FR2867137A1 - Pneumatic actuator for motor vehicle, has movable piston with bush including locking unit deactivated by plunger to push tubular unit and to vary skip distance, determined by axial position of annular unit, between sensor and reaction disk - Google Patents

Abstract

The actuator has a plunger controlled by a control rod and sliding in a movable piston (24). A sensor is arranged at a skip distance from a reaction disk through which the piston activates a master cylinder, where the distance is determined by an axial position of an annular unit of an inlet valve. The piston has a bush (62) with a locking unit (61) deactivated by the plunger to push a tubular unit (50) and to vary the skip distance.

Description

"Variable Jump Actuator with Unlockable Sleeve"

  The invention relates to a pneumatic actuator for actuating a brake master cylinder of a motor vehicle.

  The invention more particularly relates to a pneumatic brake booster for a motor vehicle, of the type which comprises a rigid casing inside which is movable a transverse wall sealingly delimiting a front chamber subjected to a first io pressure, and a rear chamber, subjected to a second pressure varying between the first pressure and a pressure greater than the first pressure, which is likely to cause an actuating rod of a master cylinder associated with the servomotor via a reaction disk, of the type which comprises a tubular movable piston which is slidably mounted in the casing and of which a front section is integral with the movable partition, of the type which comprises a control rod moving in the piston selectively according to an axial input force exerted forward against a return force exerted by a return spring, of the type of in which the movements of the control rod are able to determine the openings and closures of at least one axial valve called "intake" which is interposed between a pressure source subjected to the pressure greater than the first pressure and the chamber rear, and at least one axial valve called "rebalancing" which is interposed between the front chamber and the rear chamber, to actuate the movable partition, and of the type in which a plunger which is received in a front inner section of the piston tubular and which is integral with the end of the control rod, has at its end a feeler which is received in a bore passing through the movable piston and which is capable of directly urging the operating rod of the master cylinder by the intermediate of the reaction disk, of the type which comprises a floating tubular element, which is slidably mounted in a rear inner section of the tubular piston, and which comprises radial nt, from the axis of the piston towards its periphery, a first front annular surface forming a first transverse sealing element of the axial inlet valve and a second front annular bearing surface, of greater diameter, which forms a first transverse element of sealing of the axial rebalancing valve, of the type which comprises a second annular transverse sealing element of the axial inlet valve, complementary to the first annular bearing surface of the element, and carried by the rear end of the plunger, of the type which comprises a second annular transverse sealing element of the axial rebalancing valve, complementary to the second annular bearing surface of the element, which consists of at least one rear shoulder face delimiting the internal front and rear sections of the piston, of the type in which the axial position of the first annular bearing surface of the element determines a so-called "jump distance" between the palp eur and the reaction disk.

  Many examples of conventional servomotors of this type are known.

  In such a booster, the distance separating the first probe from the reaction disk is called the "jump distance" and corresponds to the theoretical distance that the first probe must travel before the driver of the vehicle feels the reaction force of the sensor. master cylinder, the probe coming into contact with the reaction disk due to the advance of the probe and / or the expansion of the reaction disk.

  In practice, when there is a clearance between the first probe and the reaction disk, this distance corresponds to the distance separating the first probe from the reaction disk, but it can also, when the reaction disk is initially decompressed on contact of the reaction disc, correspond to the stroke in which the first probe is likely to compress the reaction disc until the latter is fully compressed.

  In a braking situation for which a significant braking force is exerted on the control rod, the actuation of the control rod causes the displacement of the plunger comprising the first probe, which causes the maximum opening of the intake valve. and putting the back chamber to atmospheric pressure. It follows a forward movement of the movable partition, and the end of the plunger forming the first probe comes into contact with the reaction disc of elastomeric material by compressing it.

  Thus, the force exerted on the operating rod of the master cylinder when the control rod is at the end of stroke results from the assistance force which is caused by the pressure difference on each side of the movable wall. and the force exerted by the probe plunger on the reaction disc. The driver of the vehicle also experiences the braking reaction force, which is transmitted from the master cylinder to the plunger via the reaction disc.

  However, it was found that many drivers, faced with a braking situation supported, underestimated the risks incurred, and that, after braking suddenly, they relaxed their braking force while maintaining a braking effort was essential to avoid an accident.

