Abstract:
A load lifting attachment ( 30 ) for increasing the load lifting capacity of a skid-steer loader ( 62 ). A lifting mechanism ( 40 ) is supported from a C-shaped frame member ( 32 ). The frame member has a hard coupling ( 72 ) to a boom arm ( 70 ) of the skid-steer loader and a pair of caster assemblies ( 38 ) at opposed ends. A load supported on the load-bearing forks ( 42 ) of the lifting mechanism has its center of gravity proximate the swivel axis of the caster assemblies in order to minimize the resultant loads on the boom arm. The forks extend beyond the wheels ( 36 ) of the caster assemblies to allow the device to carry wide loads and to lift loads from a shelf blocking the forward movement of the wheels. An inclinometer ( 74 ) is provided to assist the operator in avoiding unstable movements. In one embodiment, the lifting capacity of a skid-steer loader is increased from 1,700 pounds to 5,000 pounds with the addition of the load lifting attachment.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to the field of load lifting equipment, and more particularly to skid-steer loaders, and specifically to an attachment for increasing the lifting and handling capability of a skid-steer loader and a method for using a skid-steer loader having such an attachment.  
         BACKGROUND OF THE INVENTION  
         [0002]    Skid-steer loaders are well known in the art. One line of skid-steer loaders is sold under the trademark “Bobcat” by the Bobcat Company. (www.bobcat.com) Skid-steer loaders are useful for handling relatively light loads. For example, a typical skid-steer loader with a standard pallet fork attachment may be rated for lifting and handling pallet loads of up to 1,700 pounds. This lifting capacity is limited by the relatively light weight of the skid-steer loader, since lifting heavier loads will cause the rear wheels of the loader to lift off of the ground. A larger, heavier loader would be capable of lifting a heavier load. However, larger, heavier machines tend to be more expensive and less maneuverable than smaller machines, so a trade-off must be made between lifting capacity and loader size.  
           [0003]    Devices have been developed to increase the lifting capability of a skid-steer loader without sacrificing the small size and weight of the loader. U.S. Pat. No. 5,685,563 describes a counterbalance system for a short wheelbase vehicle. This system uses a moveable weight connected to the vehicle frame for providing extra weight over either the front or rear wheels of the vehicle, as desired. While such a system does increase the load lifting capability of the machine, the benefit provided may not justify the complexity of the powered counterweight system.  
           [0004]    The inventor is aware of other devices for increasing the lifting capacity of a skid-steer loader. FIG. 1 is a side view of a prior art load lifting device  10  attached to the mounting plate  12  of a skid-steer loader boom arm (not shown). FIG. 2 is a front view of the same device. Pallet forks  14  are attached to a lifting frame  16 . The forks  14  are moved between a lowered position (shown) and a raised position by the action of hydraulic cylinders  18 . The parallelogram shape of the lifting frame  16  allows a caster member  20  located at a far end of the lifting frame  16  to remain in a generally vertical position as the forks  14  are raised and lowered. The load imposed by the forks is shared between the mounting plate  12  and a pair of wheels  22  attached to the caster members  20 , thereby increasing the lifting capacity of the skid-steer loader. However, the utility of this device is limited by the projection of the lifting frame  16  and wheels  22  away from the mounting plate  12 , since these structures may interfere with the forward movement of the device toward a wall or under a truck or trailer frame. Furthermore, the width of a pallet to be lifted by this device is limited to the width of the inside dimensions of lifting frame  16 , since the pallet must pass within the lifting frame structure as it is raised and lowered.  
