Pivot for a hydrodynamic bearing

Abstract

The present document relates to a pivot (20) for a hydrodynamic bearing capable of being rotated about a longitudinal axis (X), the pivot (20) comprising: a radially internal part (22) extending axially and around the longitudinal axis (X), a shaft (25) arranged around said radially internal part (22), characterized in that the shaft (25) is a truncated cylinder comprising a solid portion (26) and at least one truncated portion (30, 30-1, 30-2) defined by an average truncation depth (RT) and a truncation surface (32, 32-1, 32-2), the at least one truncated portion (30, 30-1, 30-2) opening onto the truncation surface (32, 32-1, 32-2), the truncation surface (32, 32-1, 32-2) comprising an average truncation curve segment (CT) between 2° and 180° and an average truncation length (LT) between 5% and 100% of the length of the shaft (25).

Description

Title: Pivot for hydrodynamic bearing technical field

[0001] This disclosure falls within the domain of pivots for hydrodynamic bearings. Previous technique

[0002] It is known from prior art to use a mechanical reducer to change the speed and torque ratio between the input and output shafts of a mechanical system.

[0003] Newer generations of turbofan engines, particularly those with very high bypass ratios, incorporate a mechanical gearbox, one output of which drives the shaft of a fan. Typically, the gearbox's purpose is to reduce the high rotational speed of the power turbine shaft connected to the gearbox input to a slower rotational speed for the fan shaft, which is connected to the gearbox output.

[0004] Typically, such a reduction gear comprises a central pinion, called the sun gear, a ring gear, and pinions called planet gears, which mesh between the sun gear and the ring gear. The planet gears are held by a frame called the planet carrier. The sun gear, ring gear, and planet carrier are epicyclic, as their axes of revolution coincide with the longitudinal axis of the turbomachine. Each planet gear has a different axis of revolution, equally spaced on the same operating diameter around the axis of the planet gears. These axes are parallel to the longitudinal axis.

[0005] There are several gearbox architectures. In the state of the art of double-flow turbomachinery, gearboxes are of the planetary or epicyclic type.

[0006] In other similar applications, there are so-called differential or "compound" architectures.

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

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

[0009] On a differential gearbox, no element is fixed for rotation. The ring rotates in the opposite direction to the solar panel and the satellite carrier.

[0010] Gearboxes can consist of one or more meshing stages. This meshing is achieved in various ways, such as by contact, friction, or magnetic fields.

[0011] There are several types of contact meshing such as with straight, helical or herringbone teeth.

[0012] Mechanical reducers can, for example, be gear trains, which typically use hydrodynamic bearings. The state of the art includes, in particular, application FR 31 13934.

[0013] Figure 1 illustrates an example of a hydrodynamic bearing 2 which classically comprises a pivot 4 and an external part 6 separated by liquid (such as oil or water).

[0014] During operation, the external part 6 rotates at a speed GO around a bearing axis X. A liquid pressure Fp is then generated by the relative movement of the contacting surfaces, forming a liquid film 8 over a specific segment of curve 01, defining a complete film zone. This liquid film 8 enables the bearing to support the load F exerted on the bearing 2 during operation.

[0015] As illustrated in Figure 1, the curve segment C1 defines a complete film region delimited (in the counterclockwise direction in Figure 1) between two circumferential extremities: a film break point 10a (or circumferential break end of the film) and a film reformation point 10b (or circumferential reformation end of the film). The exterior (the complement) of the curve segment C1 is a curve segment C2 delimiting a film break region 11.

[0016] Under the effect of the load F, the generated pressure Fp does not follow a uniform pressure field along the curve segment C1.

[0017] It was observed that viscous shear losses between the liquid film 8 and the outer part 6 of the bearing 2 contribute significantly to a loss of energy efficiency. It was also observed that the rupture zone of the film 1 does not bear (is not affected by) the bearing load.

