ATTACHMENT SLEEVE ON A BALL THREAD NUT

Description

TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to the field of actuators, particularly for the transport industry, especially automotive or aeronautical, particularly 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. PREVIOUS STATE OF THE ART

[0002] Document EP 2 787 248 B1 discloses a brake actuator mechanism 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 within a guide cylinder. The ball screw mechanism formed by the screw, nut, and balls requires sufficient hardness in 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 creates a risk of abrasion, exacerbated by the presence of contaminants. Therefore, the risks of mechanism failure are numerous.

[0003] Document EP 2 304 265 B1 discloses a brake actuator mechanism comprising a piston sliding within a cylinder and driven by a ball screw mechanism. The piston is made of several parts and incorporates the ball screw mechanism's nut, 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 rests. This three-part piston design aims to share certain piston components in several models of different dimensions, with the intermediate thrust piece acting as an adapter. DESCRIPTION OF THE INVENTION

[0004] The invention aims to overcome the drawbacks 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 of a brake actuator mechanism is proposed, the piston comprising a ball screw mechanism nut, defining a reference axis, an outer peripheral wall 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 fitted sliding contact with an inner guide wall of a guide cylinder of the brake actuator mechanism;remarkable in that before the sleeve is joined 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 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 carbon-rich hardened zone at least locally at the level of the nut thread.

[0006] Thermochemical treatments for abrasion and corrosion resistance and thermochemical hardening treatments impart distinct advantages to the same material. However, the implementation processes for each 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 with a thermochemical treatment involving carbon enrichment, culminating in 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 negate the effects of the quenching and tempering, release the carbon compounds, and eliminate the desired hardening effect of 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 a surface area, a subsequent hardening treatment of another part of the same part at a temperature Tc 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, or their equivalents according to other international or national standards, or of high-carbon steel such as 100Cr6, C50, or C56, or their equivalents. The thermochemical hardening treatment is preferably a gas hardening treatment. The quenching and tempering steps allow for a high surface hardness, for example, greater than 58 HRC (Rockwell hardness), while maintaining high core toughness. This treatment gives the nut thread 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 could also be a heart treatment.

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

[0009] The bushing is preferably made of steel, or 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 that can be initiated by its linear movements and particulate contamination in the guide cylinder.

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

[0011] In 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, 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 thin profile, 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 bottom wall of the nut or the bushing undergoes an additional anti-corrosion treatment, preferably the additional anti-corrosion treatment is a flake zinc coating treatment, this bottom wall being intended to bear directly or indirectly against the brake caliper.

[0013] In one embodiment, a piston slide undergoes a surface treatment before being partially inserted into a housing formed in the nut and 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 primary function is to ensure, in cooperation with a straight groove formed in the guide cylinder of the brake actuator mechanism, non-rotating translational guidance of the piston within the cylinder. Thanks to its additional treatment, the slide is resistant to abrasion and corrosion, thus extending 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 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] In one embodiment, the nut has an open external recirculation channel, at least partially closed by the bushing. This feature facilitates machining of the recirculation channel and, where applicable, assembly and insertion of the balls into the mechanism.

[0016] In 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 both 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 includes a slide projecting axially outwards, allowing the rotating piston to be fixed 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 integral with the nut and covering at least partially the outer peripheral wall of the nut, the sleeve coming into a sliding, fitted 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. [ Fig. 1 ] There figure 1 illustrates a brake actuation mechanism according to an embodiment comprising an internally recirculating piston with a closed nut. Fig. 2 ] There figure 2 illustrates a brake actuation mechanism according to an embodiment comprising an externally recirculating piston with a closed nut. Fig. 3 ] There figure 3 illustrates a brake actuation mechanism according to an embodiment comprising a piston having an open nut.

[0022] For clarity, identical or similar elements are identified by identical reference symbols across all figures. DETAILED DESCRIPTION OF IMPLEMENTATION METHODS

[0023] On the figure 1 A first embodiment of a brake actuator mechanism is illustrated.10 comprising a guide cylinder 44 fixed 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 also serves as a reference axis for the piston, allowing it to bear directly or indirectly against a brake caliper (not shown). The piston 12 includes a socket 42 and a nut 16, the nut 16 forming part of a ball screw mechanism comprising two threaded components, namely a screw 14 and the nut 16, and marbles 18.

