Mechanical gearboxes for aircraft turbines

Abstract

The present invention relates to a mechanical gearbox for an aircraft turbomachine, comprising: a sun gear having an external toothing, a ring gear having an internal toothing, and planet gears, the planet gears meshing with the sun gear and the ring gear, the planet gears each comprising a first toothing and a second toothing, the first toothing having a first average diameter meshing with the external toothing of the sun gear, the second toothing having a second average diameter different from the first average diameter meshing with the internal toothing of the ring gear, the planet gears being guided by hydrodynamic bearings, the hydrodynamic bearings each comprising a first smooth guide surface extending at least partially below the first toothing and a second smooth guide surface extending at least partially below the second toothing. The present invention also relates to an aircraft turbomachine comprising such a gearbox.

Description

Technical Field

[0001] The present invention relates to the field of mechanical gearboxes for turbomachines, in particular for aircraft, and in particular to gearboxes equipped with a geared two-stage planetary gear. Background Art

[0002] The prior art includes in particular documents US-A1-2019 / 011038, DE-A1-102017127874, US-A1-2020 / 191256, WO-A1-2010 / 092263, FR-A1-2987416, FR-A1-3011901, FR-A1-3041054, FR-A1-3058493, FR-A1-3 066792, FR-A1-3069301 and FR-A1-3076336.

[0003] The function of a mechanical transmission is to change the speed and torque ratio between the input and output axes of a mechanical system.

[0004] The new generation of multi-flow turbines, in particular those with a high bypass ratio, include a mechanical gearbox to drive the shaft of a ducted or unducted propeller (also called a "fan"). The general purpose of the gearbox is to convert the so-called high rotational speed of the shaft of the power turbine into a slower rotational speed of the shaft used to drive the propeller.

[0005] This gearbox consists of a central pinion gear, known as the sun gear, a ring gear, and pinions, known as planet gears, which engage between the sun gear and the ring gear. The planet gears are held by a frame, known as the planet carrier. The sun gear, ring gear, and planet carrier are planetary gears because their axes of rotation coincide with the longitudinal axis X of the turbine. The planet gears each have a different axis of rotation and are equally distributed around the planet gear axis, running on the same diameter. These axes are parallel to the longitudinal axis X.

[0006] There are various gearbox architectures. In the prior art for multi-flow turbines, the gearboxes are of the planetary or epicyclic type. In other similar applications, there are so-called differential or compound architectures.

[0007] -In a planetary gearbox, the planet carrier is fixed and the ring gear is the output shaft of the device, which rotates in the opposite direction to the sun gear.

[0008] -In an epicyclic gearbox, the ring gear is fixed and the planet carrier is the output shaft of the device, which rotates in the same direction as the sun gear.

[0009] - In a compound gearbox, there are no components that are rotationally attached. The ring gear rotates in the opposite direction to the sun gear and the planet carrier.

[0010] The gearbox can comprise one or more gear stages. The meshing is ensured in different ways, for example by contact, friction or magnetic fields.

[0011] In this application, "stage" or "toothing" or "teeth" refers to a set of meshing teeth and a set of complementary teeth. The toothing can be internal or external.

[0012] Planetary gears can include one or two gear stages. Single-stage planetary gears include a toothing that can be spur, spiral, or herringbone, with the teeth of these toothings located on the same diameter. This toothing mates with the sun gear and the ring gear.

[0013] The double-stage planetary gear comprises two teeth or two series of toothed parts located on different diameters. The first toothed part cooperates with the sun gear and the second toothed part cooperates with the ring gear.

[0014] Additionally, each planet gear is centered and guided for rotation about its axis by bearings carried by the planet carrier.There are a variety of bearing technologies available for this application, and this application relates in particular to the use of hydrodynamic bearings to guide planet gears in mechanical gearboxes.

[0015] In this application, a "hydrodynamic bearing" refers to a bearing comprising a body that engages with a planetary gear and surrounding which at least one oil film under pressure is located. Prior art hydrodynamic bearings for planetary gears comprise a cylindrical body with an outer cylindrical surface that extends into the inner cylindrical surface of the planetary gear. A pressurized oil film is interposed between these surfaces, preventing contact between them.

