Abstract:
Our problem relates to a master cylinder with a U-shaped seal. Our goal is to avoid defective bleeding of hydraulic braking systems. The defects arise out of the fact that the vacuum causes the lips of the seal ( 21 ) to stick together and to stick against the wall ( 41 ). To achieve this, reliefs ( 42 ) will be created on one wall ( 44 ) of one lip ( 28 ) facing the other lip ( 27 ) and on the edge face ( 50 ) of this same lip. These reliefs will prevent the lips from sticking tightly to one another and to the wall facing the edge face of the lips. This lack of sticking will prevent a vacuum from being created between the two lips and between the edge face of the lips and the wall ( 41 ). As a result, the seal will be able to return to a position in which bleeding can be performed effectively.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
       [0001]    The invention relates to a master cylinder with a U-shaped seal. It is an object of the invention to improve the efficiency with which hydraulic braking systems fitted with such a master cylinder can be bled. The invention is more specifically intended for the field of motor vehicles but could be applied to other fields. 
         [0002]    Master cylinders comprising a master cylinder body, a piston, a reservoir for brake liquid, or more generally for hydraulic fluid, and a pressure chamber, are known. The piston slides in a bore formed in the body of the master cylinder. The pressure chamber is connected to the hydraulic fluid reservoir via a hydraulic fluid pipe. 
         [0003]    The master cylinder has a seal that allows the pressure in the pressure chamber to be increased. This seal is situated between a wall delimited by the bore of the body of the master cylinder and the piston. This seal prevents hydraulic fluid from passing from the pressure chamber to the hydraulic fluid reservoir. The passage of hydraulic fluid is prevented according to the position of the piston in the bore of the master cylinder. This seal is housed in a peripheral groove. 
         [0004]    The peripheral groove is hollowed into the wall of the body of the master cylinder. This groove is circular cylindrical and faces the piston. 
         [0005]    The seal is made of rubber and has a U-shaped profile. The profile comprises a base, the bottom of the U, to which two lips, the limbs of the U are connected. These two lips extend both in the same direction from this base. 
         [0006]    A first lip, situated on the inside, is intended to face the piston. This lip extends from the smallest diameter of the base of the seal. This inner lip presses against the piston. 
         [0007]    A second lip, situated on the outside, is intended to face the wall of the body of the master cylinder. More specifically, this second lip is intended to face the wall of the peripheral groove on the opposite side of the seal to the piston. This wall is formed by the closed end of the groove. The second lip extends from the largest diameter of the base of the seal. The outer lip rests against the closed end of the groove. 
         [0008]    At rest, that is to say when a brake pedal of the vehicle is not being actuated, the pressure chamber is full of hydraulic fluid from the reservoir. The reservoir and the pressure chamber therefore communicate via a hydraulic fluid passageway. 
         [0009]    When the piston moves forward in the bore of the body of the master cylinder, that is to say when the brake pedal is actuated, hydraulic fluid is prevented from flowing from the pressure chamber into the reservoir. What happens is that as it advances along the wall of the body of the master cylinder, the piston becomes positioned relative to the seal in such a way that the seal prevents hydraulic fluid from passing from the pressure chamber to the reservoir. The hydraulic fluid in the pressure chamber is therefore isolated from the hydraulic fluid contained in the reservoir. The forward movement of the piston then causes the pressure in the pressure chamber to rise. It then follows that the hydraulic fluid is injected from the pressure chamber into the vehicle braking system. 
         [0010]    When the hydraulic braking system is being bled, the braking circuit, the outlet from the master cylinder is open. The expelled hydraulic fluid is collected in an open container positioned at a suitable location. To make this bleeding operation easier, the brake pedal is pumped hard. This pumping of the pedal forces the piston to move back and forth from its rest position to its forward position. This pumping of the pedal has the effect of discharging the liquid that is to be bled, together with any air bubbles that might be present in the hydraulic braking system. 
