Piston manufacturing method, piston and brake actuator mechanism

Abstract

A method of manufacturing a piston (12) for a brake actuator mechanism (10), the piston (12) comprising a bushing (42) and a nut (16), the nut (16) comprising an outer peripheral wall (32), the bushing (42) being subjected to a wear and corrosion resistant thermochemical treatment at a temperature Ts before the bushing (42) is secured to the outer peripheral wall (32) of the nut (16) until a nitrogen-rich wear and corrosion resistant surface layer is obtained, and the nut (16) is subjected to a thermochemical hardening treatment comprising heating to a temperature Tc which is at least 200 DEG C higher than Ts, followed by quenching and tempering to a temperature Tr which is at least 100 DEG C lower than Ts, and a carbon-rich hardened surface layer is obtained at least partially at the nut thread (27).

Description

Technical Field

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

[0002] Document EP 2787248 B1 discloses a brake actuator mechanism comprising a screw, a nut, and a ball, the ball being positioned between a helical thread on the screw and a helical thread on a 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 of the screw and nut threads. This type of piston is positioned close to the brake caliper and is subject to strong contamination from its direct external environment, which can lead to corrosion of the nut. To protect the ball screw mechanism, the piston and its guide cylinder must maintain a low level of structural clearance, which creates a risk of wear, exacerbated by the presence of contaminants. Consequently, the risk of mechanism failure is high.

[0003] Document EP 2304265 B1 discloses a brake actuator mechanism comprising a piston sliding in a cylinder and driven by a ball screw mechanism. The piston is manufactured from several parts and integrates the ball screw mechanism's nut, a solid pressure piece into which the nut is contracted, and an outer piece that is retracted onto the pressure piece. The outer piece has a base on which the frustoconical surface of the pressure piece rests. This three-piece piston design aims to integrate several piston parts across several different sizes, with the intermediate pressure piece acting as a kind of adapter. Summary of the Invention

[0004] The object of the present invention is to overcome the disadvantages of the prior art and to provide a piston which is more wear- and corrosion-resistant and economically advantageous while remaining compact.

[0005] To this end, according to a first aspect of the present invention, a method for manufacturing a piston of a brake actuator mechanism is proposed, the piston comprising a nut of a ball screw mechanism, a bushing, the nut defining a reference axis, an outer peripheral wall and a nut thread intended to form a raceway for the balls of the ball screw mechanism; the bushing is fixed to the nut and at least partially covers the outer peripheral wall of the nut, the bushing is intended to form a mating sliding contact with the inner guide wall of the guide cylinder of the brake actuator mechanism; it is worth noting that before the bushing is fixed to the outer peripheral wall of the nut, the bushing is subjected to a wear-resistant and corrosion-resistant thermochemical treatment at a temperature of Ts until a nitrogen-rich, wear-resistant and corrosion-resistant surface layer is obtained, and the nut is subjected to a thermochemical hardening treatment, the 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, and obtaining a carbon-rich hardened area at least locally at the nut thread.

[0006] Thermochemical treatments for wear and corrosion resistance and thermochemical treatments for hardening offer significant advantages to the same material. However, the methods for applying each of the two treatments are different, and regardless of the order in which they are applied to the same component, the properties imparted by one treatment can offset those imparted by the other. More specifically, assuming that we begin treating a part at the end of the treatment with a carbon-enriched thermochemical treatment (involving quenching and tempering) to increase its hardening at a tempering temperature Tr, a subsequent treatment of another portion of the same part at a temperature Ts significantly higher than the tempering temperature Tr will remove the effects of quenching and tempering, releasing carbon compounds and eliminating the hardening effect sought with the initial hardening treatment. Conversely, if the part is subjected to an initial thermochemical wear treatment at a temperature Ts involving nitrogen enrichment of the surface area of ​​the part, a subsequent hardening treatment of another portion of the same part at a temperature Tc significantly higher than Ts results in the release of nitrogen compounds that remained on the surface of the material during the initial treatment. Consequently, it is impossible to produce a single-piece part that possesses the desired properties provided by both treatments in an economically viable manner. By carrying out these two treatments 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 (e.g. 20MnCr5, 23MnB4, Scr420, 16MnCr5 or their equivalents according to other international or national standards) or high carbon steel (e.g. 100Cr6, C50 or C56 or their equivalents). The thermochemical hardening treatment is preferably carried out in a gaseous medium. The quenching and tempering stages make it possible to achieve a high surface hardness, for example a hardness exceeding 58 HRC (Rockwell hardness), while maintaining a high level of toughness in the core of the part. This treatment increases the hardness of the nut thread, making it, for example, more durable and more resistant to chipping. The thermochemical hardening treatment can be a surface treatment, but is preferably a deep treatment with a thickness exceeding 0.5 mm and preferably exceeding 2 mm. It can also be a core treatment.

