Bushing attached to a ball screw nut

Abstract

A method of manufacturing a piston (12) of a brake actuator mechanism (10), the piston (12) comprising a sleeve (42) and a nut (16), the nut (16) comprising an outer peripheral wall (32), characterized in that before securing the sleeve (42) to the outer peripheral wall (32) of the nut (16), the sleeve (42) is subjected to a thermochemical treatment for resistance to abrasion and corrosion at a temperature Ts until a nitrogen-rich surface layer for resistance to abrasion and corrosion is obtained, and the nut (16) is subjected to a thermochemical hardening treatment including heating to a temperature Tc at least 200°C higher than Ts, then quenching and tempering at a temperature Tr at least 100°C lower than Ts, and obtaining a hardened surface layer rich in carbon at least locally at the nut thread (27). (Abstract Figure: 1)

Description

Title of the invention: Sleeve attached to a ball screw nut TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to the field of actuators, in particular for the transport industry, in particular automotive or aeronautical, in particular to pistons in mechanisms driven by a worm screw, in particular a ball screw and more particularly, although not exclusively, to brake caliper pistons in braking mechanisms driven by a worm screw, in particular a ball screw. STATE OF THE PRIOR ART

[0002] In document EP 2 787 248 B1, a brake actuator mechanism is disclosed comprising a screw, a nut and balls positioned between a helical thread of the screw and a helical thread of the nut, the nut forming a piston housed in a guide cylinder. The ball screw mechanism formed by the screw, the nut and the balls requires, at the level of the threads of the screw and the nut, sufficient hardness. This type of piston is positioned close to the brake caliper, and subjected to intense pollution coming from its direct external environment, which can result in corrosion at the level of the nut. To protect the ball screw mechanism, it is necessary to maintain a small constructive clearance between the piston and its guide cylinder, which generates risks of abrasion, accentuated by the presence of pollutants. The risks of failure of the mechanism are therefore numerous.

[0003] In document EP 2 304 265 B1, a brake actuator mechanism is disclosed comprising a piston sliding in a cylinder and driven by a ball screw mechanism. The piston is made in several parts, and incorporates the nut of the ball screw mechanism, a solid thrust part in which the nut is shrunk, and an outer part shrunk onto the thrust part, the outer part having a base on which a truncated surface of the thrust part bears. This three-part construction of the piston aims to share some of the parts of the piston in several models of different dimensions, the intermediate thrust part being in some way an adaptation part. Statement of the invention

[0004] The invention aims to remedy the drawbacks of the state of the art and to propose a piston which is more resistant to abrasion and corrosion and economically advantageous, while remaining compact.

[0005] To do this, according to a first aspect of the invention, a method of manufacturing a piston of a brake actuator mechanism is proposed, the piston comprising a ball screw mechanism nut, defining a reference axis, a peri-wall outer spherical and a nut thread intended to form a raceway for balls of the ball screw mechanism; a sleeve integral with the nut and at least partially covering the outer peripheral wall of the nut, the sleeve being intended to come into adjusted sliding contact with an inner guide wall of a guide cylinder of the brake actuator mechanism;remarkable in that before securing the sleeve to the outer peripheral wall of the nut, the sleeve is subjected to a thermochemical treatment for resistance to abrasion and corrosion at a temperature Ts until a surface layer of resistance to abrasion and corrosion rich in nitrogen is obtained, and the nut is subjected to a thermochemical hardening treatment including heating to a temperature Te at least 200°C higher than Ts, then quenching and tempering at a temperature Tr at least 100°C lower than Ts, and obtaining a hardened zone rich in carbon at least locally at the nut thread. ;

