NON-RECIRCULATING BALL SCREW MECHANISM

Abstract

Ball screw mechanism (1), comprising a screw (10) defining a reference axis (100), the screw (10) comprising a helical thread forming an internal helical raceway (24) rotated radially outwards from the screw (10), opposite the reference axis (100); a nut (12) having a helical thread forming an external helical raceway (54) rotated radially inwards from the nut (12), in the direction of the reference axis (100); at least two balls (14) positioned so as to roll simultaneously on the internal and external raceways;and a first stop device (32) for the end of the stroke of the balls (14) and a second stop device (62) for the end of the stroke of the balls (14), the first stop device (32) and the second stop device (62) being axially opposed along the reference axis (100) and delimiting with the inner helical raceway (24) and the outer helical raceway (54) a helical volume for housing the balls (14), the second stop device (62) having a second fixing portion (68) fixed relative to the nut (12); the ball screw mechanism (1) being characterized in that the first stop device (32) has a first fixing portion (67) fixed relative to the screw (10). (Abstract figure: Figure 1);

Description

Title of the invention: NON-RECIRCULATING BALL SCREW MECHANISM TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to the field of ball screws, in particular applied to actuators, in particular to piston actuators, in particular for the transport industry, in particular automotive or aeronautical, and more particularly, although not exclusively, intended for driving brake calipers in braking mechanisms. PREVIOUS STATE OF THE ART

[0002] A recirculating ball screw mechanism is known to those skilled in the art. Such a mechanism comprises a threaded nut, a threaded screw, and balls, the threads of the screw and the nut each forming a helical raceway. This mechanism has a recirculation system allowing the balls to circulate in a closed circuit, partly on the raceways and partly on one or more recirculation paths, ensuring smooth and continuous motion of the mechanism without interruption.

[0003] The transition between the portion of the ball trajectory following the bearing races and the portion of the ball trajectory following the recirculation path is, however, complex to control and can cause premature wear of the balls or the ball guide walls. Furthermore, machining the recirculation paths or the recirculator housings is itself complex. In addition, the recirculation path is an "inactive" zone since the balls located there do not contribute to guiding and transmitting forces between the screw and the nut, resulting in a loss of efficiency in the mechanism.

[0004] Document CN 111022598 A shows a ball screw mechanism comprising a threaded nut, a threaded screw, and balls, the threads of the screw and the nut each forming a helical raceway. This ball screw mechanism does not recirculate the balls, meaning that the balls remain at all times on the helical raceways of the screw and the nut. To achieve this, the ball screw mechanism includes a spring at each end of the nut's raceway, so as to dampen the balls when they reach the end of their travel on said raceway.

[0005] The behavior of this mechanism is identical in both directions of rotation of the screw: the balls progress along the bearing path of the nut in the same axial direction as the screw until they reach the limit switch spring against which they reach their limit. Continued rotation of the screw causes the balls to slip, compressing the spring until a balance of forces is reached. In this phase, where the balls are no longer rolling, the efficiency of the ball screw mechanism drops sharply. Thus, the absence of ball recirculation is achieved at the cost of low efficiency in both directions of rotation, as soon as the mechanism leaves the very narrow axial travel range in which the balls are not in contact with either of the end springs. Furthermore, operating ranges in contact with one of the end springs are accompanied by wear due to friction of the balls against the raceways, which can affect service life. To obtain a sufficiently high-efficiency operating range, it becomes necessary to increase the distance between the two end springs, and therefore to increase the axial dimension of the nut, at the expense of compactness.

[0006] Finally, this symmetrical mechanism proves to be poorly suited to an application in which good efficiency is desired in at least one direction of operation and over a large axial travel operating range, even if the efficiency deteriorates when one leaves this operating range in one of the directions of movement, for example to reach a maintenance position. Description of the invention

[0007] The invention aims to remedy at least some of the disadvantages of the prior art mentioned above and to propose a ball screw mechanism whose performance in terms of efficiency and lifespan is satisfactory, while avoiding complex machining.

