Piston of an actuating mechanism with a mother and a sleeve

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 pad pistons in braking mechanisms driven by a worm screw, in particular a ball screw. PREVIOUS STATE OF THE ART

[0002] In the unpublished French patent application FR 2402914, as of the filing date of this application, a brake actuator mechanism is disclosed. This mechanism comprises a screw, a nut press-fitted into a sleeve, and balls positioned between a helical thread of the screw and a helical thread of the nut. The nut and sleeve together form a piston that slides within a guide cylinder. The piston formed by the nut and sleeve requires extremely precise dimensional control during manufacturing to ensure effective press-fitting without deformation of any of the piston components when the sleeve is assembled onto the nut. Indeed, for this application, very tight dimensional tolerances of the piston are necessary to achieve virtually play-free and friction-free sliding of the piston within the cylinder.

[0003] Documents EP 2 787 248 B1 and FR 2 753 672 A1 can likewise be cited as prior art. DESCRIPTION OF THE INVENTION

[0004] The invention aims to overcome the drawbacks of the prior art and to propose a simple solution for producing a piston that meets tight dimensional tolerances.

[0005] To this end, according to a first aspect of the invention, a piston of an actuator mechanism is proposed, the piston comprising a worm gear mechanism nut, defining a reference axis, an outer peripheral wall and a nut thread intended to cooperate, directly or via balls, with a worm gear mechanism screw; a sleeve integral with the nut and covering at least partially the outer peripheral wall of the nut, the sleeve comprising a cylindrical guide area intended to come into fitted sliding contact with an inner guide wall of a guide cylinder of the actuator mechanism;remarkable in that the nut comprises at least one annular shrink-fitting surface, the sleeve comprises at least one shrink-fitting area shrink-fitted onto the annular shrink-fitting surface, the guide area having an outside diameter greater than that of the shrink-fitting area and covering without contact or shrink-fitting a covered portion of the outer peripheral wall of the nut.

[0006] The sleeve, which includes a shrink-fit area around the nut, ensures the cohesion of the assembled piston. This area can deform freely during shrink-fitting, without exceeding the outer dimensions defined by the outer diameter of the guide area. The guide area of ​​the sleeve, on the other hand, does not deform during mounting on the nut, thus guaranteeing its dimensions. This results in a piston that does not require grinding of the sleeve's guide area after mounting the sleeve on the nut, thereby reducing the costs associated with this grinding process. Furthermore, the absence of grinding after mounting allows for a thermochemical surface treatment of the sleeve's guide area before mounting on the nut, giving it specific mechanical properties that would be lost if the guide area were to be ground.Assuming a sleeve made of a different material than the nut, the effects of differential thermal expansions between the sleeve and the nut resulting from temperature fluctuations are confined to the shrink-fit zone of the sleeve, and have no significant impact on the guiding zone.

[0007] According to one embodiment, the guiding zone and the covered portion of the outer peripheral wall of the nut are separated by an intermediate annular space generated by: an annular indentation of the covered portion of the outer peripheral wall of the nut relative to the annular shrink-fitting area; and / or an inner diameter of the guide area greater than an inner diameter of the shrink-fitting area.

[0008] According to one embodiment, the intermediate annular space has a length L1 measured parallel to the reference axis, and a depth P, measured radially between the outer peripheral wall of the nut and a cylindrical inner face of the guide zone of the sleeve, and the nut has a nut length L2 measured in an axis parallel to the reference axis between two ends of the nut, such that L 1 > 20 P ; and / or L 2 / 2 < L 1 < L 2 .

[0009] The intermediate annular space therefore does not have a large radial dimension, as the space between the sleeve and the nut is not necessarily large, but it does have a significant annular volume due to its axial dimension. This allows for the creation of a substantial and uniform intermediate annular space along a portion of the nut or sleeve, along which no shrink-fitting forces are transmitted, thus meeting the requirement of maintaining the external radial dimensions of the sleeve.

