GB2095777A - Pneumatic pressure proportioning valve - Google Patents

Abstract

A pressure reducing valve is constructed with a stepped piston 26 affording a pressure responsive area 27 on one side responsive to the pressure at an output port 23 and a smaller pressure responsive area 28 on the other side, contained between the seals 29, 30, of the two piston diameters and responsive to the pressure at an input port 22, the piston carrying elements of a self lapping valve assembly, 34, 35, 36, connected via a passage 40 to the smaller area for controlling the pressure at the output port 23 in relation to that at the input port 22. <IMAGE>

Description

SPECIFICATION Pressure proportioning valve This invention relates to a pressure proportioning valve and relates especially but not exclusively to a pressure proportioning valve for reducing pressure to brake actuators of a compressed air braking system.

In compressed air braking systems for road vehicles it is often required to present different brake pressures to brake actuators of front and rear axles and in order to achieve this a pressure proportioning valve is used to supply actuators of the front axle with a proportion of the pressure supplied to the rear axles or vice versa. One form of such a proportioning valve can comprise a dual piston Carrying a self-lapping valve one side of which piston is subject to fluid pressure at an input port and the other side of which piston is of larger area and subject to the fluid pressure at an output port which is suppiied with fluid pressure from the input port via the self-lapping valve.The self-lapping valve is arranged to close when equilibrium is created across the piston and to open a path from the delivery port to exhaust if the delivered pressure tends to exceed a predetermined proportion of the pressure at the input port. Since the piston has two diameters it requires two seals and the volume between the seals is preferably vented via a separate vent port to atmosphere.

The present invention seeks to provide an improved construction of a pressure proportioning valve which inter alia does not require such a separate vent port.

According to the present invention there is provided a pressure proportioning valve having a housing and a self-lapping double valve operable to connect a delivery port to an input port or to a vent and having a stable lap position with a predetermined ratio of fluid pressures at the input port and delivery port, said double valve being carried by a double piston having portions of different areas slideable in respective cylindrical portions of the housing and wherein the pressure at one said port acts upon an area defined between said seals and the pressure at the other said port acts opposingly thereto on the piston.

In order that the 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 accompanying drawing of which: Fig. 1 illustrates in diagrammatical form a prior known arrangement of a pressure proportioning valve Fig. 2 illustrates in diagrammatical form a pressure proportioning valve employing the present invention.

Referring to Fig. 1, the pressure proportioning valve has a main housing denoted by reference 1, having an input port 2 a pair of delivery ports 3, and exhaust port 4 and an additional vent port 5.

Sealingly moveable within the housing there is provided a stepped double piston 6 having on the underside thereof an area 7 which communicates with the delivery ports 3 and which is larger than the effective area 8 on the other side of the piston which comunicates with the input port 2. The two portions of the piston are provided with separate seals 9 and 10 the region between the seals 9 and 10 being vented via the above mentioned additional vent port 5.The piston carries a selflapping double ended valve arrangement with respective ends 11 and 12, the end 12 being urged into engagement with a valve seat 13 by a spring 14 retained between the piston and the retaining cup 1 5. On downward movement of the piston the lower end of the valve member is engageable with an exhuast valve seat 1 6 to close off the connection between the delivery ports and the exhaust port. Further downward movement after the stabilising effect of spring 14 is then effective to unseat the end 12 from the seat 13 thereby providing communication between the inlet port and the delivery port.

In operation of the arrangement of Fig. 1, with all ports vented the rest position is as shown.

When fluid pressure is applied to the inlet port 2, the double piston 7 moves downwards to carry the double valve member towards the exhaust valve seat 1 6 to then close the exhaust passage and thereafter unseat 12 from 13 to +'hereby provide communication from the in jort to the delivery ports. The delivery port pr sure thus increases and upon attainment of a fluid pressure at the delivery ports which is in a reduced proportion to the inlet port pressure predetermined in accordance with the approximate ratio of the areas 8 and 7, the piston 7 returns upwards to a position where the double valve laps with seats 13 and 1 6 closed.If the pressure at the delivery ports increases sufficiently beyond the predetermined proportion sufficiently to overcome the lap stability provided mainly by spring 14 the valve end 11 unseats from the seat 16 and venting of the delivery port pressure occurs until stable equilibrium is regained. By providing additional vent at 5, any leakage which occurs across the seal 10 or the seal 9 is vented to atmosphere. If such leakage is only small, it does not therefore affect the operation of the proportioning valve. However, this relies upon the vent 1 5 not becoming blocked with extraneous matter such as road grit, etc., and it will be seen that in the improved arrangement of Fig. 2 the risk of such a vent becoming blocked is over-come.

