Piston manufacturing method, piston, and brake actuator mechanism

Abstract

To provide a piston that overcomes the disadvantages of the prior art, is more resistant to abrasion and corrosion and is economically advantageous, while remaining compact.SOLUTION: A method of manufacturing a piston 12 for a brake actuator mechanism 10, the piston 12 including a bushing 42 and a nut 16, the nut 16 including an outer peripheral wall 32. The method comprises: prior to securing the bushing 42 to the outer peripheral wall 32 of the nut 16, subjecting the bushing 42 to an abrasion-and corrosion-resistant thermochemical treatment at a temperature Ts until a nitrogen-rich abrasion-and corrosion-resistant surface layer is obtained; and subjecting the nut 16 to a thermochemical hardening treatment that includes heating to a temperature Tc, which is higher than Ts by at least 200°C, and then quenching and tempering at a temperature Tr, which is lower than Ts by at least 100°C, to obtain a carbon-rich hardened surface layer at least locally on a thread 27 of the nut.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to the field of actuators, in particular for the transport industry, in particular the automotive or aerospace industry, in particular pistons of mechanisms driven by worm screws, in particular ball screws, and more particularly, but not exclusively, brake caliper pistons of brake mechanisms driven by worm screws, in particular ball screws. [Background technology]

[0002] EP 2787248 discloses a brake actuator mechanism comprising a screw, a nut, and a ball disposed between the helical threads of the screw and the nut, the nut forming a piston housed in a guide cylinder. The ball screw mechanism formed by the screw, nut, and ball requires sufficient hardness in the threads of the screw and the nut. This type of piston is located close to the brake caliper and is subject to direct and severe contamination from its external environment, which can lead to corrosion of the nut. To protect the ball screw mechanism, the piston and its guide cylinder must be maintained at a small structural distance, which creates an increased risk of wear due to the presence of contaminants. Therefore, the risk of mechanism failure is numerous.

[0003] EP 2304265 discloses a brake actuator mechanism with a piston that slides within a cylinder and is driven by a ball screw mechanism. The piston is made of several parts, including a nut for the ball screw mechanism, a solid pressure piece within which the nut contracts, and an outer piece that contracts onto the pressure piece. The outer piece has a bottom on which the frustoconical surface of the pressure piece rests. The purpose of this three-piece piston design is to allow some of the piston components to be shared across several different size models, with the middle pressure piece acting as a kind of adapter. Summary of the Invention [Problem to be solved by the invention]

[0004] SUMMARY OF THE INVENTION It is an object of the present invention to overcome the drawbacks of the prior art and to provide a piston that is more resistant to wear and corrosion and is economically advantageous while remaining compact. [Means for solving the problem]

[0005] To this end, according to a first aspect of the present invention, a method for manufacturing a piston for a brake actuator mechanism is proposed, the piston comprising: a nut for a ball screw mechanism defining a reference axis, an outer peripheral wall, and threads of the nut intended to form thread grooves for balls of the ball screw mechanism; and a bushing fixed to the nut and at least partially covering the outer peripheral wall of the nut, the bushing intended to be aligned with and come into sliding contact with an inner guide wall of a guide cylinder of the brake actuator mechanism, wherein before fixing the bushing to the outer peripheral wall of the nut, the bushing is subjected to a wear- and corrosion-resistant thermochemical treatment at a temperature Ts until a nitrogen-rich wear- and corrosion-resistant surface layer is obtained, and the nut is subjected to a thermochemical hardening treatment comprising heating to a temperature Tc at least 200°C higher than Ts, followed by quenching and tempering at a temperature Tr at least 100°C lower than Ts, thereby obtaining at least a localized carbon-rich hardened area in the threads of the nut.

[0006] Thermochemical treatments for wear and corrosion resistance and for hardening confer distinct advantages on the same material. However, the methods used to apply each of the two treatments are different, and regardless of the order in which they are applied to the same part, the properties imparted by one treatment will negate those imparted by the other. More specifically, if one begins processing a part to enhance its hardening by a carbon-enriched thermochemical treatment that includes quenching and tempering to a tempering temperature Tr at the end of the treatment, subsequent processing of another portion of the same part at a temperature Ts substantially higher than the tempering temperature Tr will remove the effects of the quenching and tempering, liberating carbon compounds and erasing the hardening effect intended by the initial hardening treatment. Conversely, if an initial thermochemical wear-resistant treatment is performed on a part that involves nitrogen-enriching the surface region of the part at a temperature Ts, a subsequent hardening treatment of another portion of the same part at a temperature Tc substantially higher than Ts will result in the release of nitrogen compounds retained on the surface of the material during the initial treatment. Therefore, it is not possible to obtain a single part that possesses the desired properties provided by both treatments in a cost-effective manner. By carrying out these two processes on separate parts to be assembled, namely the bushing and the nut, it is possible to provide a composite piston having all the desired properties.

