Abstract:
A pneumatic booster operated by a plunger ( 9 ) housed in a bore ( 61 ) and bearing on a moving seat ( 102 ) which co-operates selectively with a shut-off member ( 111 ). The shut-off member ( 111 ) is capable of selective co-operating with a stationary seat ( 101 ). The plunger ( 9 ) being characterized by a cylindrical body ( 91 ) and a ring ( 92 ). The ring ( 92 ) sliding between a front stop ( 911 ) and a rear stop ( 912 ) on the cylindrical body ( 91 ). A front section of the ring ( 92 ) being located between the cylindrical body ( 91 ) and the bore ( 61 ) while a rear section ( 922 ) is designed to bear against the moving seat ( 102 ). The ring ( 92 ) being temporarily retained in a stationary position in the bore ( 61 ) independent of any movement of the cylindrical body ( 91 ) through the action of a pivoting key ( 13 ) and an elastic snap ring ( 141 ) to allow permit and extended actuation of the booster after a sharp braking action has occurred.

Description:
The present invention relates to a pneumatic brake-booster. 
   More specifically, the invention relates to a brake-booster comprising: a rigid casing; a moving partition delimiting, in a sealed way, a front chamber and a rear chamber inside the casing, the front chamber in operation being permanently connected to a first source of pressure delivering a first pressure; a pneumatic piston moving with the moving partition and having a hub mounted to slide in an opening of the casing; an operating rod that can move in the hub between a position of rest and an extreme actuating position as a function of a resultant of forces which acts on it and which comprises an input force exerted in an axial actuating direction directed towards the front chamber, and a main return force exerted by a main spring in an axial return direction that is the opposite of the axial actuating direction; a plunger housed in a bore of the hub and carried along by the operating rod; and a three-way valve itself comprising a stationary annular seat formed at an internal periphery of the hub, a moving annular seat, concentric with the stationary seat and borne by the plunger and a tubular shutter concentric with the plunger and with the hub and having an annular front shut-off face, this front face co-operating selectively with the moving seat so as to isolate the rear chamber from a second source of pressure delivering a second pressure higher than the first, and this front face co-operating selectively with the stationary seat so as to allow the rear chamber to be connected to the second source of pressure. 
   BACKGROUND OF THE INVENTION 
   Devices of this type are well known in the prior art, as illustrated, for example, by 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 actual risks involved and, having braked suddenly, release their braking effort under circumstances where, on the contrary, maintaining a high effort is absolutely essential to avoid the accident. 
   This observation has led to the development of various solutions in an attempt to remedy the possibly defective behaviour of an inexperienced or panic-stricken driver. 
   The invention falls within this context and its object is to provide a simple solution to the problem of maintaining a high braking force following a sharp braking action. 
   SUMMARY OF THE INVENTION 
   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 cylindrical body secured to the operating rod and a ring mounted to slide axially with respect to the cylindrical body between front and rear stops of the body, a front section of the ring being engaged between the cylindrical body and the bore of the hub, a rear section of the ring forming a collar on which the moving seat is defined, and the front and rear stops being separated from one another by a distance that allows the ring an axial excursion, in that a secondary spring exerts on the collar a secondary return force directed in the axial return direction and urging the collar towards the rear stop, and in that the ring and the hub comprise respective first and second reversible retaining means capable of keeping the ring stationary with respect to the hub in spite of the secondary return force exerted on the ring and correspondingly of allowing the rear chamber to be connected to the second source of pressure, these retaining means being activated by the ring being driven into the bore over a travel that is at least equal to a determined minimum travel, and being deactivated at least on the condition that the ring is driven along by the front stop when this front stop moves in the axial return direction under the effect of the main return force. 
   In a preferred embodiment of the invention, the retaining means comprise a pivoting key straddling the ring and an elastically retractable stop member connected to the hub, this stop member intercepting the key and applying a determined retaining force to it when these retaining means are activated. 
   This elastically retractable stop member may, for example, consist of a flexible strand of an elastic snap ring secured to the hub. 
   Apart from the pivoting key, the retaining means may also comprise an electromagnet secured to the hub and selectively operated in such a way as to retain this pivoting key once the ring has been driven into the bore over a travel at least equal to the determined minimum travel. 
