BRAKE PRESSURE PROPORTIONING VALVE

Abstract: A brake pressure proportioning valve incorporates a failure sensitive feature whereby a secondary plunger (32) is subject to pressure at a secondary inlet port (15) and assists a primary plunger (17) to overcome the load of a biassing spring (31) when pressure at the outlet port (16) reaches a predetermined magnitude. The plungers then move to allow a metering valve ball (27) to seat and prevent pressure from a primary inlet port (14) reaching the outlet port (16) without reduction. If pressure at the secondary inlet port (15) fails, this predetermined magnitude increases. The valve is made compact by arranging the secondary plunger (32) between large and small diameter portions (18 and 21) of the primary plunger.

Description: "Brake Pressure Proportioning Valve" The invention relates to brake pressure proportioning valves and particularly to valves which can be described as "failure sensitive". It is known from GB. 1 455 826 to provide a failure sensitive brake pressure proportioning valve comprising a housing, a primary inlet port for connection to a driver-controlled source in one sub-system of a split braking system, a secondary inlet port for connection to a drivercontrolled source in another sub-system of the split braking system, an outlet port for connecttion to a brake in said one sub-system, a primary plunger having a relatively small diameter portion which defines a piston area subject to pressure at the primary inlet port and having a relatively large diameter portion which defines a piston area subject to pressure at the outlet port, metering valve means associated with the primary plunger for controlling flow between the primary inlet port and the outlet port such that pressure at the outlet port acts to move the primal plunger to close the metering valve means and primary pressure at the/ inlet port acts to move the primary plunger to open the metering valve means, and a stepped annular secondary plunger concentric with the primary plunger and defining an annular piston area subject to pressure at the secondary inlet port such that said pressure acts to oppose the load of a biassing spring, which load acts on the primary plunger in the direction to open the metering valve means. The failure sensitive nature of a valve of the kind described above arises from its operation when there is partial failure of the braking system and no pressure is available at the secondary outlet port. In normal operation pressures at the inlet ports and the outlet port rise together until a predetermined pressure is attained whereupon the primary plunger moves to allow the outlet port pressure to rise only at a reduced rate compared with inlet port pressure. In the partial failure mode pressure at the outlet port continues to rise beyond the predetermined pressure. The known valve operates satisfactorily but is expensive to manufacture and requires careful adjustment. It is thus an object of the invention to provide a brake pressure proportioning valve of the kind described which is inexpensive and does not require careful adjustment. According to the present invention there is provided a brake pressure proportioning valve of the kind described above wherein the secondary plunger is arranged between the relatively large and small diameter portions of the primary plunger. One embodiment of the invention will now be described by way of example and with reference to the accompanying drawing which shows a crosssection of a valve according to the invention. In the drawing working seals are shown conventionally and will not normally be referred to specifically. The valve comprises a housing 11 having two housing components 12 and 13 screwthreaded together. Housing component 13 has a primary inlet port 14 for connection to a drivercontrolled source in one sub-system of asplit braking system, eg. one outlet port of a tandem master cylinder. A secondary inlet port 15 in the housing component 12 is for connection to another sub-system of the split braking system, eg. another outlet port of the tandem master cylinder. An outlet port 16 in the housing component 12 is for connection to a rear brake of a vehicle to which the braking system is fitted. A primary plunger 17 hSls a relatively large diameter portion 18 which i-lslidable in the smaller diameter portion of a stepped bore 19 in housing component 12 and a relatively small diameter portion formed by a collar 21 on the main body portion of the primary plunger 17. The collar 21 is slidable in the bore 22 of a tubular housing portion 23 of housing component 13 The collar 21 flanks one side of a seal 24 which is flanked on the other side by a flange on a snap-on cage 25 which retains a valve seating spring 26 and a metering valve member in the form of a ball 27. The ball 27 is, as shown in the drawing, held spaced from a valve seat 30 formed by the adjacent end of an axial passage 28 in the primary plunger 17 by a pin 29 which extends through the passage 28 and abuts the housing component 12 at one end and the ball 27 at the other. Pin 29 includes a square section portion adjacent the ball 27 to permit guidance whilst allowing communication between the primary inlet port 14 and the outlet port 15. The primary plunger 17 is biassed into the position shown by a biassing spring 31 whose load is transmitted through an annular stepped secondary plunger 32 slidable ill the stepped bore 19 and concentric with the primary plunger 17. Thus the secondary plunger 32 is interposed between the biassing spring 31 and an abutment face 35 on the primary plunger 17. This ensures that the secondary plunger 32 is always moved with the primary plunger 17 so undetected seizing cannot occur The secondary plunger 32 defines an annular piston area formed by the difference in cross-sectional areas of the stepped bore 19 so that pressure at the secondary inlet port 15 acts to oppose the load of the biassing spring 31. Plunger portion 18 defines a piston area subject to pressure at the outlet porL 16 and the collar 21 defines a piston area subject to pressure at the primary inlet port 14. When both sub-systems of the braking system are working normally the pressures at inlet ports 14 and 15 rise substantially equally and3 with the ball 27 unseated by the pin 29, the outlet port pressure induced force imbalance on the primary plunger 17 due to the difference in piston areas, creating a force which is resisted by the secondary plunger 32. In addition there is pressure induced force imbalance on the secondary plunger 32 which adds to the force from the primary plunger 17 in opposing the load of the biassing spring 31. When the sum of the pressure induced forces matches the biassing spring load the plungers 17 and 32 move to allow ball 27 to seat and block the axial passage 28. This occurs at a predetermined master cylinder pressure. Thereafter with rising inlet port pressures the outlet. port pressure increases at a reduced rate, the metering valve means comprising ball 27 and the seat formed by the end of passage 28 opening and closing as the plungers move to and fro in a known manner to maintain the balance of forces. If the sub-system serving the secondary inlet port 15 fails, the pressure induced force imbalance on the secondary plunger 32 ceases. Hence primary inlet poft pressure will rise beyond the predetermined master cylinder pressure to allow pressure at the outlet port 16 to increase at the same rate. Eventually the pressure induced force imbalance on the primary plunger 17 will overcome the biassing spring load bt this will only occur at a pressure considesably higher than said predetermined pressure. The secondary plunger 32 is arranged between the relatively large and small diameter portions 18 and 21 of the primary plunger 17 ad this allows a construction whereby housing portion 12 has only one stepped bore 19 which need only be machined from one end. This creates small tolerance accummulations and obviates a need for adjustment of the fitted length of the biassing spring 31 to set the preload. By arranging the biassing spring 31 to encircle the tubular housing portion 23 the valve is made compact. So that seal failures can be visibly detected a radial drilling 33 in the secondary plunger 32 vents the stepped bore between the adjacent seals of the two plungers 17 and 32 (seals being shown conventionally in the drawing). A slot 34 in the end face of housing component 12 vents the chamber 35 occupied by the biassing spring 31.

