Non-recirculating ball screw mechanism

Abstract

A ball screw mechanism (1) has a screw (10) forming an inner helical raceway (24); a nut (12) forming an outer helical raceway (54), at least two balls (14) positioned so as to roll simultaneously on the inner and outer raceways, and also a first end-of-travel stop device (32) for the balls (14) and a second end-of-travel stop device (62) for the balls (14), these end-of-travel stop devices being axially opposite 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) has a second fastening portion (68) that is fixed relative to the nut (12), and the first stop device (32) has a first fastening portion (67) that is fixed relative to the screw (10).

Description

ball screw mechanism without recirculation Technical field of the invention

[0001] The invention relates to the field of ball screws, in particular applied to actuators, especially piston actuators, particularly for the transport industry, especially automotive or aeronautical, and more particularly, although not exclusively, intended for driving brake calipers in braking mechanisms. Prior 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 nut each forming a helical raceway. This mechanism features a recirculation system that allows the balls to circulate in a closed loop, partly on the raceways and partly on one or more recirculation paths, ensuring smooth and continuous motion.

[0003] The transition between the ball trajectory along the bearing races and the recirculation path is complex to control and can lead to 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 within it 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 describes a ball screw mechanism comprising a threaded nut, a threaded screw, and balls, with the screw thread and nut thread each forming a helical raceway. This ball screw mechanism is non-recirculating, meaning the balls remain on the helical raceways of the screw and nut at all times. To achieve this, the ball screw mechanism includes a spring at each end of the nut's raceway to cushion the balls as they reach the end of their travel on that raceway. Similar mechanisms are disclosed in documents DE102007042654A1 and US2021062899A1.

[0005] The behavior of this mechanism is identical in both directions of screw rotation: the balls progress along the nut's bearing path in the same axial direction as the screw until they reach the end spring, against which they come to a stop. Continued screw rotation causes the balls to slide, compressing the spring until a balance of forces is reached. During this phase, when 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 exceeds the very narrow range of axial travel within which the balls are not in contact with either of the end springs.Furthermore, the 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, thus increasing 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 it means deteriorating efficiency when moving out of this operating range in one direction of travel, for example to reach a maintenance position.

[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 including a helical thread forming an internal helical raceway rotated radially outwards from the screw, opposite the reference axis; a nut having a helical thread forming an external helical raceway rotated radially inwards from the nut, in the direction of the reference axis; and at least two balls positioned so as to roll simultaneously on the internal and external 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 second stop device having a second fixed portion relative to the nut; the ball screw mechanism being 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 separate from the two thrust bearings, their movement on the raceways is unimpeded, and the mechanism's efficiency is maximized. When the thrust bearings move closer together, the balls come into contact with them, and the efficiency drops rapidly. However, 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 axial travel operating range, and less good efficiency is accepted when moving out of this operating range in a predetermined direction, for example to reach a maintenance position.

[0010] In one embodiment, the first stop device has a first contact end with a first end ball among at least two balls. This first contact end is movable relative to the first mounting portion, depending on a bearing force exerted by the first end ball on the first contact end. The mobility of the first contact end defines a range of end-stroke travel that extends the operating range and within which relative movement between the nut and the screw is still possible, albeit at the cost of reduced efficiency. Preferably, the first contact end includes a first contact means for contacting the balls, such as a pad, so that friction between the balls and the first stop device is reduced, thereby improving the service life of both 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 mounting 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 screw. This extends the service life of the entire mechanism.

[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 its end-of-stroke range.

[0013] In one embodiment, the first spring is a helical compression spring and / or an elastic spongy body. The spring can therefore be a helical spring wound around a helical neutral axis. The spring can also consist of an elastic body, such as foam, forming a solid coil, wound around a helical neutral axis. A combination of the two types of spring is also considered. Furthermore, the spring can be of another nature, such as a hydraulic cylinder.

[0014] In one embodiment, the first fastening portion includes a first retaining means for the first spring, fixed relative to the screw and capable of holding the first spring within the inner helical thread when the first spring is not aligned with the outer helical thread. This retaining means can have different shapes depending on the chosen graduated stop. This retaining means prevents any part 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, allowing 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 able to be diffused into the mechanism either as the balls compress the spring, or when the mechanism is under stress.

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

[0016] Preferably, the second contact end includes a second contact means for contacting the balls, such as a pad, so that friction between the balls and the second thrust bearing is reduced, thereby improving the service life of both the balls and the second thrust bearing. This second thrust bearing can 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 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 screw. This extends the service life of the entire mechanism.

[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 can 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] In one embodiment, it is configured to allow the balls to roll on a helical portion of the two raceways without the balls coming into contact with either of the two thrust bearings. 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 functional range of travel without contact of the balls with either of the thrust bearings is greater than 10 mm.

