GB2084271A - Tractors - Google Patents

Abstract

A tractor, particularly a terminal tractor or dock truck has a rigidly mounted rear axle and a fifth-wheel plate 6 engageable with a semi-trailer by lifting. The braking pressure applied to the rear axle by a pneumatic braking system of the tractor is regulated in accordance with the load carried by the fifth-wheel plate 6. The load is sensed by a load sensing hydraulic cylinder 30 which is responsive to the pressure prevailing in hydraulic lifting cylinders 16 associated with the fifth-wheel plate, the load sensing cylinder being linked to a pressure regulating valve 58 in the braking system. <IMAGE>

Description

SPECIFICATION Tractors The present invention relates to tractors for use with a fifth-wheel trailer and more particularly to a terminal tractor for manoeuvring a trailer in a restricted area such as in the hold of a ship.

Tractors in the form of terminal tractors or so called dock trucks for use with fifth-wheel trailers usually comprise a fifth-wheel plate located in the -zone of the rear axle of the tractor, this axle being rigidly mounted with respect to the chassis of the tractor.

Conventional road-going vehicles such as lorries which have pneumatic braking systems frequently incorporate means for automatically controlling the pressure applied to the brake actuators according to the load being carried whereby to prevent excessive pressure being applied and therefore brake lock from occurring when the lorry is not loaded or only partly loaded.

One such system comprises a valve which is responsive to the deflection of the suspension system of an axle of the lorry and thus to the load acting on that axle, and operates progressively to regulate the air pressure applied to the brake actuators of that axle, between a minimum which occurs when the lorry is unloaded and a maximum which occurs when the lorry is fully laden. Such a system is not, however, directly suitable for use with a dock truck or similar tractor in which the load-carrying chassis is rigidly mounted on the axle.

According to the invention, there is provided a tractor including a fifth-wheel means for coupling to a trailer, lifting means for lifting the fifth-wheel means, means for sensing the load imposed on the fifth-wheel means by the trailer, and means responsive to the load sensing means for regulating the braking force which can be applied by a braking system of the tractor such that the braking force increases with increasing load.

Further according to the invention, there is provided a tractor having fifth-wheel means for coupling to a trailer, fluid actuated means for raising the fifth-wheel means, means for sensing the load imposed on the fifth-wheel means by the trailer by sensing the pressure prevailing in the fluid actuated means, and means responsive to the pressure-sensing means for controlling a pressure regulating valve in a braking system of the tractor such that the braking pressure which can be applied increases with increasing load.

Still further according to the invention, there is provided a pressure-sensing cylinder for use as the pressure sensing means in the above-defined tractor, said cylinder comprising a piston rod movable against a resilient bias in response to fluid pressure applied to the cylinder whereby the position of the piston rod is dependent on the applied fluid pressure.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a side elevation of a tractor in the form of a docile truck; * Figure 2 is a plan view of the truck; Figure 3 is a part rear elevation of the truck; Figure 4 shows, schematically, the braking system of the truck and the manner in which the braking system cooperates with hydraulic lift cylinders of a fifth-wheel plate of the truck: Figure 5 is an axial section of a hydraulic loadsensing cylinder: and Figure 6 is a side elevation of the load-sensing cylinder.

With reference to Figure 1, the dock truck comprises a front chassis portion 2 which carries the engine and the driver's cab. The front chassis portion 2 is supported on a front axle by a suitable suspension system such as a leaf spring suspension. A rear chassis portion 4 is articulated to the front chassis portion 2 and is rigidly supported on a rear axle. The rear chassis portion carries over the rear axle, a fifth-wheel plate 6 as will be described in greater detail hereinafter.

The front and rear chassis sections are articulated about a vertical axis 8 and are connected by hydraulic rams 10 so as to swing the rear chassis section about the axis 8 and thereby to steer the truck. This arrangement permits a very tight turning circle to be achieved.

The rear chassis section 4 can also pivot about a horizontal axis 1 2 relative to the front chassis section 2 as indicated in Figure 3 and this allows all of the wheels of the truck to maintain contact with the underlying surface under most conditions likely to be encountered in operation without the need for a suspension system for the rear axle.

The rear wheels of the truck are driven from the engine via a suitable transmission system and, if required, the front wheels may also be driven wheels.