[0010] In one embodiment, the nut includes a second annular shrink-fitting surface, and the sleeve includes a second shrink-fitting area in contact with the nut's second annular shrink-fitting surface. Preferably, the two annular shrink-fitting areas are axially separated, for example, located at two opposite axial ends of the guide area. In this way, the sleeve is shrink-fitted at two ends of the nut, providing a stable and reliable connection, while an intermediate, unshrink-fitted area allows it to maintain its desired radial dimensions. Preferably, no more than two shrink-fitting areas are provided.

[0011] According to one embodiment, the sleeve has a shrink-fitting end-stroke shoulder turned in an axial direction of assembly, bearing against an annular end face of the nut, which allows the nut to stop against the sleeve, allowing the correct axial positioning of the sleeve on the nut and the correspondence of the shrink-fitting areas on the annular shrink-fitting surfaces.

[0012] According to one embodiment, the nut includes a crimping mortise and the sleeve has a flap of crimping material penetrating the crimping mortise bearing against a crimping shoulder of the crimping mortise, which allows the connection between the sleeve and the nut to be locked, and thus reduces the risk of micromovements of the sleeve against the surface of the nut, these movements being able to induce wear of the shrink-fitted areas by friction corrosion.

[0013] Preferably, the nut includes a locking mortise, and the sleeve has a locking slot open to the locking mortise. The piston further incorporates a slide inserted into the locking mortise and projecting radially from the guide area. This mortise is designed to be inserted into a guide groove in the piston housing cylinder to prevent the piston from rotating within the cylinder around its reference axis, while still allowing translational movement. Moreover, the slide provides an additional connection between the sleeve and the nut, reducing the risks associated with potential movement of the sleeve on the nut.

[0014] In one embodiment, the bushing comprises a nitrogen-rich, abrasion- and corrosion-resistant surface layer, and the nut comprises a carbon-rich, hardened surface area, at least locally at the nut threads. Thus, the bushing is more resistant to contact wear caused by friction against the guide cylinder when the piston is operating within the guide cylinder, thereby increasing the strength and durability of the bushing, and consequently the piston.

[0015] In one embodiment, the piston has a piston base formed by the bushing or nut. The piston base is designed to bear against a brake pad to actuate it. It can advantageously be covered externally by a surface coating, for example, a layer of zinc flakes.

[0016] According to another aspect of the invention, a brake actuator mechanism, comprising: a guide cylinder; a ball screw mechanism, comprising a screw and a nut defining a reference axis of the brake actuator mechanism, and balls, the screw having at least one screw thread forming a raceway for the balls, the nut having a nut thread forming an external helical raceway for the balls and an external peripheral wall; and a sleeve integral with the nut and covering at least partially the external peripheral wall of the nut, the sleeve coming into fitted sliding contact with an internal guide wall of the guide cylinder, the sleeve and the nut constituting a piston as described above.

[0017] The piston preferably has a base, as mentioned previously, made by the sleeve or the nut, and intended to bear against a pad or more generally a brake component to actuate it. BRIEF DESCRIPTION OF THE FIGURES

[0018] Other features and advantages of the invention will become apparent from the following description, with reference to the attached figures, which illustrate: [ Fig. 1 ] : an axial cross-sectional view of a brake actuator mechanism comprising a ball screw mechanism including a screw, a nut and a bushing according to a first embodiment in which the nut has an annular recess. ] Fig. 2 ] : an axial cross-sectional view of the ball screw mechanism according to the embodiment of the figure 1 . [ Fig. 3 ] : a detailed cross-sectional view of a first shrink-fit zone of the nut according to the embodiment of the figure 1 . [ Fig. 4 ] : a detailed cross-sectional view of a second shrink-fit zone of the nut according to the embodiment of the figure 1 . [ Fig. 5 ] : a detailed cross-sectional view of a portion of the nut including a mortise according to the embodiment of the figure 1 . [ Fig. 6] : a detailed cross-sectional view of a first shrink-fit zone of the nut according to a second embodiment in which the sleeve has a secondary annular recess. ] Fig. 7 ] : a detailed cross-sectional view of a second shrink-fit zone of the nut according to the embodiment of the figure 6 . [ Fig. 8 ] : an axial cross-sectional view of the ball screw mechanism according to a third embodiment in which the bushing has a bottom.