[0018] Therefore, there is a need to reduce these viscous shear losses in the context of hydrodynamic bearings. There is also a need to reduce the mass and size of hydrodynamic bearings. This need is even more critical in compact environments such as turbomachinery. Summary

[0019] To this end, the present document proposes a pivot for hydrodynamic bearing suitable for rotation about a longitudinal axis, said pivot comprising: a radially internal part forming a central shaft extending axially and around the longitudinal axis, a shaft arranged around said radially internal part, characterized in that the shaft is a truncated straight cylinder comprising a solid portion without truncation or orifice and at least one truncated portion, said at least one truncated portion being defined by an average truncation depth and a truncation surface, said at least one truncated portion opening onto the truncation surface, the truncation surface comprising an average truncation curve segment between 2° and 180°, preferably between 5° and 180°, and an average truncation length between 5% and 100% of the shaft length.

[0020] The term "length" refers to a dimension along the longitudinal axis.

[0021] The terms axial, radial and circumferential are defined with respect to the longitudinal axis.

[0022] The term "curve segment" refers to the arc length, an angular sector, or a portion of the circumference.

[0023] The term "truncation depth" refers to the radial dimension along which the said truncated portion is defined (along which it extends). The term "truncation radius" also refers to the truncation depth.

[0024] The term "truncated" refers to a removal of material from a volume.

[0025] The term "truncated portion" should be understood as a truncation volume on which material is removed.

[0026] It is important to understand that we are discussing an average truncation depth, average truncation curve segment, and average truncation length, because the values ​​of truncation depth, truncation curve segment, and truncation length can vary for a truncated portion, and there may be multiple truncated portions. The term "curve segment" is analogous to an "angular segment."

[0027] The pivot can, for example, be a pivot for a hydrodynamic bearing for a mechanical gearbox of a gas turbomachine for an aircraft.

[0028] The average truncation curve segment can be between 5° and 180°, between 10° and 180°, or between 15° and 180°. The larger the average truncation segment, the lower the hydrodynamic bearing losses during operation.

[0029] The average truncation length can be between 7.5% and 100% of the shaft length, between 10% and 100% of the shaft length, or between 12.5% ​​and 100% of the shaft length. The greater the average truncation length, the lower the hydrodynamic bearing losses during operation.

[0030] In operation, the truncated portion is intended to be arranged angularly on the segment of the hydrodynamic bearing curve that does not undergo radial loading and which has low pressure, i.e. the curve segment C2 of figure 1. Therefore, it is proposed to take advantage of the absence (or low presence) of load - and therefore of resulting pressure - in this area, called the unloaded zone, to lighten the mass of the shaft by performing a truncation of the shaft.

[0031] In addition, the solid portion is intended to be arranged on the segment of the hydrodynamic bearing curve which is subjected to the load and is in contact with the liquid film, i.e. the segment of curve C2 in Figure 1.

[0032] The truncation of the shaft allows for a reduction in its mass (and therefore of the hydrodynamic bearing), as well as a reduction in viscous shear losses between the liquid film and an external part of the bearing at the level of the film rupture zone.

[0033] A truncated portion is to be understood as a removal of material which has larger and distinct dimensions (length, depth, curve segment) than a notch or an opening.

[0034] According to one aspect, at least one portion of the tree radially opposite said at least one truncated portion is complete.

[0035] The term "radially opposite" means symmetrical with respect to the longitudinal axis.

[0036] The average truncation depth of said at least one truncated portion may be between 0.01% and 100% of the difference between the radius of the tree and the radius of the radially internal part.

[0037] The greater the average truncation depth, the more the losses of the hydrodynamic bearing in operation are reduced.

[0038] The radius of the shaft is understood to be the maximum radius of its radially external surface. The radius of the radially internal part is understood to be the maximum radius of its radially external surface.

[0039] The tree can be symmetrical with respect to a median radial plane.

[0040] The term "median radial plane" refers to a plane orthogonal to the longitudinal axis that includes the axial median of the tree.

