Abstract:
A reaction device wherein a ratio of input force to output force for a brake booster is sequentially changed during a brake application. An actuation force derived from a pressure differential acts on a movable wall and is communicated through a cage as an output force to an push rod. The output force is a functional ratio of the input force as modified by a reaction disc and elastic means during a brake application. The reaction device functions as a rigid member until the input force reaches a first predetermined level Fd and a first boost ratio is in effect until a second predetermined force Fe is reached and a second boost ratio is in effect until a third predetermined input force level Ff is reached and a third boost ratio is in effect until a fourth predetermined fource level is reached.

Description:
The present invention relates mainly to a reaction device for a pneumatic brake booster with modifiable boost and to a brake booster comprising such a device. 
   BACKGROUND OF THE INVENTION 
   A pneumatic brake booster is used in a braking system to pneumatically amplify the muscle force applied to a brake pedal. The booster is arranged between the brake pedal and a master cylinder, for example a tandem master cylinder. 
   It comprises a casing divided into two chambers by a sealed moving wall, this wall being able to move by a pressure differential between the two chambers. The moving wall comprising a pneumatic piston extending radially outwards via a skirt crimped to the external periphery of the piston. The pressure differential between the two chambers is controlled by a three-way valve subject to the movement of an actuating rod controlled by a brake pedal. This three-way valve is arranged in the rear cylindrical part of the pneumatic piston. 
   The action of the actuating rod is transmitted to the pistons of a master cylinder by means of a push rod via a reaction disc made of an elastomeric material, for example rubber. The reaction disc is a means of combining three forces, a first force from the actuating rod applied via a plunger distributor which is fixed with respect to the actuating rod, the second force from the boost supplied by the pneumatic piston as a result of the movement of the moving wall by the pressure differential, and a third force being the reaction of the hydraulic braking circuit which is transmitted through the push rod. This disc allows the driver of the vehicle to adjust his action on the brake pedal by feeding information back to him from the hydraulic circuit. 
   The force applied by the driver to the pedal will be known as the input force; the force applied by the actuating rod will be known as the actuating force and the force exerted by the push rod on a piston of a master cylinder, for example a tandem master cylinder, will be known as the output force. 
   By virtue of this type of brake booster it is possible to obtain a fairly refined relationship between the input force and the output force over a certain range of input force values. 
   There are devices which allow the boost of the booster to be varied, that is to say which allow the ratio between the input force and the output force to be varied, for example so as to increase the boost above and beyond a certain input force. Patent FR 00/15943 describes a device applied to the rear cylindrical part of the pneumatic piston, the distributor is made up of two parts, a plunger on the same side as the actuating rod and a feeler on the same side as the reaction disc, a prestress spring in a deformable cage is applied between the feeler and the plunger. For an actuating force below the spring prestress, the feeler and the plunger behave like a rigid piece. For an input force that is high enough, the spring compresses, the feeler can then move axially towards the brake pedal allowing the reaction disc to expand towards the brake pedal. There is therefore a reduction in the reaction force transmitted to the actuating rod, and this increases the boost ratio. 
   However, this device entails significant modification to the internal structure of the booster because of its bulk, although for cost reasons equipment manufacturers are seeking to standardize their products. In addition, they are also seeking to reduce the size of the boosters. 
   SUMMARY OF THE INVENTION 
   In consequence, it is an object of the present invention to offer a reaction device that can be fitted to a pneumatic booster of known type. 
   It is also an object of the invention to offer a reaction device of smaller size. 
   It is also an object of the present invention to offer a pneumatic brake booster offering a modifiable boost ratio and having a standard piston body. 
   It is also an object of the present invention to offer a pneumatic brake booster that offers a modifiable boost ratio at low cost. 
   It is also an object of the present invention to offer a pneumatic brake booster that offers a modifiable boost ratio with small size. 
   It is also an object of the present invention to offer a pneumatic brake booster offering a modifiable level of boost and with simple design and manufacture. 
   The present invention advantageously can be mounted in a simple way on pneumatic pistons of boosters of standard form. 
   The present invention also has the advantage of being of a simple design and having a low cost of manufacture. 
   The present invention also has the advantage of being small in size. 