           [0005]    Other types of lifting devices are well known in the art. Fork lifts are specifically designed for the movement of pallets, and they are commercially available in a variety of sizes. The forks of a fork lift are able to carry loads wider than a pallet, but the lifting capacity of these machines is limited. Sod loaders are known for unloading pallets of sod from a trailer and for the movement of the sod around an area to be landscaped. Sod loaders typically have three wheels, with a first wheel on a front portion and two additional wheels on a rear portion that is connected to the front portion by a rotatable hitch connection. These units are steered by hydraulic rotation of the hitch connection, thereby minimizing damage to the underlying surface as the unit is turned. The wheels of the rear portion support a C-frame which in turn supports a fork lift mast for lifting the sod pallets or other wider loads. These specialized machines have limited application beyond sod handling applications due to their small size, limited traction capability and limited maneuverability when compared to a skid-steer machine.  
         SUMMARY OF THE INVENTION  
         [0006]    Thus what is needed is a skid-steer loader having an increased lifting capacity. A load lifting device is described herein as including: a skid-steer loader having a boom arm; an auxiliary lifting apparatus connected to an end of the boom arm by a hard connection, the auxiliary lifting apparatus further comprising: a generally C-shaped frame member fixedly attached to the end of the boom arm proximate a center of the C-shape; a caster assembly attached proximate each end of the C-shaped frame member; and a lifting mechanism attached to the C-shaped frame member, the lifting mechanism comprising a load-bearing member extending beyond the ends of the C-shaped frame member. A swivel axis of each caster may be disposed in a vertical plane lying proximate a center of the load-bearing member. The lifting mechanism may include a fork having a horizontal length of approximately 60 inches, and wherein the distance between the mounting device and a swivel axis of the caster assembly is approximately 30 inches. An inclinometer may be attached to the device.  
           [0007]    A load lifting attachment for a skid-steer loader is described herein as including a frame member including a mounting device for making a hard connection to a boom arm of a skid-steer loader; a caster assembly attached to the frame member opposed the mounting device; and a lifting mechanism attached to the frame member for raising and lowering a load, the lifting mechanism comprising a load-bearing member extending beyond the caster assembly in a direction away from the mounting device. A swivel axis of the caster assembly may be aligned in a vertical plane with a center of the load bearing member. The lifting mechanism may be a fork having a horizontal length of approximately 60 inches, and wherein the distance between the mounting device and a swivel axis of the caster assembly is approximately 30 inches. An inclinometer may be provided attached to the device. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The invention will be fully appreciated when the following detailed description is read with reference to the drawings, in which:  
         [0009]    [0009]FIG. 1 is a side view of a prior art lifting attachment for a skid-steer loader.  
         [0010]    [0010]FIG. 2 is a front view of the device of FIG. 1.  
         [0011]    [0011]FIG. 3 is a side view of a lifting attachment for a skid-steer loader.  
         [0012]    [0012]FIG. 4 is a front view of the device of FIG. 3.  
         [0013]    [0013]FIG. 5 is a side view of a skid-steer loader with the device of FIG. 3 attached and being operated on its rear wheels. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    An improved load lifting device  30  is illustrated in FIGS. 3 and 4. The load lifting device  30  is an auxiliary lifting device that is designed for use with a skid-steer loader in order to increase the lifting capacity of the loader. The load lifting device  30  includes a generally C-shaped frame member  32  as viewed from above. One may appreciate that the term “generally C-shaped” as used herein is meant to include a U-shape or other shapes having at least two horizontally extending portions extending away from a central portion. The frame member  32  is a load bearing member and should be sized accordingly, and may be formed of steel, aluminum or high strength composite material, for example. In one embodiment the frame member  32  has a box beam cross-section. A mounting device such as mounting plate  34  is attached to the frame member  32  proximate the center of the C-shape. The mounting plate  34  provides a means for making a hard attachment to the end of a boom arm of a skid-steer loader. The frame member  32  may be connected to the boom arm by any known type of connection, such as a bolt/nut combination, a clamp or vice arrangement, or preferably the type of universal coupler commonly supplied with skid-steer loaders. Mounting plate  34  is designed to make a hard connection with such a universal coupler. Alternatively, the frame member  32  may be attached directly to the boom arm so long as the connection is a hard connection, i.e. one which supports loads and moments in all directions so that the frame member  32  is moved together with the end of the boom arm as an integral unit.  
