or the rotor shaft of the propellers via the speed reducer. The fan shaft or the propeller shaft is supported and guided by bearings which include rolling elements such as balls or rollers.

[0005] Such speed reducers are equipped with several rotating gear wheels and / or pinions, the lubrication of which is essential for the proper functioning of the turbomachine and its efficiency. When the speed reducer is not sufficiently lubricated, friction between the teeth of the gear wheels and / or pinions or at the bearings leads to their premature wear and also causes a drop in the efficiency of the speed reducer. These bearings, gear wheels and / or pinions of the speed reducers can produce high thermal power to ensure good efficiency, requiring a lubrication system designed to send a significant flow of oil that can reach several thousand liters per hour to lubricate the speed reducer and the bearings and cool them. One of the disadvantages of such a lubrication system is the difficulty of being able to recover and evacuate such an oil flow.

[0006] The lubrication system comprises one or more casings forming an enclosure in which the speed reducer and the bearings are contained. The oil is generally evacuated towards the lower part of the enclosure, i.e. at "six o'clock". The enclosure also comprises evacuation means such as pipes which return the oil to the lubrication system. In the case of a speed reducer with an epicyclic gear train, in which the sun gear and the planet carrier are rotating while the outer ring is fixed, evacuation means are provided in the vicinity of a lower part of the outer ring so as to evacuate the oil from the internal parts of the speed reducer towards the radially internal part of the enclosure.

[0007] On the contrary, in the case of a planetary speed reducer with a rotating outer ring, evacuation is more complex. Indeed, when the outer ring rotates, the oil is projected by centrifugal effect over the entire wall of the enclosure, the volume of which is quite large. The oil progresses by gravity towards the radially internal part of the enclosure for evacuation. However, recovery is not rapid and efficient given the high flow rate of oil circulating in the enclosure and in the lubrication system.

[0008] To overcome this drawback, epicyclic gear train speed reducers have been proposed that include oil recovery systems that prevent oil from accumulating in the enclosure by forming accumulation zones and quickly evacuating the oil. Such a system includes an annular gutter arranged around the outer ring of the speed reducer, responsible for recovering the oil and channeling it towards an oil recovery chamber.

[0009] Such a gutter generally has fairly simple shapes and has a general U-shape in axial section, matching the crown of the reducer.

[0010] The crown of the reducer is generally made up of two half-flanges with reversed teeth, assembled to each other by screwing means.

[0011] However, it has been found that, during rotation of the crown, the screwing means, which protrude from the free faces of the flanges, tend to produce significant aerodynamic turbulence, in the form of swirling flows which, on the one hand, consume mechanical power in pure loss and on the other hand disturb the recovery of the oil. Objective of the invention

[0012] The present invention therefore aims to provide a recovery system comprising an improved gutter aimed at reducing these disturbances. Statement of the invention

[0013] To this end, the invention proposes a power transmission system for an aircraft turbomachine, comprising: a speed reducer comprising a sun gear rotatable about an axis X, a crown gear rotatable about the axis X, and planet gears carried by a planet carrier, meshing with the sun gear and the crown gear, said crown gear comprising two half-crowns each carrying teeth for meshing with the planet gears and comprising external annular flanges of radial orientation which are clamped against each other by first fixing means which pass through said flanges, axes Y of said first fixing means being parallel to the axis X and distributed over a circumference of diameter D1, an annular gutter for recovering centrifuged oil, extending around said flanges, this gutter having in axial section a general U shape surrounding the flanges and comprising an annular bottom wall of axis X connected to two lateral annular walls substantially perpendicular to the axis X, characterized in that: the annular lateral walls of the gutter comprise internal peripheral edges having a predetermined internal diameter D2 less than the diameter D1, the bottom wall and the lateral walls of the gutter are arranged, respectively with respect to external peripheral edges of the flanges and with respect to the first screwing means with predetermined clearances J1 and J2, said predetermined internal diameter D2 and said predetermined clearances J1, J2 of the gutter being configured so that the gutter covers the flanges and the first fixing means, and in that the gutter is formed by an axial assembly of at least two parts, a first part of which comprises at least a portion of the bottom wall, and a second part of which comprises one of the lateral walls.

