Abstract:
A hydraulic braking device for a self-propelled vehicle, particularly for agricultural vehicles, for example tractors comprising a master cylinder and a hydraulic brake booster. An actuating rod is mounted to sealingly, slide within a hydraulic boost piston that separates a hydraulic boost chamber into a front chamber and a rear chamrber. At rest the first and second chambers are in communication and filled with a same hydraulic fluid at high pressure. During a brake application, communication between the front and rear chambers is interrupted and the high pressure fluid in the front chamber communicatecd to a pressure chamber in the master cylinder to initiate a brake application and to allow the high pressure fluid in the rear chamber to act on the boost piston and develop a force that acts on an operating piston to further develop pressurized fluid in the pressure chamber during the brake application.

Description:
[0001]    The present invention relates mainly to an improved sensitivity hydraulic braking device for a self-propelled vehicle, particularly for agricultural vehicles, for example tractors.  
         BACKGROUND OF THE INVENTION  
         [0002]    Hydraulic braking device comprising a master cylinder equipped with at least one piston operated by an actuating rod connected to a brake pedal, action on the brake pedal being boosted by a hydraulic brake booster of the so-called “closed center” type equipped with a hydraulic piston supplying the assistance, the braking device also comprising a high-piressure fluid accumulator, are known. At rest, the communication between the “closed center” hydraulic brake booster of known type and the accumulator is interruipted upstream of the hydraulic piston of the booster and the booster is filled with hydraulic fluid at low pressure. During braking, the communication between the booster and the accumultor opens and the hydraulic fluid at high pressure contained in the accmulator, flows into the brake booster causing the hydraui boost piston to move towards the master cylinder and hydraulically boosting the braking. The pressure iaccumnulator is fed, for example, by a hydraulic pump.  
           [0003]    This “closed center” type of booster has the disadvantage that it does not allow fine control of the amount of braking desired because of the flow of liquid under pressure in the booster, something whichoit felt by the driver as sharp braking.  
           [0004]    In addition, this known closed center type of booster offers a very unpleasant brake pedal feel which the driver of the vehicle is sure to find uncomfortable.  
         SUMMARY OF THE INVENTION  
         [0005]    It is therefore an object of the present invention to offer a hydraulic braking device that allows better control of the degree of braking.  
           [0006]    Another object of the present invention is to offer a hydraulic braking device in which the reaction transmitted to the brake pedal gives a piedal feel that is pleasant for the driver.  
           [0007]    This object is achieved through a hydraulic braking device comprising a master cylinder and a “closed center” hydraulic brake booster, the communication between the hydraulic brake booster and the source of hydraulic fluid at high pressure being closed downstream of the hydraulic boost piston.  
           [0008]    In other words, this is a hydraulic braking device comnprising a master cylinder and a hydraulic brake booster comprising a body in which, is mounted to slide with sealing a hydraulic boost piston, said boost piston separating a first chamber from a second chamber; at rest, said chambers are in communication and filled with a hydraulic fluid at high pressure, and during a braking phase, the comnmunication between the two chambers is interrupted and the communication between the second chamber and the master cylinder is opened, causing the boost piston to move.  
           [0009]    The hydraulic device according to the present invention has the advantage of aliowing functional clearances in the braking circuit, for example at the vehicle wheel brakes, to be closed up at the very start of braking. These functional clearances are closed up in a way that is invisible to the driver, thus eliminating the impression of a braking lag.  
           [0010]    The main subject of the ievention is a hydraulic braking device comprising a master cylinder and a hydraulic brake booster, said master cylinder comprising at least one hydraulic piston defining a pressure chamber connected to brakes at the vehicle wheels, said hydraulic brake booster comprising a boost piston dividing a chamber into a first chamber and into a second chamber, means for establishing communication and means for interrupting the communication between the chambers, said braking device being actuated by an actuting rod connected to a brake pedal and moving an operating piston mounted to slide with sealing in the boost piston, said device also comprising a source of hydraulic fluid at high pressure feeding said hydraulic brake booster, characterized in that, at rest, the rear chamber and the front chamber of the hydraulic brake booster are placed in communication and filled with hydraulic brake fluid at roughly the same pressure supplied by the high-pressure source of hydraulic fluid.  