[0008] To achieve this, according to a first aspect of the invention, a ball screw mechanism is proposed, comprising: • a screw defining a reference axis, the screw comprising a helical thread forming an internal helical bearing race turned radially outwards from the screw, opposite the reference axis; • a nut having a helical thread forming an external helical bearing raceway rotated radially towards the inside of the nut, in the direction of the reference axis; and • at least two balls positioned so as to roll simultaneously on the inner and outer raceways; • a first ball endstop device and a second ball endstop device, the first and second endstop devices being axially opposed along the reference axis and defining, with the inner and outer helical raceways, a helical housing volume balls, the second stop device having a second fixed portion relative to the nut; the ball screw mechanism is remarkable in that the first stop device has a first fixed portion relative to the screw.

[0009] Since the two thrust bearings are each linked to one of the two threaded elements of the ball screw mechanism, they move closer to and further from each other depending on the relative direction of rotation between the screw and the nut. The helical volume of the ball housing is therefore variable. As long as the balls remain at a distance from the two thrust bearings, their rolling on the raceways is unimpeded, and the efficiency of the mechanism is maximized. When the thrust bearings move closer to each other, the balls come into contact with the thrust bearings, and the efficiency decreases rapidly, but this mode of operation is only encountered in one direction of rotation and over a short range.The proposed mechanism is therefore particularly suited to an application in which good efficiency is desired over a fairly wide range of axial travel, and lower efficiency is acceptable when moving outside this range in a predetermined direction, for example to reach a maintenance position.

[0010] According to one embodiment, the first stop device has a first contact end with a first end ball among at least two balls, the first contact end being movable relative to the first fastening portion, depending on a bearing force exerted by the first end ball on the first contact end. The mobility of the first contact end makes it possible to define a range of end-stroke travel that extends the operational range and within which relative movement between the nut and the screw is still possible, at the cost of reduced efficiency. Preferably, the first contact end includes a first contact means for coming into contact with the balls, a pad for example, so that friction between the balls and the first stop device is reduced, thereby improving the service life of the balls and the first stop device.This pad may, for example, have a concave face in the form of a spherical cap with a radius very slightly greater than that of the first end ball, and may, if necessary, be made of a self-lubricating material.

[0011] Preferably, the first stop device includes a first spring between the first fastening portion and the first contact end. Thanks to this first spring, the first stop device provides increasing resistance to the balls as they move in its direction, within the end-of-stroke range. In this way, the balls are gradually slowed, preventing impacts between the balls and the walls of the nut and / or the screw. The entire mechanism thus has a longer service life.

[0012] The spring preferably has a generally helical shape in the sense that it has a neutral line that forms a helix around the screw and inside the nut. This helix does not necessarily have a constant pitch, and deforms as the mechanism progresses through the end-of-stroke travel range.

[0013] According to one embodiment, the first spring is of the helical compression type and / or an elastic spongy body. The spring may therefore be a helical spring wound around a helical neutral line. The spring may also consist of an elastic body, such as foam, forming a solid coil, wound around a helical neutral line. A combination of the two types of spring is also envisaged. In addition, the spring may be of another nature, such as a cylinder.

[0014] According to one embodiment, the first fastening portion comprises a first retaining means for the first spring, fixed relative to the screw and capable of retaining the first spring in the inner helical thread when the first spring is not aligned with the outer helical thread. This retaining means may have different shapes depending on the chosen graduated stop. This retaining means prevents a portion of the first stop device from exiting the helical bearing path of the screw when said first stop device includes a flexible part such as the helical spring. The retaining means is therefore preferably rigid, enabling it to guide the elastic spring.Preferably, the first retaining means comprises a helical rod, which follows a constant, non-zero distance from the bottom of the inner helical raceway, and / or a helical sleeve, which follows a constant distance from the bottom of the inner helical raceway. The helical rod allows a helical spring to wind around it while being held within the associated raceway. Only a portion of the spring is not wound around the rod; this portion is designed to come into contact with the balls, allowing the spring to be compressed under their stress. The sleeve, like the rod, retains the spring within the associated raceway. Furthermore, the sleeve can allow the spring to be an elastic sponge, such as foam.Such a foam spring allows for the storage of lubricant within it, this lubricant being diffused into the mechanism either as the balls compress the spring, or when the mechanism is under stress.

