GB2131109A - Driver's brake valve - Google Patents

Abstract

Pedal input effort A is applied via a graduating spring 16 to a reaction piston 10 controlling one or more inlet/exhaust valve assemblies (not shown) delivering a maximum pressure lower than system pressure. Pedal stop 50 determines the maximum pressure but is spring loaded 64 to permit displacement beyond the normal stop position so enabling the brakes to be applied in an emergency, such as may occur in the event of a graduating spring failure, which might otherwise prevent application of the brakes. <IMAGE>

Description

SPECIFICATION Improved control valve This invention relates to a control valve of the type in which an input effort is applied via a graduating spring to a reaction piston controlling one or more inlet/exhaust valve assemblies delivering a maximum pressure lower than system pressure.

In such valves, the inlet valve or valves never fully open to deliver maximum supply pressure so that the internal components of the control valve are subject only to the push rod loads equivalent to the product of the limited pressure and the reaction area.

With valves of this type, particularly those with small bearing lands, the reaction piston has been known to jam or stick due to dirt or swarf, or push rod misalignment. In brake control valves, for example, dual concentric control valves of the kind described in British Patent specification No. 1,466,296 and used in fluid pressure operated braking systems embodying two independent braking circuits, jamming of the reaction piston could cause full brake failure with disastrous results.

Our co-pending British Patent application No. 8233974 aims to avoid this problem and proposes a control valve of the type referred to in which the input effort is transmitted directly to the reaction piston, when the load in the graduating spring exceeds the effort required to achieve the said limited pressure, thereby enabling the input effort to be increased to a level sufficient to free the reaction piston.

This may be achieved by designing or adjusting the valve such that a spring guide piston to which the input effort is applied bears upon the reaction piston to obtain "push-through" when the load in the graduating spring exceeds a predetermined value.

Adjustment is possible by the insertion of shims, by screw adjustment or in any other convenient manner.

In dual concentric brake valves such as described in British Patent 1,466,296, initial travel of the reaction piston closes-off communication between the delivery port or ports and the exhaust and further travel causes opening of the inlet valves. An inlet/exhaust valve lap condition is established in normal operation, at a delivery pressure appropriate to the demand (i.e. input pedal effort). Also, however, an abutment or stop for the spring guide piston is provided to ensure that the maximum limited delivery pressure is not exceeded.

It has been thought important to ensure that the inlet valves are never fully open to deliver full supply pressure since sudden braking could create uncontrollable skidding and other problems. To this end, the "pushthrough" effect proposed in our co-pending application should become effective before the guide piston reaches its stop but only by an amount sufficient to allow the travel necessary to reach the inlet/exhaust lap condition (i.e.

closure of the exhaust valves).

In the unlikely, though not inconceivable, event of a graduating spring failure however it will be understood that although the pushthrough effect would enable the reaction piston to close-off the exhaust and to reach the inlet/exhaust lap position, the inlet valve or valves would not open producing full brake failure.

In such an emergency, however, it may be better to risk skidding than to be unable to apply the brakes at all. Such a situation could occur, for example, in the event of failure of the graduating spring.

With the object of enabling this we propose, in accordance with the present invention, a control valve of the kind referred to, more particularly though, not exclusively, of the kind described in our above-mentioned copending application, having a stop for the spring guide piston determining in normal operation the maximum delivery pressure, wherein the stop is displaceable or otherwise adapted to permit travel beyond the normal stop position upon application of an input effort in excess of that required in normal operation to achieve the said maximum pressure. The stop is preferably spring loaded. In a control valve according to the present invention, therefore, when an emergency occurs for example, a graduating spring failure, an increased input effort can be applied directly to the reaction piston permitting travel beyond the normal stop position.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings of which: Figure 1 shows in cross-section, a conventional dual concentric brake valve; and Figure 2 shows, to an enlarged scale, a detail of a dual concentric brake valve according to the present invention, but otherwise constructed and arranged as the valve shown in Fig. 1.

