FR2825059A1 - DIFFERENTIATED JUMP SERVOMOTOR - Google Patents

Abstract

The invention concerns a pneumatic servo brake booster comprising a mobile piston (22) stressing an actuator rod (34) of a master cylinder (28) in response to the actuation of a plunger (56) whereof one front end, passing through the piston (22) and forming a first probe (62) is capable, in the extreme actuating position of the plunger (56), of penetrating into a reaction disc (44) interposed between the piston (22) and the actuating rod (34) to transmit to the plunger (56) the reaction force of the master cylinder (28). The invention is characterised in that it comprises a second probe (68), which is capable, when the control rod (46) is actuated at a speed higher than a predetermined speed, of being locked by a one-way clutch device (70), then of being driven by the piston to cause the second probe (68) to penetrate into the reaction disc (44) to enable the first probe (52) to apply a braking force more rapidly.

Description

(64). "Differentiated jump actuator" The invention relates to a

  pneumatic actuator for actuating a brake master cylinder of a motor vehicle. s The invention relates more particularly to a pneumatic servomotor for actuating a brake master cylinder of a motor vehicle, of the type which comprises a rigid envelope inside which is movable a transverse partition sealingly delimiting a front chamber. , subjected to a first pressure of engine depression, and a rear chamber subjected to a second pressure varying between engine depression and atmospheric pressure, of the type which comprises a movable piston, integral with! a movable partition, one front face of which is likely to request an actuating rod of the master cylinder, of the type which comprises a servomotor control rod moving in the piston selectively as a function of an axial input force exerted towards the front against a return force exerted on the rod by a return spring, of the type which includes a plunger which is arranged at the front of the control rod in the piston and which comprises at its rear end at least one rear annular seat of a three-way valve which is progressively movable between a position in which, the control rod being at rest, the front chamber and the rear chamber are in communication, and a position in which, the control rod being actuated, the second pressure prevailing in the rear chamber increases, the valve putting the rear chamber in communication with atmospheric pressure, and of the type which comprises a first feeler, forming the front end of the plunger and passing through the piston which, in the rest position of the control rod, is arranged at a first determined jump distance from a reaction disk interposed between the actuation rod of the master cylinder and the front face of the movable piston , and which is likely, when the control rod is actuated at a speed lower than a determined speed c, to be stressed by the plunger to run the first jump distance then penetrate the reaction disc so as to transmit to the plunger and the control rod

  the reaction force of the master cylinder.

  There are many examples of servo motors

of this type.

  In such a servomotor, the distance separating the first probe from the reaction disc, is called "jump distance" and it corresponds to the theoretical distance that the first probe o must travel before the driver of the vehicle feels

  the reaction force of the master cylinder.

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

fully compressed.

  In an extreme braking situation for which a maximum braking force is exerted on the control rod, actuation of the control rod causes the displacement of the plunger comprising the first feeler, which causes the maximum opening of the valve at three tracks and bringing the rear chamber to atmospheric pressure. It follows a displacement towards the front of the movable partition, and the end of the plunger forming the first probe penetrates into the reaction disc of elastomer material which is integral with the face

  rear of the movable piston by compressing it.

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

  through the reaction disc.

  However, it has been found that a good number of drivers, faced with an emergency braking situation, underestimated the risks involved, and that, after braking suddenly, they released their braking effort while maintaining a effort

  braking was essential to avoid an accident.

  Indeed, in the case of an extreme braking situation o accompanied by a rapid displacement of the control rod, the plunger can come into contact with the reaction disc and transmit to the driver a feeling of maximum braking even before the difference pressure is maximum between the front and rear pressure chambers, which can lead the driver to relax his effort, even if he should be

  maintained to benefit from the maximum braking force.

  To overcome this drawback, a servomotor has been proposed which comprises a first feeler which is slidably mounted relative to the plunger to penetrate into the reaction disc then which can, when the actuator control rod is actuated at a determined 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 when the driver

  s would have partially relaxed his effort.