[0015] According to one embodiment, the second stop device has a second contact end with a second end ball among the at least two balls, the second contact end being movable relative to the second fixing portion, depending on a bearing force exerted by the second end ball on the second contact end. The mobility of the second contact end This allows defining a limit switch travel range that extends the operational range and within which relative movement between the nut and screw is still possible, albeit at the cost of reduced efficiency. Preferably, the second moving contact end is combined with the first moving contact end discussed previously, the limit switch travel range then being delimited by two moving contact ends.

[0016] Preferably, the second contact end comprises a second contact means for contacting the balls, for example a pad, so that friction between the balls and the second stop device is reduced, thereby improving the service life of the balls and the second stop device. This second stop device may be in the form of a pad which may, for example, have a concave face in the form of a spherical cap with a radius very slightly larger than that of the second end ball, and may optionally be made of a self-lubricating material.

[0017] Preferably, the second stop device includes a second spring between the second mounting portion and the second contact end. Thanks to this second spring, the second stop device provides increasing resistance to the balls as they move in its direction, within the end-of-stroke range. In this way, the balls are gradually slowed, preventing impacts between the balls and the walls of the nut and / or the screw. The entire mechanism thus has a longer service life.

[0018] According to one embodiment, the second spring is of the helical compression spring type and / or an elastic spongy body.

[0019] In one embodiment, the second fastening portion has a second means for retaining the second spring, fixed relative to the nut and capable of holding the second spring in the outer helical thread, particularly when the second spring is not aligned with the inner helical thread. Since the second spring tends to expand and press itself against the flanks of the nut's helical thread, the second retaining means may be simpler than the first. Thus, in one embodiment, the second retaining means comprises a hook and / or a housing cavity for one end of the spring, formed in the nut.

[0020] According to one embodiment, it is configured to allow the balls to roll on a helical portion of the two raceways without said balls being in contact with either of the two stop devices. On this helical portion of the two raceways, corresponding to the functional range of axial travel, the efficiency of the mechanism is maximized. Preferably, the range functional travel without contact of the balls with either of the stop devices is greater than 10mm.

[0021] According to another aspect of the invention, a brake actuator mechanism comprising a ball screw mechanism as described above. Such a mechanism then benefits from all the advantages of the ball screw mechanism as described above.

[0022] Furthermore, according to another aspect of the invention, it relates to a ball screw mechanism, comprising: • a screw defining a reference axis, the screw comprising a helical thread forming an internal helical bearing race rotated radially in the opposite direction to the reference axis; • a nut having a helical thread forming an external helical raceway rotated radially towards the reference axis; and • at least two balls positioned so as to roll simultaneously on the inner and outer raceways; • a first ball stop device and a second ball stop device, the first stop device and the second stop device being axially opposed along the reference axis and delimiting with the inner helical raceway and the outer helical raceway a helical volume for housing the balls, the first stop device having a first fixed portion relative to the screw or nut, the second stop device having a second fixed portion relative to the screw or nut, the ball screw mechanism being notable in that the first stop device has a foam portion bearing against, or integral with, the first fixed portion and / or the second stop device has a foam portion bearing against, or integral with, the second fixed portion.

[0023] According to one embodiment, the first fastening portion is integral with the screw and the second fastening portion is integral with the nut, or vice versa, and the mechanism then conforms to the first aspect of the invention. According to another embodiment, the first fastening portion and the second fastening portion are both integral with the nut, or both are integral with the screw.

[0024] The term "foam" here refers to what has been described above as an elastic spongy body. In one embodiment, the foam portion is elastically deformable, so as to form a spring. This foam portion may or may not interface directly with the balls. Preferably, the foam portion comprises a spongy body configured to form a reservoir for liquid or paste lubricant, configured to lubricate the balls and the raceways of the mechanism. Thus, when the foam is compressed, the reservoir it forms is designed to release lubricant into the bearing raceway, thereby reducing friction when the mechanism is under stress. If necessary, the lubricant can also be reabsorbed by the foam portion when it expands.