[0007] The nut is preferably made of steel, such as 20MnCr5, 23MnB4, Scr420, 16MnCr5, or other equivalents according to international or national standards, or high-carbon steels such as 100Cr6, C50, or C56. The thermochemical hardening treatment is preferably carried out in a gaseous medium. The quenching and tempering steps allow for achieving high surface hardness, e.g., greater than 58 HRC (Rockwell hardness), while retaining a high level of toughness in the core of the part. This treatment increases the hardness of the nut's threads, making them, for example, more durable and resistant to chipping. The thermochemical hardening treatment can be a surface treatment, but preferably reaches a depth of more than 0.5 mm, preferably greater than 2 mm. It can also be a core treatment.

[0008] According to one embodiment, the thermochemical hardening treatment comprises a carburizing treatment at a temperature Tc above 900° C. and a temperature Tr below 250° C. Alternatively, it may be a carbonitriding treatment.

[0009] The bushing is preferably made of steel or a material that substantially contains steel. According to one embodiment, the wear and corrosion resistance treatment comprises a nitriding or nitrocarburizing treatment at a temperature Ts in the range of 300°C to 580°C. Due to this treatment, the piston is resistant to wear and corrosion that may be caused by its translational movement in the guide cylinder and by particulate contamination.

[0010] According to one embodiment, the outer surface of the bushing is polished before undergoing a wear- and corrosion-resistant thermochemical treatment, so that the outer surface of the bushing is completely smooth, thereby reducing friction of the piston in the guide cylinder and therefore improving efficiency, as well as increasing wear and corrosion resistance.

[0011] According to one embodiment, after the thermochemical wear and corrosion resistance treatment and thermochemical hardening treatment are completed, the bushing is fixed to the outer wall of the nut, preferably by shrinkage. In this way, initially incompatible properties are combined into a single set of components. This makes the piston harder on the nut threads and more durable at its contact surface with the guide cylinder. Due to its relatively thin thickness, the bushing provides additional properties to the piston without significantly increasing the volume of the brake actuator mechanism.

[0012] According to one embodiment, the outer surface of the bottom wall of the nut or bushing is subjected to an additional corrosion protection treatment, preferably a zinc flake coating treatment, and this bottom wall is intended to come into direct or indirect contact with the brake caliper.

[0013] According to one embodiment, the piston slider is surface-treated before being partially inserted into the housing formed in the nut and bushing. The surface treatment of the slider is preferably a nitrocarburizing treatment. While the slider may contribute to the bond between the nut and bushing, its primary function is to cooperate with a linear groove formed in the guide cylinder of the brake actuator mechanism to ensure non-rotational translational guidance of the piston within the cylinder. Due to the additional treatment, the slider is resistant to wear and corrosion, allowing the brake actuator mechanism to continue functioning for a longer period of time.

[0014] According to another aspect of the invention, there is provided a piston that is superior in that it is manufactured according to the manufacturing method as described above, and that is characterized in particular by a nitrogen-rich surface metallurgy in the bushing resulting from a thermochemical anti-wear treatment, and by a carbon-rich metallurgy that imparts high hardness to at least the threads of the nut.

[0015] According to one embodiment, the nut has an open outer recirculation channel that is at least partially closed by a bushing, this feature facilitating machining of the recirculation channel and, if necessary, assembly and introduction of the ball into the mechanism.

[0016] According to one embodiment, the bushing has a bottom, so that only the bottom of the bushing can abut against the brake caliper if necessary, and the nut can be opened at both axial ends.

[0017] According to one embodiment, the bushing has a folded portion of material at the annular end face of the nut to ensure axial positioning between the bushing and the nut.

[0018] According to one embodiment, the piston comprises an axially outwardly projecting slider for fixing the piston in rotation within the guide cylinder while allowing the piston to move in translation.

[0019] The piston described is particularly intended for vehicle brake actuators.