  Indeed, in the case of a braking situation accompanied by a rapid displacement of the control rod, the plunger may come into contact with the reaction disk and transmit to the driver a feeling of maximum braking even before the difference in speed. There is no maximum pressure between the front and rear pressure chambers, which may lead the driver to release his effort, even though it should be maintained to benefit from maximum braking effort.

  To remedy this drawback, it has been proposed a servomotor which comprises a first probe which is slidably mounted relative to the plunger to penetrate into the reaction disk and which can, when the actuator control rod is actuated at a predetermined speed, be locked relative to the movable piston so as to maintain a maximum braking force on the actuating rod of the master cylinder via the reaction disc even though the driver has partially released his effort.

  This design makes it possible to prevent the braking force from being unintentionally released. However, this design has the disadvantage of not retransmitting faithfully to the driver the feeling of braking, which results from the transmission of the reaction force of the master cylinder to the probe.

  Indeed, once the probe is locked relative to the movable piston, the reaction force from the master cylinder is transmitted in part to the movable piston and not only to the driver.

  To overcome this drawback, the invention provides a booster which comprises a device for restoring to the driver a faithful braking sensation regardless of the speed and the operating stroke of the plunger. As a result, the booster according to the invention does not require a locking device of the type previously described.

  The operating principle of the booster according to the invention is based on the principle known from the state of the art of obtaining a variation of the jump distance as a function of the speed and the operating stroke of the plunger, which are themselves increasing in proportion to the braking force exerted by the driver of the vehicle.

  This variation of the jump distance is obtained by proposing a balancing seat adapted to move a feeler slidably mounted in a bore of the piston to determine a variable volume that is likely to be filled by the reaction disk or by the probe.

  Indeed, the greater the volume to be filled in the bore is that the feeler and the reaction disk come into contact, the value of the braking force at which the jump appears to be constant, and the reaction force more , which is proportional to the pressure in the master cylinder and is transmitted from the reaction disk to the probe, is high when the probe and the reaction disk come into contact.

  This configuration makes it possible to vary the intensity of the reaction force transmitted to the driver as a function of the braking force exerted on the plunger.

  This design makes it possible to benefit from a maximum braking force in the case of emergency braking.

  Servomotors based on this principle of operation are already known from the state of the art.

  They generally provide concentric probe systems which may or may not come into contact with the reaction disk to vary the jump distance and the surface ratio between said feelers and the reaction disk, so as to vary intensity of the reaction force transmitted to the diver. In particular, a servomotor having a second concentric probe which, during a rapid application, can be pushed by the plunger and locked axially with respect to the plunger and then pushed by the plunger into the reaction disk in such a way that rapid application of a braking force can be performed, according to a new jump distance value greater than the initial value, meeting only a reduced antagonistic force transmitted by the reaction disk.

  However, such servomotors are very bulky.

  It is also known from the state of the art to remedy this drawback by taking advantage of the tubular design of the valve seats of the booster. The variation of the jump is obtained by varying the axial position of the seat of the rebalancing valve.

  For this purpose, it is known to propose a servomotor of the type described above which comprises a shuttle bushing, which is received in the front section around the plunger, which is movable only between an inactive extreme rest position and an extreme active position in which it is controlled by the plunger in axial sliding to replace the shoulder rear face so as to form a second annular transverse sealing element of the mobile re-equilibration axial valve cooperating with the second annular bearing surface of the tubular element so as to to push it to vary the axial position of the first annular bearing surface of said element and modify the so-called "jump distance" between the feeler and the reaction disk.

  The bushing is elastically biased towards its inactive position by a return spring and is retained in its rest position by an elastic clip which passes through the bushing and which can be deformed by the plunger to release the bushing.

  This design has the disadvantage of a great complexity of assembly.

  To overcome this drawback, the invention provides a booster of the type described above, in which the immobilization of the bushing is provided by rigid abutment means which can be unlocked by the plunger.

