Abstract:
A hoist for elevating cargo onto the bed of a truck includes a frame which may be mounted to the truck bed, a platform or container which is connected to the frame by a pivoting arm, and an arrangement of linkages for rotating the arm to position the platform either on the truck bed or on a loading and unloading position resting on the ground behind the truck. A threaded screw and an arm tube are connected to the arm via the linkages, with the linkages being arranged to increase the effective “flow” of the arm to provide a long travel distance of the arm. Rotational movement is imparted to the screw by electric, pneumatic and hydraulic systems to extend or retract the cantilever arm as desired.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This Continuation-in-Part application claims the benefit of U.S. patent application Ser. No. 10/477,805, filed, Nov. 14, 2003, a National application claiming the benefit of PCT/CA02/00685, having a priority date of May 14, 2001. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to hoists and more particularly to a hoist for elevating a load-bearing platform from the ground to an open bed of a pickup truck and for returning the platform to the ground using a screw driven by pneumatic or hydraulic or electric systems, each system operated directly from the battery of the pick up truck. 
       BACKGROUND OF THE INVENTION 
       [0003]    Pickup trucks and other vehicles having a cargo-carrying bed are commonly used to transport loads which are too heavy to be manually hoisted, for example, a user might wish to load a small and light vehicle such as a snowmobile, all terrain vehicles (“A.T.V.”) or riding lawn mowers. Such loads are usually loaded onto the truck by pushing or driving them up a ramp and onto the open bed of the truck. Loading of the truck bed in this manner can result in damage to the vehicles or other objects being loaded and injury to persons loading them. An example is the case of loading a vehicle onto a truck bed, using a ramp. The upper edge of the ramp must typically rest solidly on the rear bumper of the truck while the vehicle is being driven up the ramp. If the upper edge overhangs the bumper somewhat, the ramp will become unstable when the weight of the front wheels of the vehicle being loaded is on the overhang. At this point, the ramp will rock and become unstable and the driver may lose control of the vehicle. 
         [0004]    On the other hand, if the upper edge of the ramp overhangs the bumper insufficiently, vibration of the vehicle on the ramp may cause the upper edge to fall off the bumper with resulting damage to the vehicle and Injury to the driver. 
         [0005]    Similarly, a heavy load such as a large hay bale is manually loaded onto a truck bed only with great difficulty. 
         [0006]    It is known to provide a hoist to assist in the loading or unloading of a truck, consisting generally of a cargo-carrying platform or container for resting on a truck bed and a arm for moving the platform between its travelling position on the truck bed and a loading position where it rests on the ground directly behind the truck. Thus, a heavy load can be readily pushed or driven onto the platform while the platform Is resting flat on the ground. The load is then secured and the platform hoisted onto the truck bed. An actuator such as a hydraulic ram, linked to a arm, moves a platform between the travel position supported on the truck bed and loading/unloading position resting on the ground. For example, U.S. Pat. No. 4,175,904 to Airaksinen discloses a container and associated hoist mechanism, whereby the container is moved by a arm between a position on a truck bed and resting on the ground directly behind a truck bed, with the hoist arranged for moving the container between the two positions. Similar arrangements are disclosed in U.S. Pat. No. 5,601,393 to Waldschmitt and U.S. Pat. No. 6,152,674 to Ogrodnick. 
         [0007]    It is desirable to provide a hoist, including an associated container or platform, suitable for use with a relatively light vehicle such as a light truck or pickup truck, which is also capable of bearing and hoisting a relatively heavy load such as a pair of snowmobiles. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention relates to an improved hoist which includes a moveable load-bearing platform which is moveable between a pickup truck bed and a position resting at ground level, and further to a linkage arrangement for moving the platform. 
