GB2133097A - Pneumatic brake valve - Google Patents

Abstract

A control valve of the kind in which an input effort A is applied via a graduating spring 16 to a reaction piston 10 controlling one or more inlet/exhaust valve assemblies 12/14/40 delivering a maximum pressure lower than system pressure, has abutment means or the like by which the input effort is transmitted to the reaction piston when the load in the graduating spring exceeds the effort normally required to achieve the maximum pressure. Such "push-through" which enables the input effort to be increased sufficiently to free a jammed reaction piston, occurs when the guide piston 17 engages the spring seat 54 (Fig. 1), or interposed shims 52 (Fig. 3) in the dual concentric brake valve described, when sufficient input travel remains available on push-through before the guide piston 17 engages shim 50 on the casing top to operate the inlet/exhaust valve assembly so that the desired maximum delivery pressure can be achieved. <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 the full 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 maximum delivery 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 failure with disastrous results.

With the aim of avoiding this problem, we propose in accordance with the present invention, 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 a predetermined value at least equal of the effort required in normal operation to achieve the said maximum 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 an actuating member 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 delivery pressure is not exceeded.

To ensure that the inlet valves are never fully open to deliver full supply pressure the "pushthrough" effect according to the present invention preferably becomes 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).

One embodiment of the present invention will now be described by way of example with reference to the accompanying drawings of which: Figure 1 is a cross-section of a known control valve of the kind referred to, more particularly a dual concentric brake valve; Figure 2 shows to an enlarged scale a detail of the valve shown in Figure 1; and Figure 3 is a view similar to Figure 2 but of a dual concentric brake valve according to the present invention.

The valve shown in Figure 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 14 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 7 through which the thrust of the foot pedal is transmitted to the carrier piston 10.The space 18 above the annular seat 12 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-axialiy 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 1 2, 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 12 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 0.01 0 ins. which provides a restriction (to the flow of fluid through the exhaust passage) approximately equivalent to a 3 mm 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 blance 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 dpen, 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 for 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 10 to which the pedal effort is transmitted via the graduating spring 16, travels downwardly until the inlet/exhaust valve assemblies and establish a lap condition to delivery to the brake chambers, via ports 24 and 28, a pressure appropriate to the demand (i.e. pedal effort).

The point of maximum travel determined by pedal stop 50 is preset by inserting shims or otherwise adjusting the position of the pedal stop, such that at maximum travel (as shown in Figure 2) the valve delivers a maximum limited pressure (say 8 bars) below the system pressure (say, 11 bars).

Should the valve carrier piston 10 stick due to dirt, swarf or push rod misalignment, it will not travel downwardly in response to pedal effort with the result that the inlet/exhaust valve lap position is not reached, in fact, neither of the inlet/exhaust valve assemblies are operated with the result that the brakes cannot be applied.

In the valve according to the present invention shown in Figure 3, abutment means are provided such that, should the valve carrier piston 10 stick, a maximum input effort of up to 2500 Ibf, can be applied in order to free or move the valve carrier piston. In the illustrated embodiment shims 52 are introduced between the spring guide 54 on the valve carrier piston 10 and the spring guide piston 1 7 such that push-through between the spring guide piston 1 7 and the valve carrier piston 10 occurs at an input effort of say, 500 Ibf, slightly in excess of the effort required to produce, in normal operation, the desired output pressure.

At the point "push-through" occurs, the spring guide piston 1 7 is therefore spaced from the pedal stop by a distance equivalent to the travel required in order to close off communication between the delivery ports 24 and 28 and the exhaust i.e. to achieve the inlet/exhaust lap condition, so that the desired maximum delivery pressure can be achieved.

If "push-through" contact was made prior to this position (i.e. at an input effort of less than (280 Ibf) that required to achieve maximum delivery pressure in normal operation) the travel available on "push-through" would exceed that necessary to close off the exhaust with the result that the inlet valve would open to deliver full system pressure.

"Push-through" contact is only made if the valve carrier piston 10 sticks at some point in its travel to the inlet/exhaust lap condition, and only if the graduating spring 1 6 at that point, is unable to move the piston.

When the piston 10 is freed, the valve will assume the position shown in Figure 2 and delivery the maximum limited pressure (of say 8 bars) in the same manner as a conventional valve.

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 pressure lower than system pressure and wherein the input effort is transmitted directly to 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.

2. A control valve according to claim 1 wherein the graduating spring is disposed between the reaction piston and an actuating member to which the input effort is applied, and comprising abutment means operable to achieve pushthrough between the actuating member and the reaction piston when the load in the graduating spring exceeds the said predetermined value.

3. A control valve according to claim 2 wherein the abutment means is adjustable.

4. A control valve according to claim 2 or claim 3 wherein the abutment means comprises a shim or shims interposed between the actuating member and the reaction piston.

5. A control valve according to any one of preceding claims 2 to 4 and comprising a stop for the actuating member determining the said maximum delivery pressure.

6. A control 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 control valve according to any one of the preceding claims 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 Figure 3 of the accompanying drawings.