ROTOR FOR CENTRIFUGAL FAN OF A TURBOMACH

Description

Technical field of the invention

[0001] The invention relates to a rotor for a centrifugal turbomachine degasser and to a degasser comprising such a rotor. The invention further relates to a method for manufacturing this rotor. Technical background

[0002] Turbomachinery are complex systems that utilize several rotating assemblies (turbines, compressors, etc.) which must be equipped with sealing devices. These sealing devices are generally achieved through pressurized air labyrinths located near the rotating assemblies. To create these labyrinths, air is drawn directly from the turbomachine's air stream. This air then passes through the turbomachine via the various labyrinths designed for this purpose and is subsequently vented to the outside of the turbomachine to limit pressure buildup in other areas, such as the gearbox and accessory drive. This air, having passed through different areas of the turbomachine, is saturated with oil used for cooling and lubricating the bearings and gears of the rotating assemblies.To prevent the release of oil-laden air, mitigate the environmental impact of turbomachinery, reduce oil consumption, and limit the need for oil reservoir refills, it is important to install deaerators that separate the oil from the air before the air is vented outside the turbomachine. Such a deaerator is generally integrated into and driven by a mechanical power take-off from the turbomachine's accessory gearbox or reduction gear.

[0003] A centrifugal degasser typically comprises one or more centrifugal separation chambers for the air / oil mixture, arranged around a hollow shaft and delimited by an outer and an inner annular wall. The degasser also includes an axial inlet for supplying the chamber with the air / oil mixture, and a peripheral oil outlet in the outer wall. Thus, when the degasser is rotated, generally via a gear in the accessory gearbox or reduction gear, the oil is naturally driven by centrifugal force towards the oil outlet on the periphery of the degasser. An oil-free air outlet is also provided in the inner wall and connected to the hollow shaft, allowing the air to be expelled to the outside.

[0004] Some degassers, such as the one described in application WO-A1-2011 / 004023, also include filters arranged within the degasser housing to improve the capture of oil droplets and thus promote the removal of oil from the mixture. Indeed, the filters increase the available contact surface area and therefore improve the probability that an oil droplet carried by the mixture flow will adhere to a wall. These filters are generally made of a metallic foam, such as a foam marketed under the name Retimet®.

[0005] However, the performance of known degassers is generally hampered by internal pressure losses which are due to two causes in particular: the shapes of the stream, including the centrifugation chamber, through which the airflow passes during oil removal, and the presence of metallic foam.

[0006] With regard to the internal shapes of the degasser delimiting the channel taken by the airflow, the manufacturing process can then be limiting in terms of the potential for optimal geometry to be achieved.

[0007] Regarding the presence of metallic foam, the pressure losses are due to the fact that at high speeds (for example, for speeds of around 6000 to 25,000 rpm), the frontal surface made up of the metallic foam acts as a wall and the degree of penetration of air particles into the foam is low.

[0008] To address these problems and to optimize oil removal performance while limiting pressure losses through the degasser, the Applicant has already proposed in applications FR3071418 A1, WO-A1-2020 / 008153 and WO-A1-2020 / 008156 to replace the filter with an annular lattice structure which is produced by additive manufacturing.

[0009] In practice, the lattice structure is fabricated, then mounted and secured to the shaft using a nut (this is referred to as a shaft-mounted cartridge). The nut is screwed onto an external thread of the shaft and axially clamps the lattice structure against a shoulder of the shaft. The nut may include means for preventing the lattice structure from rotating relative to the shaft.

[0010] Securing the lattice structure to the shaft is relatively cumbersome because the nut and the shaft threads occupy a significant annular space in both the axial and radial directions. This space cannot be filled by the lattice structure, thus limiting its dimensions and consequently the degasser's performance. Furthermore, the threads complicate shaft manufacturing, and securing the lattice structure with a nut increases the manufacturing and assembly time of the degasser.

[0011] The present invention offers a simple, effective and economical solution to at least some of these problems. Summary of the invention

[0012] To this end, the invention relates to a rotor for a centrifugal degasser of an air / oil mixture in a turbomachine, this rotor comprising: a hollow shaft extending along an axis X and defining an internal cavity for air circulation after separation of said mixture, a pinion for rotating the hollow shaft, this pinion extending around the axis and being formed in one piece and in a first material with at least a first part of the hollow shaft, and an annular structure, preferably of lattice, extending around the axis and rotationally fixed to the shaft, this structure being made of a second material different from the first material and being configured to ensure centrifugal separation of said mixture, characterized in that said structure is made integral with the shaft by the additive manufacturing of this structure directly on at least one annular surface of the pinion which forms at least one annular support surface for this additive manufacturing.