[0014] The invention thus advantageously makes it possible to propose a power transmission system whose reducer comprises a gutter forming a fairing placed as close as possible to the crown, consequently capable of limiting the aerodynamic disturbances induced by the first fixing means during rotation of the crown. This induces a reduction in mechanical losses and an improvement in oil recovery.

[0015] According to other characteristics of the invention: the first and second parts are fixed to each other by second fixing means whose axes Z are parallel to the axis X and which are distributed over a circumference of diameter D3 greater than D1, according to a first embodiment of the invention, the first part comprises one of the side walls, a part of the bottom wall and a first annular fixing collar extending radially outwards to form a fixing part of the gutter, and the second part comprises the other side wall and another part of the bottom wall, and a second annular fixing collar, the first and second collars being configured to be fixed axially to each other.according to a second embodiment of the invention, the first part comprises the bottom wall and a single annular fixing collar extending radially outwards to form a fixing part of the gutter, the second part comprises one of the side walls of the gutter and a third part of the gutter comprises the other side wall, the bottom wall and the external peripheral edges of the annular flanges of the half-crowns are separated by the radial clearance J1 which is between 5 and 10 mm, each of the side walls is separated from the first fixing means by the axial clearance J2 which is between 4 and 8 mm, the internal diameter D2 is separated, with respect to a minimum diameter of a circumference passing through the first fixing means, by a radial clearance J3 of at most 1 mm.

[0016] The invention also relates to a method of assembling a transmission system according to the first embodiment, which comprises: a first step during which the fixing part of the first part is fixed to a fixed annular casing of the reducer, a second step during which the half-crowns of the reducer are positioned, a third step during which the second part is fixed to the first part.

[0017] The invention also relates to a method of assembling a transmission system according to the second embodiment. This method is similar to the method of assembling the transmission system according to the first embodiment, except that during the third step the third part is also fixed to the first part.

[0018] The invention finally relates to a turbomachine comprising a gas turbine coupled to a fan or to a propeller via a transmission system of the type described above. Brief description of the figures

[0019] Other characteristics and advantages of the invention will appear during the reading of the detailed description which follows for the understanding of which reference will be made to the appended drawings in which: [ Fig. 1 ] There figure 1 represents an axial and partial section of a turbomachine according to the state of the art comprising a fan module whose fan shaft is driven by a power shaft via a speed reducer installed in a lubrication enclosure; [ Fig. 2 ] There figure 2 is a schematic view illustrating the aerodynamic disturbances created by the means of fixing an external crown of a planetary speed reducer with gear train according to the state of the art during its rotation; [ Fig. 3 ] There figure 3 is a schematic, axial and partial view of a lubricant recovery gutter according to a first embodiment of the invention; [ Fig. 4 ] There figure 4 is an axial sectional view of the recovery gutter according to the first embodiment mounted in a fixed casing of the turbomachine; [ Fig. 5 ] There figure 4 is a partial phantom perspective view of the gutter of the figure 3 ; [ Fig. 6 ] There Figure 5 is a schematic, axial and partial view of a lubricant recovery gutter according to a second embodiment of the invention. Fig. 7 ] there figure 7 is a block diagram illustrating the assembly steps of a transmission system according to the invention. Detailed description of the invention

[0020] We represented at the figure 1an axial and partial sectional view of a turbomachine 10 with longitudinal axis X, in particular a double-flow turbomachine to which the invention applies. Without limiting the invention, the latter can be applied for example to a turboprop or a turbomachine comprising a pair of counter-rotating propellers rotating around a longitudinal axis and designated by the expression “open rotor”, an English acronym for unducted propeller.