[0041] In other words, by the term "median radial plane" we mean the radial plane which cuts the tree into two portions having the same longitudinal dimension.

[0042] According to a particular characteristic, said at least one truncated portion opens axially onto at least one axial end of the shaft. In other words, said at least one truncated portion extends axially so as to open axially onto at least one axial end of the shaft.

[0043] The tree may further include at least one extension zone without truncation or opening, said at least one extension zone projecting outward from a first flank of said solid portion and at least partially towards a second flank of the solid portion.

[0044] The term "flank" refers to the circumferential end face that angularly delimits the solid portion forming the truncated right cylinder. This flank may be substantially flat. It may extend substantially longitudinally and radially. It could also be curved without a preferred orientation.

[0045] According to this aspect, the solid portion can further be delimited by the outer perimeter (i.e. the periphery) of said at least one extension zone.

[0046] The first flank is located at a first circumferential end of the solid portion and the second flank at a second end of the second circumferential end of the solid portion.

[0047] Said at least one extension zone can extend from substantially an axial midpoint of the first flank of the tree.

[0048] The term "axial midpoint of the first flank of the tree" refers to the midpoint of the first flank of the tree that extends axially.

[0049] By "extending from substantially the middle of the first flank of the tree", we mean extending from a position located at plus or minus 5% of the length of the tree relative to the axial middle of the first flank of the tree.

[0050] According to a particular characteristic, said at least one extension zone extends from a segment of the first flank, preferably located at a distance from an axial end of the shaft at least equal to 30% of the length of the first flank.

[0051] In the unloaded zone, a residual pressure may remain, located circumferentially at the point where the lubricant film ruptured and axially on the pivot at the axial midline. In other words, this residual pressure may be located at the radial midline.

[0052] Thus, the presence of said at least one extension zone makes it possible to locally compensate for this residual load, while allowing an overall reduction compared to a solid tree that is not truncated.

[0053] Said at least one extension zone can extend from the first flank to the second flank.

[0054] In other words, the extension zone can extend over the entire truncation curve segment of the truncation surface.

[0055] In other words, said at least one truncation length may include: a first truncation length delimited by a first axial end of the shaft and a first axial end of the extension zone; and a second truncation length delimited by a second axial end of the extension zone and a second axial end of the bearing shaft.

[0056] The first axial end of the shaft is axially opposite to the second axial end of the shaft. The first axial end of the extension zone is axially opposite to the second axial end of the extension zone.

[0057] The extension area may include an axial dimension (a length) of between 1% and 80% of the length of the bearing shaft.

[0058] According to one aspect, said at least one extension zone can extend in such a way that the solid portion comprises a cylinder of revolution having the longitudinal axis as its center.

[0059] Said at least one extension zone may include an axial dimension decreasing or constant from the first flank and along its extension direction.

[0060] In other words, the extension zone can have an axial dimension which is maximum at the first circumferential end, and which decreases along its direction of extension, which can be circumferential.

[0061] This characteristic allows said at least one extension zone to support a possible residual pressure which is maximum circumferentially before the first circumferential end of the truncation surface.

[0062] For example, said at least one extension zone may have a general trapezoidal shape whose base is circumferentially aligned with the first circumferential endpoint of the truncation surface.

[0063] According to a particular characteristic, said at least one extension zone includes a first portion with a general trapezoidal shape whose base is circumferentially aligned with the first flank.

[0064] According to a particular characteristic, said at least one extension zone further comprises a second portion which is straight and connected to the first portion.

[0065] This document may also relate to a hydrodynamic bearing comprising a pivot of the aforementioned type mounted inside an external annular wall, the bearing comprising a preferred load direction in a given direction which passes successively through the truncated portion and then through the solid portion.

[0066] The hydrodynamic bearing may further comprise a lubricating fluid film intended to be arranged radially between the pivot shaft and the outer annular wall and circumferentially delimited by a circumferential rupture end of the fluid film and a circumferential reformation end of the fluid film, the circumferential rupture end of the fluid film and the circumferential reformation end of the fluid film being located circumferentially on either side of the preferred load direction, the pivot being oriented around its longitudinal axis so that said at least a truncated portion is radially opposite the outside of a curve segment of the fluid film.

