Background Art Conventional methods of steering an articulated forklift truck are: (a) A large displacement hydraulic motor mounted over the steering shaft. The hydraulic motor needs. to be a very large displacement in order to provide enough torque for a slow rotation speed through the 180 or more. The size of the motor required is generally so large that it is not practical to fit on an articulated forklift truck. Also, the truck must be fitted with a static brake. The hydraulic motor has no internal braking ability. For operator safety the steering must be braked when not operated.

(b) A large diameter sprocket mounted on the steering shaft driven by a smaller diameter sprocket mounted from a hydraulic motor. This system gives a gearbox

reduction effect which means that a smaller displacement hydraulic motor is possible. However, a sprocket which drives another sprocket always creates wear. This is particularly the case on an articulated forklift truck when the operator steers both left and right. This wear makes it very difficult to make small, refined adjustments to the steering. Again the truck must be fitted with a static brake.

(c) A large diameter chain type sprocket mounted on the steering shaft connected by a chain to a''small diameter chain sprocket remotely mounted and driven by a hydraulic motor. This gives a reduction effect which allows a smaller displacement hydraulic motor to be used. Also the motor can be mounted in any position away from the shaft. However, again the truck must be fitted with a static brake. Also, the length of the chain drive will naturally increase with use. When the chain becomes longer there will be a short delay when changing steering direction.

(d) A large diameter sprocket mounted on the shaft driven by a hydraulic worm gear. The hydraulic motor rotates the worm in order to rotate the sprocket. The worm is a mechanical type drive. The worm is mechanically engaged against the sprocket. The wear makes it difficult to make small, refined adjustments to the steering.

(e) A hydraulic cylinder connected to the shaft using mechanical links in order to achieve 180° rotation. In this case steering speed is inconsistent (varies

depending on where the linkage is). There are also many pivot points which need a lot of maintenance.

(f) Rack and pinion. A sprocket is mounted on the shaft and rotated using a straight rack activated using a hydraulic cylinder. The rack wears against the sprocket and makes it difficult to control refined steering correction.

It is an object of the invention to provide an articulated vehicle having a steering system-which avoids or mitigates some or all of these disadvantages.

Disclosure of the Invention According to the present invention there is provided an articulated vehicle comprising a chassis, at least one steerable ground-engaging wheel articulated to the chassis by a shaft which is rotatable relative to the chassis to steer the vehicle, and a steering mechanism mounted on the chassis for rotating the shaft, the steering mechanism comprising a pair of hydraulic cylinders coupled to the shaft and means for individually actuating the cylinders to rotate the shaft respectively in opposite directions.

Preferably, each hydraulic cylinder is coupled to the shaft by a highly elongated substantially inelastic flexible member which extends tangentially into contact with and circumferentially around the shaft in non-slip engagement therewith.

The highly elongated substantially inelastic flexible member may take the form of a chain or a length of steel cable, for example.

In a preferred embodiment the highly elongated substantially inelastic flexible member comprises a pair of chains, each associated with a respective one of the pairs of hydraulic cylinders, and being connected to one another and to the shaft at a common link.

As an alternative, the highly elongated substantially inelastic flexible member can comprise a length of chain connected at either end to a respective one of said pair of cylinders, the chain being in non-slip engagement with said shaft by means of a sprocket provided on the shaft around which the chain passes.

Preferably, the means for individually actuating the cylinders comprises a hydraulic pressure source, a respective hydraulic line connecting each of said cylinders with said pressure source, and a valve unit operable to connect the pressure source with one or other of said cylinders, or with neither cylinder.

In this embodiment, there is preferably also a respective return line from each of said cylinders to a hydraulic fluid reservoir, wherein when said valve unit connects one of said cylinders with the pressure source it connects the other of said cylinders with the reservoir.

There is preferably a pressure'relief valve located in each return line, which permits the flow of hydraulic fluid between the associated cylinder and the reservoir when the pressure of hydraulic fluid exceeds a threshold pressure. In this way the tension of the chain can be maintained, even if the chain stretches.

Advantageously, the valve unit can be an orbital steering motor.

Brief Description of the Drawings An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a plan view of a forklift truck according to the embodiment of the invention steering straight ahead ; Fig. 2 is a plan view of the forklift truck steering to the right; and Fig. 3 is a plan view of the forklift truck steering to the left.

Detailed Description of Preferred Embodiments Referring to Fig. 1, a forklift truck comprises a chassis 10 and a pair of ground-engaging wheels 12 mounted on a steering bogie 14. The bogie 14 is articulated to the chassis 10 by a substantially vertical shaft 16 which is fixed to the bogie 14 and rotatable in bearings (not shown) relative to the

chassis to steer the vehicle. As so far described the forklift truck is of conventional and well-known construction and does not need further description.