           [0011]    Another subject of the invention is a braking device characterized in that the piston has a skirt fixed to a rod driven by the actuating rod.  
           [0012]    Another subject of the invention is a braking device characterized in that the communication between the chamber of the hydraulic booster and the chamber of the master cylinder is via ducts and a system of valves.  
           [0013]    Another subject of the invention is a braking device characterized in that the system of valves comprises a first valve equipped with a shutter collaborating with a valve seat actuated by a push-rod as the operating piston moves in the rod of the hydraulic boost piston and a second valve equipped with a shutter collaborating with a seat actuated by a difference in hydraulic pressure between the chamber of the hydraulic booster and the chamber of the master cylinder.  
           [0014]    Another subject of the invention is a braking device characterized in that the rod of the hydraulic boost piston comprises a first part of larger outside diameter facing toward the brake pedal and a second part of smaller relative outside diameter, axially opposed to the first part.  
           [0015]    Another subject of the invention is a braking device characterized in that it comprises an exhaust chamber able to be placed in communication with the front chamber of the hydraulic booster by first comrmunication means and with the chamber of the master cylinder by second communication means.  
           [0016]    Another subject of the invention is a braking system characterized in that it comprises two braking devices as described hereinabove, connected by a pressure-equalizing device.  
           [0017]    The present invention will be better understood with the aid of the description which follows and with the attached drawings in which the front and the rear correspond respectively to the left and to the right of the drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a view in longitudinal section of a hydraulic braking device according to the present invention, in the rest position;  
         [0019]    [0019]FIG. 2 is a view in section of a hydraulic braking device of FIG. 1 in a first braking position; and  
         [0020]    [0020]FIG. 3 is a view in longitudinal section of the hydraulic braking device of FIG. 1 in a second braking position. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    [0021]FIG. 1 shows a hydraulic braking device D 1  according to the present inivention comprising a body  1  of axisymmetric overall shape and of axis X. The body  1  is pierced with a bore  3  formed at the rear by a larger-diameter first chamber  5  and at the front by a smaller-diameter second chamber  7 . The first chamber  5  forms a hydraulic brake booster, the second chamber  7  forms a master cylinder  13 , the chamber  7  being connected to at least one brake at the wheels by a duct  9  made at right angles to the axis X in the body  1 . The hydraulic booster  11  comprises a hydraulic boost piston  15  mounted to slide with sealing in the chamber  5 , the piston  15  beingn guided in translation along the axis X by a ring  17  closing off a rear longitudinal first end  19  of the chamber  5 . The hydraulic boost piston  15  comprises a rod  23  to which a skirt  25  is fixedly mounted, the rod  23  formed of a tube comprising, at a longitudinal first end facing towards the rear, a larger-diameter first part  27  and, at a longitudinal second end facing toward the front, a smaller-diameter second part  29 , the larger-diameter first part  27  being connected to the smaller-diameter second part  29  by a conical section  26  the cone vertex of which faces forward, able to collaborate with an orifice  33  of conical longitudinal section made in the central part of the skirt  25 . In consequence, the skirt  25  is secured to the rod  23  towards the rear by the collaboration of the conical section  26  and of the passage  33 , and towards the front by a circlip  35 . Sealing between the rod and the skirt is achieved by means of a seal, for example an O-ring  37 , sealing between the skirt  25  and the interior periphery of the chamber  5  is achieved by means of a seal, for example. an O-ring  39 , and sealing between the rod and the ring  17  is achieved for example by means of seals, for example lip seals  41 . The front longitudinal end of the rod  23  sealingly enters the chamber  7  of the master cylinder, sealing between the chamber  7  and the chamber  5  being achieved by a ring  43  having a passage  45  in which the rod  23  slides with sealing, the sealing being afforded by seals, for example an O-ring, the ring  43  being mounted fixedly, for example forcibly, in the chamber  5  in a sealed way, sealing being achieved using a seal, for example an O-ring  49 .  
