This invention relates to a synchronised steering mechanism for a vehicle such as a forklift truck.

The conventional method of steering two wheels together is to fit a solid mechanical connection link, e. g. a track rod, between the two steered wheels. By moving one wheel with an hydraulic cylinder the other wheel also moves. In some cases the hydraulic steering cylinder also acts as the connecting link.

However, this is not always possible if the construction of the machine does not allow for a connecting link. In such cases manufacturers fit a separate hydraulic cylinder to steer each wheel individually. In order to lock the hydraulic system both cylinders operate through one or more hydraulic flow divider (s) fitted in the circuit. The only two versions of flow dividers available on the market are either rotary or spool design.

This system has the following disadvantages: 1. Flow dividers only operate efficiently (supplying an equal amount of flow) within a certain flow range.

When steering slowly (low flow) they do not operate effectively, nor when steering quickly (high flow).

2. Flow dividers heat the hydraulic oil in the system.

When hydraulic oil is pumped through a divider it is forced through small orifices.

3. Flow dividers are viscosity sensitive. The viscosity of hydraulic oil changes with temperature.

The hotter the oil the less efficient the flow divider.

4. When the vehicle is driving but not steering for long periods of time the wheels get out of line due to leakage across the spool.

5. All types of flow dividers produce a pressure drop across themselves which reduces the efficiency of the circuit.

6. A flow divider will let more oil flow to the wheel of least resistance. This can happen when the hydraulic oil pressure needed to steer one wheel is much higher than the other. This means that the wheel with least resistance turns faster than the loaded wheel.

It is therefore an object of the invention to provide a vehicle with steerable wheels in which such disadvantages are overcome or mitigated.

Accordingly, the present invention provides a vehicle having a pair of wheels each steerable by a respective hydraulic cylinder such that each wheel is turned to an angle determined by the position of the piston head in the respective cylinder, wherein each cylinder has a full bore side and a rod side with one of the full bore side and rod side of one of the cylinders being hydraulically connected to one of the full bore side and rod side of the other cylinder, and wherein the vehicle further includes means for supplying hydraulic

fluid under pressure selectively to the other of the full bore side and rod side of the one cylinder or to the other of the full bore side and rod side of the other cylinder.

Embodiments 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 forklift truck steering system in a first embodiment of the invention with the wheels of the truck being turned towards each other; Fig. 1A shows right hand hydraulic cylinder of Fig. 1 to illustrate the operation of valve means in its piston head when it is fully extended; Fig. 2 is a plan view the forklift truck steering system of the first embodiment with the wheels of the truck being turned away from each other; Fig. 2A shows the hydraulic cylinder of Fig. 2 to illustrate the operation of valve means in its piston head when it is fully retracted; Fig. 3 is a plan view of forklift truck steering system in a second embodiment of the invention with the wheels of the truck being turned in a first direction; Fig. 4 is a plan view the forklift truck steering system of the second embodiment with the wheels of the truck being turned in the opposite direction;

Fig. 5 is a plan view of forklift truck steering system in a third embodiment of the invention with the wheels of the truck being turned towards each other; Fig. 6 is a plan view the forklift truck steering system of the third embodiment with the wheels of the truck being turned away from each other; Fig. 7 is a plan view of a forklift truck in sideways driving mode; and Fig. 8 is a plan view of a forklift truck in forward/reverse driving mode.

In the figures of the various embodiments the same or equivalent components have the same reference numerals.

Referring to Figs. 1 and 2 of the drawings, a forklift truck comprises a chassis 10 having left and right steerable front wheels 12L, 12R respectively and a single steerable rear wheel 14. Each front wheel 12L, 12R is mounted to a respective wheel support 16 which is pivotted to the chassis 10 for rotation about a substantially vertical axis 18, and is steered by rotating the corresponding wheel support 16 relative to the chassis 10 about the axis 18. The rear wheel 14 is also mounted to a wheel support 20, and this latter is also pivotted to the chassis 10 for rotation about a substantially vertical axis 22 to allow steering of the rear wheel 14. The rear wheel 14 is arranged at the apex of a substantially isoceles triangle of which the notional line joining the front wheels 12L, 12R forms the base.

