Mechanical reduction gear for a turbine engine

Abstract

The invention relates to a mechanical reduction gear comprising: - a sun gear having a first axis of rotation; - a ring gear extending around the sun gear; - planet gears that are meshed with the sun gear and the ring gear, these planet gears being held by a planet carrier rotating about the first axis; and - a ring gear carrier (36) comprising a first attachment flange (362) for attachment to the ring gear and a second attachment flange (364), wherein the ring gear carrier comprises, between its flanges (362, 364), an annular strip (360) that has through-holes (366), these through-holes defining therebetween connecting bars (368) configured to transmit a torque transmitted by the ring gear in a first direction about the first axis, and to break when a torque greater than a predetermined threshold is transmitted by the ring gear in a second direction (D2) about the first axis opposite to the first direction.

Description

[0001]DESCRIPTION TITLE: MECHANICAL REDUCER FOR A TURBOMACHINE Field of the invention The present invention relates to the field of mechanical reducers for a turbomachine, in particular for an aircraft, as well as a turbomachine comprising such a mechanical reducer. Technical background The state of the art includes in particular documents US-A1-2018 / 038448 and US-A1-2018 / 297157. The role of a mechanical reducer is to modify the speed and torque ratio between the input shaft and the output shaft of a mechanical system. The new generations of double-flow turbomachines, in particular those having a high bypass ratio, comprise a mechanical reducer to drive the shaft of a fan. Usually, the speed reducer aims to transform the so-called fast rotation speed of the shaft of a power turbine into a slower rotation speed for the shaft driving the fan. Such a reducer comprises a central pinion,called sun gear, a crown and pinions called satellites, which are meshed between the sun gear and the crown gear. The satellites are held by a frame called a planet carrier. The sun gear, the crown gear and the planet carrier are planetary gears because their axes of revolution coincide with the longitudinal axis X of the turbomachine. The satellites each have a different axis of revolution and are equally distributed on the same operating diameter around the axis of the planetary gears. These axes are parallel to the longitudinal axis X. There are several gearbox architectures. In the state of the art of double-flow turbomachines, the gearboxes are of the planetary or epicyclic type. In other similar applications, there are architectures called differential or compound. x On a planetary gearbox, the planet carrier is fixed and the crown gear constitutes the output shaft of the device which rotates in the opposite direction to the sun gear. x On an epicyclic gearbox, thecrown is fixed and the planet carrier constitutes the output shaft of the device which rotates in the same direction as the sun gear. x On a differential reducer, no element is fixed in rotation. The crown rotates in the opposite direction of the sun gear and the planet carrier. The reducers can be composed of one or more meshing stages. This meshing is ensured in different ways such as by contact, by friction or even by magnetic fields. There are several types of contact meshing such as with straight, helical or herringbone teeth. In operation, a failure of a satellite bearing (hydrodynamic or rolling type) can cause the fan to jam, which can lead to an increase in the drag of the turbomachine and impair the maneuverability of the aircraft. To prevent such a jamming of the fan, it was proposed in document FR-A1-3066004) to form the crown by two half-crowns, so as tobe able to be uncoupled from the satellite when the satellite exerts on each half-crown a disengaging action whose amplitude is greater than a determined amplitude value. For this, this document proposes to integrate a fusible stop connected both to the crown carrier and to one of the half-crowns and a ball thrust bearing to retain the half-crowns in a coupled position with the satellite during normal operation of the reducer. This fusible stop breaks by the disengaging action, in which the predetermined amplitude which is exceeded corresponds to a blockage of a component of the turbomachine (such as a blockage of a satellite bearing) and the axial force applied to the crown is oriented towards the outside of the reducer. However, this design can be complex to implement in particular by adding additional elements (fusible stop, ball thrust, spring, etc.) to be assembled in the reducer and thus cumbersome the reducer. In this context, it isinteresting to overcome the drawbacks of the prior art, by proposing a solution making it possible to maintain the rotation of a fan during a blockage of a component of the mechanical reducer of a turbomachine (resulting from a failure of a satellite bearing or a satellite for example). Summary of the invention The present invention proposes a simple and economical solution to at least some of the aforementioned problems. To this end, the invention