[0013] The invention simplifies the rotor manufacturing process by eliminating the prior art step of screwing a nut onto the shaft. The annular structure is automatically attached to the shaft by its additive manufacturing process, directly onto the pinion. Therefore, no special fasteners or fastening steps are required, simplifying and reducing the rotor manufacturing time. Furthermore, eliminating the nut eliminates the need for shaft threading, a requirement of the prior art, and optimizes the available space for the annular structure, which can thus be larger than those of the prior art. This, in turn, improves the degassing performance.Finally, using two different materials for the annular structure on the one hand, and for the pinion and at least part of the shaft on the other, allows these materials to be chosen according to the mechanical and chemical resistance requirements and the lifespan of the different parts of the rotor.

[0014] The rotor according to the invention may comprise one or more of the following features, taken individually or in combination with each other: the pinion is located at a longitudinal end of said first part of said shaft and said at least one annular surface is located on a side of the pinion opposite this first part; said at least one annular surface is perpendicular to said axis; said structure is produced by additive manufacturing on a single annular surface of the pinion, this surface being flat and extending from the inner periphery of the pinion to the outer periphery of the pinion; the pinion has an outer periphery comprising teeth, an inner periphery connected to the shaft, and an intermediate annular web extending between its inner and outer peripheries and having a thickness measured along said axis that is less than the thicknesses of said peripheries measured along the same axis; the annular structure is produced in one piece and in said second material with at least a second part of said shaft;the first material is chosen from a steel that can be hardened by heat treatment by carburizing or nitriding, for example of type E16NCD13 and E32CDV13; these grades of steel allow surface hardening by heat treatment (carburizing or nitriding); the choice of this grade makes it possible to obtain the mechanical properties necessary for the good behavior of the mesh in operation; the second material is a stainless steel, for example of type 17-4; PH ; insofar as the structure may be in contact with the ambient air outside the turbomachine, this material helps to prevent oxidation and any risk of damage.

[0015] The present invention also relates to a centrifugal degasser for a turbomachine air / oil mixture, comprising a rotor as described above.

[0016] The present invention also relates to a method for manufacturing a rotor as described above.

[0017] The process includes the following steps: a) manufacturing in one piece and in a first material of the pinion and at least part of the hollow shaft, b) additive manufacturing, for example on powder beds, of the annular structure in a second material directly on at least one annular surface of the pinion.

[0018] The process according to the invention makes it possible to obtain the rotor as described above, for a centrifugal degasser of an air / oil mixture in a turbomachine. This rotor comprises: a hollow shaft extending along an axis X and defining an internal cavity for air circulation after separation of said mixture, a pinion for rotating the hollow shaft, this pinion extending around the axis and being formed in one piece and in a first material with at least a first part of the hollow shaft, and an annular structure, preferably of lattice, extending around the axis and rotationally fixed to the shaft, this structure being made of a second material different from the first material and being configured to ensure centrifugal separation of said mixture, said annular structure is made integral with the shaft by the additive manufacturing of this structure directly on at least one annular surface of the pinion which forms at least one annular support surface for this additive manufacturing.

[0019] The rotor obtained by the manufacturing process may thus include at least one or more of the rotor-related characteristics described above. These characteristics may be considered individually or in combination.

[0020] The pinion may be located at one longitudinal end of said first part of said shaft and said at least one annular surface is located on one side of the pinion opposite this first part.

[0021] The pinion may have an outer periphery comprising teeth, an inner periphery connected to the shaft, and an intermediate annular web extending between its inner and outer peripheries. This annular web may have a thickness measured along said axis that is less than the thicknesses of said peripheries measured along the same axis.

[0022] Preferably, step a) includes machining a block of metal alloy, and also preferably the treatment of this block after machining by carburizing or nitriding.

[0023] Advantageously, step b) includes the simultaneous fabrication of the annular structure and a second part of said tree.

[0024] This second part of said tree can be located at the inner periphery of the annular structure and extends upwards from its annular surface and in line with the first part of said tree.