[0020] According to another aspect of the present invention, there is provided a brake actuator mechanism comprising: a guide cylinder that defines a reference axis of the brake actuator mechanism; a ball screw mechanism including a screw and a nut centered on the reference axis; and a ball, wherein the screw has at least one screw thread that forms a screw groove for the ball, and the nut has nut threads that form a screw groove for the ball and an outer circumferential wall; and a bushing fixed to the nut and at least partially covering the outer circumferential wall of the nut, the bushing coming into close contact with an inner guide wall of the guide cylinder and sliding thereon, wherein the piston is formed by the bushing and the nut as described above. [Brief explanation of the drawings]

[0021] Other features and advantages of the present invention will become apparent from the following disclosure, taken in conjunction with the accompanying drawings.

[0022] [Figure 1] FIG. 1 shows a brake actuating mechanism according to one embodiment with an internal recirculating piston with a closed nut. [Figure 2] FIG. 2 shows a brake actuating mechanism according to one embodiment that includes an outer recirculating piston with a closed nut. [Figure 3] FIG. 3 shows a brake actuating mechanism according to one embodiment comprising a piston with an open nut.

[0023] For greater clarity, identical or similar elements are identified by the same reference numerals in all figures. DETAILED DESCRIPTION OF THE INVENTION

[0024] 1 shows a first embodiment of a brake actuator mechanism 10 comprising a fixed guide cylinder 44 that defines a reference axis 100 for the brake actuator mechanism 10, and a piston 12 that translates within the guide cylinder 44 along the reference axis 100, which is also the reference axis of the piston, to directly or indirectly support a brake caliper (not shown). The piston 12 comprises a bushing 42 and a nut 16, which is part of a ball screw mechanism that comprises two threaded components, namely a screw 14 and a nut 16, and a ball 18.

[0025] The screw 14 is preferably made of metal, for example, steel such as 20MnCr5, 23MnB4, Scr420, 16MnCr5, or other equivalents according to 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 portion 22, and a screw body 24. The screw body 24 has a larger diameter than the screw head 20, and the connecting portion 22 provides a connection between the screw body 24 and the screw head 20. The connecting portion 22 may be frustoconical, or preferably cylindrical, and form a first flat shoulder 26. The screw head 20 is designed to be rotatably mounted on the output shaft of an electric or gear motor and may have a non-circular interface, for example, having a hexagonal shape of 4, 6, or 8.

[0026] The screw body 24 has threads 25 that form an internal helical thread about a reference axis 100 of the ball screw mechanism, the internal helical thread facing radially outward from the reference axis 100. Additionally, the screw 14 has an open central cavity 28 to reduce the overall weight of the brake actuator mechanism 10 and to provide a reservoir for grease contained in the ball screw mechanism.

[0027] The nut 16 is made of steel, such as 20MnCr5, 23MnB4, Scr420, 16MnCr5, or other equivalents according to international or national standards, or a high-carbon steel such as 100Cr6, C50, or C56, or an equivalent. The nut 16 is generally cylindrical and has a reference axis 100 as its central axis. The nut 16 forms an external helical thread around the reference axis 100 and has nut threads 27 directed radially toward the reference axis 100. The nut 16 has a cylindrical outer surface 32 in which a locking mortise 64 is formed.

[0028] Additionally, nut 16 is closed in the sense that it has a base 17, an outer closure surface 34 that may have a recess 35, and is configured for direct or indirect contact with a brake caliper (not shown). Outer closure surface 34 further has a flange 72 that projects radially from outer peripheral wall 32 to form a flange shoulder 72'. Flange 72 further limits deformation of outer closure surface 34 under mechanical stress, for example, when brake actuator mechanism 10 is actuated.

[0029] One of the two threaded components, i.e., screw 14 or nut 16, may further comprise means 40 for recirculating balls 18, which may comprise one or more recirculators passing through the threads of the threaded component respectively, or a pair of recirculators located at the ends of a recirculation channel spanning one or more turns of the threads of screw 14 and nut 16, as shown in Figure 1. The system may also operate as a non-recirculating system.

[0030] The balls 18 may be made of, for example, steel or ceramic and are sized and positioned to circulate in a closed circuit between the outer helical thread groove of the nut 16 and the inner helical thread groove of the screw 14, as well as by a recirculation means 40, if desired, and preferably without separators between the balls 18.

