Piston of an actuator mechanism, comprising a nut and a bushing

Abstract

Piston (12) of an actuator mechanism, the piston (12) comprising a nut (16) of a worm gear mechanism, the nut (16) comprising an outer peripheral wall (32) and a nut thread (27) intended to cooperate, directly or via balls, with a worm gear mechanism screw; a sleeve (42) integral with the nut (16) and covering at least partially the outer peripheral wall (32) of the nut (16), the sleeve (42) comprising a cylindrical guide area (44) intended to come into fitted sliding contact with an inner guide wall of a guide cylinder (84) of the actuator mechanism (10L), the nut (16) comprising at least one annular shrink-fitting surface (52), the sleeve (42) comprising at least one shrink-fitting surface (62) shrink-fitted onto the annular shrink-fitting surface (52).The guiding zone (44) has an outer diameter greater than the shrink-fit zone (62) and covers, without contact or shrink-fitting, a covered portion (80) of the outer peripheral wall (32) of the nut (16). (Abstract figure: 2).

Description

Title of the invention: Piston of an actuator mechanism, comprising a nut and a bushing. TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to the field of actuators, in particular for the transport industry, especially automotive or aeronautical, in particular 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[0002] In the unpublished French patent application, bearing application number FR 2402914, a brake actuator mechanism is disclosed, comprising a screw, a nut shrink-fitted in a sleeve, and balls positioned between a helical thread of the screw and a helical thread of the nut. The nut and the sleeve form a piston sliding in a guide cylinder. The piston formed by the nut and the sleeve requires particularly precise dimensional control during manufacturing to ensure effective shrink-fitting without deformation of any of the piston elements during assembly of the sleeve onto the nut. Indeed, for this application, it is necessary that the dimensional tolerances of the piston be very tight to achieve virtually play-free and friction-free sliding of the piston within the cylinder. Description of the invention

[0003] The invention aims to remedy the drawbacks of the prior art and to propose a simple solution for producing a piston that meets tight dimensional tolerances.

[0004] 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 at least partially covering 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; notable 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 surface of shrink fitting, the guide area having an outside diameter greater than that of the shrink fitting area and covering without contact or without shrink fitting a covered portion of the outer peripheral wall of the nut.

[0005] The sleeve, which includes a shrink-fitted area around the nut, ensures the cohesion of the assembly corresponding to the 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, to give 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 expansion 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.

[0006] 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-fit bearing surface; and / or • an inner diameter of the guiding zone greater than an inner diameter of the shrink-fit zone.

[0007] 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 area of ​​the sleeve, and the nut has a nut length L2 measured along an axis parallel to the reference axis between two ends of the nut, such that • Ll>20P ; and / or • L2 / 2 <L1<L2.

[0008] 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 non-negligible annular volume due to its axial dimension. This makes it possible to create a substantial and uniform intermediate annular space over a portion of the nut. or of the sleeve along which no shrink-fitting force is reflected, in order to meet the need to maintain the external radial dimensions of the sleeve.

[0009] According to one embodiment, the nut comprises a second annular shrink-fitting surface, and the sleeve comprises a second shrink-fitting area in contact with the second annular shrink-fitting surface of the nut. Preferably, the two annular shrink-fitting areas are axially separated from each other, for example, located at two opposite axial ends of the guide area. In this way, the sleeve is shrink-fitted on two ends of the nut, which provides a stable and reliable connection, while having an intermediate, unshrink-fitted area to the nut that allows it to maintain its desired radial dimensions. Preferably, no more than two shrink-fitting areas are provided.

[0010] 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.

[0011] According to one embodiment, the nut includes a crimping mortise and the sleeve has a crimping material flap penetrating the crimping mortise bearing against a crimping shoulder of the crimping mortise, which makes it possible to lock the connection between the sleeve and the nut, and thus to reduce 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.