[0067] The term "outside of a curve segment" refers to its complement over a full circumference (360°). The outside of the curve segment of the liquid film defines the curve segment of an unloaded area of ​​the hydrodynamic bearing.

[0068] According to this aspect, the truncation curve segment of the truncation surface of said at least one truncated portion is included in the curve segment of the unloaded zone of the hydrodynamic bearing.

[0069] Put another way, the truncation curve segment of the truncation surface is radially aligned with at least part of the outside of the curve segment of the liquid film.

[0070] The unloaded zone of the hydrodynamic bearing defines a curve segment on which material can be removed due to the absence or minimal presence of load. Therefore, the pivot can be oriented so that its truncated portion (and thus its truncation surface) is radially aligned with the unloaded zone of the bearing.

[0071] The term "external wall" refers to a radially external wall, the external wall thus surrounding the pivot.

[0072] The liquid film can be arranged in an annular space provided between the pivot shaft and the outer wall.

[0073] The radially internal portion may include at least one liquid supply orifice intended to supply said annular space.

[0074] The solid portion may include a curve segment extending over a first angle greater than or equal to a second angle over which the curve segment of the liquid film extends.

[0075] In this way, we ensure that the liquid film is radially aligned with the solid portion of the pivot shaft, and not radially aligned with said at least one truncated portion.

[0076] A first circumferential end of said at least a truncated portion can be arranged substantially at the level of the circumferential end of rupture of the liquid film.

[0077] By "arranged substantially at the circumferential rupture end of the liquid film", we mean arranged at a position located at plus or minus 5% of the length of the circumferential rupture end of the liquid film.

[0078] A first circumferential end of said at least a truncated portion may be arranged between 2° and 135° with respect to the preferred load direction along a direction of rotation of the pivot.

[0079] The average truncation curve segment of the truncation surface can be a function of the load applied to the hydrodynamic bearing.

[0080] A significant load applied to the bearing results in an extension of the liquid film curve segment. Therefore, the greater the load applied to the bearing, the smaller the average truncation segment can be, i.e., the smaller it can be.

[0081] This document may also relate to a method for positioning the pivot of a bearing of the aforementioned type, the method comprising: Determine the preferred load direction; and Orient the pivot around its longitudinal axis so that said at least one truncated portion is radially aligned with the outside of a curve segment of the liquid film.

[0082] According to a particular characteristic, the aforementioned level is obtained following the aforementioned positioning process.

[0083] This document may also relate to a mechanical reducer of an aircraft gas turbomachine, comprising an outer ring gear, at least one hydrodynamic bearing of the type mentioned above and at least one satellite pinion meshing with a central pinion and with the outer ring gear and mounted freely to rotate on a satellite carrier, said at least one satellite pinion being capable of rotating around a satellite axis by means of said at least one hydrodynamic bearing.

[0084] This document may also relate to an aircraft gas turbomachine comprising a mechanical reducer of the type described above, the central pinion of which surrounds and is rotationally fixed to a shaft of a turbomachine compressor. Brief description of the drawings

[0085] Other features, details, and advantages will become apparent upon reading the detailed description below and analyzing the attached drawings, on which:

[0086] [Fig. 1] illustrates a schematic diagram of a hydrodynamic bearing according to the previous technique,

[0087] [Fig. 2] represents a perspective view of a pivot according to a first embodiment of this document,

[0088] [Fig. 3] represents a perspective view of a pivot according to a second embodiment of this document, and

[0089] [Fig. 4] represents a perspective view of a pivot according to a third embodiment of this document. Description of the implementation methods

[0090] Reference is now made to figure 2 which represents a pivot according to a first embodiment of this document.