[0006] The thermochemical treatment for resistance to abrasion and corrosion and the thermochemical treatment for hardening attribute distinct advantages to the same material. However, the methods for implementing each of the two treatments differ and, whatever the order in which these treatments are implemented on the same part, the properties provided by one erase the properties provided by the other. More specifically, assuming that one begins to treat a part to increase its hardening by a thermochemical treatment with carbon enrichment involving at the end of the treatment quenching and tempering at a tempering temperature Tr, the subsequent treatment of another part of the same part at a temperature Ts substantially higher than the tempering temperature Tr will eliminate the effects of quenching and tempering, release the carbon compounds and eliminate the hardening effect sought with the initial hardening treatment.Conversely, if an initial thermochemical treatment for abrasion resistance is carried out on a part at a temperature Ts involving enrichment of a surface area of ​​the part with nitrogen, a subsequent hardening treatment of another part of the same part at a temperature Te significantly higher than Ts has the consequence of releasing the nitrogen compounds retained on the surface of the material during the initial treatment. It is therefore not possible to have a single-piece part possessing the desired properties provided by the two treatments, in an economically viable manner. By carrying out these two treatments each on a separate part, namely the sleeve and the nut, intended to be assembled, it becomes possible to propose a composite piston having all the desired properties.

[0007] The nut is preferably made of steel, for example 20MnCr5, 23MnB4, Scr420, 16MnCr5 steel or their equivalents according to other international or national standards, or high carbon steel such as 100Cr6, C50 or C56 ... equivalents. The thermochemical hardening treatment is preferably a gaseous medium treatment. The quenching and tempering steps make it possible to achieve a high surface hardness, for example a hardness greater than 58 HRC (Rockwell hardness), and to maintain a high toughness in the core of the part. Thanks to this treatment, the nut thread has increased hardness, which allows it to be more durable by resisting chipping for example. The thermochemical hardening treatment can be a surface treatment, but is preferably a deep treatment over a thickness exceeding 0.5 mm, and preferably exceeding 2 mm. It can also be a core treatment.

[0008] According to one embodiment, the thermochemical hardening treatment includes a carburizing treatment, the temperature Te being greater than 900°C, the temperature Tr being less than 250°C. Alternatively, it may be a carbonitriding treatment.

[0009] The sleeve is preferably made of steel, or of a material essentially containing steel. According to one embodiment, the abrasion and corrosion resistance treatment includes nitriding or nitrocarburizing, the temperature Ts being between 300°C and 580°C. Thanks to this treatment, the piston resists abrasion and corrosion, likely to be initiated by its translational movements and particulate pollution in the guide cylinder.

[0010] According to one embodiment, an outer face of the sleeve is ground before being subjected to the thermochemical treatment for resistance to abrasion and corrosion, so that the outer surface of the sleeve is perfectly smooth, which allows a reduction in the friction of the piston in the guide cylinder and therefore an improvement in efficiency, as well as an increase in the resistance to abrasion and corrosion.

[0011] According to one embodiment, after completion of the thermochemical treatment for resistance to abrasion and corrosion and the thermochemical hardening treatment, the sleeve is secured, preferably by shrink fitting, to the outer peripheral wall of the nut. In this way, the initially incompatible properties are combined in a single set of single-piece parts. The piston is then harder at the nut thread and more resistant at its contact surface with the guide cylinder. The sleeve, due to its relatively small thickness, makes it possible to provide an additional property to the piston without considerably increasing the volume of the brake actuator mechanism.

[0012] According to one embodiment, an outer surface of a bottom wall of the nut or the sleeve undergoes an additional anti-corrosion treatment, preferably the additional anti-corrosion treatment is a lamellar zinc coating treatment, this bottom wall being intended to come into direct or indirect contact with the brake caliper.

[0013] According to one embodiment, a slide of the piston undergoes a surface treatment before being partially inserted into a housing formed in the nut and the sleeve, preferably the surface treatment of the slide is nitrocarburizing. The slide may, if necessary, contribute to the cohesion between the nut and the sleeve, but its main function is to ensure, in cooperation with a rectilinear groove formed in the guide cylinder of the brake actuator mechanism, translational guidance without rotation of the piston in the cylinder. Thanks to its additional treatment, the slide is resistant to abrasion and corrosion, allowing an increased service life for the brake actuator mechanism.

[0014] According to another aspect of the invention, it relates to a piston which is remarkable in that it is manufactured according to the manufacturing method as described above. This piston is characterized in particular by a surface metallurgical state at the level of the sleeve, rich in nitrogen, a consequence of the thermochemical treatment for abrasion resistance, and by a metallurgical state rich in carbon conferring a high hardness at least at the level of the thread of the nut.