[0024] The screw 14is preferably metallic, for example steel such as 20MnCr5, 23MnB4, Scr420, 16MnCr5 or their equivalents according to other international or national standards, or high carbon steel such as 100Cr6, C50 or C56 or their equivalents, and may include a screw head 20, a connecting section 22 and a screw body 24. The screw body 24 has a diameter greater than the screw head 20, the connecting portion 22 creating the connection between the screw body 24 and the screw head 20. This connecting section 22 can 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.

[0025] The screw body 24 features a screw thread 25 which forms an internal helical rolling path around the reference axis 100 of the ball screw mechanism, the internal helical raceway being rotated radially in the opposite direction to the reference axis 100. Furthermore, the screw 14 presents an open central cavity 28 allowing for a lighter overall brake actuator mechanism 10, and to provide a receptacle for grease contained in the ball screw mechanism.

[0026] 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 generally has a cylindrical shape whose central axis is the reference axis 100.The nut 16 features a nut thread 27 which forms an external helical rolling path around the reference axis 100, and rotated radially towards the reference axis 100. The nut 16 presents an outer peripheral face 32 cylindrical in which a locking mortise is formed 64.

[0027] Furthermore, the nut 16 is of the closed type in the sense that it has a background 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 features a collar 72, projecting radially from the outer peripheral wall 32, which forms a collar shoulder 72'. The collar 72It also helps to limit any potential deformation of the outer closing face 34 under mechanical stress during the activation of the brake actuator mechanism 10 For example.

[0028] One of the two threaded components, namely the screw 14 or the nut 16, may also be equipped with recirculation means 40 marbles 18, which may include one or more recirculators, each passing through a thread of the threaded component, as illustrated in the figure 1 , or pairs of recirculators arranged at the ends of a recirculation channel that spans one or more turns of the screw's bearing races 14 and the nut 16. The system could also operate on a system without recirculation.

[0029] The marbles 18They can, 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 internal helical bearing race of the screw 14, thus, where applicable, through recirculation methods 40, preferably without separators between the marbles 18.

[0030] The socket 42 is metallic, for example made of steel, such as 20MnCr5, 23MnB4, Scr420, 16MnCr5 or their equivalents according to other international or national standards, or high carbon steel such as 100Cr6, C50 or C56 or their equivalents. The socket 42 presents a cylindrical inner face 48 reinforced on at least part of the outer peripheral wall 32 of the nut 16. The socket 42 presents an outer face of the socket 49,and a thickness between the cylindrical inner face 48 and the outer face of the socket 49 is on the order of 1 mm. The socket 42 features 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. Furthermore, the socket 42 may feature a socket shoulder 50 which rests axially on the annular end face 36 of the nut 16, opposed to the bottom 17 of the nut.

[0031] The brake actuator mechanism 10 also includes a slider 46, clamped in the locking mortise 64, projecting radially towards the guide cylinder 44 through the blocking slot 66,relative to the outer face of the socket 49.

[0032] The guide cylinder 44 consists of a metal base, for example made of steel, and includes a preferably flat annular base 52, a guiding 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.

[0033] The guide body 54 is a cylinder whose central axis is the reference axis 100. The guide body 54 includes an internal guide wall 58, rotated radially towards the reference axis 100, in sliding contact with the socket 42.

[0034] The inner sealing skirt 56 presents a cylindrical inner face 57,rotated radially towards the reference axis 100, defining an intermediate space 59. The inner range of guidance 57 is positioned opposite and a short distance from the screw shaft 20, in order to create a dynamic, non-contact seal in this area, in order to retain the lubricating grease in the guide cylinder 44.

[0035] The annular base 52, the guide body 54 and the inner guide skirt 56 define an annular space 62.

[0036] The guide body 54 presents an open annular end 63 including a chamber 74. The guide body 54 includes an axial locking groove 60 longitudinal, 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 slider 46 in sliding contact, in order to block the piston 12 rotating relative to the guide cylinder 44, while allowing it a translational movement within the guide cylinder 44.

