In the braking systems of articulated vehicles, there are typically two airlines which supply air under high pressure from the tractor unit to the trailer: the "yellow line"or"service line"and the"red line"or "emergency line".

Air is supplied along the yellow line only when a brake pedal in the tractor unit is depressed by the driver.

This air is supplied to air chambers in the trailer, one air chamber typically being associated with each wheel or set of wheels of the trailer. Increased air pressure in an air chamber causes a cam brake to brake the corresponding wheel. The brakes are then released when air supply along the yellow line is terminated, when the brake pedal is released. This will be referred to herein as the"service braking system".

In addition to the air chambers, the trailer has an air tank, which normally contains air under high pressure.

Air may be released from this tank via a valve, to the air chambers, in order to activate the trailer brakes independently of the yellow line. In normal operation, the red line supplies air under high pressure to the valve in the air tank, keeping the valve closed, so that the trailer brakes are not activated by release of air from the tank.

If the supply of air along the red line is interrupted and pressure in the red line falls, the valve in the air tank is no longer kept closed and air is supplied from the tank to the air chambers to activate the trailer brakes. In particular, air supply along the red line is interrupted when a connector in the red line between the tractor unit and trailer is disconnected when the tractor unit and trailer are uncoupled, with the result that the trailer brakes are automatically applied when the trailer is loose from the tractor unit.

When the air supply along the red line is restored, for example when the connector between the trailer and tractor unit is reconnected, the valve in the air tank is closed and an exhaust valve is opened, thus allowing air to escape from the air chambers and the trailer brakes to be released, so that the system returns to its pre-interruption state. This will be referred to herein as the"emergency braking system".

A problem with such an emergency braking system, however, is that the combination of trailer and tractor unit can become free-wheeling if the handbrake of a tractor unit has not been applied before reconnecting the connector in the red line between trailer and tractor unit on coupling the trailer and tractor unit.

Because the trailer is braked prior to reconnection by the emergency braking system, it is a simple error for a driver to move the tractor unit into position relative to the trailer and then rely on the trailer

brakes to keep both the tractor unit and trailer stationary during the coupling operation, forgetting that during this operation the emergency braking system will be deactivated, releasing the trailer brakes.

Many costly and some fatal accidents have occurred because of this simple human error.

It is an aim of the present invention to address this problem.

Accordingly, the present invention provides a valve for inclusion in an emergency air supply line to the trailer braking system of an articulated vehicle, the valve having: a first air inlet, connectable to an upstream side of the emergency air supply line; a first air outlet, connectable to a downstream side of the emergency air supply line; a closing member, biased towards a closed position in which passage of air through the valve between the first air inlet and first air outlet is substantially prevented; a second air inlet, connectable to a service air supply line of the trailer braking system; and an actuator, operable in use by passage of air into the valve via the second air inlet to urge the closing member towards an open position, in which passage of air between the first air inlet and first air outlet is permitted, in which position the closing member may then in use be held against the bias by air pressure in the valve between the first inlet and the

first outlet.

Hereinafter, for convenience only, the first air inlet may also be referred to as the"red inlet", the first air outlet as the"red outlet", and the second air inlet as the"yellow inlet".

When such a valve is included in the emergency air supply line to a trailer braking system, the emergency braking system is not deactivated automatically when the emergency line is connected, by virtue of the closing member being biased towards a closed position which prevents passage of air through the valve.

Rather, the emergency braking system is deactivated only when air passes into the valve via the yellow inlet from the service air supply line. This occurs when the brake pedal of the tractor unit is depressed, which of course applies the trailer brakes via the service braking system, with the effect that the trailer is still braked when the emergency braking system is released. Thus the trailer cannot become free-wheeling when the emergency line is connected.

Thereafter, the closing member is retained in an open position by air pressure in the emergency line, to keep the emergency braking system in a deactivated state until the line is disconnected again.

The valve may be included at any position along the emergency and service lines, either on the tractor unit side or the trailer side. For convenience of retro-

fitting to existing vehicles, the valve can be included near the point at which the tractor unit and trailer sides of the line are connected when the tractor unit and trailer are coupled.