   Thus, these objects are achieved by a reaction device according to the present invention comprising a housing in which there is arranged a reaction disc made of incompressible or practically incompressible materials, delimited: 
   a) at a first longitudinal end by a first surface, 
   b) at a second end opposite the first end, by a second radially external surface for application of the pneumatic piston and a third surface for application of the plunger delimited by the second surface, and 
   c) by a fourth radial surface extending from the first surface to the second and third surfaces. 
   The reaction device also comprises at least one elastic means able to be deformed, and means of prestressing the elastic means, the elastic means allowing, for a certain value of input force higher than a certain fixed value, an increase in the volume available for the reaction disc which takes place in a collection of the first and of the second surfaces. 
   The main subject of the present invention is a reaction device for a brake booster comprising a reaction disc arranged in a receiving housing delimited at a first longitudinal end by a first surface, at a second end opposite the first end, by a second radially external surface and a third surface delimited by the second surface, and by a fourth radial surface extending axially from the first surface to the second and third surfaces, the reaction device also comprising at least one elastic means which can be deformed and which allows the available volume of the reaction disc to increase for an input force above a certain force value, and determining a first boost ratio for an input force below the predetermined value, characterized in that the increase in the volume available for the reaction disc takes place in a collection of the first and of the fourth surfaces and in that the reaction device determines at least a second boost ratio for an input force value higher than the predetermined value. 
   Another object of the present invention is a reaction device, characterized in that the fourth surface is at least partially defined by a radial end of the said push rod. 
   Another object of the present invention is a reaction device, characterized in that the first surface is defined by an annular washer and at least one floating piece. 
   Another object of the present invention is a reaction device, characterized in that the first surface is formed by a piece secured to one axial end of the push rod. 
   Another object of the present invention is a reaction device, characterized in that an elastic means of which there is at least one, is prestressed so as to provide the reaction device with rigidity for an input force below a certain force value. 
   Another object of the present invention is a reaction device, characterized in that an reaction device changes the boost ratio according to the value of the input force. 
   Another object of the present invention is a reaction device, characterized in that the device increases the boost ratio for an actuating force value higher than a certain force value. 
   Another object of the present invention is a reaction device, characterized in that the prestress of an elastic means is greater than or equal to 150 newtons. 
   Another object of the present invention is a reaction device, characterized in that an elastic means is a spring. 
   Another object of the present invention is a reaction device, characterized in that an elastic means is a cylindrical spring. 
   Another object of the present invention is a reaction device, characterized in that an elastic means is an elastic ring. 
   Another object of the present invention is a reaction device, characterized in that an elastic means is a conical spring. 
   Another object of the present invention is a reaction device, characterized in that an annular washer and a floating piece of which there are at least one, form means of prestressing an elastic means of which there is at least one. 
   Another object of the present invention is a reaction device, characterized in that an annular washer is crimped onto the body of the push rod. 
   Another object of the present invention is a reaction device, characterized in that an floating piece comprises means of centring an elastic means. 
   Another object of the present invention is a reaction device, characterized by at least two coaxial elastic means. 
   Another object of the present invention is a reaction device, characterized in that a first elastic means has a preload lower than the preload of a second elastic means. 
   Another object of the present invention is a reaction device, characterized in that an floating piece comprises a central first piece and an annular second piece which can move axially one with respect to each other. 
   Another object of the present invention is a reaction device, characterized by first guide means that collaborate with second guide means of a pneumatic brake booster. 
   Another object of the present invention resides in a pneumatic brake booster comprising a casing divided into two chambers by a sealed moving wall comprising a pneumatic piston in which there is arranged a three-way valve actuated by an actuating rod which can be connected at a first longitudinal end to a brake pedal and at a second longitudinal end opposite the first end to a distributor, characterized in that the distributor comes into contact with a reaction device as defined previously. 
   Another object of the present invention resides in a pneumatic brake booster, characterized in that the reaction device defined previously is fixed to the pneumatic piston by means of a washer. 
   Another object of the present invention is a pneumatic brake booster, characterized in that the washer is fixed by being trapped between a return spring and the pneumatic piston. 
   Another object of the present invention is a pneumatic brake booster, characterized in that the pneumatic piston comprises guide means collaborating with guide means of the reaction device defined previously. 