[0015] According to one embodiment, the nut has an open external recirculation channel, closed at least in part by the sleeve. This feature makes it easier to machine the recirculation channel and, where appropriate, to assemble and introduce the balls into the mechanism.

[0016] According to one embodiment, the sleeve has a bottom. In this configuration, the bottom of the sleeve can, if necessary, provide support against the brake caliper alone, and the nut can be open at its two axial ends.

[0017] According to one embodiment, the sleeve has a flap of material on an annular end face of the nut, which ensures axial positioning between the sleeve and the nut.

[0018] According to one embodiment, the piston comprises a slide projecting axially outwards making it possible to fix the piston in rotation in the guide cylinder, while allowing its translation.

[0019] The piston thus described is intended in particular for vehicle braking actuators.

[0020] According to another aspect of the invention, it relates to a brake actuator mechanism, comprising a guide cylinder defining a reference axis of the brake actuator mechanism; a ball screw mechanism, comprising a screw and a nut centered on the reference axis, and balls, the screw having at least one screw thread forming a raceway for the balls, the nut having a nut thread forming a raceway for the balls and an outer peripheral wall; and a sleeve secured to the nut and at least partially covering the outer peripheral wall. outer spherical of the nut, the sleeve coming into adjusted sliding contact with an inner guide wall of the guide cylinder; remarkable in that the sleeve and the nut constitute a piston as described above. BRIEF DESCRIPTION OF THE FIGURES

[0021] Other characteristics and advantages of the invention will emerge on reading the description which follows, with reference to the appended figures.

[0022] [Fig.l] [Fig.l] illustrates a brake actuation mechanism according to one embodiment comprising an internally recirculating piston having a closed nut.

[0023] [Fig.2] [Fig.2] illustrates a brake actuation mechanism according to a mode of construction comprising an external recirculating piston comprising a closed nut.

[0024] [Fig.3] [Fig.3] illustrates a brake actuation mechanism according to a mode of construction comprising a piston having an open nut.

[0025] For clarity, identical or similar elements are identified by identical reference signs throughout the figures. DETAILED description of embodiments

[0026] In [Fig.l] is illustrated a first embodiment of a brake actuator mechanism 10 comprising a fixed guide cylinder 44 defining a reference axis 100 of the brake actuator mechanism 10 and a piston 12 sliding in translation in the guide cylinder 44 along the reference axis 100, which is also a reference axis of the piston, to bear directly or indirectly against a brake caliper (not shown). The piston 12 comprises a bushing 42 and a nut 16, the nut 16 being part of a ball screw mechanism comprising two threaded components, namely a screw 14 and the nut 16, and balls 18.

[0027] The screw 14 is preferably metallic, for example made of steel such as 20MnCr5, 23MnB4, Scr420, 16MnCr5 steels or their equivalents according to other international or national standards, or high carbon steel such as 100Cr6, C50 or C56 steel or their equivalents, and may comprise a screw head 20, a connecting portion 22 and a screw body 24. The screw body 24 has a diameter greater than the screw head 20, the connecting portion 22 providing the connection between the screw body 24 and the screw head 20. This connecting portion 22 may be frustoconical, preferably cylindrical, and forms a first flat shoulder 26. The screw head 20 is shaped to be secured in rotation to an output shaft of an electric motor or a geared motor, and may present, for example, a non-circular interface, for example with four, six or eight sides.

[0028] The screw body 24 has a screw thread 25 which forms an inner helical raceway around the reference axis 100 of the ball screw mechanism, the inner helical raceway being rotated radially away from the axis of reference 100. In addition, the screw 14 has an open central cavity 28 making it possible to lighten the entire brake actuator mechanism 10, and to provide a receptacle for grease contained in the ball screw mechanism.

[0029] The nut 16 is made of steel, for example 20MnCr5, 23MnB4, Scr420, 16MnCr5 steel or their equivalents according to other international or national standards, or high carbon steel such as 100Cr6, C50 or C56 steel or their equivalents. The nut 16 has a generally cylindrical shape whose central axis is the reference axis 100. The nut 16 has a nut thread 27 which forms an external helical raceway around the reference axis 100, and facing radially towards the reference axis 100. The nut 16 has a cylindrical external peripheral face 32 in which a locking mortise 64 is formed.