[0016] One of the problems with mechanical gearboxes is the risk of planetary gear misalignment. This problem is magnified in two-stage planetary gears, as significant torques are exerted on the running planetary gears. Taking these torques into account when sizing the bearings that can support them is complex. The design of a stage with at least one helical tooth requires the use of supports to absorb axial forces. However, the installation of these supports results in significant axial overload requirements. The available overload dimensions under the teeth are uneven. If there are two teeth on a planetary gear, one is smaller than the other, forcing conventional hydrodynamic bearings to be undersized.

[0017] Therefore, there is a need to manage the ability to withstand the torques applied to the bearings of the planetary gears, which are often difficult to predict and size and create the risk of excessive oil consumption, arcing, and bearing seizure. Summary of the Invention

[0018] The present invention relates to a mechanical gearbox for a turbomachine, in particular an aircraft turbomachine, comprising:

[0019] a sun gear having an axis of rotation and comprising an external toothing,

[0020] a ring gear extending around the sun gear and comprising an internal toothing,

[0021] a plurality of planetary gears meshing with the sun gear and the ring gear, each of the planetary gears comprising a first toothing having a first average diameter and a second toothing having a second average diameter different from the first average diameter of the first toothing, the first toothing meshing with the external toothing of the sun gear and the second toothing having the second average diameter meshing with the internal toothing of the ring gear, the planetary gears being guided by hydrodynamic bearings carried by a planet carrier,

[0022] It is characterized in that the fluid dynamic bearing for guiding each planet gear includes a first smooth guide surface and a second smooth guide surface, wherein the first smooth guide surface extends at least partially below the first tooth connection portion around the rotation axis of the planet gear, and the second smooth guide surface is different from the first surface and extends at least partially below the second tooth connection portion around the rotation axis of the planet gear.

[0023] Therefore, the present invention provides the possibility of guiding teeth of different diameters by means of different guiding surfaces.It will be understood that the first oil film is inserted between the first surface and the planetary gears and the second oil film is inserted between the second surface and the planetary gears.

[0024] For example, the second surface is configured to primarily bear radial and tangential forces applied to the planetary gear during operation.For example, the first surface is configured to absorb residual torque and meshing forces of the first toothing portion.

[0025] Since the guide surfaces of the bearing are different and preferably have different diameters, the bearing can have a stepped shape, which facilitates the integration of an axial support into the bearing for cooperating with the planetary gears. It will therefore be understood that this support does not need to be provided on any other element of the gearbox (e.g. the ring gear carrier).

[0026] The present invention may provide several advantages, including:

[0027] -Optimize lubrication oil consumption of hydrodynamic bearings,

[0028] - reducing pressure peaks in the corners of the oil film of hydrodynamic bearings, and

[0029] - For example, optimizing excessive oil film thickness in hydrodynamic bearings.

[0030] The present invention is suitable for:

[0031] -Multi-speed gearbox,

[0032] - Gearboxes called epicyclic, planetary or differential,

[0033] - straight, spiral or herringbone teeth,

[0034] - any type of planet carrier, whether of the integral type or of the cage and carrier type,

[0035] -Planetary gear bearings of smooth or hydrodynamic type.

[0036] According to the invention, the first surface is located on a first axial portion of the body of the fluid dynamic bearing and the second surface is located on a second axial portion of the body of the fluid dynamic bearing, the two portions being connected together by a first annular web of the body.

[0037] The gearbox according to the invention may include one or more of the following features, taken alone or in combination with one another:

[0038] the first surface has a third diameter D26c2 or a third average diameter smaller than D32, and the second surface has a fourth diameter D26c1 or a fourth average diameter different from D26c1 and smaller than D28;

[0039] - a first average diameter D32 of the first toothing portion is greater than a second average diameter D28 of the second toothing portion;

[0040] - the first axial portion of the body of the hydrodynamic bearing has an inner cylindrical surface with a diameter of the fifth diameter D32a, and the second axial portion of the body of the hydrodynamic bearing has an inner cylindrical surface with a diameter of the sixth diameter D28a;

[0041] --D32a is smaller than D28a;

[0042] --D32a is greater than D28a;

[0043] - a first annular web comprising a cylindrical rim for axially supporting the planetary gears mounted on hydrodynamic bearings,