   The present invention will be better understood with the aid of the following description and of the appended figures in which the front and the rear are depicted respectively by the left-hand and right-hand parts of the figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a view in longitudinal section of a pneumatic brake booster of known type; 
       FIG. 2  is a curve representing the variation in the output force of the booster as a function of the input force for a booster of the type with two boost ratios; 
       FIG. 3  is a view in longitudinal section of a pneumatic brake booster comprising a first alternative form of a reaction device according to the present invention; 
       FIG. 4  is a detail view of the reaction device of  FIG. 3 ; 
       FIG. 5  is a view in longitudinal section of a second alternative form of the first embodiment of a reaction device which can be used in a booster according to the present invention; 
       FIG. 6  is a view in longitudinal section of a third alternative form of the first embodiment of a reaction device which can be used in a booster according to the present invention; 
       FIG. 7  is a view in longitudinal section of a fourth alternative form of the first embodiment of a reaction device which can be used in a booster according to the present invention; 
       FIG. 8  is a view in longitudinal section of a third embodiment of a reaction device which can be used in a booster according to the present invention; 
       FIG. 9  is a view in longitudinal section of a third embodiment of a reaction device which can be used in a booster according to the present invention; 
       FIG. 10  is a curve representing the variation in output force as a function of the input force for a pneumatic brake booster comprising the reaction device of  FIG. 9 ; 
       FIG. 11   a  is a view in longitudinal section of a fourth embodiment of a reaction device which can be used in a booster according to the present invention; 
       FIG. 11   b  is a three-quarters perspective view from the front of the housing of the reaction disc of the reaction device according to  FIG. 11   a,    
       FIG. 12   a  depicts curves of the variation in actuating force as a function of the compression of the elastic means for three different thrust forces and three different reaction disc diameters with the same fixed reaction disc elastic modulus; and 
       FIG. 12   b  represents curves of the variation in actuating force as a function of the compression of the elastic means for three different thrust forces and three different reaction disc diameters with a fixed reaction disc elastic modulus which differs from the one used for the measurements depicted on curves  12   a.   
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The same references are used in all the figures for the same pieces or the pieces which perform the same function. 
   All the elements described are pieces which have symmetry of revolution of axis XX′. 
   The term expansion is understood in this description as being an elastic operation, at the end of which the reaction disc which will be defined hereinafter returns to its initial shape. 
     FIG. 1  shows a pneumatic brake booster of known type, comprising a casing  1  formed of a first  3  and of a second  5  shells which are crimped together in a sealed fashion and each of which comprises a central opening  6 ,  8  facing each other, the interior space delimited by the casing  1  being divided into a front chamber  7  and a working chamber  9  by a sealed moving wall  11 . This wall  11  is equipped in its central part with a pneumatic piston  13  extending axially towards the rear and in its radially external part with a skirt  15  crimped to the external periphery of the piston  13 . The wall  11  is sealed by means of a rolling seal  16 , for example made of elastomer, and fixed to the periphery of the piston  13  and to the periphery of the casing  1 . A spring  18  is mounted in compression between the rear face of the shell  3  and the front face of the piston  13  so as to return the wall to the retreated position in the rest phase. 
   The front chamber  7  is connected by sealed means (not depicted) to a source of partial vacuum. At rest, the working chamber  9  is in communication with the front chamber  7 , whereas at the time of braking it can be placed in communication with an environment at atmospheric pressure. 
   The communications between the two chambers and with the environment are performed by means of a three-way valve  19  arranged in the cylindrical rear part of the pneumatic piston  13 . The valve  19  comprises a valve seat  20  borne by an actuating rod  21  connected to a brake pedal (not depicted), a plunger distributor  22  comprising a plunger  23  and a feeler  25 , the plunger  23  receiving, in its rear end, a front end of the actuating rod  21  which is the opposite end to the brake pedal end and which carries a first valve  27 . 
   A second valve  29  is formed by the rear periphery of the pneumatic piston  13 . The feeler and the plunger are connected by a spring  30  mounted in compression in a deformable cage  32  and having an inside diameter greater than the outside diameter of the feeler and of the plunger. The cage  32  comprises a first sleeve directed towards the rear fixed to the feeler, advantageously by force, and a second sleeve of smaller size facing the first sleeve fixed to the plunger, advantageously by force. 
