Abstract:
This invention relates to a housing for receiving a reaction disk in a pneumatic servomotor. The housing and reaction disk co-operate to resist too fast a forward travel of a piston when the brake is actuated and thereby avoid an undesired reopening of a passage connecting a front chamber with a rear chamber in a brake booster during a brake application. The reaction disk ( 39 ) is located in an inner space of the housing ( 47 ) that has a greater size than the volume of the disk ( 39 ). The disk ( 39 ) is initially deformed to completely fill the housing ( 47 ) by an input force corresponding to a desired brake application to limit the forward speed of the pneumatic piston and only thereafter as a reaction force develops during the brake application, does the reaction force balance the input force applied to the disk ( 39 ) during a brake application.

Description:
This invention mainly relates to a receiving housing for a reaction disk and to a pneumatic servomotor for an assisted braking, including such housing fitted with its reaction disk. 
     BACKGROUND OF THE INVENTION 
     It is quite usual to amplify the force, which a driver exerts on a brake pedal, by means of a pneumatic servomotor (also called a “booster”), comprising a variable-volume front chamber, separated from a variable-volume rear chamber by a tight flexible diaphragm and by a rigid skirt driving a pneumatic piston which bears, through a push rod, on the primary piston of a tandem master cylinder of a hydraulic braking system. The front chamber, directed towards the tandem master cylinder, is hydraulically connected to a vacuum source whereas the rear chamber, in the opposite direction to the front chamber, is hydraulically connected, using a valve control, to a propellant fluid source, typically atmospheric-pressure air. 
     At rest, that is when the driver does not actuate the brake pedal, the front and rear chambers are interconnected, while the rear chamber is isolated from the atmospheric pressure. On braking, first the front chamber is isolated from the rear chamber and then air is admitted into the rear chamber. 
     Unfortunately, when the brake pedal is depressed, the sudden opening of the air-supply port of the rear chamber causes the pneumatic piston to move forward too fast. The initial resistance to the travel of this pneumatic piston is quite weak. As a matter of fact, on the one hand, there is a great pressure difference between the front and the rear chambers of the servomotor and, on the other hand, the operational clearances of the tandem master cylinder are taken up. This swift advance of the pneumatic piston may cyclically open the passage connecting up the front chamber and the rear chamber, which generates a noise resembling that of wipers set in motion on a dry windscreen and therefore called a “wiper noise” in the art. Such noise, although normal, is considered quite unpleasant by the users who fear some dysfunctioning of the system. Besides, the reopening of the passage connecting up the front and rear chambers of the servomotor reduces the assistance efficiency at the beginning of the braking process. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to provide a servomotor for an assisted braking, ensuring a noiseless running, especially by a reduction of the so-called “wiper noise” to the minimum. 
     Another object of this invention consists in providing a servomotor for an assisted braking with a quite high efficiency. 
     It is also an object of the present invention to provide a servomotor for an assisted braking, ensuring a quick-acting assistance. 
     It is another object of this invention to provide a braking system including a pneumatic assistance servomotor and a tandem master cylinder, equipped with superassistance means for emergency braking situations, of the types described in EP 0 662 894 and FR 2 751 602, and having a low variation of the starting threshold of superassistance means in the case of an emergency braking situation. 
     These objects are achieved, in accordance with this invention, by a pneumatic servomotor for an assisted braking, fitted with means capable of resisting too fast a forward travel of the pneumatic piston when the brake is actuated, which otherwise might lead to an undesired reopening of the passage connecting the front chamber with the rear chamber of the assistance servomotor. 
     These objects are achieved according to this invention, in that the compliance of a reaction disk is turned into account so as to resist too fast a forward travel of the piston. In the preferred embodiment of the invention, the substantially incompressible reaction disk is accommodated inside a housing, the inner space of which is greater than the volume of the disk. The deformation of the disk, during an initial braking stage, will result in a such a deformation of the disk as to enable it to fill completely its housing, which makes it act conventionally in the course of a subsequent braking stage. The applying of the reaction disk onto the distributor plunger or onto any other part, interlocking with the control rod for the control of the flaps, limits the forward speed of the pneumatic piston, when the brake is actuated, and prevents or restricts the undesired reopenings of the connecting passage between the front and rear chambers of the assistance servomotor. 
