GB2037917A

GB2037917A - Master Cylinder for a Hydraulic Brake System

Info

Publication number: GB2037917A
Application number: GB7942531A
Authority: GB
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1978-12-28
Filing date: 1979-12-10
Publication date: 1980-07-16
Also published as: GB2037917B; DE2856417A1; JPS5591445A

Abstract

A master cylinder has a recuperation valve formed by a bore 20 and groove 21 in the piston rod 18, and a seal in the end wall 5. The seal consists of a hard running ring 15 and a soft back-up ring 16. The seal is retained by axial pressure of the return spring 9 (Fig. 1) or a circumferential lip 26 (Fig. 2). <IMAGE>

Description

SPECIFICATION A Master Cylinder for a Hydraulic Brake System This invention concerns a master cylinder for hydraulic brake systems for motor vehicles. In a known master cylinder described in U.S. Patent Specification No. 2590430, the brake pressure is generated by a low-compression piston and a high-compression piston. These pistons are connected to each other via a spring coupling, an axial clearance being provided between the pistons. The known master cylinder is complicated and prone to failure in service because a ring seal, having a soft sealing lip, is provided for a piston rod where it enters an end wall of the working chamber which end wall is rigidly attached to the housing.Such a seal is not capable of withstanding the heavy stresses encountered in service, particularly because it must serve simultaneously as a non-return valve for pressure medium flowing from a low-pressure chamber to a high-pressure chamber.

According to the present invention there is provided a master cylinder for hydraulic brake systems for motor vehicles, having a piston rod which extends through a working chamber and which is mounted axially displaceably in an end wall of the working chamber, which end wall accommodates a ring seal through which the piston rod passes and which comprises a hard running ring engaging the piston rod and a soft back-up ring, the hard ring having an axial width which is greater than the axial distance between two control edges of an annular groove provided in the piston rod, a central supply passage being provided in the piston rod to connect a fluid reservoir to the annular groove, such that the fluid reservoir is placed in communication with the working chamber when the brakes are released, and when the brakes are applied the hard ring passes over the said annular groove such that the said communication is interrupted.

The master cylinder in accordance with the present invention has the advantage that, owing to the special construction and arrangement of the seal, a long service life of the ring seal is ensured. In a preferred embodiment the necessary equalisation of fluid quantity and pressure is effected via the end wall directly from the brake fluid reservoir.

In a further preferred embodiment of the master cylinder, it is possible to supply highpressure fluid to a further hydraulic device without increasing the number of seals necessary.

The present invention will now be described further by way of example only with reference to the accompanying drawings, in which: Figure 1 shows the master cylinder according to the present invention with a first embodiment of the ring seal; Figure 2 shows a different type of construction of the ring seal, and Figure 3 shows a different type of connection of the fluid reservoir to that of the embodiment of Figure 1.

A master cylinder 1 comprises a substantially cylindrical housing 2, having a cylinder bore 3 for receiving an operating piston 4, and an end wall 5; between the two is a working chamber 6. The operating piston 4 is displaceable by means of a brake pedal 8 via a connecting rod 7 against the force of a return spring 9 arranged in the working chamber 6. The other end of the spring 9 abuts the end wall 5, so that the latter is in constant abutment with a snap ring 10, which is fitted in a recess in the bore 3 and serves as a fixed stop.

To one side of the working chamber there is connected a brake line 12, leading to a wheel brake cylinder 11, and, to its other side, there is connected a bleed port 1 3.

Mounted in the end wall 5 is a ring seal 14, comprising a hard running ring 15 and a soft back-up ring 1 6. An end region 1 7 of a piston rod 18, which is attached to the operation piston 4 and extends through the working chamber 6, is movable in the end wall 5, and is provided, in the region of the ring seal 14, with an annular groove 19, with whose base there communicates a supply passage 20, which extends coaxially through the end region. The axial distance between two control edges 21 and 22, which delimit the width of the annular groove 19, is less than the effective axial width of the hard running ring 15, that is, the axial width of the hard ring 1 5 in contact with the piston rod.

In the embodiment of Figure 1, the return spring 9 presses a flat disc 23 against a shoulder 24. In the embodiment of Figure 2, a special shoulder 25 on the housing is provided for the return spring 9, and the disc 23 is clamped by a clamping flange 26 against an end wall 5' The end wall 5, 5' is also provided with a plurality of circumferentially spaced through bores 27, whose outlets communcating with the working chamber 6 are covered by a disc 41 (see Fig. 2) and the rear face of a packing ring 28, in the manner of a non-return valve. The disc 41 may be attached to the packing ring 28 by vulcanising. Alternatively, however, it may be replaced by a hard insert in the back of the packing ring 28. Axially arranged to the rear of the end wall there is provided a fluid reservoir 29.