         [0011]    As the piston moves along the wall of the master cylinder, during brake bleedings, a depression is created in the peripheral groove in which the seal is positioned. This depression is created as a result of the sharp forward movements of the piston in the bore. This depression will cause the lips of the seal to detach from the walls against which they respectively rest. This detachment may, in some cases, cause the lips to move closer to one another and cause the lips to move closer toward the wall facing an edge face of these lips. The inner lip and the outer lip will stick together. The two limbs of the U come into contact with one another at their ends. A vacuum will then be created between the lips of the seal in the thus-closed U. This vacuum forces the lips to remain stuck together as the piston returns to the rest position. This vacuum, is irreversible because, when the depression in the groove disappears, at best, the returning hydraulic fluid forces the two lips to remain stuck together. Once deformed in this way, the seal no longer seals. 
         [0012]    Furthermore, a vacuum may also be created between the lips and the wall against which the edge face of the lips is stuck. This vacuum also forces the lips to remain stuck to the wall of the groove facing the edge face of the lips as the piston returns to the rest position, and once again the seal fails to perform its sealing function. The hydraulic fluid contained in the pressure chamber can then seep between the wall of the master cylinder body and the piston, as far as the hydraulic fluid reservoir. 
         [0013]    The only way to avoid this lack of sealing is to bleed the brakes at a limited throughput and with a limited rate of pedal travel, otherwise the seal will no longer prevent hydraulic fluid from passing from the pressure chamber to the hydraulic fluid reservoir. It will be appreciated that bleeding in this way becomes a far more tricky operation. The operator has to have appropriate training and this training is an irksome constraint. 
         [0014]    In an attempt to prevent these lips from sticking together it might be possible to increase the rigidity of the lips. To do that, an internal skeleton could be incorporated into the lips. Seals such as this with an internal skeleton could be produced using a dual molding technique. However, that would lead to complicated manufacture and a not-insignificant increase in cost. 
         [0015]    Another solution might be to create passageways that cause the outside of the lips to communicate with the inside of the lips. These passageways would make it possible to prevent a vacuum from forming between the lips and between the lips and the wall when the pedal is being pumped. However, this solution would reduce the sealing of the lips and the seal would therefore no longer seal satisfactorily. In addition, producing such passageways in a seal of this size would have a tendency to weaken the structure of the seal. These solutions are therefore not acceptable for solving the problem such as it stands. 
         [0016]    In order to solve this problem, in the invention, reliefs are made to prevent a vacuum from being created between the two lips. Thus, as the lips move closer together, the lips cannot stick tightly together because the reliefs prevent the lips from making a perfect seal with one another or with the wall facing the edge face of the lips. The fact that a perfect seal is not made between these lips and/or wall prevents the vacuum from being created. The vacuum that could create a depression between the two lips is removed via a communicating space allowing the outside and the inside of the lips to communicate with one another, this space being formed by virtue of the reliefs. 
         [0017]    The invention of course can apply to a master cylinder that has several incorporated seals of this type. In addition, the invention can be applied to all kinds of master cylinder, including tandem master cylinders. 
         [0018]    One subject of the invention is therefore a brake master cylinder comprising a piston sliding in a chamber of a master cylinder body, the body comprising an inlet allowing hydraulic fluid to enter the chamber, and a smooth seal of circular shape with a U-shaped profile, the U-shaped profile being formed by an inner lip and an outer lip of the seal, the seal being in contact via the inner lip with the periphery of the piston, and via the outer lip with the body, characterized in that it comprises localized reliefs produced on one lip. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The invention will be better understood from reading the following description and from examining the accompanying figures. These are given merely by way of nonlimiting indication of the invention. The figures show: 
           [0020]      FIG. 1 : a longitudinal section through a master cylinder, in this instance a tandem master cylinder, fitted with seals according to the invention; 
           [0021]      FIG. 2 : a longitudinal section through part of a master cylinder, in this instance a tandem master cylinder, fitted with a known seal, at rest; 
           [0022]      FIG. 3 : a longitudinal section through part of a master cylinder, in this instance a tandem master cylinder, fitted with a known seal during brake bleeding; 
           [0023]      FIG. 4 : a schematic perspective view of a seal according to the invention; 
           [0024]      FIG. 5 : a schematic perspective view of an alternative form of the seal according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]      FIG. 1  depicts a longitudinal section of a master cylinder  1 , in this instance a tandem master cylinder, fitted with seals according to the invention. The master cylinder  1  here is a tandem master cylinder but the invention does of course apply to cases in which the master cylinder is a single master cylinder. A tandem master cylinder  1  such as this comprises a body  2  in which there is a bore  3 . This tandem master cylinder also comprises a primary hydraulic circuit fitted with a primary piston  4  and a secondary hydraulic circuit is fitted with a secondary piston  5 . The primary piston  4  and the secondary piston  5  slide along a wall  6  delimited by the bore  3  of the body  2 . The primary piston  4  and the secondary piston  5  slide longitudinally along an axis of insertion  7  of a push rod (not depicted here) inserted into the master cylinder  1 . The tandem master cylinder  1  is actuated by the push rod which is itself actuated when a brake pedal (not depicted here, is depressed. A receptacle  10  accommodates this push rod. This receptacle  10  is formed at a rear end  11  of the primary piston  4 . A front end  12  of the primary piston is intended to be positioned facing the secondary piston  5 . Normally, the axis  7  is an axis that coincides with an axis of movement of the primary piston  4  and with an axis of movement of the secondary piston  5 . 