         [0015]    Wheels  36  are rotatably connected to opposed ends of the frame member  32 , preferably as part of a corresponding pair of caster assemblies  38 . Any commercially available caster assembly  38  having sufficient weight bearing capacity may be used. The caster assembly is preferably selected to have a height such that the frame member  32  may be moved under a trailer for off-loading material from the trailer.  
         [0016]    A lifting mechanism  40  is attached to frame member  32  for raising and lowering a load, such as a pallet. In the embodiment illustrated in FIGS. 3 and 4, the lifting mechanism  40  includes a load-bearing member such as tines or forks  42  having a generally L-shape, as are known in the art for handling pallets. The forks  42  may be spaced at a fixed distance from each other, or may be adjustably attached to a fork carriage  44  so that the spacing between the forks  42  may be conveniently changed at the discretion of the operator. The forks  42  and/or fork carriage  44  are slidably supported by mast  46  to move upward and downward between lowered and raised positions. The forks  42  are illustrated as being in a lowered position. Power to raise and lower the forks  42  and any associated load is provided by a hydraulic lift cylinder  48  connected between the fork carriage  44  and the mast  46  and/or frame member  32 . Lift cylinder  48  may be a commonly available double-acting fork lift cylinder. Lift cylinder  48  advantageously receives pressurized hydraulic fluid from an auxiliary hydraulic circuit commonly available on skid-steer loaders through quick-disconnect hydraulic line connectors as are known in the art. Appropriate double-acting controls are provided to the operator of the skid-steer loader for raising and lowering the lifting mechanism  40 . The lift cylinder  48  may be provided with a mechanical advantage by a chain drive  50 , as are common on fork lift masts, to double the range of motion available from the lift cylinder  48 . An electric motor or a pneumatic actuator may be used in place of a hydraulic lift cylinder  48 .  
         [0017]    The operation of lifting device  30  begins by connecting the mounting plate  34  to the lifting boom arm coupler of a skid-steer loader. Hydraulic connections are then made between the skid-steer loader auxiliary hydraulic system and the lift cylinder  48 . The forks  42  are raised off of the ground and the skid-steer loader is operated in normal fashion to move the lifting device  30  from place to place. The hard connection provided by mounting plate  34  and the action of the casters  38  allow the lifting device  30  to follow the movement of the skid-steer loader as a single integral unit.  
         [0018]    The lifting device may be moved to position the forks  42  under a load, and the lifting mechanism  40  energized to raise and lower the load as desired. Weight applied to the forks  42  is transferred to the frame member  32  which is supported by both the mounting plate  34  and the wheels  36 . Because the load is partially supported by the wheels  36 , the magnitude of the force exerted onto the skid-steer boom arm is reduced when compared to a standard pallet fork attachment which has no such additional support mechanism. The relative positions of the casters  38  and the center of gravity (CG) of the load tend to reduce the force/moment applied by the load onto the mounting plate  34 . Ideally, if the CG of the load is directly above the caster support location as viewed in FIG. 3, there will be zero force/moment imposed on the mounting plate  34 . Accordingly, the location of caster  38  is selected so that the point of contact P with the ground is disposed in a vertical plane lying proximate a center of the horizontal portion of the forks  42 , i.e. the center of gravity of a load to be supported on the forks  42 . One will appreciate that as the skid-steer loader is moved alternately from the forward direction to the rearward direction, the position of the point of contact P will move relative to the frame member  32  due to the rotational action of the casters  38 . In one embodiment, the center of the horizontal portion of forks  42  is aligned along a vertical plane with the swivel axis A of the caster  38 . In this embodiment, the expected center of gravity CG of a load on the forks  42  will remain proximate the point of contact P for all positions of caster  38 . In one such embodiment, lifting device  30  is configured to have the point of contact P of the wheels  36  to be approximately 30 inches forward of the mounting plate  34 , with the length of the horizontal portion of forks  42  being approximately 60 inches. Such a device may have a fork lifting capacity of 5,000 pounds when connected to a skid-steer loader that otherwise would have a fork lifting capacity of only 1,700 pounds. The prior art device of FIGS. 1 and 2 also functioned to reduce the load exerted onto the boom arm when compared to a standard pallet fork attachment, however, because of the forwardly extended wheel position, the benefit was not as great as is obtained with the present invention. Other embodiments of the present invention may be envisioned where the center of the horizontal portion of forks  42  extends away from the hard contact location beyond the point of contact P or alternatively does not extend as far as the point of contact P, depending upon the specific requirements of the application regarding weight to be lifted, shape of the load to be lifted, and surrounding obstructions.  