The arrangement described above with front and rear articulated chassis sections may be substantially as described in U.S. Patent Specification No. 4 111 273. The details of the articulation system form no part of the present invention and reference may be made to the aforesaid U.S. Patent Specification for further details.

The fifth-wheel plate 6 is carried by a pair of laterally spaced arms 14 pivotally mounted on the rear chassis section for movement about a horizontal axis between raised and lowered positions as illustrated in Figure 1. It will be seen from Figure 1 that the horizontal pivotal axis of the arms 14 is located slightly rearwardly of the vertical articulation axis 8 and extends transversely of the truck. Each arm 14 is of channel shaped cross-section with the open side of the channel facing downwards and one operative end of a respective hydraulic lift cylinder 1 6 is received within the interior of the arm 14 at a position remote from the horizontal pivotal axis of the arm. The hydraulic cylinder 1 6 extends downwardly and rearwardly from the arm and is pivotally attached to the rear chassis portion 4 at the other end of the cylinder.The two hydraulic cylinders 16 are connected in parallel in a suitable hydraulic circuit and areactuable to raise and lower the arms 14 and thereby the fifth-wheel plate 6 and the trailer coupled thereto. The fifthwheel plate 6 is mounted at the rear end portion of the arms 14 on a transverse shaft which spans the arms 14, the mounting of the plate 6 being such as to permit the plate 6 to be tilted relative to the arms 14 so that the plate 6 can maintain a substantially horizontal attitude, irrespective of the elevation of the arms. The plate itself is of conventional form to receive detachably a corresponding standard king pin type coupling of a trailer or of an adapter such as so-called "goose neck" to be coupled to a trailer.

The construction of the arms 14, the fifthwheel plate 6 and the cylinders 1 6 may be as described in U.S. Patent Specification No.

4 111 273.

The truck has a conventional pneumatic braking system for braking the front and rear wheels of the vehicle. The braking system is illustrated purely schematically in Figure 4, which shows brake actuators 20 associated with the wheels of the tractor connected via a valve 22 to a compressed air tank 24. The valve 22 is operated by a pedal in the driver's cab to feed compressed air from the tank 24 to the actuators 20. The basic braking system is of an entirely conventional construction and incorporates various safety protection devices to safeguard the system in the event of failure in the brake lines and a parking brake system. These are entirely conventional and their construction and arrangement will be readily understood by those skilled in the art and no further description will be given here.

The braking system also incorporates a control valve which is responsive to the load on the fifthwheel of the truck for progressively regulating the pressure which can be applied to the brake actuators of the rear axle, between a minimum pressure which occurs when the fifth-wheel is unloaded, and a maximum which occurs when the fifth-wheel is carrying a full load. The manner in which this is achieved will now be described, also with reference to Figure 4.

Figure 4 shows schematically the two hydraulic lift cylinders 16 which are coupled to the two arms 14 carrying the fifth-wheel plate 6. The two hydraulic cylinders 1 6 are connected in parallel by a hydraulic line 1 6a to a conventional hydraulic control system comprising a pump and appropriate valving (not shown). As will be apparent the pressure in the hydraulic cylinders 16 and the hydraulic line 1 6a will be proportional to the vertical load acting on the fifth-wheel plate 6 and that this load can therefore be sensed by sensing the pressure in the hydraulic line 1 6a. This is achieved by means of a hydraulic load-sensing cylinder 30 which is connected into the hydraulic line. The detailed construction of this cylinder will now be described with reference to Figures 5 and 6.

As Figure 5 shows, the hydraulic load sensing cylinder 30 comprises a cylinder body 32 which receives a piston 34 having a piston rod 36. An annular casing 38 is attached to the end of the cylinder body 32 remote from the head end and carries a fitting 40 to enable hydraulic fluid to be fed into the piston rod end of the cylinder, this fitting being connected to the hydraulic line 16a.

The piston rod 36 extends through the casing 38 and an annular body 42 carrying packing rings 44 on its inner and outer surfaces is interposed between the piston rod 36 and the casing 38 to provide a fluid tight seal. A sleeve 46 screwed into the free end portion of the casing 38 serves to retain the sealing body 42 and also acts as a seat for one end of a compression spring 48 which surrounds the projecting end portion of the piston rod 36 and is seated at its other end on a profiled collar 50 which is retained on the piston rod 36 by a nut 52 engaged with a corresponding thread at the free end portion of the piston rod 36.