[0021] The turbomachine 10 comprises, in a known manner, an external nacelle (not shown) receiving a gas turbine 12 upstream of which a fan 14 is mounted. In the present invention, and generally, the terms "upstream" and "downstream" are defined with respect to the direction of circulation of the gases in the turbomachine which is substantially parallel to the longitudinal axis X. Similarly, the terms "radial", "internal", "external", "lower", "upper", "below" and "above" are defined with respect to a radial axis R perpendicular to the longitudinal axis X and with respect to the distance from the longitudinal axis X. The gas turbine 12 comprises, for example, from upstream to downstream, a low-pressure compressor 16, a high-pressure compressor, a combustion chamber, a high-pressure turbine and a low-pressure turbine (not shown).The high-pressure compressor is connected to the high-pressure turbine via a high-pressure shaft to form a high-pressure spool. Similarly, the low-pressure compressor 16 is connected to the low-pressure turbine via a low-pressure shaft 18 to form a low-pressure spool. The low-pressure shaft 18 and the high-pressure shaft are centered on the longitudinal axis X.

[0022] The fan 14 is here shrouded by a fan casing (not shown) secured to the external nacelle. The fan 14 compresses the air entering the turbomachine which is divided into a primary flow P circulating in an annular primary vein 20, which passes through the gas turbine 12 and a secondary flow S circulating in an annular secondary vein 22 surrounding the gas turbine 12. In particular, the primary vein 20 and the secondary vein 22 are separated by an annular inter-vein casing 24 surrounding the gas turbine 2. The fan 14 comprises a series of blades 26 which each extend radially outwards and are delimited radially by the fan casing.

[0023] A fan module comprising the fan 14 is arranged upstream of an internal casing 28 of the turbomachine which is traversed by a rotor shaft 30, here the fan shaft which is centered on the longitudinal axis X. The fan shaft 30 is driven in rotation by a power shaft of the turbomachine via a power transmission system, making it possible to reduce the rotational speed of the fan 14. This power shaft is the low pressure shaft 18 in the present example. The power transmission system allows the arrangement of a fan with a large diameter which causes an increase in the bypass ratio.

[0024] The power transmission system includes a speed reducer 32 formed by a gear train and known by the English acronym RGB for " GearBox Discount”. The speed reducer here is a planetary reducer.

[0025] This includes, as schematically illustrated on the figure 1 , a pinion here called sun 34, satellites 36, a planet carrier 38, and an external crown 40, conventionally constituted by two half crowns 40a, 40b, assembled by their flanges 42a, 42b using first axial fixing means such as screws and nuts, the internal teeth of which mesh with the satellites 36. In the present example, the input of the speed reducer 32 is coupled to the low pressure shaft 18 while the output of the speed reducer 32 is coupled to the fan shaft 30.

[0026] The speed reducer 32 comprises an input shaft 44, centered on the longitudinal axis X, integral in rotation with the low pressure shaft 18 and with the sun 34 along the longitudinal axis X. The external crown 40, also centered on the longitudinal axis X, is integral in rotation with the fan shaft 30 around the longitudinal axis X. The satellites 36, for example five in number, are carried by the planet carrier 38 which is mounted fixed. The satellites 36 each rotate around an axis substantially parallel to the longitudinal axis X. Each of the satellites 36, produced in the form of a pinion, has teeth which mesh with those of the sun 34, in the form of a toothed wheel, and with the external crown 40, each half-crown of which is provided with internal teeth of pitches reversed relative to each other. In operation, the solar 34 is driven in rotation by the low pressure shaft 18 according to a first rotation speed.The satellites 36 are rotated by the sun gear 34 around their axis at a second rotational speed. The outer ring gear 40 which meshes with the satellites 36 is rotated around the longitudinal axis X and drives the fan shaft 30. The outer ring gear 40 rotates at a third rotational speed and in a direction opposite to that of the sun gear 34.

[0027] Alternatively, the speed reducer 32 could be a differential speed reducer with an epicyclic gear train. In this case, the sun gear, the planet gears, the planet carrier and the outer ring gear would all be movable.