[0044] - the first annular web extends in a plane perpendicular to the axis of rotation of the planet gear;

[0045] - the first annular web comprises an annular part having a C-shaped cross section;

[0046] - each of the planetary gears comprises a tubular body connected to the first toothing by a second web, the second annular web comprising a through hole for the passage of oil;

[0047] - the first surface extends at least partially around the second surface;

[0048] - the length of the first surface is greater than 20% of the length of the second surface;

[0049] - the first surface and the second surface are offset;

[0050] - at least one of the first surface and the second surface is cylindrical;

[0051] - At least one of the first surface and the second surface is oval in cross-section.

[0052] The invention also relates to an aircraft turbomachine comprising a gearbox as described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Other features and advantages will become apparent from the following description of non-limiting embodiments of the invention and with reference to the accompanying drawings, in which:

[0054] Figure 1 is a schematic axial cross-sectional view of an aircraft turbine,

[0055] Figure 2 is a partial view of an axial cross section of a mechanical gearbox,

[0056] Figure 3 is a schematic axial cross-sectional view of a mechanical transmission equipped with a geared two-stage planetary gear, and illustrates the prior art,

[0057] Figure 4 yes Figure 3 A schematic perspective view of the body of a fluid dynamic bearing of a gearbox,

[0058] Figure 5 is a schematic partial view of an axial section of a fluid dynamic bearing and a planetary gear of a first embodiment of a mechanical transmission according to the present invention,

[0059] Figure 6 yes Figure 5 A schematic perspective view of a body of a fluid dynamic bearing;

[0060] Figure 7 is a schematic partial view of an axial section of a fluid dynamic bearing and a planetary gear of a second embodiment of a mechanical transmission according to the present invention,

[0061] Figure 8is a schematic partial view of an axial section of a fluid dynamic bearing and a planetary gear of a third embodiment of a mechanical transmission according to the present invention,

[0062] Figure 9 is a schematic partial view in axial section of a fluid dynamic bearing and a planetary gear of a fourth embodiment of a mechanical gearbox according to the present invention,

[0063] Figure 10 yes Figure 9 A schematic perspective view of a body of a fluid dynamic bearing;

[0064] Figure 11A and Figure 11B is a schematic cross-sectional view of a hydrodynamic bearing and planetary gears, Figure 11A shows the case where the guiding surfaces of the bearings are coaxial, Figure 11B These surfaces are shown as off-axis variations, and

[0065] Figure 12A and Figure 12B is a schematic cross-sectional view of a hydrodynamic bearing and planetary gears, Figure 12A shows the case where the guiding surface of the bearing is cylindrical, Figure 12B Variations are shown where the cross-section of these surfaces is elliptical. DETAILED DESCRIPTION

[0066] Figure 1 A turbomachine 1 is depicted, generally comprising an axis of rotation X, a fan S, 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 an exhaust nozzle 1h. The high-pressure compressor 1b and the high-pressure turbine 1d are connected by a high-pressure shaft 2 and, together with the high-pressure shaft, form the high-pressure (HP) body. The low-pressure compressor 1a and the low-pressure turbine 1e are connected by a low-pressure shaft 3 and, together with the low-pressure shaft, form the low-pressure (LP) body.

[0067] The fan S is driven by a fan shaft 4 which is driven by the LP shaft 3 via a gearbox 6. This gearbox 6 is generally of the planetary or epicyclic type.

[0068] The following description relates to a planetary type gearbox in which the ring gear is rotatably movable.

[0069] A gearbox 6 is positioned in the upstream portion of the turbine. A fixed structure is arranged to form a housing E around the gearbox 6, which structure schematically comprises an upstream portion 5a and a downstream portion 5b, which constitute the engine housing or stator 5. This housing E is closed upstream by seals at the location of the bearings that allow the passage of the fan shaft 4, and downstream by seals at the location that allows the passage of the LP shaft 3.