  For this purpose, the invention proposes a servomotor of the type described above, characterized in that it comprises a shuttle bushing, which is received in a bore of the front section around the plunger, which is movable in axial sliding between: a position advanced inactive end in which first locking means, formed at a first end of a substantially longitudinal track of the sleeve in which slides a radial finger of the plunger, cooperate with the finger to immobilize the sleeve against a first stop against a rearward biasing force exerted by a spring interposed between the bore and the shuttle sleeve, E and a retracted active extreme position, obtained after the first locking means have been deactivated to allow sliding of the finger in the track and the recoil of the sleeve under the bias of the return spring, wherein the sleeve is immobilized by a second stop, to substitute for the rear shoulder face and to form a second annular transverse sealing element of the mobile re-equilibration axial valve cooperating with the second annular bearing surface of the tubular element in order to push it to vary the axial position of the the first annular range of said element and change the distance "d" called "jump" between the probe and the reaction disk.

  According to other features of the invention: the plunger carries a ring which is slidably mounted on a first section of the plunger arranged behind the probe, which can be urged forward by a shoulder face of a second section of the plunger arranged behind the first section, and which comprises at least one radial finger which is received in at least one associated track of the actuating sleeve, a rear end of which comprises the first locking means, - the socket shuttle is additionally rotatably mounted in the bore of the front section and each track comprises: E a front portion parallel to the axial direction, which allows the recoil of the socket, E a rear part, inclined in ramp with respect to the axial direction at least one inclined edge forms the first locking means which is capable of being deactivated when the plunger is urged and causes the finger to slide ring along said inclined edge of the rear part of the track i0 so that it escapes from the first locking means, - the sleeve comprises two diametrically opposed tracks and the ring comprises two diametrically opposite radial fingers, - the front section internal piston has a tubular bearing in which the plunger slides and around which the shuttle bushing is slidably mounted and rotating, the servomotor comprises a radial key which protrudes outside the piston so as to bear on the actuator casing when the piston is in the rest position, which has the shape of a jumper whose branches pass through four longitudinal slots two by two opposite the outer wall of the piston, pass through an angular clearance two opposite longitudinal slots of the sleeve shuttle and two opposite longitudinal lumens of the tubular bearing, which overlaps the plunger behind the second tr onon, and which is arranged permanently in contact with the front end of the longitudinal slots of the shuttle bushing under the effect of the spring return force, so that the extreme positions of the key between the ends of the lights of the outer wall of the piston determine the first and second stops, - the return spring is interposed between the rear end of the shuttle bushing and a shoulder face of the inner front section of the piston arranged behind the opposite longitudinal slots of the piston of the outer wall of the piston.

  Other features and advantages of the invention will appear on reading the detailed description which follows for the understanding of which reference will be made to the accompanying drawings in which: - Figure 1 is a sectional view of a servomotor according to a 2A and 2B are detailed sectional views of a servomotor according to the invention shown in a rest position; FIGS. 3A and 3B are sectional views in detail of a Servomotor according to the invention shown leaving its rest position at the beginning of the opening of the intake valve before the appearance of hydraulic pressure in the brake circuit, - Figures 4A and 4B are sectional views of a servomotor according to the invention shown in equilibrium, the inlet valve being closed, - Figures 5A and 5B are sectional views of a booster according to the invention shown in a saturation braking position, - the figure 6 is a perspective view showing the internal elements of the movable piston of the booster according to the invention; - FIG. 7 is a perspective view showing the shapes of the piston of the booster according to the invention.

  In the following description, like reference numerals denote like parts or having similar functions.

  By convention, the terms "front" or "rear" respectively designate elements or positions respectively oriented towards the left or the right of FIGS. 1 to 7.

  FIG. 1 shows the assembly of a conventional pneumatic brake booster comprising a servomotor 11 for braking assistance for a motor vehicle intended to actuate a master cylinder 22.

  In known manner, the servomotor 11 comprises a rigid casing 12 inside which is movable a transverse partition 14, biased rearwardly by a spring 15, which delimits in a leaktight manner a front chamber 16, subjected to a first pressure "PI", and a rear chamber 18, subjected to a second pressure "P2" varying between the first pressure "PI" and a pressure "Pa" greater than the first pressure "PI". The movable partition 14 is capable of urging a rod 20 for actuating the master cylinder 22 associated with the servomotor 11 via a reaction disc 28.