         [0022]    The skirt  25  divides the chamber  5  into a rear first chamber  53  and a front second chamber  51 . The rod  23  defines a passage  55  in which there is mounted to slide with sealing an operating piston  59  equipped at a first end, facing the rear, with an actuating rod  60  connected to an actuating means, for example a brake pedal  62 . The front chamber  51  and the rear chamber  53  are, at rest, placed in communication via the bore  55  and ducts  65 ,  67  made in the rod  23  at right angles to the axis X.  
         [0023]    The operating piston  59  comprises means  69  of comrnmunication between the front chamber  51  and the chamber  7  of the master cylinder, the communication means  69  are formed of ducts  71  made on the exterior periphery of the operating piston  59  and parallel to the axis X, of a bore  73  made in the longitudinal front part of the operating piston  59  and connected to the duct  71  by a transverse passage  75 . The bore  73  made in the front part of the operating piston  59  comprises a first valve  77  and a second valve  79 , in the non-limiting example depicted in FIGS. 1, 2 and  3 , the first valve  77  is hydraulically operated, the second valve  79  being mechanically operated.  
         [0024]    The first valve  77  comprises a valve seat  81  formed, for example, of a shoulder made in the interior periphery of the bore  73  and a shutter formed for example of a ball  83 , the ball  83  being pressed against the valve seat  81  by a spring  85 . The second valve  79  is formed of a valve seat  87  formed of a longitudinal first end of a sleeve  89  forcibly. mounted in the front part of the bore  73  and a shutter, for example a ball  91 , applied to the valvti seat  87  by the spring  85 . The valve  79  is opened by means of a rod  93  mounted fixedly with respect to a hydraulic piston  95  of the master cylinder  13 . The spring  85  is mounted in compression between the first ball  83  and the second ball  91  via a first and a socond bearing means  94 . The master cylinder  13  comprises a hydraulic piston  95  mounted to slide with sealing in the chamber  7  of the master cylinder for example by means of O-rings arranged in grooves made in the exterior periphery of the piston  95 . The piston  95  is more or less socket-shaped, its interior periphery having an annular projection  97  in which the front end of the operating piston  59  slides. The annular projection  97  forms a front shoulder  99  and a rear shoulder  101  dividing the inside of the socket into a front part  103  of smaller inside diameter and a larger-diameter rear part  105 .  
         [0025]    The rod  93  for mechanically opening the second valve  79  arranged in the operating piston  59  is carried by a sleeve  107  the rear end of which forms a base trapped between the shoulder  99  and a spring  96  mounted in compression in the chamber  7 . The spring  96  is a spring for returning the hydraulic piston  95  to the rest position, the longitudinal rest position of the piston  95  being defined by a stop  109 , for example a circlip, rhounted in a groove made in the internal periphery of the chamber  7 , a seal, for example a flat seal  111  is mounted to bear between the cirlip  109  and the rear longitudinal end of the piston  95 . The return spring  113 , when the operating piston  59  is in the rest position, is mounted between a rear longitudinal and of the sleeve  107  and a shoulder  115  made on the exterior periphery of the operating piston  59 , the rest position of the operating piston being defined by a stop  117 , for example a circlip, mounted in a groove made in the internal periphery of the rear part of the bore  55  of the rod  23 .  
         [0026]    The chamber  5  comprises an exhaust chamber  119  delimited by the second guide ring  43  and by a part of the body  1  projecting into the chamber  5  and forming a rear longitudinal end of the chamber  7 . The exhaust chamber  119  is able to be placed in communication with the front chamber  51  and the chamber  7  of a master cylinder via, respectively, first  121  and second  123  communication means. The first communication means  121  comprises a first exhaust duct  125  made in the operating piston  59  at right angles to the axis X and opening into the bore  73  and into the exterior surface of the operating piston  59 , and also comprises second exhaust ducts  127  made in the rod  23  of the boost piston  15  at right angles to the axis X and opening into the exhaust chamber and into the bore  55 . Comrnunication between the first exhaust ducts  125  and the second exhaust ducts  127  can be interrupted by a seal  126 , for example an O-ring mounted fixedly in a groove  128  made in the internal surface of the rod  23 .  
