Bushing attached to a ball screw nut

Abstract

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 level of the nut thread (27). (Abstract figure: 1)

Description

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

[0001] The invention relates to the field of actuators, particularly for the transport industry, especially automotive or aeronautical, and specifically 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. PRIOR TECHNOLOGY

[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 forms a piston housed in a guide cylinder. The ball screw mechanism formed by the screw, nut, and balls requires sufficient hardness at the screw and nut threads. This type of piston is positioned near the brake caliper and is subjected to intense contamination from its immediate external environment, which can lead to corrosion of the nut. To protect the ball screw mechanism, a small clearance must be maintained between the piston and its guide cylinder, which generates a risk of abrasion, exacerbated by the presence of contaminants. The risks of mechanism failure 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 of several parts and incorporates the nut of the ball screw mechanism, a solid thrust piece into which the nut is shrink-fitted, and an outer part shrink-fitted onto the thrust piece. The outer part has a base against which a frustoconical surface of the thrust piece bears. This three-part embodiment of the piston aims to share certain piston components in several models of different dimensions, the intermediate thrust piece acting as an adapter. Description of the invention

[0004] The invention aims to remedy the disadvantages of the prior art and to offer a piston that is more resistant to abrasion and corrosion and economically advantageous, while remaining compact.

[0005] To this end, according to a first aspect of the invention, a method for manufacturing a piston for a brake actuator mechanism is proposed, the piston comprising a ball screw mechanism nut, defining a reference axis, a perimeter wall outer sphere and a nut thread intended to form a bearing race for balls of the ball screw mechanism; a sleeve integral with the nut and covering at least partially the outer peripheral wall of the nut, the sleeve being intended to come into sliding contact with an inner guide wall of a guide cylinder of the brake actuator mechanism;remarkable in that before the sleeve is fixed to the outer peripheral wall of the nut, the sleeve is subjected to a thermochemical treatment for abrasion and corrosion resistance at a temperature Ts until a nitrogen-rich abrasion and corrosion resistance surface layer 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 carbon-rich hardened zone at least locally at the level of the nut thread.

[0006] Thermochemical abrasion and corrosion resistance treatment and thermochemical hardening treatment impart distinct advantages to the same material. However, the implementation processes for each of the two treatments differ, and regardless of the order in which these treatments are applied to the same part, the properties imparted by one negate the properties imparted by the other. More specifically, assuming that a part is initially treated to increase its hardness by a thermochemical treatment with carbon enrichment, which at the end of the treatment involves quenching and tempering at a tempering temperature Tr, the subsequent treatment of another part of the same part at a temperature Ts significantly higher than the tempering temperature Tr will eliminate the effects of quenching and tempering, release the carbon compounds, and negate the hardening effect desired by the initial hardening treatment.Conversely, if an initial thermochemical abrasion resistance treatment is applied to a part at a temperature Ts, resulting in nitrogen enrichment of one surface area, a subsequent hardening treatment of another part of the same part at a temperature Te significantly higher than Ts will release the nitrogen compounds retained on the material's surface during the initial treatment. Therefore, it is not economically viable to produce a single-piece component possessing the desired properties imparted by both treatments. By performing these two treatments on separate components, namely the bushing and the nut, intended for assembly, it becomes possible to offer a composite piston exhibiting all the required 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 of high carbon steel such as 100Cr6, C50 or C56 steel or their equivalents equivalents. The thermochemical hardening treatment is preferably a gaseous process. Quenching and tempering steps allow for high surface hardness, for example, a hardness greater than 58 HRC (Rockwell hardness), while maintaining high core toughness. Thanks to this treatment, the nut thread has increased hardness, making it more durable by resisting chipping, for example. The thermochemical hardening treatment can be a surface treatment, but is preferably a deep treatment to a thickness exceeding 0.5 mm, and preferably exceeding 2 mm. It can also be a through-hardening 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 containing essentially steel. In one embodiment, the abrasion and corrosion resistance treatment includes nitriding or nitrocarburizing, with the temperature Ts being between 300°C and 580°C. Thanks to this treatment, the piston resists abrasion and corrosion, which can be initiated by its translational movements and particulate contamination in the guide cylinder.

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

[0011] According to one embodiment, after completion of the thermochemical abrasion and corrosion resistance treatment 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, one-piece assembly. The piston then becomes harder at the nut threads and more resistant at its contact surface with the guide cylinder. The sleeve, due to its relatively small thickness, provides an additional property to the piston without significantly increasing the volume of the brake actuator mechanism.

[0012] According to one embodiment, an external surface of a base wall of the nut or socket undergoes an additional anti-corrosion treatment, preferably a zinc flake coating treatment, this base wall being intended to bear directly or indirectly against the brake caliper.

[0013] According to one embodiment, a piston slide 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 optionally contribute to the cohesion between the nut and the sleeve, but its main function is to ensure, in cooperation with a straight groove formed in the guide cylinder of the brake actuator mechanism, translational guidance without rotation of the piston within the cylinder. Thanks to its additional treatment, the slide is resistant to abrasion and corrosion, thus increasing the service life of the brake actuator mechanism.