Referring to Fig. 2, this again has a main housing 21 an inlet port 22, a pair of delivery ports 23 and an exhaust port 24. A piston 26 has an area 27 (generated by dimension B) subject to the pressure at the delivery ports and an annular area 28 (generated by dimension A) subject to pressure at the inlet port. The area 28 is of appreciably less area than the area 27 the ratio being such as to determine the approximate proportioning ratio of the valve. The larger diameter portion of the piston has a seal 29 and a smaller diameter portion of the piston has a seal 30 separating a region 31 which is at input port pressure from a region 32 which is always at exhaust port pressure.Carried within the piston 26 there is a sealingly slideable tubular valve member 33 with a closure surface 34 which is engageable with either or both of seat 35 formed in the piston and seat 36 formed on a tubular upstand 37 communicating with the exhaust port 24. The upstand 37 is so engageable by extending upwardly through a central aperture 38 in the piston and the valve member 33 is urged against the seat 35 by a housed spring 39. A passage 40 in the wall of piston 26 provides a path to the valvefom region 31.

In operation of the arrangement shown in Fig.

2, fluid pressure applied at the inlet port 22 is effective on the smaller area 28 of the piston 26 to tend to move the piston downwards to cause the closure surface 34 to engage with seat 36 to close off the exhaust port and further downward movement causes the member 33 to be sealingly moved in the piston to unseat the surface 34 from the seat 35 thereby providing a communication between the inlet port 22 via region 31 and passage 40 to delivery ports 23. Upon attainment of the predetermined proportion between the pressures at the inlet port 22 and the delivery ports 23, the valve arrangement comprising closure surface 34 and seats 36 and 35 are lapped together thereby closing off the delivery port both from the exhaust port and the inlet port.

Any tendency for the delivered pressure to increase beyond the predetermined proportion of the pressure of the inlet port, to an extent to overcome the stability afforded by spring 39, causes a piston assembly to move upwards thereby briefly opening the delivery port to the exhaust port to permit partial venting to reestablish equilibrium.

In the event of the seal 30 leaking, since this seal is now located to separate a region 31 at inlet port pressure from a region 32 at exhaust port pressure at all times, such leakage if only small does not affect the operation of the proportioning valve. Further, in the vent of the seal 29 exhibiting a similar small leakage, overcharging at the delivery ports is prevented by the above mentioned lifting of the piston to permit venting of the excess pressure to the exhaust 4.

It is therefore seen that not only is the improved arrangement a compact and convenient arrangement to manufacture but also a need for an additional vent path for venting a region between the two seals of the pressure responsive member is avoided.

A typical application of the pressure proportioning valve described above with reference to Fig. 2, is to provide a predetermined ratio of braking pressure to brake actuators of a heavy road vehicle fitted with air brakes or air over hydraulic brakes. In such an application the air pressure for operating the brakes on a front axle may be applied also to the input port 22 of the proportioning valve and the pressure for controlling the rear brakes may then be derived from delivery ports 23 so that rear axle braking pressures are a predetermined proportion of front axle braking pressures.

Whilst in accordance with the example of the proportioning valve described above in accordance with the invention the area of the pressure responsive member between the seals (seals 29 and 30) thereof is designed to be less than the opposing area, so that ports 23 are used as delivery ports for pressures fed in at port 22 between the seals an alterative example may be readily envisaged. In such alternative the area between the seals may be larger than the opposing area and the self-lapping double valve rearranged so that the input pressure is applied to the said opposing area allowing the reduced delivered pressure to be derived from the region between the seals.

Such an arrangement may have similar advantages to Fig. 2 in regard to the possibility of minor leakage around the seals.

Claims (8)

1. A pressure proportioning valve having a housing and a self-lapping double valve operable to connect a delivery port to an input port or to a vent and having a stable lap position with a predetermined ratio of fluid pressures at the input port and delivery port, said double valve being carried by a double piston having portions of different areas slideable in respective cylindrical portions of the housing and wherein the pressure at one said port acts upon an area defined between said seals and the pressure at the other said port acts opposingly thereto on the piston.

2. A pressure proportioning valve having a housing with an input port and a delivery port, a self-lapping double valve operable to connect the delivery port to the input port or to a vent, the double valve having a valve member carried by a moveable pressure responsive member having an inner portion sealingly moveable in the housing to provide a vented area and a surrounding area subject to the pressure at the input port is smaller than an opposing area of the moveable member subject to pressure at the delivery port.

3. A pressure proportioning valve as claimed in any preceeding claim, the pressure responsive member comprising a stepped piston with different diameter portions sealingly moveable in respective bores of the housing.

4. A pressure proportioning valve as claimed in claim 2 or 3 said valve member having means to urge it towards closure of an aperture between the input and delivery ports and being engageable with a valve seat to close a path between the delivery port and the vent port.

5. A pressure proportioning valve as claimed in claim 4, said valve member having a pssage through which when disengaged from said seat provides a path between said vented area and the opposing area.

6. A pressure proportioning valve as claimed in claim 3, 4 or 5 a path to the vent being provided by a passage from the delivery port through said valve seat to a port in the housing.

7. A pressure proportioning valve as claimed in claim 4, 5 or 6 a path to the vent being provided from the delivery port via the passage through the valve member and a port in the housing.

8. A pressure proportioning valve substantially as described herein with reference to Fig. 2 (and Fig. 3) of the accompanying drawings.