         [0009]    I have invented an improved hoist that will elevate a platform from the ground to an open bed of a pickup truck. A vehicle such as a snowmobile, an A.T.V. and a riding mower can be driven onto the platform while the platform is resting solidly on the ground. The driver can then get off the vehicle and the platform. The hoist will then safely elevate the platform, bearing the vehicle resting on the platform and place these onto the open bed of the pickup truck. The vehicle is then in position to be transported on the pickup truck. Reversing the action of the hoist allows the vehicle to be unloaded from the pickup truck. 
         [0010]    The hoist can be bolted into the bed of a pickup truck, can be operated without invading the bed of the pickup truck or requiring holes in the bed of the pickup truck other than the bolt securing holes, and the hoist can be easily removed, freeing the pickup truck for other tasks. 
         [0011]    Briefly the hoist of my invention includes a moveable load-bearing platform, a framework adapted to be mounted on the open bed of a truck and having a longitudinally extending stringer; a cantilever arm being dogleg L-shaped and pivotal relative to the stringer and being pivotally interconnected to the platform; and an actuator which is pivotal relative to the stringer and which, when retracting, causes the cantilever to pivot toward the actuator with resulting raising of the platform. The actuator comprises a longitudinally extending member such as a threaded screw, which may be operated by an electric motor driven directly from the truck battery, or pneumatic, or hydraulic devices may also be employed. An arrangement of linkages connects the actuator with the cantilever dogleg L-shaped arm to increase the leverage and “reach” of the actuator thereby permitting the actuator to be relatively short in comparison with the travel distance required of the cantilever dogleg L-shaped arm. For example, it is desirable to provide a sufficient movement of the cantilever dogleg L-shaped arm to lower the platform significantly below the truck wheels. This permits loading of watercraft, or of the platform on an irregular surface. The linkage arrangement comprises a first connecting link member interposed between the actuator and the cantilever dogleg L-shaped arm, this first link member being pivotally joined at a first end to the actuator. The second end of the link member is pivotally engaged to the cantilever dogleg L-shaped arm, at a point close to the pivot mount of the cantilever dogleg L-shaped arm to the stringer. A second, generally upstanding anchoring link is pivotally mounted at its lower end to the stringer, at a point between the actuator and the cantilever dogleg L-shaped arm. At its upper end the upstanding link is pivotally joined to the pivot mount joining the actuator and the first link. The actuator and first link form a scissors-like arrangement which unfolds as the actuator extends, thus increasing the “reach” and lifting force of the actuator. The second link serves to maintain longitudinal rigidity of the link system, i.e. to permit the horizontal thrust of the actuator to be transmitted to the cantilever dogleg L-shaped arm. 
         [0012]    In one preferable version, the links are arranged such that they meet at an elbow junction at approximately a 90 degree angle when the actuator and cantilever dogleg L-shaped arm are extended for depositing the platform at ground level. In this position, a retractive force of the actuator may be translated into an upward lifting action of the second link arm acting on the main cantilever dogleg L-shaped arm. The second link arm joins with the cantilever dogleg L-shaped arm at an angle of between 45 and 90 degrees when in this extended position, thereby enhancing the upward lifting and inertial action of the actuator. This shallow angle is desirable to permit the first link to transfer the weight of the cantilever dogleg L-shaped arm against the frame when the actuator retracts or extends. 
         [0013]    In another version the actuator is a threaded screw and an arm tube threadably supported on the screw. 
         [0014]    Preferably, the platform includes a at least one roller or wheel mounted at its distal end, to facilitate fore and aft movement of platform as the same as loaded and unloaded. A further desirable feature is fore and aft upstanding members at the respective fore and aft ends of the frame, for elevating and supporting the platform above the truck bed, thus permitting space between the truck bed and the platform for the various linkages and actuator. The rearward (aft) member desirably includes one or more rollers to ease the sliding of the platform over the rearward supporting member. 
         [0015]    Directional references such as “horizontal” and “vertical” are used for convenience of description. It will be understood that such references are used in relation to the device on a substantially horizontally-disposed truck, and that such terms also include a reasonable and mechanically effective range of angles or positions. 