[0070] Figure 2 A gearbox 6 is shown, which can take the form of different architectures depending on whether certain parts are fixed or rotating. The input of the gearbox 6 is connected to the LP shaft 3, for example by means of internal splines 7a. The LP shaft 3 thus drives a series of planetary pinions, known as sun gears 7. Traditionally, the axis of rotation of the sun gear 7 coincides with the axis of rotation X of the turbine, and the sun gear drives a series of pinions, known as planet gears 8, which are evenly distributed around the axis of rotation X and have the same diameter. This diameter is equal to twice the distance between the running centers of the sun gear 7 and the planet gears 8. For this type of application, the number of planet gears 8 is usually limited to between three and seven.

[0071] The planetary gear 8 assembly is held together by a frame called a planet carrier 10. Each planetary gear 8 rotates about its own axis Y and meshes with a ring gear 9. In this planetary configuration, the planetary gear 8 assembly is held by the planet carrier 10, which is attached to the engine housing or stator 5. Each planetary gear drives a ring gear, which is mounted to the fan shaft 4 via a ring gear carrier 12.

[0072] Each planet gear 8 is rotatably mounted by means of a bearing 11 (e.g., a bearing or hydrodynamic bearing type). Each bearing 11 is mounted on one of the shafts 10b of the planet carrier 10, and all shafts are positioned relative to each other using one or more structural frames 10a of the planet carrier 10. There are multiple shafts 10b and bearings 11, the number of which is equal to the number of planet gears. The shafts 10b and frames 10a can be disassembled into multiple components for reasons of operation, assembly, manufacturing, inspection, maintenance, or spare parts.

[0073] For the same reasons as above, the gearbox toothing can be broken down into a number of spirals, each with a mid-plane P. In the example shown, the ring gear is broken down into two ring gear halves:

[0074] - Upstream gear ring half 9a, consisting of a rim 9aa and an attachment flange half 9ab. Rim 9aa carries the upstream helical section of the gearbox toothing. This upstream helical section meshes with the helical sections of the planetary gears 8, which in turn mesh with the helical section of the sun gear 7.

[0075] - Downstream gear ring half 9b, consisting of a rim 9ba and an attachment flange half 9bb. Rim 9ba carries the downstream helical section of the gearbox toothing. This downstream helical section meshes with the helical sections of the planetary gears 8, which in turn mesh with the helical section of the sun gear 7.

[0076] The attachment flange halves 9ab and 9bb of the upstream ring gear 9a and 9b form the ring gear attachment flange 9c. The ring gear 9 is attached to the ring gear carrier by assembling the ring gear attachment flange 9c and the ring gear carrier attachment flange 12a by means of bolt mountings, for example.

[0077] Figure 2 The arrows in the figure depict the oil flow through the gearbox 6. Oil enters the distributor 13 from the stator assembly 5 and into the gearbox 6 via various mechanisms, which are not specified in this illustration as they are specific to one or more types of architecture. The distributor is divided into two sections, each typically repeated with the same number of planetary gears. The injector 13a lubricates the gearing, while the arm 13b lubricates the bearings. Oil is supplied to the injector 13a and discharged through the end 13c to lubricate the gearing. Oil is also supplied to the arm 13b and flows through the supply opening 13d for the bearings. The oil then flows through the shaft into one or more buffer areas 10c and out through the hole 10d to lubricate the bearings of the planetary gears.

[0078] Figure 3 A gearbox 6 of an aircraft turbomachine according to the prior art is shown.

[0079] The gearbox 6 comprises a planet carrier 10 which is configured to be rotatable about an axis X and which is unitary, ie formed in one single piece.

[0080] The planet carrier 10 includes a cage 14 and a shaft portion 15 .

[0081] The shaft portion 15 is generally tubular and extends along the axis X and comprises a free longitudinal end shown on the left in the drawings and an opposite longitudinal end intended for connection to the cage 14 .

[0082] The shaft portion 15 includes an external toothing 15 a for engaging with, for example, a fan.

[0083] The cage 14 comprises two annular shrouds 14a, 14b, which are parallel and spaced apart and extend perpendicularly to the axis X. The shrouds 14a, 14b are of substantially circular shape and are centered on the axis X.

[0084] The shield 14a on the left in the drawing, which is referred to as the first shield, is connected to the shaft portion 15. The other shield 14b is referred to as the second shield.

