Abstract:
A lift truck ( 10 ) including a pair of rear wheels ( 16 ) mounted on a body ( 12 ), and a pair of front wheels ( 36 ) rotatably mounted on opposing ends of an axle ( 40 ), such that the axes of rotation ( 42 ) of the front wheels are fixed relative to each other, the axle being capable of rolling relative to the body and being pivotable through substantially 90 degrees either side of a straight ahead position relative to the body to steer the truck, in which all four wheels can be driven.

Description:
BACKGROUND OF INVENTION  
         [0001]    The present invention relates to lift trucks, in particular forklift trucks.  
           [0002]    Lift trucks are known and fall into two broad categories, building trucks and warehouse type trucks.  
           [0003]    Building trucks are used outside to transport material around building sites. Typically, building trucks will have a mast with vertically moveable forks upon which the material can be lifted and carried.  
           [0004]    Building sites have contrasting types of terrain, such as soft ground, rough ground and surfaces having loose material, in addition to the possibility of the terrain being wet and sloping. Therefore, building trucks have particular features which enable them to negotiate such terrain.  
           [0005]    To operate over rough terrain, the building truck must have a suitably high ground clearance in addition to the suspension properties of pneumatic tires. The tire footprint loading of the building truck must be sufficiently low to prevent it from sinking into soft ground. Consequently, high flotation tires are employed, and the weight of the building truck is minimized, the weight being determined by the structural strength requirements of the truck.  
           [0006]    To maintain traction over the various terrains, particularly when conditions are wet and the surface is inclined, building trucks employ drive to all four wheels. This is particularly necessary when taking into consideration the low weight of the building truck. Furthermore, the center of gravity is located at the approximately the central position of the building truck to load all four wheels evenly, thus ensuring each wheel is capable of providing tractive force.  
           [0007]    Building trucks must also be sufficiently maneuverable, and generally have the ability to turn the forks up to a maximum of about 45 degrees either side of the forward direction. However, angles significantly above 45 degrees are not used since they tend to make the building truck unstable.  
           [0008]    Building trucks are powered by gasoline or diesel driven internal combustion engines where the harmful emissions are diluted in the atmosphere.  
           [0009]    Building trucks are not required to lift loads to very great heights because: (a) other forms of lifting devices are available, e.g., cranes; and (b) lifting a load to a significant height, when the building truck is on uneven ground is dangerous since the truck may tip over as the load is raised. The latter is particularly significant on building trucks, which have pneumatic tires (and hence allow roll or pitch of the truck body as the load is raised) and, as mentioned previously, building trucks are designed to be light (in line with strength requirements) and have a relatively high ground clearance (thus raising the center of gravity of the truck).  
           [0010]    Conversely, warehouse type trucks are required to lift loads to significant heights and thus use a telescopic mast and fork arrangement to load and unload goods in loading bays, typically in the aisles of warehouses. To maximize space efficiency in the warehouse, the aisles are narrow, and goods are stacked vertically in the loading bay.  
           [0011]    The conditions and the requirements of a lift truck in a warehouse, and the associated problems these conditions bring, contrast strongly with those conditions found on a building site and the requirements of a building truck.  
           [0012]    Warehouses are inside and hence dry, the warehouse floor is generally hard and smooth, usually of concrete, as well as being flat. Therefore, there are no associated traction problems associated with the warehouse type truck when moving along an aisle, and for this reason warehouse type trucks employ drive to the two rear wheels.  
           [0013]    Operating inside a warehouse requires warehouse type trucks to be powered by power sources where there are no harmful emissions, such as gas fueled internal combustion engines or electric motors. Therefore, it would be inappropriate to use a lift truck powered by a gasoline or diesel fueled internal combustion engine in a warehouse.  
           [0014]    To maintain the stability of the warehouse type truck, particularly when it is loading and unloading goods into the loading bays, extra weight is added to the rear of the warehouse type truck. The combined extra weight and the weight of the heavy goods exhibits high loading on the tires of the warehouse type truck, and hence solid tires are used, i.e. non-pneumatic tires, which do not significantly deform during loading and unloading. Such tires exert high loadings on the ground, which must be sufficiently strong to resist such loads, hence, the use of concrete.  
           [0015]    The added extra weight results in the center of gravity being located towards the rear of the truck. Thus, the stability of the truck is improved when steering at high steer angles.  
           [0016]    The narrow nature of the warehouse aisles requires the warehouse type truck to operate in a severely restricted space, hence the warehouse type truck must be able to load and unload goods into loading bays, which are at 90 degrees to the aisle. This requires the front wheels and forks to be positioned at an angle of 90 degrees relative to the rear wheels.  