[0012] Preferably, the nut includes a locking mortise, and the sleeve has a locking slot open to the locking mortise. The piston further comprises a slide inserted into the locking mortise and projecting radially from the guide area. This mortise is intended to be inserted into a guide groove in the piston housing cylinder to prevent any rotation of the piston within the cylinder around the reference axis, while still allowing translational movement. Moreover, the slide provides an additional means of connection between the sleeve and the nut, reducing the risks associated with potential movement of the sleeve on the nut.

[0013] According to one embodiment, the sleeve 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 sleeve is more resistant to contact wear caused by friction against the guide cylinder when the piston is operating within the guide cylinder, thereby making the sleeve, and consequently the piston, more robust and durable.

[0014] According to one embodiment, the piston has a piston base formed by the sleeve or the 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.

[0015] 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 for the brake actuator mechanism, and balls, the screw having at least one thread forming a raceway for the balls, the nut having a thread forming an external helical raceway for the balls and an external peripheral wall; and • a sleeve attached to the nut and covering at least partially the outer peripheral wall of the nut, the sleeve coming into sliding contact adjusted with an inner guide wall of the guide cylinder, the sleeve and the nut constituting a piston as described above.

[0016] The piston preferably has a bottom, as mentioned previously, formed 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

[0017] Other features and advantages of the invention will become apparent from the following description, with reference to the accompanying figures, which illustrate: • [Fig.l]: 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 mode of the realization of [Fig.1]. • [Fig.3]: a detailed cross-sectional view of a first shrink-fitting zone of the nut according to the embodiment of [Fig.1]. • [Fig. 4]: A detailed cross-sectional view of a second shrink-fitting zone the nut according to the embodiment of [Fig.l]. • [Fig.5]: a detailed cross-sectional view of a part of the nut including a mortise according to the embodiment of [Fig.1]. • [Fig.6]: a detailed cross-sectional view of a first shrink-fit area 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-fitting area the nut according to the embodiment of [Fig.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.

[0018] For clarity, identical or similar elements are identified by identical reference signs throughout the figures. DETAILED description of implementation methods

[0019] Figures 1 to 5 illustrate a first embodiment of a brake actuator mechanism 10 comprising a fixed guide cylinder 84 and a piston 12 sliding in translation within the guide cylinder 84 along a reference axis 100, which is also a reference axis for the piston, to bear directly or indirectly against a brake pad (not shown). The piston 12 includes a sleeve 42 and a nut 16, the nut 16 being part of a ball screw mechanism comprising two threaded components, namely a screw 14 and the nut 16, and balls.

[0020] The guide cylinder 84 consists of a metal base, for example made of steel, and includes a cylindrical guide body 86, 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 to a second open annular end 90B, over a predetermined distance. The locking groove 88 is configured to accommodate a sliding slide 92, in order to lock the piston 12 against rotation relative to the guide cylinder 84, while allowing it translational movement within the guide cylinder 84. In addition, the guide body 86 may include a positioning flange 94.

[0021] The screw 14 is preferably metallic, for example made of steel such as 20MnCr5, 23MnB4, Scr420, 16MnCr5 or their equivalents according to other international or national standards, or high carbon steel such as 100Cr6, C50 or C56 or their equivalents, and may comprise a screw head 20 and a screw body 24, optionally connected by a connecting portion 22. The screw body 24 may have a diameter greater than the screw head 20. The screw head 20 is shaped to be rotationally fixed to an output shaft of an electric motor or geared motor, and may have, for example, a non-circular interface, for example with four, six or eight sides.

[0022] The screw body 24 has a screw thread 25 which forms an internal helical raceway around the reference axis 100 of the ball screw mechanism, the internal helical raceway being rotated radially in the opposite direction to the reference axis 100. In addition, the screw 14 has an open central cavity 28 allowing to lighten the whole of the brake actuator mechanism 10.

