Abstract:
A pneumatic booster operated by a plunger ( 8 ) located in a piston ( 5 ) with a rear section ( 82 ) which operates a valve ( 91,92,93 ) to develop a force on the piston ( 5 ) to apply a boost force to force-transmission members ( 12,14 ). The plunger ( 8 ) has a first section ( 81 ) connected to the rear section ( 82 ) by a spring ( 15 ). The front section ( 81 ) and the force-transmission members ( 12,14 ) defining a retention assembly ( 16,17 ) for securing the front section ( 81 ) to the piston ( 5 ) when a boost force exceeds a predetermined threshold while the plunger ( 8 ) is in a maximum actuating position to allow for extended actuation of the booster following a sharp braking action.

Description:
The present invention relates to a pneumatic brake-booster. 
     More specifically, the invention relates to a pneumatic brake-booster comprising: a rigid casing; a moving partition delimiting, in leaktight fashion, a front chamber and a rear chamber inside the casing, the front chamber in operation being subjected to a first pressure and the rear chamber being selectively connected to the front chamber or subjected to a second pressure higher than the first; a pneumatic piston moving with the moving partition; an operating rod moving in the piston as a function of an input force selectively exerted in an axial actuating direction orientated towards the front chamber, and of a return force exerted by a main spring in an axial return direction which is the opposite of the axial actuating direction, the return force urging the operating rod into a return position, and the input force urging the operating rod into an intermediate actuating position or into an extreme actuating position, depending on whether the input force is applied at a rate that is lower or higher than a determined limiting rate; a plunger housed in the piston and driven along by the operating rod; a three-way valve itself comprising an annular seat borne by a rear section of the plunger, this valve connecting the rear chamber to the front chamber when the operating rod is in the position of rest, and subjecting the rear chamber to the second pressure when the operating rod is in one of its actuating positions; force-transmission means capable of receiving and of transmitting at least a boost force exerted by a front face of the piston when the valve subjects the rear chamber to the second pressure, these force-transmission means themselves comprising reaction means capable of reacting against the input force with a reaction force that depends on the boost force. 
     BACKGROUND OF THE INVENTION 
     Devices of this type are well known in the prior art, as illustrated, for example, in patent documents U.S. Pat. No. 3,470,697, FR-2,532,084 and FR-2,658,466. 
     Recent research has shown that a good many drivers, when confronted with an emergency braking situation, underestimate the risks actually involved and, having braked sharply, release their braking effort under circumstances in which, on the contrary, it is absolutely essential that they maintain a substantial braking effort in order to avoid the accident. 
     This observation has led to the development of various solutions in an attempt to alleviate any possible shortcomings in the behaviour of an inexperienced or panic-stricken driver. 
     SUMMARY OF THE INVENTION 
     The invention falls within this context and is intended to provide a simple solution to the problem of maintaining a substantial braking force after a violent braking action. 
     To this end, the booster of the invention which in other respects is in accordance with the definition given in the above preamble, is essentially characterized in that the plunger comprises a front section connected to the rear section by a secondary spring exerting, between the front and rear sections, a mutual-contact force that is lower than the return force, and in that the front section and the force-transmission means comprise respective reversible retention means capable of securing the front section to the piston when the boost force exceeds a determined threshold while the operating rod is in its extreme actuating position. 
     In one possible embodiment of the invention, the retention means comprise an elastic annular layer axially subjected to the boost force, this layer having an axial thickness and an inside diameter which are smaller, the higher the boost force. 
     For example, reaction means comprise a reaction disc made of elastomeric material, the retention means comprise a rigid disc constituting a front face of the front section of the plunger, and the annular layer consists of a thickening of the reaction disc which delimits, in the reaction disc, a housing capable of trapping the rigid disc when the boost force exceeds the determined threshold while the operating rod is in its extreme actuating position. 
     In this case, the housing and the rigid disc preferably have complementary frustoconical shapes. 
     The retention means may also comprise a shoulder with the external shape, for example at least partially cylindrical, of the front section. 