  This design has the drawback of requiring a significant effort from the driver, since in order for the probe to penetrate the reaction disc at the end of its travel, the actuating force must be exerted against the reaction force.

master cylinder.

  This is particularly disadvantageous in the event of emergency braking, that is to say in the case of the rapid application of a braking force for which the duration of the actuation must

be as short as possible.

  The invention remedies the drawbacks of the two aforementioned designs by proposing a booster according to the first design to which is added a second feeler which, during rapid application, can be pushed by the plunger and locked axially with respect to the piston, then be pushed by the piston in the reaction disc so that the rapid application of a braking force can be carried out, according to a new jump distance of value greater than the initial value, by meeting only a reduced antagonistic force transmitted by the reaction disk, so that the application of the force of

  braking can be performed more quickly.

  To this end, the invention provides a servomotor as described above, characterized in that it comprises a tubular sleeve, which is slidably mounted on the plunger, which comprises a front section which crosses the piston and one end of which comprises a second probe, and which is capable, when the control rod is actuated at a speed greater than a determined speed, of being pushed by the plunger and locked by a one-way clutch device according to a determined axial position relative to the piston, then to be driven by the piston to drive the second probe to penetrate the reaction disc so as to allow the first probe to apply a rapid braking force against an antagonistic reaction force of the master cylinder which is reduced along a second determined jump distance, greater than the

first jump distance.

  According to other characteristics of the invention: - the bush comprises a rear tubular section which is slidably mounted in a rear bore complementary to the piston, - the unidirectional clutch device comprises a key, which is mounted in a recess of the through piston the piston perpendicularly to its axis, and a substantially annular intermediate portion of which surrounds the bush with play and is capable, when the input force is exerted on the control rod at a speed greater than the determined speed, be driven by the movable piston to tilt about a generally transverse axis so as to cooperate with the periphery of the rear tubular section of the socket to block said socket in a determined axial position, - the rear tubular section of the socket comprises at least a transverse blocking face, which is turned axially towards the rear to form a stop for u n pin which o is carried by an upper part of the key and which extends radially towards the sleeve, so as to ind exe r axially the determined position for blocking the sleeve and the second probe, - the rear section of the sleeve has a groove of substantially frustoconical profile, a front shoulder face forming the transverse locking face, - the lug of the key is. in section through an axial plane, shaped according to an angular sector complementary to the frustoconical profile of the groove of the socket, - the key has a lower part substantially in the shape of an "L", diametrically opposite the lug, which extends radially from its annular part and which is resiliently biased against an abutment face of the casing of the booster by means of a return spring interposed between a front face of the recess of the piston and a front face of said lower part, - the second feeler is of an outside diameter greater than that of the rear section of the sleeve, - the reaction disc is received in a cup, coaxial with the piston, which is integral with the rear end of the actuating rod and of which a rear tubular part is slidably mounted in a coaxial annular groove on the front face of the piston, - the key has substantially, in section through an axial plane, the shape of a "T", the vertical branch of substantially radial orientation comprises the annular intermediate part and the lower part substantially in the shape of "L", and the upper horizontal branch of which is received with clearance between the two opposite walls of the recess and comprises protruding from her

end before the erot.

  Other characteristics and advantages of the invention

  will appear on reading the following detailed description for the

  o understanding of which reference will be made to the appended drawings in which: - Figure 1 is an overall view in axial section of a pneumatic brake booster produced in accordance with the invention, - Figure 2 is a view of detail in axial section of the booster of FIG. 1 shown in a rest position of the control rod, - FIG. 3 is a detail view in axial section of the booster of FIG. 1 shown in a position of application at speed slow braking force on the control rod, - Figure 4 is a detail view in axial section of the booster of Figure 1 shown during application at high speed of a braking force on the control rod s according to a first step of starting actuation of the control rod, - Figure 5 is a detail view in axial section of the actuator of ia Figure 1 shown during application at high speed of an effort of f reinforcement on the control rod according to a second intermediate step of actuation of the

control rod.