[0091] The pivot 20 with longitudinal axis X includes a radially internal part 22 forming a central shaft 22 which extends around an axial passage 24.

[0092] According to the illustrated aspect, the radially internal part 22 is an annular part whose surface is closed.

[0093] The radially internal part 22 comprises truncated conical axial ends 22a (only one is visible in the figure) which are connected to first cylindrical portions 22b themselves connected to second cylindrical portions 22c by radial shoulders 22d which extend radially outwards, so that the radial section decreases progressively towards the truncated conical axial ends 22a.

[0094] The term "radial section" refers to a section along a radial cutting plane, i.e. orthogonal to the X-axis.

[0095] The pivot 20 further includes a shaft 25 forming a truncated straight cylinder which is integral with the radially internal part 22 by means of an annular connecting part 28 which connects the second cylindrical portions 22c of the radially internal part 22 of the pivot 20 to the shaft 25.

[0096] The shaft 25 and the radially internal portion 22 together define an annular groove opening at at least one of the axial ends of the pivot 20, preferably at each of its axial ends. The bottom of the annular groove is formed by an axial end of the connecting annular portion 28.

[0097] The connecting part 28 is in the form of a radial shoulder which extends radially outwards.

[0098] Tree 25 includes a solid portion 26 which is devoid of truncation or openings.

[0099] The solid portion 26 includes axial flanges 26a at its axial ends such that a flange delimits its free radially internal surface 26b on the circumference of the solid portion 26. This flange extends axially to the axial ends of the solid portion 26 of the shaft 25.

[0100] Tree 25 further includes a truncated portion 30 which includes a truncation surface 32.

[0101] The truncated portion 30 opens radially outwards onto the truncation surface 32 which is delimited: circumferentially by a first circumferential end 32a and a second circumferential end 32b, which define a truncation curve segment CT and coincide with a first flank 26d and a second flank 26e of the solid portion 26 respectively; and axially by a first axial end 32c and a second axial end 32d, which coincide with the axial ends of the solid portion 26.

[0102] The first flank 26d and the second flank 26e delimit a curve segment of the solid portion 26.

[0103] The length between the first axial end 32c and the second axial end 32d is a truncation length LT of the truncation surface 32.

[0104] The truncated portion 30 further includes a truncation depth RT which, according to the first embodiment, is the difference in radius between a radially external surface 26c of the shaft 25 and a radially external surface 28a of the connecting portion 28.

[0105] According to the aspect illustrated: the average truncation depth is equal to 100% of the difference between the radius of the shaft 25 and the radius of the radially internal part 22; and the average truncation length is equal to 100% of the length of the shaft 25.

[0106] It is to be understood that this truncation depth RT can be variable in the truncated portion 30, for example less than the truncation depth RT of the first embodiment.

[0107] This truncation depth RT corresponds here to the radial dimension (Le. thickness) of the solid portion 26 of the tree 25: the tree 25 is here truncated over its entire thickness.

[0108] The pivot 20, and more particularly the shaft 25, is symmetrical with respect to a median radial plane P which cuts the shaft 25 into two parts of equal lengths.

[0109] By removing this material (the truncated portion 30), the mass of the pivot 20 is reduced. During its integration into a hydrodynamic bearing, the pivot 20 is oriented so that that the truncated portion 30 is oriented radially towards the rupture zone of film 1 1 in figure 1. This rupture zone of film 1 1 corresponds to an unloaded area of ​​the bearing.

[0110] Put another way, when it is integrated into a hydrodynamic bearing, the pivot 20 is oriented so that the truncation curve segment CT is included in the curve segment C2 of the film rupture zone 1 1.

[0111] According to this embodiment, the first circumferential end 32a (and therefore the first flank 26d) is radially aligned with the break point of the film 10a in Figure 1. In other words, the first circumferential end 32a is radially aligned with the break point of the film 10a.

[0112] In operation, this helps to reduce viscous shear losses between the fluid film 8 and an external part 6 of the bearing which is in contact with the liquid film 8.