[0023] The nut 16 is made of steel, for example 20MnCr5, 23MnB4, Scr420, 16MnCr5 steel or their equivalents according to other international or national standards, or of high-carbon steel such as 100Cr6, C50 or C56 steel or their equivalents. The nut 16 has an overall cylindrical shape with its central axis being the reference axis 100. The nut 16 has a nut thread 27 which forms an external helical bearing race 30 around the reference axis 100, and is radially rotated towards the reference axis 100.

[0024] The nut 16 is of the closed type in that it has a nut seat 17, formed by an external closing face 34, which also forms a piston seat. The nut 16 has a generally cylindrical external peripheral face 32 extending from the external closing face 34 to an annular end face 36, over a length L2. The external peripheral face 32 has a crimping mortise 63, a locking mortise 64, and a covered portion 80. The crimping mortise 63 has a crimping shoulder 74, rotated in an axial assembly direction 210. The covered portion 80 has an annular indentation 46 radially inward. The crimping mortise 63 and the locking mortise 64 may be considered one and the same.The annular recess 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 guide zone 44 of the sleeve 42 described subsequently. The length L1 is greater than the depth P such that L > 20P, and L2 is greater than the length L1 such that L < 2P. annular recess 46 can be obtained during a molding of the nut 16 or by machining for example.

[0025] Furthermore, the outer peripheral face 32 of the nut 16 comprises two distinct annular shrink-fitting surfaces 52, arranged on either side of the annular recess 46. The two annular shrink-fitting surfaces 52 open axially onto the annular recess 46. Of the two annular shrink-fitting surfaces 52, a first annular shrink-fitting surface 52A is located near the outer closing face 34, while a second annular shrink-fitting surface 52B is located near the end annular face 36. The second annular shrink-fitting surface 52B is connected to the end annular face 36 by means of a chamfer 59. The two annular shrink-fitting surfaces 52 are configured to receive the sleeve 42 in tight contact.

[0026] Furthermore, the outer closing face 34 may have a recess 35, the base of the nut 16 being configured to make direct or indirect contact with the brake pad (not shown in the figures). The outer closing face 34 also has a flange 72, projecting radially from the outer peripheral wall 32 , which forms a flange shoulder 72'. The flange 72 also helps to limit any deformations of the outer closing face 34 under mechanical stresses when the brake actuator mechanism 10 is activated, for example.

[0027] One of the two threaded components, namely the screw 14 or the nut 16, may further be equipped with ball recirculation means 40, which may comprise one or more recirculators each passing through one or more threads of the threaded component, as illustrated in [Fig. 1], or pairs of recirculators arranged at the ends of a recirculation channel which spans one or more turns of the raceways of the screw 14 and the nut 16. In this embodiment, the ball recirculation is internal, i.e. comprising at least one recirculator each passing through one thread of the screw 14.

[0028] The balls can for example be made of steel or ceramic, and are dimensioned and positioned to circulate in a closed circuit between the outer helical raceway 30 of the nut 16 and the inner helical raceway 25 of the screw 14, as well as, where appropriate, by the recirculation means 40, preferably without separators between the balls.

[0029] The sleeve 42 is metallic, for example made of steel, such as 20MnCr5, 23MnB4, Scr420, 16MnCr5 steel or their equivalents according to other international or national standards, or of high-carbon steel such as 100Cr6, C50 or C56 steel or their equivalents. The sleeve 42 has a radially opposed cylindrical inner face 48 and a radially opposed cylindrical outer face 49, extending from a first end 57 of the sleeve 42 to a second end 58 of the sleeve 42. The sleeve 42 has a locking slot 66, such as 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.

[0030] The sleeve 42 has an inwardly facing crimping material flap 76, designed to bear against the crimping mortise 63, at the level of the crimping shoulder 74 of the crimping mortise 63. In addition, the sleeve 42 may optionally have a sleeve shoulder 50 located at the second end 58 of the sleeve 42, rotated in an axial assembly direction 210. The sleeve shoulder 50 rests axially on the annular end face 36 of the nut 16, opposite the bottom 17 of the nut 16. The crimping material flap 76 and the sleeve shoulder 50 have generally parallel bearing surfaces and, joined by the cylindrical inner face 48 of the sleeve 42, form a crimping groove 78. The crimping groove 78 is configured to receive in contact tightened at least part of nut 16.