[0025] According to one embodiment, the ball screw mechanism includes a retaining means configured to hold the foam in the associated helical raceway, preferably the retaining means comprising a waterproof sleeve and / or a rod. Preferably, the retaining means extends over a portion of either of the two raceways, at a constant distance from a bottom of the associated raceway. BRIEF DESCRIPTION OF THE FIGURES

[0026] Other features and advantages of the invention will become apparent from the following description, with reference to the attached figures.

[0027] [Fig.1] Fig.1 illustrates a non-recirculating ball screw mechanism, according to a first embodiment, in a loosened position.

[0028] [Fig.2] Fig.2 illustrates the mechanism of the ball screw, according to the first mode of realization, in a tight position.

[0029] [Fig.3] Fig.3 illustrates the mechanism of the ball screw, according to the first mode of realization, in an isometric view.

[0030] [Fig.4] Figure 4 illustrates a variant of the first embodiment, comprising means of contact.

[0031] [Fig.5] Fig.5 illustrates a detail of the previous figure.

[0032] [Fig.6] Fig.6 illustrates the mechanism, in an exploded view, according to another method of construction comprising two foam springs, one of which has a central light.

[0033] [Fig.7] Fig.7 illustrates the mechanism, in an exploded view, according to another method of construction comprising a foam spring and a metal spring.

[0034] [Fig.8] Fig.8 illustrates the mechanism, in an exploded view, according to another mode of construction including a retaining sleeve for the springs.

[0035] For clarity, identical or similar elements are identified by identical reference signs throughout the figures. DETAILED description of implementation methods

[0036] Figure 1 illustrates a first embodiment of a ball screw mechanism 1 for a brake actuator mechanism intended for driving brake calipers in braking mechanisms. The ball screw mechanism 1 comprises two threaded components forming a linear actuator mechanism, namely a screw 10 and a nut 12, aligned on a reference axis 100 of the ball screw mechanism 1, which is also a reference axis 100 of the nut 12 and the screw 10, and the balls 14.

[0037] The screw 10 is preferably metallic, for example made of steel, and has a screw head 16, a connecting portion 18, and a screw body 20. The screw body 20 has a diameter larger than the screw head 16, the connecting portion 18 providing the connection between the screw body 20 and the screw head 16. This connecting portion 18 may be frustoconical. The screw head 16 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, or a cavity for housing a key for connecting to the output shaft. The screw body 20 has a screw thread 22 which forms an internal helical raceway 24 around the reference axis 100, the internal helical raceway 24 being rotated radially in the opposite direction to the reference axis 100.The internal helical raceway 24 has a bottom 26 and two opposing screw flanks 28 formed by the screw thread 22. The internal helical raceway 24 extends between two end portions of the screw body 20.

[0038] The internal helical raceway 24 has, at a first end 30 located in the end portion of the screw body 20 closest to the connecting portion 18, a first stop device 32 for the end of travel of the balls 14. The first stop device 32 is intended to axially close the internal helical raceway 24 and retain the balls 14 in the ball screw mechanism 1, and more particularly in the internal helical raceway 24. The first stop device 32 extends from a first fixing portion 67 to a first contact end 36 configured to come into contact with a first end ball 14A. The first stop device 32 is gradual, so that it opposes more resistance to the balls 14 as they roll in the direction of the end portion of the screw body 20 closest to the connecting portion 18.More specifically, the first stop device 32 comprises a first spring 34 and a first retaining means 40.

[0039] The first spring 34 is a mechanical component made from elastic materials, for example steel, designed to return to its original shape after being deformed by the balls 14. The first spring 34 is configured to absorb the shocks of the balls 14 at the end of their travel at the end portion of the screw body 20 closest to the connecting portion 18. The first spring 34 is made of, preferably composed of, metal, preferably steel, and has a rigidity that determines the force required to deform said first spring 34. The first spring 34 is helical and of the "compression" type, that is to say, it is coil-shaped and is compressed under the load of the balls 14 in the path of internal helical bearing 24 and that it recovers its shape when the load of the balls 14 is less than the stiffness of the first spring 34. The first spring 34 extends between the first contact end 36 and a first retaining portion 38 linked to the first retaining means 40 of the first stop device 32. The first spring 34 is further held in the internal helical raceway 24 by this first retaining means 40.

