Fork lift truck with a single front wheel

A fork lift truck (10) has a truck body (12), a lift mechanism (14) connected to the truck body (12) by means of a vertically extending pivot (52) and means (24) for turning the lift mechanism (14) relative to the truck body (12) about said pivot (52) to steer the truck (10), the truck body (12) having a pair of rear ground engaging wheels (16) mounted on transverse axes, the lifting mechanism (14) having a single ground engaging front wheel (40) mounted centrally on a transverse axis, the front wheel (40) having independent drive means (44, 46).

This application is a national stage completion of PCT/GB2004/002242 filed May 26, 2004 which claims priority from British Application Serial No. 0312343.7 filed May 30, 2003.

FIELD OF THE INVENTION

The present invention relates to fork lift trucks of the kind designed for use in narrow aisles of warehouses and the like, where the truck is to deposit loads in and remove loads from the face of a stack, in a direction transverse to the length of the aisle, that is at right angles to the face of the stack.

BACKGROUND OF THE INVENTION

In order to maximise the storage area of a warehouse, it is desirable to make the aisles of the minimum width possible. The aisles must however be wide enough to permit the manoeuvring of fork lift trucks to deposit a load in or remove a load from the stacks.

In order to improve the manoeuvrability of the fork lift trucks and thus reduce the aisle width, GB 2234214, the disclosure of which is incorporated herein by reference thereto, discloses a fork lift truck with two parts that are pivoted together. The rear part comprises a truck body which carries the driver, propulsion unit and counterweights to balance loads carried by a lifting mechanism mounted on the front part. A pair of driven wheels are provided on the truck body and a pair of non-driven wheels are provided on the front part, as close as possible to the load bearing part of the lift mechanism. The truck is steered by turning the front part relative to the truck body, about the pivot axis.

In order to permit loads to be deposited or removed from the stacks at right angles to the aisles, the front part is preferably capable of being turned at 90° or more to the truck body. As the front wheels approach 90°, the drive from the rear wheels will cause the front wheels to slide sideways along the aisle, rather than steering the truck towards the position in the stack into which a load is to be deposited or from which a load is to be removed.

In order to overcome this problem, it has been proposed, for example as disclosed in GB 2263088 or GB 2255941, the disclosure of which is incorporated herein by reference thereto, to provide differential drive to the rear wheels, in order to produce a steering effect.

A more effective approach, as disclosed in GB 2265344 and EP 1201596, the disclosure of which is incorporated herein by reference thereto, has been to drive the front wheels, instead of or in addition to the rear wheels. However in order to provide stability, it is necessary for the weight distribution in this type of lift truck to be very much to the rear of the truck. It is consequently necessary with front wheel drive systems of this type, to provide an articulated front axle to ensure that both front wheels remain in driving engagement with the floor, in spite of irregularities in the floor surface. This will generate further stability problems, particularly with elevated loads and in practice articulation of the front axle must be limited to provide a maximum upward and downward movement of each wheel, of about 25 mm. Even when the front axle is articulated in this manner, wheel spin is libel to occur if there are variations in the floor surface in excess of 20 mm in 1.5 m, which is typical for a newly laid warehouse floor.

According to one aspect of the present invention, a fork lift truck comprises a truck body, a lift mechanism connected to the truck body by means of a vertically extending pivot and means for turning the lift mechanism relative to the truck body about said pivot to steer the truck, the truck body having a pair of rear ground engaging wheels mounted on transverse axes, the lifting mechanism having a single ground engaging front wheel mounted centrally on a transverse axis, characterised in that the front wheel has independent drive means.

SUMMARY OF THE INVENTION

The present invention provides a front wheel drive fork lift truck which will overcome the steering problems associated with rear wheel drive trucks of this type. Furthermore as the single front wheel will always be in driving engagement with the floor, irrespective of irregularities in the surface of the floor, there is no need for articulation of the front axle and the problems associated therewith.