[0030] In addition, the nut 16 is of the closed type in the sense that it has a bottom 17, with an outer closing face 34 which may have a recess 35, and is configured to come into direct or indirect contact with a brake caliper (not shown in the figures). The outer closing face 34 also has a collar 72, projecting radially from the outer peripheral wall 32, which forms a collar shoulder 72'. The collar 72 also makes it possible to limit any deformations of the outer closing face 34 under mechanical stresses during activation of the brake actuator mechanism 10 for example.

[0031] One of the two threaded components, namely the screw 14 or the nut 16, may further be equipped with recirculation means 40 for the balls 18, which may comprise one or more recirculators each passing through a thread of the threaded component, as illustrated in [Fig.l], or pairs of recirculators arranged at the ends of a recirculation channel which spans one or more turns of the raceways of the screw 14 and the nut 16. The system could also operate on a system without recirculation.

[0032] The balls 18 may for example be made of steel or ceramic, and are sized and positioned to circulate in a closed circuit between the outer helical raceway of the nut 16 and the inner helical raceway of the screw 14, as well as, where appropriate, by the recirculation means 40, preferably without separators between the balls 18.

[0033] The sleeve 42 is metallic, for example made of steel, for example 20MnCr5, 23MnB4, Scr420, 16MnCr5 steel or their equivalents according to other international or national standards, or high carbon steel such as 100Cr6, C50 or C56 steel or their equivalents. The sleeve 42 has a cylindrical inner face 48 shrunk onto at least a portion of the outer peripheral wall 32 of the nut 16. The sleeve 42 has an outer face of the sleeve 49, and a thickness between the cylindrical inner face 48 and the outer face of the sleeve 49 is of the order of 1 mm. The sleeve 42 has a locking slot 66, such as a generally rectangular through hole, located near the annular end face 36 of the nut 16. The locking slot 66 provides access to the locking mortise 64 of the nut 16. In addition, the sleeve 42 may have a sleeve shoulder 50 which rests axially on the annular end face 36 of the nut 16, opposite the bottom 17 of the nut.

[0034] The brake actuator mechanism 10 also comprises a slider 46, shrunk into the locking mortise 64, projecting radially towards the guide cylinder 44 through the locking slot 66, relative to the outer face of the sleeve 49.

[0035] The guide cylinder 44 is made of a metal base, for example steel, and comprises a preferably flat annular base 52, a guide body 54 projecting axially from the outer periphery of the annular base 52, and an inner sealing skirt 56 projecting axially from the inner periphery of the annular base 52.

[0036] The guide body 54 is a cylinder whose central axis is the reference axis 100. The guide body 54 comprises an inner guide wall 58, facing radially towards the reference axis 100, in sliding contact with the sleeve 42.

[0037] The inner sealing skirt 56 has a cylindrical inner face 57, facing radially towards the reference axis 100, delimiting an intermediate space 59. The inner guide surface 57 is positioned opposite and at a short distance from the screw rod 20, in order to constitute in this zone a dynamic seal without contact, in order to maintain the lubricating grease in the guide cylinder 44.

[0038] The annular base 52, the guide body 54 and the inner guide skirt 56 delimit an annular space 62.

[0039] The guide body 54 has an open annular end 63 comprising a recess 74. The guide body 54 comprises a longitudinal axial locking groove 60, extending from the open annular end 63 towards the annular base 52, over a predetermined distance, for example 9 / 10 of the height of the inner guide wall 58. The locking groove 60 is configured to accommodate the slide 46 in sliding contact, in order to lock the piston 12 in rotation relative to the guide cylinder 44, while allowing it a translational movement in the guide cylinder 44.