Preferably, the closing member includes a first piston movable in a first chamber of the valve, the first chamber lying between the red inlet and red outlet.

More preferably, the red inlet and red outlet are spaced apart in the direction of movement of the piston. When the piston is in its closed position, this feature allows the red inlet and red outlet to be isolated by a seal between the piston and a wall of the chamber.

Preferably the closing member is spring biased towards the closed position, more preferably by a compression spring.

In a preferred embodiment, the valve also includes a second outlet, connectable to a downstream side of the service air supply line (hereinafter referred to as the "yellow outlet"). This feature allows passage of air along that line through the valve. Alternatively, the valve could be connected to a dead-end branch from the service air supply line, but this would entail more complex connection to the service line.

The actuator preferably includes a second piston, slidable in a second chamber of the valve, the yellow inlet (and in the preferred embodiment also the yellow

outlet) being in communication with the second chamber.

When the valve is included in the service air supply line (rather than being connected to a branch from it), the valve desirably includes means for preventing the second piston from restricting air flow between the yellow inlet and yellow outlet. This is so that the valve does not impede normal operation of the service brake system. Such means may include a stop in the chamber, to prevent the piston from blocking air passage between the inlet and outlet, or a through-hole in the piston, for passage of air between the yellow inlet and outlet.

Desirably, movement of the actuator is imparted to the closing member via a link. More desirably, the piston and actuator are in line, movement being imparted by the link along that line. Alternatively, however, in some embodiments the link could be pivoted. It is also possible that movement of the actuator could be imparted to the closing member hydraulically.

There may be a conduit by which air could potentially leak between on the one hand the red inlet and red outlet (the"red side") and on the other hand the yellow inlet and, if present, yellow outlet (the "yellow side"), e. g. in some embodiments in which a link is provided to impart movement from the actuator to the closing member. In such a case, the valve preferably includes a seal between the red and yellow sides, in order to reduce or prevent leakage of air between the service and emergency air supply lines.

In a further preferred embodiment, the valve includes a vent, located between the red and yellow sides and isolated by seals from the red and yellow inlets and outlet (s). In the event of leakage of air in the valve from the red side towards the yellow side, where the valve is in use connected to a service air supply line, such a vent allows the leaking air to escape from the valve. Otherwise, as will be appreciated, such leakage could (potentially hazardously) activate the service braking system without the driver having depressed the brake pedal.

The valve may include an override, operable (desirably manually) to move the closing member into the open position in case of failure of the actuator to do so.

Such an override may be lockable, to keep the closing member in the open position. Such a feature allows the valve to be completely disabled, and the emergency braking system will then function as if no valve were present.

The valve may include a bypass which, when the closing member is in its closed position, allows air to escape from the trailer side (i. e. downstream side) of an emergency air supply line connected to the valve. Such a bypass eliminates the possibility that by moving very rapidly to its closed position upon initial reduction of pressure in the red line, the closing member may prevent sufficient reduction in pressure on the trailer side of the valve to activate the emergency braking system.

A bypass may be provided by configuring the valve such that when the closing member is in the closed position, the red outlet is in communication with an exhaust outlet. In such an embodiment, the closing member must of course prevent communication between either the red inlet or the red outlet and the exhaust outlet, when it is in the closed position.

From a second aspect, the present invention provides a system for safely deactivating an emergency braking system of a trailer an articulated vehicle, the system including: an emergency air supply line, in which is fitted a valve, biased towards a closed position in which air passage through the emergency air supply line is prevented; and release means, operable from a tractor unit of the articulated vehicle, for urging the closing member towards an open position in which passage of air through the emergency air supply line is permitted, thereby to release the emergency braking system of the trailer, in which open position the closing member may in use then be held against the bias by air pressure in the emergency air supply line.

Preferably the valve is as defined in the first aspect of the invention, the release means being provided by the service air supply line, the yellow inlet of the valve and the actuator. However, other embodiments may for example include mechanical linkages between the tractor unit and the valve or electronically operable

actuators.