   To further reduce the intensity of the residual input force that the driver has to apply in order to keep the booster of the invention in its actuating state after a sharp braking action, it may also be advantageous to envisage for the plunger to comprise a push-rod selectively given a sliding movement with respect to the cylindrical body, arranged in front of the cylindrical body in the actuating direction, and having a shoulder co-operating with the hub to limit the movement of this push-rod with respect to the hub in the axial return direction, to envisage for the cylindrical body to keep the push-rod in a forwards position, in the actuating direction, when the operating rod is in its extreme actuating position, the cylindrical body and the push-rod then being in axial contact one against the other, and to envisage for the shoulder of the push-rod to co-operate with the hub to cause the axial contact between the cylindrical body and the push-rod to cease when the operating rod returns to its position of rest. 
   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 view in section of a booster according to the invention; 
       FIG. 2  is a view in section of the plunger of a booster according to the invention, seen at rest; 
       FIG. 3  is a view in section of the plunger of a booster according to the invention, seen in its standard operating mode; 
       FIG. 4  is a view in perspective of the plunger and of the associated members of a booster in accordance with the invention, illustrating the pivoting key and the elastic snap ring; 
       FIG. 5  is a front-on view of the pivoting key associated with a view in section of the elastic snap ring; 
       FIG. 6  is a view in section of the pneumatic piston seen in the direction marked by the arrows  6 — 6  of  FIG. 3 ; 
       FIG. 7  is a view in section of the pneumatic piston seen in the direction marked by the arrows  7 — 7  of  FIG. 6 ; 
       FIG. 8  is a view in section of the plunger of a booster in accordance with the invention, seen in its specific operating mode, this view depicting the retaining means in their activated state; 
       FIG. 9  is a view in section of the plunger of a booster in accordance with the invention, seen in its specific operating mode, this view depicting the retaining means in their activated state and during co-operation; 
       FIG. 10  is a view in section of the plunger of a booster in accordance with the invention, seen in its specific operating mode, this view depicting the retaining means during deactivation; and 
       FIG. 11  is a view in section of the plunger of a booster in accordance with a second embodiment of the invention, this view depicting the retaining means in their activated state and during co-operation. 
   

   DESCRIPTION OF THE INVENTION 
   The pneumatic booster of the invention comprises, as is 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 a sealed way and therein delimits a front chamber  3  and a rear chamber  4 , the front chamber  3  in operation being connected to a first source of pressure D which delivers 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 and 2 ) and an extreme actuating position ( FIG. 8 ), the position of this rod depending both on an input force Fe which is applied to this rod in an axial actuating direction X+ by the brake pedal (not depicted) and on a return force Frp exerted in the opposite direction X− by a main return spring  71 . 
   A plunger  9 , driven along by the operating rod  7 , is mounted to slide in a bore  61  of the hub  6  to control the state of a three-way valve  10 . 
   This valve  10  consists essentially ( FIG. 2  et seq) of a stationary annular seat  101  formed at the internal periphery of the hub  6 , a moving annular seat  102  which is concentric with the stationary seat  101  and borne by the plunger  9 , and a tubular shutter  11  which is itself concentric both with the plunger and with the hub. 
   The tubular shutter  11  has an annular front shut-off face  111  intended to co-operate, depending on the position of the plunger  9 , either with the moving seat  102 , or with the stationary seat  101 . 
   When the booster is at rest ( FIGS. 1 and 2 ), the front face  111  of the shutter  11  presses against the moving seat  102  and isolates the rear chamber  4  from a second source of pressure A, generally consisting of the atmosphere, and which delivers a pressure Pa higher than the pressure Pd of the first source D. 
   By contrast, when an input force Fe appreciably higher than the return force Frp of the spring  71  is exerted on the rod  7 , this force Fe causes the plunger  9  to move in the actuating direction X+ so that the front face  111  of the shutter  11  becomes detached from the moving seat  102 , pressing against the stationary seat  101  ( FIG. 3 ), the rear chamber  4  then finding itself connected to the source of pressure A. 
   The admission of air into the rear chamber pushes the moving partition  2  back in the actuating direction X+ and generates a boost force used to actuate the master cylinder  20 . 
   According to the invention, as shown best by  FIG. 2  et seq, the plunger  9  comprises a cylindrical body  91  secured to the operating rod  7 , and a ring  92 . 
   The ring  92  is mounted to slide axially in a sealed way over the outside of the cylindrical body  91  between a front stop  911  and a rear stop  912 , both borne by the body  91 , the front  911  and rear  912  stops being separated from one another by a distance that allows the ring  92  an axial excursion over the body  91 . 
   A front section  921  of the ring  92  is engaged in the annular space which separates the cylindrical body  91  from the bore  61  of the hub, and completely fills this space in a radial direction. 
   A rear section  922  of the ring  92  is shaped as a collar and bears the moving seat  102 . 