  In the description which follows, reference figures

  identical designate identical parts or have similar functions. By convention, the terms "front", "rear", upper "," lower "respectively designate elements or positions oriented respectively to the left, the right, the

top, or bottom of the figures.

  s FIG. 1 shows a set of a pneumatic brake booster for a motor vehicle. In known manner, the pneumatic actuator 10 comprises a rigid casing 12 inside which is movably mounted a transverse partition 14 which delimits a front chamber 16 in leaktight manner, subjected to a first pressure "P." whose value is equal to the value of the vacuum of the vehicle engine, and a rear chamber 18 subjected to a second pressure "P2". The second pressure "P2" is s likely, as ii will be described later, to vary between the value of engine depression "P '" and the value of the atmospheric pressure "Pa The front chamber 16 is supplied with pressure" P. "by via a vacuum duct 20 which is connected for example to an intake manifold (not shown) of an engine

of the vehicle.

  The pneumatic booster 10 comprises a movable piston 22 which is integral with the movable partition 14. Inside the casing 12, the movable partition 14 is biased elastically towards the rear by a return spring 24 which bears on

  the casing 12 and on a front face 26 of the movable piston 22.

  The servomotor 10 is coupled to a braking master cylinder 28 which is also connected to a braking circuit of the

  vehicle via hydraulic lines 30 and 32.

  In particular, the front face 26 of the movable piston 22 is coupled to an actuating rod 34 of the master cylinder 28 by means of a cup 36, coaxial with the piston 22, which is integral with the rear end 38 of the actuating rod 34 and a rear tubular portion 40 of which is slidably mounted in a coaxial annular groove 42 arranged in the front face 26 of the piston 22. The cup 36 receives a reaction disc 44 in

  elastomeric material whose function will be described later.

  The piston 22 comprises a control rod 46, which is for example connected to a brake pedal (not shown) of the vehicle by means of a coupling sleeve 48 which is arranged at its rear free end. The rod 46 is capable of moving in the movable piston 22, selectively as a function of an axial input force exerted towards the front on the control rod 46. The actuating force is exerted against a return force which is exerted on the rod 46 by a return spring 50 which is interposed between the movable piston 22 and the control rod 46. Sealing against dust and foreign bodies from the rear end of the movable piston 22 is ensured by a bellows 51, which is mounted on the rear end 53 of the casing 12, and which is crossed by the rod

command 46.

  The other free front end of the control rod 46 is in the form of a ball joint 52 which is received in a complementary housing 54 of a plunger 56 substantially

  cylindrical which is slidably mounted in the movable piston 22.

  The plunger 56 has at its rear end at least one annular seat 58 at the rear of a three-way valve 60 which is progressively softened between a position in which, the control rod 46 being at rest, the front chamber 16 and the rear chamber 18 are in communication, and a position in which, the control rod 46 being actuated, the second pressure "P2" prevailing in the rear chamber 18 increases, the valve 60 putting the rear chamber 18 in communication with the

o atmospheric pressure "Pa".

  The operation of the three-way valve 60 being known from the state of the art, it will not be described more explicitly

in this description.

  In known manner, the servomotor 10 comprises a first feeler 62, forming the front end of the plunger 56 and passing through the piston 22. More particularly, the first feeler 62 opens into the front face 26 of the piston 22 opposite the

  s reaction disc 44 which is arranged in the cup 36.

  As illustrated more particularly in FIG. 2, the plunger 56 therefore comprises a front section 61 whose end forms the first feeler 62, an intermediate section 63 with a diameter greater than the front section, and a rear section 65 whose

  o the rear end forms the seat 58 of the valve.