Accordingly, in the drawings only the parts of the forklift truck relevant to the invention are shown.

For example, the non-steerable ground-engaging wheels on the chassis have been omitted, as well as the forklift mast, fork, and other conventional parts.

A steering mechanism is mounted on the chassis 10 for rotating the shaft. The steering mechanism comprises a pair of single acting hydraulic cylinders 18A, 18B.

One end 20A, 20B of each cylinder is fixed to the chassis 10 and the piston rod 22A, 22B of each cylinder is coupled to the shaft 16 by a short length of chain 24A, 24B respectively. The chains 24A, 24B extend tangentially into contact with the shaft 16 on opposite sides thereof and then pass circumferentially around the shaft in opposite directions to terminate at a common link 26 which is fixed to the shaft.

A steering motor in the form of a conventional steering orbital unit 28 has four ports P, T, A and B. Ports A and B are connected to the piston rod side of the hydraulic cylinders 18A and 18B respectively, port P is connected to a source of hydraulic oil under pressure, and port T to a tank 30. A respective non-return valve 32 is connected in series in each hydraulic line connecting the unit 28 to the cylinders 18A and 18B, and a respective. pressure relief valve 34A, 34B is connected in parallel with each valve 32A, 32B. The unit 28 is rotatable such that port P may be connected

selectively to cylinder 18A (Fig. 2) via port A, or to cylinder 18B (Fig. 3) via port B, or to neither cylinder (Fig. 1). When the port P is connected to one of the ports A or B (Figs. 2 and 3), the other port B or A is connected to port T.

Starting from the straight ahead position, Fig. 1, when the port P is connected to port A, Fig. 2, oil under pressure is forced into the piston rod side of the cylinder 18A via the respective non-return valve 32A.

This retracts the piston rod 22A and thereby pulls the chain 24A away from the shaft 16. Due to the attachment of the chain 24A to the fixed link 26, this movement of the chain rotates the shaft 16, and hence the bogie 14, in a clockwise direction to steer the forklift truck to the right.

In a similar manner, when the port P is connected to cylinder 18B, Fig. 3, oil under pressure is forced into the cylinder 18B via the respective non-return valve 32B to retract the piston rod 22B and thereby rotate the shaft 16 and bogie 14 in an anticlockwise direction to steer the forklift truck to the left.

In each case, in order to allow the piston rod 22A or 22B to retract when oil under pressure is supplied to the corresponding cylinder 18A or 18B, it is necessary that oil be allowed to flow out of the other cylinder 18B or 18A respectively to allow the piston rod of that other cylinder to extend. This is because the total distance between the piston rod 22A and piston rod 22B, as measured along the chains, is fixed. This is

achieved in each case by the pressure relief valves 34A, 34B, which allows hydraulic oil to flow out of a cylinder to the tank 30 when the cylinder's port A or B is connected to port T and the pressure of the oil therein exceeds a certain threshold pressure set by the relief valve. This ensures that the chains 24A, 24B are kept under tension at all times, even if the chains should increase in length over time, and provides precise steering control.

The diameter of the shaft 16, the diameter of the hydraulic cylinders 18A, 18B and the displacement of the steering motor 28 govern the steering speed of the mechanism. If desired, the diameter of the shaft can be locally increased where it is engaged by the chains 24A, 24B, for example by fixing a collar around the shaft a that point, to provide for a desired steering speed. Alternatively, the diameter of the shaft could be locally reduced where it is engaged by the chains.

Although the foregoing has used chains to couple the hydraulic cylinders to the shaft, it is possible to use any highly elongated substantially inelastic flexible member such as steel cable. Also, instead of using two chains 24A, 24B connected to a common fixed link 26, it would be possible to use a single chain passing around a sprocket fixed to the shaft 16, one end of the chain being connected to one cylinder and the other end of the chain being connected to the other cylinder. What is important is that the chain or chains or other inelastic flexible member (s) be in non-slip engagement with the circumference of the shaft so that pulling the

chain rotates the shaft without any significant lost motion or play in the mechanism.

The advantages of the above embodiment are that steering is precise and controlled due to the absence of slack in the system, even where the components are subject to wear over time. Further, the steering speed is constant for all angles of the bogie 14 which allows the system to be used over a full steering range of 184°. Also, the steering motor 28 provides a hydraulic brake, so that no separate steering brake is necessary.

The invention is not limited to the embodiments described herein which may be modified or varied without departing from the scope of the invention.