         [0016]    Expressions herein such as “substantially” or “approximately” in reference to dimensions and angles of the invention, cover a range of plus or minus 20% of any expressed length or 15 degrees from any expressed angle. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0017]    The hoist of the invention is described with reference to the accompanying drawings and photographs. In the drawings: 
           [0018]      FIG. 1  is an elevation of the hoist in combination with a platform; 
           [0019]      FIG. 2  is a plan view of the lower framework of the hoist; 
           [0020]      FIG. 3  is an elevation of the hoist and the platform in position to be loaded; 
           [0021]      FIG. 4  is an elevation of the hoist and the platform in position to be transported; 
           [0022]      FIG. 5  is an elevation of the lower framework from the side; 
           [0023]      FIG. 6  is a plan view of a roller illustrated in  FIG. 2 , In enlarged scale; 
           [0024]      FIG. 7  is a perspective view of the hoist position of the device, with the platform not shown, mounted to the bed of a pickup truck; 
           [0025]      FIG. 8  is a perspective view of the device mounted in a bed of a pickup truck, showing in particular the platform portion; 
           [0026]      FIG. 9  is a plan view of a second embodiment of the hoist  FIG. 10  is a perspective view of the screw device as it would be mounted in a bed of a pickup truck, not show, powered by an electric motor; 
           [0027]      FIGS. 11A-C  show a perspective, side, and bottom view, respectively, of the screw, screw arm, screw coupler, gear box, and the electric motor in operative position; 
           [0028]      FIG. 12  is a schematic view of the components and their wiring connections; 
           [0029]      FIG. 13  is a perspective view of the screw device as it would be mounted powered by a pneumatic motor; 
           [0030]      FIGS. 14   a - c  show a perspective, side, and bottom view, respectively, of the screw, screw arm, screw coupler pneumatic motor, motor mount, and electric pneumatic pump in operative position; 
           [0031]      FIG. 15  is a schematic view of the electrical components and the pneumatic drive components; 
           [0032]      FIG. 16  is a perspective view of the screw as it would be mounted powered by a hydraulic motor; 
           [0033]      FIGS. 17   a - c  show perspective, side, and bottom views, respectively, of the screw, screw arm, screw coupler, hydraulic motor, electric hydraulic pump and motor mount in operative position; and, 
           [0034]      FIG. 18  is a schematic view of the electrical and hydraulic drive components. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0035]    With reference to  FIGS. 1 ,  2  and  8  the hoist of the invention, generally  10 , is shown in conjunction with a platform, generally  12 . Hoist  10  has a lower framework  11  which includes a pair of stringers  14 ,  16 , which are fastened to the open bed of a pickup truck  2  and extend longitudinally from the forward wall of the box adjacent to the cab of pickup truck  2  to the rear edge of the bed. Lower framework  11  is sized to conveniently fit within the bed of pickup truck  2 . It will be seen that the frame can be scaled up or down in size to permit mounting on a desired size pickup truck  2 . The frame is preferably removably mounted by the bed, of pickup truck  2  for example by bolting to the bed of pickup truck  2 . The invention is illustrated and described herein in association with a conventional pickup truck  2 . It will be understood that any vehicle having a suitable bed will accommodate the invention. 
         [0036]    As illustrated in  FIGS. 7 and 8 , the rear gate of pickup truck  2  is removed to accommodate hoist  10  and platform  12 . 