[0028] As illustrated by the figure 1, first and second bearings 46 and 48 make it possible to support and guide in rotation the fan shaft 10 in order to support the radial and axial loads to which it is subjected. These first and second bearings 46, 48 comprise rolling elements which are each mounted between a first and a second ring. Each first ring is mounted on the fan shaft 30 and each second ring is mounted on a ferrule 50. The rolling elements of the first bearing 46, mounted in the vicinity of the speed reducer 32, are here balls while the rolling elements of the second bearing 48 are rollers. The first and second bearings 46, 48 are mounted upstream of the speed reducer 32. More precisely, the second bearing 48 is mounted upstream of the first bearing 46. Similarly, a third bearing 52 guides the low pressure shaft 18 in rotation. This third bearing 18 is located downstream of the speed reducer 32.The third bearing 52 also comprises an inner ring carried by the low pressure shaft 18 and an outer ring carried by a downstream support 24.

[0029] The first, second, and third bearings 46, 48, 52, as well as the gear wheels and pinions of the speed reducer 32 are contained in an annular lubrication enclosure 56 formed by one or more fixed casing(s) 58 of the turbomachine. The shell 50 and the downstream support 54 are fixed to the annular fixed casing 58. This fixed casing 26 is itself secured to the internal casing 28 of the turbomachine. The enclosure 56 extends axially and radially on either side of the speed reducer 32. As illustrated in the figure 1 , this enclosure 56 is crossed at least partly axially by the fan shaft 30 and by the low pressure shaft 18.

[0030] In the enclosure 56 circulates a lubricant such as oil for permanently lubricating and cooling the bearings and the rotating members of the speed reducer during operation of the turbomachine to ensure the proper functioning of the bearings and the speed reducer 32. For this, a lubrication system (not shown) comprises pipes which spray oil through the sun gear 34. The oil is injected into the rotating members at the level of the satellites 36 and the sun gear 34 then of the external ring gear 40 by centrifugation using ejection means (not shown). The oil is also injected at the level of the first, second and third bearings 56, 48, 52.

[0031] In such a reducer 32, the oil is generally projected by centrifugal force through the crown 40, the two half-crowns 40a, 40b of which generally have oil passages. The oil then flows by gravity to a low point in the enclosure 56 where it is to be pumped again to the reducer.

[0032] To prevent the oil from stagnating for too long on the walls of the enclosure 56, an annular oil recovery gutter 62 is generally arranged around the crown 40, which is responsible for collecting it.

[0033] However, it has been found that this recovery needs to be improved because the first fixing means which allow the assembly of the half-crowns 40a, 40b to one another cause aerodynamic disturbances between their flanges 42a, 42b and the gutter 62, in the form of swirling flows which dissipate mechanical power to no avail.

[0034] As illustrated by the figure 2 , which represents a flange 42a of the half-crown 40a provided with first fixing means 60, when the crown 40 is rotated in the direction W, a force F is exerted on the first fixing means 60, opposite to the movement due to the interaction between the air and the first fixing means. This force F creates on the first fixing means 60 pressure fields P, and depression D represented by rectangles on the figure 2 .

[0035] These pressure differentials have the consequence of disturbing the flow on the sides of the flanges 40a, 40b, in the form of swirling flows which have the consequence of causing the appearance of mechanical energy losses. Indeed, the flow of air is disturbed around this entire geometry, with an inlet from the enclosure 56 outside the crown 40 towards the first fixing means 60 when the crown 40 accelerates its rotation, and an outlet of the air towards the enclosure 56 when the crown 40 slows down.

[0036] The invention overcomes this drawback by proposing a gutter 62 limiting these drawbacks.

[0037] To this end, the invention provides a power transmission system of the type previously described comprising an improved gutter 62.