[0040] Here, the retaining means 40 is a generally helical rod 40', which runs along the bottom 26 of the inner helical raceway 24 without contact, and which is fixedly connected to the screw 10, and more particularly to the connecting portion 18 of the screw 10. The rod 40' can be fixed in a fixing recess in the connecting portion 18 of the screw 10, provided for this purpose, for example a blind hole 42, which can be machined. The connecting portion 18 further includes a groove 44 configured to allow the rod 40' to access the inner helical raceway 24 from the fixing recess. The first spring 34 is held in the inner helical raceway 24 by the rod 40', the latter extending along a part of the raceway from the inside of said first spring 34.Without this retaining means 40, the spring, given its elastic properties, would escape from the ball screw mechanism 1 as soon as the internal helical race 24 is no longer radially aligned with the nut 12, for example, when the ball screw mechanism 1 is in a loosened position. The first spring 34 is fixedly connected to the rod 40' by fitting said first spring 34 onto the rod 40'. In other words, the first spring 34 comprises a fixed part, held in place by the retaining means 40, which is the rod 40'.

[0041] The nut 12 comprises a nut body 46, preferably metallic, for example made of steel, and generally cylindrical in shape. The nut body 46 has an inner annular face 48, facing the reference axis 100, and an outer annular face 50, facing away from the reference axis 100. The nut 12 may have a closed or open bottom 52, arranged axially opposite the screw head 16. Since the ball screw mechanism 1 serves as a piston in the brake actuator mechanism, if the bottom 52 of the nut 12 is open, the outer annular face 50 can then accommodate a closed-bottom sleeve so as to accommodate a brake caliper. In any embodiment, the outer annular face 50 can accommodate a sleeve, for example, to provide the piston with one or more properties that the nut 12 cannot possess, such as corrosion resistance.For example, the outer annular surface 50 of the nut 12 and the sleeve may be as described in applications FR 2402914 or FR 2404264.

[0042] The inner annular face 48 has a nut thread 53 extending between two end portions of the nut 12. The nut thread 53 forms a bearing race The external helical raceway 54 is wound about the reference axis 100 and rotated radially inwards. The external helical raceway 54 has a raceway bottom 56 and two opposing raceway sides 58 formed by the nut thread 53. The external helical raceway 54 is obtained, for example, by continuous turning between two ends of the external helical raceway 54.

[0043] The external helical raceway 54 has, at a second end 60, intended to be furthest from the first end 30 of the ball 14's travel in a mechanism 1 operating position, a second stop device 62. The second stop device 62 is intended to axially close the external helical raceway 54 and retain the balls 14 in the ball screw mechanism 1, and more particularly in the external helical raceway 54. The second stop device 62 extends from a second mounting portion 68 to a second contact end 66 configured to come into contact with a second end ball 14B. The second stop device 62 is graduated, so that it offers more resistance to the balls 14 as they roll in the direction of the second end 60.More specifically, the second stop device 62 includes a second spring 64 and a second retaining means 72.

[0044] The second spring 64 is configured to absorb the shocks of the balls 14 at the end of their travel at the second end 66. The second spring 64 has the same characteristics as the first spring 34, that is to say that it is made of steel and is coil-shaped.

[0045] The second spring 64 extends between the second contact end 66, and a second retaining portion 69 linked to the second retaining means 72. The second retaining means 72 is preferably located in a wall of the inner annular face 48 of the nut 12, at the level of the end of the outer helical raceway 54 associated with the second end 60. The second retaining means 72 can be formed by a hook for example, or a counterform of the second fixing portion 68 into which said second fixing portion 68 is press-fitted, the connection being made by shrink fitting.

[0046] In addition, the ball screw mechanism 1 includes a lubricant to ensure the proper functioning and longevity of the system. The lubricant may be, for example, grease.

