FIELD OF THE INVENTION

The present invention relates to a tandem master cylinder for a vacuum brake booster assembly in automotive vehicles, and in particularly to a piston for the tandem master cylinder.

BACKGROUND OF THE INVENTION

The tandem master cylinder for a vacuum brake booster assembly in automotive vehicles generally comprises a master cylinder body enclosing a bore in which a primary piston and a secondary piston are slidably disposed respectively. The primary piston and the secondary piston define with the bore a respective compressible chamber which is associated with a respective vehicle braking circuit. Generally, the primary piston and the secondary piston are made from metal such as aluminum alloy. To reduce weight and cost of the tandem master cylinder, it was proposed that at least the secondary piston of the primary piston and the secondary piston be molded from plastic.

US2005/0060997A1 discloses such a tandem master cylinder in which the secondary piston is molded from plastic. However, the plastic piston disclosed in US2005/0060997A1 has a complicated structure and is difficult to be manufactured by molding process. Furthermore, the number of parts in the assembled tandem master cylinder is so many that it is not easy to assemble the tandem master cylinder.

To overcome the deficiency caused by the complicated structure of the secondary piston, a piston as shown FIGS 1 and 2 was proposed. The piston 1 as shown FIGS 1 and 2 comprises a cylindrical body 3 within which a hollow chamber 5 is formed. The hollow chamber 5 is blacked off by a partitioning portion 7 adjacent one end of the cylindrical body 3 and is open at an opposite end. A projection 9 for mounting a secondary return spring is formed on a side of the partitioning portion 7 facing the open end of the hollow chamber 5 and a recess 11 for receiving a primary pin is formed on an opposite side of the partitioning portion 7. To re-feed a compressible chamber of the secondary piston with the brake fluid, a plurality of holes 13 are formed in the cylindrical body 3. However, to form a plurality of holes in the cylindrical body by molding process, the molds for molding the piston are complicated in structure and the process for molding the piston is troublesome. SUMMARY OF THE INVENTION

It is an object of the present invention to provide a piston for the tandem master cylinder which is capable of be manufactured by molding process without requiring complicated molds. Furthermore, the process for molding the piston is relatively simple.

It is another object of the present invention to provide a tandem master cylinder which has a simple structure and the reduced number of parts, thereby being capable of being manufactured in a low-cost manner.

According to a first aspect of the present invention, there is provided a piston for a tandem master cylinder comprising:

a cylindrical body within which a hollow chamber is formed, the hollow chamber being blacked off by a partitioning portion adjacent one end of the cylindrical body and open at an opposite end; and

at least one slot formed in an outer wall of the cylindrical body at the open end of the hollow chamber.

According to a second aspect of the present invention, there is provided a tandem master cylinder for a vacuum brake booster assembly in automotive vehicles, comprising at least one piston as mentioned in the first aspect of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a clear understanding of the present invention, a preferred, non-limiting embodiment of a piston for a tandem master cylinder and a tandem master cylinder comprising such a piston will be described by way of example with reference to the accompanying drawings, in which some parts are removed for clarity.

In the drawings:

FIG. 1 is a perspective view showing a piston for the tandem master cylinder in the prior art;

FIG. 2 is a sectional view of the piston as shown in FIG. 1;

FIG. 3 is a perspective view showing a piston for a tandem master cylinder according to the present invention; FIG. 4 is a sectional view of the piston as shown in FIG. 3; and

FIG. 5 is a schematically sectional view of the tandem master cylinder according to the present invention, showing the tandem master cylinder in a rest position. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS 3 and 4 show a piston 21 for a tandem master cylinder according to the present invention. The piston 21 according to the present invention comprises a cylindrical body 23 within which a hollow chamber 25 is formed. The hollow chamber 25 is blacked off by a partitioning portion 27 adjacent one end of the cylindrical body 23 and is open at an opposite end 28. A projection 29 for mounting a return spring is formed on a side of the partitioning portion 27 facing the open end 28 of the hollow chamber 25 and a recess 31 for receiving a pin is formed on an opposite side of the partitioning portion 27.