[0038] In a known manner, as illustrated by the figures 3 to 6, the centrifuged oil recovery gutter 62 is annular, with an X axis. It extends around the flanges 42a, 42b and has an axial section with a general U shape surrounding the flanges 42a, 42b. The first fixing means 60 comprise, for example, screws 68 which pass through the flanges 42a, 42b and which tighten the flanges against each other by means of nuts 70. Axes Y of the first fixing means and therefore of the screws 68 are parallel to the X axis and distributed over a circumference of diameter D1.

[0039] Also in a known manner, the gutter 62 comprises an annular bottom wall 64 of axis X connected to two lateral annular walls 66a, 66b substantially perpendicular to the axis X.

[0040] To limit the turbulence produced by the first fixing means 60 during the rotation of the crown 40 between the flanges 42a, 42b and the gutter 62, the latter is configured to constitute a fairing of the flanges 42a, 42b of the crown 40.

[0041] To this end, firstly, the lateral annular walls of the gutter 66a, 66b comprise internal peripheral edges 72a, 72b having a predetermined internal diameter D2 less than the diameter D1. In this way, the gutter 62 covers the majority of the first screwing means 60, thus limiting the phenomenon of suction of air from outside the gutter 62 inside the latter.

[0042] Furthermore, the gutter 62 is configured to fit as closely as possible the first fixing means 60. For this purpose, the bottom wall 64 is arranged, relative to external peripheral edges 43a, 43b of the flanges 42a, 42b, with a predetermined clearance J1, and the side walls 66a, 66b of the gutter 62 are arranged, relative to the first fixing means 60, that is to say relative to the ends of the screws 68, with a predetermined clearance J2.

[0043] Advantageously, the predetermined internal diameter D2 and the predetermined clearances J1, J2 of the gutter 62 are configured so that the gutter 62 covers the flanges 42a, 42b and the first fixing means 60, i.e. the screw 68. In this way, the gutter 62 advantageously induces a flow of air free from disturbances between the first fixing means 60 and the gutter 62 during rotation of the crown 40.

[0044] The determination of this internal diameter D2 and the predetermined clearances J1, J2 results from a calculation using software for simulating air flows between the first fixing means 60, the flanges 42a, 42b, and the gutter 62. These internal diameters D2 and clearances J1, J2 are thus calculated as precisely as possible to minimize the possibilities of swirling flows, and therefore be adjusted so that the gutter 62 is arranged as close as possible to the first fixing means 60, the flanges 42a, 42b, while maintaining a minimum distance from them taking into account the axial movements of the crown 40 during its rotation, the manufacturing dispersions thereof over its diameter, and the expansion of these parts during their operation.

[0045] The gutter 62 being arranged as close as possible to the first fixing means 60 of the flanges 42a, 42b, it is advantageous that this gutter 62, to allow its mounting, is formed by an axial assembly of at least two parts, 74a, 74b of which a first part 74a comprises at least one part 64a of the bottom wall 64, and of which a second part 74b comprises one of the side walls 66b. The first and second parts 74a, 74b are fixed to each other by second fixing means 76 comprising for example screws whose axes Z are parallel to the axis X and which are distributed over a circumference of diameter D3 greater than D1. The second fixing means 76 comprise for example screws 78.

[0046] According to a first embodiment of the invention which has been shown in figures 3 , 4 And 5, the first part 74a comprises one of the side walls 66a, a portion 64a of the bottom wall 64 and a first annular fixing collar 80a extending radially outwards to form a portion 82 for fixing the gutter 62. As illustrated in Figure 5 , which represents the gutter 62 on the other side, it is by this fixing part 82 that the gutter 62 is fixed to the annular casing 58 which surrounds the reducer.

[0047] In this first embodiment of the invention, the second part 74b comprises the other side wall 66b and another part 64b of the bottom wall 64, and a second annular fixing collar 80b which is intended to be fixed to the first collar 80a. It will be understood that the first and second collars 80a, 80b are configured to be fixed axially to each other. For this purpose, and in a manner not limiting the invention, the first collar 80a comprises an annular groove 84 in which the second collar 80b is received, and the two collars are assembled by the second fixing means 76, that is to say crossed by the screws 78 which assemble them to each other.