The front wheel may be driven, for example by a hydraulic or electric motor. The motor is preferably coupled directly to the wheel and the motor or a gearbox associated therewith partially built into the wheel to reduce the bulk and minimise the width of the mechanism. The power source for the wheel motor and also for the steering mechanism by which the lifting mechanism is turned about the pivot and for the lifting mechanism itself, for example a hydraulic pump driven by an engine, an engine driven electrical generator or a battery pack, is housed in the truck body. The rear wheels may also be driven in addition to the front wheel.

In accordance with a preferred embodiment of the invention, the front wheel is mounted centrally on a transverse axis as far forward towards the load bearing part of the lifting mechanism as possible, in order to maximise the load bearing capability of the truck.

DETAILED DESCRIPTION OF THE INVENTION

As illustrated inFIGS. 1 and 2, a fork lift truck10comprises a truck body12and lifting mechanism14.

The truck body12has a pair of rear wheels16mounted on a common axis, which is transverse to the longitudinal axis of the lift truck. The wheels16have solid tires18. The truck body provides a cabin20having a seat22, steering controls24, drive control pedals26and lifting controls28. Means, for example, a battery pack or engine driven generator or hydraulic pump, for providing power to the various systems of the truck10are also mounted in the truck body12, together with counter balance weights.

The lifting mechanism14comprises a telescopic mast30comprising several rails32, which may be moved in telescopic manner. A fork carriage34is mounted on the mast30for movement longitudinally of the rails32. A pair of load engaging forks36are provided on the fork carriage34. Drive means (not shown), for example hydraulic motors or rams, or electric motors are provided for extending the mast and for moving the fork carriage. Furthermore, means, for example a hydraulic ram (not shown) may be provided for tilting the mast30, backwards from the vertical, in conventional manner.

A single front wheel40is mounted on the lifting mechanism14, on a fixed axle which is transverse to the longitudinal axis of the truck when in the straight ahead position. The front wheel40is mounted beneath the mast30centrally of the lifting mechanism14and as far forward towards the forks36as possible, without fouling loads mounted on the forks36. The wheel40has a solid tire42.

An electric motor44is mounted coaxially of the wheel40and is coupled to the wheel40by means of a gearbox46which is built partially into the hub of the wheel40, in order to reduce the overall width of the wheel40/motor44/gearbox46unit.

The lifting mechanism14is pivotally connected to an arm50which extends forwardly from the front of the truck body12, by means of a vertical bearing tube assembly52. A steering mechanism, for example a hydraulic or electric motor and gear or chain mechanism, or hydraulic rams (not shown), is provided for turning the lifting mechanism14relative to the truck body12under control of the steering control24, in order to steer the fork lift truck10.

FIGS. 3 to 5show a typical manoeuvre required to deposit or remove a load in bay62of a stack60. The fork lift truck10is driven along an aisle64between two stacks60. With the lifting mechanism14in the straight ahead position, similar to that illustrated inFIG. 3. As the truck10approaches the bay62, the truck10is manoeuvred by turning lifting mechanism14, so that the truck body is close into the stack60but angled away from the bay62, as illustrated inFIG. 3. The lifting mechanism14is then turned towards the bay62, while the truck10is driven forward by motor44, so that the forks36gradually move into the bay62, as illustrated inFIG. 4. Eventually the forks36are disposed at right angles to the stack60, as illustrated inFIG. 5. The truck10may then be driven by motor44, while reducing the steering angle, so that the forks36enter the bay62at right angles to the stack, so that a load mounted thereon may be deposited in the bay62or a load may be removed from the bay62.

The fork lift truck10is at its least stable position when the lifting mechanism14is positioned at 90° to the line X-X joining the points of contact of the front wheel40and inside rear wheel16with the ground. In this position the load mounted on the forks36will produce a moment about the line X-X. In order to balance the load carried by the truck10, the centre of gravity of the truck must be positioned as far rearwardly as possible, in order to maximise the distance y between the centre of gravity and line X-X.