         [0037]    A rod  18  extends between stringers  14 , 16 . A cantilever dogleg L-shaped arm  19  is pivotally mounted to rod  18  between stringers  14 ,  16  at first pivot mount  17 . The cantilever is a rigid unitary member comprising of three interconnected segments, an inner segment is  19   a , an intermediate segment  19   b  and an outer segment  19   c . The first angle, between intermediate segment  19   b  and outer segment  19   c  is about 90 degrees while the second angle, between inner segment  19   a  and intermediate segment  19   b  is about 160 degrees forming the dogleg portion of the L-shaped cantilever arm. While the preferred first angle is about 90 degrees, it is contemplated that a range of angles would be suitable, for example, between 60 degrees and 120 degrees. An angle of substantially 90 degrees is preferable to permit the cantilever to fit snugly against the front wall of platform  12  when platform  12  is in the “travel” position on the bed of pickup truck  2  (e.g.  FIG. 4 ). It will be seen that platform  12  may include a front wall which is canted away from the vertical, and this would lead to a preference that the angle of cantilever arm  19  between intermediate segment  19   b  and outer segment  19   c  is correspondingly changed. The second angle, between intermediate segment  19   b  and inner segment  19   a  of the cantilever arm  19 , may range between 180 and 140 degrees. The provision of an angle at this position is desirable for permitting the arrangement of linkage, described below, to effectively operate. However, if the first angle is adjusted suitably to other than 90 degrees, it is contemplated that the second angle may be about 180 degrees. Cantilever dogleg L-shaped arm  19  may comprise a rigid metal beam, having a box-like or I-beam cross-sectional configuration. Pivot mount  120  on cantilever arm  19  provides means for attachment of cantilever arm  19  to platform  12 . 
         [0038]    An actuator  22  is pivotally mounted to stringers  14 ,  16  adjacent to their forward ends. Actuator  22  may be a hydraulic cylinder  24  and hydraulic ram  26  as disclosed in copending application Ser. No. 10/477,805, or it may be a threaded screw  126  and an arm tube  128 , which may be operated by an electric motor  122 , or by a pneumatic pump  172 , or by hydraulic pump  191 . Battery  150  of pickup truck  2  is used to operate these devices. The free end of piston  26  is pivotally connected at pivotal interconnection  39  to first and second links  30 ,  32 . First link  30  is pivotally connected to rod  34  which extends between stringers  14 , 16 . Second link  32  is pivotally connected to cantilever dogleg L-shaped arm  19  at  36 . 
         [0039]    First link  30  and second link  32  are arranged to increase the lifting power of actuator  22  for rotating cantilever dogleg L-shaped arm  19 . In particular, the purpose of first link  30  and second link  32  is to convert a generally horizontal force of actuator  22  into a substantially vertical force for retracting cantilever dogleg L-shaped arm  19 , particularly when cantilever arm  19  is in the fully extended position. For this purpose, first link  30  and second link  32  are arranged such that first link  30  is mounted to the frame at a position between actuator  22  and cantilever dogleg L-shaped arm pivot mount  17 , and second link  32  is mounted to cantilever arm  19  at a position removed from its pivot mount  17 . As will be seen in  FIG. 3 , when cantilever dogleg L-shaped arm  19  is generally fully extended, first link  30  and second link  32  meet at a substantially right angle at pivotal interconnection  39 . First link  30  is about the same length as second link  32 , thereby permitting the links to be disposed at about the same angle (i.e. about 45 degrees from the horizontal), when cantilever dogleg L-shaped arm  19  is in the extended position as seen in  FIG. 3 . This reasonably shallow angle permits first link  30  to transfer a forward thrust in an efficient manner to the frame, when actuator  22  is retracted. This permits first link  30  and second link  32  to provide lift and inertia to cantilever dogleg L-shaped arm  19  as actuator  22  is retracted (and similarly as actuator  22  is extended to lower cantilever dogleg L-shaped arm  19 ). The linkage arrangement also makes it easier for actuator  22  to be stopped at any position between the extended and retracted positions. It will be seen that a preferred system provides for first link  30  and second link  32 , and their respective contact points to be arranged such that second link  32  meets the inner segment  19   a  at an angle of between about 350 and 90.degrees from the vertical, when inner dogleg segment  19   a  of cantilever arm  19  is horizontal. As suitable arrangement provides an angle of about 45.degree. It will however be seen that the right angle is not critical and first link  30  and second link  32  may meet at a range of angles when inner segment  19   a  is horizontal and still serve their desired function. Further, cantilever dogleg L-shaped arm  19  may extend further downwardly from the position shown at  FIG. 3 , with first link  30  and second link  32  and actuator  22  rotating downwardly in a corresponding fashion. In effect, first link  30  and second link  32  define a folding arm with a pivotal elbow. The elbow is defined by pivotal interconnection  39 , with the free end of the piston  26  being pivotally joined to the arm at the elbow joint. Pivotal interconnection  39  may comprise a single pivot connecting the actuator arm and first link  30  and second link  32 , or alternatively it may comprise two spaced apart pivots. In one example, the extended actuator length is 477/8 inches, first link  30  has a pivot-to-pivot length of 11.5 inches and second link  32  has a pivot-to-pivot length of 12 inches. 