   The booster also comprises a push rod  31  actuating a piston of a master cylinder (neither depicted) fixed in front of the booster by means of a washer  32 ′ trapped between the spring  18  and the front face of the piston  13  and in its rear axial end directed towards the brake pedal forming a cup carrying a reaction disc  33  capable, under braking, of collaborating with the front face of the feeler  25 , the reaction disc  33  is made of a deformable, practically incompressible, material, for example an elastomer and preferably rubber. 
   A pneumatic brake booster is, in the known way, characterized by a boost ratio equal to the ratio between the output force (Fs) and the input force (Ft) and which is determined by the ratios of the areas of the reaction disc  33  and of the feeler  25 . 
   For obvious safety reasons, it is desirable to increase this boost ratio when there is a braking action with an input force higher than a determined value, corresponding to an emergency situation, so as to boost the action of the driver on the brake pedal still further. 
   The booster is therefore characterized by a first and at least one second boost ratio. 
   We shall now explain the operation of the booster of known type with reference to  FIGS. 1 and 2 . 
   At rest, the front chamber  7  and the working chamber  9  are in communication and subjected to the partial vacuum of the front chamber  7 . 
   During braking, the action of the driver on the brake pedal causes the actuating rod to move, causing the communication between the chambers  7  and  9  to close by applying the second valve  29  to the valve seat  20  and by opening the communication between the working chamber and the environment at atmospheric pressure by lifting the first valve  27  off the valve seat  20 . A pressure difference therefore arises between the front chamber  7  and the working chamber  9  and this has the effect of carrying the wall  11  towards the master cylinder. The distributor  22  formed of the feeler  25  and of the plunger  23  behaves like a rigid piece, the reaction disc  33  combines the force applied by the actuating rod  21  via the feeler  25 , the boost force applied by the pneumatic piston and the reaction of the hydraulic circuit which is transmitted by the push rod  31 . The booster provides a first boost ratio (AB). 
   For sufficient input force, the spring  30  compresses, the feeler  25  is then able to move with respect to the plunger  24  towards the brake pedal, allowing the reaction disc  33  to expand towards the brake pedal. 
   There is therefore a change in the distribution of contact pressures in the surface of the rubber, reducing the contact pressure between the distributor  22  and the reaction disc  33 , corresponding to a second booster ratio (BC). 
   At the end of the braking phase, the actuating rod  31  moves towards the rear, allowing the entire booster to return to its rest position, and in particular, the reaction disc returns to its initial shape. 
     FIG. 3  shows a brake booster equipped with a reaction device according to a first embodiment of the present invention, comprising a casing  1  formed of a first shell  3  and a second shell  5  each of which respectively has a central opening  6 ,  8  and which is divided into two chambers  7 ,  9  by a sealed moving wall  11 . The moving wall  11  in its central part has a pneumatic piston  13  extending axially towards the rear and in its radially external part has a rigid skirt  15  crimped to the external periphery of the piston  13 , sealing being achieved by means of a rolling seal  16  fixed hermetically to the casing  1  and to the pneumatic piston  13 . 
   A spring  18  is mounted in compression between the rear face of the shell  3  and the front face of the piston  13  so as to return it at rest. 
   The front chamber  7  is connected by sealed means (not depicted) to a source of partial vacuum. 
   A three-way valve  19  is arranged in the cylindrical part of the pneumatic piston  13  and is controlled by means of a control rod  21  connected to a brake pedal (not depicted). It comprises a valve seat  20  borne by the actuating rod  21  and formed by an annular surface, a spring is mounted in compression resting against the rear face of the valve  19  so as to apply the valve seat  20  to the valves  27 ,  29 . 
   The action of the actuating rod  21  is transmitted to the hydraulic braking circuit by means of a reaction device  35  and of a plunger distributor  22  coaxial with the actuating rod  21  receiving, in a rear opening of approximately thrustoconical shape, the front end of the actuating rod  21 . This rear end forming a first valve  27 . 
   The pneumatic piston  13  comprises a rear cylindrical end forming a second valve  29  and, in its front face, axial-guidance means  24  capable of collaborating with axial-guidance means of the reaction device  35 , these advantageously being formed by a radial notch  26 . 