  In the rest position of the control rod 46 which is shown in FIG. 2, the first probe 62 is arranged at a first determined jump distance "d1" from the reaction disc 44. When the rod the control rod 46 is actuated at a speed lower than a determined speed, the first probe 62 is capable, as will be seen later, of accompanying the piston 22 according to the first jump distance "d1" up to that, the piston 22 having completely compressed the reaction disc 44 in the cup 36, the latter comes into contact with the first feeler 62 and transmits the reaction force of the master cylinder 28 to the control rod 46 as

shown in Figure 3.

  According to the invention, the servomotor 10 comprises a tubular sleeve 64, which is slidably mounted on the plunger 56, which comprises a front section 66 which passes through the piston and whose end 68 comprises a second feeler, and which is capable, when the control rod 46 is actuated at a speed greater than the determined speed, to be pushed by the plunger 56 and blocked by a one-way clutch device 70 in a determined axial position relative to the piston 22, then to be interlocked by the piston 22 to drive the second probe 68 to penetrate the reaction disc 44 as illustrated in FIG. 4, so as to allow the first probe 62 to apply a rapid braking force against an antagonistic force of reaction of master cylinder 28 which is reduced along a second determined jump distance "d2", greater

  at the jump distance "1", as shown in Figure 5.

  For this purpose, the sleeve 64 is slidably mounted in the piston 22, and it is. in the preferred embodiment of the invention, fully coaxial with the plunger 56. For this purpose, the sleeve 64 comprises a rear tubular section 72 which is slidably mounted in a rear bore 74 complementary to the piston 22. o In the embodiment preferred of the invention, the second feeler 68 is coaxial with the plunger 56. This arrangement is not limiting of the invention, and the second feeler 68 may not be coaxial with the plunger 56, provided that the front section 66 of the socket 64, which carries the second probe 68,

either coaxial with the plunger 56.

  Furthermore, in the preferred embodiment of the invention, the second probe 68 came integrally with the section 66 of the sleeve 64. This arrangement is not limiting of the invention, and the second probe 68 could, alternatively (not shown), be attached to the front section 66 of the socket 64. Advantageously, the second probe 68 has an outside diameter which is greater than that of the rear section 72 of the socket and which is substantially intermediate between that of the disc

  44 and that of the first probe 62.

  Furthermore, the rear section 72 of the sleeve 64 has a bore 76 which has a diameter corresponding to that of the

  intermediate section 63 of the plunger 56.

  The unidirectional clutch device 70 comprises a key 78, which is mounted in a recess 80 of the piston 22 passing through the piston 22 perpendicularly to its axis A, and of which a substantially annular intermediate portion 82 surrounds the bush 64 with play. The key 78 comprises a lower part 84 substantially in the shape of an "L" which extends radially from the annular part 82 and which is resiliently biased against an abutment face 86 of the casing 12 of the booster 10 by means of a return spring 88 interposed between a front face 90 of the recess 80 of the piston 22

  s and a front face 92 of the lower part 84.

  More particularly, the spring 88 is interposed between the front face 90 of the recess 80 and a vertical branch 94 of the lower part 84 in the form of "L". A horizontal branch 96 of the lower part 84 in the shape of an "L" is more particularly intended to come into abutment against a stop face 86 of

the casing 12 of the servomotor 10.

  Advantageously, the abutment face 86 of the casing 12 is for example carried by a ring 98 which is received in the cylindrical casing 12 of the booster 10. The ring 98 is for example mounted tight in the rear end of the casing 12. As illustrated in FIG. 4, the key 78 is likely, when the input force is exerted on the control rod 46 at a speed greater than the determined speed, to be driven by the movable piston 22 to tilt about a generally transverse axis so as to cooperate with the periphery of the rear tubular section 72 of the sleeve 64 to block said sleeve 64

  relative to the piston 22 according to a determined axial position.

  To this end, the rear tubular section 72 of the sleeve 64 has at least one transverse locking face 100, which is turned axially towards the rear to form a stop for a lug 102 which is carried by an upper part of the key 78 and which extends radially towards the socket 64, so as to axially index the determined locking position of the socket 64 and

o of the second probe 68.