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

[0015] According to one embodiment, the nut has an open external recirculation channel, closed at least partially by the sleeve. This feature facilitates machining of the recirculation channel and, where applicable, assembly and insertion of the balls into the mechanism.

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

[0017] According to one embodiment, the sleeve has a material flap 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 includes a slide projecting axially outwards allowing the piston to be fixed 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 thread forming a bearing race for the balls, the nut having a thread forming a bearing race for the balls and an outer peripheral wall; and a sleeve integral with the nut and at least partially covering the outer peripheral wall outer sphere of the nut, the sleeve coming into fitted 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 features and advantages of the invention will become apparent from the following description, with reference to the attached figures.

[0022] [Fig.1] Fig.1 illustrates a brake actuation mechanism according to an 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 embodiment comprising an external recirculating piston with a closed nut.

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

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

[0026] Figure 1 illustrates 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 within 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 includes a sleeve 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 or their equivalents according to other international or national standards, or of high-carbon steel such as 100Cr6, C50 or C56 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 rotationally fixed to an output shaft of an electric motor or geared motor, and may have, 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 internal helical raceway around the reference axis 100 of the ball screw mechanism, the internal helical raceway being rotated radially in the opposite direction to the axis of reference 100. In addition, the screw 14 has an open central cavity 28 allowing to lighten the whole of the brake actuator mechanism 10, and to offer 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 of high-carbon steel such as 100Cr6, C50 or C56 steel or their equivalents. The nut 16 has an overall cylindrical shape whose central axis is the reference axis 100. The nut 16 has a nut thread 27 which forms an external helical bearing race around the reference axis 100, and is radially rotated towards the reference axis 100. The nut 16 has a cylindrical external peripheral face 32 in which a locking mortise 64 is formed.

[0030] Furthermore, the nut 16 is of the closed type in that it has a base 17, with an outer closing face 34 which may have a recess 35, and is configured to make direct or indirect contact with a brake caliper (not shown in the figures). The outer closing face 34 also has a flange 72, projecting radially from the outer peripheral wall 32, which forms a flange shoulder 72'. The flange 72 also helps to limit any deformation of the outer closing face 34 under mechanical stresses during the 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 include one or more recirculators each passing through a thread of the threaded component, as illustrated in [Fig.1], 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 can, for example, be made of steel or ceramic, and are dimensioned 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, such as 20MnCr5, 23MnB4, Scr420, 16MnCr5 steel or their equivalents according to other international or national standards, or of high-carbon steel such as 100Cr6, C50 or C56 steel or their equivalents. The sleeve 42 has a cylindrical inner face 48 that is shrink-fitted onto at least part of the outer peripheral wall 32 of the nut 16. The sleeve 42 has an outer face of the sleeve 49, and the thickness between the cylindrical inner face 48 and the outer face of the sleeve 49 is approximately 1 mm. The sleeve 42 has a locking slot 66, like a through hole, generally rectangular, located near the annular end face 36 of the nut 16. The locking slot 66 gives 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 includes a slide 46, press-fitted into the locking mortise 64, projecting radially towards the guide cylinder 44 through the locking slot 66, relative to the outer face of the bushing 49.

[0035] The guide cylinder 44 consists of a metallic base, for example made of steel, and includes 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 includes an inner guide wall 58, rotated 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, rotated radially towards the reference axis 100, defining an intermediate space 59. The inner guide bearing 57 is positioned opposite and at a short distance from the screw rod 20, in order to constitute in this area a dynamic non-contact seal, in order to retain the lubricating grease in the guide cylinder 44.

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

[0039] The guide body 54 has an open annular end 63 comprising a chamber 74. The guide body 54 includes 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 against rotation relative to the guide cylinder 44, while allowing it translational movement within the guide cylinder 44.

[0040] The brake actuator mechanism 10 further comprises an annular bellows 76, including an annular base of the bellows 78 configured to fit into the recess 74, and a bellows head 80 configured to be pinched between the flange shoulder 72' of the flange 72 and the sleeve 42, radially bearing against the outer peripheral wall 32. This annular bellows 76 prevents the intrusion of contaminants into 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 pressed into the sleeve 42 in an axial assembly direction 210 until the annular end face 36 of the nut 16 abuts against the shoulder of the sleeve 50 or until it reaches an axial position that ensures the annular bellows 76 remains in position. The sleeve 42, thus pressed onto the nut 16, forms a single-piece assembly. The assembly is performed with angular indexing such 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 provides access to the locking mortise 64.

[0042] The slide 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 slide 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 with respect to the locking groove 60 of the guide body 54 of the locking cylinder 44, while the slide 46 enters the locking groove 60. The outer face of the sleeve 49 then enters 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, within functional clearances, in translation parallel to the reference axis 100 in the locking groove 60. The slide 46 then locks 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 bushing 42 and the slide 46 are inserted into the guide cylinder 44 and reach their operating position, the first flat shoulder 26 of the connecting portion 22 of the screw 14 butts against the inner guide skirt 56, while the screw shank 20 is housed in the intermediate space 59.