     FIGS. 4 and 5  show two alternative forms of a first embodiment of a reaction device of the booster of  FIG. 3  comprising an elastic means  30 , means  45  of prestressing the elastic means  30 , and a reaction disc  33 . The prestressing means  45  comprise a cage  37  formed by a sleeve  38 , an annular washer  47  and a floating piece  49 . The sleeve  38  comprises, at a first end facing forwards, a first sleeve of smaller diameter  39  and, at a second end facing backwards, a second sleeve of larger diameter  41  connected to the first sleeve  39  by a shoulder  43 , the axial front end of the first sleeve of smaller diameter  39  being closed and forming a base of a push rod  31  coaxial with the actuating rod  21 , the sleeve  38  and the push rod  31  being coaxial. 
   The annular washer  47  coaxial with the push rod  31  is mounted fixedly in the second sleeve  41 , resting against the shoulder  43  and advantageously crimped to the internal radial surface of the sleeve  39 . The floating piece  49  is mounted in axial translation in the passage  51  made in the washer  47 , advantageously mounted coaxially with the plunger distributor  22  and of an outside diameter equal to the inside diameter of the passage  51 . 
   The floating piece  49  comprises retaining means  53  situated some non-zero distance away from the axial ends of the piece  49  and advantageously formed of a flange  55  extending radially outwards and with an outside diameter greater than the inside diameter of the opening  51 . In the second alternative form of the first embodiment depicted in  FIG. 5 , the retaining means  53  also comprise means  54  of centring the spring  30  which are formed by an annular depression  54  made in the front face of the flange  55 . 
   The elastic means  30 , advantageously a spring, is mounted in compression between the rear-facing face of the sleeve  39  and the front annular face of the flange  55 . 
   The reaction disc is arranged so that it rests on a face  57  formed by the rear-facing face of the annular washer  47  and by the rear-facing face of the floating piece  49 . 
   The axial dimension of the larger-diameter sleeve  41  is greater than the sum of the axial dimensions of the washer  47  and of the reaction disc  33 , leaving a free rear axial end of the sleeve  41  forming the guide means  59  of the reaction device  35  capable of collaborating with the guide means  24  of a pneumatic piston of a booster. 
   The reaction device is fixed to the pneumatic piston  13  by means of an annular washer  32 , the central passage of which advantageously extends axially forwards in the form of a sleeve bordering the smaller-diameter sleeve  39  of the sleeve  37 . 
   We shall describe the operation of the pneumatic brake booster comprising the device  35  according to the present invention. 
   At rest, the chambers  7  and  9  are subjected to the partial vacuum. 
   Upon braking, action on the brake pedal moves the actuating rod which closes the communication between the front chamber  7  and the working chamber  9  by applying the second valve  29  to the valve seat  20  then opens the communication between the working chamber  9  and the environment at atmospheric pressure by lifting the first valve  27  off the seat  20 . The piston  13  is then carried forwards by the difference in pressure between the two chambers  7  and  9 . 
   The reaction disc  33  then receives the actuating force Fa of the actuating rod via the distributor  22 , the boost force Fp of the pneumatic piston  13  which can actually move with respect to the reaction disc by virtue of the axial-guidance means  24  formed by a radial notch, and the reaction force Fr of the hydraulic braking circuit. 
   For an input force lower than a certain input force Fe, the reaction means  35  behaves like a rigid piece, the reaction device determining a first boost ratio. 
   For an input force higher than the value of the force Fe, the spring  30  compresses, allowing the floating piece  49  to slide forwards, the reaction disc  33  therefore expands forwards into the opening  51 . There is therefore a change to the distribution of contact pressures in the rubber surface, reducing the contact pressure between the distributor  22  and the reaction disc  33  and the reaction device determining a second boost ratio. 
   For an even higher value of input force, the floating piece  49  comes to rest against the push rod  31  and the booster then provides a boost ratio equal to the first boost ratio obtained for an input force lower than Fe. 
   The variation in boost ratio depends on the elastic modulus of the rubber of which the reaction disc is made and on the geometry of the reaction disc and of the housing of the reaction disc. 
   At the end of the braking phase, the actuating rod  21  moves backwards, allowing the entire booster to return to its rest position, and in particular, the reaction disc returns to its initial shape. 