[0031] The bushing 42 is made of metal, for example, steel, such as 20MnCr5, 23MnB4, Scr420, 16MnCr5, or equivalents according to international or national standards, or high-carbon steel such as 100Cr6, C50, or C56. The bushing 42 has a cylindrical inner surface 48 that is recessed onto at least a portion of the outer peripheral wall 32 of the nut 16. The bushing 42 has a bushing outer surface 49, and the thickness between the cylindrical inner surface 48 and the bushing outer surface 49 is on the order of 1 mm. The bushing 42 has a locking slot 66, such as a generally rectangular through-hole, located adjacent the annular end face 36 of the nut 16. The locking slot 66 allows access to a locking mortise 64 in the nut 16. Additionally, the bushing 42 may feature a bushing shoulder 50 that axially rests on the annular end face 36 of the nut 16 opposite the nut bottom 17.

[0032] The brake actuator mechanism 10 also includes a slider 46 retracted within a locking mortise 64 that projects radially through the locking slot 66 toward the guide cylinder 44 against the outer surface 49 of the bushing.

[0033] The guide cylinder 44 comprises a metal base, for example steel, preferably a flat annular base 52, a guide body 54 protruding axially from the outer periphery of the annular base 52, and an inner seal skirt 56 protruding axially from the inner periphery of the annular base 52.

[0034] The guide body 54 is a cylinder having a central axis on the reference axis 100. The guide body 54 includes an inner guide wall 58 that faces radially toward the reference axis 100 and that slides into contact with the bushing 42.

[0035] The inner seal skirt 56 has a cylindrical inner surface 57 that faces radially toward the reference axis 100 and defines an intermediate space 59. The inner guide surface 57 is positioned a short distance opposite the screw shaft 20 and forms a dynamic, contactless seal in this area to retain the lubricating grease within the guide cylinder 44.

[0036] The annular base 52 , the guide body 54 , and the inner guide skirt 56 define an annular space 62 .

[0037] The guide body 54 has an open annular end 63 with a chamfer 74. The guide body 54 includes a longitudinal locking groove 60 extending from the open annular end 63 toward the annular base 52 over a predetermined distance, e.g., 9 / 10 of the height of the inner guide wall 58. The locking groove 60 is configured to receive in sliding contact the slider 46 to rotationally lock the piston 12 relative to the guide cylinder 44 while allowing translational movement within the guide cylinder 44.

[0038] Brake actuator mechanism 10 further features an annular bellows 76 including an annular bellows base 78 configured to fit within chamfer 74 and a bellows head 80 configured to be sandwiched between flange shoulder 72' of flange 72 and bushing 42 and to radially abut outer peripheral wall 32. The annular bellows 76 prevents the ingress of contaminants into guide cylinder 44 by providing a primary seal. The annular bellows 76 is optional and, therefore, need not be incorporated into brake actuator mechanism 10 if brake actuator mechanism 10 is intended to operate in a clean environment.

[0039] Once the piston 12 of the brake actuator mechanism 10 is assembled, the nut 16 is forcefully inserted into the bushing 42 in the axial assembly direction 210 until the annular end face 36 of the nut 16 abuts the bushing shoulder 50, or until an axial position is reached that ensures the annular bellows 76 is held in place. The bushing 42 shrinks onto the nut 16 to form a unitary assembly. Assembly is performed at an angle such that the locking mortise 64 of the nut 16 and the locking slot 66 of the bushing 42 are positioned opposite one another, allowing the locking slot 66 to move in and out of the locking mortise 64.

[0040] The slider 46 is then inserted through the locking slot 66 into the locking mortise 64 of the nut 16 .

[0041] The screw 14 is then inserted into the nut 16 of the piston 12 using an incremental spiral motion to insert the balls 18 one by one.

[0042] The subassembly consisting of the screw 14 and piston 12 with attached 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 bushing 42 and the locking mortise 64 of the nut 16 must be inserted facing the locking groove 60 of the guide body 54 of the guide cylinder 44 at the same time that the slider 46 enters the locking groove 60. The outer surface 49 of the bushing then slides into contact with the inner guide wall 58 of the guide body 54.

[0043] The slider 46 inserted into the locking groove 60 has only one degree of freedom within the functional gap as it translates parallel to the reference axis 100 within the locking groove 60. The slider 46 then locks the piston 12 in rotation about the reference axis 100 while allowing it a translational degree of freedom parallel to the reference axis 100.

[0044] When the piston 12, bushing 42 and slider 46 are inserted into the guide cylinder 44 and reach their operating positions, the screw shaft 20 is accommodated in the intermediate space 59, and at the same time, the first flat shoulder 26 of the connecting portion 22 of the screw 14 abuts against the inner guide skirt 56.

[0045] Finally, an annular bellows 76 may be installed to provide a primary seal for the brake actuator 10 .