[0019] For clarity, identical or similar elements are identified by identical reference symbols across all figures. DETAILED DESCRIPTION OF IMPLEMENTATION METHODS

[0020] On the figures 1 to 5 A first embodiment of a brake actuator mechanism is illustrated. 10 comprising a guide cylinder 84 fixed and a piston 12 sliding in translation within the guide cylinder 84 along a reference axis 100,which also serves as a reference axis for the piston, allowing it to bear directly or indirectly against a brake pad (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.

[0021] The guide cylinder 84 consists of a metal base, for example made of steel, and includes a guide body 86 cylindrical, preferably with a circular base, centered on the reference axis 100. The guide body 86 includes a longitudinal locking groove 88, extending from a first open annular end 90A towards a second open annular end 90B, over a predetermined distance. The locking groove 88 is configured to accommodate a slider 92in sliding contact, in order to block the piston 12 rotating relative to the guide cylinder 84, while allowing it a translational movement within the guide cylinder 84. Furthermore, the guide body 86 may include a positioning flange 94.

[0022] The screw 14 is 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 and a screw body 24, where applicable connected by a linking section 22. The screw body 24 may have a diameter greater than the screw head 20. The screw head 20is 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.

[0023] 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.

[0024] The nut 16is made of steel, for example 20MnCr5, 23MnB4, Scr420, 16MnCr5 steel or their equivalents according to other international or national standards, or high-carbon steel such as 100Cr6, C50 or C56 steel or their equivalents. The nut 16 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 30 around the reference axis 100, and rotated radially towards the reference axis 100.

[0025] The nut 16 is of the closed type in the sense that it has a nut base 17, achieved by an external closing face 34, and creating a piston bottom. The nut 16 presents an outer peripheral face 32 generally cylindrical extending from the outer closing face 34to an annular end face 36, over a length L2. The outer peripheral face 32 features a crimping mortise 63, a locking mortise 64 and a portion covered 80. The crimping mortise 63 features a crimping shoulder 74, rotated in an axial direction of assembly 210. The covered portion 80 features an annular indentation 46 radially inwards. The crimping mortise 63 and the locking mortise 64 can be confused. The annular indentation 46 extends over a length L1 parallel to the reference axis 100 and has a depth P measured radially between a bottom of the annular recess 46 and a cylindrical inner face 48 at the level of a guidance zone 44 socket 42described below. The length L1 is greater than the depth P such that L 1 > 20 P and L2 is greater than the length L1 such that L 2 2 < L 1 < L 2 The annular recess 46 can be obtained during nut molding 16 or by machining, for example.

[0026] Furthermore, the outer peripheral face 32 of the nut 16 includes two annular shrink-fitting supports 52 distinct, arranged on either side of the annular recess 46. The two annular shrink-fit bearing surfaces 52 open axially onto the annular recess 46. Among the two annular shrink-fitting ranges 52, a first annular fretting range 52A is positioned near the outer closing face 34 while a second annular fretting range 52B is positioned near the annular end face 36.The second annular fretwork support 52B is linked to the annular end face 36 by means of a chamfer 59. The two annular shrink-fit bearing surfaces 52 are configured to accommodate the socket in tight contact 42.

[0027] Furthermore, the outer closing face 34 may present a hollow 35, the bottom of the nut 16 configured to make direct or indirect contact with the brake pad (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 72 It also helps to limit any potential deformation of the outer closing face 34under 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 balls, which may include one or more recirculators, each passing through one or more threads 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. In this embodiment, the ball recirculation is internal, that is to say, comprising at least one recirculator, each passing through a thread of the screw 14.

[0029] The balls can be made of steel or ceramic, for example, and are sized and positioned to circulate in a closed circuit between the external helical raceway 30 of the nut 16 and the internal helical raceway 25 of the screw 14, thus, where applicable, through recirculation methods 40, preferably without separators between the marbles.

[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 and a cylindrical outer face 49 radially opposite, and extending from a first extremity 57 socket 42at a second extremity 58 socket 42. The socket 42 features a locking slot 66, like a through hole, generally rectangular or oblong, located near the annular end face 36 of the nut 16. The locking slot 66 provides access to the locking mortise 64 of the nut 16.