[0031] Furthermore, the sleeve 42 has a guide zone 44 and two shrink-fit zones 62, namely a first shrink-fit zone 62A located near the first end 57 of the sleeve 42 and a second shrink-fit zone 62B located near the second end 58 of the sleeve 42. The guide zone 44 is designed, via its cylindrical outer face 49, to come into a sliding, close-fitting contact with the inner guide wall of the guide cylinder 84. The guide zone 44 is designed, via its cylindrical inner face 48, to cover, without shrink-fitting and preferably without contact, the covered portion 80 of the nut 16. Each shrink-fit zone 62 is designed to come into tight contact with the outer peripheral face 32 of the nut 16 at the two annular shrink-fit bearing surfaces 52.The guide zone 44 and the two shrink-fit zones 62 are optionally joined by chamfers to facilitate the insertion of the piston 12 into the guide cylinder 84.

[0032] More specifically, the first shrink-fit zone 62A is intended to come into tight contact with the first annular shrink-fit bearing surface 52A, while the second shrink-fit zone 62B is intended to come into tight contact with the second annular shrink-fit bearing surface 52B. The guide zone 44 has an outer diameter greater than the outer diameter of the first shrink-fit zone 62A in order to limit any deformations related to the assembly processes of the sleeve 42 onto the nut 16 in a diameter less than or equal to the diameter of the guide zone 44. The guide zone 44 has an outer diameter greater than or equal to the outer diameter of the second shrink-fit zone 62B since the second shrink-fit zone 62B is located near the second end 58 of the sleeve 42, including the sleeve shoulder 50, the curvature of which stiffens the sleeve 42 in this region of the sleeve 42.

[0033] The brake actuator mechanism also includes the slide 92, press-fitted into the locking mortise 64, projecting radially towards the guide cylinder 84 through the locking slot 66, relative to the outer face of the bushing 42.

[0034] When the piston 12 of the brake actuator mechanism 10 is assembled, the nut 16 is inserted forcefully into the sleeve 42 without risk of snagging thanks to the various chamfers it includes. Insertion occurs in the axial direction of assembly 210, until the annular end face 36 of the nut 16 abuts against the shoulder of the sleeve 50. The sleeve 42, via the two shrink-fit zones 62, is shrink-fitted onto the nut 16 at the two annular shrink-fit surfaces 52, thus forming a single-piece assembly composed of the nut 16 and the sleeve 42, namely the piston 12. The first shrink-fit zone 62A is shrink-fitted onto the first annular shrink-fit surface 52A, the second shrink-fit zone 62B is shrink-fitted onto the second annular shrink-fit surface 52B, and the guide zone 44 is positioned opposite, without contact with, the outer peripheral wall 32 of the nut 16 due to the annular recess 46. The wall outer peripheral 32 of the nut 16 and the cylindrical inner face 48, due to the annular recess 46, are then separated by an intermediate annular space 56.

[0035] The intermediate annular space 56 has the dimensions of the annular recess 46, such as its length or depth. The intermediate annular space 56 compensates for deformations of the sleeve 42 during its shrink-fitting to the nut, and / or of the nut 16 during any increase in the volume of the nut 16 caused by temperature variations when the piston 12 is operating in its operating environment. Thus, the diameter of the sleeve 42, and more specifically the outer diameter of the guide area 44, namely the outer face of the sleeve 49, is maintained at a relatively constant value, so as to preserve the desired properties during the production of the brake actuator mechanism 10.

[0036] The assembly is carried out with angular indexing such that: • the locking mortise 64 of the nut 16 and the locking slot 66 of the sleeve 42 are located opposite each other, and the locking slot 66 allows access to the locking mortise 64; and • the crimping mortise 63 of the nut 16 and the crimping material flap 76 of the sleeve 42 are made opposite each other.

