GB2053397A - Tractor/trailer braking systems - Google Patents

Abstract

A foot brake valve 11 controls a relay valve 14 (top of the figure) to pressurise a trailer brake connection S. A hand brake valve 12, for tractor spring-applied brakes, also applies an upright pressure for the same valve 14, so that the trailer service brakes are used for parking. However, there is a dump valve 16 in the trailer emergency brake connection E; this valve opens when there is pressure in the connection 17 (connected via relay valve 18 to hand brake valve 12) and no pressure in the connection S. The hand brake pressure is applied to the dump valve 16 either by an external loop-back between tractor connections 17, SD, or by a internal connection 'x'. <IMAGE>

Description

SPECIFICATION Fluid pressure-operable vehicle braking systems This invention relates to fluid pressure operable vehicle braking systems and relates particularly to tractor vehicle braking systems which provide pressures to brakes on the tractor and also provide service and emergency brakes on a trailer when connected to the tractor vehicle.

Tractor vehicle braking systems have been proposed in which a foot valve is provided which is usually a dual circuit valve providing service brake pressures on the tractor vehicle and controlling the application of service brake control pressure via a service line to a trailer when connected to the tractor. Also usually provided there is an emergency line which is normally pressurised to maintain an emergency brake system on the trai!er vehicle in a fully released condition, it being understood that a collapse of emergency line pressure is required to cause very rapid emergency braking on the trailer.

Such a tractor vehicle braking system is usually also provided with a secondary brake control valve which may also operate to control parking brakes on the tractor and/or trailer. In one widely acceptable form of system, the trailer brakes are controlled by means of two lines, namely, a service line controlling the trailer service and secondary braking and an emergency line which has the aforementioned function.

In a further widely accepted system, a third trailer line is provided on the tractor for connection to trailer secondary brakes so that the trailer braking can be controlled by the further tractor driver's brake valve independently of the service line. In the latter case, on failure of the service brakes, the vehicle driver is easily able to operate the secondary brake control valve and can readily apply brakes on the tractor and trailer together.

However, in the case of a two-line braking system, it will be appreciated that, in the event of failure of the service line, the only trailer braking which is available is that which becomes effective in the event of venting of the emergency line. In view of this, it has been proposed to include in the emergency line for a tractor vehicle, a so-called Supply Dump Valve which is responsive to a noncorrespondence between a pressure signal in the service line and a pressure derived from the driver's foot valve which controls the relay valve supplying the service line. In such a case, a fracture of the service line results in loss of pressure when the next service application occurs and this causes the emergency line to be rapidly vented to atmosphere to initiate emergency braking on the trailer.

Having regard to the desirability for towing vehicles and trailers to be universally interchangeable as far as possible, it is considered desirable that a two-line tractor may be connectable to a three-line trailer and vice versa.

As far as the tractor vehicle is concerned, it may readily be appreciated that a third (or secondary) control line may be provided on a tractor vehicle by introducing a relay valve which is responsive to an output pressure of the secondary control valve to provide an appropriate secondary braking control pressure on the third line. In such a case, it may be assumed that if double diaphragm actuators are employed on the trailer, normal operation with three lines does not cause over braking due to the non-compounding effect of such actuators.

A problem arises however in such a system to the extent that, as mentioned above, a supply dump valve is provided to ensure emergency braking in the event of service line failure for trailer operation with two lines. In the event of such a service line failure, proper operation of the secondary brake cannot be effected owing to predominant emergency braking on the trailer.

According to the present invention there is provided a tractor vehicle fluid pressure operable braking system including a first driver-operable value to apply fluid pressure for controlling tractor vehicle service braking and for controlling further valve means to apply fluid pressure to a trailer service line for operating brakes of a trailer vehicle when connected, a trailer emergency line to be pressurised to hold-off trailer emergency brakes when connected, a second driver-operable valve operable to apply fluid pressure for controlling tractor secondary braking and for controlling said further valve means to apply fluid pressure to said trailer service line and additional valve means responsive to a pressure signal from the second driver-operable valve in the absence of a corresponding pressure signal in the service line to vent the emergency line.

By virtue of the invention, it is possible to include a third trailer line, the pressure to which is controlled by said second driver-operable valve, said third trailer line being operable to control secondary braking on a three-line trailer when connected.

In accordance with a particular embodiment of the invention, said additional valve means may be provided with one control port-connected to the trailer service line of the system and a further port which is substantially ineffective for three-line operation but is connected to the secondary (or third) trailer line for two-line operation.

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 which illustrates a two-line braking system with modification rendering it adaptable for use with a three-line trailer.

The system is provided with three reservoirs, 1, 2 and 3, supplied by respective protection valves 4, 5 and 6 from a compressor 7. A suitable compressor unloader valve 8, a wet tank 9 with drain valve 10 and alcohol injector 1 9 are shown.