[0031] The socket 42 features a flap of crimping material 76 inwards, designed to rest against the crimping mortise 63, at the level of the crimping shoulder 74 of the crimping mortise 63. Furthermore, the socket 42 may optionally feature a shoulder 50 socket 42 located at the second end 58 socket 42, rotated in an axial direction of assembly 210. The shoulder 50 socket 42rests axially on the annular end face 36 of the nut 16, opposed to the bottom 17 of the nut 16. The crimping material flap 76 and the shoulder 50 socket 42 have generally parallel bearing faces and are joined by the cylindrical inner face 48 socket 42, form a crimping groove 78. The crimping groove 78 is configured to accommodate at least part of the nut in tight contact 16.

[0032] Furthermore, the socket 42 presents a guidance zone 44 and two shrink-fitting zones 62, namely a first shrink-fitting zone 62A located in the vicinity of the first extremity 57 socket 42 and a second shrink-fitting area 62B located in the vicinity of the second end 58 socket 42.The guidance zone 44 is intended, via the cylindrical outer face 49, to come into sliding contact adjusted with the inner guide wall of the guide cylinder 84. The guidance zone 44 is intended, via the cylindrical inner face 48, to be covered without reinforcement, and preferably without contact, the covered portion 80 of the nut 16. Each shrink-wrapping zone 62 is designed to come into close contact with the outer peripheral face 32 of the nut 16 at the level of the two annular shrink-fit bearing surfaces 52. The guidance zone 44 and the two shrink-fitting zones 62 are, where applicable, joined by chamfers to facilitate piston insertion 12 in the guide cylinder 84.

[0033] More specifically, the first shrink-fitting area 62Ais intended to come into close contact with the first annular bearing surface 52A while, the second shrink-wrapping zone 62B is intended to come into close contact with the second annular shrink-fit bearing surface 52B. The guidance zone 44 has an outer diameter greater than the outer diameter of the first shrink-fit zone 62A in order to limit any potential deformations related to the socket assembly processes 42 on the nut 16 in a diameter less than or equal to the diameter of the guide zone 44. The guidance zone 44 has an outside diameter greater than or equal to the outside diameter of the second shrink-fit zone 62B since the second shrink-fitting zone 62B is located in the vicinity of the second end 58 socket 42, including the socket shoulder 50, whose curvature stiffens the socket 42in this region of the socket 42.

[0034] The brake actuator mechanism 10 also includes the slider 92, clamped in the locking mortise 64, projecting radially towards the guide cylinder 84 through the blocking slot 66, relative to the outer face of the socket 42.

[0035] When the piston 12 of the brake actuator mechanism 10 is assembled, the nut 16 is forced into the socket 42 without risk of snagging thanks to its various chamfers. Insertion is made in the axial direction of assembly. 210, until the annular end face 36 of the nut 16 butts against the shoulder of the socket 50. The socket 42, through the two shrink-wrapping zones 62, is fitted onto the nut 16,at the level of the two annular shrink-fit bearing surfaces 52, which allows for the creation of a single-piece assembly consisting of the nut 16 and the socket 42, namely the piston 12. The first shrink-fitting area 62A is fretted onto the first annular fretting support 52A, the second shrink-fitting zone 62B is fretted onto the second annular fretting support 52B, and the guidance zone 44 is located opposite, without contact with, the outer perimeter wall 32 of the nut 16 due to the annular indentation 46. The outer perimeter wall 32 of the nut 16 as well as the cylindrical inner face 48, due to the annular indentation 46, are then spaced by an intermediate annular space 56.

[0036] The intermediate annular space 56 presents the dimensions of the annular recess46 such as its length or depth. The intermediate annular space 56 allows to compensate for deformations of the socket 42 during its tightening to the nut, and / or the nut 16 in the event of a possible increase in the volume of the nut 16 caused by temperature variations when the piston 12 is in operation within its operating environment. Thus, the diameter of the socket 42, and more specifically the outer diameter of the guide zone 44, namely the outer face of the socket 49, is maintained at a relatively constant value, so as to preserve the desired properties during the production of the brake actuator mechanism 10.