  The key 78 therefore has substantially, in section through an axial plane, the shape of a "T", the vertical branch of substantially radial orientation comprises the annular intermediate portion 82 and the lower portion 84 substantially in the form of "L", and of which the upper horizontal branch 83, forming the upper part, is received with clearance between the two opposite walls 90, 91 of the recess 80 and comprises projecting from its front end

lug 102.

  Advantageously, the rear section 72 of the sleeve 64 comprises a groove 104 of substantially frustoconical profile, one front shoulder face forming the transverse locking face 100. This provision is not limitative of the invention. In the preferred embodiment of the invention, the transverse blocking face is annular, but it could consist of an angular portion or sector of annular element, or alternatively of a flange protruding from the socket 64, without modify

  properties and advantages of the invention.

  Advantageously, as illustrated in FIGS. 1 to 5, the frustoconical groove 104 has a profile which gradually joins

  the cylindrical periphery of the rear section 72 of the sleeve 64.

  Furthermore, the pin 102 of the key 78 is. in section through an axial plane, shaped along an angular sector which is o complementary to the frustoconical profile of the groove 104 of the sleeve 64. The frustoconical profiles of the lug 102 and of the groove 104 of the sleeve 64 are particularly advantageous because they allow, during the tilting of the key 78, to guide the erpot 102 of the key 78 to its stop position against the front face

shoulder 100 of sleeve 64.

  In this configuration, the servomotor 10 is capable of operating according to different configurations which have been

shown in Figures 2 to 5.

  o In the rest position of the control rod 46 which is shown in Figures 1 and 2, the horizontal branch 96 of the lower part 84 of the key 78 is supported on the abutment face

  86 of the ring 98 integral with the casing 12 of the booster 10.

  A transverse pin 106, which passes through a lumen (not shown) of the sleeve 64 and the plunger 56, is. in the rest position of the rod 46, pressing on a front face 107 of the annular intermediate portion 82 of the key 78 to define the rest position of the plunger 56, the first feeler 62 of which forms with the reaction disc 44 the first jump distance

"d1".

  In this way, as illustrated in FIGS. 3 to 5, when an input force is applied from rear to front following an extreme travel to the control rod 46, the lower part 84 of the key 78 leaves its support on the abutment face 86. As a result, the key 78 rocks around a generally transverse axis and counterclockwise to cooperate with the periphery of the sleeve 64. s If the input force is applied relatively slowly, ie in the case of normal braking shown in FIG. 3, the pressures "P." and "P2" which prevail in the front and rear chambers of the servomotor 10 balance each other at substantially the same speed as that with which the plunger 56 advances under the effect of the actuation of the control rod 46. Therefore , the plunger 46 and the piston 22 advance at substantially the same speed. As soon as the lower part 84 of the key 78 leaves its support on the abutment face 86, the lug 102 of the key 78 falls back onto

  the periphery of the front section 66 of the sleeve 64 without blocking it.

  The piston 22 compresses the reaction disc 44 and the latter deforms to fill the cavity formed in the socket 64 by the jump distance "d1". The distance "d1" is completely covered when the reaction disc 44 can no longer compress at the

contact of the first probe 62.

  o On the other hand, if the input force is applied at a speed greater than a determined speed, that is to say in the case of emergency braking, the pressures "P." and "P2" which prevail in the front and rear chambers of the actuator 1û are balanced at a speed which is lower than that with which the plunger 56

  advance under the effect of actuation of the control rod 46.

  Therefore, according to a first step of starting actuation of the control rod which is shown in Figure 4, the plunger 56, which slides forward relative to the piston 22 initially stationary, first pushes the sleeve 64 by through its rear section 65, and the two feelers 62 and 64 advance simultaneously. The piston 22 turns in turn and as soon as the lower part 84 of the key 78 leaves its support lO on the abutment face 86, the shell 102 of the key 78 falls back into the groove 104 of the socket which it locks relative to piston 22 in

  bearing on its transverse shoulder face 100.