         [0040]    With reference to  FIGS. 2 ,  5  and  6 , a first transverse bar  40  extends upwardly from the forward ends of each stringer  14 , 16  and a second transverse bar  42  extends upwardly from the rear ends of each stringer  14 , 16 . First transverse bar  40  and second transverse bar  42  are mounted to upstanding supports  43 . Rollers  44 ,  46  are mounted at the upper ends of upstanding supports  43 . First transverse bar  40  and second transverse bar  42  provide a support for platform  12 . Rollers  44 ,  46  are preferably journalled in a conventional roller bearing mount, on axles  49 . Axle mounts  47  support rollers  44 ,  46  on second transverse bar  42 . Rollers  44 ,  46  have a generally hourglass shape to permit platform rails  61  to “track” on rollers  44 ,  46  as platform  12  slides on or off the bed of pickup truck  2 . 
         [0041]    Hoist  10  illustrated in  FIGS. 1 through 8  has a single cantilever dogleg L-shaped arm. In a second embodiment, shown at  FIG. 9 , hoist  90  has two cantilever dogleg L-shaped arms  19  and associated actuators  22  and first links  30  and second links  32  in a paired side-by-side arrangement. The number of cantilever dogleg L-shaped arms is a matter of choice and will depend upon such factors as the proposed maximum loaded weight of the platform, the distribution of the weight and so on. 
         [0042]    With reference to  FIGS. 3 and 8 , platform  12  includes side walls  52 ,  54 , a front wall  56 , a rear gate  58  and rollers or wheels  60 ,  62  which extend from the rear of platform  12 . A pair of side rails  59  forms part of lower framework  11  of platform  12 . The outer end of cantilever dogleg L-shaped arm  19  is pivotally connected to front wall  56  of platform  12  by way of pivot mount  20 . The platform bed may include apertures to receive vehicle wheel blocks to prevent the carried vehicle from rolling. Platform  12  may also include anchors to hold tie-downs to stabilize a load on platform  12 . Rear gate  58  swings open about either side or bottom-mounted hinges to permit a vehicle to be driven onto platform  12  or a load to be rolled or slid thereon. Alternatively, rear gate  58  may be lifted out of side brackets  63  for complete removal. 
         [0043]    The elevated first transverse bar  40  and second transverse bar  42  to support platform  12 , elevated above the linkages and threaded screw  100  of the hoist. 