[0037] 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 sleeve 42 and the nut 16. The slide 92 is then inserted into the locking mortise 64 of the nut 16 through the locking slot 66. When the crimping mortise 63 coincides with the locking mortise 64, the slide 92 can come into contact with the crimping material flap 76, allowing for further securing between the sleeve 42 and the nut 16.

[0038] The screw 14 is then inserted into the nut 16 of the piston 12, by a progressive helical movement.

[0039] The subassembly consisting of the screw 14 and the piston 12 equipped with the slide 92 is then inserted into the guide cylinder 84 in the axial direction of assembly 210. To do this, the locking slot 66 of the sleeve 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 slide 92 enters into the locking groove 88. The guide area 44 of the outer face of the sleeve 42 then enters into sliding contact in translation without rotation with the inner guide wall of the guide body 86.

[0040] The slide 92 inserted in the locking groove 88 has only one degree of freedom, within functional clearances, in translation parallel to the reference axis 100 in the locking groove 88. The slide 92 then locks the piston 12 in rotation around the reference axis 100, while allowing it a degree of freedom of translation parallel to the reference axis 100.

[0041] In operation, a rotational movement of the screw 14 around the reference axis 100, driven in rotation at the screw head 20 by a motor, generates a translational movement of the piston 12 in a direction which is a function of the direction of rotation of the screw 14.

[0042] According to a second embodiment, illustrated in 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 recess 46 in the nut 16. In addition, the sleeve has a secondary annular recess 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', having the same advantageous characteristics as when it is generated by the annular recess 46 in the nut 16.

[0043] According to a third embodiment, illustrated in [Fig.8], the 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 includes two annular end faces 36 of the nut 16. The piston then includes a piston base formed by the sleeve 42, and more particularly by a sleeve base 82. The nut is then inserted into the sleeve 42 in a second axial assembly direction 200, opposite in direction to the axial assembly direction 210 but in the same direction. The shoulder 50 of the sleeve 42, projecting axially relative to the annular end face 36 of the nut 16 opposite the bottom of the sleeve 82, if present, is then made by folding down material after assembly of the sleeve 42 on the nut 16. This shoulder 50 is however optional.

[0044] In all the embodiments described above, the bushing 42 can undergo, before being assembled onto the nut 16, a thermochemical treatment to resist abrasion and corrosion at a temperature Ts until a nitrogen-rich, abrasion- and corrosion-resistant surface layer is obtained. The treatment to obtain such a layer includes nitriding or nitrocarburizing, with the temperature Ts being between 300°C and 580°C. Nitriding and / or nitrocarburizing allow nitride to form on the surface when a part is placed in a treatment atmosphere very rich in nitrogen at temperature Ts, enabling the formation of a different surface material. Thanks to this treatment, the outer face 49 of the bushing 42 of the piston 12 resists abrasion and corrosion that could occur under operating conditions, when the piston 12 slides in translation within the guide cylinder 84.Nitrocarburizing does not change anything. the flatness of a surface, it is therefore possible to rectify the outer face 49 of the sleeve 42 before the application of the thermochemical treatment.

[0045] Similarly, the nut 16 can be subjected to a thermochemical hardening treatment including heating to a temperature Te 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 level of the nut thread 27 on the inner surface of the nut 16. Thanks to this treatment, the nut thread 27 has increased hardness both on the surface and throughout, making it more durable by resisting chipping, for example.However, carburizing alters the flatness of a workpiece surface; therefore, a grinding, hard turning, or hard milling step is necessary on the inner surface of nut 16 after the application of the thermochemical treatment.

[0046] Furthermore, the part in contact with the brake pad, i.e., the piston base, consisting of the outer closing face 34 or the sleeve base 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 outer closing face 34 or the sleeve base 82 more resistant to corrosion during the actuation of the brake actuator mechanism 10. Alternatively, in the embodiment shown in Figures 1 to 4, the case hardening treatment of the nut may provide the outer closing face 34 with sufficient corrosion resistance.