     In this case, the retention means advantageously comprise a rigid insert located in the elastic annular layer and interacting with the shoulder with the external shape of the front section in order to combine the front section with the piston when the boost force exceeds the determined threshold while the operating rod is in its extreme actuating position. 
     In other embodiment of the invention, the retaining means comprise a rigid sleeve arranged inside an elastic sleeve wich delimits a housing capable of trapping the rigid sleeve when the boost force exceeds the determined theshold while the operating rod is in its extreme actuating position. 
    
    
     Other features and advantages of the invention will emerge clearly from the description thereof given hereafter by way of non-limiting indication, with reference to the appended drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectioned view of a booster in accordance with a first embodiment of the invention; 
     FIGS. 2,  3  and  4  are detailed sectioned views of part of the booster illustrated in FIG. 1, seen at various stages of operation; 
     FIG. 5 is a sectioned view of a booster in accordance with a second embodiment of the invention; 
     FIGS. 6,  7  and  8  are detailed sectioned views of part of the booster illustrated in FIG. 5, seen at various stages of operation, and 
     FIG. 9 is a sectional view of the rear part of a booster in accordance with a third embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A pneumatic booster in accordance with the invention comprises, in a way known per se, a front shell  1   a  and a rear shell  1   b  which together form a rigid casing  1 . 
     A moving partition  2  divides the inside of this rigid casing  1  in leaktight manner and therein delimits a front chamber  3  and a rear chamber  4 , the front chamber  3  being, when in operation, subjected to a first, relatively low, pressure Pd. 
     A pneumatic piston  5  moves with the moving partition  2  and has a hub  6  mounted to slide in an opening  19  of the casing  1 . 
     The booster is operated by an operating rod  7  that can move in the hub  6  between a position of rest (FIGS. 1,  2 ,  5 ,  6  and  9 ) and an extreme actuating position (FIGS.  3  and  7 ), the position of this rod in the hub depending, in particular, both on an input force Fe which is applied to this rod, in an axial actuating direction X+ by the brake pedal (not depicted), on the rate at which this input force Fe is applied, on a return force Fr exerted in the opposite direction X− by a main return spring  71 , and on a reaction force in the same direction as the return force. 
     When the input force Fe is applied slowly to the operating rod  7 , the latter adopts an intermediate actuating position somewhere between its position of rest (FIGS. 1,  2 ,  5 ,  6  and  9 ) and its extreme actuating position (FIGS.  3  and  7 ), the operating rod reaching its extreme actuating position only when the input force Fe is applied at a rate higher than a given limiting rate. 
     A plunger  8 , carried along by the operating rod  7 , is mounted to slide in a bore  61  in the hub  6 , to control the status of a three-way valve  9 . 
     This valve  9  essentially consists of a stationary annular seat  91  formed at the internal periphery of the hub  6 , of a moving annular seat  92 , concentric with the stationary seat  91  and borne by a rear section  82  of the plunger  8 , and of a tubular shutter  93 , which is itself concentric both with the plunger and with the hub. 
     The tubular shutter  93  has an annular front shut-off face intended to interact, according to the position of the plunger  8 , either with the moving seat  92  or with the stationary seat  91 . 
     When the booster is at rest (FIGS. 1,  2 ,  5 ,  6  and  9 ), the shutter  93  presses against the moving seat  92  and isolates the rear chamber  4  from a source of pressure, generally consisting of the atmosphere, which delivers a pressure Pa higher than the pressure Pd to which the front chamber  3  is subjected. 
     By contrast, when an input force Fe appreciably higher than the return force Fr of the spring  71  is exerted on the rod  7 , this force Fe causes the plunger  8  to move in the actuating direction X+, so that the shutter  93  comes away from the moving seat  92  and presses on the stationary seat  91  (FIG.  3 ), the rear chamber  4  thus finding itself subjected to the pressure Pa. 
     The entry of air into the rear chamber pushes the moving partition  2  back in the actuating direction X+ and generates a boost force Fa which is exerted on the front face  51  of the piston  5 . 
