Plastic primary piston with insert for a tandem penetration-type master cylinder and a master cylinder equipped with such a piston

Abstract

A primary piston of molded plastic material equipped with a metallic insert and having grooves, mounted in a master cylinder comprising at least a primary piston and a secondary piston mounted in the bore hole of the master cylinder. These pistons can create pressure, respectively, in a primary pressure chamber and in a secondary pressure chamber due to the action of a push rod on the primary piston.

Claims

1. A primary piston for a tandem penetration-type brake master cylinder, comprising: a primary piston of molded plastic material and having an overmolded stamped sheet-metal insert situated between a forward cavity and a cavity, and also having at least one groove on the surface emerging on a forward face; wherein the metallic insert has at least one hole, and wherein the hole is substantially parallel to the axis of the piston.

2. The primary piston of claim 1, wherein the primary piston is made of a thermoset plastic material.

3. The primary piston of claim 1, wherein, after injection of the plastic material, the hole of the insert forms a blind hole in the piston.

4. The primary piston of claim 1, wherein the surface of the piston is ground by machine.

5. The primary piston of claim 1, wherein the primary piston is made of plastic materials capable of damping noise.

6. A master cylinder, comprising: a master cylinder arrangement: and at least a primary piston and a secondary piston mounted in a bore hole of the master cylinder arrangement, wherein the primary piston is of molded plastic material and having a metallic insert situated between the forward cavity and the primary pressure chamber and in that it also has at least one groove on a surface emerging on the forward face, wherein the metallic insert has at least one hole, and wherein the hole is substantially parallel to the axis of the piston.

7. The master cylinder of claim 6, wherein the insert of the primary piston is made of a magnetizable material, so that the piston emits a magnetic field capable of being detected by a magnetic field sensor.

8. The master cylinder of claim 6, wherein the primary piston is made of a thermoset plastic material.

9. The master cylinder of claim 6, wherein, after injection of the plastic material, the hole of the insert forms a blind hole in the piston.

10. The master cylinder of claim 6, wherein the surface of the piston is ground by machine.

11. The master cylinder of claim 6, wherein the primary piston is made of plastic materials capable of damping noise.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows an axial cutaway of a master cylinder known to the state of the art and previously described.

(2) FIG. 2 shows an isometric view with a partial cutaway of an embodiment of a master-cylinder piston according to the invention, with grooves situated in the front of the piston.

(3) FIG. 3 shows a partial axial view of an embodiment of a portion of a master cylinder according to the invention, secondary piston, and springs, which are not shown.

DETAILED DESCRIPTION

(4) 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. Brake fluid supply access holes 12 and 13 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. When the master cylinder is at rest, the primary piston is in the position shown in FIG. 1. The 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 chambers 9 and secondary pressure chamber 8. At rest, cups 4 and 6 enable communication between access holes 12 and 13 and primary and secondary pressure chambers 9 and 8, which are thereby supplied with brake fluid.

(5) When braking force is applied in direction D by a 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 hole-shaped passages 15, and cup 4 blocks passages 14. Because the primary and secondary pressure chambers are isolated from holes 12 and 13, a pressure is established in chambers 9 and 10, this pressure being proportional to the force exercised along direction D by push rod 16, which is 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 on the primary piston and requires a minimum thickness of material E between the receiving cavity of push rod 17 and forward cavity 19 of primary piston 2 emerging in pressure chamber 9.

(6) In FIGS. 2 and 3 can be seen a master-cylinder primary piston 20 according to the exemplary embodiments and/or exemplary methods of the present invention, characterized in that primary piston 20 has body 21 of molded plastic material equipped with an overmolded stamped sheet-metal insert situated between a forward cavity 25 and a receiving cavity of push rod 27 and in that it also has at least one groove 24 on a surface 28 emerging on forward face 26. Grooves 24 are of sufficient length so that, when the master cylinder is at rest, the grooves allow brake fluid to pass beneath a seal point 42 of cup 41 and emerge in annular chamber 44 situated between cups 31 and 41. These grooves form passages between the pressure chamber and the annular chamber connected to the reservoir 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. Plastic body 21 and metallic insert 22 confer increased mechanical resistance upon the primary piston and allow the thickness E of the plastic materials to be reduced, making piston 20 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 allows the master cylinder itself to be more compact and results in savings on master-cylinder materials.

(7) 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.

(8) According to another beneficial characteristic, body 21 of the piston is easily made by molding thermoset plastic materials, which allow surface 28 to be ground by machining in order to improve the reliability of the master cylinder.

(9) According to another beneficial characteristic, metallic insert 22 has at least one hole 29 to maintain the insert in the mold before injection of the plastic material, the axis of the hole being advantageously appreciably parallel to the X-axis of the primary piston. After injection of plastic material, hole 29 of insert 22 forms a blind hole in piston 20.

(10) According to another advantageous characteristic, the master cylinder has primary piston 20 and insert 22, made of magnetizable material, so that the piston emits a magnetic field capable of being detected by a magnetic-field sensor.

REFERENCE KEY

(11) 1 body 2 primary piston 3 secondary piston 4 cup 5 cup 6 cup 7 spring 8 spring 9 secondary pressure chamber 10 primary pressure chamber 11 bore hole 12 hole 13 hole 14 grooves 15 holes 16 push rod 17 primary piston cavity 18 cup 19 forward cavity 20 primary piston according to the invention 21 piston body 22 insert 23 spring centerer 24 groove 25 forward cavity 26 front face of piston 27 push-rod receiving cavity 28 piston surface 29 hole 31 cup 32 hole 41 cup 42 seal point 44 annular chamber 100 tandem master cylinder E thickness of material between the two cavities S piston diameter X axis of the master cylinder and pistons