[0037] The assembly is performed with angular indexing such that: the locking mortise 64 of the nut 16 and the locking slot 66socket 42 are located opposite each other, and the locking slot 66 allows access to the locking mortise 64 ; and the crimping mortise 63 of the nut 16 and the crimping material flap 76 socket 42 are carried out opposite each other.

[0038] The crimping material flap 76, initially oriented parallel to the reference axis 100, is then folded radially inwards into the crimping mortise 63 so as to secure the socket 42 and the nut 16. The slider 92 is then inserted into the locking mortise 64 of the nut 16 through the blocking slot 66. When the crimping mortise 63 is confused with the locking mortise 64, the slider 92can come into contact with the crimping material flap 76 allowing additional securing between the socket 42 and the nut 16.

[0039] The screw 14 is then inserted into the nut 16 piston 12, by a progressive helical movement.

[0040] The subassembly consisting of the screw 14 and the piston 12 equipped with the slider 92 is then inserted into the guide cylinder 84 in 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 into the locking groove 88 of the guide body 86 of the guide cylinder 84, while the slider 92 penetrates the locking groove 88. The guidance zone 44from the outer face of the socket 42 then enters into sliding contact, translationally without rotation, with the inner guide wall of the guide body. 86.

[0041] The slider 92 inserted into the locking groove 88 possesses only one degree of freedom, apart from functional play, in translation parallel to the reference axis 100 in the locking groove 88. The slider 92 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.

[0042] 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.

[0043] According to a second embodiment, illustrated on the figures 6 And 7 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 indentation 46 of the nut 16. In addition, the socket features a secondary annular indentation 46' radially outwards, at the level of the cylindrical inner face 48. The intermediate annular space 56 is then generated by the secondary annular recess 46', exhibiting the same advantageous characteristics as when generated by the annular recess 46 of the nut 16.

[0044] According to a third embodiment, illustrated on the figure 8the brake actuator mechanism 10 differs from that described in the first two embodiments in that the nut 16 does not include a nut base 17. The nut 16 then comprises two annular end faces 36 of the nut 16. The piston then includes a piston base formed by the bushing 42, and more specifically by a socket base 82. The nut is then inserted into the socket 42 in a second axial direction of assembly 200, opposite in direction to the axial direction of assembly 210 but in the same direction. The shoulder 50 socket 42, projecting axially relative to the annular end face 36 of the nut 16 opposite the base of the socket 82, If present, it is then achieved by folding down material after assembly of the socket 42 on the nut 16. This shoulder 50However, it is optional.

[0045] In all the embodiments described above, the socket 42 may undergo, 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 12resists abrasion and corrosion that could occur under operating conditions, when the piston 12 slides in translation within the guide cylinder 84. 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.

[0046] Similarly, the nut 16may be 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 16. Thanks to this treatment, the nut thread 27 It 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.16 after the application of the thermochemical treatment.

[0047] Furthermore, the part in contact with the brake pad, in other words the bottom of the piston, constitutes the outer closing face 34 or the base of the socket 82, may undergo 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 34 or the base of the socket 82 more resistant to corrosion during the operation of the brake actuator mechanism 10. Alternatively, in the implementation of figures 1 to 4 The case hardening treatment of the nut can give the outer closing face 34 sufficient corrosion resistance.

[0048] 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.

[0049] According to another, unillustrated variant, the means of recirculation 40 are formed at the level of the nut 16 which features an external recirculation channel and recirculators, allowing external recirculation of the balls, i.e., within the thickness of the nut 16. The recirculation channel can be opened and closed during the assembly of the brake actuator mechanism. 10 by the cylindrical inner face 48 socket 42.

[0050] According to another variant not shown, the brake actuator mechanism 10may feature an annular bellows, comprising an annular base configured to fit into an annular chamber of the guide cylinder 84, and a bellows head 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 or in an annular groove formed on the solid-bottomed socket of the embodiment illustrated in figure 8 This annular bellows prevents contaminants from entering the guide cylinder. 84 by creating a primary seal.

[0051] According to another variant not illustrated, the nut 16 includes only one ring span 52 and the socket 42 includes only one shrink-fitting area 62.