[0048] According to a second embodiment of the invention which has been shown in the figure 6, the first part 74a comprises the bottom wall 64 and a single annular fixing collar 80 extending radially outwards to form a part 82 for fixing the recovery device, as previously. The second part 74b comprises one of the side walls 66a of the gutter 62 and a third part 74c of the gutter comprises the other side wall 66b. The second part 74b and the third part 74c are fixed on either side of the first part 74a, not as previously at the level of the annular collar 80, but at the axial ends of the bottom wall 64, by means of second fixing means 76, that is to say preferably by means of screws 78 with axes Z. As previously, the axes Z of the screws 68 are parallel to the axis X and distributed over a circumference of diameter D3 greater than D1.

[0049] By way of example, and in a non-limiting manner of the invention, in either of these two embodiments, the bottom wall 64 and the external peripheral edges 43a, 43b of the annular flanges 42a, 42b of the half-crowns 40a, 40b are separated by a radial clearance J1 of between 5 and 10 mm. This clearance corresponds to a common value for a standard reducer, it can nevertheless be lower if the dimensioning of the oil evacuation system allows it.

[0050] Each of the side walls 66a, 66b is separated from the first fixing means 68 by an axial clearance J2 of between 4 and 8 mm.

[0051] This range of values ​​corresponds on the one hand to a minimum safety value of 4 mm to prevent possible axial displacement of the crown accompanying axial displacement of the reducer. On the other hand, the maximum value of 8 mm makes it possible to avoid a high air speed gradient between the first fixing means 68 and the gutter, which would reduce the fairing effect of the gutter and risk allowing the flow to escape from the gutter.

[0052] The internal diameter D2 is separated, with respect to a minimum diameter D4, from a circumference passing through the first screwing means 68, which has been shown in figures 3 And 6 by a radial clearance J3 of at most 1 mm. It will therefore be understood that the side walls 66a, 66b extend in line with the majority of the first fixing means 68.

[0053] In this configuration, as illustrated by the figure 3 and the figure 7, a transmission system comprising a gutter 62 according to the first embodiment of the invention can be assembled by a method comprising firstly a first step ET1 during which the fixing part 82 of the first part 74a is fixed to the annular fixed casing 58 of the reducer. Then, during a second step ET2, the half-crowns of the reducer are positioned. This operation being an assembly operation of the reducer widely known in the art, it will not be detailed further in the present description.

[0054] Then, during a third step ET3, the second part 74b is fixed to the first part 74a. In particular, the second collar 80b of the second part 74b is positioned in the annular groove 84 of the first collar 80a of the first part 74a, and the two collars are then assembled by the second fixing means 76, that is to say crossed by the screws 78 which assemble them to each other.

[0055] The assembly of a transmission system according to the second embodiment of the invention is substantially similar. As illustrated by figure 6 and the figure 7, it can be produced according to a method comprising a first step ET1 during which the fixing part 82 of the first part 74 is fixed to the annular fixed casing 58 of the reducer. Then, during a second step ET2, the half-rings 40a, 40b of the reducer are put into position. Finally, during a third step ET3, the second and third parts 74b and 74c are fixed to the first part 74a. In particular, the side walls 66a and 66b are fixed to the ends of the bottom wall 64 using the second fixing means 76, that is to say in the case which has been shown here, using the screws 78.

[0056] The invention therefore advantageously makes it possible to have a power transmission system comprising an oil recovery gutter 62 minimizing aerodynamic losses. Such a power transmission system can be used in the context of a turbomachine comprising a gas turbine coupled to a fan or to a propeller via this power transmission system.