All these embodiments share the advantage that the emergency trailer braking system is not deactivated until the driver is in the tractor unit, so any free- wheeling of the vehicle can rapidly be stopped. The valve according to the first aspect has the added advantage that the service braking system is activated when the emergency braking system is deactivated. Not only is this simple for the driver, but free-wheeling may be entirely avoided. There is the further advantage of the valve that it may very conveniently be fitted to the air supply lines of existing tractor units and/or trailers.

In further aspects, the present invention provides tractor units and/or trailers fitted with valves or systems according to the first two aspects.

Embodiments of the present invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows in cross section a valve embodying the first aspect of the invention, with a closing member (here a piston) in a closed position; Figure 2 shows the valve of figure 1 when air is supplied to the valve from a service air supply line, with the closing member consequently in an open position (an override shown in figure 1 being omitted);

and Figure 3 shows the valve of figure 2 when air supply from the service air supply line has ceased, but the closing member remains in an open position due to air pressure in an emergency air supply line.

A valve 1 embodying the first aspect of the present invention includes first and second chambers 2,4, in which are slidable first and second pistons 6,8, respectively. The pistons are aligned relative to each other along their respective directions of movement in the chambers. The chambers communicate via a narrow through-hole 10 in the valve, an axis of the through- hole also being aligned along the direction of movement of the pistons. A rod 12 is located in the through- hole 10 and may be used to transmit movement of the second piston 8 to the first piston 6, as will be described in more detail below.

In the following description, the valve is connected as described below to the red (emergency) and yellow (service) lines of an articulated vehicle. The function and operation of these lines is described in the introduction. However, in its broadest aspects, the invention is not to be construed as being limited to the valve thus connected.

The first chamber 2 has an inlet 14 and an outlet 16, which may be connected to the red line. The inlet 14 and outlet 16 will therefore be referred to as the

"red"inlet and outlet. The valve includes connectors (not shown) adapted for airtight connection of the red inlet and outlet to corresponding connectors on the red line. Suitable connectors may for example include male and female threaded sleeves. The valve may be fitted on a trailer of an articulated vehicle.

The first piston 6 is biased by a helical compression spring 18 to a closed position, shown in figure 1, in which passage of air through the red inlet 14, first chamber 2 and red outlet 16 of the valve is prevented, thereby preventing passage of air along the red line.

A seal 20 between the first piston 6 and a cooperating side wall 3 of the chamber 2 substantially eliminates leakage between the inlet and outlet when the piston is in the closed position.

The second chamber 4 has an inlet 22 and an outlet 24, which may be connected to the yellow line. The inlet 22 and outlet 24 will therefore be referred to as the "yellow"inlet and outlet. Again, the valve preferably includes connectors adapted for airtight connection of the yellow inlet and outlet to corresponding connectors on the yellow line.

The second chamber 4 includes a stop 26 which prevents the second piston 8 from blocking air passage through the chamber between the yellow inlet and outlet. As shown, the stop 26 includes a projection from an end wall into the bore of the chamber, leaving an annular space 28 in the chamber around the projection. The

yellow inlet and outlet communicate with the annular space.

Seals 30 between the second piston 8 and a cooperating side wall 5 of chamber 4 substantially eliminate leakage of air past the second piston 8. Under pressure in the yellow line (as occurs when a driver of the articulated vehicle depresses the brake pedal in the tractor unit), the second piston 8 is therefore pushed away from the stop 26 towards the other end of chamber 4.

A rod 12 is longitudinally aligned with the direction of movement of the pistons 6,8 in the chambers 2,4, such that movement of the second piston 8 in the second chamber 4 away from stop 26 imparts movement to the first piston 6 in the first chamber 2, against the bias of the spring 18, towards an open position in which passage of air between the red inlet and outlet is permitted. Figure 2 illustrates the state when the second piston 8 has been moved away from stop 26 by pressure in the yellow line, thereby moving the first piston 6 to an open position.