[0047] Naturally, the examples shown in the figures and discussed above are given by way of illustration only and are not intended to be limiting. It is explicitly intended that the different embodiments illustrated may be combined to propose others.

[0048] According to another variant not illustrated, the recirculation means 40 are constituted at the level of the nut 16 which has an external recirculation channel and recirculators, allowing the external recirculation of the balls, i.e. in the thickness of the nut 16. The recirculation channel can be opened, and closed during the assembly of the brake actuator mechanism by the cylindrical inner face 48 of the sleeve 42.

[0049] According to another variant not shown, the brake actuator mechanism 10 may have 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 flange shoulder 72' of the flange 72, and the sleeve 42, radially supported on the outer peripheral wall 32 or in an annular groove formed on the solid-bottomed sleeve of the embodiment illustrated in [Fig.8]. This annular bellows prevents the intrusion of contaminants into the guide cylinder 84 by providing a primary seal.

[0050] According to another variant not illustrated, the nut 16 comprises only one annular bearing surface 52 and the sleeve 42 comprises only one shrink-fit area 62.

[0051] According to another variant not illustrated, the brake actuator mechanism 10 has the annular recess 46 of the nut 16 and the secondary annular recess 46' of the sleeve 42. The intermediate annular space 56 is then generated by the annular recess 46 and the secondary annular recess 46'.

Claims

Demands

1. Piston (12) of an actuator mechanism, the piston (12) comprising - a worm gear mechanism nut (16), defining a reference axis (100), an outer peripheral wall (32) and a nut thread (27) intended to cooperate, directly or via balls, with a worm gear mechanism screw; - a sleeve (42) integral with the nut (16) and covering at least partially the outer peripheral wall (32) of the nut (16), the sleeve (42) comprising a cylindrical guide zone (44) intended to come into fitted 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-fitting surface (52), the sleeve (42) comprises at least one shrink-fitting area (62) shrink-fitted onto the annular shrink-fitting surface (52), the guide area (44) having an outside diameter greater than that of the shrink-fitting area (62) and covering without contact or shrink-fitting a covered portion (80) of the outer peripheral wall (32) of the nut (16).;

2. 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 indentation (46) of the covered portion (80) of the outer peripheral wall (32) of the nut (16) with respect to the annular shrink-fitting surface (52); and / or - an inner diameter of the guide zone (44) greater than an inner diameter of the shrink-fitting zone (62).

3. 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 guide zone (44) of the sleeve (42), and the nut (16) has a length nut L2 measured in an axis parallel to the reference axis (100) between two ends of the nut (16), such that - £1>2OP; and / or “ < L1 < El

4. Piston (12) according to any one of the preceding claims, characterized in that the nut comprises a second annular shrink-fitting surface (52), and the sleeve (42) comprises a second shrink-fitting area (62) in contact with the second annular shrink-fitting surface (52) of the nut (16).

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

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

8. Piston (12) according to any one of the preceding claims, characterized in that the nut (16) comprises a locking mortise (64) and the sleeve (42) has a locking slot (66) open on the locking mortise (64), the piston further comprising a slide (92) inserted in the locking mortise (64) and projecting radially from the guide area (44).

9. Piston (12) according to any one of the preceding claims, characterized in that the sleeve (42) comprises a nitrogen-rich abrasion and corrosion resistance surface layer, and the nut (16) comprises a carbon-rich hardened surface area at least locally at the nut thread (27).

10. Piston (12) according to any one of the preceding claims, characterized in that it has a piston bottom.

11. 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 external helical raceway (30) for the balls and an external peripheral wall (32); and - a sleeve (42) integral with the nut (16) and at least partially covering the external peripheral wall (32) of the nut (16), the sleeve (42) coming into a sliding, fitted contact with an internal guide wall of the guide cylinder (84); characterized in that the sleeve (42) and the nut (16) constitute a piston (12) according to any one of the preceding claims.

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