[0113] The axial passage 24 can be a liquid supply orifice.

[0114] Figure 3 represents a pivot according to a second embodiment of this document.

[0115] In this second embodiment, the pivot 20 further includes an extension zone 36 which extends circumferentially from the first flank 26d. The extension zone 36 includes a first portion 36a with a general trapezoidal shape (along a tangential section plane) whose base 36a-1 is circumferentially aligned with the first flank 26d (and also the first circumferential end 32a).

[0116] It is to be understood that pivot 20 can include several extension zones 36, even if only one extension zone 36 is represented.

[0117] Therefore, the axial dimension of the extension zone 36 is maximum at the level of the first flank 26d and decreases or is constant along the direction of extension (here circumferential), i.e. from the first flank 26d to the second flank 26e.

[0118] The extension zone 36 further includes at its circumferential end a second portion 36b which is straight and connected to the first portion 36a.

[0119] According to this embodiment, the extension zone 36 is also symmetrical with respect to the radial plane P.

[0120] During operation, a residual pressure resulting from the load applied to the bearing may remain in the unloaded area. This residual pressure follows a gradient and results in a liquid pressure (oil for example) lower than the ambient pressure. This liquid pressure can be reduced to a liquid saturation pressure value, at which point the liquid transforms into its gaseous state: axially at the middle of the pivot 20, i.e. at the radial plane P comprising the axial median of the shaft 25; and circumferentially at the point of rupture of the lubricant film 10a, and therefore at the first circumferential end 32a and the first flank 26d.

[0121] The fact that the liquid pressure can be reduced to a saturation pressure value for said liquid is described in the following document: Liu, G., Fan, Z., Li, X., & Gu, D. (2018). Estimate of saturation pressures of crude oil by using ensemble-smoother-assisted equation of state. Industrial & Engineering Chemistry Research, Vol 57 / issue 49.

[0122] The general trapezoidal shape of the extension zone 36 thus takes into account the shape of the pressure field to compensate for this residual pressure.

[0123] The presence of the extension zone 36 redefines the outline of the truncated portion 30 and the associated truncation surface 32 which is now further delimited by the free perimeter of the extension zone 36.

[0124] Reference is now made to figure 4 which represents a pivot according to a third embodiment.

[0125] In this third embodiment, the extension zone 36 extends over the entire truncation curve segment CT of the truncation surface 32, namely from the first circumferential end 32a to the second circumferential end 32b (i.e. from the first flank 26d to the second flank 26e).

[0126] More specifically, the second straight portion 36b of the extension zone 36 extends circumferentially to the second circumferential extremity 32b.

[0127] With the presence of this new extension zone 36, the tree 25 includes a first truncated portion 30-1 and a second truncated portion 30-2 associated with respective truncation surfaces 32-1, 32-2.

[0128] The first truncated portion 30-1 and the second truncated portion 30-2 are located on either side axially of the extension zone 36.

[0129] In figure 4, the first truncated portion 30-1 and the second truncated portion 30-2 are identical due to the symmetry of the shaft 25 with respect to the radial plane P.

[0130] In operation, this allows the extension zone 36 to ensure that the residual pressure is compensated over the entire truncation curve segment 32e.

[0131] In the embodiments of Figures 2 to 4, the truncated portion(s) 30 open axially (along X) onto the two axial ends of the shaft 25. For example, the groove 26a of the solid portion 26 does not extend over the entire circumference: the truncated portion 30 opens axially onto a part of it.