[0085] The shields 14a, 14b are connected to each other by a material bridge 16 defined between the shields 14a, 14b, and the housing 18 of the shield is configured to accommodate the planetary gears 8. The housing 18 opens radially outward at the outer periphery of the cage 14 and also opens radially inward through the inner tubular wall 20 of the cage 14. The material bridge 16 can be solid or partially recessed, as shown in FIG. Figure 3 shown.

[0086] The wall 20 extends from the first shroud 14 a towards the second shroud 14 b around the axis X. Here, the wall extends substantially along the axial extension of the shaft portion 15 . The wall 20 internally delimits a space 22 for accommodating the sun gear 7 .

[0087] This space 22 includes two adjacent parts. A first part 22a is surrounded by a wall 20 including an inner cylindrical surface 22a for mounting a bearing 23 for guiding the end of the sun gear 7. A second part 22b, located at the opening of the housing 18, accommodates the opposite end of the sun gear 7, which includes an external toothing 7b for meshing with the planetary gears 8. The sun gear 7 also includes an internal toothing 7a for coupling to a shaft (e.g., the shaft of a turbine).

[0088] Each housing 18 includes a first portion 18a located on one side of the first shield 14a and a second portion 18b located on one side of the second shield 14b. The housing 18 is open at both portions 18a and 18b on the outer periphery of the cage 14, and is open only at the second portion 18b on the inner periphery of the cage 14.

[0089] The shrouds 14 a , 14 b comprise aligned apertures or holes 24 for mounting the planetary gears 8 and, in particular, for mounting hydrodynamic bearings 26 for guiding these planetary gears 8 .

[0090] Each hydrodynamic bearing 26 comprises a body 27 around which a pressurized oil film is located.

[0091] exist Figure 4 The body 27 of the bearing 26 is shown separately in FIG. This body has a generally cylindrical shape extending along the axis Y, and its longitudinal ends comprise extensions 26a housed in the hole 24 forming the bearing seat.

[0092] The body 27 may also be tubular and include an internal oil flow hole 26 b which is generally in communication with an oil supply duct leading to an outer cylindrical surface 26 c of the body to form an oil film between this surface 26 c and the inner cylindrical surface of the planetary gear 8 .

[0093] As mentioned above, the planetary gears 8 are of the gearing double-stage type and each comprises a tubular body 8 a connected to a first external toothing 32 by a web 30 , the body 8 a itself being equipped with a second toothing 28 .

[0094] The teeth 28 , 32 are arranged adjacent to one another and, more specifically, the teeth 28 , 32 are respectively situated in two planes perpendicular to the axis Y.

[0095] The second toothed portion 28 on the left side in the drawing is located on the side of the first shield 14a and is therefore located at the position of the first portion 18a of the housing. Figure 3 As shown, the toothing 28 meshes with the ring gear 9 .

[0096] The first toothed portion 32 on the right side in the drawing is located on the side of the second shield 14b and is therefore located at the position of the second portion 18b of the housing. Figure 3 As shown, the toothing 32 meshes with the toothing 7 b of the sun gear 7 .

[0097] The ring gear 9 is carried by a ring gear carrier which is not shown in the drawings.

[0098] The present invention provides a solution for managing the torque applied to the hydrodynamic bearings 26 of a running planetary gear.

[0099] Figure 5 and Figure 6 A first embodiment of a gearbox according to the invention, in particular a hydrodynamic bearing 26 and a planetary gear 8 therefor, is shown.

[0100] Gearbox includes Figure 3 All the above characteristics are relevant, as long as they do not contradict or conflict with the characteristics below.

[0101] therefore, Figure 5 and Figure 6 Reference numerals used in Figure 3 Reference numerals that have been used throughout refer to identical or similar elements.

[0102] The following description relates to the planetary gear 8 and its pilot hydrodynamic bearing 26 , but it will be understood that the description applies to all planetary gears and hydrodynamic bearings of the gearbox.

[0103] The planetary gear 8 is of the two-stage gearing type and comprises a tubular body 8 a connected to a first external toothing 32 by a web 30 , the body 8 a itself being equipped with a second toothing 28 .

[0104] The teeth 28 , 32 are arranged adjacent to one another and, more specifically, respectively lie in two planes perpendicular to the axis Y.

