GB2027850A - Fluid pressure proportioning valve - Google Patents

Abstract

A varible load valve for modulating the pressure in a vehicle braking system according to the weight of the load carried. The valve comprises a self-lapping double valve arrangement 5, 6, 7 operative to vent brake system pressure in inverse proportion to the weight of the load. The relative disposition of the double valve arrangement is controlled through mechanical linkage 8, 9 by deflection of the vehicle suspension, and the arrangement is provided with a piston 10 acted on by inlet pressure so that the highest position of seat carrier 7 is maintained and the brake system pressure is thus unmodulated in the event of disruption of the link means. <IMAGE>

Description

SPECIFICATION Fluid pressure proportioning valve This invention relates to a fluid pressure proportioning valve and more especially it relates to a variable load valve for fluid pressure braking systems.

Fluid pressure proportioning valves for braking systems have already been proposed which include a fluid pressure-responsive assembly, one side of which is subject to the fluid pressure at an input port and the other side of which is subject to the fluid pressure at an output port, the assembly operating a self-lapping double valve arrangement to, in resonse to an increase of input port pressure, open a flow path for fluid to the output port in a sense to correspondingly increase the pressure at the output port and in response to decreasing input port pressure to open a vent path to correspondingly reduce the output port pressure, the arrangement also having a part which is movable by a positioning member to different positions to determine a proportioning ratio for the valve.

Certain such valves have been found to have a shortcoming to the extent that when used for variable load valves in braking systems, in the event of a controlling linkage to the positioning member becoming disconnected, the valve has tended to move to a position of lowest proportioning ratio for the output pressure in relation to the input pressure.

According to the present invention there is provided a fluid pressure proportioning valve which includes a fluid pressure-responsive assembly, one side of which is subject to the fluid pressure at an input port and the other side of which is subject to the fluid pressure at an output port, said pressure-responsive assembly operating a self-lapping double valve arrangement to, in response to an increase of pressure at the input port, open a flow path for fluid to the output port in a sense to correspondingly increase the pressure thereat and in response to decreasing fluid pressure at the input port to open a vent path to correspondingly reduce the output port pressure, the valve arrangement having a part which is movable by a positioning member to different positions to determine a proportioning ratio for the valve and means responsive to fluid pressure at the input port to act on said part and substantially more than counteract the reaction via said part of the positioning member.

It has been found that by virtue of the provision of a means responsive to pressure at the input port to act on the said part as aforesaid, in the event of failure of a connection to the controlling positioning member, the proportioning ratio moves towards a maximum value for significant levels of input pressure.

In order that the present invention may be more clearly understood and readily carried into effect, the same will be further described by way of example with reference to the single figure of the accompanying drawing which shows a typical fluid pressure proportioning valve modified in accordance with the invention.

Referring to the drawing, the load proportioning valve which is shown is a variable load valve for fluid pressure vehicle braking systems and in normal operation, the input port would receive fluid pressure from a driver's foot-operated brake valve and the resultant pressure arriving at the output port would be proportioned in accordance with the setting of a control lever connected to an unsprung part of the vehicle such that its position is dependent upon the load on the vehicle.

The variable load valve is constructed with upper and lower body members denoted by references 1 and 2 respectively and which are bolted together to include a control piston 3, a flexible diaphragm 4 and a double valve arrangement comprising a spring-loaded valve member 5 which normally engages against a seat 6 and is further engageable by an internal tubular part 7 which is sealingly slidable in the lower body member 2 and is engageable in turn with a ball pivot 8 carried by a pivoted internal member of the valve actuable by a lever 9 external of the body members.

The lever 9 is connected to the pivot spindle of ball pivot 8 via a resilient coil portion 9a which absorbs road shocks.

In the lower part of the housing, a piston 10 is provided which, when subjected to fluid pressure beneath it derived from the input port, moves upwards to engage the lower extremity of the member 7 and provides only restricted clearance as between the ball pivot 8, the member 7 and an upper projection 11 of the piston 10.

The control piston 3 carries radially extending fingers such as 14, the upper edges of which, in the position shown, abut the interior of the body member 1 and which, on downward movement of the control member, move between corresponding fingers 1 8 against which the diaphragm 1 4 rests in the position shown. The variable diaphragm arrangement is a known arrangement in which, upon downward movement of the piston, the diaphragm is increasingly engaged by the radial fingers 1 4 and decreasingly engaged by the inwardly projecting fins 1 8. The effect of pressure beneath the diaphragm on the piston arrangement therefore increases the greater is the downward deflection of the piston.A similar such arrangement is described in the Specification of British Patent No. 1,029,183 and therefore requires no further detailed description. Referring however to the piston 10 which is slidable in a cylinder 1 5 connected by an external pipe 1 2 to the input port 13, the size of the piston 10 is arranged such that for appreciable levels of input pressure at the input port 1 3 from a driver's foot valve, the piston 10 moves upwards to exert a force on the part 7 which is substantially greater than the reaction which would otherwise exist between the ball pivot 8 and the part 7.The effect of this is that for a given load-setting position of the lever 9, an increasing pressure at the input port 1 3 causes a downward deflection of the piston 3 to a point where the valve member 5 engages the part 7 which closes-off the exhaust path through the central bore 1 9 of the part 7. Further downward movement of the piston 3 results in the valve member 5 lifting from the seat 6 to provide a communication between the input port 1 3 and the output port 1 7 which is also connected to the region beneath the diaphragm 4. When the pressure beneath the diaphragm 4 attains a value such as to counteract the downward force on the piston 3, the valve member 5 is carried to a position in which it seals-off both the part 7 and the seat 6.This condition is reached when the set level of proportioning as between the pressure at the input port and the output port is attained. A reduction of input port pressure enables the piston 3 to rise to unseat the valve member 5 from the part 7 and allow a proportion of the pressure to be released from beneath the diaphragm 4 until an equilibrium condition is again attained.

By virtue of the provision of the piston 10 with such a surface area as to substantially more than counteract the reaction of the part 7 on the ball pivot 8, failure of a linkage controlling the lever 9 results in a progressive upward movement of the part 7 unconstrained by other than the weight of the lever 9 with increasing input port pressure. Such failure therefore produces an increased ratio of output to input port pressures for significant levels of input pressure. It is therefore not necessary to provide alternative means for securing the necessary degree of braking in the event of failure of the linkage to the lever 9.

Claims (2)

1. A fluid pressure proportioning valve which includes a fluid pressure-responsive assembly, one side of which is subject to the fluid pressure at an input port and the other side of which is subject to the fluid pressure at an output port, said pressure-responsive assembly operating a self-lapping double valve arrangement to, in response to an in crease of pressure at the input port, open a flow path for fluid to the output port in a sense to correspondingly increase the pressure thereat and in response to decreasing fluid pressure at the input port to open a vent path to correspondingly reduce the output port pressure, the valve arrangement having a part which is movable by a positioning member to different positions to determine a proportioning ratio for the valve and means responsive to fluid pressure at the input port to act on said part and substantially more than counteract the reaction via said part on the positioning member.

2. A pressure proportioning valve substantially as described herein with reference to the accompanying drawing.