  The servomotor 11 comprises a tubular movable piston 24 which is slidably mounted in the casing 12 and a front section of which is integral with the movable partition 14. A front face 26 of the front section is arranged facing a cup 30, integral with the rear end of the actuating rod 20, wherein lo is housed the reaction disc 28.

  The servomotor comprises an axis control rod 32 "A" moving in the piston 24 selectively as a function of an axial input force exerted forward against a return force exerted by a spring 34 reminder.

  The movements of the control rod 32 are capable of determining the openings and closures of at least one three-way axial valve 40 which is capable of selectively bringing the rear chamber 18 into communication with a pressure source subjected to the "Pa" pressure. "greater than the first pressure" PI ", in particular the ambient medium at atmospheric pressure" Pa "for, during a so-called" admission "phase, causing the moving part 14 to move forward, or to put in communication the rear chamber 18 with the front chamber 16 subjected to the pressure "P", during a rebalancing phase, allow the return to the rear of the movable partition 14 to its rest position .

  A plunger 36 is received in a front internal section 51 of the tubular piston 24. This plunger 36 is integral with the end of the control rod 32, and it comprises an end comprising a feeler 42 which is received in a bore 44 passing through the front face 26 of the piston 24 and which may directly urge the actuating rod 20 of the master cylinder 22 through the reaction disc 28.

  In the rest position of FIGS. 2A and 2B, the probe 42 is arranged at a distance "d" from the jump of the reaction disc 28. During braking, at the end of the stroke of the control rod 36, or when a braking force is applied at a speed greater than the speed of displacement of the movable wall 14, the feeler 42 comes into contact with the reaction disk 28 and retransmits to the driver the reaction force that it receives from the rod 20 for actuating the master cylinder 22.

  Such a servomotor does not allow to vary the lo jump distance.

  The invention overcomes this disadvantage by providing a servomotor 11 remote variable jump. Advantageously, the servomotor 11 according to the invention makes it possible to vary the jump distance by taking advantage of a known design of the state of the art of servomotor comprising axial tubular valves.

  As illustrated in FIGS. 2B to 4B, in a known manner, the servomotor 11 comprises a tubular valve 46 called "intake" and a tubular valve 48 called "rebalancing".

  The movements of the control rod 32 are capable of determining the openings and closures of the axial intake valve 46, which is interposed between the pressure source subjected to the pressure "Pa" greater than the first pressure "PI" and the chamber 18, and the axial valve 48 rebalancing which is interposed between the front chamber 16 and the rear chamber 18, to actuate the movable partition 14.

  The detail of the internal ducts of the piston 24 making it possible to put in communication the front chamber 16 and the rear chamber 18 which are not the subject of the present invention and which are moreover widely known from the state of the art, they have not not shown in Figures 2 and following.

  More particularly, the piston 24 comprises a floating tubular element 50, which is slidably mounted in a rear internal portion 52 of the tubular piston 24, and which comprises radially, from the axis "A" of the piston 24 towards its periphery, a first bearing annular ring 54 forming a first transverse sealing element of the axial inlet valve 46 and a second front annular surface 56 of greater diameter, which forms a first transverse sealing element of the axial valve rebalancing 48.

  The servomotor 11 further comprises a second annular transverse sealing element 58 of the axial intake valve 46, complementary to the first annular bearing surface 54 of the element 50, and carried by the rear end of the plunger 36, and a second annular transverse sealing element 60 of the equilibrium axial check valve 48, complementary to the second annular bearing surface 56 of the element 50, which consists of at least one rear shoulder face delimiting the internal front and rear sections 51, 52 of the piston.

  According to this known configuration, the axial position of the first annular surface 54 of the element 50, on which the plunger 36 rests, determines a distance "d" called "jump" between the probe 42 and the reaction disk 28.