[0052] According to another variant not shown, the brake actuator mechanism 10presents the annular recess 46 of the nut 16 and the secondary annular recess 46' socket 42. The intermediate annular space 56 is then generated by the annular recess 46 and the secondary annular recess 46'.

Claims

1. A piston (12) of an actuator mechanism, the piston (12) comprising - a nut (16) of a worm gear mechanism, defining a reference axis (100), an outer peripheral wall (32) and a nut thread (27) intended to cooperate, directly or via balls, with a screw of the worm gear 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) comprising a cylindrical guide zone (44) intended to come into close sliding contact with an inner guide wall of a guide cylinder (84) of the actuator mechanism (10); characterized in that the nut (16) comprises at least one annular shrink-fit bearing surface (52), the bushing (42) comprises at least one shrink-fit zone (62) shrink-fitted onto the annular shrink-fit bearing surface (52), the guide zone (44) having an outer diameter greater than that of the shrink-fit zone (62) and overlapping, without contact or shrink-fitting, a covered portion (80) of the outer peripheral wall (32) of the nut (16).

2. The piston (12) according to claim 1, characterized in that the guide zone (44) and the covered portion (80) of the outer peripheral wall (32) of the nut (16) are separated by an intermediate annular space (56) generated by: - an annular recess (46) in the covered portion (80) of the outer peripheral wall (32) of the nut (16) relative to the annular shrink-fit bearing surface (52); and / or - an inner diameter of the guide zone (44) greater than an inner diameter of the shrink-fit zone (62).

3. The piston (12) according to claim 2, characterized in that the intermediate annular space (56) has a length L1 measured parallel to the reference axis (100), and a depth P, measured radially between the outer peripheral wall (32) of the nut (16) and a cylindrical inner face (48) of the bushing guide zone (44) of the bushing (42), and the nut (16) has a nut length L2 measured along an axis parallel to the reference axis (100) between two ends of the nut (16), such that - L 1 > 20 P ; and / or - L 2 2 < L 1 < L 2 .

4. The piston (12) according to any of the preceding claims, characterized in that the nut comprises a second annular shrink-fit bearing surface (52), and the bushing (42) comprises a second shrink-fit zone (62) in contact with the second annular shrink-fit bearing surface (52) of the nut (16).

5. The piston (12) according to claim 4, characterized in that the two annular shrink-fit zones (62) are located at two opposite axial ends of the guide zone (44).

6. The piston (12) according to any of the preceding claims, characterized in that the bushing (42) has a shrink-fit end-of-stroke shoulder (50) turned in an axial assembly direction (210), bearing against an annular end face (36) of the nut (16).

7. The piston (12) according to any of the preceding claims, characterized in that the nut (16) comprises a crimping mortise (63) and the bushing (42) has a flap of crimping material (76) penetrating the crimping mortise (63) and bearing against a crimping shoulder (74) of the crimping mortise (63).

8. The piston (12) according to any of the preceding claims, characterized in that the nut (16) comprises a locking mortise (64) and the bushing (42) has a locking slot (66) open onto the locking mortise (64), the piston further comprising a slipper (92) inserted into the locking mortise (64) and projecting radially with respect to the guide zone (44).

9. The piston (12) according to any of the preceding claims, characterized in that the bushing (42) comprises a nitrogen-rich abrasion- and corrosion-resistant surface layer, and the nut (16) comprises a carbonrich hardened surface zone at least locally at the nut thread (27).

10. The piston (12) according to any of the preceding claims, characterized in that the piston has a piston crown.

11. A brake actuator mechanism (10), comprising: - a guide cylinder (84); - a ball screw mechanism, comprising a screw (14) and a nut (16) defining a reference axis (100) of the brake actuator mechanism (10), coaxial with the guide cylinder, and balls, the screw (14) having at least one screw thread (25) forming a raceway for the balls, the nut (16) having a nut thread (27) forming an outer helical raceway (30) for the balls 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 close sliding contact with an inner guide wall of the guide cylinder (84); characterized in that the bushing (42) and the nut (16) constitute a piston (12) according to any one of the preceding claims.