As will be apparent from Figures 5 and 6, the compression spring 48 applies an axial bias to the piston rod 3Bin a sense to eause thgpiston rod 36 to extend from the cylinder, and the hydraulic fluid fed into the cylinder acts in a sense to retract the piston rod 36 against the force of the spring 48. The distance by which the piston rod 26 extends from the cylinder will therefore be dependent on the pressure prevailing in the cylinder and thus the pressure prevailing in the hydraulic line. Therefore, the position of the piston rod 36 will be dependent on a vertical load applied to the fifth-wheel plate 6, and will be representative of that load.

The piston rod 36 is connected by a simple mechanical link 54, such as a rod, to an external operating lever 56 of a variable load pressure regulating valve 58 incorporated in the brake line leading to the brake actuators of the rear axle. The operating lever is of resilient material such as spring steel and is connected to a pressure regulating valve arrangement within the valve casing which progressively regulates the ratio of the pressures prevailing at the inlet and outlet ports of the valve in accordance with the deflection of the operating lever 56. Such a valve is known per se and is described in detail in British Patent Specifications Nos. 1012059 and 1029183 to which reference may be made for a fuller understanding of its mode of operation. A suitable valve for this purpose may be the model KY 807/1 manufactured by Westinghouse Limited.

The valve 58 is so linked to the piston rod 36 of the load-sensing cylinder 30 that, when the piston rod 36 is in a position corresponding to an unloaded condition of the fifth-wheel plate 6, a maximum pressure differential will exist between the inlet and outlet ports of the valve 58 so that the pressure applied to the brake actuators of the rear axle will be a minimum. As the load on the fifth-wheel plate 6 progressively increases, the piston rod 36 of the load-sensing cylinder 30 will move correspondingly and this, in turn, controls the valve 58 so that the pressure differential between the inlet and outlet ports is progressively reduced to correspondingly increase the pressure at the outlet port.

As particularly described, there is thus provided a terminal tractor having a pneumatic braking system with means for regulating the braking pressure in accordance with the load.

Conventional components are used in the braking system, with a conventional variable-load pressure-regulating valve, despite the tractor having a rigid rear axle, rather than a springmounted axle; this is achieved by means of the hydraulic load sensing cylinder which senses the load applied to the fifth-wheel and transduces this load into a displacement which is applied to the operating lever of the pressure regulating valve.

Claims (7)

1. A tractor including fifth-wheel means for coupling to a trailer, lifting means for lifting the fifth-wheel means, means for sensing the load imposed on the fifth-wheel means by the trailer, and means responsive to the load sensing means for regulating the braking force which can be applied by a braking system of the tractor such that the braking force increases with increasing load.

2. A tractor having fifth-wheel means for coupling to a trailer, fluid actuated means for raising the fifth-wheel means, means for sensing the load imposed on the fifth-wheel means by the trailer by sensing the pressure prevailing in the fluid actuated means, and means responsive to the pressure-sensing means for controlling a pressure regulating valve in a braking system of the tractor such that the braking pressure which can be applied increases with increasing load.

3. A tractor according to claim 2, wherein the pressure-sensing means comprises a pressuresensing cylinder including a piston rod movable against a resilient bias in response to fluid pressure applied to the cylinder whereby the position of the piston rod is dependent on the applied fluid pressure, the control means being responsive to the position of the piston rod.

4. A tractor according to claim 3, wherein the means for controlling the pressure-regulating valve comprises a mechanical operating member of the valve, mechanically linked to the piston rod.

5. A tractor according to any one of claims 2 to 4, wherein the fifth wheel means is above at least one rear axle of the tractor, and the pressure regulating valve is operative to control the braking pressure only at the wheels of the or each rear axle.

6. A pressure sensing cylinder for use as the pressure sensing meaftsX a trelstor according-to any one of claims 2 to 5, said cylinder including a piston rod movable against a resilient bias in response to fluid pressure applied to the cylinder whereby the position of the piston rod is dependent on the applied fluid pressure.

7. A tractor substantially as hereinbefore described with reference to the accompanying drawings.

,8. A pressure sensing cylinder substantially as hereinbefore described with reference to Figures 5 and 6 of the accompanying drawings.