[0105] The second toothing 28 located on the left in the drawing is intended to mesh with the ring gear 9. The first toothing 32 located on the right in the drawing is intended to mesh with the toothing 7b of the sun gear 7.

[0106] In the example shown, the web 30 has a C-shaped cross section, with the opening of the C oriented axially toward the toothing 28. This particular shape provides the planetary gear 8 with a certain flexibility in the radial direction by virtue of the elastic deformation of the web.

[0107] The toothing 28 has a second diameter or second average diameter D28 which, in the example shown, is smaller than the first diameter or first average diameter D32 of the toothing 32 .

[0108] Toothing 28 is located on an axial portion or section of planet gear 8 that has an inner cylindrical surface 28a having a sixth diameter D28a. Similarly, toothing 32 is located on an axial portion or section of planet gear 8 that has an inner cylindrical surface 32a having a fifth diameter D32a.

[0109] Here, D32a is greater than D28 and D28a.

[0110] In addition, from Figure 8 As can be seen, surface 32a may extend at least partially around surface 28a.

[0111] exist Figure 6 1 and 2. The body 27 of the hydrodynamic bearing 26 is shown in isolation. This body has a generally cylindrical shape and is staged along the axis Y. The longitudinal ends of the body 27 comprise extensions 26a housed in the bores 24 of the planetary gears, as previously described.

[0112] The body 27 may also be tubular and include an internal oil flow hole 26b connected to an oil supply conduit (not shown) leading to surfaces 26c1, 26c2 for guiding the body to form an oil film between these surfaces 26c1, 26c2 and the surfaces 28a, 32a of the planetary gear 8.

[0113] The surface 26c1 extends inside the surface 28a and is opposed to the surface 28a with a predetermined gap depending on the thickness of the oil film interposed between these surfaces 26c1, 28a.

[0114] The surface 26c1 is located on the axial section or portion 34 of the body 27 and has a fourth diameter D26c1 and a length L26c1.

[0115] The surface 26c2 extends inside the surface 32a and is opposed to the surface 32a, with a predetermined gap depending on the thickness of the oil film interposed between these surfaces 26c2, 28a.

[0116] Surface 26c2 is located on the axial section or portion 36 of body 27 and has a third diameter D26c2 and a length L26c2.

[0117] Here, D26c2 is greater than D28 and D28a.

[0118] Here, L26c2 is smaller than L26c1 and is, for example, at least 20% of L26c1.

[0119] In addition, from Figure 5 It can be seen that the surface 26c2 can extend at least partially around the surface 26c1, thereby enabling improved load bearing capacity.

[0120] exist Figure 5 and Figure 6 In the embodiment shown, the portions 34 , 36 of the bearing surfaces 26 c 1 and 26 c 2 of the body 27 are connected together by a first annular web 38 extending in a plane perpendicular to the axis Y.

[0121] Preferably, the first web 38 comprises a cylindrical edge 40 for axially supporting the planetary gear 8, such as Figure 5 The edge 40 can be engaged with the toothed portion 28 or the axial end of the body 8a on the inner periphery of the first web 38 by pressing.

[0122] The outer periphery of the first web 38 can be connected directly to the portion 36 or connected to the portion 36 via an annular member 42, which includes an annular groove 44, which is axially open on the side of the toothed portion 28 ( Figure 5 ), or the annular member has a substantially C-shaped axial cross section ( Figure 7 These arrangements, especially Figure 7 The arrangement shown in FIG. 2 allows for a certain flexibility in the radial direction of the body 27 of the bearing 26 .

[0123] It will therefore be understood that both the planetary gear 8 and the body 27 of the bearing can have a certain flexibility, which is advantageous since this flexibility allows independent guidance of the two toothings 28, 32 of the planetary gear 8 and, in particular, independent stiffness, excitation and load bearing in the bearing for each of the toothings.

[0124] exist Figure 7 In the example shown, the part with a C-shaped cross section comprises an opening 46 which opens on the side of the toothing 28. The part 42 can extend over a radial dimension which is approximately 10% to 40% of the radial dimension of the first web 38.

[0125] Figure 8 Another alternative embodiment is shown, in which the second web 30 ′ of the planetary gear 8 has a frustoconical shape and here widens on the side of the toothing 32 . This makes it possible to absorb the meshing forces of the toothing 32 .