[0025] The said annular surface of the pinion may be unique. Brief description of the figures

[0026] Other features and advantages of the invention will become apparent upon reading the detailed description that follows, for an understanding of which reference should be made to the accompanying drawings in which: [ Fig.1 ] there figure 1shows a schematic perspective view of a centrifugal degasser for a turbomachine, cut along a plane of symmetry; Fig. 2 ] there figure 2 shows a partial schematic perspective view, cut along a plane of symmetry, of a degasser centrifuge chamber. figure 1 ; Fig.3 ] there figure 3 shows a schematic perspective view of a portion of a lattice structure of the centrifuge chamber of the figure 2 ; Fig. 4 ] there figure 4 shows a schematic perspective view of a manufacturing process, and in particular an assembly process, of a centrifugal degasser rotor, according to the technique prior to the present invention; [ Fig. 5 ] there figure 5 shows a schematic and partial cross-sectional view of a pinion on which an annular lattice structure is produced by additive manufacturing, and illustrates a method for manufacturing a rotor according to the invention; Fig. 6 ] there figure 6shows a schematic perspective view of a pinion and part of a shaft, for implementing the method according to the invention; and [ Fig. 7 ] there figure 7 shows another schematic perspective view of the pinion and shaft portion of the figure 6 . Detailed description of the invention

[0027] In the figures, the scales and proportions are not strictly respected for the purposes of illustration and clarity.

[0028] A centrifugal degasser for a particular aircraft turbomachine is shown on the figure 1 .

[0029] This degasser includes in particular a part 1 that rotates around a longitudinal axis X.

[0030] As shown in more detail on the figure 2, part 1 has a structural part which includes a first shell 2 surrounded by a second shell 3. The space between the two shells 2, 3, forms a vein 4 of revolution around the X axis, intended to circulate a mixture of air and oil to be separated.

[0031] The channel 4 has an axial inlet 5 for the entry of the air and oil mixture to be separated. This axial inlet 5 corresponds to one end of a first section 6 of the channel 4, which extends essentially axially in order to centrifuge the mixture. The first axially extending section of the channel 6 acts as the centrifugation chamber because it is there that the centrifugal force is exerted with the greatest force on the air / oil mixture. It is therefore referred to as the centrifugation chamber 6 in the following description.

[0032] The vein 4 further comprises a plurality of compartments distributed circumferentially around the X-axis. The compartments are formed by radially extending longitudinal partitions 7. Advantageously, these longitudinal partitions 7 connect the first 2 and the second shell 3, thus forming a bond that secures them. Each compartment communicates with the axial inlet 5 of the mixture. The axial partitions 7 form fins that rotate the incoming mixture into the adjacent compartments. At its second axial end, the centrifugation chamber 6 is axially closed by a portion 3a of the second shell 3, substantially perpendicular to the X-axis, and has a radial opening 9 towards the X-axis between the first 2 and the second shell 3.The second shell 3 forms a radially external wall 3b of the centrifugal chamber 6, which is substantially annular, between the inlet 6 and the portion 3a of the second shell that axially delimits the centrifugal chamber 6 at its second end. The centrifugal chamber 6 has a plurality of radial oil outlets 8, in the form of through-holes provided in the radially external wall 3b, and is configured to be able to discharge the oil separated from the mixture by the effect of the centrifugal force of the degasser. Each compartment of the stream 4 is connected to one or more radial oil outlet(s) 8.

[0033] The first shell 2 forms a radially internal wall of the vein compartments within the centrifuge chamber 8. It stops axially before the axial portion 3a of the second shell 3, starting from the vein inlet 6, to provide the inward radial opening 9 at the second end of the centrifuge chamber 6. Its shape can be optimized to promote oil separation and minimize pressure losses, particularly at the bend formed at the radial outlet. In the example shown, the radially internal wall 2 is substantially annular starting from the axial inlet 5 and has an axial end 2a opposite the radial inlet 5, forming a rounded circumferential rim or plateau at the second end of the centrifuge chamber 6.This shape of the axial end 2a of the first shell tends to send the fluid radially outwards at the passage of the bend formed in the vein 4 at the outlet of the centrifugation chamber 6, in order to optimize the flow of the air / oil mixture flux.