[0040] The brake actuator mechanism 10 further has an annular bellows 76, comprising an annular base of the bellows 78 configured to be inserted into the recess 74, and a bellows head 80 configured to be pinched between the collar shoulder 72' of the collar 72, and the sleeve 42, radially bearing on the outer peripheral wall 32. This annular bellows 76 makes it possible to prevent the intrusion of pollutants inside the guide cylinder 44 by providing a primary seal. The annular bellows 76 is optional, and may therefore not be integrated into the mechanism brake actuator 10 if the latter is intended to operate in an unpolluted environment.

[0041] When the piston 12 of the brake actuator mechanism 10 is assembled, the nut 16 is forcibly inserted into the sleeve 42, in an axial assembly direction 210, until the annular end face 36 of the nut 16 abuts against the sleeve shoulder 50 or until it reaches an axial position ensuring that the annular bellows 76 is held in position. The sleeve 42 thus shrunk onto the nut 16 makes it possible to form a single-piece assembly. The assembly is carried out with angular indexing so that the locking mortise 64 of the nut 16 and the locking slot 66 of the sleeve 42 are located opposite each other, and the locking slot 66 allows access to the locking mortise 64.

[0042] The slider 46 is then inserted into the locking mortise 64 of the nut 16 through the locking slot 66.

[0043] The screw 14 is then inserted into the nut 16 of the piston 12, by a progressive helical movement allowing the balls 18 to be inserted one by one.

[0044] The subassembly consisting of the screw 14 and the piston 12 equipped with the slider 46 is then inserted into the guide cylinder 44 in the axial assembly direction 210. To do this, the locking slot 66 of the sleeve 42 and the locking mortise 64 of the nut 16 must be inserted opposite the locking groove 60 of the guide body 54 of the locking cylinder 44, while the slider 46 enters the locking groove 60. The outer face of the sleeve 49 then comes into sliding contact with the inner guide wall 58 of the guide body 54.

[0045] The slide 46 inserted in the locking groove 60 has only one degree of freedom, apart from functional clearances, in translation parallel to the reference axis 100 in the locking groove 60. The slide 46 then blocks the piston 12 in rotation around the reference axis 100, while leaving it one degree of freedom of translation parallel to the reference axis 100.

[0046] When the piston 12, the sleeve 42 and the slider 46 are inserted into the guide cylinder 44 and reach their position of use, the first flat shoulder 26 of the connecting portion 22 of the screw 14 abuts on the inner guide skirt 56, while the screw rod 20 is housed in the intermediate space 59.

[0047] Finally, the annular bellows 76 can be mounted to ensure the primary sealing of the brake actuator 10.

[0048] In operation, a rotational movement of the screw 14 around the reference axis 100, driven in rotation at the level of the screw head 20 by a motor, generates a translational movement of the piston 12 in a direction which is a function of the direction of rotation of the screw 14.

[0049] According to another embodiment, illustrated in [Fig.2], the actuator mechanism brake actuator mechanism 10 differs from that described in the first embodiment in that the brake actuator mechanism 10 does not have an annular bellows 76 or a recess 74. In addition, the recirculation means 40 are formed at the nut 16 which has an external recirculation channel 41 and recirculators 41', allowing external recirculation of the balls 18. The recirculation channel 41 is here open, and closed during assembly of the brake actuator mechanism 10 by the cylindrical inner face 48 of the sleeve 42. In the absence of a bellows, sealing is achieved at this level by the sliding contact between the sleeve 42 and the inner guide wall 58 of the guide body 54.

[0050] According to a third embodiment illustrated in [Fig. 3], the brake actuator mechanism 10 differs from that described in the first embodiment in that the brake actuator mechanism 10 does not have an annular bellows 76 or a counterbore 74. Furthermore, the nut 16 is of the open type, and does not have an outer closing face 34. Furthermore, the sleeve 42 does not have the sleeve shoulder 50 which rests axially in the axial direction 200 opposite the assembly direction 210 on the annular end face 36 of the nut 16.

[0051] The sleeve 42 then has a closed bottom 68, preferably flat, which comes to bear against an upper end annular surface of the nut 70. The closed bottom of the sleeve 68 then presses directly or indirectly against the brake pad when the brake actuator mechanism 10 is actuated. Conversely, when the brake actuator mechanism 10 is not actuated and therefore the piston 12 is in the free position, it is the end annular face 36 of the nut 16 which abuts against the base 52.