relates to a mechanical reducer for a turbomachine, in particular an aircraft, this reducer comprising: - a sun gear having a first axis A of rotation, - a ring which extends around the sun gear and said first axis A, - satellites which are arranged around the first axis A, between the sun gear and the ring gear, and which are meshed with the sun gear and the ring gear, these satellites each having a second axis B of rotation parallel to the first axis A and being held by a planet carrier movable in rotation around said first axisA, and - a crown carrier having a generally annular shape around the first axis A, the crown carrier comprising a first annular flange for attachment to the crown and a second annular attachment flange intended to be attached to a member of the turbomachine. According to the invention, the crown carrier comprises, between its flanges, an annular band around the first axis A which has through-holes distributed around this first axis A, these holes defining between them connecting bars between a first annular part of the crown carrier comprising the first flange, and a second annular part of the crown carrier comprising the second flange, these connecting bars being configured to transmit a torque transmitted by the crown in a first direction D1 around the first axis A, and to break when a torque greater than a predetermined threshold is transmitted by the crown in a second direction D2 around the first axis A, opposite to the firstdirection D1. The invention thus proposes to modify the crown carrier so as to maintain the rotation of a fan of the turbomachine during a blockage of a component of the reducer (for example of a satellite bearing or of the satellite). For this, the crown carrier integrates an annular band with through-holes and connecting bars. These connecting bars, on the one hand, are defined between the through-holes, and on the other, make it possible to connect a first part of the crown carrier and a second part of the crown carrier (opposite the first part) together. These connecting bars are preferably robust to tensile forces and fragile to compressive forces (or otherwise said bending forces). Thus, in normal operation of the reducer, the connecting bars are capable of transmitting a torque transmitted by the crown in a first direction D1 around the first axis A. In this configuration, the first direction D1 of transmission oftorque around the first axis A generates forces, for example in traction, on the connecting bars. This makes it possible to keep the connecting bars intact and robust (and consequently the crown carrier intact and functional), so as to maintain the connection between the crown and the crown carrier and ensure the normal operation of the reducer. In abnormal operation of the reducer (for example in the event of failure of the planet bearing(s)), the connecting bars are capable of breaking when the torque transmitted by the crown is greater than a predetermined threshold and in a second direction D2 around the first axis A, opposite to the first direction D1. Indeed, the direction of transmission of the torque by the crown to the crown carrier is reversed, for example during a failure of the planet bearing. This reversal of the direction of torque transmission generates forces, for example compression, on the connecting bars and causes a breakage of theseconnecting rods, so as to first break the crown carrier. This makes it possible to separate the connection between the crown and the crown carrier so that the crown becomes mobile in rotation around the first axis A. The rotation of the satellite(s) having the fault remains blocked. In this way, the solar directly drives the planet carrier and / or the crown carrier through the blocked satellite(s), so that the fan (connected to the planet carrier and / or the crown carrier for example via a drive shaft) of the turbomachine can continue to rotate. Consequently, in the event of a satellite blockage, the fan blockage is prevented. The problems of drag of the turbomachine on the aircraft wings and of nuisance to the maneuverability of the aircraft are limited. The connecting rods therefore make it possible to provide a mechanical fuse function (in abnormal operation described above) while allowing to have a systemrigid when the crown carrier is connected to the crown and the satellites are not blocked (in normal operation). The reducer also comprises one or more of the following characteristics, taken alone or in combination: - the connecting bars are identical; - the connecting bars comprise first connecting bars identical to each other, and second connecting bars which are identical to each other and different from the first connecting bars; - the first connecting bars are parallel to the first axis A and the second connecting bars are inclined at an angle relative to the first axis A; - some or all of the connecting bars are inclined at an angle relative to the first axis A; - the angle of inclination is between 5° and 75°; - the slots each have a general triangular, elliptical, annular, or polygonal shape; - the slots comprise first slots