[0008] According to one embodiment, the thermochemical hardening treatment comprises a carburizing treatment, wherein the temperature Tc is higher than 900° C. and the temperature Tr is lower than 250° C. Alternatively, it may be a carbonitriding treatment.

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

[0010] According to one embodiment, the outer face of the bushing is ground before being subjected to a thermochemical treatment for wear and corrosion resistance, so that the outer face of the bushing is completely smooth, thereby reducing piston friction in the guide cylinder and thus improving efficiency, as well as increasing wear and corrosion resistance.

[0011] According to one embodiment, after the thermochemical wear and corrosion resistance treatment and the thermochemical hardening treatment, the bushing is preferably fixed to the outer peripheral wall of the nut by shrinkage. In this way, initially incompatible properties are brought together in a single set of parts. This makes the piston harder at the nut threads and more resistant at its contact surface with the guide cylinder. Due to its relatively low thickness, the bushing provides additional properties to the piston without significantly increasing the bulk of the brake actuator mechanism.

[0012] According to one embodiment, the outer surface of the bottom wall of the nut or bushing, which is intended to abut directly or indirectly against the brake caliper, is subjected to an additional anti-corrosion treatment, preferably a zinc flake coating treatment.

[0013] According to one embodiment, the piston slider undergoes a surface treatment before being partially inserted into the housing formed by the nut and bushing; preferably, the slider's surface treatment is nitrocarburizing. The slider may facilitate the connection between the nut and bushing, but its primary function is to cooperate with a straight groove formed in the guide cylinder of the brake actuator mechanism to ensure non-rotational translational guidance of the piston in the cylinder. Due to its additional treatment, the slider is wear- and corrosion-resistant, allowing the brake actuator mechanism to last longer.

[0014] According to another aspect of the invention, it relates to a piston notable for being manufactured according to the manufacturing method described above. The piston is characterized in particular by a nitrogen-rich surface metallurgy at the bushing (caused by a thermochemical anti-wear treatment) and a carbon-rich metallurgy imparting a high hardness at least at the nut thread.

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

[0016] According to one embodiment, the bushing has a bottom. In this configuration, the bottom of the bushing can (if necessary) rest against the brake caliper alone, and the nut can be open at both axial ends.

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

[0018] According to one embodiment, the piston comprises an axially outwardly projecting slide for rotatably fixing the piston in the guide cylinder while allowing a translational movement of the piston.

[0019] The described piston is particularly intended for use in a vehicle brake actuator.

[0020] According to another aspect of the present invention, a brake actuator mechanism includes a guide cylinder, a ball screw mechanism, and a bushing. The guide cylinder defines a reference axis of the brake actuator mechanism; the ball screw mechanism includes a screw having at least one thread forming a raceway for the ball, a nut centered on the reference axis, and a ball; the screw having at least one thread forming a raceway for the ball; the nut having a nut thread forming a raceway for the ball and an outer peripheral wall; a bushing secured to the nut and at least partially covering the outer peripheral wall of the nut, the bushing forming close contact with an inner guide wall of the guide cylinder; it is noteworthy that the bushing and the nut constitute a piston as described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Other features and advantages of the present invention will become apparent upon reading the following disclosure and referring to the accompanying drawings.