Claims

Demands

1. Pivot (20) for a hydrodynamic bearing capable of being rotated about a longitudinal axis (X), said pivot (20) comprising: a radially internal portion (22) forming a central shaft extending axially and about the longitudinal axis (X), a shaft (25) arranged about said radially internal portion (22), characterized in that the shaft (25) is a truncated straight cylinder comprising a solid portion (26) without truncation or orifice and at least one truncated portion (30, 30-1, 30-2), said at least one truncated portion (30, 30-1, 30-2) being defined by an average truncation depth (RT) and a truncation surface (32, 32-1, 32-2), said at least one truncated portion (30, 30-1, 30-2) opening onto the truncation surface. (32, 32-1, 32-2), the truncation surface (32, 32-1, 32-2) comprising a mean truncation curve (CT) segment between 2° and 180°, preferably between 5° and 180°,and an average truncation length (TL) of between 5% and 100% of the length of the tree (25), wherein the tree (25) further comprises at least one extension zone (36) devoid of truncation or opening, said at least one extension zone (36) projecting outward from a first flank (26d) of the solid portion (26) and at least partially toward a second flank (26e) of the solid portion (26), said at least one extension zone (36) projecting from substantially an axial midpoint of the first flank (26d) of the tree (25).

2. Pivot (20) according to the preceding claim, wherein the average truncation depth (RT) of said at least one truncated portion (30, 30-1, 30-2) is between 0.01% and 100% of the difference between the radius of the shaft (25) and the radius of the radially internal part (22).

3. Pivot (20) according to any one of the preceding claims, wherein the shaft (25) is symmetrical with respect to a median radial plane (P).

4. Pivot (20) according to claim 1, wherein said at least one extension zone (36) extends from the first flank (26d) to the second flank (26e).

5. Pivot (20) according to any one of the preceding claims, wherein said at least one extension zone (36) comprises an axial dimension decreasing or constant from the first flank (26d) and along its extension direction.

6. Pivot (20) according to the preceding claim, wherein said at least one extension zone (36) comprises a first portion (36a) of general trapezoidal shape whose base (36a-1) is circumferentially aligned with the first flank (26d).

7. Pivot (20) according to the preceding claim, wherein said at least one extension zone (36) further comprises a second portion (36b) which is straight and connected to the first portion (36a).

8. Pivot (20) according to any one of the preceding claims, wherein said at least one truncated portion (30, 30-1, 30-2) opens axially onto at least one axial end of the shaft (25).

9. Hydrodynamic bearing (2) comprising a pivot (20) according to any one of the preceding claims mounted inside an external annular wall, the bearing comprising a preferred load direction along a given direction which passes successively through the truncated portion and then through the solid portion.

10. Hydrodynamic bearing (2) according to the preceding claim, further comprising a lubricating liquid film (8) intended to be arranged radially between the shaft (25) of the pivot (20) and the outer annular wall and circumferentially delimited by a circumferential rupture end of the liquid film (10a) and a circumferential reformation end of the liquid film (10b), the circumferential rupture end of the liquid film (10a) and the circumferential reformation end of the liquid film (10b) being located circumferentially on either side of the preferred load direction, the pivot (20) being oriented about its longitudinal axis (X) such that said at least one truncated portion (30, 30-1, 30-2) is radially opposite the outside of a curve segment of the liquid film (8).

11. Hydrodynamic bearing (2) according to the preceding claim, wherein the solid portion (26) comprises a curve segment extending over a first angle greater than or equal to a second angle over which the curve segment of the liquid film (8) extends.

12. Hydrodynamic bearing (2) according to claim 10 or 11, wherein a first circumferential end (32a) of said at least a truncated portion (30, 30-1, 30-2) is arranged substantially at the circumferential rupture end of the liquid film (10a).

13. Hydrodynamic bearing (2) according to claim 10 or 11, wherein a first circumferential end (32a) of said at least one truncated portion (30, 30-1, 30-2) is arranged between 2° and 135° with respect to the preferred load direction along a direction of rotation of the pivot.

14. Mechanical reducer of an aircraft gas turbomachine, comprising an outer ring, at least one hydrodynamic bearing (2) according to any one of claims 9 to 13 and at least one satellite pinion meshing with a central pinion and with the outer ring and mounted freely for rotation on a satellite carrier, said at least one satellite pinion being capable of rotating about a satellite axis through said at least one hydrodynamic bearing (2).

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