[0037] The brake actuator mechanism 10 It also features an annular bellows 76, including an annular base of the bellows 78 configured to fit into the chamber 74, and a bellows head 80 configured to be pinched between the collar shoulder 72' of the collar 72, and the socket 42, radially supported on the outer peripheral wall 32. This annular bellows 76prevents pollutants from entering the guide cylinder 44 by creating a primary seal. The annular bellows 76 is optional, and therefore may not be integrated into the brake actuator mechanism 10 if the latter is intended to operate in an unpolluted environment.

[0038] When the piston 12 of the brake actuator mechanism 10 is assembled, the nut 16 is forced into the socket 42, in an axial assembly direction 210, until the annular end face 36 of the nut 16 butts against the shoulder of the socket 50 or up to an axial position that ensures the annular bellows 76 remains in position. The socket 42 thus fitted onto the nut 16allows for the creation of a single-piece assembly. The assembly is performed with angular indexing so that the locking mortise 64 of the nut 16 and the locking slot 66 socket 42 are located opposite each other, and the locking slot 66 allows access to the locking mortise 64.

[0039] The slider 46 is then inserted into the locking mortise 64 of the nut 16 through the blocking slot 66.

[0040] The screw 14 is then inserted into the nut 16 piston 12, by a progressive helical movement allowing the balls to be inserted one by one 18.

[0041] The subassembly consisting of the screw 14 and the piston 12 equipped with the slider 46 is then inserted into the guide cylinder 44in the axial direction of assembly 210. To do this, the locking slot 66 socket 42 and the locking mortise 64 of the nut 16 must be inserted in relation to the locking groove 60 of the guide body 54 of the locking cylinder 44, while the slider 46 penetrates the locking groove 60. The outer face of the socket 49 then enters into sliding contact with the inner guide wall 58 of the guide body 54.

[0042] The slider 46 inserted into the locking groove 60 possesses only one degree of freedom, apart from functional play, in translation parallel to the reference axis 100 in the locking groove 60. The slider 46 then blocks the piston 12 rotating around the reference axis 100,while allowing it a degree of freedom of translation parallel to the reference axis 100.

[0043] When the piston 12, the socket 42 and the slide 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 stops against the inner guide skirt 56, while the screw rod 20 is housed in the intermediate space 59.

[0044] Finally, the annular bellows 76 can be fitted to provide primary sealing of the brake actuator. 10.

[0045] During 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 12in a direction that depends on the direction of rotation of the screw 14.

[0046] According to another embodiment, illustrated on the figure 2 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 nor of chambering 74. Furthermore, the means of recirculation 40 are formed at the level of the nut 16 which features an external recirculation channel 41 and recirculators 41', allowing external recirculation of the balls 18. The recirculation channel 41 is open here, and closed during the assembly of the brake actuator mechanism 10 by the cylindrical inner face 48 socket 42. In the absence of a bellows, the seal is achieved at this point by the sliding contact between the sleeve42 and the inner guide wall 58 of the guide body 54.

[0047] According to a third embodiment illustrated on the figure 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 nor of chambering 74. Furthermore, the nut 16 is of the open type, and does not have an external closing face 34. Furthermore, the socket 42 does not have the socket 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.

[0048] The socket 42 then presents a closed bottom 68,preferably flat, resting against an annular surface at the upper end of the nut 70. The closed bottom of the socket 68 then presses directly or indirectly against the brake pad when the brake actuator mechanism 10 is activated. Conversely, when the brake actuator mechanism 10 is not activated and therefore the piston 12 is in a free position; it is the annular end face 36 of the nut 16 which bumps against the base 52.

[0049] In all the embodiments described above, the socket 42 undergones, before its assembly onto the nut 16,A thermochemical treatment is used to provide resistance to abrasion and corrosion at a temperature Ts until a nitrogen-rich, abrasion- and corrosion-resistant surface layer is obtained. The treatment to achieve such a layer includes nitriding or nitrocarburizing, with the temperature Ts ranging from 300°C to 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, resulting in the formation of a different surface material. This treatment provides the outer surface with superior resistance. 49 socket 42 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 socket 42 before the application of the thermochemical treatment.