         [0026]    The primary piston  4 , the secondary piston  5  and the wall  6  of the master cylinder together delimit a primary pressure chamber  13 . The secondary piston  5  and the wall  6  of the master cylinder  1  together delimit a secondary pressure chamber  14 . The primary piston  4  is intended to compress a hydraulic fluid contained in the primary pressure chamber  13 . The secondary piston  5  is intended to compress a hydraulic fluid contained in the secondary pressure chamber  14 . 
         [0027]    The hydraulic fluid contained in the primary pressure chamber  13  comes from a primary hydraulic fluid reservoir  15 . The hydraulic fluid contained in the secondary pressure chamber  14  comes from a secondary hydraulic fluid reservoir  16 . The primary reservoir  15  supplies hydraulic fluid to the primary pressure chamber  13  via a primary pipe  17 . The secondary reservoir  16  supplies hydraulic fluid to the secondary pressure chamber  14  from a secondary pipe  18 . The primary pipe  17  and the secondary pipe  18  are hollowed into the body  2  of the master cylinder and open respectively into the primary pressure chamber  13  and into the secondary pressure chamber  14 . The body of the master cylinder  1  therefore has inlets via which hydraulic fluid is supplied to the pressure chambers  13  and  14 . 
         [0028]    The master cylinder  1  also comprises at least one seal. As a preference, each chamber may comprise two seals. These seals are made of rubber. Each of the seals has a U-shaped profile. Each of these seals forms a cylindrical annulus. Each annulus is developed in a plane perpendicular to the axis  7  of movement of the piston. 
         [0029]    In this instance, the primary chamber  13  comprises a first seal  19  and a second seal  21 . The secondary chamber  14  comprises a third seal  20  and a fourth seal  22 . The invention is aimed at any one of these seals, or several or all of them. 
         [0030]    The seal  19  and the seal  20  are positioned respectively at an inlet  8  of the primary chamber  13  and at an inlet  9  of the secondary chamber  14 . The seal  19  seals the primary chamber  13  against the outside of the master cylinder. The seal  20  seals the secondary chamber  14  against the primary chamber  13 . 
         [0031]    The seal  21  is positioned between the wall  6  of the bore  3  of the body  2  and the primary piston  4 . The seal  22  is positioned between the wall  6  of the bore  3  of the body  2  and the secondary piston  5 . The seal  21  and the seal  22  are situated respectively in a primary peripheral groove  23  and in a secondary peripheral groove  24 . The seal  21  and the seal  22  are positioned respectively facing the primary piston  4  and the secondary piston  5 . The primary groove  23  and the secondary groove  24  are hollowed into the wall  6  of the body  2 . These grooves  23  and  24  are cylindrical and face the piston. The grooves  23  and  24  are hollowed out facing the piston. A generatrix of the grooves  23  and  24  lies in a plane perpendicular to the axis of movement of the primary piston  4 . 