           [0017]    A problem arises when power is applied to the rear wheels when the front wheels are angled at 90 degrees to the rear wheels, in that the front wheels tend to slip sideways relative to their normal direction of rotation, as opposed to steering the warehouse type truck into the loading bay as required. The truck tends to shuffle along the aisle resulting in the load becoming misaligned with its loading bay and the operator has to reverse and try again. This problem is more acute as the horizontal distance between the point about which the front wheels steer and the horizontal axis of the front wheels reduces. Thus, it is difficult to maneuver the goods into the loading bay.  
         SUMMARY OF INVENTION  
         [0018]    An object of the present invention is to provide an improved form of lift truck where this problem is reduced.  
           [0019]    Thus, according to the present invention, there is provided a lift truck including a pair of rear wheels mounted on a body, and a pair of front wheels rotatably mounted on opposing ends of an axle, such that the axes of rotation of the front wheels are fixed relative to each other, the axle being capable of rolling relative to the body and being pivotable through substantially 90 degrees either side of a straight ahead position relative to the body to steer the truck, in which all four wheels can be driven.  
           [0020]    A warehouse lift truck is provided comprising (1) a body, (2) a pair of rear wheels mounted on the body, (3) a pivot portion pivotably mounted on the body, wherein the pivot portion is pivotable through substantially 90 degrees either side of a straight-ahead position relative to the body to steer the truck, (4) an axle, (5) a pair of front wheels rotatably mounted on opposing ends of the axle, such that the axes of rotation of the front wheels are fixed relative to each other, and (6) means for driving each of the four wheels, wherein the axle is rollably mounted on the pivot portion so as to compensate for undulations in the flatness of the surface over which the wheels travel. Each front and rear wheel preferably has a solid tire.  
           [0021]    The lift truck preferably further comprises a power source and wherein the means for driving each of the four wheels comprises at least one front motor for driving the front wheels and at least one rear motor for driving the rear wheels, wherein the at least one front motor and at least one rear motor are supplied power from the power source. Preferably, there are two front motors wherein the front wheels are each driven by a respective front motor. Alternatively or in addition, there are two rear motors wherein the rear wheels are each driven by a respective rear motor.  
           [0022]    In one embodiment, the power source is an internal combustion engine. The engine preferably utilizes a gas fuel. The engine preferably drives hydraulic pump. The hydraulic pump is preferably a variable displacement pump, for example, swash-plate type pump. In this embodiment, the motors are preferably hydraulic motors to drive the wheels.  
           [0023]    In another embodiment, the power source is a battery. The motors are electric motors.  
           [0024]    In either of these embodiments, one rear wheel is preferably drivable in a reverse direction while the other rear wheel is driven in a forward direction.  
           [0025]    In one preferred embodiment of the lift truck, the two front wheels have a front track, the two rear wheels have a rear track, and the front track is less than the rear track. In another preferred embodiment and preferably in addition, the two rear wheels have a rear track and the horizontal distance between the point about which the front wheels steer and the horizontal axis of the front wheels is equal to or less than one-half of the rear track.  
           [0026]    In yet another embodiment, the lift truck preferably further comprises a lifting device mounted on the pivot portion, wherein the front axle is rollably mounted on the pivot portion so as to compensate for undulations in the flatness of the surface over which the wheels travel, thereby the front axle is capable of rolling relative to the body and lifting device. The lifting device preferably comprises a mast. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0027]    The invention will now be described, by way of example only, with reference to the accompanying drawings.  
         [0028]    [0028]FIG. 1 is a side elevational view of a lift truck according to the present invention.  
         [0029]    [0029]FIG. 2 is a plan view of the lift truck of FIG. 1.  
         [0030]    [0030]FIG. 3 is an exploded partial front view of the lift truck of FIG. 1 showing various axes.  
         [0031]    [0031]FIG. 4 is a schematic view of the drive arrangement of the lift truck of FIG. 1.  
         [0032]    [0032]FIG. 5 is a plan view of the lift truck of FIG. 1 when loading goods into a loading bay. 
     
    
     DETAILED DESCRIPTION  
       [0033]    With reference to FIG. 1 and FIG. 2, there is shown a lift truck  10  comprising a body  12  and a lifting device  14 .  
         [0034]    The body  12  comprises a pair of rear wheels  16 , the rear wheels  16  having solid tires  18 , a cabin  20  housing a seat  22 , steering controls  24 , pedals  26  and lifting controls (not shown).  
         [0035]    The lifting device  14  comprises a mast  28 , forks  30 , and a mechanism (not shown) for raising and lowering the forks  30  on the mast  28 . The mechanism consists of a conventional telescopic construction and can be driven hydraulically. The lifting device  14  is pivotally attached to the body  12  at a pivot hinge  32  having a pivot axis  34 .  