The valve shown in Fig. 1 is adapted to be actuated by a foot pedal (not shown) as indicated by the arrow A. Having a sealed sliding fit in the upper end of the housing 1 is a reaction or valve carrier piston 10, the lower end of which has an inwardly-directed flange formed on its upper side with an annular seat 12, engageable by the head of a rubber-faced poppet-type inlet valve element 14. The valve element 1 4 has a sliding fit on a sleeve 7 which is clamped within the valve carrier piston 10 for movement therewith, a graduating spring 1 6 being interposed between the carrier piston 10 and an actuating member in the form of a graduating spring guide piston 1 6 through which the thrust of the foot pedal is transmitted to the carrier piston 10.The space 1 8 above the annular seat 1 2 communicates with an inlet port 20 in the housing wall whilst the space 22 below said seat communicates with a delivery port 24 in said wall, these ports being associated with one of the braking circuits. At the lower end of the housing there is a second inlet valve assembly of similar construction controlling communication between an inlet port 26 and a delivery port 28 connected into the second braking circuit, the head of the valve element 30 in this instance engaging a seat 32 on the upper end of a seating structure 34 which is fixed preferably by forming it integral with the housing. The valve element 30 slides upon a tubular guide 36 which is secured within the lower part of the housing structure.

Located between the two valve assemblies is a co-axially disposed tubular structure 38 the ends of which constitute annular exhaust valve elements 40 and co-act respectively with the heads of inlet valves 14,30 on a smaller diameter than does the inlet valve seats 12,32. The exhaust valve structure is carried by a balancing piston 42 which is displaceable within a chamber 44 formed in the housing, the spaces at opposite sides of the piston communicating respectively with the two delivery ports 24,28.

The bores of the exhaust valve structure 38 of the lower inlet valve 30 together form an axial exhaust passage 46 in permanent communication at its lower end with atmosphere via an exhaust elbow 48.

Normally the heads of the inlet valves 14, 30 engage their seats 12, 32 but are spaced from the exhaust valve elements 40 whereby the inlet ports 20,26 are sealed off but the delivery ports 24,28 are in communication with the axial exhaust passage. Depression of the foot pedal graduating spring guide piston 1 7 acting through the graduating spring 1 6 displaces the carrier piston 10 and seating structure taking with it the upper inlet valve 14 which is spring loaded on to its seat 1 2.

This inlet valve 14 then engages the tubular exhaust structure and displaces it into engagement with the lower inlet valve 30. The tubular exhaust structure 38 is disposed between adjacent ends of the valves 14 and 30 and in operation acts as a thrust member transmitting force therebetween. Thus the axial exhaust passage is shut off from both delivery ports and upon continued depression of the foot pedal the seat 1 2 at the upper end of the housing moves away from the associated inlet valve 14 whilst at the lower end of the housing, the inlet valve 30 is moved off its co-acting seat 32 by the displaced exhaust valve structure whereby both inlet ports are then connected to their related delivery ports and air under pressure is conducted to the brakes.

In operation of the valve, an extension restrictor 48 attached to the graduating spring guide piston 1 7 by screw thread means 49 enters the balance piston 42 exhaust passage 46. Typically, the clearance between the external diameter of the extension restrictor 48 and the bore of the exhaust passage 46, is of the order of 0.010 ins. which provides a restriction (to the flow of fluid through the exhaust passage) approximately equivalent to a 3mm diameter orifice. Further movement downwards will simultaneously open the primary and secondary inlet valves 14 and 36 and pressurise the output ports. As this travel increases the exhaust restrictor 48 penetrates deeper into the balance piston exhaust passage 46, so that by virtue of flow losses the resistance to exhaust flow upon release of the brakes increases with greater penetration.

This resistance to exhaust flow from delivery port 24 increases the pressure above the balancing piston 42 so imposing an equivalent restriction on the exhaust flow from delivery port 28.

On partial removal of the input load the inlet valves will seat and the exhaust valves will open, releasing the required amount of air. However, the time taken to obtain release now depends on the release orifice available, and how far the restrictor has penetrated the exhaust passageway 46. The size and shape of the restrictor plunger may be varied (to adjust the clearance and depth of penetration) to obtain a required release characteristic. It should be noted that as the output pressure is increased the degree of restriction (i.e. resistance to flow) upon exhaust is correspondingly increased.

The restriction creates a lag in the response of the vehicle brakes to a reduction in pedal load so that oscillatory feedback arising fro example, due to "Cab nod" in large commercial vehicles is largely prevented. There is no restriction to exhaust flow when the brake pedal is fully released.

As described above, when the brake pedal is depressed, the reaction or valve carrier piston 12 to which the pedal effort is transmitted via the graduating spring 14, travels downwardly until the inlet/exhaust valve assemblies 16 and 1 8 establish a lap condition to deliver to the brake chambers, via outlet ports 20 and 22, a pressure appropriate to the demand (i.e. pedal effort).