The reservoir 1 provides a source of fluid pressure for one side of a first driver-operable dual foot valve 11, the other side of the foot valve being supplied from the reservoir 2. The reservoir 3 supplies fluid pressure for the secondary and parking functions of the vehicle in that it supplies a second driveroperable spring brake control valve denoted by reference 12.The latter valve has an inverse output on the line 13 for controlling secondary spring brakes of the tractor via a differential protection valve 14 for preventing compounding of the secondary braking with service braking produced by the first valve 1 The valve 12 has a further upright output which forms one input to a further valve means comprising a multi-relay valve denoted by reference 14, the output of which feeds the trailer service line S. -A second input to the relay valve is derived from the other side of the dual foot valve 11. In addition to supplying one side of the dual foot valve 11, the reservoir 2 supplies a relay valve 1 5 controlled by the pressure in the third reservoir to normally provide an output pressure on the emergency line E via an additional valve means 16.The valve means 16 is a so-called supply dump valve means 16. The valve means 16 is a so-called supply dump valve and has one control input connected to the service line S and has a reference input connected to an additional coupler SD which is optionally connectabie via a loop-back (shown dotted) to a secondary line coupler 17. The secondary line coupler 1 7 is supplied with fluid pressure from a relay valve which is controlled by the output of the hand valve 12 and supplied by the reservoir 3.

Combined service and spring brake actuators are denoted by reference 20 and require no further discussion.

In operation of the system, operation by the driver of the foot valve 11 applies fluid pressure from the reservoir 1 to the service brakes of the tractor vehicle and from reservoir 2 to the multirelay valve 14 to provide fluid pressure on the service line S. At the same time, fluid pressure in the reservoir 3 enables the relay valve 1 5 to supply emergency line pressure to the emergency line of the tractor vehicle and in the event of a trailer being connected, the pressure in the service line S is able to control service braking of a trailer connected thereto. In the event of fracture of the emergency line, emergency braking on the trailer immediately occurs in well-known manner.On operation of the hand valve 12, a normal hold-off pressure on the line 13 is appropriately reduced in accordance with the position of the handle and spring brakes are applied on the tractor. At the same time, a corresponding upright fluid pressure is applied to the mutli-relay valve 14 thereby initiating braking on the trailer via the service line S. In the event of a failure in the service line, on subsequent operation of the secondary hand valve 12 by a driver realising that a loss of service braking has occurred, generates a reference pressure via the secondary coupler 17, the loopback, coupler SD to the supply dump valve 16 and the supply dump valve therefore senses the loss of pressure in the service line and initiates emergency braking on the trailer.

In the case of three-line operation, the service breaking operates as described above but in the event of a secondary braking operation by the driver, secondary braking on the three-line trailer is initiated via the third line 1 7. Since under these conditions, the loopback connector is not included, the supply dump valve does not receive a reference pressure and does not operate.

Accordingly, the emergency line operates purely as an emergency line and has no secondary braking function.

If desired, in a basic two-line tractor/trailer system in which items 17 and 18 would not be fitted, a connection may be provided as indicated at "x" between the control output of the second control valve 12 and reference line to the supply dump valve 12. In that case, in the event of the driver operating the secondary braking after realising that a fault such as a service line failure had occurred, the supply dump valve is supplied with a secondary control pressure as a reference signal and emergency braking is initiated on the trailer in response to operation of the secondary valve under such conditions.

It will be appreciated that the trailer braking system can be of well-known two-line or threeline type appropriate for use with either of the above fluid pressure operable tractor vehicle system proposals.

Claims (4)

1. A tractor vehicle fluid pressure-operable braking system for tractor/trailer vehicle operation, including a first driver-operable valve to apply fluid pressure for controlling tractor vehicle service braking and for controlling further valve means to apply fluid pressure to a trailer service line for operating brakes of a trailer vehicle when connected, a trailer emergency line to be pressurised to hold-off trailer vehicle emergency brakes when connected, a second driver-operable valve operable to apply fluid pressure for controlling tractor vehicle secondary braking and for controlling the further valve means to apply fluid pressure to said trailer service line and additional valve means responsive to a fluid pressure signal from the second driver-operable valve in the absence of a corresponding fluid pressure signal in the trailer service line to vent the emergency line.

2. A tractor vehicle fluid press-operable braking system as claimed in Claim 1, wherein the output of the second driver-operable valve is connected to a relay valve for supplying a trailer secondary line, the system including means for connecting the output of the relay valve to the additional valve means to provide said pressure signal from the second driver-operable valve in the case of twoline operation.

3. A tractor vehicle fluid pressure-operable braking system as claimed in Claim 1, wherein the output of the second driver-operable valve is connected to the additional valve means to provide the fluid pressure signal from the second driver-operable valve.

4. A tractor vehicle fluid pressure-operable braking system, substantially as described herein, with reference to the accompanying drawing with or without the optional connection "x".