         [0036]    The lift truck  10  further comprises front wheels  36 , which are mounted below the lifting device  14 . The front wheels  36  have solid tires  38 , and are rotatably mounted on opposite ends of an axle  40 . The front wheels  36  rotate about a common horizontal axis  42  to provide forward and reverse motion to the lift truck  10 . Axis  42  is positioned at horizontal distance D forward of pivot axis  34 .  
         [0037]    [0037]FIG. 3 shows that the axle  40  is pivotally fixed to the lifting device  14  via a pin and hole arrangement  44  at the mid-point of the axle  40 . Thus, the axle  40  can roll relative to the lifting device  14 , with the forward direction of the lift truck  10  being taken as the reference axis for the rolling direction (R). This rolling of the axle  40  compensates for small undulations in the flatness of the ground by allowing one wheel  36  to move up or down and correspondingly the other wheel  36  to move down or up relative to the pin and hole arrangement  44 . Preferably, the amount of roll is less than plus or minus 10 degrees, for example, plus or minus 6 degrees, and more preferably less than plus or minus 5 degrees, for example, plus or minus 3 degrees. For example, if the axle  40  were about 500 mm long, the movement of one wheel  36  by plus or minus 25 mm or plus or minus 50 mm would correspond to approximately to an angle of plus or minus 3 degrees or plus or minus 6 degrees of roll, respectively.  
         [0038]    Herein, the word “roll” and its variations rolling and rollably are used in the sense of pitch, yaw and roll. For example, your head moves in pitch when you make a “yes”motion. Your head moves in yaw when you make the “no”motion. Your head moves in roll when it moves “side-to-side”.  
         [0039]    The pin and hole arrangement  44  is arranged relative to the pivot hinge  32  such that when the lifting device  14  pivots about the pivot axis  34 , the axle  40 , and hence the front wheels  36 , also pivot about the pivot axis  34  to steer the truck.  
         [0040]    Consideration of FIG. 2 shows that the track T (which is defined as the transverse distance between the centerlines of two wheels on same axis) of the rear wheels, is greater than the track t of the front wheels. Furthermore, the horizontal distance D is less than half the track T of the rear wheels.  
         [0041]    The lifting device  14  and hence the front wheels  36  are driven about the pivot axis  34  by a hydraulic steer arrangement (not shown).  
         [0042]    [0042]FIG. 4 shows a drive arrangement  46 , which is provided for driving the front wheels,  36  and the rear wheels  16 . Means is provided for driving any wheel in a forward or reverse direction.  
         [0043]    The power source for the drive system in this embodiment is an internal combustion engine  48  powered by a gas fuel. The internal combustion engine  48  provides power to a hydraulic pump  50 . The hydraulic pump  50  feeds a control unit  52 , which in this embodiment is a valve arrangement (not shown), the control unit  52  providing a variable output to four hydraulic motors  54 , each hydraulic motor  54  driving an associated front wheel  36  or rear wheel  16 . The control unit  52  is arranged such that it can regulate power to each of the four wheels.  
         [0044]    In other embodiments, the power source could be a battery (instead of the internal combustion engine  48 ), which in turn drives four electric motors (instead of hydraulic motors  54 ).  
         [0045]    [0045]FIG. 5 shows the operation of the lift truck  10  when loading goods  56  into a loading bay  58 , the lift truck  10  being positioned in a narrow aisle  60 .  
         [0046]    Consideration of FIG. 5 shows the position  42 ″ of the axis  42  relative to the body  12 , when the front wheels are positioned at substantially 90 degrees to the body  12 . It can be seen that the horizontal distance D is less than ½ T, the track of the rear wheels.  
         [0047]    To maneuver the goods  56  into the loading bay  58 , the lifting device  14  is rotated about the pivot axis  34 . It is necessary to maneuver the lift truck  10  such that front wheels  36  are substantially at 90 degrees to the lift truck body  12  in order to position the goods  56  in the loading bay  58  since the aisle  60  is relatively narrow when compared with the width of the body  12 .  
         [0048]    When loading the goods  56  it is necessary to drive the front wheels  36  into the loading bay  58 . By regulating and controlling the power to the front and rear wheels via the control valve arrangement, a balance can be obtained that provides enough power to the front wheels  36  to enable the front wheels  36  to drive into the loading bay  58 .  
         [0049]    The ability to control the power to the front wheels  36  prevents the front wheels from being forced sideways in the direction of arrow A along the aisle, i.e., in the direction of the rear wheels when the front wheels are positioned substantially at 90 degrees relative to the rear wheels.  
         [0050]    The ability to drive the rear wheels in opposite directions while simultaneously driving the front wheels also considerably increases the maneuverability of the lift truck, enabling goods to be loaded and unloaded in confined spaces such as in the narrow aisle of a warehouse.