Patent Publication Number: US-7900732-B2

Title: Fork lift truck with a single front wheel

Description:
This application is a divisional of U.S. Application. Ser. No. 10/555,921 filed on Nov. 4, 2005, which is a National Stage completion of PCT/GB2004/002242 filed on May 26, 2004 which claims priority from British Patent Application Serial No. GB 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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is now described, by way of example only, with reference to the accompanying drawings, in which:— 
         FIG. 1  is a perspective view of one embodiment of a fork lift truck in accordance with the present invention; 
         FIG. 2  is a diagrammatic side elevation of the fork lift truck shown in  FIG. 1 ; 
         FIGS. 3 to 5  are diagrammatic plan views of the fork lift truck shown in  FIG. 1 , at various stages of a maneuvering operation; 
         FIG. 6  is a circuit diagram for a three wheel drive lift truck in accordance with an alternative embodiment of the present invention; and 
         FIG. 7  is a diagrammatic plan view of an alternative embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As illustrated in  FIGS. 1 and 2 , a fork lift truck  10  comprises a truck body  12  and lifting mechanism  14 . 
     The truck body  12  has a pair of rear wheels  16  mounted on a common axis, which is transverse to the longitudinal axis of the lift truck. The wheels  16  have solid tyres  18 . The truck body provides a cabin  20  having a seat  22 , steering controls  24 , drive control pedals  26  and lifting controls  28 . Means, for example, a battery pack or engine driven generator or hydraulic pump, for providing power to the various systems of the truck  10  are also mounted in the truck body  12 , together with counter balance weights. 
     The lifting mechanism  14  comprises a telescopic mast  30  comprising several rails  32 , which may be moved in telescopic manner. A fork carriage  34  is mounted on the mast  30  for movement longitudinally of the rails  32 . A pair of load engaging forks  36  are provided on the fork carriage  34 . 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 mast  30 , backwards from the vertical, in conventional manner. 
     A single front wheel  40  is mounted on the lifting mechanism  14 , on a fixed axle which is transverse to the longitudinal axis of the truck when in the straight ahead position. The front wheel  40  is mounted beneath the mast  30  centrally of the lifting mechanism  14  and as far forward towards the forks  36  as possible, without fouling loads mounted on the forks  36 . The wheel  40  has a solid tyre  42 . 
     An electric motor  44  is mounted coaxially of the wheel  40  and is coupled to the wheel  40  by means of a gearbox  46  which is built partially into the hub of the wheel  40 , in order to reduce the overall width of the wheel  40 /motor  44 /gearbox  46  unit. 
     The lifting mechanism  14  is pivotally connected to an arm  50  which extends forwardly from the front of the truck body  12 , by means of a vertical bearing tube assembly  52 . 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 mechanism  14  relative to the truck body  12  under control of the steering control  24 , in order to steer the fork lift truck  10 . 
       FIGS. 3 to 5  show a typical manoeuvre required to deposit or remove a load in bay  62  of a stack  60 . The fork lift truck  10  is driven along an aisle  64  between two stacks  60 . With the lifting mechanism  14  in the straight ahead position, similar to that illustrated in  FIG. 3 . As the truck  10  approaches the bay  62 , the truck  10  is maneuvered by turning lifting mechanism  14 , so that the truck body is close into the stack  60  but angled away from the bay  62 , as illustrated in  FIG. 3 . The lifting mechanism  14  is then turned towards the bay  62 , while the truck  10  is driven forward by motor  44 , so that the forks  36  gradually move into the bay  62 , as illustrated in  FIG. 4 . Eventually the forks  36  are disposed at right angles to the stack  60 , as illustrated in  FIG. 5 . The truck  10  may then be driven by motor  44 , while reducing the steering angle, so that the forks  36  enter the bay  62  at right angles to the stack, so that a load mounted thereon may be deposited in the bay  62  or a load may be removed from the bay  62 . 