[0126] Furthermore, although this feature can be Figure 5 or Figure 7A web 30 of the type shown is present, but here the third diameter D26c2 and the fifth diameter D32a of the surfaces 26c2 and 32a are smaller than D28 and larger than D28a and D26c1.

[0127] This embodiment makes it possible to reduce the Reynolds number of the oil film located between the surfaces 26c2 and 32a.

[0128] The surfaces 26c1 and 26c2 of the body 27 of the bearing are connected by a radial annular surface 48 which forms an axial stop for cooperating with the planetary gear 8 ( Figure 8 and Figure 10 ) and thus replaces the edge 40 described above.

[0129] Figure 9 An alternative embodiment with Figure 8 The alternative embodiment differs in that the second web 30' includes through-holes 50 for the passage of oil, and in particular for its drainage. These holes are preferably evenly distributed about the axis Y and are inclined so that their axial ends, located on the side of the toothing 32, open at the level of the annular joint edge of the surfaces 48 and 26c2. The opposite axial end opens on the annular surface of the second web 30', located on the side of the toothing 28.

[0130] Figure 11A and Figure 12A are the same, and show that for example Figure 5 Cross-sectional views of the planetary gear 8 and the hydrodynamic bearing 26 are shown. These figures make it possible to show that the surfaces 26c1 and 26c2 are cylindrical and coaxial with each other and with the axis Y. These figures also show the radial clearances between these surfaces and the surfaces 28a, 32a of the planetary gears, which are occupied by the oil film H.

[0131] Figure 11B The variation in FIG. 2 shows a case where surfaces 28 a, 32 a, 26 c 1 are coaxial with one another and aligned along axis Y, while surface 26 c 2 is offset from the axis (offset Δ). Unlike the oil film between surfaces 26 c 2 and 32 a, this results in the oil film H between surfaces 28 and 26 c 1 not having the same radial thickness throughout its entire extent around axis Y.

[0132] Figure 12BThe variation in FIG. 1 shows a case where surfaces 28a and 32a are cylindrical, while surfaces 26c1 and 26c2 have elliptical cross-sections. Surfaces 26c1 and 26c2 may each have two diametrically opposed vertices, with the vertex of surface 26c1 being angularly offset relative to the vertex of surface 26c2 about axis Y. This also results in the oil film H between surfaces 28a and 26c1 and between surfaces 26c2 and 32a not having the same radial thickness throughout their entire range about axis Y, as shown in the accompanying drawings.

[0133] Figure 12B The variations in make it possible to reduce the pressure peaks in the corners of the oil film and to reduce losses and oil flows.

[0134] Figure 12B The characteristics can be Figure 11B and combined with the features of each of the above embodiments.

[0135] The present invention can provide many advantages, including:

[0136] Make mechanical gearboxes run healthier;

[0137] oReducing the pressure in the oil film of hydrodynamic bearings;

[0138] oIncrease the minimum thickness of the oil film in these bearings;

[0139] oReducing the maximum thickness of the oil film in these bearings;

[0140] oConvert the torque in the bearing into radial force;

[0141] Optimized operation:

[0142] oBetter control of bearing lubricant consumption;

[0143] oImprove the efficiency of the gearbox;

[0144] oReducing the available space in the gearbox.

Claims

1. A mechanical gearbox (6) for a turbine (1), the mechanical gearbox comprising: - a sun gear (7) having an axis of rotation (X) and comprising an external toothing (7b), - a ring gear (9) extending around the sun gear (7) and comprising an internal toothing (9d), - a plurality of planetary gears (8), said planetary gears meshing with said sun gear (7) and said ring gear (9), and said planetary gears each comprising a first toothing portion (32) having a first average diameter (D32) and a second toothing portion (28) having a second average diameter (D28), said second average diameter (D28) being different from the first average diameter (D32) of said first toothing portion (32), said first toothing portion meshing with said external toothing portion (7b) of said sun gear (7), said second toothing portion (28) having said second average diameter (D28) meshing with said internal toothing portion (9d) of said ring gear (9), said planetary gears (8) being guided by hydrodynamic bearings (26) carried by a planet carrier (10), Characterized in that the fluid dynamic bearing (26) for guiding each planetary gear (8) comprises a first smooth guide surface (26c2) and a second smooth guide surface (26c1), the first smooth guide surface extending at least partially below the first toothing portion (32) around the rotation axis (Y) of the planetary gear (8), the second smooth guide surface being different from the first smooth guide surface (26c2) and extending at least partially below the second toothing portion (28) around the rotation axis (Y) of the planetary gear, and wherein the first smooth guide surface (26c2) is located on a first axial portion (36) of the body (27) of the fluid dynamic bearing (26), the second smooth guide surface (26c1) is located on a second axial portion (34) of the body (27) of the fluid dynamic bearing (26), the first axial portion (36) and the second axial portion (34) being connected together by a first annular web (38) of the body of the fluid dynamic bearing (26).