[0047] When the ball screw mechanism 1 is assembled, the inner helical raceways 24 and outer helical raceways 54 define a helical raceway of the mechanism. The helical raceway of the mechanism therefore extends between the first end 30 of the end of the stroke and the second end 60 of the end of the stroke.

[0048] When the ball screw mechanism 1 is in a so-called "loose" position (illustrated in [Fig.1]), i.e. the position in which the screw 10 is least engaged in the nut 12, the first end ball 14A and the second end ball 14B are respectively not in contact with the spring 34 of the first stop device 32 and the spring 64 of the second stop device 62.

[0049] As the ball screw mechanism 1 approaches a so-called "tightened" position (illustrated in [Fig. 2]), that is, the position in which the screw 10 is fully engaged in the nut 12, all the balls 14, namely the first 14A, the second 14B, and those between the first 14A and the second 14B, move freely. When the balls 14 are free-moving, they roll, thereby reducing, among other things, the friction between the screw 10 and the nut 12. The first stop device 32 and the second stop device 62 move closer together, and the second end ball 14B comes into contact with the spring 64 of the second stop device 62. Very quickly, the first spring 34 comes into contact with the first end ball 14A. The balls 14 then continue to advance, as the screw 10 sinks into the nut 12, in a combination of sliding and rolling, in a constrained stroke by the two springs 34, 64.

[0050] The efficiency of the ball screw mechanism 1 is optimal when all the balls 14 roll only (do not slip). The mechanism 1 is therefore dimensioned so that, during operation, the balls 14 are continuously in free motion. The dimensioning includes the length of the inner and outer helical raceways, as well as the length of each of the two thrust bearings and the number of balls 14 in the mechanism 1. The ball screw mechanism then moves between the loosened position and an intermediate position.

[0051] The balls 14 are intended to move in constrained travel only when the ballast mechanism is placed in a position considered extreme, when it is necessary to make modifications to it, for example modify the brake pads or the brake caliper.

[0052] According to a variant of the first embodiment, illustrated in Figures 4 and 5, the first stop device 32 has a first contact means 80 and the second stop device 62 has a second contact means 82. These contact means 80, 82 are configured to reduce friction between the balls 14 and the contact ends 36, 66, and thus premature wear of the balls 14 and / or the stop devices 32, 62. The contact means 80, 82 are identical and fixedly connected to the associated contact end by means of a pin 84, for example. They have a concave receiving surface 83, positioned opposite the balls, so as to distribute the force during impact between the balls and the receiving surface 83.

[0053] According to other embodiments, illustrated in Figures 6, 7, and 8, the first spring 34 and the second spring 64 are elastic spongy bodies, preferably cylindrical with a circular base, such as foam coils 70 ([Fig. 6]). The foam springs 34, 64 may be solid or have a central opening, so as to allow the passage of a retaining means in the form of a rod 40', for example ([Fig. 7]). However, if the first spring 34 is a solid cylinder, the first retaining means 40 is then a sheath 90 in which a portion of the first foam spring 34 may or may not slide. In other words, the foam can retract by retracting the 90 sleeve. The 90 sleeve can also be used when the spring is helical and made of metal, so as to keep said spring in the associated raceway while reducing friction between the spring and the raceways.The retaining means can also consist of the 90 mm sleeve in combination with the 40 mm rod ([Fig. 8]). This helps to extend the service life of the bearing races and the spring.

[0054] Furthermore, the second spring 64 can be made of foam independently of the first spring 34. In other words, the second spring 64 can be made of foam while the first spring 34 is, for example, a steel helical spring or a foam cylinder, and vice versa. Such a foam spring allows, for example, a portion of the lubricant contained within the mechanism 1 to be stored. Consequently, the lubricant can then be released by the foam spring when it is compressed, for example, when the mechanism is forced into a closed position.

[0055] 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 can be combined to propose others.

[0056] According to an unillustrated variant, the first stop device 32 comprises a helical cylinder, the first spring 34 being the piston of said cylinder. The same applies to the second stop device 62.

[0057] According to another variant not shown, the connecting portion 18 does not include the groove 44. In this case, the blind hole 42 into which the rod 40' is fixed is directly present in the inner helical raceway. This reduces the number of machining operations required for the parts composing the ball screw mechanism 1, thereby reducing the production cost of said mechanism 1.