[0047] Finally, the annular bellows 76 can be mounted to provide the primary seal 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 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 The brake actuator 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 chamber 74. Furthermore, the recirculation means 40 are formed at the nut 16, which has an external recirculation channel 41 and recirculators 41', allowing the external recirculation of the balls 18. The recirculation channel 41 is open here and is 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 point 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 chamber 74. In addition, the nut 16 is of the open type and does not have an external closing face 34. Furthermore, the sleeve 42 does not have the sleeve shoulder 50 which rests axially in the axial direction 200 opposite to 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 bears against an annular surface at the upper end 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 a free position, it is the annular end face 36 of the nut 16 that abuts against the base 52.

[0052] In all embodiments described above, the bushing 42 undergoes, before being assembled onto the nut 16, a thermochemical treatment to resist abrasion and corrosion at a temperature Ts until a nitrogen-rich, abrasion- and corrosion-resistant surface layer is obtained. The treatment to obtain such a layer includes nitriding or nitrocarburizing, with the temperature Ts being between 300°C and 580°C. Nitriding and / or nitrocarburizing allow nitride to form on the surface when a part is placed in a treatment atmosphere very rich in nitrogen at temperature Ts, enabling the formation of a different surface material. Thanks to this treatment, the outer face 49 of the bushing 42 of the piston 12 resists abrasion and corrosion that could occur under operating conditions, when the piston 12 slides in translation within the guide cylinder 44.Since nitrocarburizing does not alter the flatness of a surface, it is therefore possible to rectify 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, of Preferably above 900°C. The thermochemical hardening treatment, for example, surface or through case hardening, then includes quenching and tempering at a temperature Tr at least 100°C lower than Ts, preferably below 200°C. This treatment produces a hardened surface layer, rich in carbon, at least locally at the thread of the nut 27 on the inner surface of the nut 30. Thanks to this treatment, the nut thread 27 has increased hardness both on the surface and throughout, making it more durable and resistant to scaling, for example. However, case hardening alters the flatness of a workpiece surface; therefore, a grinding, hard turning, or hard milling step is necessary on the inner surface of the nut 30 after the thermochemical treatment.

[0054] In the first two embodiments, the part in contact with the brake caliper or its actuation mechanism, i.e., the outer closing face 34, preferably undergoes the application of an additional surface coating. This additional treatment is, for example, the application of zinc flakes to the surface or another surface treatment process. This additional treatment makes the outer closing face 34 more resistant to pressure 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] The thermochemical hardening treatment (case hardening / 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] The thermochemical treatment for abrasion and corrosion resistance (nitriding / nitrocarburizing) involves the introduction of nitrogen into the surface layer of the material. It can produce surface layers that are highly wear-resistant, particularly when nitrides, such as iron nitride (Fe3N) or chromium nitride (Cr2N), form on the surface.

[0057] If the thermochemical treatment for abrasion and corrosion resistance is carried out after the sleeve 42 is assembled onto the nut 16 and after the thermochemical hardening treatment of the nut 16, it is not possible to thermally insulate the nut, so the nut threads 27 are brought to a temperature close to Ts, sufficient to negate the carbon fixation effect obtained by quenching and tempering. Conversely, if the thermochemical hardening treatment is carried out after the sleeve 42 is assembled onto the nut 16 and after the thermochemical treatment for abrasion and corrosion resistance, it is not possible to thermally insulate the sleeve 42, so it is brought to a temperature close to Te, well above Ts. which releases the nitrogen compounds fixed to the surface of the sleeve 42 by the thermochemical treatment for abrasion and corrosion resistance. This is why these two incompatible treatments are carried out on both 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 by way of illustration only and are not intended to be limiting. It is explicitly intended that the different embodiments illustrated may be combined to propose others.

[0059] According to an unillustrated variant, the external recirculation channel 41 is located in the screw 14.

[0060] According to another variant, the bushing 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 properties required to reduce friction between the bushing 42 and the guide cylinder 44, the resin being able to form the bushing 42 and / or the guide cylinder 44 by molding or 3D printing for example.

Claims

Demands

1. Method of manufacturing a piston (12) of a brake actuator mechanism (10), the piston (12) comprising - a nut (16) of a ball screw mechanism, 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 covering at least partially the outer peripheral wall (32) of the nut (16), the sleeve (42) being intended to come into fitted 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 abrasion and corrosion resistance treatment at a temperature Ts until a nitrogen-rich abrasion and corrosion resistance surface layer 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 carbon-rich hardened zone at least locally at the level of the nut thread (27).

2. A process 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. A process 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. A 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. A 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 shrink fitting, to the outer peripheral wall (32) of the nut (16).

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

7. A 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 slide (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 a nitrocarburization.

10. Piston (12) characterized in that it is manufactured according to the manufacturing process 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 material flap (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) integral with the nut (16) and covering at least partially the outer peripheral wall (32) of the nut (16), the sleeve (42) coming into sliding contact adjusted 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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