[0022] Figure 1 A brake actuation mechanism including an internal recirculation piston with a closed nut is shown according to one embodiment.

[0023] Figure 2 A brake actuation mechanism including an external recirculation piston with a closed nut is shown according to one embodiment.

[0024] Figure 3 A brake actuation mechanism according to one embodiment is shown that includes a piston with a split nut.

[0025] For greater clarity, the same or similar elements are identified by the same reference numerals throughout the drawings. DETAILED DESCRIPTION

[0026] Figure 1 A first embodiment of a brake actuator mechanism 10 is shown, comprising a fixed guide cylinder 44 and a piston 12. The fixed guide cylinder 44 defines a reference axis 100 of the brake actuator mechanism 10. The piston 12 slides in translation along the reference axis 100 (also the reference axis of the piston) in the guide cylinder 44 to bear directly or indirectly against a brake caliper (not shown). The piston 12 comprises a bushing 42 and a nut 16, which is part of a ball screw mechanism comprising two threaded components, namely, a screw 14 and a nut 16, as well as a ball 18.

[0027] The screw 14 is preferably metal, for example steel (such as 20MnCr5, 23MnB4, Scr420, 16MnCr5 or its equivalent according to other international or national standards) or high carbon steel (such as 100Cr6, C50 or C56 or its equivalent), and the screw 14 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 (preferably cylindrical) and forms a first flat shoulder 26. The screw head 20 is designed to be rotationally attached to the output shaft of the motor or gear motor and may have a non-circular interface (for example, having four, six or eight hexagons).

[0028] The screw body 24 has a screw thread 25 that forms an inner spiral raceway around the reference axis 100 of the ball screw mechanism, the inner spiral raceway facing radially away from the reference axis 100. In addition, 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 the grease contained in the ball screw mechanism.

[0029] Nut 16 is made of steel (e.g., 20MnCr5, 23MnB4, Scr420, 16MnCr5, or equivalents thereof according to other international or national standards) or high-carbon steel (e.g., 100Cr6, C50, or C56, or equivalents thereof). Nut 16 is generally cylindrical, with reference axis 100 as its central axis. Nut 16 has nut threads 27 that form an outer helical raceway around reference axis 100 and face radially toward reference axis 100. Nut 16 has a cylindrical outer peripheral surface 32 in which a locking mortise 64 is formed.

[0030] Furthermore, the nut 16 is of a closed type, as it includes a base 17 with an outer closed face 34 that may include a recess 35, and is configured to directly or indirectly contact a brake caliper (not shown). The outer closed face 34 further includes a flange 72 that protrudes radially from the outer peripheral wall 32, forming a flange shoulder 72'. The flange 72 further limits any deformation of the outer closed face 34 under mechanical stress, for example, when the brake actuator mechanism 10 is activated.

[0031] One of the two threaded parts (i.e. the screw 14 or the nut 16) may further be provided with means 40 for recirculating the balls 18, which means 40 may comprise one or more recirculators (e.g. Figure 1 ) or a pair of recirculators arranged at the end of the recirculation channel that span one or more turns of the raceways of the screw 14 and nut 16. The system can also operate on a non-recirculating system.

[0032] The balls 18 can be made of, for example, steel or ceramic and are sized and positioned to circulate in a closed loop between the outer helical raceway of the nut 16 and the inner helical raceway of the screw 14 and, if necessary, through the recirculation device 40, preferably without separators between the balls 18.

[0033] The bushing 42 is metallic, for example, made of steel (e.g., 20MnCr5, 23MnB4, Scr420, 16MnCr5, or equivalents thereof according to other international or national standards) or high-carbon steel (e.g., 100Cr6, C50, or C56, or equivalents thereof). The bushing 42 has a cylindrical inner face 48 that is contracted onto at least a portion of the outer peripheral wall 32 of the nut 16. The bushing 42 has an outer face 49, and the thickness between the cylindrical inner face 48 and the outer face 49 is approximately 1 mm. The bushing 42 has a generally rectangular locking groove 66, such as a through-hole, located adjacent to the annular end face 36 of the nut 16. The locking groove 66 allows access to the locking mortise 64 of the nut 16. The bushing 42 may also have a bushing shoulder 50 that rests axially on the annular end face 36 of the nut 16, opposite the base 17 of the nut.