identical to each other, and secondidentical to each other and different from the first lights; - the first lights and the second lights are arranged in a staggered manner, each of the first lights being located between two of the second lights, and each of the second lights being located between two of the first lights; - the number of connecting bars is greater than or equal to 30, and preferably greater than or equal to 50; - the annular band has a length along the first axis A, which represents 10 to 30% of a total length of the crown carrier along this first axis A; - each of the connecting bars has a maximum width measured in a tangential direction with respect to a circumference centered on the first axis A, which is less than or equal to a maximum width of each of the lights measured in a tangential direction with respect to a circumference centered on the first axis A; - the connecting bars are oriented so as to be stressed in tension during thetransmission of torque in the first direction, and in compression when the crown carrier is subjected to a torque in the second direction; -- the turbomachine member is a turbomachine stator or a drive shaft, for example connected to (or forming) a second propeller stage of a fan of the turbomachine, -- the second annular fixing flange is intended to be fixed to the stator of the turbomachine, -- the second annular fixing flange is intended to be fixed to the drive shaft of the fan, for example this drive shaft forms a second propeller stage of the fan. The invention further relates to a turbomachine, in particular an aircraft turbomachine, comprising a mechanical reducer according to one of the particularities of the invention. The invention further relates to an aircraft equipped with such a turbomachine. Brief description of the figures The invention will be better understood, and other aims, details, characteristics and advantages thereof will appearmore clearly on reading the detailed explanatory description which follows, of embodiments of the invention given as purely illustrative and non-limiting examples, with reference to the appended schematic drawings in which: Figure 1 is a half schematic axial sectional view of a first example of a turbomachine according to the invention; Figure 2 is a half schematic axial sectional view of a second example of a turbomachine according to the invention; Figure 3a is a half schematic axial sectional view of an epicyclic type mechanical reducer of the turbomachine of Figure 1; Figure 3b is a half schematic axial sectional view of a differential type mechanical reducer of the turbomachine of Figure 2; Figure 4a is a schematic perspective view of a first embodiment of a crown carrier of the reducer of Figure 3a or Figure 3b; Figure 4b is a schematic perspective view of a second embodiment of acrown carrier of the reducer of Figure 3a or Figure 3b; Figure 4c is a schematic perspective view of a third embodiment of a crown carrier of the reducer of Figure 3a or Figure 3b; Figure 5a schematically represents normal operation of the reducer of Figure 3a or Figure 3b; Figure 5b schematically represents abnormal operation of the reducer of Figure 3a or Figure 3b, for example in the event of failure of a satellite bearing of the reducer of Figure 3a, Figure 3b or Figure 5a; Figure 6a is a partial schematic perspective view of the crown carrier of Figure 4a in normal operation; Figure 6b is a partial schematic perspective view of the crown carrier of Figure 4a in abnormal operation; Figure 7a is a partial schematic perspective view of the crown carrier of Figure 4c in normal operation; Figure 7b is a schematic viewpartial and perspective view of the crown carrier of Figure 4c in abnormal operation. Elements having the same functions in the different implementations have the same references in the figures. Detailed description of the invention By convention, in the description below, the terms "longitudinal" and "axial" qualify the orientation of structural elements extending in the direction of a longitudinal axis (such as a turbomachine). The terms "radial" or "vertical" qualify an orientation of structural elements extending in a direction perpendicular to the longitudinal axis. The terms "inner" and "outer", and "internal" and "external" are used with reference to a positioning relative to the longitudinal axis. Thus, a structural element extending along the longitudinal axis has an inner face facing the longitudinal axis and an outer surface, opposite its inner surface. Likewise, by convention in the presentapplication, the terms "upstream" and "downstream" are defined in relation to the direction of gas flow in the turbomachine. Conventionally, a turbomachine 1 (figures 1 or 2) comprises a gas generator G (or engine) upstream of which a fan S is mounted. The turbomachine 1 may extend along a longitudinal axis X. Figures 1 and 2 illustrate two types of turbomachine 1 each intended to be mounted on an aircraft. More particularly, figure 1 schematically illustrates a dual-flow turbomachine comprising the fan S with ducted propellers and extending around the axis X. Of course, the