The front wheels 12L, 12R are steered in synchronism by hydraulic cylinders 24L, 24R respectively, as will be described, while the rear wheel 14 is independently steerable by a further hydraulic cylinder 26. The operation and control of the rear wheel 14 may be conventional and will not be further described.

Each cylinder 24L and 24R has a conventional piston head 28 and piston rod 30. The cylinder 24L has its piston rod 30 pivotted to the left hand wheel support 16 at point 32L and its base pivotted to the chassis 10 at point 34L. Likewise, the cylinder 24R has its piston rod 30 pivotted to the right hand wheel support 16 at point 32R and its base pivotted to the chassis 10 at point 34R. Note that the pivot point 32L is on the left hand side of its wheel support 16 while the pivot point 32R is on the right hand side of its wheel support 16.

The full bore side 36L of the cylinder 24L is hydraulically connected directly to the rod side (annulus) 38R of the cylinder 24R via a hydraulic line 40, and this circuit is primed with hydraulic oil (the oil in the circuit 38R/40/36L is herein referred to as synchronising oil). Hydraulic oil under presssure may be supplied selectively to the full bore side 36R of the cylinder 24R via a hydraulic line 42 or to the rod side 38L of the cylinder 24L via a hydraulic line 44.

A standard multi-way valve 46, operated in conventional fashion by a steering wheel 48 (Fig. 7), determines whether hydraulic oil is supplied to the line 42 or to

the line 44. The oil is pumped under pressure to the valve 46, from a tank 50, by a pump 52.

The total volume available on the rod side 38R of cylinder 24R is substantially equal to the total volume available on the full bore side 36L of cylinder 24L.

However, the stroke of each of the cylinders may be different, and the working surface area of the rod side 38R of cylinder 24R may be different to the working surface area of the full bore side 36L of cylinder 24L.

In operation, when the steering wheel is positioned to cause the valve 46 to connect the hydraulic oil under pressure to the line 42, Fig. 1, the hydraulic cylinder 24R extends. This in turn rotates the right hand wheel support 16 in an anti-clockwise direction, as indicated by the arrow, so that the wheel 12R is also turned in an anti-clockwise direction. At the same time, as the cylinder 24R extends, synchronising oil is driven out of the rod side 38R of the cylinder 24R via the line 40 into the full bore side 36L of the cylinder 24L, while oil from the rod side 38L of the cylinder 24L is exhausted via the valve 46 to the tank 50. This causes the cylinder 24L to extend in synchronism with the cylinder 24R, which in turn rotates the left hand wheel support 16 in a clockwise direction, as indicated by the arrow, so that the wheel 12L is also turned in a clockwise direction. Accordingly the wheels 12L and 12R turn towards one another in synchronism, the cylinder 24R acting as a master and the cylinder 24L acting as a slave. The angle to which each wheel turns is determined by the position of the piston head in the respective cylinder and therefore the amount of oil

which is supplied to the full bore side 36R of the cylinder 24R via the line 42.

However, when the steering wheel is positioned to cause the valve 46 to connect the hydraulic oil under pressure to the line 44, Fig. 2, the hydraulic cylinder 24L retracts. This in turn rotates the left hand wheel support 16 in an anti-clockwise direction, as indicated by the arrow, so that the wheel 12L is also turned in an anti-clockwise direction. At the same time, as the cylinder 24L retracts, synchronising oil is driven out of the full bore side 36L of the cylinder 24L via the line 40 into the rod side 38R of the cylinder 24R, while oil from the full bore side 36R of the cylinder 24R is exhausted via the valve 46 to the tank 50. This causes the cylinder 24R to retract in synchronism with the cylinder 24L, which in turn rotates the right hand wheel support 16 in a clockwise direction, as indicated by the arrow, so that the wheel 12R is also turned in a clockwise direction. Accordingly the wheels 12L and 12R turn away from one another in synchronism, the cylinder 24L now acting as the master and the cylinder 24R acting as the slave. As before, the angle to which each wheel turns is determined by the position of the piston head in the respective cylinder and therefore the amount of oil which is supplied to the rod side 38L of the cylinder 24L via the line 44.