         [0019]    The frame member  32  and the caster assembly  38  are also selected to allow forks  42  to extend beyond the forward-most position of wheels  36  away from the location of attachment to the skid-steer loader. This enables the lifting device  30  to be used to lift and to move objects that may be wider than the distance between the wheels  36 . An object such as a length of pipe may be placed onto the forks  42  forward of the wheels  36  at a location  46  on the forks  42  and extending beyond the width of the wheels  36  and frame member  32 . Furthermore, the geometry of device  30  enables the operator to lift a pallet or other object placed on a shelf where it is impossible to move the wheels  36  under such shelf. This situation may occur, for example, when a pallet is loaded onto a trailer and the wheels of the trailer block the movement of wheels  36  under the trailer. To achieve such a lift, the forks  46  are first moved at least partially under the object until the wheels  36  or frame member  32  strike the blocking structure. The lifting mechanism  40  is then energized sufficiently to engage the forks  46  under the object so that, as the skid-steer loader is backed away from the shelf, the object will slide somewhat over the edge of the shelf. The upward force on the forks  46  is then reduced and the skid-steer loader is moved forward against the blocking structure to further extend the forks  46  under the load. This process is repeated as necessary until the forks  46  are fully inserted under the load, whereupon the load is lifted and moved in normal fashion. In this manner a load having a magnitude larger than the rating of a standard pallet fork attachment may be moved from a position that is not accessible to the prior art device illustrated in FIGS. 1 and 2.  
         [0020]    [0020]FIG. 5 illustrates a lifting apparatus  60  including the lifting device  30  of FIGS. 3 and 4 attached to a prior art skid-steer loader  62 . The skid-steer loader moves on a pair of front wheels  64  and a pair of rear wheels  66 . As is known in the art, the front and rear wheels  64 ,  66  on each respective side of a skid-steer loader are driven together with a chain drive. The vehicle is turned by driving the paired wheels on opposite sides of the loader  62  in opposite directions. This type of operation causes the wheels to skid during a turn, thus the name skid-steer loader. Such skidding will destroy sod or other surfaces over which the loader is operated. The lifting apparatus  60  is illustrated in a mode of operation where the front wheels  64  are lifted off of the ground surface  68  by moving the boom arm  70  downward beyond the point where the two portions of universal coupler  72  align naturally. Because frame member  32  is held in position by wheels  36 , the reaction to the downward movement of boom  70  is an upward movement of front wheels  64 . In this position, the lifting apparatus  60  may be turned by driving the opposed rear wheels  66  in opposite directions. The hard connection provided by coupler  72  and the action of casters  38  allow the lifting apparatus  60  to make smooth turns without significant damage to the underlying surface  68 . Since this type of operation is inherently less stable than operation with all six wheels on the ground, an inclinometer  74  may be provided at a convenient location on the lifting apparatus  60  in view of the operator so that movement on hills exceeding a predetermined slope may be avoided. Furthermore, the hydraulic lifting mechanism  40  may include automatic controls for limiting the weight that may be lifted on the forks  42 .  
         [0021]    While the preferred embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those of skill in the art without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.