         [0027]    The second means  123  able to place the chamber  7  of the master cylinder and the exhaust chamber  119  in communication, comprise a lip seal  129  mounted in the larger-diameter rear part  105  of the hydraulic piston  95  of the master cylinder, able to intercept a larger-diameter rear portion  131  of the front longitudinal end of the rod  23  relative to a smaller-diameter front portion  133 , the smaller-diameter portion  133  extending over a length L. The lip seal  129  is held in position by a stack of rings  135  and by a stop, for example a circlip, mounted in a groove made in the internal periphery of thb larger-diameter rear part  105  of the hydraulic piston  95 . The exhaust chamber  119  comprises an exhaust orifice  137  allowing the brake fluid to be returned to a brake fluid reservoir (not depicted).  
         [0028]    This first device D 1  is advantageously associated with an identical second device, in a brake system for a tractor (not shown). Each device is connected to a separate wheel brake associated with a wheel axle and a pressure balancing device (not shown) located between the devices to equalize pressures there between. A communication duct  139  made in the wall of the chamber  7  of each master cylinder at right angles to the axis X and a second communication duct  141  made in the hydraulic piston  95  in the smaller-diameter part  10  provides commnunication between the first and second devices D 1 , and the balancing device. The duct  139  is isolated from the chamber  7 , and therefore from the-brake fluid by two seals, O-rings  143  mounted in grooves made in the exterior periphery of the piston  95 . During braking, the chambrer  7  is placed in communication with the duct  139  via the duct  141 , the seals  143  having traveled beyond the axial position of the duct  137  through the advance of the piston  95 , no longer provide sealing between the duct  137  and the chamber  7 .  
         [0029]    The first and second devices D 1  are each actuated :by a respective separate actuating means, for example a pedal  62 , connected to the actuating rod, it being possible for the pedals to be actuated sirnultaneoutsly by removable connecting means. When a tractor is equipped with first and second devices D 1  when driven along a road rather than over agricultural land, for example a field, the pedals  62  are connected together and can be actuated together so as to brake both tractor wheel axles. When moving around a field, the pedals  62  can be actuated independently so as to assist with the movement of the tractor in bends bv.applying a braking action that can differ across the two axles.  
         [0030]    The device also comprises a source (not depicted) of hydraulic fluid at high pressure of roughly between 15 and 30 bar and advantageously between 18 and 20 bar, fed to the hydraulic brake booster. The source, for example comprising a pressure accumulator fed via a hydraulic pump, is in constant communication with the rear chamber  53  of the booster. In cionsequence, in the rest phase, the rear chamber  53  and the front chamber  51  are filled with hydraulic fluid of the same pressure originating from the high-pressure fluid source.  
         [0031]    We shall now describe the way in which the hydraulic braking device according to the present invention works.  
         [0032]    At rest, as illustrated in FIG. 1, the operating piston  59  bears against the stop  117 . The relative position of the operating piston  59  and of the rod  23  or more particularly that of the transverse duct  65  of the rear chamber  53  and the position of the longitudinal duct  71  of the operating piston  59 , allows communication between the rear chamber  53  and the front chamber  51  via the duct  65 , the duct  67  and the duct  71 . Communication between the front chamber  51  of the hydraulic booster and the chamber  7  of the master cylinder  13  is interrupted by the system of valves  77 ,  79  which is arranged in the front part of the longitudinal end of the operating piston  59 .  
         [0033]    Comnmunication between the front chamber  51  of the hydraulic booster  11  and the exhaust chamber  119  is interrupted by the O-ring  126  mounted in the groove  128  made in the interior periphery of the rod  23 , the O-ring  126  being arranged between the first exhaust duct  125  made in the body of the operating piston  59  and the second exhaust duct  127  made in the body of the rod  23 .  