Claims

1. A power transmission system for an aircraft turbomachine, comprising: - a speed reducer (32) comprising a sun pinion (34) which can rotate about an axis X, a ring gear (40) which is mobile in rotation about the axis X, and planet pinions (36) carried by a planet carrier (38), meshing with the sun pinion (34) and the ring gear (40), said ring gear (40) comprising two half-ring gears (40a, 40b) each carrying teeth for meshing with the planet pinions (36) and comprising radially oriented external annular flanges (42a, 42b) which are tightened against each other by first attachment means (60), which pass through said flanges (42a, 42b), axes Y of said first attachment means (60) being parallel to the axis X and distributed over a circumference of diameter D1, - an annular channel (62) for recovering centrifuged oil, extending around said flanges (42a, 42b), this channel having in axial cross-section a general U shape surrounding the flanges (42a, 42b) and comprising an annular bottom wall (64) of axis X connected to two annular lateral walls (66a, 66b) substantially perpendicular to the axis X, characterised in that: - the annular lateral walls (66a, 66b) of the channel (62) comprise internal peripheral edges (72a, 72b) having a predetermined internal diameter D2 less than the diameter D1, - the bottom wall (64) and the lateral walls (66a, 66b) of the channel (62) are arranged, respectively, with respect to external peripheral edges (43a, 43b) of the flanges (42a, 42b) and with respect to the first attachment means (60) with predetermined clearances J1 and J2, - said predetermined internal diameter D2 and said predetermined clearances J1, J2 of the channel (62) being configured such that the channel (62) covers the flanges (42a, 42b) and the first attachment means (60), - and in that the channel (62) is formed by an axial assembly of at least two parts (74a, 74b), a first part (74a) of which comprises at least one portion of the bottom wall (64), and a second part (74b) of which comprises one of the lateral walls (66b).

2. The transmission system according to the preceding claim, characterised in that the first and second parts (74a, 74b) are attached to each other by second attachment means (76) whose axes Z are parallel to the axis X and which are distributed over a circumference of diameter D3 greater than D1.

3. The transmission system according to one of the preceding claims, characterised in that: - the first part (74a) comprises one of the lateral walls (66a), a portion (64a) of the bottom wall (64) and a first annular attachment collar (80a) extending radially outwardly to form an attachment portion (82) of the channel (62), and - the second part (74b) comprises the other lateral wall (66b) and another portion (64b) of the bottom wall (64), and a second annular attachment collar (80b), the first and second collars (80a, 80b) being configured to be axially attached to each other.

4. The transmission system according to one of claims 1 or 2, characterised in that: - the first part (74a) comprises the bottom wall (64) and a single annular attachment collar (80) extending radially outwardly to form an attachment portion (82) of the channel (62), - the second part (74b) comprises one of the lateral walls (66a) of the channel (62) and, - a third part (74c) of the channel (62) comprises the other lateral wall (66b).

5. The transmission system according to one of the preceding claims, characterised in that the bottom wall (64) and the external peripheral edges (43a, 43b) of the annular flanges (42a, 42b) of the half-ring gears (40a, 40b) are separated by the radial clearance J1 which is between 5 and 10 mm.

6. The transmission system according to any of the preceding claims, characterised in that each of the lateral walls (66a, 66b) is separated from the first attachment means (60, 62) by the axial clearance J2 which is between 4 and 8 mm.

7. The transmission system according to any of the preceding claims, characterised in that the internal diameter D2 is separated, relative to a minimum diameter of a circumference passing through the first attachment means (60), by a radial clearance J3 of at most 1 mm.

8. A method for assembling a transmission system according to one of claims 1 or 2, characterised in that it comprises: - a first step (ET1) in which the attachment portion (82) of the first part (74a) is attached to an annular stationary casing (58) of the reducer, - a second step (ET2) in which the half-ring gears (40a, 40b) of the reducer are brought into position, - a third step (ET3) in which the second part (74b) is attached to the first part (74a).

9. The method for assembling a transmission system according to the preceding claim taken in combination with claim 4, characterised in that during the third step (ET3) the third part (74c) is also attached to the first part (74a).

10. A turbomachine (10) comprising a gas turbine (12) coupled to a fan (14) or to a propeller by means of a transmission system according to any of claims 1 to 7.