  According to the invention, the piston 24 comprises a shuttle sleeve 62, which is received in a bore 49 of the front inner section 51 around the plunger 36. This shuttle sleeve 62 is movable in axial sliding between: an inactive extreme end position , represented in FIGS. 2A and 2B, in which first locking means 61, formed at a first end 63 of a substantially longitudinal track 65 of the bushing 62 in which a radial pin 76 of the plunger 36 slides, cooperate with the finger 76 to immobilize the sleeve 62 against a first stop against a return force exerted to the rear by a spring 94 interposed between the bore 49 and the shuttle bushing 62, and a retracted active extreme position, shown in the figures 4A, 4B, 5A, 5B obtained after the first locking means have been deactivated to allow sliding of the finger 76 in the track 65 and the recoil of the socket 62 under the bias of the return spring 94, wherein the sleeve 62 is immobilized by a second stop, to substitute for the shoulder rear face 60 and to form a second annular transverse sealing element 64 of the movable re-equilibrium axial valve 48 cooperating with the second annular bearing surface 56 of the tubular element 50 in order to push it to vary the axial position of the first annular bearing surface 54 of said element 50 and to modify the "d" so-called "jump" distance between the probe 42 and the reaction disk 28.

  More particularly, to control the axial sliding towards the rear of the shuttle bushing 62, the plunger 36 carries a ring 74 which is slidably mounted on a first section 78 of the plunger 36 arranged behind the probe 42, which is capable of being urged forward by a shoulder face 80 of a second section 82 of the plunger 36 arranged behind the first section 78, and which comprises at least one radial finger 76 which is received in at least one associated track 65 of the shuttle bushing 62, a rear end 63 comprises the first locking means 61.

  To enable the socket 62 to be unlocked with the aid of the preceding aid, the shuttle bushing 62 is additionally rotatably mounted in the bore 49 of the front section 51 and each track 65 comprises: a front portion 68 parallel to the axial direction "A ", which allows the recess of the sleeve 62, - a rear portion 70, inclined in ramp with respect to the axial direction" A ", of which at least one inclined edge 61 forms the first locking means which is likely to be disabled when the plunger 36 is biased and that it causes the sliding of the finger 76 of the ring 74 along said inclined edge 61 of the rear portion 70 of the track 65 so that it escapes from the first locking means 61.

  In order to prevent jamming of the finger 76 of the ring 74 in the track 65, and in order to evenly distribute the forces on the ring 74, the shuttle sleeve 62 has two diametrically opposed tracks 65 and the ring 74 has two fingers 76 diametrically opposed radials.

  It will therefore be understood that, when the plunger 36 is biased, it in turn urges the ring 74 whose fingers 76 each urge the inclined edges 61 of the track 65 and cause a rotation of the shuttle bushing 62. Simultaneously, the fingers 76 slide along the rear portions 70 of the tracks 65 until they reach the front portions 68 of the tracks 65. The shuttle bushing 62 is then free to slide axially rearward and to replace the rear face 60 shoulder of the piston 24, then constituting a second annular transverse sealing element 64 of the mobile re-equilibration valve 48 which cooperates with the second annular bearing surface 56 of the tubular element 50 in order to repel it.

  According to the invention, to provide a simple and satisfactory guide of the plunger 36 and the shuttle bushing 62, the inner front section of the piston comprises a tubular bearing 92 in which the plunger 36 slides and around which the shuttle bushing 62 is mounted. sliding and rotating. This configuration makes it possible to propose one and the same guidance for the shuttle bushing 62 and for the plunger 36.

  To determine the first and second stops, the servomotor comprises a radial key 96 which protrudes outside the piston 24 so as to bear on the casing 12 of the servomotor 11 when the piston 24 is in the rest position, which has the shape of a jumper whose branches 98 pass through four longitudinal slots 99 two opposite two of the outer wall 100 of the piston 24, pass through with angular play two opposite longitudinal slots 102 of the shuttle bushing 62 and two opposite longitudinal slots 104 of the tubular bearing 92, which overlaps the plunger 36 behind the second section 82, and which is arranged permanently in contact with the end 106 before the longitudinal lumens 102 of the shuttle bushing 62 under the effect of the return force of the spring 94, so that the extreme positions of the key 62 between the ends 108, 110 of the slots 99 of the outer wall 100 of the piston 24 determine the first and second stop.

  To allow the return bushing 62 to be returned, the return spring 94 is interposed between the rear end 112 of the shuttle bushing 62 and a rear face 114 of the key 96.

  In this way, when applying a braking force on the control rod 32, the jump distance varies rapidly.