         [0034]    Communication via the means  123  between the chamber  7  and the exhaust chamber  119  is open, the lip seal  129  at rest not intercepting the larger-diameter rear portion  131 .  
         [0035]    When the driver performs a braking action, he depresses the brake pedal which moves the actuating rod  60 , moving the operating piston  59  axially forward. The relative movement of the operating piston  59  with respect to the rod  23  interrupts the communication between the rear chamber  53  and the front chamber  51  of the hydraulic booster. What actually happens is that the portion of the hydraulic piston  59  that is situated behind the ducts  71  intercepts duct  65  as the operating piston  59  moves forward. The axial movement of the piston  59  with respect to the hydraulic piston  95  of the master cylindc  13  causes the second valve  79  to open by the pressing of the rod  93  secured to the sleeve  107  mounted fixed with respect to the hydraulic piston  95  on the ball of the second valve seat  79 , the ball  91  lifting off the valve seat  87  against the action of the spring  85 .  
         [0036]    The difference in pressure between the front chamber  51  of the hydraulic booster  11  and the chamber  7  of the master cylinder causes the first valve  77  to open, the ball  83  on the first valve seat  77  lifting off the valve seat  81  against the action of the spring  85 . In consequence, the hydraulic fluid contained in the front chamber  51  under a high pressure relative to the hydraulic fluid contained in the chamber  7 , flows through the communication means  69  into the chamber  7 , more particularly, the fluid-flows from the chamber  51  to the chamber  7  through the duct  67 , the duct  71 , the transverse bore  75  and the bore  73  made in the front part of the longitudinal end of the operating piston  59  through the first and second valves  77 ,  79 , which are open.  
         [0037]    There is therefore a reduction in pressure in the front chamber  51  of the hydraulic booster, and in consequence there is forward axial movement of the hydraulic boost piston  15  because of the difference in pressure between the rear chamber  53  aind the front chamber  51 .  
         [0038]    Up to this moment, the brake fluid has been flowing into the exhaust chamber  119  through the communication means  123 , preventing pressure from appearing in the chamber  7 . However, the pressure of the fluid contained in the chamber  7  and in the brake circuit is high enough to allow the functional clearances of the braking circuit to be taken up.  
         [0039]    Thereafter, the advance of the boost piston  15  causes the closing of the second communication means  123 , through the advance of the rod  23  of the boost piston  15  with respect to the lip seal  129  mounted fixedly, in the hydraulic piston of the master cylinder  95 . Once the boost piston  15 , and therefore the rod  23 , have moved axially forward, the lip seal  129  intercepts the larger-diameter rear portion  131 , sealingly tupting the communication between the chamber  7  of the master cylinder and the exhaust chamber  119  and allowing the pressure in the chamber  7  of the master cylinder  15  to rise and the master cylinder to exert a reaction on the operating piston  59  by applying a brake fluid pressure to the front transverse end of the operating piston  59 , this reaction adding to the reaction offered by the spring  116 .  
         [0040]    There is therefore a relative movement between the rod  23  of the hydraulic boost piston  15  and the operating piston  59 , more particularly the hydraulic piston  15  has a position further advanced with respect to the operating piston  59 , and the re-opening of the communication between the rear chamber  53  and the front chamber  51  of the hydraulic booster and therefore a supply of high-pressure brake fluid to the front chamber  51 .  
         [0041]    Next, as the pressure in the chamber  7  of the master cylinder combines with the reaction of the spring  85 , this closes the first valve  77  by pressing the ball  83  onto the seat  81 .  
