GB2120335A - Tandem master cylinder for vehicle braking systems - Google Patents

Abstract

The cylinder comprises two aligned pistons (16, 18) slidably received in a bore (14), two working chambers (22, 24) the first of which is divided into two compartments (30, 32) by a fixed partition member (34), said compartments communicating with each other through a valve (42) which is normally open and is designed to close as a function of a braking parameter, e.g. deceleration. The second piston (18) has a stepped face adjacent to the first working chamber (22) and includes a first portion of working area (S1) projecting into the first compartment (30) and a second portion of working area (S2) which defines with the partition member (34) the second compartment (32). When the valve is closed the pressure from chamber (24) feeding the rear brakes is proportionally less than the pressure from chamber (22) feeding the front brakes. In a second embodiment (not shown) the piston (18) is in the form of a sleeve, the partition member (34) is located in the sleeve and fixed to end wall (20) and valve 42 is located adjacent the partition member. <IMAGE>

Description

SPECIFICATION Tandem master cylinder The invention relates to a tandem master cylinder and more particularly to a master cylinder of this type which is capable, under certain conditions, of delivering fluid at two different pressures to two braking circuits.

Such a tandem master cylinder is known from French Patent Application NO 2 379 412, which is provided with a bore in which the first and second pistons slide, a first pressure chamber being defined between the two pistons and a second pressure chamber between the second piston and the bottom of the bore. According to this former application, the first pressure chamber is divided into two compartments by a partition wall, namely a first compartment adjacent to the first piston and the second compartment adjacent to the second piston, and a pressure regulating valve is mounted between the two compartments.

In particular, this regulating valve may be lodged in the partition wall.

It is an object of the present invention to improve and simplify the construction of such a master cylinder while retaining its advantageous functional characteristics, and this object is achieved, according to this invention, and in a tandem master cylinder of the kind comprising a casing which is provided with a bore in which are slidably mounted two pistons, namely a first piston located near the open end of the bore and a second piston located between the first piston and the bottom of the bore, a first working chamber being defined between said pistons and a second working chamber being defined between the second piston and the bottom of the bore, said first chamber being divided into two compartments by a fixed partition member, namely a first compartment adjacent to the first piston and a second compartment adjacent to the second piston, and a valve mounted in a passage communicating said compartments with each other, said valve being normally open and being designed to close as a function of a braking parameter, thanks to the fact that the face of the second piston which is adjacent to the first working chamber is stepped so as to include two concentric portions of working area, namely a first portion which extends beyond the partition member and projects into said first compartment thus being submitted to the fluid pressure generated therein, and a second portion which defines with said partition member said second compartment and is responsive only to the fluid pressure established in the later.

In a first preferred embodiment of the invention, in which the partition member consists of a sealed annular wall, the first portion of working area comprises the end face of an axial projection of the second piston which slidably projects through a central opening of said annular wall, while the second portion of working area includes an annular shoulder formed on the second piston at the foot of said axial projection.

In a second preferred embodiment of the invention, in which the partition member consists of a head integral with a rod which projects axially through the second piston and bears against the bottom of the bore, the first portion of working area comprises the end face of a tubular sleeve extending from said second piston and surrounding said head, while the second portion of working area includes an inner annular shoulder formed on the second piston at the foot of said sleeve.

The invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a sectional view of a first embodiment of a master cylinder according to the invention; and Figure 2 is a sectional view of a second embodiment of a master cylinder according to the invention.

Figure 1 shows a master cylinder 10 comprising a casing 12 which is provided with a blind bore 14 in which two pistons are slidably mounted: A first piston 16, at the open end of the bore, and a second piston 18, between the first piston and the bottom 20 of the bore.

A first working chamber 22 is defined between the two pistons and a second working chamber 24 is defined between the second piston and the bottom of the bore; the first chamber is connected to the front brakes of the vehicle via an orifice 26, while the second chamber is connected to the rear brakes of the vehicle via an orifice 28. These brakes are not shown.