turbomachine may be a single-flow turbojet or may be a turboprop equipped with a single unducted propeller or a pair of counter-rotating, unducted propellers, known by the English expression "open rotor". Figure 2 schematically illustrates the “open-rotor” type turbomachine comprising the fan S with propellers, respectively,upstream Sa and downstream Sb which are unducted. In the example of Figure 2, the fan S called a double propeller fan Sa, Sb is upstream of the gas generator G. Alternatively (not shown), the double propeller fan S Sa, Sb can be downstream of the gas generator G. The invention can be applied to other fields in which a mechanical speed reducer is used. The gas generator G comprises, in a conventional manner and, from upstream to downstream, a low pressure compressor 1a, a high pressure compressor 1b, an annular combustion chamber 1c, a high pressure turbine 1d, a low pressure turbine 1e and optionally an exhaust nozzle 1f. The high pressure compressor 1b and the high pressure turbine 1d are connected by a high pressure shaft 1bd and form with it a high pressure (HP) body. The low pressure compressor 1a and the low pressure turbine 1e are connected by a low pressure shaft 1ae and together form a low pressure (LP) body.With reference to Figure 1, the fan S may be shrouded by a fan casing 2s carried by an external nacelle 2. The fan S generates, from an air flow F entering the fan, a primary air flow which circulates in a primary vein V1 opening into the exhaust nozzle 1f and a secondary air flow which circulates in a secondary vein V2, around the primary vein V1, opening into an ejection nozzle 2f of the nacelle 2. With reference to Figure 2, the fan S may be unshrouded (i.e. for example without a fan casing 2s). The fan S also generates, from an air flow F entering the fan, a primary air flow which circulates in a primary vein V1 emerging from the turbomachine 1, for example via an exhaust nozzle (not shown in FIG. 2). The fan S can be driven by a fan shaft 1s which is driven by the low pressure shaft 1ae by means of a mechanical reducer 3.In the example of Figure 1, the blower S can be connected to the mechanical reducer 3 by a single fan shaft 1s. In this configuration, the blower S comprises a single propeller stage in which the blower is connected to the mechanical reducer 3 by the fan shaft 1s. In the example of Figure 2, the blower S can comprise two propeller stages. In particular, the fan S can be connected to the mechanical reducer 3 by the fan shaft 1s which can form a first stage of propellers of the fan S, and a drive shaft 50 which can form a second stage of propellers of the fan S. In other words, each upstream propeller Sa of the fan S can be connected to the mechanical reducer 3 by the fan shaft 1s to form the first stage of propellers, and each downstream propeller Sb of the fan 50 can be connected to the mechanical reducer 3 by the drive shaft 50 to form the second stage of propellers. The turbomachine 1 can be equipped with themechanical reducer 3 formed of a gear train and known by the English acronym RGB for “Reduction Gear Box”. The mechanical reducer 3 is positioned in the upstream part of the turbomachine 1 following the circulation of the gases of the turbomachine. A fixed structure 4 is arranged so as to form an enclosure 40 surrounding the mechanical reducer 3. A lubricant mist reigns in the enclosure 40. This enclosure 40 is advantageously but not limited to closed upstream by seals at the level of an upstream bearing P allowing the fan shaft 1s to pass through, and downstream by seals at the level of the crossing of the low pressure shaft 1ae. The fixed structure 4 can schematically comprise, here, an upstream part 4a and a downstream part 4b. With reference to FIG. 1, the fixed structure 4 can form at least in part a turbomachine stator 5. The stator 5 may be, for example, an inlet casing of the turbomachine. With reference to figures 3a and 3b, themechanical reducer 3 can be of the epicyclic or differential type. The mechanical reducer 3 can comprise three components which are a sun gear 31, planet gears 32 and a planet carrier 33 which are rotatable. The rotation speed of one of these components depends in particular on the difference in speeds of the other two components. At the input, the mechanical reducer 3 is connected to the low pressure shaft 1ae, for example by means of splines. Thus the low pressure shaft 1ae drives the sun gear 31 (or internal planetary gear). Conventionally, the sun gear 31, of which a first axis of rotation A which is coincident with that of the longitudinal axis X of the turbomachine, drives the planet gears 32, which are equally distributed on the same diameter around the first A. This diameter is equal to twice the operating center distance between the sun gear and the planet gears. The number of planet gears is generally defined between three and seven for this type of application.solar 31 is integral in rotation with the low pressure shaft 1ae. All the satellites 32 are held by a frame called a planet carrier 33. Each satellite 32 rotates around its own second axis of rotation B. Each