  From the rest position which has been shown in FIGS. 2A and 2B, the application of a braking force on the control rod 32 causes the plunger 36 to advance. The shoulder face 80 of the second section 82 of the plunger 36 urges the ring 74 whose fingers 76, by urging in turn the inclined edges 61 of the tracks 65 causes the rotation of the shuttle bushing 62 as shown in Figures 3A and 3B.

  When the actuating sleeve 62 pivots, the fingers 76 of the ring 74 slide in the rear portions 70 of the tracks 65, then reach the front portions 68 of the said tracks 65. When the fingers 76 have reached the front portions 68 of the tracks 65, the return spring 94 pushes the sleeve 62 backwards. The rear face 64 of the sleeve 62 comes into contact with the second annular surface 56 of the element 50 and replaces the second transverse element 60 consisting of the rear face delimiting the front and rear sections 51, 52 of the piston 24. Then, the shuttle bushing 62 pushes the floating element 50.

  Indeed, the piston 24 advances simultaneously due to the introduction of the atmospheric pressure "P2" in the rear chamber 18, greater than the pressure "PI" prevailing in the front chamber 16.

  The advance of the piston 24 allows the key 96, which remains resting on the casing 12 of the booster, to slide back and forth in the slots 99 of the first stop position determined by the front face 108 of the lights 99 to the second stop position determined by the rear face 110 of the lights 99.

  The stroke of the key 96 also determines the stroke of the bushing 62 because the front face 106 of the slot 102 of the bushing 62 is always in contact with the key 96 due to the force exerted by the return spring 94. stroke of the socket 62 corresponds to the increase of the jump distance.

  Thus, as illustrated in FIGS. 4A and 4B, the advance of the piston 24 conjugated to the recoil of the shuttle bushing 62 enables the annular transverse sealing element 58 of the plunger 36 to rest on the element 50 in a position axial offset backward from its initial position. As a result, the feeler 42 is also shifted by a stroke "X" from its initial position, and the distance "d" of jump is increased by the stroke "X".

  In the case of the application of a saturation braking force which has been shown in FIGS. 5A and 5B, the plunger 46 advances abruptly before the piston 24 has advanced due to the delay in the balancing of the pressures between the chambers before 16 and rear 18. In this case, the key 96 remains supported on the casing 12 of the booster 11 and the shuttle bushing 62 pivots bearing on the key 96 while remaining stationary axially without changing the jump distance.

  The invention therefore advantageously allows to benefit from a servomotor having a distance "d" of variable jump.

Claims (7)