The first working chamber 22 is divided into two compartments 30, 32 by a partition member 34 integral with the casing 12; in this embodiment, this member consists of an annular wall 35 fixed in the bore 14. The first compartment 30 is adjacent to the first piston 1 6 whereas the second compartment 32 is adjacent to the second piston 1 8.

In addition, a cavity 36 is formed in the casing 12 and communicates with the two compartments 30 and 32 via two passages 38 and 40, respectively, said cavity containing a valve 42 which is controlled as a function of a braking parameter. In this case, the valve is an inertia valve whose seat 44 is located at the mouth of the passage 40 and whose plug consists of a ball 46 able to move, under the influence of the inertia forces, along a ramp 48 inclined at an angle a when the deceleration due to braking (the bottom 20 of the master cylinder being supposed directed towards the front of the vehicle) exceeds a predetermined value.

The second piston 1 8 is provided with an extension 50 in the form of an axial projection 51 which passes through the partition wall 35 and projects into the first compartment 30, so that the piston 1 8 presents a first cross sectional area Sa which is exposed to the pressure in the first compartment 30, a second, annular cross sectional area S2 exposed to the pressure in the second compartment 32, and a third cross sectional area S3 which is equal to the sum of the preceding areas and is exposed to the pressure in the second chamber 24.

Lastly, two springs 52 and 54 are provided to return the two pistons towards the outside of the bore to a rest position, such as that shown, in which the pistons 1 6, 1 8 uncover two orifices 56 and 58 which communicate with the reservoir of fluid (not shown).

The above described master cylinder functions in the following manner: when braking is initiated with the adi of a brake pedal (not shown), the piston 1 6 is displaced towards the interior of the bore 14, together with the piston 18, which causes the orifices 56 and 58 to close and the chambers 22 and 24 to be put under pressure, and also causes the brakes of the vehicle to be applied.

More precisely, the advance of the piston 1 6 causes a pressure P1 to appear in the first compartment 30 and this pressure exerts a force P,S1 on the piston 18.

Furthermore, for as long as the deceleration remains slight, the valve 42 remains open and this pressure P1 also reigns in the second compartment 32, via the passage 38, the cavity 36 and the passage 40. This pressure exerts an additional force P1S2 on the piston 1 8.

In opposition, the pressure P2 which exists in the chamber 24 exerts a reaction P2S3 on the piston 1 8. Now it will be noted that as S3 is equal to S1 + S2, the pressure P2 remains equal to the pressure P1 and the front and rear brakes of the vehicle are supplied with fluid at the same pressure.

However, if the deceleration which results from braking exceeds a certain predetermined value, the ball 46 moves along the ramp 48 and comes into contact with the seat 44 so that the valve 42 is closed.

Under these conditions, the pressure in the second compartment 32 can rise no higher and the pressures P1 and P2 evolve differently, as follows: the pressure Pc will be designated as the pressure which exists in the compartments 30 and 32 and also in the chamber 24 at the moment when the valve 42 closes.

Initially, the increase in pressure P1 has the effect of increasing the force acting on the cross sectional area S1 of the piston 1 8 and of reducing simultaneously the force acting on the cross sectional area S2 by reducing the pressure of the liquid trapped in the compartment 32.

During this phase and until the pressure in this latter compartment is nullified, the pressure P2 existing within the chamber 24 remains equal to the pressure Pc.

When the pressure P1 exceeds a value PL such that PLUS1 = PCS3 or S3 P,= PCx S1 a second phase of operation commences during which the piston 1 8 is only subjected to two forces P1S1 and P2S3.

Thus the pressure P2 increases in proportion to the pressure P1 in the ratio of the cross sectional areas S1 and S3.

In this way, the pressure P2 which is delivered to the rear brakes will be less than the pressure P delivered to the front brakes, as is desirable in order to avoid locking the rear wheels.

It will be noted that the invention enables these advantages to be obtained with a particularly simple and compact construction; the master cylinder 10 may be mounted in place of a conventional tandem master cylinder without further modification to the installation.