     The input force Fe and the boost force Fa are applied together to the force-transmission members which uses them to actuate the master cylinder  20 , these force-transmission members comprising reaction members capable of reacting against the input force Fe with a reaction force that depends on the boost force Fa. 
     The specific nature of the force-transmission and reaction members varies with the type of booster in question, but the present invention can be implemented irrespective of the type of reaction used in the booster to which it is fitted. 
     For example, FIG. 1 illustrates a booster which, in a known way, uses a reaction disc  13  made of an elastomeric material housed in a cup  11  and which receives both the input force Fe transmitted by the plunger  8  and the boost force Fa exerted by the front face  51  of the pneumatic piston  5 . 
     The reaction disc  13  reacts against the input force Fe with a reaction force which varies with the boost force Fa, and the cup  11 , which is joined to a push rod  10 , transmits to the primary hydraulic piston  200  of the master cylinder  20  the resultant of these forces, of the return force Fr and of the elastic force of a piston-return spring  35 . 
     FIG. 5 by contrast illustrates a so-called “hydraulic reaction” booster, also known, such a booster using a hydraulic reaction piston  14  which receives the input force Fe decreased, in particular, by the return force Fr, and which slides in a hydraulic cylinder  12  designed to receive the boost force Fa exerted by the front face  51  of the pneumatic piston  5 . 
     According to the invention, the plunger  8  has a front section  81  connected to the rear section  82  by a secondary spring  15 , while the front section  81  of the plunger and the force-transmission members comprise respective reversible retention means capable of securing the front section  81  to the piston when the boost force Fa exceeds a determined threshold S while the operating rod is in its extreme actuating position. 
     More specifically, the secondary spring  15 , which for example consists of a compressively preloaded helical spring, brings the front and rear sections  81 ,  82  closer together by exerting on them a mutual contact force Fc that is lower than the return force Fr exerted by the return spring  71  of the operating rod  7 . 
     In the preferred embodiments of the invention, as illustrated, the retention means comprise an elastic annular layer  16  axially subjected to the boost force Fa, this layer  16  having an axial thickness E and an inside diameter Di which are lower, the higher the boost force Fa. 
     In the case of a booster that employs a reaction disc  13 , as illustrated in FIGS. 2 to  4 , the retention means also comprise, for example, a rigid disc  811  constituting a front face of the front section  81  of the plunger  8 , the annular layer  16  in this case consisting of a thickening of the reaction disc  13  which delimits, in the reaction disc, a housing  101  capable of trapping the rigid disc  811  when the boost force Fa exceeds the determined threshold S while the operating rod is in its extreme actuating position, the housing  101  and the rigid disc  811  possibly having complementary frustoconical shapes. 
     The way in which the invention, as may be explained with reference to FIGS. 2 to  4 , functions is as follows. 
     When the booster is at rest (FIG.  2 ), the operating rod  7  is pulled in the return direction X− by the force Fr exerted by the spring  71 , which means that the moving annular seat  92  is pressed against the shutter  93 . 
     Under these conditions, the rear chamber  4  communicates with the front chamber  3  and is isolated from atmospheric pressure Pa. 
     If an input force Fe which changes slowly (this state is not illustrated) is applied to the operating rod  7 , the moving seat  92  moves slowly away from the shutter  93 , and this opens the rear chamber  4  to the atmosphere and causes the plunger  8  to move in the actuating direction X+. 
     As, in this movement, the front face  51  of the plunger  8  is pressed on the periphery of the reaction disc  13 , the central part of this disc, in which the housing  101  is formed, is pushed back in the return direction X− until it encounters the rigid disc  811 , this central part of the reaction disc then exerting on the rigid disc  811  a reaction force that is directed in the opposite direction X−. The operating rod is therefore in an intermediate actuating position. 
     If, by contrast, an input force Fe that changes quickly (FIG. 3) is applied to the operating rod  7 , the latter then adopts its extreme actuating position and the rigid disc  811  becomes inserted in the housing  101  before the pneumatic piston  5  has had time to press against the periphery of the reaction disc  13 . 