         [0032]    The primary seal  21  is able to prevent hydraulic fluid from passing from the primary pressure chamber  13  as far as the primary reservoir  15 . The secondary seal  22  is for controlling the passage of hydraulic fluid from the primary pressure chamber  14  to the secondary reservoir  16 . 
         [0033]    The primary piston  4  and the secondary piston  5  are returned to their position of rest by a primary return spring  25  and by a secondary return spring  26 , respectively. 
         [0034]      FIG. 2  is a longitudinal section through part of a master cylinder, in this instance a tandem master cylinder,  1  fitted with a known seal  21 , at rest. 
         [0035]    The primary seal  21  in this instance is made of rubber and has a U-shaped profile. The primary seal  21  is positioned about the axis of movement of the primary piston  4 . The profile has a base  29 , the bottom of the U, to which two lips  27  and  28 , the limbs of the U, are connected. These two lips  27  and  28  extend from this base  29  in one and the same direction. The external lip  27  and the internal lip  28  between them at rest delimit an opening  30 . This opening  30  in this instance faces towards the secondary chamber  14 . 
         [0036]    A first lip  27 , situated on the outside, is intended to face the wall  6  of the body  2  of the master cylinder  1 . More specifically, this second lip  27  is intended to face the wall of the primary groove  23  on the opposite side of the seal  21  to the piston  4 . This wall is formed by the closed end of the groove  23 . The outer lip  27  extends from the largest diameter of the base  29  of the seal. The outer lip  27  presses against the closed end of the groove  23 . 
         [0037]    A second lip  28 , situated on the inside, is intended to face the piston  4 . This lip extends from the smallest diameter of the base  29  of the primary seal  21 . This inner lip  28  rests against the piston  4 . More specifically, at rest, the internal lip  28  may be positioned so that it rests against a point on the primary piston  4  such that the hydraulic fluid seeps from the primary pressure chamber  13  as far as the primary reservoir  15  through a primary port  31 . The primary piston  4  and the secondary piston  5  are equipped with a primary port  31  and with a secondary port  32 , respectively. Each of these ports opens at a first end  33  facing the wall  6  of the master cylinder and at a second end  34  facing the corresponding chamber. 
         [0038]    The primary seal  21  may be stuck to the primary piston  4  inside a groove  35 . This groove in this instance is hollowed into a surface of the primary piston  4  facing the seal  21 . The same may be apply to the secondary piston  5 . Only the primary groove  35  is described. 
         [0039]    The primary piston  4  is positioned in the bore with the end of the internal lip  28  of the primary seal  21  opposite the base  29  of the primary seal  21  positioned against a groove  35 . The inner lip  28  may press against the groove  35  in the closed end of the groove  35 . Another primary groove (not depicted) could be created at a position close to the seal  19  in addition to or in place of the aforementioned primary groove  35 . 
         [0040]    The primary groove  35  forms a first surface  36 , a second surface  37  and a third surface  38  which are joined together as a continuum. The first surface  36  is made continuous and flat at an angle to the axis  7 . This angle of inclination is such that the junction between the surface  36  and the remainder of the surface formed by the primary piston  4  is closer to the secondary piston  5  than is the junction between the surface  36  and the surface  37 . This surface is close to the secondary piston  5  by comparison with the surfaces  37  and  38 . The second surface  37  is flat and relatively parallel to the axis  7 . The surface  37  is located between the surface  36  and the surface  38 . The third surface  38  is made continuous and flat and at an angle to the axis  7 . This surface  38  connects the second surface  37  to a remainder of the surface formed by the primary piston  4 . The angle of inclination of the surface  38  is such that the junction between the surfaces  37  and  38  is closer to the secondary piston  5  than is the junction between the surface  38  and the remainder of the surface formed by the primary piston  4 . The angle of inclination of the surfaces  36  and  38  is created in such a way that the primary seal  21  resting against the surfaces  36  and  38  does not impede the movement of the primary piston  4 . 
