Abstract:
A primary piston is described of molded plastic and equipped with a functional metallic insert and at least one groove. The primary piston is installed in a master cylinder comprising at least the primary piston and a secondary piston, these being mounted in the bore hole of a master cylinder. These pistons can create pressure in a primary pressure chamber and a secondary pressure chamber, respectively, due to the action of a push rod on the primary piston.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a master-cylinder primary piston for an automobile and to vehicle master cylinders equipped with such a piston. 
       BACKGROUND INFORMATION 
       [0002]    Master cylinders known to the state of the art have a primary piston and a secondary piston generally made of aluminum, both of which are installed in series in an axial bore hole of a brake master-cylinder body, generally made of aluminum and machined. Such a master cylinder is described in French Patent No. 2,827,244. A push rod is used to actuate the displacement of the primary piston. The primary piston serves to pressurize a primary pressure chamber and a secondary piston serves to pressurize a secondary pressure chamber. Primary and secondary springs tend to push the pistons in the direction opposite displacement, thereby ensuring the increase in pressure. 
         [0003]    The bore hole of the master cylinder is supplied with brake fluid from two supply access holes that are connected to a brake fluid reservoir. The supply access holes are used to supply the primary and secondary pressure chambers. These holes emerge in annular chambers, annular seals known as “cups” being provided on either side of the annular chambers. 
         [0004]    The supply of brake fluid to the pressure chambers occurs when the pistons are at rest. The pistons are then in the position shown in  FIG. 1 . Supply occurs by passages provided in the piston walls, which enable the supply access holes and the annular chambers to communicate with the interior of the primary and secondary pistons emerging respectively in the primary and secondary pressure chambers. When the pistons are moved axially forward (direction of arrow D in  FIG. 1 ), the piston passages cross the seals, isolating the supply chambers and enabling brake pressure to be established in the primary and secondary pressure chambers. 
         [0005]    The master cylinder assembly is capable of being installed on a brake-assist servomotor. When the pistons are displaced along the direction of arrow D by a push rod that exercises a selective force on the primary piston, cup  4  isolates the primary pressure chamber of the primary supply access hole and cup  6  isolates the secondary pressure chamber of the supply access hole. When the force on the push rod is released, the volume of brake fluid accumulated in the brakes and springs of the master cylinder pushes the pistons into rest position. At times, when the push rod is rapidly released, the brake fluid contained in the pressure chambers of the master cylinder can drop below atmospheric pressure due to the action of the springs, which push the pistons more rapidly than the ability of brake fluid to flow through the master cylinder. When the pistons reach rest position, communication between the reservoir at atmospheric pressure and the chambers of the master cylinder is directly established and a sudden surge of brake fluid occurs, which generates noise in the master cylinder, known as a “fluid hammer.” To improve the performance of master cylinders, it is necessary to provide aluminum master-cylinder pistons with specific shapes, which shapes can result in significant additional costs due to the complexity of their production. 
       SUMMARY 
       [0006]    An object of the present invention is to provide a plastic primary piston for a master cylinder that is easy to produce, economical, and capable of resisting mechanical stress. 
         [0007]    An object of the present invention is a master-cylinder piston installed in a brake master cylinder of the type described above, comprising at least the primary piston and a secondary piston installed in a bore hole of the master cylinder. These pistons allow a pressure to be created in a primary pressure chamber and a secondary pressure chamber, respectively, by the action of a push rod on the primary piston, characterized in that the primary piston is of molded plastic material and equipped with a metal insert situated in a receiving cavity of the push rod, where the push rod exercises a force to displace the primary piston and wherein the insert has a specific shape capable of accommodating, on the one hand, the push rod and, on the other hand, an exterior shape enabling it to be maintained in the receiving cavity of the push rod and wherein the piston is preferably made of a thermoset plastic and, more preferably, of phenolic resin filled with glass fibers. The specific shape and hardness of the insert is capable of accommodating the push rod and of resisting deformation of the push rod due to the application of the brakes. The metal insert provides the primary piston with increased mechanical resistance and helps to reduce the thicknesses of plastic materials and make the primary piston more compact. 
         [0008]    Another beneficial characteristic is that the body of the primary piston is easily made by injection molding, which allows complex shapes to be produced, such as: grooves, ribs, non-cylindrical holes. 
         [0009]    According to another beneficial characteristic, the body of the piston is easily made by injection molding and the primary piston leaving the mold is ready for assembly without requiring any finishing work, unlike the aluminum piston, which requires additional machining. 
         [0010]    According to another beneficial characteristic, the body of the piston is easily made by molding thermoset plastic materials, which allow the surface to be ground by machining to improve the reliability of the master cylinder. 
         [0011]    According to another beneficial characteristic, the body of the piston is easily made by injection molding plastic materials, which allow noises caused by the brake fluid to be damped. 
         [0012]    According to another beneficial characteristic, the insert is made of magnetizable material so that the piston emits a magnetic field capable of being detected by a magnetic-field sensor. 
         [0013]    According to another beneficial characteristic, the exterior shape of the metal insert has at least one protrusion whose exterior shape is of slightly greater dimension for forced insertion into the bottom of the receiving cavity of the push rod. 