[0034] The vein 4 has a second part 10 which communicates with the centrifugation chamber 6 by the radial opening 9 between the first 2 and the second 3 shells and which is configured to guide the fluid towards a radial outlet 11 in an empty cylindrical space, which extends axially between the limits of the centrifugation chamber 6. The first 2 and the second 3 shells form collars 12, 13, which limit said empty cylindrical space.

[0035] These collars 12, 13 are configured to connect part 1 to a hollow shaft 14, shown on the figure 1, which causes the part to rotate. The cross-section of the vein 4 along a longitudinal plane has an angled shape optimized to guide the oil-free air towards the internal radial outlet 12.

[0036] Part 1 is used in a degasser which includes a pinion 15 for rotating the shaft 14 and part 1. In the example shown, the pinion 15 includes a web 16 which is fixedly connected to the hollow shaft 14 and which includes openings opposite the axial inlet 5 for the passage of the mixture into the compartments of the vein 4.

[0037] Part 1 further comprises at least one honeycomb lattice structure 17 housed in the centrifugation chamber 6.

[0038] The centrifugation chamber 6 can comprise two successive distinct spaces: a free space 18 located upstream with reference to the flow of the mixture in the chamber 6, and a space 19 filled by the structure 17. The free space 18 is supplied with mixture through the opening of the compartment on the axial inlet 5 and it opens into the space 19 filled by the structure 17. The space 19 filled by the structure 17 opens into the second part 10 of the vein.

[0039] As indicated by the arrow F1 on the figure 1The air / oil mixture thus enters the moving part 1 through the openings in the web 16. In the free space 18, the longitudinal partitions 7 cause the mixture to rotate. During the passage of the flow F1 through this first part 18 of the enclosure, a first oil removal phase is carried out by centrifugal force. The lattice structure 17 serves to capture any oil droplets not extracted during the first phase. By centrifugal force, the oil is discharged towards the radial outlets 8 through the structure 17, as illustrated by the arrows F2. This second oil removal phase is further carried out in the space 19 occupied by the structure 17 without significant pressure losses due to the axial impact of the oil droplets and the lattice structure.

[0040] Next, the oil-free air having passed through structure 17 in vein 4 arrives in hollow shaft 14 to be evacuated.

[0041] Structure 17, for example, is formed by the repetition, along three spatial dimensions, of a single motif arranged so that the gaps between the material communicate in such a way as to organize paths traversing the lattice material in three spatial dimensions. These paths exhibit bends, pinches, and / or bifurcations. Several embodiments of such a structure or lattice are conceivable, such as the one illustrated in the figure 3 .

[0042] The configurations shown on the Figures 1 And 2 are not limiting. In particular, structure 17 can be formed in one piece with shells 2, 3 so that part 1 forms a monobloc assembly.

[0043] Part 1 is then advantageously produced using an additive manufacturing method, as described in application WO-A1-2019 / 063458, which allows for the creation of the complex shapes shown in the example, particularly to facilitate the separation of oil droplets from the mixture while minimizing pressure losses. Additive manufacturing can be carried out using a known method such as laser powder bed fusion.

[0044] In the current technique, after manufacturing part 1 and therefore the lattice structure 17, it is engaged on one end of the shaft 14 and fixed to it using a nut 20 (cf. figure 1 ).

[0045] The rotor 21 of the degasser is then formed by assembling the parts of the figure 4 , namely the shaft 14 attached to the pinion 15, the part 1 and its annular lattice structure 17 which can form a single unit, and the nut 20.

[0046] The invention proposes an optimized rotor 21 and in particular simplified insofar as it does not include a fixing element of the lattice structure 17 on the pinion 15 or the shaft 14.

[0047] THE figures 5 to 7 illustrate one embodiment of the invention, namely on the one hand a rotor 121 for a centrifugal degasser, and on the other hand a method for manufacturing this rotor 121.

[0048] The rotor 121 essentially comprises three parts, namely: a hollow shaft 114 extending along an axis X and defining an internal cavity for air circulation after separation of the air / oil mixture, a pinion 115 for rotating the hollow shaft 114, this pinion 115 extending around the axis X and being formed in one piece and in a first material with at least a first part 114a of the hollow shaft 114, and an annular lattice structure 117 extending around the axis X and rotationally fixed to the shaft 114, this structure 117 being made of a second material different from the first material and being configured to ensure the centrifugal separation of the mixture, as mentioned above.