Claims: Claims 1. A brake pressure propoutoning valve comprising a housing (11), a primary inlet port (14) for connection to a driver-controlled source in one sub-system of a split braking system, a secondary inlet port (15) for connection to a driver-controlled source in another sub-system of the split braking system, an outlet port (16) for connection to a brake in said one sub-system, a primary plunger (17) having a relatively small diameter portion (21) which defines a piston area subject to pressure at the primary inlet port and having a relative large diameter portion (18) which defines a piston area subject to pressure at the outlet port, metering valve means (27) associated with the primary plunger for controlling flow between the primary inlet port and the outlet port such that pressure at the outlet port acts to move the primary plunger to close the metering valve means and pressure at the primary inlet port acts to move the primary plunger to open the metering valve means, and a stepped annular secondary plunger (32) concentric with the primary plunger and defining an annular piston area subject to pressure at the secondary inlet port such that said pressure acts to oppose the load of a biassing spring (31), which load acts on the primary plunger in the direction to open the metering valve means, characterised in that the secondary plunger is arranged between said large and small diameter portions (18 and 21) of the primary plunger (17). 2. A valve according to Claim 1, characterised in that the secondary plunger (32) is interposed between the biassing spring (31) and an abutment face (35) of the primary plunger (17) such that the load of the biassing spring is transmitted through the secondary plunger. 3. A valve according to Claim 1 or Claim 2, characterised in that the relatively small diameter plunger portion (21) is slidable in the bore (22) of a tubular housing portion (23) which is encircled by the biassing spring (31). 4. A valve according to any preceding Claim 1, characterised in that the metering valve means comprises a metering valve member (27) which co-operates with a valve seat (30) formed in an axial passage (28) in the primary plunger (17) and a pin (29) which extends through the axial passage to abut the housing (11) at one end and themetering valve member at the other end. 5. A valve according to any preceding Claim, characterised in that the relative small diameter plunger portion is formed by a collar (21) on a main body portion of the primary plunger.

Inventors: BAILEY, WALTER WILLIAM
Applicants: AUTOMOTIVE PRODUCTS PUBLIC LIMITED COMPANY
Application year: 1982
Main classification: B60T11
IPC codes: B60T8, B60T11