second axis B is parallel to the first axis A. Each satellite 32 meshes with a ring gear 35 (or external planetary gear). The planet carrier 33 may be integral in rotation with the fan shaft 1s (as illustrated in FIGS. 1, 2, 3a and 3b). The ring gear 35 may be fixed (for example integral with the stator 5 with reference to FIGS. 1 and 3a) or movable in rotation relative to the first axis A (for example connected to the second propeller stage of the fan S in particular by the drive shaft 50 with reference to FIGS. 2 and 3b). The mechanical reducer 3 may comprise an annular deflector 6 extending around the first axis A. This deflector 6 may be fixed to the crown 35 and configured to convey oil exiting radially towardsthe outside of the crown thanks to centrifugal forces. At the output of the epicyclic reducer (figure 3a), the set of satellites 32 drives the planet carrier 33 in rotation around the first axis A. The crown 35 is fixed to the stator 5 via a crown carrier 36. The planet carrier 33 is fixed and integral in rotation with the fan shaft 1s. At the output of the differential reducer (figure 3b), the set of satellites 32 drives the planet carrier 33 in rotation around the first axis A. The planet carrier 33 is therefore integral in rotation with the fan shaft 1s which forms in particular the first propeller stage of the fan S. The ring gear 35 is integral with the drive shaft 50, for example by means of the ring gear carrier 36, which forms in particular the second propeller stage of the fan S. Thus, the two outputs (namely the fan shaft 1s and the drive shaft 50) drive two different propeller stages of the fan. In particular, theplanet carrier 33 drives the fan shaft 1s via the first upstream propeller stage Sa and the ring gear 35 drives the drive shaft 50 via the second downstream propeller stage Sb. Each satellite 32 is mounted to rotate freely using a bearing 34, for example of the rolling bearing or hydrodynamic bearing type. Generally, a hydrodynamic bearing is supplied with "low" pressures (usually less than 10 bars). The rotation of the bearing makes it possible to increase the pressure of the oil wedge and to separate the satellites and the bearings. Each bearing is mounted on one of the axes of the planet carrier 33 and all the axes are positioned relative to each other using one or more structural frames of the planet carrier 33. Each satellite 32 meshes with external teeth of the sun 31 and internal teeth of the crown 35. The internal teeth of the crown 35 can be straight (parallel to the longitudinal axis) or herringbone. There are a numberof axes and bearings equal to the number of satellites. For reasons of operation, assembly, manufacturing, control, repair or replacement, the axes and the chassis can be separated into several parts. For the same reasons mentioned above, the teeth of a reducer 3 can be separated into several helices. Still with reference to figures 3a and 3b, the crown 35 is separated into two half-crowns 35a, 35b: - a front half-crown 35a consisting of a rim 35aa and a half-fixing flange 35ab. On the rim 35aa is the front helix of the teeth of the reducer. This front helix meshes with that of the satellite 32 which meshes with that of the solar 31; - a rear half-crown 35b consisting of a rim 35bb and a half-fixing flange 35ba. On the rim is the rear helix of the gear teeth. This rear helix meshes with that of the satellite 32 which meshes with that of the solar 31. The half-fixing flange 35ab of thefront crown and the half-fixing flange 35ba of the rear crown can form a third fixing flange 350 of the crown 35. This third fixing flange 350 is annular and extends radially outwards. Of course, the crown 35 can be formed from a single piece. In this case, the crown 35 comprises a (single) third annular fixing flange 350 which extends radially outwards. Still with reference to figures 3a and 3b, the mechanical reducer 3 comprises a crown carrier 36. This crown carrier 36 connects the crown 35 to a member of the turbomachine (for example the stator 5 in the case of figure 3a or the drive shaft 50 of the second propeller stage of the fan in the case of figure 3b). The crown carrier 36 makes it possible to transfer the torque from the mechanical reducer 3 to this member of the turbomachine. The crown carrier 36 is annular and centered on the first axis A. For this purpose, the crown carrier 36 comprises a first flangeannular fixing flange 362 to the crown 35, in particular to the third annular fixing flange 350, using a bolted assembly for example. This first flange 362 can extend radially inwards. Alternatively, the first flange 362 extends radially outwards. The crown carrier 36 comprises a second annular fixing flange 364 intended to be fixed to the turbomachine member using a bolted assembly for example. This second flange 364 can extend radially outwards. Alternatively, the second flange 364 extends radially inwards. The crown carrier 36 may comprise a first part 365 connected to the first flange 362 and a second part 367 connected to the second flange 364. Advantageously, the crown carrier 36 may comprise flexibility means (not illustrated in the figures) which are configured so as, on the one hand, to limit the overloads in the turbomachine due to thedisplacements of certain members thereof and / or of the mechanical reducer 3, and on the other hand, obtain a uniform and stable distribution of the dynamic loads. The flexibility means may comprise at least one bellows. The crown carrier 36 may comprise a portion provided with several bellows. One of the particularities of the invention is that the crown carrier 36 comprises an annular band 360 located between the first 362 and second 364 flanges. The annular band 360 extends around the first axis A. Examples of crown carriers 36 of the invention are illustrated in FIGS. 4a, 4b, 4c, 6a, 6b, 7a and 7b. Although the examples of the figures represent a single annular band 360, the crown carrier 36 of the present application may comprise several annular bands 360 juxtaposed with one another or spaced apart from one another, for example by an annular wall extending axially along the first axis A. In particular, the first 365 and second 367 parts of thecrown carrier 36 are connected to each other by the annular band 360. Advantageously, the first 362 and second 364 flanges, the first 365 and second 367 parts and the annular band 360 may be monobloc (i.e. made of one piece). The annular band 360 may have a length L360 along the first axis A. This length may represent from 10 to 30% of a total length L36 of the crown carrier 36 along this first axis A. The annular band 360 comprises through-holes 366 distributed around this first axis A. The through-holes 366 may each have a general triangular (figures 4c, 7a and 7b), elliptical (figure 4b), annular (or circular) or polygonal shape. For example, the polygonal shape of the through-lights 366 may be quadrangular (Figure 4a, 6a and 6b) or pentangle (i.e., star-shaped). The through-lights 366 may comprise first lights 366a identical to each other (Figures 4a, 4b, 6aand 6b). The through-lights 366 may comprise first lights 366a identical to each other, and second lights 366b identical to each other and different from the first lights 366a (Figures 4c, 7a and 7b). The first lights 366a and the second lights 366b may be arranged in a staggered manner. In this case, each of the first lights 366a is located between two of the second lights 366b, and each of the second lights 366b is located between two of the first lights 366a (Figures 4c, 7a and 7b). Each of the through-lights 366 may have a first maximum width l366. This first width l366 is measured in a tangential direction relative to a circumference centered on the first axis A. The number of through-lights 366 may be greater than or equal to 30, and preferably greater than or equal to 50. The through-lights 366 define between them connecting bars 368 between the first 365 and second 367 parts of thecrown carrier 36. The connecting bars 368 may be identical to each other (Figures 4a, 4b, 7a and 7b). The connecting bars 368 may comprise first connecting bars 368a identical to each other, and second connecting bars 368b which are identical to each other and different from the first connecting bars 368a (Figures 4c, 6a and 6b). The first connecting bars 368a may be parallel to the first axis A and the second connecting bars 368b may be inclined at an angle α relative to the first axis A (Figures 4c, 6a and 6b). At least some or all of the connecting bars 368 may be inclined at an angle α relative to the first axis A. The angle α of inclination may be between 5° and 75°. Each of the connecting bars 368 may have a second maximum width l 368. This second maximum width l368 may be less than or equal to the first maximum width l366 of each of the through lights 366. The first l366 and second l 368maximum widths are measured in a tangential direction relative to a circumference centered on the first axis A. The number of connecting bars 368 may be greater than or equal to 30, and preferably greater than or equal to 50. The number of connecting bars 368 may be identical to or greater than the number of through-holes 366 of the annular band. Figures 4, 6a and 6b illustrate a first embodiment of the crown carrier 36 of the invention, in which the through-holes 366 have a generally quadrangular shape. The through-holes 366 are first holes 366a identical to each other. The connecting bars 368 are first connecting bars 368a identical to each other. All of the connecting bars 368 are inclined at the angle α. This angle α may be between 35° and 55°. Preferably, the angle α is approximately 45°. Figure 4b illustrates a second embodiment of the crown holder 36 of the invention.The crown holder 36 of the second embodiment differs from the crown holder 36 of the first embodiment by the shape of the through-holes 366. Indeed, the through-holes 366 of FIG. 4b have a generally elliptical shape. FIGS. 4c, 7a and 7b illustrate a third embodiment of the crown holder 36 of the invention. The crown holder 36 of the third embodiment differs from the crown holder 36 of the first and second embodiments by the through-holes 366 and the connecting bars 368. The through-holes 