  It should be noted that from this moment the axial position of the second probe 68 relative to the piston 22 is then s unequivocally determined by the engagement of the erpot

102 in throat 104.

  Then, according to a second intermediate step of actuation of the control rod shown in FIG. 5, the piston 22 moves under the effect of the increase in pressure in the rear chamber 18 of the booster. Therefore, the second probe 68, moved by the sleeve 64 locked relative to the piston 22, penetrates further into the reaction disc 44 while the first probe 62 remains substantially stationary. As a result, the second probe 68 locally compresses the reaction disc, which has the effect of locally decompressing the disc.

  reaction 44 at the first probe 62.

  The jump distance being characterized by the distance which separates the rest position of the first probe 62 from the position of the probe 62 in which it compresses the resection disc 44, the probe 62 can therefore still travel, a second jump distance "d2 ", which is greater than the jump distance" d1 "

previously mentioned.

  According to a third final step of actuation of the control rod 46, the first palletizer 62 can therefore locally compress the reaction disk 44 along the distance "d2" until said disk 44 is no longer compressible. At the end of this third step, the booster therefore presents itself in a configuration similar to that which has been represented in FIG. 4, except for the fact that the piston 22 has

  advanced in to the casing 12 of the servomotor 10.

  In the present case, this second distance "d2" corresponds substantially to the distance separating the first probe 62 from the second probe 68, but it would be possible for the first probe 62 to compress the reaction disc 44 only when it has crossed the second probe 68, this depending on the mechanical characteristics of the elastomeric material which is

  formed the reaction disc 44.

  Anyway, the distance "d2" will be traveled by the first feeler 62 in the presence of an antagonistic reaction force of the master cylinder which is reduced since the reaction disc 44 will be kept compressed at the level of the second feeler

  68 which is locked by the key 78.

  This configuration therefore allows the driver to apply a braking force very quickly without having to provide excessive force to oppose the opposing force of reaction of the master cylinder. The servomotor 10 is therefore characterized by two differentiated jump distances, namely a first jump distance "d1" reserved for the application of a braking force at slow speed, and a second jump distance "d2" reserved for the application of a

  braking force at high speed.

  It will be understood that the complete relaxation of the control rod 46, which causes the piston 22 to recoil, causes the return of the lower part 84 of the key bearing on the abutment face 86 of the ring 98 and consequently causes unlocking the socket 64 in a similar configuration

to that of figure 2.

  The invention therefore advantageously makes it possible to benefit

  maximum braking force in all circumstances.

Claims (10)