[0034] The brake actuator mechanism 10 further comprises a slider 46 which is retracted relative to the outer face of the bushing 49 into the locking mortise 64 , protruding radially towards the guide cylinder 44 through the locking slot 66 .

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

[0036] The guide body 54 is a cylindrical body whose central axis is the reference axis 100. The guide body 54 includes an inner guide wall 58 that radially faces the reference axis 100 and is in sliding contact with the bushing 42.

[0037] The inner sealing skirt 56 has a cylindrical inner face 57 facing radially toward the reference axis 100, thereby delimiting an intermediate space 59. The inner guide surface 57 is positioned opposite and a short distance from the screw shaft 20 to form a dynamic, non-contact seal in this region for retaining the 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 including a chamfer 74. The guide body 54 includes a longitudinal axial locking groove 60 that extends from the open annular end 63 toward the annular base 52 by a predetermined distance (e.g., 9 / 10) exceeding the height of the inner guide wall 58. The locking groove 60 is configured to receive the slider 46 in a sliding contact manner to lock the piston 12 in rotation relative to the guide cylinder 44 while allowing the piston 12 to move in translation within the guide cylinder 44.

[0040] Brake actuator mechanism 10 further features an annular bellows 76 including an annular bellows base 78 configured to fit into chamfer 74 and a bellows head 80 configured to be sandwiched between flange shoulder 72' of flange 72 and bushing 42, radially abutting outer peripheral wall 32. Annular bellows 76 prevents contaminants from entering guide cylinder 44 by providing a primary seal. Annular bellows 76 is optional and, therefore, may not be integrated into brake actuator mechanism 10 if brake actuator mechanism 10 is intended to operate in a non-contaminated environment.

[0041] When the piston 12 of the brake actuator mechanism 10 is assembled, the nut 16 is forcibly 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 that the annular bellows 76 is retained in place. The bushing 42 is shrunk onto the nut 16 to form a single-piece assembly. Assembly is performed by angular indexing so that the locking mortise 64 of the nut 16 and the locking groove 66 of the bushing 42 are positioned relative to each other and the locking groove 66 allows access to the locking mortise 64.

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

[0043] The screw 14 is then inserted into the nut 16 of the piston 12, using a gradual helical motion to insert the balls 18 one by one.

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

[0045] The slide 46 inserted into the locking groove 60 has, within the functional clearance, only one degree of freedom in translation in the locking groove 60 parallel to the reference axis 100. The slide 46 then locks the piston 12 in rotation about the reference axis 100 while allowing the piston 12 a degree of freedom in 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 use position, the first flat shoulder 26 of the connecting portion 22 of the screw 14 abuts the inner guide skirt 56 , while the screw shaft 20 is accommodated in the intermediate space 59 .

[0047] Finally, an annular bellows 76 may be fitted to provide a primary seal for the brake actuator 10 .

[0048] In operation, rotational movement of the screw 14 about the reference axis 100 (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 .

[0049] according to Figure 2 In another embodiment shown in FIG, brake actuator mechanism 10 differs from brake actuator mechanism 10 described in the first embodiment in that brake actuator mechanism 10 does not have annular bellows 76 and does not have chamfer 74. Furthermore, a recirculation device 40 is formed at nut 16, having an external recirculation channel 41 and a recirculator 41', thereby allowing external recirculation of balls 18. Here, recirculation channel 41 is open and then closed again when brake actuator mechanism 10 is assembled using cylindrical inner face 48 of bushing 42. In the absence of a bellows, sealing is achieved at this point by sliding contact between bushing 42 and inner guide wall 58 of guide body 54.