[0049] According to the invention, the structure 117 is made integral with the shaft 114 by the additive manufacturing of this structure 117 directly on at least one annular surface 122 of the pinion 115 which forms at least one annular support surface for this additive manufacturing (cf. figure 5 ).

[0050] THE figures 6 and 7 show a first manufacturing step of the rotor 121, which consists of manufacturing in one piece and in a first material the pinion 115 and at least a part 114a of the hollow shaft 114.

[0051] In the example shown, the pinion 115 is located at a longitudinal end of the first part 114a of the shaft 114. The annular surface 122 on which the lattice structure 117 is formed is located on a side of the pinion 115 opposite to this first part 114a of the shaft 114.

[0052] The annular surface 122 is here perpendicular to the X axis. This surface 122 is unique here (the gable 115 does not include other surfaces intended to receive the lattice structure 117).

[0053] The pinion 115 has an outer periphery comprising a toothed area 123, an inner periphery connected to the shaft 114, and an intermediate annular web 116 extending between its inner and outer peripheries.

[0054] The veil 116 preferably has a thickness E1 measured along the X axis which is less than the thicknesses E2, E3 of the peripheries measured along the same X axis.

[0055] Unlike the prior art in which the annular structure 117 is manufactured and then attached and fixed to the shaft 114 and the pinion 115, here the structure 117 is manufactured and attached simultaneously to the pinion 115. For this, the structure 117 is made in a second material by additive manufacturing directly on the surface 122 of the pinion 115.

[0056] As schematically represented in the figure 5Additive manufacturing can be of the powder bed type. As is well known to a person skilled in the art specializing in this field, a laser beam 125 emitting a laser beam 126 scans the surface of a powder bed 127 which is contained in a bin 128. The bin 128 comprises several superimposed layers of powder and the top layer of powder 129 is scanned by the laser beam 126 following a predetermined pattern to melt the powder and generate a bead of molten material.

[0057] The pinion 115 forms a support platform for additive manufacturing and includes a first layer of powder regularly distributed over its surface 122. The platform 115 is mobile in the tray 128 along the X axis which is oriented vertically.

[0058] Once the first layer has melted, the gear 115 is lowered into the tank 128, and a new layer of powder is spread over the gear 115 and the already molten layer. The laser 126 is used again to melt this new layer, and this process is repeated as many times as necessary until the annular structure 117 is fully formed by stacking strands of molten material one on top of the other along the X-axis.

[0059] There figure 5 This allows us to observe that an upper part 114b of the shaft 114 is produced simultaneously with the annular structure 117 by additive manufacturing. This second part 114b is located on the inner periphery of the structure 117 and extends upwards from the surface 122 of the pinion 115, continuing from the first part 114a of the shaft 114 located below the pinion 115 and in the container 128.

[0060] The pinion 115 and the shaft portion 114a are preferably machined from a block of metal alloy prior to additive manufacturing. The material of the pinion 115 and the shaft portion 114a is preferably selected from E16NCD13 and E32CDV13. This material is preferably nitrided or case-hardened to make its external surface harder. Hardening the external surface of the pinion 115, and in particular its teeth 123, is indeed important for optimizing its service life.

[0061] The material for the 117 lattice structure is preferably stainless steel, for example type 17-4 PH. Unlike aluminum, for example, steel is well suited to laser melting and allows for the creation of thin structures without the risk of fusion defects.

[0062] To date, no additive manufacturing material exists that guarantees sufficient mechanical properties for fatigue resistance. Therefore, additive manufacturing materials cannot be used to produce the pinion 115 and its teeth 123. It is thus important that the pinion 115 be made from a material other than that used for additive manufacturing, and the solution has been found to produce this pinion by machining it from a solid block of material, as mentioned above.

[0063] Certain finishing or machining operations on the pinion 115 can be carried out after the additive manufacturing of the lattice structure 117. This is particularly the case for drilling the pinion 115 and the shaft 114 along the X-axis to create the internal cavity of the shaft, which is shown in dashed lines in figures 6 and 7This is also the case for the openings in the web 116 of the pinion 115, which are shown as dashed lines in these figures. Creating these holes and openings after additive manufacturing ensures good material continuity and facilitates the support of the layers and powder during the additive manufacturing process. It is also possible to consider reworking the bearing surfaces of the shaft 114's guide bearings after additive manufacturing, and grinding the teeth 123 of the pinion 115 after this additive manufacturing process, to correct any potential deformation and ensure perfect coaxiality of the different parts of the rotor 121.

