Abstract:
A straddle carrier comprises a frame ( 10 ) having opposite substantially parallel sides ( 12, 14 ) and which is configured for straddling a container to be lifted and transported between said opposite sides. The carrier has three ground wheels ( 24, 26 ) and ( 28 ) to enable the frame to be positioned over the container. The ground wheels comprise a first ground wheel ( 24 ) disposed at least approximately centrally on one side ( 12 ) of the frame, and second and third ground wheels ( 26, 28 ) disposed at opposite ends of the other side ( 14 ) of the frame. The second and third wheels ( 26, 28 ) are steerable by rotation about respective substantially vertical axes ( 40, 42 ).

Description:
TECHNICAL FIELD 
     This invention relates to straddle carriers. 
     BACKGROUND ART 
     A straddle carrier is a vehicle for use in e.g. port terminals and intermodal yards used for stacking and moving intermodal containers. Straddle carriers pick up and carry containers while straddling their load and, depending upon capacity, they have the ability to stack containers up to four high. They are capable of relatively low speeds (typically up to 30 km/h) with a laden container, and are generally not road-going. 
     Conventionally, straddle carriers are equipped with four ground wheels. When the carrier is not used on a level surface, a suspension system is required to ensure the four wheels stay in contact with the ground at all times. However, due to cost, the majority of straddle carriers have no suspension system. Without a suspension system a wheel may lose contact with the ground, therefore losing traction and braking and reducing stability. 
     DISCLOSURE OF THE INVENTION 
     According to the present invention there is provided a straddle carrier comprising a frame having opposite substantially parallel sides and which is configured for straddling a container to be lifted and transported between said opposite sides, a plurality of ground wheels to enable the frame to be positioned over the container, and means for lifting the container within the frame, wherein the ground wheels comprise a first ground wheel disposed at least approximately centrally on one side of the frame, and second and third ground wheels disposed at or adjacent opposite ends of the other side of the frame, the second and third wheels being steerable by rotation about respective substantially vertical axes. 
     In a preferred embodiment the first wheel is non-steerable with its rotational axis extending across the width of the frame. 
     Preferably the second and third wheels are synchronized for steering by substantially equiangular amounts in opposite directions of rotation about their vertical axes. 
    
    
     