Unlike the prior piston 1, at least one slot 33 is formed in an outer wall of the cylindrical body 23 at the open end 28 of the hollow chamber 25. Preferably, as shown in FIGS 3 and 4, two slots 33 are formed diametrically opposite to each other in the outer wall. It should be understood that the number of the slots 33 may be more than two. When there are a plurality of slots 33, the plurality of slots 33 are preferably distributed uniformly in the outer wall of the cylindrical body 23. By replacing the holes passing through the cylindrical body of the prior piston with the slots formed in the outer wall of the cylindrical body, the piston according to the present invention is capable of be manufactured by molding process without requiring complicated molds. Furthermore, the process for molding the piston is relatively simple. The piston 21 according to the present invention may be molded from plastic or metal such as aluminum alloy by molding process. When the piston 21 according to the present invention is molded from plastic by molding process, PPA with 45%-50% glass fibre may be used. It should be understood that it is possible to use PPS with 45% glass fibre. Preferably, the end portion 24 of the cylindrical body 23 in which the slots 33 are formed has an outer diameter slightly less than that of the other portion of the cylindrical body 23. Of course, the end portion 24 of the cylindrical body 23 in which the slots 33 are formed may have the same outer diameter as that of the other portion of the cylindrical body 23. In this manner, the outer surface of the cylindrical body 23 of the piston 21 manufactured by molding process hardly has any parting line or burr. To enhance the strength of the piston 21, a plurality of reinforcing ribs 35 extending from the partitioning portion 27 to an inner surface of the cylindrical body 23 are provided around the recess 31. FIG. 5 is a schematically sectional view of the tandem master cylinder according to the present invention, showing the tandem master cylinder in a rest position. The tandem master cylinder 41 according to the present invention generally comprises a master cylinder body 43 enclosing a bore 45. The bore 45 is blocked off at one end by an end wall 47. A primary piston and a secondary piston are slidably disposed in the bore 45. Since the plastic 21 according to the present invention is preferably used as the secondary piston in the tandem master cylinder 41, only the secondary piston is shown and the primary piston is not shown in Fig 5. However, it goes without saying that the plastic 21 according to the present invention may be used as the primary piston in the tandem master cylinder 41. A secondary return spring 49 is interposed between the end wall 47 of the bore 45 and the partitioning portion 27 of the secondary piston 21. Preferably, a telescopic rod 51 passes through the secondary return spring 49 so that one end of the telescopic rod 51 bears against the end wall 47 of the bore 45 and the other end of the telescopic rod 51 engages with the projection 29. The telescopic rod 51 helps to hold the secondary return spring 49 in position. A primary return spring 53 is interposed between the partitioning portion 27 of the secondary piston 21 and the primary piston (not shown).

The secondary piston 21, the master cylinder body 43 and the end wall 47 of the bore 45 define a secondary compressible chamber 55 which is associated with a secondary vehicle braking circuit by a hole 57 formed in the master cylinder body 43. The primary piston, the master cylinder body 43 and the secondary piston 21 define a primary compressible chamber 59 which is associated with a primary vehicle braking circuit. The secondary vehicle braking circuit and the primary vehicle braking circuit are connected to a braking assembly arranged at the wheels of the vehicle respectively. Further, a secondary supply chamber 61 is defined between the secondary piston 21 and the master cylinder body 43. The secondary supply chamber 61 communicates with a brake fluid reservoir 63 via a passage 65. Of course, a primary supply chamber (not shown) may be formed similarly.

A first lip seal 67 is disposed between an outer surface of the secondary piston 21 and an inner surface of the bore 45 to isolate the secondary supply chamber 61 and the secondary compressible chamber 55 from the primary compressible chamber 59. A second lip seal 69 is disposed between the outer surface of the secondary piston 21 and the inner surface of the bore 45 and adjacent the slots 33 formed in the outer wall of the cylindrical body 23 of the secondary piston 21 so that the brake fluid may flow from the secondary supply chamber 61 to the secondary compressible chamber 55 when the tandem master cylinder is in the rest position as shown in FIG 5 and no brake fluid flows from the secondary supply chamber 61 to the secondary compressible chamber 55 when the tandem master cylinder is actuated during braking action.

It was well known that a vacuum brake booster assembly in automotive vehicles generally comprises a brake booster and a tandem master cylinder. The tandem master cylinder is controlled by a brake pedal actuated by a driver in a passenger compartment of an automotive vehicle by way of a control rod. A brake booster is commonly interposed between the brake pedal and the tandem master cylinder and transmits the braking effort and boost force to the primary piston by a push rod which connects to a rear end of the primary piston.

During braking action, the primary piston slides in the bore 45 towards the secondary piston 21 under the braking effort and boost force, causing the secondary piston 21 to slide towards the end wall 47 of the bore 45. With the sliding of the secondary piston 21 towards the end wall 47, the second lip seal 69 forms a close contact with the outer surface of the secondary piston 21 to isolate the secondary supply chamber 61 from the secondary compressible chamber 55 so that no brake fluid flows from the secondary supply chamber 61 to the secondary compressible chamber 55. With the further sliding of the secondary piston 21 towards the end wall 47, the fluid pressure in the secondary compressible chamber 55 and therefore in the secondary vehicle braking circuit increases, causing the braking assembly to brake the wheels of the vehicle. The braking level increases with the displacement of the primary and secondary pistons in the master cylinder body. When the braking action is released, the secondary piston 21 and the primary piston go back to the rest position as shown in FIG 5 under the action of secondary return spring 49 and the primary return spring 53 respectively. If necessary, the brake fluid from the brake fluid reservoir 63 may flow towards the secondary compressible chamber 55 via the passage 65, the secondary supply chamber 61, a gap between the second lip seal 69 and the end portion 24 of the secondary piston 21 as well as the slots 33 formed in the outer wall of the cylindrical body 23 of the secondary piston 21 to supply the secondary compressible chamber 55 with the brake fluid. Although the preferred embodiments of the invention were described in detail taken in conjunction of the accompanying drawings in a non-limiting sense, it should be understood that various changes and modifications could be made without departing from the scope defined by accompanying claims of the invention.