     The main subject of this invention is a receiving housing for a reaction disk, said housing being defined by stationary walls and by moving walls and accommodating an incompressible reaction disk of a braking servomotor, said reaction disk comprising a contact area associated with an element driven by a brake pedal, characterised in that, when the contact is made between the reaction disk and the element driven by the brake pedal, the housing of the reaction disk has an area which is taken by the incompressible reaction disk, and an area filled with a compressible fluid, enabling, in a first braking stage (jump stage) of the braking process, an elastic deformation of the reaction disk so that the latter may resiliently exert a mechanical reaction on said element driven by the brake pedal, and in that, during s second braking stage (the actual braking operation), the incompressible reaction disk will fill its housing in a substantially complete manner. 
     The present invention also deals with a housing, characterised in that it comprises two adjacent coaxial cylindrical areas, the first area having a first diameter corresponding to that of the reaction disk, and the second area having a second diameter, which is smaller than the first diameter. 
     Another subject matter still of the present invention is a housing, characterised in that the second area is radially bounded by a washer. 
     According to another aspect of the invention, a servomotor for an assisted braking is characterised in that it includes a housing. 
     The present invention also deals with a servomotor, characterised in that the housing is fitted with a reaction disk, including a radially-outer annular area which defines, together with the walls of the housing, an area of said housing to be filled with a compressible fluid. 
     Another subject matter still of the present invention is a servomotor, characterised in that the element driven by the brake pedal is a distributor plunger and in that the latter comprises a stub to be applied onto the reaction disk. 
     According to another aspect of the invention, a servomotor is characterised in that the reaction disk comprises at least one passage for the supply and discharge of the compressible fluid, respectively into and from the compressible area of the housing for the reaction disk. 
     Another subject of the invention is a servomotor, characterised in that, at rest, the element, which is driven by the brake pedal, is in contact with the reaction disk. 
     This invention also deals with a servomotor, characterised in that the element driven by the brake pedal comes into contact with the reaction disk, just as the flap closes for the tight separation of the front and rear chambers of the servomotor. 
     Another subject matter still of the present invention is a servomotor, characterised in that the reaction disk includes a resilient reinforcing insert. 
     Other features and advantages of the present invention will be apparent from the following detailed description, by way of example and by no means as a limitation, when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal sectional view of an assistance servomotor of a well-known type; 
     FIG. 2 is a longitudinal sectional view of a detail from FIG. 1, at rest; 
     FIG. 3 is a view of the servomotor, similar to FIG. 1, but in a state in which the front and rear chambers are mutually isolated, while the rear chamber is supplied with air; 
     FIG. 4 is a similar view, in the event of an undesired opening of the connecting passage between the front chamber and the rear chamber; 
     FIG. 5 is a longitudinal sectional view of the central part in a first embodiment of a servomotor according to the present invention; 
     FIG. 6 is a longitudinal sectional view of a second embodiment of a servomotor according to the present invention; 
     FIG. 7 is an enlarged view of the central part according to FIG. 6; 
     FIG. 8 is a perspective view of a third embodiment of a reaction disk, capable of being implemented in a device according to this invention; 
     FIG. 9 is a longitudinal sectional view of a third embodiment of a reaction disk, capable of being implemented in a servomotor according to the present invention; 
     FIG. 10 is a longitudinal sectional view of a fourth embodiment of a reaction disk, capable of being implemented in a servomotor according to the present invention; and 
     FIG. 11 is a longitudinal sectional view of a fifth embodiment of a reaction disk, capable of being implemented in a servomotor according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIGS. 1 through 11, the same reference numerals will designate the same elements. 
     FIG. 1 shows a pneumatic servomotor  1  for an assisted braking of a well-known type, comprising a front chamber  3 , separated from a rear chamber  7  by a tight diaphragm  5 . The diaphragm  5  rests on a rigid skirt  9 , driving a pneumatic piston  11  at the time of the volume variation of said chambers. A control rod  13 , capable of being driven in the direction of the arrow  15  by a brake pedal (not shown), bears on a distributor plunger  17 . This distributor plunger  17 , as is more visible in FIGS. 2-4, controls a valve fitted with a first flap  19  shutting off, on actuation, a passage  21  connecting the front chamber  3  with the rear chamber  7  of the servomotor  1 , and a second flap  23  opening, on actuation, the air-supply passage of the rear chamber  7  of said servomotor. The valve further comprises a return spring  25  for the flap and a spring  27 , closing the valve when the brake is not actuated. The pneumatic piston  11  drives, in the direction of the arrow  15 ′, a push rod  29  bearing on a primary piston of a tandem master cylinder (not shown). 