2. The mechanical gearbox (6) according to claim 1, wherein: The first smooth guide surface (26c2) has a third diameter (D26c2) or a third average diameter, which is smaller than the first average diameter (D32) of the first toothed portion (32), and the second smooth guide surface (26c1) has a fourth diameter (D26c1) or a fourth average diameter, which is smaller than the second average diameter (D28) of the second toothed portion (28).

3. The mechanical gearbox (6) according to claim 1 or 2, wherein: The first average diameter (D32) of the first toothed portion (32) is greater than the second average diameter (D28) of the second toothed portion (28).

4. The mechanical gearbox (6) according to claim 1 or 2, wherein: The first annular web (38) comprises a cylindrical edge (40) for axially supporting the planetary gear (8) mounted on the hydrodynamic bearing (26).

5. The mechanical gearbox (6) according to claim 1 or 2, wherein: The first annular web (38) extends in a plane perpendicular to the axis of rotation (Y) of the planet gear (8).

6. The mechanical gearbox (6) according to claim 1 or 2, wherein: The first annular web (38) comprises an annular member (42) having a C-shaped cross section.

7. The mechanical gearbox (6) according to claim 1 or 2, wherein: Each of the planetary gears (8) comprises a tubular body (8a) connected to the first toothing portion (32) via a second web (30'), the second web (30') comprising a through hole (50) for passage of oil.

8. The mechanical gearbox (6) according to claim 1 or 2, wherein: The first smooth guide surface (26c2) extends at least partially around the second smooth guide surface (26c1).

9. The mechanical gearbox (6) according to claim 1 or 2, wherein: The length (L26c2) of the first smooth guide surface (26c2) is greater than 20% of the length (L26c1) of the second smooth guide surface (26c1).

10. The mechanical gearbox (6) according to claim 1 or 2, wherein: The first smooth guide surface (26c2) and the second smooth guide surface (26c1) are offset.

11. The mechanical gearbox (6) according to claim 1 or 2, wherein: At least one of the first smooth guide surface (26c2) and the second smooth guide surface (26c1) is cylindrical.

12. The mechanical gearbox (6) according to claim 1 or 2, wherein: A cross section of at least one of the first smooth guide surface (26c2) and the second smooth guide surface (26c1) is elliptical.

13. The mechanical gearbox (6) according to claim 1 or 2, wherein: The first axial portion (36) of the body (27) of the fluid dynamic bearing (26) has an inner cylindrical surface (32a) having a fifth diameter (D32a), and the second axial portion (34) of the body (27) of the fluid dynamic bearing (26) has an inner cylindrical surface (28a) having a sixth diameter (D28a), the fifth diameter (D32a) being smaller than the sixth diameter (D28a).

14. The mechanical gearbox (6) according to claim 1 or 2, wherein: The first axial portion (36) of the body (27) of the fluid dynamic bearing (26) has an inner cylindrical surface (32a) having a fifth diameter (D32a), and the second axial portion (34) of the body (27) of the fluid dynamic bearing (26) has an inner cylindrical surface (28a) having a sixth diameter (D28a), the fifth diameter (D32a) being greater than the sixth diameter (D28a).

15. The mechanical gearbox (6) according to claim 1, wherein The turbine (1) is an aircraft turbine.

16. An aircraft turbomachine (1) comprising a mechanical gearbox (6) according to any one of claims 1 to 15.

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