       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 perspective view of a straddle carrier according to the embodiment of the invention. 
         FIGS. 2(A)  to (C) are schematic top views of the carrier of  FIG. 1  showing steering in various directions. 
         FIGS. 3(A)  and (B) are side views showing the carrier lifting loads of different lengths. 
         FIG. 4  is a schematic end view of the carrier of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to the drawings, the straddle carrier comprises a steel frame  10  which, in end view,  FIG. 4 , is generally in the form of an inverted “U”. The frame  10  has opposite substantially parallel sides  12 ,  14  joined at the top by cross members  16 , an open base  18  and opposite open ends  20 . This structure forms a kind of tunnel which allows the frame to straddle a container  22  to be lifted and transported. In  FIG. 1  (dashed lines) and in  FIG. 4  the container  22  is shown on the ground, while in  FIG. 1  (solid lines) and in  FIG. 3  the container  22  is shown lifted within the frame. The frame  10  is mounted on three ground-wheels  24 ,  26  and  28  to allow the frame to be driven over the container  22  while the latter is resting on the ground, as shown in  FIG. 4 . The construction and operation of the wheels  24 - 28  will be described in more detail below. 
     The container  22  is attached to the frame by four flexible steel members  30 , e.g. chains or cables. In use, one end of each member  30  is attached to a respective bottom corner of the container  22  and the member  30  passes over a respective pulley  32  at the top of the frame  10  where its other end is attached to one end of a respective hydraulic cylinder  34 . By retracting the cylinders  34  the container  22  can be lifted off the ground for transport,  FIGS. 1 and 3 , and by extending the cylinders the container can be lowered to the ground once again,  FIGS. 1 and 4  (the members  30  are not shown in  FIG. 4 ). 
     Since the flexible members  30  can be swung over an angle shown in dashed lines in  FIG. 3(A) , they can be connected to different lengths of container  22 , as shown in  FIGS. 3(A) and 3(   b ), so that the carrier is capable of lifting and transporting loads of different lengths. The straddle carrier is also capable of lifting the container on and off haulage vehicles, or stacking them two or more high provided the internal clearance height of the frame  10  is sufficient to raise the container to the required height. For transportation, the container  22  can be lifted right to the top of the frame  10  where it is held tight against the underside of the cross members  16 . This secures and stabilises the container. 
     The straddle carrier is operated by an operator seated in a cabin  36  mounted on the outside of the frame  10 , between the wheels  26 ,  28 . Behind the cabin  36  is a hydraulic power unit  38  which, under operator control, supplies hydraulic power to the cylinders  34  via a hydraulic circuit, not shown. The operator in the cabin  36  is able to raise and lower the container  22  by contracting and extending the cylinders  34  in known manner. The container  22  can be lifted high enough for the operator to have good visibility underneath it—see  FIG. 3 . 
     The straddle carrier&#39;s ground wheels comprise a single wheel  24  which is disposed below the side  12  at least approximately centrally between the opposite open ends  20  of the frame  10 , and two wheels  26 ,  28  which are disposed below the side  14  respectively at or adjacent the opposite open ends of the frame. The three wheels lie approximately on an equilateral triangle whose apex is at the wheel  24  and whose base extends between the wheels  26  and  28 . The wheel  24  is fixed, i.e. it is non-steerable, its rotational axis X ( FIG. 3 ) extending across the width of the frame perpendicular to the sides  12 ,  14 . However, each of the wheels  26 ,  28  is steerable by rotation about respective substantially vertical axes  40  and  42 . 
     The steering of the wheels  26 ,  28  is synchronized such that under the control of an operator&#39;s steering wheel  44  in the cabin  36  the wheels  26 ,  28  rotate about their vertical axes  40 ,  42  by substantially equiangular amounts in mutually opposite directions of rotation. The rotation of the steerable wheels  26 ,  28  about their vertical axes is effected by hydraulic cylinders  46 ,  48  which are connected to the hydraulic power unit  38  and controlled by the steering wheel  44 . The direction and amount by which the steering wheel  44  is turned determines the direction in which the wheels  26 ,  28  are rotated about their vertical axes, and through what angle. 
     In addition to the hydraulic circuits necessary to operate the lifting cylinders  34  and steering cylinders  46  and  48 , the carrier also includes a hydraulic circuit connected to one or more hydraulic drive motors associated respectively with one or more of the wheels  24 - 28  to allow the carrier to be driven over the ground under operator control. The hydraulic motor(s) and drive circuitry is not shown, but the manner of its implementation will be readily known to the skilled man. 
     Referring to  FIG. 2 , when driving forward straight ahead,  FIG. 2(A) , all three wheels are aligned parallel to the sides  12 ,  14 . When driving forward and the operator turns the steering wheel  44  clockwise, the straddle carrier steers clockwise,  FIG. 2(B) , the wheels  26 ,  28  turning in mutually opposite directions by equal angles. When driving forward and the operator turns the steering wheel  44  anti-clockwise, the straddle carrier steers anti-clockwise,  FIG. 2(C)  the wheels  26 ,  28  again turning in mutually opposite directions by equal angles (although in this case the direction of rotation of each wheel is opposite that for clockwise steering). At the extreme limits of steering, which are the wheel positions shown in  FIGS. 2(B) and 2(C) , the carrier can turn within its own circle. 
     In a modification of the above embodiment one or more of the wheels  24 ,  26  and  28  may comprises one of a pair of wheels disposed coaxially side-by-side, the pair of wheels being driven and/or steered as a single unit. 
     In another modification the operator&#39;s cabin could be removed and the straddle carrier controlled remotely. 
     Although in the foregoing embodiment the ground wheels are driven by hydraulic motors (not shown) and steered by hydraulic cylinders  46  and  48 , and the flexible steel lifting members  30  are operated by hydraulic cylinders  34 , all powered from the hydraulic power unit  38 , any or all of these mechanisms could alternatively be operated by electric motors powered by heavy duty rechargeable batteries in the unit  38 . 
     The three main advantages of the three-wheel straddle carrier described above are:
         1. No suspension system is required. All three wheels will remain in contact with the ground irrespective of the terrain.   2. Traction and braking are maintained. All three wheels will remain in contact with the ground irrespective of the terrain, therefore none of the wheels will lose traction or braking.   3. The carrier is more maneuverable. The straddle and its load can turn within its own circle.       

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