     FIG. 6  shows a third alternative form of the first embodiment of the reaction device according to the present invention comprising an elastic means  530 , means  45  of prestressing the elastic means  530  and a reaction disc  33 . The prestressing means  45  comprise a cage  37  formed by a cylindrical sleeve  38  of axis XX′ the front axial end of which forms the base of a push rod  31 , by an annular washer  47  equipped with a central opening  51 , and by a floating piece  49 . 
   The annular washer  47  is mounted fixedly in the sleeve  38 , advantageously by crimping to the periphery of the sleeve  38 . 
   The floating piece  49  is mounted, advantageously so it is coaxial with the plunger distributor  22 , in axial translation in the opening  51  of the washer  47  and of an outside diameter equal to the inside diameter of the passage  51 . 
   The floating piece  49  comprises retaining means  53  situated a non-zero distance away from the axial ends of the piece  49  and formed by a flange  55  extending radially outwards and with an outside diameter greater than the inside diameter of the opening  51 . 
   The elastic means  530  is advantageously a conical spring with the cone facing forwards mounted in compression between the rear face of the front end wall of the cage  37  and the annular front face of the flange  55 . The reaction disc is arranged resting against the face  57  formed by the rear face of the annular washer  47  and by the rear face of the floating piece  49 . The reaction device comprises means  59  of guidance with respect to the piston  13  which are capable of collaborating with the guide means  24  of the pneumatic piston  13 , formed by the cylindrical rear end of the cage  37 . 
   The reaction device is fixed to the pneumatic piston  13  by means of an annular washer  32 , the central passage of which advantageously extends axially forwards in the form of a sleeve bordering the push rod  31 . 
   The way in which the device  135  applied to a brake booster according to the present invention works is the same as the method of operation of the reaction device of  FIGS. 4 and 5 . 
   This alternative form of the first embodiment of the reaction device according to the present invention advantageously makes it possible to considerably reduce the axial size of the reaction device. 
     FIG. 7  shows a fourth alternative form of the first embodiment of the reaction device  35 , in which the pneumatic piston  13  and the distributor  22  are depicted in part. The device  35  comprises a reaction disc  33 , means  45  of prestressing an elastic means  30  comprising a sleeve  60  equipped with an interior shoulder  61 , with a floating piece  49  mounted in translation in the sleeve  60  and of an outside diameter equal to the inside diameter of the passage defined by the shoulder  61 , and with a sleeve  63 . The sleeve  63  has a front axial end closed off and forming a base  64  of a push rod  31  which is formed integrally with it. The sleeve  63  enters the sleeve  60  via its rear end and is fixed to the sleeve  60 , for example by force or by crimping so as to close off the open front end of the sleeve  60 . 
   All the parts of the device of  FIG. 7  are coaxial with the axis XX′. 
   The floating piece  49  includes stop means  53  formed by an annular surface  71  that extends radially outwards. 
   The reaction disc  33  is mounted so that it rests, on the same side as the rear end of the sleeve  60 , against the rear face, and a spring  30  is mounted in compression between the rear face of the front end of the sleeve  63  and the front face of the annular surface  71  of the stop means  53  of the floating piece  49 . 
   The sleeve  60  comprises at its rear axial end guide means  59  capable of collaborating with the guide means  24  formed in the front face of the pneumatic piston  13 . 
   The device  35  also comprises means  59  of guidance with respect to the pneumatic piston  13  which are capable of collaborating with the axial-guidance means  24  of the piston  13 . 
   The way in which the device  35  of  FIG. 7  applied to a brake booster according to the present invention works is the same as the method of operation of the device  35  described in  FIGS. 5 and 6 . 
   The advantage of this embodiment is the simplicity of manufacture of the reaction device according to the present invention. 
     FIG. 8  shows a second embodiment of a reaction device  135  according to the present invention, comprising a reaction disc  33 , an elastic means  30  and means of prestressing an elastic means comprising a sleeve  72  equipped at a first end directed forwards with a first part  73  of smaller diameter and at a second end directed backwards with a second part  75  of larger diameter, the first part  73  being connected to the second part by a shoulder  77 . The part  73  of smaller diameter comprises, at a forward facing end, an annular surface  79  extending inwards and delimiting a passage  74 . 