  1. Pneumatic actuator (10) for actuating a master cylinder (28) for braking a motor vehicle, of the type which comprises a rigid casing (12) inside which a transverse partition (14) is movable sealingly delimiting a front chamber (16), subjected to a first pressure (P.) of engine depression, and a rear chamber (18) subjected to a second pressure (P2) varying between engine depression o and atmospheric pressure ( Pa), of the type which comprises a movable piston (22), integral with the movable partition (14), one front face (26) of which is capable of urging an actuating rod (34) of the master cylinder (28), of the type which comprises a rod (46) for controlling the i5 servomotor (10) moving in the piston (22) selectively as a function of an axial input force exerted forward against a force of return exerted on the rod (46) by a return spring (50), of the type which includes a plunger (56) which is agen at the front of the control rod (46) in the piston (22) and which comprises at its rear end at least one annular seat (58) rear of a three-way valve (60) which is progressively movable between a position in which, the control rod (46) being at rest, the front chamber (16) and the rear chamber (18) are in communication, and a position in which, the control rod (46) being actuated, the second pressure (P2) prevailing in the rear chamber (18) increases, the valve (60) putting the rear chamber (18) in communication with the atmospheric pressure (Pa) 'and of the type which comprises a first feeler (62), forming the front end of the plunger (56) and passing through the piston (22) which, in the rest position of the control rod (46), is arranged at a determined first jump distance (d1) from a disc (44 ) of reaction interposed between the rod (34) for actuating the master cylinder (28) and the front face (26) of the movable piston ( 22), and which is likely, when the control rod (46) is actuated at a speed lower than a determined speed, to be urged by the plunger (56) to cover the first jump distance (d1) and then penetrate the reaction disc (44) so as to transmit to the plunger (56) and the rod (46)   controls the reaction force of the master cylinder (28).   characterized in that it comprises a tubular sleeve (64), which is slidably mounted on the plunger (56), which comprises a front section (66) which crosses the piston (22) and one end of which comprises a second feeler ( 68), and which is likely, when the control rod (46) is actuated at a speed greater than a determined speed, to be pushed by the plunger (56) and blocked by a one-way clutch device (70) according to a determined axial position relative to the piston (22), then to be started by the piston (22) to drive the second probe (68) to penetrate the reaction disc (44) so as to allow the first probe (52 ) to apply a rapid braking force against an antagonistic reaction force of the master cylinder (28) which is reduced along a second determined jump distance (d2), greater   at the first jump distance (d1).   2. Booster (10) according to the preceding claim, characterized in that the sleeve (64) has a rear toboggan section (72) which is slidably mounted in a bore   rear (74) complementary to the piston (22).   3. Booster (10) according to the preceding claim, characterized in that the one-way clutch device (70) comprises a key (78), which is mounted in a recess (80) of the piston (22) passing through the piston (22 ) perpendicular to its axis (A), and of which a substantially annular intermediate portion (82) surrounds the bushing (64) with play and is susceptible, when the input force is exerted on the control rod (46) at a speed greater than the determined speed, to be driven by the movable piston (22) to tilt around a generally transverse axis so as to cooperate with the periphery of the rear tubular section (72) of the sleeve (64) for block said socket (64) according to the position s axial determined.   4. Booster (10) according to the preceding claim, characterized in that the rear tubular section (72) of the sleeve (64) comprises at least one transverse face (100) for locking, which is turned axially towards the rear to form a stop for a lug (102) which is carried by an upper part of the key (78) and which extends radially towards the socket (64), so as to axially index the determined locking position   the socket (64) and the second probe (68).   5. Booster (10) according to the preceding claim, characterized in that the rear section (72) of the sleeve has a groove (104) of substantially frustoconical profile, a front shoulder face forming the transverse face (100) of blocking. 6. Booster (10) according to the preceding claim, characterized in that the lug (102) of the key is. in section through an axial plane, shaped according to an angular sector complementary to the frustoconical profile of the groove (104) of the sleeve (64) 7. Booster (10) according to any one of   Claims 3 to 6, characterized in that the key (78) comprises   a lower part (84) substantially in the shape of an "L", diametrically opposite the erpot (102), which extends radially from its annular part (82) and which is resiliently biased against a face (86) of abutment of the casing (12) of the booster (10) by means of a return spring (88) interposed between a front face (90) of the recess (80) of the piston (22 ) and a front face (92) of said lower part.   8. Booster (10) according to any one of   Claims 2 to 7, characterized in that the second probe (68)   has an outer diameter greater than that of the rear section (72) of the sleeve (64).   9. Booster (10) according to any one of   previous claims, characterized in that the disc (44)   s resection is received in a cup (36), coaxial with the piston (22), which is integral with the rear end (38) of the actuating rod (34) and of which a rear tubular part (40) is mounted sliding in a coaxial annular groove (42) of the front face (26) of the piston (22).   o 10. Booster (10) according to one of claims   previous, characterized in that the key (78) has substantially, in section through an axial plane, the shape of a "T" whose vertical branch of substantially radial orientation comprises the annular intermediate part (82) and the lower part (84) substantially in the shape of an "L", and the upper horizontal branch (83) of which is received with clearance between the two opposite walls (90, 91) of the recess (80) and has its projection