[0021] According to another aspect of the invention, a brake actuator mechanism includes 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 including a helical thread forming an internal helical raceway rotated radially opposite 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 internal and external 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] In 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. In 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 was described above as an elastic spongy material. In one embodiment, the foam portion is elastically deformable, forming a spring. This foam portion may or may not interface directly with the balls. Preferably, the foam portion comprises a spongy material configured to form a reservoir for liquid or paste lubricant, designed to lubricate the balls and raceways of the mechanism. Thus, when the foam is compressed, the reservoir it forms is configured to release the lubricant into the 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] In 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] Laillustrates a non-recirculating ball screw mechanism, according to a first embodiment, in a loosened position.

[0028] Laillustrates the mechanism of the ball screw, according to the first embodiment, in a tightened position.

[0029] Laillustrates the mechanism of the ball screw, according to the first embodiment, in an isometric view.

[0030] Laillustre illustrates a variant of the first embodiment, including means of contact.

[0031] This illustrates a detail of the previous figure.

[0032] Laillustre the mechanism, in an exploded view, according to another embodiment comprising two foam springs, one of which has a central light.

[0033] Laillustrates the mechanism, in an exploded view, according to another embodiment comprising a foam spring and a metal spring.

[0034] Laillustre the mechanism, in an exploded view, according to another embodiment including a retaining sheath for the springs.

[0035] For clarity, identical or similar elements are identified by identical reference symbols across all figures. Detailed description of implementation methods

[0036] A first embodiment of a ball screw mechanism 1 for a brake actuator mechanism intended for driving brake calipers in braking mechanisms is illustrated. 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 of the balls 14.

[0037] The screw 10 is preferably metallic, for example 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, with 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, such as four, six, or eight sides, or a cavity for a key connecting it to the output shaft. The screw body 20 has a screw thread 22 that forms an internal helical raceway 24 around the reference axis 100, the internal helical raceway 24 being radially rotated 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 designed to axially close the internal helical raceway 24 and retain the balls 14 in the ball screw mechanism 1, and more specifically 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 offers 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 device32 includes a first spring34 and a first retaining means40.

[0039] The first spring 34 is a mechanical component made from elastic materials, such as 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 portion of the screw body 20 closest to the connecting portion 18. The first spring 34 is composed, preferably made, of metal, preferably steel, and has a stiffness that determines the force required to deform said first spring 34. The first spring 34 is helical and of the "compression" type, that is, it is coil-shaped, it compresses under the load of the balls 14 in the internal helical raceway 24, and it returns to its original 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 retained 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 mounting 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 mounting recess. The first spring 34 is retained in the inner helical raceway 24 by the rod 40', which extends along a portion 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 attached 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 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 to receive a brake caliper. Regardless of the 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 bushing 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 an external helical raceway 54 around the inwardly radially rotated reference axis 100. 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 designed to axially close the external helical raceway 54 and retain the balls 14 within the ball screw mechanism 1, and more specifically within 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 gradual, so that it offers increasing resistance to the balls 14 as they roll in the direction of the second end 60.More specifically, the second stop device62 includes a second spring64 and a second retaining means72.

[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, 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 connected 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 may 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 mechanism1 includes a lubricant to ensure the proper functioning and longevity of the system. The lubricant can 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 on the), that is to say 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 "tight" position (illustrated in the figure), 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, thus reducing, among other things, the friction between the screw 10 and the nut 12. The first thrust bearing 32 and the second thrust bearing 62 move closer together, and the second end ball 14B comes into contact with the spring 64 of the second thrust bearing 62. Very quickly, the first spring 34 comes into contact with the first end ball 14A. The balls14 then continue to move forward, as the screw10 sinks into the nut12, in a combination of sliding and rolling, in a stroke constrained by the two springs34, 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 free to move. 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 1 then moves between the loosened position and an intermediate position.

[0051] The balls 14 are intended to move under constrained travel only when the mechanism 1 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 bearing surface 83, positioned opposite the balls, so as to distribute the force during impact between the balls and the bearing 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, for example ( ). The foam springs 34, 64 may be solid or have a central opening to allow the passage of a retaining means in the form of a rod 40', for example ( ). However, if the first spring 34 is a solid cylinder, the first retaining means 40 is then a sleeve 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 sleeve 90. The sleeve 90 can also be used when the spring is helical and made of metal, so as to retain said spring in the associated raceway while reducing friction between the spring and the raceways.The retaining device can also consist of the 90 mm sleeve in combination with the 40 mm rod. 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 for illustrative purposes only and are not exhaustive. It is explicitly intended that the different embodiments illustrated can be combined to create other solutions.

[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 components of the ball screw mechanism 1, thereby lowering 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 device32 has a first fixed portion67 relative to the nut10, or in which both fixed portions67,68 are fixed relative to the screw12.

Claims

Ball screw mechanism (1), comprising: a screw (10) defining a reference axis (100), the screw (10) having 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).; 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). 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 pad for example. 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). 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. Ball screw mechanism (1) according to claim 5, characterized in that the first fixing portion (67) comprises a first retaining means (40) of the first spring (34), fixed with respect 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. 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). 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). 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. 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). 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. 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. 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. 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. 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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