366 of the third embodiment have a generally triangular shape, in particular of the right-angled triangle type with the three rounded corners. The through-holes 366 comprise first holes 366a identical to each other and second holes 366b which are identical to each other and different from the first holes 366b.The first slots 366a and the second slots 366b are arranged in a staggered manner, wherein each of the first slots 366a is located between two of the second slots 366b, and each of the second slots 366b is located between two of the first slots 366a. The connecting bars 368 of the third embodiment thus comprise first connecting bars 368a that are identical to each other, and second connecting bars 368b that are identical to each other and different from the first connecting bars 368a. The first connecting bars 368a are parallel to the first axis A and the second connecting bars 368b are inclined at the angle α. This angle α may be between 35° and 55°. Preferably, the angle α is approximately 45°. We will now describe the operation of the mechanical reducer 3 (epicyclic and / or differential) of the invention with reference to figures 5a, 5b, 6a, 6b, 7a and 7b.According to the invention, the connecting bars 368 are configured, on the one hand, to transmit a torque transmitted by the crown 35 in a first direction D1 around the first axis A, and on the other hand, to break when a torque greater than a predetermined threshold is transmitted by the crown 35 in a second direction D2 around the first axis A, opposite to the first direction D1. By "predetermined threshold" is meant the moment when the torque transmitted by the crown 35 to the crown carrier 36 is in the first direction D1 during normal operation of the mechanical reducer 3. For example, this predetermined threshold is exceeded when the crown carrier 36 is subjected to a torque in the second direction D2, in particular in the event of failure of the satellite bearing 34.The connecting bars 368 can be oriented so as to be stressed in tension during the transmission of the torque in the first direction D1, and in compression when the crown carrier 36 is subjected to a torque in the second direction D2. Figures 5a, 6a and 7a schematically represent the normal operation of the mechanical reducer 3, and in particular the forces exerted on each of the components of the mechanical reducer 3 (illustrated by single arrows in the figures). As described above with reference to the reducer of Figures 3a and 3b, the rotation of the sun gear 31 (driven by the low pressure shaft 1ae) rotates the planet gears 32 inside the crown 35 which remains stationary. The rotation of the planet gears 32 rotates the planet carrier 33 so as to rotate the fan S via the fan shaft 1s.The movable crown 35 in the case of the differential reducer drives the drive shaft 50 so as to also rotate the fan S. The rotating satellites 32 transmit an axial force to the crown 35 which is directed towards the outside of the mechanical reducer. The crown 35 in turn transmits a torque to the crown carrier 36 in the first direction D1 around the first axis A. This first direction D1 (illustrated by double arrows in the figures) of torque transmission generates for example tensile forces (illustrated by dotted arrows in Figure 6a or Figure 7a) on the connecting bars 368. The tensile forces make it possible to keep the connecting bars 368 robust, so as to ensure the operation of the mechanical reducer 3.Figures 5b, 6b and 7b schematically represent the abnormal operation of the mechanical reducer 3, and in particular the forces exerted on each of the components of the mechanical reducer. For example, in the event of failure of the satellite bearing(s) 34, the direction of transmission of the torque by the crown wheel 35 to the crown carrier 36 is reversed. Thus, the crown wheel 35 transmits a torque to the crown carrier 36 in the second direction D2 around the first axis A. This second direction D2 (illustrated by double arrows in Figures 5b, 6b and 7b) of torque transmission generates, for example, compressive forces (illustrated by dotted arrows in Figure 6b or Figure 7b) on the connecting bars 368 and causes a break in these connecting bars 368, so as to break the crown carrier 36.This makes it possible to separate the connection between the crown 35 and the crown carrier 36 so as to rotate the crown 35 (whether it is fixed in the case of the epicyclic reducer or mobile in the case of the differential reducer) around the first axis A. The rotation of the satellite(s) 32 having the fault remains blocked. In other words, the crown 35 (and consequently the crown carrier 36) can rotate freely at a speed independent of a rotational speed of the sun gear 31. For example, the rotational speed of the crown 35 can be controlled by an air flow passing through the second propeller stage of the fan S. In this way, the sun gear 31 makes it possible to directly drive the planet carrier 33 (via the fan shaft 1s) and / or the crown carrier 36 (via the drive shaft 50) in rotation through the blocked planet carrier(s) 32 to rotate the fan S (via the fan shaft 1s).