[0050] according to Figure 3 In the third embodiment shown, the brake actuator mechanism 10 differs from the brake actuator mechanism 10 described in the first embodiment in that the brake actuator mechanism 10 has neither the annular bellows 76 nor the chamfer 74. Furthermore, the nut 16 is open and does not have the outer closed surface 34. Furthermore, the bushing 42 does not have the bushing shoulder 50 that rests axially on the annular end surface 36 of the nut 16 in the axial direction 200 opposite the assembly direction 210.

[0051] The bushing 42 then has a closed bottom 68 (preferably flat) that rests against the annular upper end surface of the nut 70. When the brake actuator mechanism 10 is actuated, the closed bushing bottom 68 then presses directly or indirectly against the brake pad. Conversely, when the brake actuator mechanism 10 is not actuated and the piston 12 is therefore in a free position, the annular end face 36 of the nut 16 abuts against the base 52.

[0052] In all the embodiments described above, before being assembled on the nut 16, the bushing 42 undergoes a thermochemical treatment at a temperature Ts to resist wear and corrosion until a nitrogen-rich, wear- and corrosion-resistant surface layer is obtained. The treatment to obtain this layer consists of nitriding or nitrocarburizing, wherein the temperature Ts is in the range of 300°C to 580°C. Nitriding and / or nitrocarburizing enable nitrides to be formed on the surface when the component is placed in a very nitrogen-rich treatment atmosphere at a temperature Ts, allowing the formation of different materials on the surface. Thanks to this treatment, the outer face 49 of the bushing 42 of the piston 12 resists the wear and corrosion that may occur under operating conditions when the piston 12 slides in translation in the guide cylinder 44. Since nitrocarburizing does not change the flatness of the surface, the outer face 49 of the bushing 42 can be ground before applying the thermochemical treatment.

[0053] Similarly, the nut 16 is subjected to a thermochemical hardening treatment comprising heating to a temperature Tc of at least 200° C. above Ts, preferably at least 900° C. above Ts. The thermochemical hardening treatment (for example of the surface or core carburizing type) then comprises quenching and tempering at a temperature Tr of at least 100° C. below Ts, preferably below 200° C. This treatment produces, at least locally, a hardened, carbon-rich surface layer on the nut thread 27 of the inner surface of the nut 30. As a result of this treatment, the nut thread 27 has an increased surface and depth hardness, making it more durable, for example by resisting chipping. However, carburizing alters the flatness of the workpiece surface, so it is necessary to perform a grinding, hard turning or hard milling step on the inner surface of the nut 30 after the thermochemical treatment has been applied.

[0054] In the first two embodiments, the part that comes into contact with the brake caliper or its actuating mechanism (i.e., outer sealing surface 34) is preferably given an additional surface coating. For example, this additional treatment involves applying zinc flakes to one or more surfaces. This additional treatment makes outer sealing surface 34 more resistant to pressure when brake actuator mechanism 10 is actuated. In the third embodiment, this additional treatment is unnecessary due to the corrosion protection provided by the thermochemical treatment of the lining.

[0055] Thermochemical hardening treatment (carburizing / quenching / tempering) involves the introduction of carbon into at least one surface layer of the steel material, which is then fixed by quenching and tempering. The aim is to increase the carbon content at least close to the surface to give the part the desired hardness.

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

[0057] If the thermochemical treatment for wear and corrosion resistance is performed after the bushing 42 is assembled on the nut 16 and after the thermochemical treatment for hardening the nut 16, it is impossible to thermally isolate the nut, so that the nut thread 27 is brought to a temperature close to Ts, which is sufficient to eliminate the carbon bonding effect obtained by quenching and tempering. Conversely, if the thermochemical hardening treatment is performed after the bushing 42 is assembled on the nut 16 and after the thermochemical wear and corrosion resistance treatment, it is impossible to thermally isolate the bushing 42, so that the bushing 42 is brought to a temperature close to Tc, which is much higher than Ts, and the bushing 42 releases nitrogen compounds attached to the surface of the sleeve 42 by the thermochemical wear and corrosion resistance treatment. For this reason, these two incompatible treatments are performed on the two parts (nut 16 and bushing 42) before they are assembled.

[0058] Naturally, the examples shown in the figures and discussed above are provided for illustrative and non-limiting purposes only. It is expressly provided that it is possible to combine the various illustrated embodiments in order to provide other embodiments.