   The push rod  31  comprises at a forward facing first end a first part  78  of smaller cross section and, at a second end which is the opposite end to the first end, a second part of larger cross section  78 ′ forming a base  80  of outside diameter smaller than the inside diameter of the larger-diameter second part  75 , the space between the base  80  and the sleeve  75  defining an annular passage  76 . The piece  78  is fixed into the passage  74 , for example by force-fitting. 
   The device  135  also comprises means  54  of guidance with respect to the pneumatic piston  13  which are capable of collaborating with the axial-guidance means  24  of the piston  13 . 
   The prestressing means also comprise an annular ring  81  of an outside diameter roughly equal to the inside diameter of the part  75  of the sleeve  72  which part is slideably mounted in the part  75 , the ring  81  is pierced towards the rear with a first bore  83  of larger diameter and towards the front with a second bore  85  of smaller diameter, the two bores  83 ,  85  being connected by a shoulder  87 . The inside diameter of the bore  83  is roughly equal to the outside diameter of the part  78 ′ of the push rod  31  and the inside diameter of the bore  85  is roughly equal to the outside diameter of the base  80 . The ring  81  also comprises a part  89  at its front end that has a frustoconical interior shape with the cone shape facing forwards. The ring  81  is slideably mounted in the part  75  of the sleeve and around the push rod  21 . The elastic means  30 , advantageously a cylindrical spring, is mounted in compression between the rear face of the annular surface  79  and the end wall of the part  89 . 
   The reaction disc  33  is mounted so that it rests against the surface formed by the rear faces of the base  80  and of the ring  81 . 
   The reaction device is fixed to the pneumatic piston  13  by means of an annular washer  32  the central passage of which runs axially forwards in the form of a sleeve  32 ″ bordering the smaller-diameter part  73  of the sleeve  72 . 
   We are now going to describe the operation of the reaction device  135  according to  FIG. 8  incorporated into a pneumatic brake booster. 
   Upon braking, for an input force value below a certain force Fe, the reaction device  135  behaves like a rigid piece. The reaction disc acts in a known way and combines the forces Fa, Fp and Fr and transmits the entire reaction to the actuating rod. The reaction device determines a first boost ratio. 
   For an input force value higher than Fe, the spring  30  deforms, allowing the ring  81  to slide forwards and therefore allowing the reaction disc  33  to expand forwards into the external annular part  76  defined by the outside diameter of the base  80  and the inside diameter of the part  75  of the sleeve  72 . 
   There is therefore a change in the distribution of contact pressures in the surface of the rubber, and this reduces the contact pressure between the distributor  22  and the reaction disc  33 , the reaction device determining a second boost ratio. 
   For a certain input force, the ring  81  comes to rest against the shoulder  77  and the booster therefore supplies a boost level equal to the first boost level obtained for a force Fa lower than the prestress of the spring. 
   Unlike the case of the first embodiments, the expansion of the reaction disc takes place via an external annular part  76  rather than via the central passage  51 . 
     FIG. 9  shows a third embodiment of a reaction device which once again allows the boost ratio of the booster to be modified and in which the pneumatic piston  13  and the distributor  22  are depicted in part. 
   The reaction device  235  differs from the reaction device  35  of the first embodiment in that it comprises a composite floating piece  49  comprising a central first piece  249  and an annular second piece  349  sliding about the first piece  249 . The first piece  249  includes stop means  253  that rest against and engage the front face of the second piece  349  and the second piece  349  includes stop means  353  that rest against and engage against the annular washer  47 . A first spring  230  is mounted in compression between the rear face of the end wall of the cage  37  and the stop means  253 , a second spring  330 , with a prestress higher than that of the first spring  230 , is mounted in compression between the rear face of the end wall of the cage  37  and the stop means  353 , the spring  330  has an inside diameter greater than the outside diameter of the spring  230 . 
   We are going to describe the operation of a pneumatic brake booster comprising the third embodiment of the reaction device according to the present invention. 
     FIG. 10  shows the change in boost ratios as a function of the actuating force for the booster comprising the device according to the third embodiment. 