[0064] The invention thus makes it possible to create a robust, non-removable connection between the lattice structure 117 and the gable 115, thereby maximizing the volume of the cells in this structure. Additive manufacturing of the structure 117 allows, for example, the production of cells or lattice strands with small dimensions, for example, between 0.4 mm and 0.7 mm in diameter.

Claims

1. A rotor (121) for a centrifugal breather for a turbomachine air / oil mixture, this rotor comprising: - a hollow shaft (114) extending along an axis (X) and defining an internal air circulation cavity after separation of said mixture, - a pinion (115) for rotating the hollow shaft (114), this pinion (115) extending about the axis (X) and being formed in one piece and from a first material with at least one first portion (114a) of the hollow shaft (114), and - an annular structure (117), preferably in the form of a lattice, extending around the axis (X) and secured in rotation to the shaft (114), this structure (117) being made of a second material different from the first material and being configured to ensure a centrifugal separation of said mixture, characterised in that said structure (117) is made secured to the shaft (114) by the additive manufacture of this structure directly on at least one annular surface (122) of the pinion (115) which forms at least one annular support surface for this additive manufacture.

2. The rotor (121) according to claim 1, wherein the pinion (115) is located at a longitudinal end of said first portion (114a) of said shaft (114) and said at least one annular surface (122) is located on a side of the pinion (115) opposite to this first portion (114a).

3. The rotor (121) according to claim 1 or 2, wherein said at least one annular surface (122) is perpendicular to said axis (X).

4. The rotor (121) according to one of the preceding claims, wherein said structure (117) is produced by additive manufacturing on a single annular surface (122) of the pinion (115), this surface (122) being flat and extending from the internal periphery of the pinion (115) to the external periphery of the pinion (115).

5. The rotor (121) according to one of the preceding claims, wherein the pinion (115) has an external periphery comprising a toothing (123), an internal periphery connected to the shaft (114), and an intermediate annular web (116) extending between its internal and external peripheries and having a thickness (E1) measured along said axis (X) which is less than the thicknesses (E2, E3) of said peripheries measured along the same axis (X).

6. The rotor (121) according to any of the preceding claims, wherein the annular structure (117) is made in one piece and from said second material with at least one second portion (114b) of said shaft (114).

7. The rotor (121) according to one of the preceding claims, wherein the first material is chosen from a steel that is hardenable by thermal treatment of case-hardening or nitriding, for example of the E16NCD13 and E32CDV13 type.

8. The rotor (121) according to one of the preceding claims, wherein the second material is a stainless steel, for example of the 17-4 PH type.

9. A centrifugal breather for a turbomachine air / oil mixture, comprising a rotor (121) according to one of the preceding claims.

10. A method for manufacturing a rotor according to one of claims 1 to 8, characterised in that it comprises the steps of: a) manufacturing the pinion (115) and at least one portion (114a) of the hollow shaft (114) in a single piece and from a first material, b) additive manufacturing, for example on powder beds, of the annular structure (117) in a second material directly on at least one annular surface (122) of the pinion (115).

11. The method according to the preceding claim, wherein the pinion (115) is located at a longitudinal end of said first portion (114a) of said shaft (114) and said at least one annular surface (122) is located on a side of the pinion (115) opposite to this first portion (114a).

12. The method according to claim 10 or 11, wherein the pinion (115) has an external periphery comprising a toothing (123), an internal periphery connected to the shaft (114), and an intermediate annular web (116) extending between its internal and external peripheries and having a thickness (E1) measured along said axis (X) which is less than the thicknesses (E2, E3) of said peripheries measured along the same axis (X).

13. The method according to one of claims 10 to 11, wherein step a) comprises machining a metal alloy block, and also preferably treating this block after machining by case-hardening or nitriding.

14. The method according to any of claims 10 to 13, wherein step b) comprises simultaneously manufacturing the annular structure (117) and a second portion (114b) of said shaft (114).

15. The method according to the preceding claim, wherein the second portion (114b) is located at the internal periphery of the structure (117) and extends towards the top of the pinion (115) from its surface (122) and in the extension of the first portion (114a) of the shaft (114).