     The mode of operation of the servomotor of the known type will now be described with reference to FIGS. 2 through 4. 
     FIG. 2 shows the system at rest, i.e. when the driver does not depress the brake pedal. The seats of the flaps  19  and  23  are axially offset in such a way that the forward travel of the control rod  13  in the direction of the arrow  15 , in the first place, closes the flap  19  and then opens the flap  23 . For a given position of the control rod  13 , the flap  23  opens and closes periodically in order to set the push rod  29  in the desired position and, as a result, the desired pressure in the hydraulic circuit of the braking system. 
     At rest, the flap  19  is permanently open whereas the flap  23  is permanently closed. Thus, the same low pressure exists in the front chamber  3  and in the rear chamber  7 . 
     When the driver depresses the brake pedal, the control rod  13  moves forward, as shown in FIG. 3, and it drives the distributor plunger  17 , thus closing the flap  19  and opening the flap  23 . The atmospheric-pressure air rushes into the rear chamber  7 . The pressure difference drives the skirt  9  which, in turn, moves the pneumatic piston  11  forward. In the first instance, the pneumatic piston does not meet with a noticeable resistance to its forward travel, till all the mechanical clearances at the tandem master cylinder of the hydraulic braking system are compensated. Thus, as represented in FIG. 4, such piston moves forward faster that said distributor plunger  17 , borne by the control rod  13 , which undesiredly causes the flap  19  to open. When the pressures are equalized between the front chamber  3  and the rear chamber  7 , the piston is slowed down, thus allowing the distributor plunger  17  to close the flap  19  (state represented in FIG.  3 ), with the result that the pneumatic piston  11  moves forward, which means the reappearance of the state illustrated in FIG. 4, in which the flap  19  is open. The cyclic changeover from one state to the other one, as respectively shown in FIGS. 3 and 4, gives rise to the so-called “wiper noise”, which is quite undesired and, moreover, it leads to an undesired supply of the front chamber  3  with air, thus reducing the efficiency of the pneumatic assistance servomotor. 
     After the initial braking stage, the push rod  29  drives, in the direction of the arrow  15 ′, a primary piston of a tandem master cylinder (not shown) for the supply of the brake cylinders with a pressure brake fluid, so as to apply the friction elements onto the disks and/or the drums. The reaction of the brake and of the tandem master cylinder gives rise to a mechanical reaction, which prevents the reopening of the flap  19 , during the second braking stage (the actual braking operation). 
     The end of the push rod  29 , in the opposite direction to that facing the tandem master cylinder, bears a cup  33  constituting, together with the front face  35  of the central part of the pneumatic piston  11  and with the front face  37  of the distributor plunger  17 , a receiving housing for a reaction disk  39 , made of an almost incompressible material (typically an elastomer). The reaction disk  39  of the servomotor  1  of a known type entirely fills its receiving housing, with the exception of a gap  41 , remaining between a rear face  43  of the reaction disk and a front face  37  of the distributor plunger  17 . 
     The reaction disk  39  of the servomotor  1  of a known type does not transmit the reaction, generated at the tandem master cylinder, to the distributor plunger  17  during the initial braking stage, the so-called jump stage. 
     The pneumatic servomotor  31  for an assisted braking (FIG. 6) according to this invention differs from the servomotor  1  of the well-known type, shown in FIGS. 1 through 4, in that it comprises means for delaying, at the beginning of the braking operation (typically before the mechanical clearance take-up at the tandem master cylinder) the forward travel of the pneumatic piston  11  in the direction of the arrow  15 ′, so as to prevent it from moving faster than the distributor plunger  17  and consequently in order to preclude any undesired opening, in the course of the braking operation, of the flap  19  arranged between the front chamber  3  and the rear chamber  7  of the servomotor  31  according to the present invention. 