[0050] Similarly, the nut 16 is subjected to a thermochemical hardening treatment including heating to a temperature Tc at least 200°C higher than Ts, preferably at least 900°C higher. The thermochemical hardening treatment, for example of the surface or through case hardening type, then includes quenching and tempering at a temperature Tr at least 100°C lower than Ts, preferably lower than 200°C. This treatment results in 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 27It has increased hardness both on the surface and throughout, making it more durable and resistant to chipping, for example. However, case hardening alters the flatness of a workpiece's surface, so a grinding, hard turning, or hard milling step is necessary on the nut's inner surface. 30 after the application of the thermochemical treatment.

[0051] In the first two embodiments, the part in contact with the brake caliper or its actuation mechanism, in other words 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 exterior closing face more durable. 34 more resistant to pressure during the actuation of the brake actuator mechanism10. For the third embodiment, this additional treatment is not necessary, due to the anti-corrosion properties provided by the thermochemical treatment of the socket.

[0052] The thermochemical hardening treatment (case hardening / quenching / tempering) involves introducing carbon into at least one surface layer of a steel material, followed by its fixation through quenching and tempering. The goal is to increase the carbon content, at least near the surface, to give the part the desired hardness.

[0053] 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 highly wear-resistant surface layers, particularly when nitrides, such as iron nitride (Fe3N) or chromium nitride (Cr2N), form on the surface.

[0054] If the thermochemical treatment for abrasion and corrosion resistance is carried out after the bushing has been assembled 42 on 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 thread 27 are brought to a temperature close to Ts, sufficient to negate the carbon fixation effect achieved by quenching and tempering. Conversely, if the thermochemical hardening treatment is carried out after the bushing has been assembled 42 on the nut 16 And after the thermochemical treatment for abrasion and corrosion resistance, it is not possible to thermally insulate the socket 42, so that it is brought to a temperature close to Tc, well above Ts, which releases the nitrogen compounds fixed to the surface of the socket 42by 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 socket 42, before their assembly.

[0055] Naturally, the examples shown in the figures and discussed above are given for illustrative purposes only and are not exhaustive. It is explicitly intended that the different embodiments illustrated can be combined to create other solutions.

[0056] According to an unillustrated variant, the external recirculation channel 41 is located in the screw 14.

[0057] According to another variant, the socket 42 and / or the guide cylinder 44are each composed of a metal base treated according to the thermochemical treatment described above and of resin having the desired properties in order to reduce friction between the socket 42 and the guide cylinder 44, the resin that can form the socket 42 and / or the guide cylinder 44 for example by molding or 3D printing.

Claims

1. A 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) for forming a raceway for balls (18) of the ball screw mechanism; - a bushing (42) secured to the nut (16) and at least partially covering the outer peripheral wall (32) of the nut (16), the bushing (42) being intended to come into tight sliding contact with an inner guide wall (58) of a guide cylinder (44) of the brake actuator mechanism (10); characterized in that prior to securing the bushing (42) to the outer peripheral wall (32) of the nut (16), the bushing (42) is subjected to a thermochemical abrasion and corrosion resistance treatment at a temperature Ts until a nitrogen-rich abrasion- and corrosion-resistant surface layer 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, followed by quenching and tempering to a temperature Tr at least 100°C lower than Ts, and obtaining a carbon-rich hardened zone at least locally at the nut thread (27).

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

3. The 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. The method according to any one of the preceding claims, characterized in that an outer face of the bushing (49) is ground before being subjected to the thermochemical abrasion and corrosion resistance treatment.

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

6. The 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 bushing (42) undergoes an additional anti-corrosion treatment.

7. The method according to claim 6, characterized in that the additional anticorrosion treatment is a zinc flake coating treatment.

8. The 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 bushing (42).

9. The method according to claim 8, characterized in that the surface treatment of the slider (46) is nitrocarburizing.

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

11. The 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 bushing (42).

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

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

14. The piston (12) according to any one of claims 10 to 13, characterized in that it comprises an axially outwardly projecting slider (46).

15. A 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 bushing (42) secured to the nut (16) and at least partially covering the outer peripheral wall (32) of the nut (16), the bushing (42) coming into tight sliding contact with an inner guide wall (58) of the guide cylinder (44); characterized in that the bushing (42) and the nut (16) constitute a piston (12) according to any one of claims 10 to 14.