This arrangement allows the forklift truck to operate in a sideways driving mode, Fig. 7. As seen in Fig. 7, in the sideways driving mode the rear wheel 14 may be fixed in the orientation shown and the vehicle may be steered along arcs indicated schematically by the tyre

tracks A and B according to the setting of the front wheels 12L and 12R.

To allow for a small amount of internal leakage, or for the cylinders 24L, 24R not being sized correctly, two non-return correction valves 54A and 54B are fitted in the piston head 28 of one of the cylinders, in this case the cylinder 24R.

If cylinder 24R becomes fully extended before cylinder 24L is fully extended then the non-return valve 54A fitted in the piston head 28 of cylinder 24R next the cylinder cap is pushed off its seat, Fig. 1A. This allows oil to flow freely across the piston head from the full bore side 36R of cylinder 24R into the synchronised circuit 38R/40/36L between the two cylinders. This will only happen until cylinder 24L extends fully.

Similarly if cylinder 24R becomes fully retracted before cylinder 24L is fully retracted then the non- return valve 54B fitted in the piston head 28 of cylinder 24R next the base of the cylinder is pushed off its seat, Fig. 1B. This allows the excess synchronising oil to flow freely across the piston head 28 and out through the line 42 until cylinder 24L completely retracts.

In a second embodiment of the invention, Figs. 3 and 4, the arrangement is identical to the first embodiment with the sole exception that the cylinder 24L has its piston rod 30 pivotted to the left hand wheel support 16 at point 32L'which in this case is on the right

hand side of the wheel support. This means that the wheels 12L and 12R will turn in the same direction rather than in opposite directions as for the first embodiment. Thus, Fig. 3 shows both the wheels 12L, 12R turning in sychronism in an anti-clockwise direction when hydraulic fluid under pressure is supplied to the line 42, while Fig. 4 shows both the wheels 12L, 12R turning in sychronism in a clockwise direction when hydraulic fluid under pressure is supplied to the line 44.

This arrangement allows the forklift truck to operate both in a sideways driving mode and, as seen in Fig. 8, in a conventional forward/reverse driving mode. As seen in Fig. 8, in the forward/reverse driving mode the front wheels 12L and 12R may be fixed in the orientation shown and the vehicle may be steered using the rear wheel 14.

Figs. 5 and 6 show a third embodiment of the invention.

This differs from the first embodiment in that the rod side 38R of the cylinder 24R is connected via the line 40 to the rod side 38L of the cylinder 24L rather than to the full bore side 36L (thus the synchronising oil is contained in 38R/40/38L), in that the line 44 is connected to the full bore side 36L of the cylinder 24L rather than to the rod side 38L, and in that the the cylinder 24R has its piston rod 30 pivotted to the right hand wheel support 16 at point 32R'which in this case is on the left hand side of the wheel support.

This means that when oil under pressure is applied to the line 44 the cylinder 24L will extend while the

cylinder 24R will retract. This will turn the wheel 12L clockwise and the wheel 12R anti-clockwise, Fig. 5.

Conversely, when oil under pressure is applied to the line 42 the cylinder 24R will extend while the cylinder 24L will retract. This will turn the wheel 12L anti- clockwise and the wheel 12R clockwise, Fig. 6.

In a variant (not shown) of the third embodiment, the line 40 connects the full bore side 36L of the cylinder 24L to the full bore side 36R of the cylinder 24R (thus the synchronising oil is contained in 36R/40/36L), and the lines 42 and 44 are connected to the rod sides 38R and 38L respectively of the cylinders. In this case, when oil under pressure is applied to the line 44 the cylinder 24L will retract while the cylinder 24R will extend. This will turn the wheel 12L anti-clockwise and the wheel 12R clockwise. Conversely, when oil under pressure is applied to the line 42 the cylinder 24R will retract while the cylinder 24L will extend.

This will turn the wheel 12L clockwise and the wheel 12R anti-clockwise.

In all embodiments, both wheels do not need to steer through the same angle if this is not required, and the non-return correction valves can be fitted in either cylinder or in both.

It is an advantage of the invention that synchronism of the steerable wheels is achieved without the need for mechanical links such as track rods nor the use of flow dividers. Further, if one wheel turns easily the remaining pressure is transferred to the other wheel

automatically. Thus providing the energy where it is required while keeping the system synchronised.

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