         [0041]    At rest, that is to say when the brake pedal is not being actuated, the primary seal  21  is positioned resting against the groove  35  downstream of the primary port  31 . The primary port  31  may be hollowed from the second surface  37  of the groove. The primary seal  21  may be positioned resting against the first surface  36  and against the second surface  37  simultaneously. Alternatively, the primary seal  21  may be positioned resting against the first surface  36  only or against the second surface  37  only. The primary piston  4  is positioned with respect to the primary seal  21  and with respect to the primary pipe  17  in such a way that the hydraulic fluid flows from the primary pressure chamber  13  toward the primary reservoir  15  via the primary port  31  and via the primary pipe  17 . 
         [0042]    During brake bleeding, the primary piston  4  is thrust suddenly into the bore  3  of the master cylinder towards the secondary piston  5  and longitudinally with respect to the axis of the primary piston  4 . The primary piston  4  slides along the wall  6  of the bore  3  of the master cylinder and therefore along the primary seal  21 . The primary seal  21  is therefore positioned resting against the primary piston between the primary port  31  and the primary pipe  17 . The primary seal  21  is positioned upstream of the primary port  31 , longitudinally with respect to an axis passing through the primary port and relatively parallel to the axis of the piston, in the direction of forward travel of the primary piston. The hydraulic fluid can no longer pass through the primary port  31  because the primary seal  21  is positioned resting against the primary piston and therefore blocks access of the liquid through the primary port  31 . The pressure in the primary chamber  13  therefore rises. The primary seal  21  is positioned resting against the rest of the surface of the primary piston  4 . However, the primary seal  21  could be positioned resting against the second or third surface of the primary groove  35 , always in such a way as to prevent hydraulic fluid from passing from the primary pressure chamber  13  to the primary reservoir  15 . 
         [0043]      FIG. 3  is a partial longitudinal section through a master cylinder, in this instance a tandem master cylinder, fitted with a known seal during brake bleeding. When the hydraulic system is being bled, pumping of the pedals causes sharp movements of the primary piston  4  towards the primary chamber  13 . These sharp forward movements of the primary piston  4  create a depression in the groove  23 . This depression sucks out the hydraulic fluid contained between the lips  27  and  28 , leading to a depression between the lips  27  and  28 . The depression between the lips  27  and  28  causes the lips  27  and  28  to detach from the walls against which they are resting. The lips  27  and  28  stick together. A vacuum  39  is created between the wall of the lips facing one another. This vacuum keeps the lips  27  and  28  stuck together. 
         [0044]    A vacuum  40  may also be created between the edge face of the lips  27  and  28  and the wall  41  facing the edge face of the lips  27  and  28 . This vacuum  40  keeps the lips  27  and  28  stuck to the wall  41 . 
         [0045]    As the primary piston  4  returns to its rest position, the depression that has been created in the groove  23  disappears because the hydraulic fluid contained in the reservoir  15  and in the chamber  13  comes back into contact with one another. However, the vacuums  39  and  40  still keep the lips jammed together and against the wall. In addition, the difference in pressure between the inside of the groove  23 , which is returned to a normal pressure because the depression that was therein has disappeared, and the vacuums  39  and  40 , keep the lips  27  and  28  stuck even move firmly together and even more firmly against the wall  41 . In this situation, the seal  21  no longer provides an effective seal between the primary reservoir  15  and the primary chamber  13 . As this sealing is no longer established, bleeding the brakes becomes less effective, if not impossible. 
         [0046]      FIG. 4  depicts a schematic perspective view of a seal  21  according to the invention. A seal such as this may be positioned equally well on a simple master cylinder or a tandem master cylinder. According to the invention, the lips of a seal  21  have reliefs  42 . One particular embodiment of these reliefs  42 , as shown in  FIG. 4 , is that they are in the form of protrusions  43 , of ribs  43 . These reliefs may, however as we shall see later, be in the form of hollows. 
         [0047]    The ribs  43  according to  FIG. 4  extend over the surface  44  of the inner lip  28  facing the outer lip  27 . These ribs  43  extend along an axis parallel to the axis of movement of the piston, continuously over at least the entire height of the inner lip  28 . Furthermore, these ribs  43  may be extended to form a pip  45  on the edge face  50  of the inner lip  28 . 