As the lifting mechanism14is rotated and the truck10is driven by the motor44, the speed of the inside rear wheel16will reduce with increasing steering angle, until when the point of intersection A of the axis of the front wheel40with the axis of the inside rear wheel16coincides with the point of contact of the inside rear wheel16with the ground, the inside rear wheel16will be stationary, the truck10pivoting about the inside rear wheel16. When the steering angle increases beyond this point, the inside read wheel16will rotate backwards.

While in the above embodiment only the front wheel40is driven, in an alternative embodiment, all three wheels16,40may be driven independently by individual electric motors. When all three wheels16,40are driven in this manner, the individual electric motors are preferably connected to a power source, in a manner such that under the forces generated by the drive applied to the front wheel40and outside rear wheel16, the inside rear wheel16will automatically slow down as the steering angle increases and will eventually reverse, the power to the inside rear wheel16being automatically diverted to one or both of the other wheels16,40.

For example, as illustrated inFIG. 6, electric motors160,162powering the rear wheels16of a truck10, may be connected in series, to a suitable power source164, for example a battery or an engine driven generator. The electric motor44driving the front wheel40, is connected to the power source164, in parallel with the electric motors160,162. The power source164is connected to the motors44,160,162, by switch means66by which the power may be reversed, to reverse the motors44,160,162. The circuit also includes a start switch167and means168controlled by the drive control pedal26, to control the speed of the motors44,160,162.

With this arrangement, as the truck10turns, the increasing load applied to the inside rear wheel16, causes the motor60driving that wheel16to slow down. This in turn causes an increase in the current in the circuit connecting motors60,62and an increase in the torque applied by motor62to the outside rear wheel16. The inside and outside rear wheels16will thus automatically run at different speeds, as the truck10turns.

According to a further embodiment, the motor44driving the front wheel40, may also be connected in series with the motors60,62driving the rear wheels16, so that the torque applied to the front wheel40will also increase, as the truck10turns.

In the embodiment illustrated inFIG. 7, the front and rear wheels16,40are driven by hydraulic motors70,72,74, respectively. The hydraulic motors70,72,74are built into the hubs of the wheels16,40. Hydraulic fluid is supplied under pressure to the hydraulic motors70,72,74by means of a hydraulic pumps76,78mounted in the truck body12. The hydraulic pumps76,78are driven by an internal combustion engine80powered by a fuel gas or similar fuel. Hydraulic fluid is pumped from a reservoir82, by means of a low pressure auxiliary pump76, to a high pressure pump78. A distribution block84is provided to permit automatic variation in the flow of hydraulic fluid to the motors70,72,74to control the speed and direction of the motors70,72,74, by means of feed and return lines86,88. Flexible hydraulic pressure hoses90are provided in the hydraulic lines86,88between the distribution block84and motor70driving the front wheel40, in order to permit pivoting of the lifting mechanism14.

The speed of the truck10is controlled by engine speed and adjusting the angle of the swash plate of pump78. The direction of motion of the truck10is controlled by means of solenoids, which reverse the direction of flow in lines86,88from the pump78.

Other systems of the fork lift truck10, for example the steering, the mast extension means, lifting mechanism and means for tilting the mast, are powered by hydraulic fluid from independent source.

The hydraulic motors72,74driving the rear wheels16of the truck10are connected to the pump78in series, so that as the inside rear wheel16slows down when the truck10is turning, the flow rate of fluid to the motor72driving that wheel16will reduce, while the flow rate of fluid to the motor74driving the outside rear wheel16will increase. The outside rear wheel16will thus be automatically driven at a speed greater than that of the inside rear wheel16.

The hydraulic motor70driving the front wheel40may be connected to the pump76in series with the motors72and74, or may be connected to the pump78or a separate pump, by a separate, parallel hydraulic circuit.

Various modifications may be made without departing from the invention. For example while in the embodiment described with reference toFIGS. 1 to 5, an electric motor is used to drive the front wheel, a hydraulic motor or other suitable drive means may be used. Similarly, in the three wheel drive embodiments described with reference toFIGS. 6 and 7, the electric or hydraulic motors may be replaced by other suitable drive means.

The present invention is also applicable to pedestrian operated fork lift trucks in which the operator walks behind the truck.