         [0042]    The relative movement between the boost piston  15  and the operating piston  59  causes the opening of the first communication means  121 , by placing the.first exhaust duct  125  of the operating piston  59  in communication with the second exhaust duct  127  of the rod  23 . The high-pressure hydraulic fluid contained in the chamber  51  of the hydraulic booster therefore flows through the first communication means  121  into the exhaust chamber  119 . This causes a difference in pressure between the rear chamber  53  and the front chamber  51  of the hydraulic booster and therefore causes the boost piston  15  to advance. The forward axial movement of the boost piston  15  causes, through the bearing of the front longitudinal end of the rod  23  on the rear shoulder  101  borne by the annular projection  97  of the hydraulic piston  95 , the forward axial movement of the hydraulic piston  96  and an increase in the pressure in the chaimber  7 , in the braking circuit allowing braking. The pressure in the chamber  7  of the master cylinder  13  is proportional to the force applied to the brake pedal by the driver, in accordance with the following formula (I):.  
           F−F 2 −F 3=( P 3 *A 7)+ F 1 −[P 53*( A 51 −A 27)]+[ P 51*( A 51 −A 29)]  (I) 
         [0043]    F being the force applied by the pedal, F2 being the load on the spring  116 , F3. being the reaction of the spring  85 , P3 being the pressure in the chamber  7 , A3 being the cross-sectional area of the hydraulic piston  95 , F1 being the reaction of the return spring  113 , P53 being the pressure in the rear chamber  53 , P51 being the pressure in the front chamber  51 , A7 being the cross-sectional area of the chamber  7 , A51 being the cross-sectional area of the front chamber  51  and also that of the :chamber  53 , A29 being the cross-sectional area of the smaller-diameter part  29  of the  10  rod  23  and A27 being the cross-sectional area of the larger-diameter part  27  of the rod  23 .  
         [0044]    The boosting afforded by the hydraulic booster will increase with the force F and the pressure in the chamber  7  of the master cylinder in accordance with formula (I) until the moment when the pressure in the front chamber  51  of the booster is equal to atmospheric pressure. The device then enters saturation phase. In this saturation phase, any variation by the driver of the force F on the brake pedal causes a variation in pressure P in the chamber  7  of the master cylinder equal to:  
           P=F/A 7 
         [0045]    The return of the device according to the present invention to the rest position is as follows: the driver releases the brake pedal, which, under the action of the spring  116 , causes the operating piston  59  to return to the rest position and the second valve  79  to close, and causes the reopening of the communication between the rear chamber  53  and the front chamber  51  and therefore the re-equalizing of the pressures between the chamber  53  and the chamber  51  and causes the boost piston  15  to be returned to the rest position.  
         [0046]    The device according to the present invention is now ready for a further braking phase.  
         [0047]    Should the high-pressure brake fluid source malfunction, the device operates like a conventional master cylinder without boost. When the driver depresses the brake pedal, the rear end of the operating piston  59  comes to bear against a shoulder formed on the internal periphery of the bore  51  of the rod  23 , moving the rod  23  and closing the conimunication between the chamber  7  of the master cylinder and the exhaust chamber  119  by the interception of the seal  129  by the longitudinal end of the rod  23 . The front longitudinal end comes to bear against the rear shoulder  101  formed on the internal periphery of the piston  95  and moves the piston forward. The advance of the piston has the consequence of sending brake fluid to the brakes, the valve  77  being closed because it is not subjected to a pressure difference and because the communication between the chamber  7  of the master cylinder and the :exhaust chamber  119  is closed.  
         [0048]    We have indeed produced a braking device allowing progressive braking, by flow, proportional to the braking force applied by the driver to the brake pedal, of the high-pressure hydraulic fluid into the chamber  7  of the master cylinder and into the braking circuit via the duct  9 , and which offers the driver a pleasant pedal feel.  
         [0049]    Of course, the use of any type of valve other than the ball valves  77  and  79  does not constitute departure from the scope of the present invention.  
         [0050]    The difference in cross section between the larger-diameter first-part  27  of the rod  23  situated behind the skirt  25  and the smaller-diameter second part  29  of the rod  23  situated in front of the skirt  25  advantageously makes it possible to keep the boost piston  15  in a backward position of rest. This obviously saves adding a return spring to the device of the present invention.  
         [0051]    The present invention applies in particular to the agricultural-vehicle industry.  
         [0052]    The present invention applies mainly to the agricultural-vehicle braking system industry.