[0046] In operation, rotational movement of the screw 14 about the reference axis 100 , which is rotationally driven by the motor at the screw head 20 , produces translational movement of the piston 12 in a direction based on the direction of rotation of the screw 14 .

[0047] According to another embodiment shown in 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 the annular bellows 76 and the chamfered portion 74. Furthermore, the recirculation means 40 is formed in the nut 16 and has an outer recirculation channel 41 and a recirculator 41', allowing for outer recirculation of the balls 18. Here, the recirculation channel 41 is open and is closed again when the brake actuator mechanism 10 is assembled by the cylindrical inner surface 48 of the bushing 42. In the absence of the bellows, sealing is achieved at this point by the sliding contact between the bushing 42 and the inner guide wall 58 of the guide body 54.

[0048] According to the third embodiment shown in 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 or a chamfer 74. Furthermore, the nut 16 is open and does not have an outer closure surface 34. Furthermore, the bushing 42 does not have an axially mounted bushing shoulder 50 in the axial direction 200 opposite the assembly direction 210 on the annular end face 36 of the nut 16.

[0049] Furthermore, bushing 42 has a preferably flat closing bottom 68 that abuts the annular top end surface of nut 16. Closing bushing bottom 68 then presses directly or indirectly against the brake pads when brake actuator mechanism 10 is actuated. Conversely, when brake actuator mechanism 10 is not actuated and piston 12 is therefore in a free position, annular end surface 36 of nut 16 abuts base 52.

[0050] In all of the above embodiments, the bushing 42 undergoes a thermochemical treatment at a temperature Ts to obtain a nitrogen-rich, wear- and corrosion-resistant surface layer before assembly onto the nut 16. The treatment for obtaining such a layer involves nitriding or nitrocarburizing, with temperatures Ts ranging from 300°C to 580°C. Nitriding and / or nitrocarburizing allows nitrides to form on the surface when the component is placed in a highly nitrogen-rich treatment atmosphere at temperature Ts, allowing the formation of a different material on the surface. This treatment allows the outer surface 49 of the bushing 42 of the piston 12 to resist wear and corrosion that may occur under operating conditions when the piston 12 translates and slides within the guide cylinder 44. Because the nitrocarburizing treatment does not alter the surface flatness, the outer surface 49 of the bushing 42 can be polished before the thermochemical treatment.

[0051] Similarly, the nut 16 undergoes a thermochemical hardening process, which involves heating to a temperature Tc at least 200°C above Ts, preferably at least 900°C above Ts, followed by a thermochemical hardening process, e.g., of the surface or core carburizing type, followed by quenching and tempering at a temperature Tr at least 100°C below Ts, preferably 200°C below Ts. This process produces a hardened, carbon-rich surface layer, at least locally, on the nut threads 27 of the nut 16. This process increases the surface and depth hardness of the nut threads 27, making them more durable, e.g., by resisting chipping. However, because the carburizing process alters the flatness of the workpiece surface, the interior surface of the nut 16 after the thermochemical treatment must be subjected to a grinding, hard turning, or hard milling process.

[0052] In the first two embodiments, the portion of the bushing that contacts the brake caliper or its actuation mechanism, i.e., the outer closure surface 34, preferably has an additional surface coating. This additional treatment can be, for example, the application of zinc flakes to the surface or another surface treatment method. This additional treatment makes the outer closure surface 34 more resistant to the pressures exerted when the brake actuator mechanism 10 is actuated. In the third embodiment, this additional treatment is not necessary due to the corrosion-resistant properties provided by the thermochemical treatment of the bushing.

[0053] Thermochemical hardening (carburizing / quenching / tempering) involves the introduction of carbon into at least one surface layer of a steel material, followed by hardening and tempering to fix it. The aim is to increase the carbon content, at least near the surface, to give the part the desired hardness.

[0054] Thermochemical treatments (nitriding / nitrocarburizing) to resist wear and corrosion involve the introduction of nitrogen into the surface layer of a material, which can produce a highly wear-resistant surface layer, especially when nitrides such as iron nitride (Fe3N) or chromium nitride (Cr2N) are formed at the surface.