As an additional point, it will be observed that the second compartment 32 increases in volume during the last phase of operation. Given that the seals 59 and 60 which seal this compartment are immersed in liquid on both sides, there is therefore no risk of causing air to enter the compartment 32. Thus, according to choice, either a cavitation phenomenon may be deliberately allowed to develop during this increase in volume, or the feeding of the compartment 32 may be provided for, which may be obtained by making the seal 60 around the piston 1 8 in the form of a lip seal.

Then, as soon as the pressure in the compartment 32 is reduced below atmospheric pressure, some liquid will flow from the chamber 62, which communicates with the fluid reservoir, into the compartment as a result of the lifting of the lip of the seal 60.

The second embodiment shown in Figure 2 includes a number of identical elements which are designated by the same reference numbers.

In this embodiment, the partition member 34 which separates the first compartment 30 from the second compartment 32 consists of a head 33 which is fixed to the end of a rod 64 which projects axiaily through the second piston 1 8 and bears against the bottom 20 of the bore.

Furthermore, the extension 50 of the piston 1 8 takes the form of a tubular sleeve 49 located between the head 33 and the bore 14.

In addition, the valve 42 is arranged as follows: the cavity 36 is enclosed in a perforated cage 66 which is fixed to the head 33 and projects into the compartment 30. The passage 40 which leads into the compartment 32 is arranged in the head 33 and the valve seat 44 is fixed to the head at the mouth of the passage 40. The ball 46 is retained inside the case 66 and is able to roll on the ramp 48 formed by the wall itself of the cage 66.

The inclination of the ramp 48 is obtained by mounting the master cylinder inclined at an angle a relative to the horizontal.

The second piston 1 8 thus presents several areas of cross section: S, is the annular area of the extension 50 included between the head 33 and the bore 14, S2 is the annular area of the compartment 32 surrounding the rod 64, and S3 is the annular area of the chamber 24 surrounding the rod 64.

It follows that S3 is equal to the sum St + S2 The different stages of operation of this apparatus are identical to those described above relating to the apparatus shown in Figure 1.

Claims (4)

1. A tandem master cylinder comprising a casing (12) which is provided with a bore (14) in which are slidably mounted two pistons, namely a first piston (1 6) located near the open end of the bore and a second piston (18) located between the first piston and the bottom (20) of the bore, a first working chamber (22) being defined between said pistons and a second working chamber (24) being defined between the second piston and the bottom of the bore, said first chamber (22) being divided into two compartments by a fixed partition member, namely a first compartment (30) adjacent to the first piston (16) and a second compartment (32) adjacent to the second piston (18), and a valve (42) mounted in a passage communicating said compartments with each other, said valve being normally open and being designed to close as a function of a braking parameter, characterised in that the face of the second piston (18) which is adjacent to the first working chamber (22) is stepped so as to include two concentric portions of working area, namely a first portion (St) which extends beyond the partition member (34) and projects into said first compartment (30) thus being submitted to the fluid pressure generated therein, and a second portion (S2) which defines said partition member said second compartment (32) and is responsive only to the fluid pressure established in the latter.

2. A tandem master cylinder according to claim 1, in which the partition member consists of a sealed annular wall (35, 59), characterised in that the first portion of working area (S1) comprises the end face of an axial projection (51) of the second piston (18) which slidably projects through a central opening of said annular wall, while the second portion of working area (S2) includes an annular shoulder formed on the second piston (18) at the foot of said axial projection.

3. A tandem master cylinder according to claim 1, in which the partition member consists of a head (33) integral with a rod (64) which projects axially through the second piston (18) and bears against the bottom (20) of the bore (14), characterised in that the first portion of working area (S,) ) comprises the end face of a tubular sleeve (49) extending from said second piston and surrounding said head, while the second portion of working area (S2) includes an inner annular shoulder formed on the second piston at the foot of said sleeve.

4. A tandem master cylinder substantially as described and as shown in the accompanying drawings.