         [0048]    Such ribs  43  prevent the inner lip  28  from sticking tightly against the outer lip  27  because the shapes of the outer lip  27  and of the ribs  43  do not complement one another. As any sticking-together of the inner lip  28  and of the outer lip  27  during the brake bleeding is no longer tight, the vacuum  39  between these two lips  27  and  28  when the pedals are pumped is no longer created. The two lips  27  and  28  therefore no longer remain stuck together. 
         [0049]    Likewise, the presence of the pips  45  on the edge face of the inner lip  28  prevents the lips  27  and  28  from sticking tightly against the wall  41  for the same reason that the shapes of the pips  45  and of the wall  41  do not complement one another. There is therefore a space between the wall  41  and the inner lip  28 . This space prevents a depression, the vacuum  40 , from being created between the lips and the wall  41 . 
         [0050]    The differences in pressure between the inside of the lips and the outside of the lips therefore disappear. The lips  27  and  28  may return to a free position and seal effectively. The hydraulic braking system can be bled under optimal conditions. 
         [0051]    The number of reliefs  42  present may vary. However, with just one rib  43 , the passageway that avoids the vacuum is limited. This single passage way removes pressure slowly. The seal  21  takes longer to return to its position. Having six or eight ribs  43  is enough to ensure quick and effective removal of pressure. The inner lip  28  here has six ribs  43 . 
         [0052]    The reliefs  42  may be arranged in groups. The ribs  43 , in  FIG. 4 , are positioned in pairs on the outer part  44  of the inner lip  28 . For each pair of reliefs  42 , the space  47  separating the two reliefs is, for example, equal to about half the height  48  of the inner lip, as measured in the direction of movement of the piston. 
         [0053]    Having pairs of ribs provides a better flow space as the lips begin to stick together. What happens is that the small space between the two ribs  43  prevents the lips from sticking together within this space  47 . Likewise, the space  49  between the pips  45  prevents the lips from sticking tightly to the wall  41  inside this space  49 . These spaces therefore allow the seal to return more quickly to a bleeding position and therefore allow better quality bleeding. 
         [0054]      FIG. 4  depicts a seal  21  in which the ribs  43  are continuous from the edge face  50  of the inner lip  28  to the wall  44  of the inner wall  28  facing the outer lip  27 . However, the reliefs  42  could be situated on the wall of absolutely any lip facing the other lip. Thus, the ribs  43  may be situated on the wall  51  of the outer lip  27  facing the inner lip  28  using the same method. 
         [0055]    According to  FIG. 4 , the reliefs  42  are continuous from the edge face of the lip to a wall of the lip facing the other lip. According to another embodiment of the invention, the ribs  43  present on the lips and the pips  45  present on the edge face of the lips may be independent. Such independency allows the pips  45  to be positioned as required on the edge face of the lips, for example positioning them offset from the ribs  43 . 
         [0056]      FIG. 5  depicts a schematic perspective view of a seal  21  according to the invention. According to the invention, the reliefs  42  present on one lip of the seal  21  may have a hollow shape. In this instance, these hollows are in the form of grooves  52  made on the edge face  50  of the inner lip  28 . These grooves  52  extend over the entire thickness  53  of the edge face  50  of the inner lip  28 , providing a passage that regulates the pressure between the space between the lips and the wall  41 . Furthermore, grooves  54  may be made on the outer wall  44  of the inner lip  28 . These grooves  54 , which extend over the entire height  48  of the inner lip  28 , regulate the pressure between the lips and thus prevent the creation of a vacuum  39  that keeps the lips stuck together while the brakes are being bled. 
         [0057]    In  FIG. 5 , the grooves  52  and the grooves  53  are independent, although like the rib-shaped reliefs, the grooves  52  and  54  may lie in the continuation of one another. 
         [0058]    The grooves  52  situated on the edge face  50  of the inner lip  28  do not prevent the seal  21  from sealing. This is because that surface of the wall  41  that is situated under the grooves  52  continues to seal by pressing against the piston  4 , the grooves  52  being just deep enough to allow the pressure to even out without preventing the inner lip  28  from sealing. 
         [0059]    Furthermore, the grooves  54  may be situated on the inner wall  51  of the outer lip  27  so as to act as passages for regulating the pressure between the lips.