   A secondary return spring  12  exerts on the collar  922  a secondary return force Frs directed in the axial return direction X− and urging the collar  922  towards the rear stop  912 . 
   A pivoting key  13  is engaged in a transverse peripheral groove  93  of the ring  92  so as to straddle this ring  92 , to move with it along the axis of the directions X− and X+, and be able to incline with respect to the plane formed by the groove  93 , as shown by a comparison between  FIGS. 2 and 8 , when its base  130  encounters a stop  610  formed in a cutout in the hub  6  (see  FIG. 7 ). 
   An elastically retractable stop member  141  is also connected to the hub  6  so as to intercept the key  13  when the ring  92  is driven into the bore  61  over a travel that exceeds the travel through which this ring moves in the case of normal actuation of the booster, illustrated in  FIG. 3 . 
   The elastically retractable stop member  141  consists, for example, of a flexible strand forming a bent-over end of an elastic snap ring  14  which, incidentally, is of essentially circular shape, engaged in a groove  62  of the hub  6 . Each of the ends  141 ,  142  of the snap ring  14  is preferably engaged in a corresponding cut  131 ,  132  of the key  13  so as to achieve optimum guidance of these ends (see  FIG. 5 ). 
   Thus, in the case illustrated in  FIG. 8  in which the booster is actuated sharply enough for the ring  92  to be driven into the bore  61  over a travel that exceeds a determined minimum travel C, the key  13  engages in the stop member  141 , which then applies to the key  13 , and therefore to the ring  92 , a determined retaining force capable of keeping the ring  92  stationary with respect to the hub  6  in spite of the secondary return force Frs exerted by the spring  12 , and correspondingly of allowing the rear chamber  4  to be connected to the second source of pressure A. 
   Under these conditions, the valve  10  remains open, even when the driver partially releases his braking effort, as shown in  FIG. 9 . 
   To allow the booster to return to its state of rest, the front stop  911  has to move, under the effect of the main return force Frp exerted by the spring  71 , and drive the ring  92  along in the axial return direction X−, which assumes that the driver reduces his braking effort to the point where the main return force Frp and the secondary return force Frs can together manage to overcome the input force Fe and the retaining force exerted by the stop member  141  on the key  13 , the latter then disengaging from the stop member as shown by  FIG. 10 . 
     FIG. 11  illustrates one embodiment of the invention, by virtue of which it is possible, after the booster has been actuated sharply, to keep the valve  10  open even if the driver completely releases his braking effort. 
   To this end, the booster in accordance with this embodiment comprises, in addition to the pivoting key  13  and possibly the stop member  141 , an electromagnet  15  which is secured to the hub  6  and which is operated selectively to apply to the pivoting key a retaining force that is capable, either by itself or assisted by the retaining force exerted by the stop member  141 , of keeping the ring  92  in an actuating position as illustrated in  FIG. 11 . 
   As this figure also shows, the key  13  is then preferably equipped with an extension  133  via which it can be attracted by the electromagnet  15 . 
     FIGS. 2 ,  3  and  8  to  11  also illustrate additional features which make it possible to reduce still further the value of the residual input force that the driver needs to apply in order to keep the booster of the invention in its actuating state after intense braking. 
   To this end, the plunger  9  comprises a push-rod  94  which is mounted to slide with respect to the cylindrical body  91  and which is arranged in front of the latter in the actuating direction X+. 
   When the operating rod  7  is in its position of rest ( FIG. 2 ), the cylindrical body  91  and the push-rod  94  are not axially in contact one against the other, in other words, they are axially distant from one another. 
   The push-rod  94  has a shoulder  95  capable of coming into rear abutment against the hub  6  in order to limit the movement of this push-rod  94  with respect to the hub  6  in the axial return direction X−. 
   When the operating rod  7  is in its extreme actuating position ( FIG. 8 ), the cylindrical body  91  keeps the push-rod  94  in an advanced position, in the actuating direction X+, the cylindrical body  91  and the push-rod  94  then being in axial contact one against the other, and the shoulder  95  of the push-rod being detached from the hub  6 . 
   By contrast, when the operating rod  7  returns to its position of rest after a sharp braking action ( FIG. 9 ), the shoulder  95  of the push-rod  94  co-operates again with the hub  6  to make the axial contact between the cylindrical body  91  and the push-rod  94  cease. 
   In this way, the reaction forces which are applied to the push-rod  94  in the return direction X− are not transmitted to the operating rod  7 , which means that keeping this rod in the position as illustrated in  FIG. 9  for which the booster remains actuated requires only a very small effort on the part of the driver.