     As shown in FIGS. 5 through 7, the servomotor  31  for an assisted braking according to the present invention comprises a housing  47  for the reaction disk  39 , the inner space of the former being greater than the incompressible volume of the reaction disk  39 , when the front face  37  of the distributor plunger  17  is in contact with the rear face  43  of the reaction disk  39 , so as to allow an elastic deformation of the reaction disk  39  owing to a thrust exerted by the distributor plunger  17 , in the direction of the arrow  15 ′, in the course of the initial braking stage. The reaction force, which is resiliently exerted by the reaction disk, during said initial braking stage, prevents or limits undesired reopenings of the flap  19 . 
     In the example according to FIG. 5, the reaction disk  39  has a cylindrical shape and its receiving housing  47  comprises a first rear cylindrical area with an axis  49 , corresponding to the axis of the servomotor, and the diameter of which is equal to the inner diameter of the cup  33 , and an adjoining second front cylindrical area  51 , with an axis  49  and the diameter of which is smaller than the diameter of the first rear cylindrical area. 
     The front cylindrical area  51  is radially bounded by a washer  53  or by a shoulder of the cup  33 , providing a support for the peripheral area of the front face  55  of the reaction disk  39 . 
     In an alternative embodiment, the front face  37  of the distributor plunger  17  bears permanently on the rear face  43  of the reaction disk  39 . 
     In an advantageous manner, the contact between the reaction disk  39  and the front face  37  of the distributor plunger  17  takes place right after the closing of the flap  19  and before the opening of the flap  23 . 
     It should be noted that, in FIG. 5, the reaction disk is represented in the non-stressed condition whereas, in FIGS. 6 and 7, it is illustrated in a partially stressed state, in order to show the gap provided in the receiving housing  47  of the reaction disk  39 . 
     In the embodiment in accordance with FIGS. 6 and 7, the housing  47  of the reaction disk  39  has a substantially cylindrical shape, and the reaction disk has, on its front face  55 , a radially-outer annular area  57 , capable of bearing on the rear face of the wall of the cup  33 , perpendicular to the axis  49 . The area  57  defines an area  51 , either empty or filled with air, forwardly of the reaction disk  39  inside the housing  47 . 
     In the advantageous example as shown in FIGS. 6 and 7, the front face  37  of the distributor plunger  17  comprises a central stub  59  for a localized elastic deformation of the reaction disk  39 . 
     FIG. 8 represents a reaction disk  39  having a plane rear face and a front face  55  provided with a peripheral annular area  57 , in which radial passages  61  have been made for the air circulation, so as to avoid the so-called suction effect. In the illustrated advantageous example, the reaction disk  39  exhibits three passages  61  at regular angular spacings. In an advantageous manner, the passages  61  are prolonged, parallel to the axis  49 , into the radially-outer part of the cylinder. 
     FIG. 9 represents a reaction disk  39  having an area  63  either empty or filled with a compressible fluid, such as plain air or nitrogen. Thus, at rest, the outer volume of the reaction disk  39  of FIG. 9 is capable of entirely filling the receiving housing  47  for said disk, while preserving an adequate elastic property for the control of the forward travel of the moving elements during the initial braking stage. 
     FIG. 10 shows an embodiment of a reaction disk including a peripheral groove, for instance in the shape of a capital V. 
     The reaction disks for an implementation in the pneumatic servomotors according to this invention may be made of any conventional material intended for reaction disks, such as elastomers, more particularly rubber. Besides, as shown in FIG. 11, they may be fitted with a reinforcing insert  67 , consisting of a resilient material, such as a metal, in an advantageous manner spring steel, plastics or composites. Advantageously too, the shape of the reinforcing insert  67  is adjusted to the resilient behaviour required of the reaction disk. Therefore, disk-shaped reinforcing inserts may be used or, as illustrated, said reinforcing inserts may look like a portion of a sphere, advantageously with spaced radial notches. Alternatively, the reinforcing insert  67  will be star-shaped. 
     The servomotor according to the present invention can be installed in any braking system whatsoever and, in particular, in braking systems using a single master cylinder or, preferably, a tandem master cylinder. And it is more specially advantageous in the case of a master system for a superassisted emergency braking, as disclosed in patent applications EP 0 662 894 and FR 2 751 602, which are included herein by way of reference. 
     The present invention concerns, more particularly, the motor car industry. 
     And the present invention mainly applies to the design and manufacturing of motor vehicle braking systems.