The point of maximum travel in normal operation is determined by pedal stop 50 and is preset by inserting shims or otherwise adjusting the position of the pedal stop, such that at maximum travel the valve delivers a maximum pressure (say, 8 bars) below the system pressure (say, 11 bars).

The valve is so designed that in the absence of pedal effort (and in normal operation) the distance between the lower end of the spring guide piston 1 7 and the spring carrier 54 on the reaction piston 10 is slightly greater than the sum of the graduating spring travel and twice the exhaust travel. This distance may be adjusted as proposed in our above-mentioned British Patent application No. 8233974 and as shown in Fig. 2 by the insertion of shims 52 or in any other convenient manner, such that the said distance is substantially equal to the said sum providing "push-through" to the inlet/exhaust lap position at maximum pressure in the event the reaction piston sticks or jams.

Should the graduating spring 1 6 fail, it will be understood that the spring guide piston 1 7 can be moved down to the normal pedal stop position so that in both of the cases described above (i.e. the valves shown in Figs. 1 and 2), the exhaust valves can be closed. Without the graduating spring however it is not possible for the inlet valves to open even where shims are inserted as shown in Fig. 2 to move the reaction piston 10 into the inlet/exhaust lap position for maximum limited delivery pressure.

To guard against complete brake failure in the event of graduating spring failure, the pedal stop 50 is spring loaded (as shown in Fig. 2) by a series of disc springs 64 carried on a shoulder 66 in the housing and affording further travel of the spring guide piston 1 7 (i.e. travel beyond the normal pedal stop position), by an amount sufficient to open the inlet valves. The springs 54 are such that the inlet effort required to achieve further travel is considerably in excess preferably by 50% or more of the maximum input effort in normal operation (say 500 Ibf). Typically, the input effort required will be in the region of 800 Ibf increasing to 1 200 Ibf at maximum further travel.

In Fig. 2, push-through contact occurs when the spring guide piston 1 7 is spaced from the pedal stop 50 by an amount equal to the exhaust travel, so that the pedal stop travel is required is equal to the exhaust travel plus the travel required to open the inlet valves. If shims 52 are not fitted, the pedal stop travel required is greater by an amount equal (in a valve such as shown in Fig. 1) to the distance between the lower end of the spring guide piston 1 7 and the spring carrier 54 on the reaction or valve carrier piston 10 when, in normal operation, the spring guide piston 1 7 abuts the pedal stop.

Claims (8)

1. A control valve in which an input effort is applied via a graduating spring to a reaction piston controlling one or more inlet/exhaust valve assemblies delivering a maximum pres sure lower than system pressure and compris ing an actuating member to which the input effort is applied, and a stop for the actuating member determining in normal operation the maximum delivery pressure, wherein the stop is displaceable or otherwise adapted to permit travel beyond the normal stop position upon application of an input effort in excess of that required in normal operation to achieve the said maximum pressure.

2. A valve according to claim 1 wherein the stop is spring loaded.

3. A valve according to claim 2 wherein the stop comprises a series of disc springs carried on a shoulder in the valve housing.

4. A valve according to any one of claims 1 to 3 wherein the stop is displaceable by an amount sufficient to operate the or each inlet/ exhaust valve assembly.

5. A valve according to any one of claims 1 to 4 and comprising abutment means operable to achieve push-through between the actuating member and the reaction piston when the load in the graduating spring exceeds a predetermined value at least equal to the effort required in normal operation to achieve the said maximum pressure.

6. A valve according to claim 5 wherein the abutment means are operable to achieve push-through when the actuating member is spaced from the stop by a distance equal to the reaction piston travel necessary to operate the inlet/exhaust valve assembly.

7. A valve according to any one of claims 1 to 6 wherein the valve is a dual concentric brake valve for controlling the fluid pressure in a primary and a secondary braking circuit, the valve having an inlet/exhaust valve assembly for each circuit each assembly being operable in response to movement of the actuating member to admit fluid under pressure to an outlet port for applying the brakes and to release fluid under pressure from the brake circuit to exhaust, and having a balancing piston responsive to differential pressure between the two circuits to match the pressure in the two circuits.

8. A control valve constructed and arranged substantially as hereinbefore described and as illustrated in Fig. 2 of the accompanying drawings.