The rod 12 is located in the narrow through-hole 10, by which the two chambers communicate. However, such communication is merely a by-product of the means by which movement is imparted by piston 8 to piston 6, and is not a desired feature in itself. Rather, leakage of air between the chambers is to be avoided, in order that air from the red line (which is normally

maintained at high pressure) does not leak into the yellow line, in which pressure is modulated to activate and deactivate the service braking system, thereby applying and releasing the trailer brakes. Seals 32 between the rod 12 and a cooperating side wall of the through-hole 10 substantially prevent such leakage.

Once the first piston 6 has been moved to the open position in the first chamber 2, and air passage between the red inlet and red outlet is permitted, high air pressure in the red line maintains the piston in the open position, against the bias of spring 18, until such time as the air pressure in the red line drops, whereupon the spring moves the first piston 6 back to the closed position, as shown in figure 1. Note that the maintenance of the first piston 6 in the open position is not dependent on high pressure in the yellow line. As shown in figure 3, pressure in the yellow line may drop (e. g. when the driver releases the brake pedal), and the second piston 8 may return to a position abutting projection 26, without the first piston 6 returning to the closed position.

Because the valve prevents air passage between the red inlet and outlet when the first piston 6 is in its closed position, reconnection of the red line following disconnection does not immediately result in air being supplied from the tractor unit to an air tank in the emergency braking system of the trailer (see introduction above) to deactivate the emergency braking system. Rather, deactivation occurs only when the

first piston 6 moves to its open position, which as described above depends on air being supplied along the yellow line.

As will be evident from this description, the rod 12 may be attached to either or neither of the pistons, or both. Furthermore, the valve may arranged in any orientation, though the strength of spring 18 will have to be chosen according to whether or not it biases piston 6 towards the closed position with the benefit of gravity. Preferably the valve is orientated as shown in the figures, with the first chamber 2 generally above the second chamber 4.

As shown in the figures, the chamber 2 also has a projection 34 from one end, around which also is an annular space 36. The red outlet 16, but not the red inlet 14, communicates with this annular space. The red inlet is sufficiently distant from the annular space, along the direction of movement of the piston 6 in the first chamber 2, that the seal 20 isolates the red inlet and red outlet when the piston is in the closed position, in order to reduce leakage of air along the red line. Excessive such leakage could prevent proper function of the valve and allow premature deactivation of the emergency braking system.

In an alternative embodiment (not shown) the red inlet and an exhaust outlet communicate with the first chamber 2 near respective ends of the chamber, the red outlet communicating with the chamber near its middle.

When the piston is in a closed position, towards one end of the chamber, passage of air between the red outlet and exhaust outlet is permitted, to allow complete release of pressure on the trailer side of the red line, but passage of air between the red inlet and red outlet is prevented, as with the above embodiment.

When the piston is in an open position, near the other end of the chamber, the converse applies, i. e. passage of air between the red inlet and red outlet is permitted, for supply of air to the emergency braking system, but passage of air between the red outlet and exhaust outlet is not permitted, to prevent leakage of air from the red line.

In the second chamber 4, near where the narrow through- hole 10 opens into the chamber, is a vent 40 which communicates with an annular space 38 in the chamber wall 5. The vent is permanently isolated from the yellow inlet 22 and outlet 24 by the second piston 8 and seals 30, so as to prevent loss of pressure from the yellow line (which could of course hinder operation of the service braking system). The vent 40 allows any air which may leak from the first chamber 2 via the narrow through-hole 10 into the second chamber 4 (despite seals 32) to be vented outside the valve rather than to leak into the yellow line, where it may activate the service braking system unwantedly.

The valve is fitted with a handle 42, shown only in figure 1, which is connected to the first piston 6 and extends outside the valve. The piston may be manually

moved by means of the handle to the open position, to disable the valve. A lug 44 on a shaft 46 of the handle may engage a hook 48 in the chamber 2 to retain the piston in the open position. Other restraining means than the lug and hook will be evident to the skilled person.