[0052] In all the embodiments described above, the sleeve 42 undergoes, before its assembly on the nut 16, a thermochemical treatment in order to resist abrasion and corrosion at a temperature Ts until a surface layer of resistance to abrasion and corrosion rich in nitrogen is obtained. The treatment in order to obtain such a layer includes nitriding or nitrocarburizing, the temperature Ts being between 300°C and 580°C. Nitriding and / or nitrocarburizing allows the nitride to form on the surface when a part is placed in a treatment atmosphere very rich in nitrogen at the temperature Ts, allowing the formation of a different material on the surface. Thanks to this treatment, the outer face 49 of the sleeve 42 of the piston 12 resists the abrasion and corrosion which could occur under the conditions of use, when the piston 12 slides in translation in the guide cylinder 44.Since nitrocarburizing does not modify the flatness of a surface, it is therefore possible to grind the outer face 49 of the sleeve 42 before applying the thermochemical treatment.

[0053] Similarly, the nut 16 is subjected to a thermochemical hardening treatment including heating to a temperature Te at least 200°C higher than Ts, preferably at least higher than 900°C. The thermochemical hardening treatment, for example of the surface or core carburizing type, then includes quenching and tempering at a temperature Tr at least 100°C lower than Ts, preferably lower than 200°C. This treatment makes it possible to obtain a hardened surface layer, rich in carbon at least locally at the nut thread 27 of the inner surface of the nut 30. Thanks to this treatment, the nut thread 27 has increased hardness on the surface and in depth, which allows it to be more durable by resisting chipping for example. However, carburizing modifies the flatness of a surface of a part, it is therefore necessary to carry out a grinding, hard turning or hard milling step on the inner surface of the nut 30 after the application of the thermochemical treatment.

[0054] In the first two embodiments, the part in contact with the brake caliper or its actuating mechanism, in other words the outer closing face 34, preferably undergoes the application of an additional surface coating. This additional treatment is for example an application of flake zinc to the surface or other surface treatment method. This additional treatment makes it possible to make the outer closing face 34 more resistant to pressures during the actuation of the brake actuator mechanism 10. For the third embodiment, this additional treatment is not necessary, due to the anti-corrosion properties provided by the thermochemical treatment of the sleeve.

[0055] Thermochemical hardening treatment (carburizing / quenching / tempering) involves introducing carbon into at least one surface layer of a steel material and then fixing it by quenching and tempering. The objective is to increase the carbon content at least near the surface to give the part the desired hardness.

[0056] Thermochemical treatment for abrasion and corrosion resistance (nitriding / nitrocarburizing) involves the introduction of nitrogen into the surface layer of the material. It can produce highly wear-resistant surface layers, particularly when nitrides, such as iron nitride (Fe3N) or chromium nitride (Cr2N), are formed on the surface.

[0057] If the thermochemical treatment for resistance to abrasion and corrosion is carried out after assembly of the sleeve 42 on the nut 16 and after the thermochemical treatment for hardening the nut 16, it is not possible to thermally insulate the nut, so that the nut thread 27 is brought to a temperature close to Ts, sufficient to destroy the carbon fixing effect obtained by quenching and tempering. Conversely, if the thermochemical hardening treatment is carried out after assembly of the sleeve 42 on the nut 16 and after the thermochemical treatment for resistance to abrasion and corrosion, it is not possible to thermally insulate the sleeve 42, so that it is brought to a temperature close to Te, much higher than Ts, which releases the nitrogen compounds fixed to the surface of the sleeve 42 by the thermochemical treatment for resistance to abrasion and corrosion. This is the reason why these two incompatible treatments are carried out on the two parts, namely the nut 16 and the sleeve 42, before their assembly.

[0058] Naturally, the examples shown in the figures and discussed above are given for illustrative purposes only and are not limiting. It is explicitly provided that the different embodiments illustrated can be combined with each other to propose others.

[0059] According to a variant not illustrated, the external recirculation channel 41 is located in the screw 14.