     The fork lift truck  10  is at its least stable position when the lifting mechanism  14  is positioned at 90° to the line X-X joining the points of contact of the front wheel  40  and inside rear wheel  16  with the ground. In this position the load mounted on the forks  36  will produce a moment about the line X-X. In order to balance the load carried by the truck  10 , 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 mechanism  14  is rotated and the truck  10  is driven by the motor  44 , the speed of the inside rear wheel  16  will reduce with increasing steering angle, until when the point of intersection A of the axis of the front wheel  40  with the axis of the inside rear wheel  16  coincides with the point of contact of the inside rear wheel  16  with the ground, the inside rear wheel  16  will be stationary, the truck  10  pivoting about the inside rear wheel  16 . When the steering angle increases beyond this point, the inside read wheel  16  will rotate backwards. 
     While in the above embodiment only the front wheel  40  is driven, in an alternative embodiment, all three wheels  16 , 40  may be driven independently by individual electric motors. When all three wheels  16 , 40  are 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 wheel  40  and outside rear wheel  16 , the inside rear wheel  16  will automatically slow down as the steering angle increases and will eventually reverse, the power to the inside rear wheel  16  being automatically diverted to one or both of the other wheels  16 , 40 . 
     For example, as illustrated in  FIG. 6 , electric motors  160 ,  162  powering the rear wheels  16  of a truck  10 , may be connected in series, to a suitable power source  164 , for example a battery or an engine driven generator. The electric motor  44  driving the front wheel  40 , is connected to the power source  164 , in parallel with the electric motors  160 ,  162 . The power source  164  is connected to the motors  44 ,  160 ,  162 , by switch means  66  by which the power may be reversed, to reverse the motors  44 ,  160 ,  162 . The circuit also includes a start switch  167  and means  168  controlled by the drive control pedal  26 , to control the speed of the motors  44 ,  160 ,  162 . 
     With this arrangement, as the truck  10  turns, the increasing load applied to the inside rear wheel  16 , causes the motor  60  driving that wheel  16  to slow down. This in turn causes an increase in the current in the circuit connecting motors  60 ,  62  and an increase in the torque applied by motor  62  to the outside rear wheel  16 . The inside and outside rear wheels  16  will thus automatically run at different speeds, as the truck  10  turns. 
     According to a further embodiment, the motor  44  driving the front wheel  40 , may also be connected in series with the motors  60 ,  62  driving the rear wheels  16 , so that the torque applied to the front wheel  40  will also increase, as the truck  10  turns. 
     In the embodiment illustrated in  FIG. 7 , the front and rear wheels  16 , 40  are driven by hydraulic motors  70 ,  72 ,  74 , respectively. The hydraulic motors  70 ,  72 ,  74  are built into the hubs of the wheels  16 , 40 . Hydraulic fluid is supplied under pressure to the hydraulic motors  70 ,  72 ,  74  by means of a hydraulic pumps  76 , 78  mounted in the truck body  12 . The hydraulic pumps  76 , 78  are driven by an internal combustion engine  80  powered by a fuel gas or similar fuel. Hydraulic fluid is pumped from a reservoir  82 , by means of a low pressure auxiliary pump  76 , to a high pressure pump  78 . A distribution block  84  is provided to permit automatic variation in the flow of hydraulic fluid to the motors  70 ,  72 ,  74  to control the speed and direction of the motors  70 ,  72 ,  74 , by means of feed and return lines  86 , 88 . Flexible hydraulic pressure hoses  90  are provided in the hydraulic lines  86 , 88  between the distribution block  84  and motor  70  driving the front wheel  40 , in order to permit pivoting of the lifting mechanism  14 . 
     The speed of the truck  10  is controlled by engine speed and adjusting the angle of the swash plate of pump  78 . The direction of motion of the truck  10  is controlled by means of solenoids, which reverse the direction of flow in lines  86 , 88  from the pump  78 . 
     Other systems of the fork lift truck  10 , 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 motors  72 ,  74  driving the rear wheels  16  of the truck  10  are connected to the pump  78  in series, so that as the inside rear wheel  16  slows down when the truck  10  is turning, the flow rate of fluid to the motor  72  driving that wheel  16  will reduce, while the flow rate of fluid to the motor  74  driving the outside rear wheel  16  will increase. The outside rear wheel  16  will thus be automatically driven at a speed greater than that of the inside rear wheel  16 . 
     The hydraulic motor  70  driving the front wheel  40  may be connected to the pump  76  in series with the motors  72  and  74 , or may be connected to the pump  78  or 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 to  FIGS. 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 to  FIGS. 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.