Alternatively, as shown in Figure 3, it is possible, by means of a different construction of the end wall, to arrange the fluid reservoir on the master cylinder in the conventional manner. Such an end wall is here designated by the reference numeral 30. It has a radial passage 31 leading to an annular groove 32 in the cylindrical outer wall of the end wall 30. The annular groove 32 is in communication with a passage 33 through the housing, which passage is connected in a conventional manner to the fluid reservoir arranged above the master cylinder.

The method of operation is as follows: When the brake is released, the movable members of the master cylinder are in the positions shown in the drawing. The working chamber 6 is connected to the fluid reservoir 29, 34 via the supply passage 20, so that a pressure below atmospheric pressure, which may result, for example, from a slight leak at a screwed joint or in a brake cylinder, on the return stroke of the piston 4 does not induce outside air into the brake system. On its left-hand side the hard ring 1 5 abuts the cylindrical outer wall of the end region 17.

When the brakes are applied, the piston 4 moves to the left. The end region 1 7 is displaced further inside the end wall 5, 5', 30, and the hard ring 15 5 of the ring seal 14 reaches the control edge 22 without having uncovered the control edge 21 so that the working chamber 6 is then cut off from the fluid reservoir 29, 34. The pressure, which is increased as a result of the displacement of the piston in the working chamber 6, is transmitted via the brake line 21 to the wheei brake cylinder 12. The brakes are applied.

When the pressure on the brake pedal 8 is relaxed, the return spring 9 moves the piston 4 back towards its original position. Pressure equalisation between the brake system and the fluid reservoir 29, 34 is able to take place via the disc 41 and packing 28, which lift away from the outlets of the through bores 27. This is important, because the fluid sealed in the master cylinder and the rest of the brake system is subject to influences which may result in variation both in quantity and in pressure.

Lastly, as shown in Figure 1, the operating piston 4 is provided with a piston seal similar in principle to the ring seal 14. The operating-piston ring seal 35 also comprises a hard running ring 36 and a soft back-up ring 37. The back-up ring 37 operates as an expander and presses the hard ring 36 tightly and rigidly against the cylindrical wall of the working chamber. In this wall there is provided an annular groove 38, which is narrower than the effective width of the hard ring 36. From the base of the annular groove 38 a passage 39 leads to an ancillary device 40 (not shown).

It is evident that, in a brake application phase, the ancillary device 40 is supplied with pressurised fluid. On full application of the brakes, however, the ring seal 35 overruns the annular groove 38, and the supply of fluid to the ancillary device 40 is interrupted. The advantage of the pressure resistance of the ring seal 35 is evident in the event of the dismantling of the master cylinder 1 in the above-described manner, since no other ring seal could withstand the high pressures. ~~~~~~~~~~~~~~~~~ Because of the substantial load-bearing capacity of the ring seals 14 and 35, the master cylinder according to the present invention is usable preferably in brake systems provided with anit-locking devices, in which substantial back pressures may be created, particularly in the pressure-reduction phase.

Claims

1. A master cylinder for hydraulic brake systems for motor vehicles, having a piston rod which extends through a working chamber and which is mounted axially displaceably in an end wall of the working chamber, which end wall accommodates a ring seal through which the piston rod passes and which comprises a hard running ring engaging the piston rod and a soft back-up ring, the hard ring having an axial width which is greater than the axial distance between two control edges of an annular groove provided in the piston rod, a central supply passage being provided in the piston rod to connect a fluid reservoir to the annular groove, such that the fluid reservoir is placed in communication with the working chamber when the brakes are released, and when the brakes are applied the hard ring passes over the said annular groove such that the said communication is interrupted.

2. A master cylinder as claimed in claim 1, in which the piston rod carries a piston, which serves as a movable wall, to define one end of the working chamber in the master cylinder, and whose other end is sealed by the end wall, which is rigidly attached to the housing.

3. A master cylinder as claimed in claim 1 or 2, in which the end wall is provided with a plurality of circumferentially spaced through bores, whose outlets are controllable, in the manner of a nonreturn valve, by a disc and the rear face of a packing ring.

4. A master cylinder as claimed in any of claims 1 to 3, in which an additional hydraulic device is connected to the working chamber, and whose connecting passage has its outlet at the base of an annular groove in the wall of the cylinder, and the piston seal is a ring seal, comprising a hard running ring and a soft back-up ring which hard bearing track ring has a greater width than the annular groove in the wall of the cylinder.

5. A master cylinder constructed and arranged substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.