[0059] In a variant not shown, the external recirculation channel 41 is located in the screw 14 .

[0060] In another variation, the bushing 42 and / or the guide cylinder 44 are each formed of a metal base treated according to the above-mentioned thermochemical treatment and a resin including desired properties so as to reduce friction between the bushing 42 and the guide cylinder 44, and the resin can be formed into the bushing 42 and / or the guide cylinder 44 by, for example, molding or 3D printing.

Claims

1. A method of manufacturing a piston (12) of a brake actuator mechanism (10), comprising: A nut (16) for a ball screw mechanism is provided, the nut (16) defining a reference axis (100), an outer peripheral wall (32), and a nut thread (27), the nut thread (27) being used to form a raceway for balls (18) of the ball screw mechanism; fixing a bushing (42) to the nut (16) so that the bushing at least partially covers the outer peripheral wall (32) of the nut (16), the bushing (42) being intended to form a close sliding contact with an inner guide wall (58) of a guide cylinder (44) of the brake actuator mechanism (10); and Before the bushing (42) is fixed to the outer peripheral wall (32) of the nut (16), the bushing (42) is subjected to a thermochemical wear and corrosion resistance treatment at a temperature Ts until a nitrogen-rich wear and corrosion-resistant surface layer is obtained, and the nut (16) is subjected to a thermochemical hardening treatment, which comprises heating to a temperature Tc of at least 200°C higher than Ts, followed by quenching and tempering to a temperature Tr of at least 100°C lower than Ts, and obtaining a carbon-rich hardened area at least locally at the nut thread (27).

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

3. The method according to claim 1 or 2, characterized in that The thermochemical anti-wear and anti-corrosion treatment comprises nitriding or nitrocarburizing, and the temperature Ts is between 300°C and 580°C.

4. The method according to claim 1 or 2, characterized in that The outer surface of the bushing (49) is ground before being subjected to the thermochemical wear and corrosion resistance treatment.

5. The method according to claim 1 or 2, characterized in that After the thermochemical wear and corrosion resistance treatment and the thermochemical hardening treatment are completed, the bushing (42) is fixed to the outer peripheral wall (32) of the nut (16).

6. The method according to claim 1 or 2, characterized in that After the thermochemical wear and corrosion resistance treatment and the thermochemical hardening treatment are completed, the bushing (42) is fixed to the outer peripheral wall (32) of the nut (16) by shrinking.

7. The method according to claim 1 or 2, 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.

8. The method according to claim 1 or 2, 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, wherein the additional anti-corrosion treatment is a zinc flake coating treatment.

9. The method according to claim 1 or 2, characterized in that The slide (46) of the piston (12) is subjected to surface treatment before being partially inserted into a housing formed in the nut (16) and the bushing (42).

10. The method according to claim 1 or 2, characterized in that The slide (46) of the piston (12) is subjected to nitrocarburizing before being partially inserted into a housing formed in the nut (16) and the bushing (42).

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

12. The piston (12) according to claim 11, characterized in that The nut (16) has an open external recirculation channel (41) which is at least partially closed by the bushing (42).

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

14. The piston (12) according to any one of claims 11 to 12, characterized in that The bushing (42) has a material fold (50) on the annular end face (36) of the nut (16).

15. The piston (12) according to any one of claims 11 to 12, characterized in that The piston (12) includes a sliding member (46) protruding axially outward.

16. 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 ball (18) and a screw (14) and a nut (16) centered about the reference axis (100), the screw (14) having at least one screw thread (25) forming a raceway for the ball (18), the nut (16) having an outer peripheral wall (32) and a nut thread (27) forming a raceway for the ball (18); and a bushing (42) fixed to the nut (16) and at least partially covering the outer peripheral wall (32) of the nut (16), the bushing (42) forming a close sliding contact with the inner guide wall (58) of the guide cylinder (44); It is characterized in that the bushing (42) and the nut (16) constitute the piston (12) according to any one of claims 11 to 15.

Images