[0058] It should be noted that one or more foam cushions can also be implemented in a configuration in which the first stop device 32 has a first fixing portion 67 fixed relative to the nut 10, or in which both fixing portions 67, 68 are fixed relative to the screw 12.

Claims

Demands

1. Ball screw mechanism (1), comprising: - a screw (10) defining a reference axis (100), the screw (10) comprising a helical thread forming an internal helical raceway (24) rotated radially outwards from the screw (10), opposite the reference axis (100); - a nut (12) having a helical thread forming an external helical raceway (54) rotated radially inwards from the nut (12), in the direction of the reference axis (100); and - at least two balls (14) positioned so as to roll simultaneously on the internal and external raceways;- a first stop device (32) for the end of the stroke of the balls (14) and a second stop device (62) for the end of the stroke of the balls (14), the first stop device (32) and the second stop device (62) being axially opposed along the reference axis (100) and delimiting with the inner helical raceway (24) and the outer helical raceway (54) a helical volume for housing the balls (14), the second stop device (62) having a second fixing portion (68) fixed relative to the nut (12); the ball screw mechanism (1) being characterized in that the first stop device (32) has a first fixing portion (67) fixed relative to the screw (10).;

2. Ball screw mechanism (1) according to claim 1, characterized in that the first stop device (32) has a first contact end (36) with a first end ball (14A) among the at least two balls (14), the first contact end (36) being movable relative to the first fixing portion (67), as a function of a bearing force exerted by the first end ball (14A) on the first contact end (36).

3. Ball screw mechanism (1) according to claim 2, characterized in that the first contact end (36) comprises a first contact means (80) intended to come into contact with the balls (14), a skate for example.

4. Ball screw mechanism (1) according to claim 3, characterized in that the first stop device (32) comprises a first spring (34) between the first fixing portion (67) and the first contact end (36).

5. Ball screw mechanism (1) according to claim 4, characterized in that the first spring (34) is of the helical compression spring type and / or an elastic spongy body.

6. Ball screw mechanism (1) according to claim 5, characterized in that the first fastening portion (67) comprises a first retaining means (40) for the first spring (34), fixed relative to the screw (10) and capable of retaining the first spring (34) in the inner helical thread when the first spring (34) is not opposite the outer helical thread.

7. Ball screw mechanism (1) according to claim 6, characterized in that the first retaining means (40) comprises a helical rod (40') which follows at a constant and non-zero distance a bottom of the inner helical raceway (24) and / or a helical sleeve which follows at a constant distance a bottom of the inner helical raceway (24).

8. Ball screw mechanism (1) according to any one of claims 1 to 7, characterized in that the second stop device (62) has a second contact end (66) with a second end ball among the at least two balls (14), the second contact end (66) being movable relative to the second fixing portion (68), as a function of a bearing force exerted by the second end ball on the second contact end (66).

9. Ball screw mechanism (1) according to claim 8, characterized in that the second contact end (66) comprises a second contact means (82) intended to come into contact with the balls (14), a pad for example.

10. Ball screw mechanism (1) according to claim 9, characterized in that the second stop device (62) comprises a second spring (64) between the second fixing portion (68) and the second contact end (66).

11. Ball screw mechanism (1) according to claim 10, characterized in that the second spring (64) is of the helical compression spring type and / or an elastic spongy body.

12. Ball screw mechanism (1) according to claim 11, characterized in that the second fixing portion (68) has a second retaining means (72) for the second spring (64), fixed relative to the nut (12) and suitable for retaining the second spring (64) in the external helical thread.

13. Ball screw mechanism (1) according to claim 12 or 13, characterized in that the second retaining means (72) comprises a hook and / or a housing cavity for one end of the spring, formed in the nut.

14. Ball screw mechanism (1) according to any one of the preceding claims, characterized in that it is configured to allow the balls (14) to roll on a helical portion of the two raceways without said balls being in contact with either of the two stop devices.

15. Brake actuator mechanism characterized in that it comprises a ball screw mechanism (1) according to any one of the preceding claims.

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