[0055] If the thermochemical treatment for wear and corrosion resistance is performed after the bushing 42 is assembled to the nut 16 and after the thermochemical treatment for hardening the nut 16, the nut cannot be thermally isolated, and therefore the nut threads 27 are brought to a temperature close to Ts, sufficient to eliminate the carbon bonding effect achieved by quenching and tempering. Conversely, if the thermochemical hardening treatment is performed after the bushing 42 is assembled to the nut 16 and after the thermochemical wear and corrosion resistance treatment, the bushing 42 cannot be thermally isolated, and therefore it is brought to a temperature close to Tc, sufficient to exceed Ts, and release the nitrogen compounds attached to the surface of the bushing 42 by the thermochemical wear and corrosion resistance treatment. For this reason, these two incompatible treatments are performed on the two components, the nut 16 and the bushing 42, before they are assembled.

[0056] Of course, the examples shown in the figures and described above are provided for illustrative and non-limiting purposes only, and it is expressly submitted that various illustrated embodiments can be combined to provide other embodiments.

[0057] In a variant not shown, the outer recirculation channel 41 is arranged within the screw 14 .

[0058] In another variation, the bushing 42 and / or the guide cylinder 44 are each constructed from a metal base that has been treated according to the thermochemical treatment described above and a resin with the desired properties to reduce friction between the bushing 42 and the guide cylinder 44, and the resin can be shaped into the bushing 42 and / or the guide cylinder 44, for example by molding or 3D printing.

Claims

1. A method of manufacturing a piston (12) for a brake actuator mechanism (10), the piston (12) comprising: a nut (16) of said ball screw mechanism defining a reference axis (100), an outer peripheral wall (32) and a nut thread (27) for forming a thread groove for the balls (18) of said ball screw mechanism; a bushing (42) fixed to said nut (16) and at least partially covering said outer peripheral wall (32) of said nut (16), said bushing (42) being intended to come into close, sliding contact with an inner guide wall (58) of a guide cylinder (44) of said brake actuator mechanism (10); a thermochemical wear- and corrosion-resistant treatment at a temperature Ts before fastening the bushing to the outer peripheral wall of the nut until a nitrogen-rich wear- and corrosion-resistant surface layer is obtained, and a thermochemical hardening treatment comprising heating the nut to a temperature Tc at least 200° C. above Ts, followed by quenching and tempering at a temperature Tr at least 100° C. below Ts, to obtain at least locally carbon-rich hardened areas in the threads of the nut.

2. 2. The method of claim 1, wherein the thermochemical hardening treatment comprises a carburizing treatment, the temperature Tc being greater than 900°C, and the temperature Tr being less than 250°C.

3. 3. The method according to claim 1, wherein the wear and corrosion resistance treatment comprises a nitriding or nitrocarburizing treatment, and the temperature Ts is between 300°C and 580°C.

4. 4. A method according to any one of claims 1 to 3, characterized in that the outer surface (49) of the bushing is polished before being subjected to the thermochemical wear and corrosion resistance treatment.

5. 5. The method according to claim 1, wherein after completion of the thermochemical wear and corrosion resistance treatment and the thermochemical hardening treatment, the bushing (42) is fixed to the outer peripheral wall (32) of the nut (16), preferably by shrinkage.

6. 6. A method according to any one of claims 1 to 5, characterized in that the outer surface of the bottom wall (17) of the nut (16) or bushing (42) is subjected to an additional anti-corrosion treatment.

7. 7. The method of claim 6, wherein the additional corrosion protection treatment is a zinc flake coating treatment.

8. 8. The method according to claim 1, wherein the slider (46) of the piston (12) is subjected to a surface treatment before being partially inserted into a housing formed in the nut (16) and the bushing (42).

9. The method of claim 8, wherein the surface treatment of the slider (46) is nitrocarburizing.

10. A piston (12), characterized in that it is manufactured according to the manufacturing method according to any one of claims 1 to 9.

11. 11. The piston (12) of claim 10, wherein said nut (16) has an open outer recirculation channel (41) at least partially closed by said bushing (42).

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

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

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

15. A brake actuator mechanism (10), comprising: - a guide cylinder (44) defining the reference axis (100) of said brake actuator mechanism (10); a ball screw mechanism comprising a screw (14) and a nut (16) centered on said reference axis (100), and a ball (18), said screw (14) having at least one screw thread (25) forming a thread groove for said ball (18), said nut (16) having a nut thread (27) forming a thread groove for said ball (18), and an outer peripheral wall (32); a bushing (42) fixed to the nut (16) and covering at least partly the outer peripheral wall (32) of the nut (16), said bushing (42) coming into close and sliding contact with the inner guide wall (58) of the guide cylinder (44); A brake actuator mechanism, characterized in that the bushing (42) and the nut (16) constitute a piston (12) as claimed in any one of claims 10 to 14.

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