     Under these conditions, when the pneumatic piston  5  presses on the annular layer  16  at the periphery of the reaction disc  13 , this layer  16  is made to flow towards the centre of the reaction disc, reducing its inside diameter Di, and keeps the rigid disc  811  trapped in the housing  101 . 
     When the brake pedal (not depicted) is released gradually, and although the input force Fe is reducing (FIG.  4 ), the rigid disc  811  remains trapped in the housing  101  as long as the seat  92  of the valve does not move the shutter  93  off the stationary seat  91 , which means that maximum boost force continues to be applied to the master cylinder  20  during this phase of brake release. 
     In fact, the application of this boost force does not cease until the seat  92  of the valve has retreated far enough in the direction X− to move the shutter  93  off the stationary seat  91 , the rear chamber  4  then being connected to the front chamber  3  again. 
     In the embodiment of FIGS. 5 to  8 , the annular layer  16  is arranged between the front face  51  of the pneumatic piston  5  and the hydraulic cylinder  12 , and the retention means comprise a shoulder  810  of the, at least partially cylindrical, external shape of the front section  81  of the plunger  8 . 
     A rigid insert  17  may also be arranged in the elastic annular layer  16  to interact with the shoulder  810 . 
     When the booster is at rest (FIG.  6 ), the operating rod  7  is pulled in the return direction X− by the force Fr exerted by the spring  71 , which means that the moving annular seat  92  is pressed against the shutter  93 . Under these conditions, which are identical to those illustrated in FIG. 2, the rear chamber  4  communicates with the front chamber  3  and is isolated from atmospheric pressure Pa. 
     If an input force Fe changing slowly (state not illustrated) is applied to the operating rod  7 , the moving seat  92  moves slowly away from the shutter  93 , and this opens the rear chamber  4  to the atmosphere. The operating rod is then in an intermediate actuating position. 
     The venting of the rear chamber  4  to atmosphere in turn causes the plunger  8  to move in the actuating direction X+ and correspondingly causes the reaction piston  14  to move in the return direction X−, this reaction piston then exerting on the front section  81  of the plunger  8  a reaction force directed in this same direction X−. 
     Bearing in mind the small relative movement of the plunger  8  compared with the pneumatic piston  5 , the relative position of the shoulder  810  and of the annular layer  16  when the booster is in this state remains practically that illustrated in FIG.  6 . 
     If, by contrast, an input force Fe that changes quickly (FIG. 7) is applied to the operating rod  7 , the latter then adopts its extreme actuating position and the shoulder  810  becomes placed in front of the insert  17  before the pneumatic piston  5  has had time to press against the hydraulic cylinder  12  and therefore make the reaction piston  14  retreat. 
     Under these conditions, when the pneumatic piston  5  presses on the annular layer  16  (FIG.  7 ), this layer  16  is made to flow towards the shoulder  810 , pushing the insert or each insert  17  back towards this shoulder and thus combining the front section  81  of the plunger  8  with the pneumatic piston  5  so that they move in translation as one. 
     When the brake pedal (not depicted) is released gradually, and although the input force Fe is reducing (FIG.  8 ), the shoulder  810  remains trapped by the insert or inserts  17  as long as the seat  92  of the valve has not moved the shutter  93  off the stationary seat  91 , which means that a maximum boost force continues to be applied to the master cylinder  20  during this phase of brake release. 
     The application of this boost force does not cease until the seat  92  of the valve has retreated far enough in the direction X− to move the shutter  93  off the stationary seat  91 , the rear chamber  4  then being connected to the front chamber  3  again. 
     In the embodiment of FIG. 9, the retaining means comprise a rigid sleeve  8110  arranged inside an elastic sleeve  16  which delimits a housing capable of trapping the rigid sleeve  8110  when the boost force Fa exceeds the determined threshold S while the operating rod  7  is in its extreme actuating position. 
     By virtue of the invention, it is thus possible to keep the valve  9  wide open for the greatest possible proportion of the brake release phase that follows an emergency braking operation, and therefore to continue to apply a braking force that is very much stronger than the one that would be obtained, without the invention, by the braking effort actually supplied by the driver.