  1. Servomotor (11) pneumatic brake assist for a motor vehicle, the type which comprises a rigid casing (12) within which is movable a partition (14) transversely delimiting a front chamber (16). ), subjected to a first pressure (P1), and a rear chamber (18), subjected to a second pressure (P2) varying between the first pressure (P1) and a pressure (Pa) greater than the first pressure (P1) , which is capable of urging an actuating rod (20) of a master cylinder (22) associated with the servomotor (11) via a reaction disc (28) of the type which comprises a piston (24). ) movable tubular 15 which is slidably mounted in the casing (12) and a front portion is secured to the movable partition (14), the type which comprises a rod (32) for control moving in the piston (24) selectively according to an axial input force exerted forward against an effort return device exerted by a return spring (34) of the type in which the movements of the control rod (32) are capable of determining the openings and closures of at least one axial valve (46) called "intake" which is interposed between a pressure source subjected to the pressure (Pa) greater than the first pressure (P1) and the rear chamber (18), and at least one axial valve (48) called "rebalancing" which is interposed between the front chamber (16) and the rear chamber (18), for actuating the moving partition (14), and of the type in which a plunger (36) which is received in a front inner section (51) of the piston (24) tubular and which is secured to the end of the rod (32) control, has at its end a probe (42) which is received in a bore (44) passing through the movable piston (24) and which is likely to solicit directly the rod (20) for actuating the master cylinder (22) via the reaction disk (28), the e which comprises a floating tubular element (50), which is slidably mounted in a rear internal section (52) of the tubular piston (24), and which comprises radially, from the axis (A) of the piston (24) towards its periphery a first, forward annular bearing surface (54) forming a first transverse sealing element of the axial inlet valve (46) and a second, larger diameter, annular front surface (56) which forms a first transverse sealing element of the axial rebalancing valve (48) of the type which comprises a second annular transverse sealing element (58) of the inlet axial valve (46) complementary to the first annular bearing surface (54) of the element (50), and carried by the rear end of the plunger (36), of the type which comprises a second annular transverse sealing element (60) of the axial equilibration valve (48) complementary to the second annular bearing surface (56) of the element (50), which consists of at least ins a shoulder rear face delimiting the inner front sections (51) and rear (52) of the piston (24), of the type in which the axial position of the first annular bearing surface (54) of the element (50) determines a so-called "jump" distance between the probe (42) and the reaction disc (28), characterized in that it comprises a shuttle bushing (62), which is received in a bore (49) of the front section (51) around the plunger (36), which is movable in axial sliding between: - an advanced inactive end position in which first locking means (61), formed at a first end (63) of a substantially longitudinal track (65) of the sleeve (62) in which slides a radial finger (76) of the plunger, cooperate with the finger (76) to immobilize the sleeve (62) against a first stop (108) against a return force exerted to the rear by a spring (94) interposed between the bore (51) and the shuttle bushing (62), - and u a retracted active extreme position, obtained after the first locking means (61) have been deactivated to allow the finger (76) to slide in the track (65) and the socket (62) to recoil under the bias of the spring ( 94), in which the sleeve (62) is immobilized by a second stop (110), to substitute for the rear shoulder face (60) and to constitute a second annular transverse sealing element (64) of the valve (48) movable rebalancing axial cooperating with the second bearing surface (56) annular of the tubular element (50) to push it to vary the axial position of the first bearing surface (54) annular of said element (50) and modify the distance "d" called "jump" between the probe (42) and the disc (28) reaction.   2. Servomotor (11) according to the preceding claim, characterized in that the plunger carries a ring (74) which is slidably mounted on a first section (78) of the plunger (36) arranged behind the probe (42), which is capable of being urged forward by a shoulder face (80) of a second section (82) of the plunger (36) arranged behind the first section (78), and which comprises at least one radial finger ( 76) which is received in at least one associated track (65) of the shuttle bushing (62), a rear end (63) of which has the first locking means (61).   3. Actuator (11) according to the preceding claim, characterized in that the shuttle sleeve (62) is additionally rotatably mounted in the bore (49) of the front section (51) and in that each track (65) comprises: - a front portion (68) parallel to the axial direction (A), which allows the recess of the sleeve (62), - a rear portion (70), inclined in a ramp with respect to the axial direction (A), which at at least one inclined edge (61) forms the first locking means which is capable of being deactivated when the plunger (36) is urged and causes the finger (76) of the ring (74) to slide along said edge (61) inclined the rear portion (70) of the track (65) to escape from the first locking means (61).   4. Servomotor (1 1) according to one of claims 2 or 3, characterized in that the sleeve (62) comprises two tracks (65) diametrically opposed and in that the ring (74) comprises two radial fingers (76) diametrically opposed.   5. Servomotor (11) according to any one of the preceding claims, characterized in that the internal front section (51) of the piston comprises a tubular bearing (92) in which the plunger (36) slides and around which the sleeve shuttle (62) is slidably and rotatably mounted.   6. Booster according to one of the preceding claims, characterized in that it comprises a radial key (96) which projects outside the piston (24) so as to bear on the casing (12) of the booster (11) when the piston (24) is in the rest position, which has the shape of a jumper whose branches (98) pass through four longitudinal slots (99) two by two opposite the outer wall (100) of the piston ( 24), traverse with an angular clearance two opposite longitudinal slots (102) of the shuttle bushing (62) and two opposite longitudinal slots (104) of the tubular bearing surface (92), which overlaps the plunger (36) behind the second section (82), and which is arranged permanently in contact with the end (106) before the longitudinal slots (102) of the shuttle bushing (62) under the effect of the return force of the spring (94), way that the extreme positions of the key (96) between the ends (108, 110) of the lumi res of the outer wall of the piston determine the first and second abutments.   7. Booster (11) according to any one of the preceding claims characterized in that the return spring (94) is interposed between the rear end (112) of the shuttle bushing (62) and a rear face (114) of the key (96).