         [0014]    The thickness of the insert is designed to resist a master-cylinder test pressure of 40 MPa, this thickness taking into account the diameter of the primary piston and the bearing section of the push rod. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is an axial cutaway of a master cylinder known to the state of the art and previously described, 
           [0016]      FIG. 2  is an isometric view with a partial cutaway of an embodiment of a master-cylinder piston according to the invention, with grooves on the front of the piston, 
           [0017]      FIG. 3  is a partial axial view of an embodiment of a portion of a master cylinder according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    In  FIG. 1 , therefore, we find brake master cylinder  100  having bore hole  11  in which primary piston  2  and secondary piston  3  and primary spring  7  and secondary spring  8  have been placed. Pistons  2  and  3  serve to pressurize, respectively, primary pressure chamber  9  and secondary pressure chamber  10  from brake fluid supply access holes  12  and  13 , which are intended to be connected to a brake fluid reservoir (not shown). On either side of access hole  12 , cups  3  and  4  are provided and, on either side of access hole  13 , cups  5  and  6  are provided. Whenever the master cylinder is at rest, the primary piston is in the position shown in  FIG. 1 . The exterior piston walls are equipped with passages  14  and  15  and enable holes  12  and  13  to communicate with the interior of the piston and primary pressure chamber  9  and secondary pressure chamber  8 . When at rest, cups  4  and  6  allow communication between access holes  12  and  13 , primary and secondary pressure chambers  9  and  8  then being supplied with brake fluid. 
         [0019]    Whenever, under the effect of a braking force exercised in direction D by push rod  16  placed in cavity  17  of primary piston  2 , primary piston  2  is moved in the direction of arrow D, cup  6  blocks passages  15  and cup  4  blocks passages  14 . Because primary and secondary pressure chambers are thereby isolated from holes  12  and  13 , a pressure is established in chambers  9  and  10 , this pressure being proportional to the force exercised in direction D by push rod  16  placed in cavity  17  of primary piston  2 . The external diameter S of primary piston  2  forms a section on which the pressure of the primary chamber acts. In cavity  17  of the primary piston, push rod  16  applies a force to generate a pressure in the master cylinder but on a diameter that is appreciably smaller, at a minimum 4 times smaller. This creates significant stress at the primary piston and requires a minimum thickness of material E between the receiving cavity of push rod  27  and forward cavity  19 , where a primary spring pack is found, including of two spring ends  71 ,  72 , rod  73 , and the primary spring. 
         [0020]      FIGS. 2 and 3  show a master-cylinder primary piston according to the invention, characterized in that primary piston  20  is of molded plastic material and equipped with a metallic insert  22  situated behind forward cavity  25  of the primary piston and in a receiving cavity of push rod  27 , where the push rod exerts a force to move the primary piston and generate pressure in the master cylinder, and wherein piston  20  has at least one groove  24  and the insert has the shape of a spherical cap capable of accommodating the push rod and an external shape capable of maintaining it in the receiving cavity of push rod  27 . The hardness of insert  22  is capable, on the one hand, of accommodating the push rod and, on the other, of resisting deformation of the push rod due to the application of the brakes. 
         [0021]    Grooves  24  are of sufficient length so that, when the master cylinder is at rest, the grooves allow brake fluid to pass beneath seal point  42  of cup  41  and emerge in annular chamber  44  situated between cups  31  and  41 . These grooves  24  form passages between the pressure chamber and the annular chamber connected to the reservoir, not shown, by hole  32 . It would have been possible to realize grooves  24  of aluminum but this would have resulted in significant additional costs. The use of plastic materials helps to reduce costs because the shapes of the grooves can be incorporated into the mold. The use of those same plastic materials requires that thicknesses be increased and additional material be used to overcome the difference in the mechanical resistance of the materials. The plastic body and metallic insert confer upon the primary piston an increased mechanical resistance and allow the thickness E of the plastic materials to be reduced and piston  20  to be more compact and, therefore, the invention, through the placement of the insert between forward cavity  25  and the receiving cavity of push rod  27 , provides the benefits of the mechanical resistance of aluminum and the ease of manufacture of complex shapes such as grooves through the use of plastic materials that can be molded. Moreover, the compactness of the primary piston also allows the master cylinder to be more compact and results in savings on master-cylinder materials. 
         [0022]    Another advantageous characteristic is that body  21  of piston  20  is easily made by injection molding, which allows for the realization of complex shapes such as grooves  24 . 
         [0023]    According to another advantageous characteristic, the body of the piston is easily made by molding thermoset plastic materials, which allow surface  28  to be ground by machining to improve the reliability of the master cylinder. 
         [0024]    According to another characteristic, the exterior surface  30  of metal insert  22  has at least one protrusion whose dimension is slightly greater than its exterior shape for forced insertion into the bottom of the receiving cavity of the push rod and which cooperates with the interior surface  29  of cavity  27 . 
         [0025]    According to another characteristic, the exterior surface  30  of metal insert  22  has irregular anchors that cooperate with interior surface  29  of cavity  27 . 
         [0026]    According to another advantageous characteristic, insert  22  is cylindrical and its exterior surface  30  has at least one rib cooperating with interior surface  29  of cavity  27 . 
         [0027]    According to another advantageous characteristic, the master cylinder has a primary piston  20  and an insert  22  made of magnetizable material so that the piston emits a magnetic field capable of being detected by a magnetic-field sensor.