[0060] According to another variant, the sleeve 42 and / or the guide cylinder 44 are each composed of a metal base treated according to the thermochemical treatment described above and of resin comprising the desired properties in order to reduce friction between the sleeve 42 and the guide cylinder 44, the resin being able to form the sleeve 42 and / or the guide cylinder 44 by molding or 3D printing for example.

Claims

Claims

1. A method of manufacturing a piston (12) of a brake actuator mechanism (10), the piston (12) comprising - a ball screw mechanism nut (16), defining a reference axis (100), an outer peripheral wall (32) and a nut thread (27) intended to form a raceway for balls (18) of the ball screw mechanism; - a sleeve (42) integral with the nut (16) and at least partially covering the outer peripheral wall (32) of the nut (16), the sleeve (42) being intended to come into adjusted sliding contact with an inner guide wall (58) of a guide cylinder (44) of the brake actuator mechanism (10);characterized in that before securing the sleeve (42) to the outer peripheral wall (32) of the nut (16), the sleeve (42) is subjected to a thermochemical treatment for resistance to abrasion and corrosion at a temperature Ts until a surface layer of resistance to abrasion and corrosion rich in nitrogen is obtained, and the nut (16) is subjected to a thermochemical hardening treatment including heating to a temperature Te at least 200°C higher than Ts, then quenching and tempering at a temperature Tr at least 100°C lower than Ts, and obtaining a hardened zone rich in carbon at least locally at the nut thread (27).;

2. Method according to claim 1, characterized in that the thermochemical hardening treatment includes a carburizing treatment, the temperature Te being greater than 900°C, the temperature Tr being less than 250°C.

3. Method according to claim 1 or 2, characterized in that the abrasion and corrosion resistance treatment includes nitriding or nitrocarburizing, the temperature Ts being between 300°C and 580°C.

4. Method according to any one of the preceding claims, characterized in that an external face of the sleeve (49) is ground before being subjected to the thermochemical treatment for resistance to abrasion and corrosion.

5. Method according to any one of the preceding claims, characterized in that after completion of the thermochemical treatment for resistance to abrasion and corrosion and the thermochemical hardening treatment, the sleeve (42) is secured, preferably by shrinking, to the outer peripheral wall (32) of the nut (16).

6. Method according to any one of the preceding claims, characterized in that an outer surface of a bottom wall (17) of the nut (16) or the sleeve (42) undergoes an additional anti-corrosion treatment.

7. Method according to claim 6, characterized in that the additional anti-corrosion treatment is a lamellar zinc coating treatment.

8. A method according to any one of the preceding claims, characterized in that a slider (46) of the piston (12) undergoes a surface treatment before being partially inserted into a housing formed in the nut (16) and the sleeve (42).

9. Method according to claim 8, characterized in that the surface treatment of the slide (46) is nitrocarburization.

10. Piston (12) characterized in that it is manufactured according to the manufacturing method of any one of the preceding claims.

11. Piston (12) according to claim 10, characterized in that the nut (16) has an open external recirculation channel (41), closed at least in part by the sleeve (42).

12. Piston (12) according to claim 10 or 11, characterized in that the sleeve (42) has a bottom (68).

13. Piston (12) according to any one of claims 10 to 12, characterized in that the sleeve (42) has a flap of material (50) on an annular end face (36) of the nut (16).

14. Piston (12) according to any one of claims 10 to 13, characterized in that it comprises a slide (46) projecting axially outwards.

15. Brake actuator mechanism (10), comprising: - a guide cylinder (44) defining a reference axis (100) of the brake actuator mechanism (10); - a ball screw mechanism, comprising a screw (14) and a nut (16) centered on the reference axis (100), and balls (18), the screw (14) having at least one screw thread (25) forming a raceway for the balls (18), the nut (16) having a nut thread (27) forming a raceway for the balls (18) and an outer peripheral wall (32); and - a sleeve (42) secured to the nut (16) and at least partially covering the outer peripheral wall (32) of the nut (16), the sleeve (42) coming into adjusted sliding contact with an inner guide wall (58) of the guide cylinder (44); characterized in that the sleeve (42) and the nut (16) constitute a piston (12) according to any one of claims 10 to 14.

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