   For a value of input force below a certain force Fe, the reaction device  235  behaves as a rigid piece, the reaction device determining a first boost ratio represented by the curve portion DE. For an input force value higher than Fe and lower than a certain force Fd, the spring  230  compresses, allowing the reaction disc  33  to expand through the central passage  51 , by the sliding-forwards of the floating piece  249 . The reaction device determines a second boost ratio represented by the curve portion EF. For an input force higher than Fd, the spring  330  compresses, allowing the reaction disc  33  to expand axially forwards over an annular surface by displacement of the annular piece  349 , the reaction device determines a third boost ratio represented by the curve portion FG. Next, for even higher input values, the front end of the floating piece  349  comes up against the rear face of the end wall of the cage  37  and the first boost ratio DE is had again. The start of this last stage appears according to the settings of the device, the start may lie before the saturation regional in the saturation region, the saturation region being the moment when the pressure in the working chamber reaches atmospheric pressure when there is no more additional boost, any increase in the actuating force resulting only in an increase in the output force by the value of the actuating force. 
   It must be clearly understood that the means  45  of prestressing at least one elastic means  30 ,  230 ,  330  are not restricted to an annular washer resting against a shoulder and fixed by crimping, but to any means of immobilization in a precise position, such as a washer and a screw for example. 
   It must be clearly understood that a device allowing firstly the annular ring and secondly the floating piece to move could be provided. 
     FIGS. 11   a  and  11   b  depict a fourth embodiment of the reaction device according to the present invention allowing the reaction disc to extend radially for an input force higher than a force Fe. The device comprises a reaction disc  33 , a housing  91  for the reaction disc  33  and an elastic means  430 . 
   The housing  91  for the reaction disc is a receiving cup  91  secured via its axial front face to a push rod  31  of axis XX′, the cup  91  being coaxial with the push rod  31 . 
   The cup  91  comprises a front face  93  in the form of a disc perpendicular to the axis XX′, with a coaxial sleeve  95  extending therefrom, the sleeve  95  includes, in its lateral periphery, angularly distributed recesses  97 , advantageously uniformly distributed, and an elastic means  430  in the form of a preloaded elastic ring equipped on its interior face  99  with projecting tabs  101  collaborating with the recesses  97 . 
   The reaction device is fixed to the pneumatic piston  13  by means of an annular washer  32  the central passage of which advantageously extends axially forwards in the form of a sleeve  32 ′ bordering the push rod  31 . 
   We are going to describe the operation of a pneumatic brake booster comprising the fourth embodiment of the reaction device according to the present invention. 
   The input forces below a certain force value Fe, the cup behaves as a rigid piece, the reaction device determining a first boost ratio. 
   For an input force higher than Fe, the ring deforms allowing the reaction disc  33  to expand via the recesses  97 . There is then a change in the distribution of contact pressures in the surface of the rubber, reducing the contact pressure between the distributor  22  and the reaction disc  33 , the reaction device determining a second boost ratio. 
     FIGS. 12   a  and  12   b  show the change in the value of the actuating force as a function of the compression of the elastic means  30 ,  230 ,  330 ,  430 ,  530 . 
   Measurements of the force applied to the distributor as a function of the compression of the elastic means were taken, varying the output force, the elastic modulus of the reaction disc  33  and the diameter of the reaction disc  33 . The actuating force is directly proportional to the input force on the brake pedal. 
   These characteristics show that when the elastic means compresses, the actuating force decreases and therefore, for the same output force, the boost ratio increases. 
   This proves the effectiveness of the reaction device according to the present invention applied to a pneumatic brake booster in causing the boost ratio of the booster to vary, the variation in boost ratio depending on the characteristics of the reaction disc. 
   The prestress on an elastic means is preferably greater than or equal to 150 newtons. 
   It must be clearly understood that the force value Fe given as being the limiting value of the input force for switching from a first boost ratio to a second boost ratio depends on the settings of the devices, on the size of the reaction disc, on the elastic modulus of the rubber of the reaction disc and on the elastic means used. The same is true of the value of the force Fd. 
   It must also be clearly understood that the present invention does not restrict itself to a reaction device with two or three different boost ratios but with the number of successive boost ratios needed to ensure effective and comfortable braking. 
   The present invention applies in particular to the motor industry. 
   The present invention applies mainly to the braking industry.