Claims

CLAIMS 1. Mechanical reducer (3) for a turbomachine (1), in particular for an aircraft, this reducer (3) comprising: - a sun gear (31) having a first axis (A) of rotation, - a ring gear (35, 35a, 35b) which extends around the sun gear (31) and said first axis (A), - satellites (32) which are arranged around the first axis (A), between the sun gear (31) and the ring gear (35, 35a, 35b), and which are meshed with the sun gear (31) and the ring gear (35, 35a, 35b), these satellites (32) each having a second axis (B) of rotation parallel to the first axis (A) and being held by a planet carrier (33) movable in rotation around said first axis (A), and - a ring gear carrier (36) having a generally annular shape around the first axis (A), the crown carrier (36) comprising a first annular fixing flange (362) to the crown and a second annular fixing flange (364) intended to be fixed to a member (5, 50) of the turbomachine,characterized in that the crown carrier (36) comprises, between its flanges (362, 364), an annular band (360) around the first axis (A) which has through-holes (366) distributed around this first axis (A), these holes (366) defining between them connecting bars (368) between a first annular part (365) of the crown carrier comprising the first flange (362), and a second annular part (367) of the crown carrier comprising the second flange (364), these connecting bars (368) being configured to transmit a torque transmitted by the crown (35, 35a, 35b) in a first direction (D1) around the first axis (A), and to break when a torque greater than a predetermined threshold is transmitted by the crown (35, 35a, 35b) in a second direction (D2) around the first axis (A), opposite to the first direction (D1).

2. Reducer according to claim 1,characterized in that the connecting bars (368) are identical.

3. Reducer according to claim 1 or 2, characterized in that the connecting bars (368) comprise first connecting bars (368a) identical between, them, and second connecting bars (368b) which are identical to each other and different from the first connecting bars (368a).

4. Reducer according to the preceding claim, characterized in that the first connecting bars (368a) are parallel to the first axis (A) and the second connecting bars (368b) are inclined at an angle (α) relative to the first axis (A).

5. Reducer according to any one of claims 1 to 4, characterized in that some or all of the connecting bars (368) are inclined at an angle (α) relative to the first axis (A).

6. Reducer according to claim 4 or 5, characterized in that the angle (α) of inclination is between 5° and 75°.

7. Reducer according to any one of the preceding claims, characterized in that the slots (366) each have a generally triangular, elliptical, annular, or polygonal shape. 8.Reducer according to any one of the preceding claims, characterized in that the slots (366) comprise first slots (366a) identical to each other, and second slots (366b) identical to each other and different from the first slots (366a).

9. Reducer according to the preceding claim, characterized in that the first slots (366a) and the second slots (366b) are arranged in a staggered pattern, each of the first slots (366a) being located between two of the second slots (366b), and each of the second slots (366b) being located between two of the first slots (366a).

10. Reducer according to any one of the preceding claims, characterized in that the number of connecting bars (368) is greater than or equal to 30, and preferably greater than or equal to 50.

11. Reducer according to any one of the preceding claims, characterized in that the annular band (360) has a length (L360) along the first axis (A), which represents 10 to 30% of a total length (L 36 ) of the crown carrier along this first axis (A).

12. Reducer according to any one of the preceding claims, characterized in that each of the connecting bars (368) has a maximum width (l368) measured in a tangential direction with respect to a circumference centered on the first axis (A), which is less than or equal to a maximum width (l 366) of each of the lights (366) measured in a direction tangential to a circumference centered on the first axis (A).

13. Reducer according to any one of the preceding claims, characterized in that the connecting bars (368) are oriented so as to be stressed in tension during the transmission of the torque in the first direction (D1), and in compression when the crown carrier (36) is subjected to a torque in the second direction (D2).

14. Turbomachine (1), in particular for an aircraft, comprising a mechanical reducer (3) according to any one of the preceding claims.

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