         [0044]    The operation of hoist  10  is as follows: platform  12  is positioned to rest on the ground directly behind pickup truck  2  as seen in  FIG. 3 . For this purpose, actuator  22  is extended, thereby rotating the cantilever dogleg L-shaped arm  19  into its extended position. The load to be carried by pickup truck  2  is then placed on platform  12 . If, for example, the load is a snowmobile or an A.T.V., the vehicle is driven onto platform  12 . Rear gate  58  of platform  12  is then closed. The vehicle, or other load, is secured to platform  12  by means of wheel blocks, tie downs or other suitable securing means. Actuator  22  is then activated to retract piston  26 . As piston  26  retracts, first link  30  and second link  32  fold together and cantilever dogleg L-shaped arm  19  is rotated forwardly. As actuator  22  is retracted, the thrusting retractive force of piston  26 , which is disposed in a near horizontal position, is converted by means of first link  30  and second link  32  into a generally upward retractive force in the manner explained above. The leading edge of platform  12  is drawn forwardly and upwardly, with rails  61  of platform  12  sliding over rollers  44 ,  46 , as illustrated in  FIG. 1 . The rear edge of platform  12  is supported by wheels  60 ,  62 . When platform  12  reaches the position illustrated in  FIG. 1 , platform rails  61  contacts rollers  44 ,  46  and platform  12  then rolls forward in the box of the pickup truck  2 , with continued forward movement of platform  12  causing wheels  60  and  62  to leave the ground until platform  12  is fully supported in a generally horizontal position on first transverse bar  40  and second transverse bar  42 . 
         [0045]    Preferably, a control unit  80  (see  FIG. 1 ) is provided which controls the operation of actuator  22 . Control unit  80  includes conventional controls for controlling threaded screw  100 , including a remote unit  82  (seen in  FIG. 1  as connected by a dotted line to control unit  80 ) for actuation by a user standing along side or behind pickup truck  2 . Control unit  80  may include an emergency shut-off switch which is triggered by e.g. a sudden increase in load indicative of a jam. Remote unit  82  may include a “dead man” switch. Control unit  80  and threaded screw  100  are powered from the electric power supply of pickup truck  2 , either by way of a cigarette lighter attachment or a dedicated electrical connection. 
         [0046]    When outer segment  19   c  of the cantilever dogleg L-shaped arm  19  is vertical as illustrated in  FIG. 4 , platform  12  will be horizontal and fully supported by first transverse bar  40  and second transverse bar  42 . Preferably, platform  12  then will be retained within the confines of the truck bed and not overhang the edges of pickup truck  2 . 
         [0047]    It will be seen that any suitable vehicle may be used in association with this invention for carrying hoist  10 , such as a flat-bed truck, a farm or construction vehicle, a towed vehicle such as a flat-bed trailer, and any other suitable vehicle. 
         [0048]    With reference to FIGS.  10 , 11 A-C and  12 , hoist  110  has a lower framework  111  which includes a pair of stringers  114 ,  116 , which are fastened to the open bed of a pickup truck  2  (see  FIG. 7 ) and extend longitudinally from the forward wall of the box adjacent to the cab of pickup truck  2  to the rear edge of the bed. Lower framework  111  is sized to conveniently fit within the bed of pickup truck  2 . 
         [0049]    A rod  118  extends between stringers  114 ,  116 . A cantilever dogleg L-shaped arm  119  is pivotally mounted to rod  118  at its base or bottom end  117  between stringers  114 ,  116 . Cantilever arm  119  is a rigid unitary member comprising of three interconnected segments, an inner segment, the dogleg, is  119   a , an intermediate segment  119   b  and an outer segment  119   c . The first angle, between intermediate segment  119   b  and outer segment  119   c  is about 90 degrees while the second angle, between inner segment  119   a  and intermediate segment  119   b  is about 160 degrees forming the dogleg portion of L-shaped cantilever arm  119 . While the preferred first angle is about 90 degrees, it is contemplated that a range of angles would be suitable, for example, between 60 degrees and 120 degrees. An angle of substantially 90 degrees is preferable to permit the cantilever to fit snugly against the front wall of platform  12  when platform  12  is in the “travel” position on the bed of pickup truck  2  (e.g.  FIG. 4 ). Cantilever dogleg L-shaped arm  119  may comprise a rigid metal beam, having a box-like or I-beam cross-sectional configuration. 
         [0050]    Pivot mount  120  on cantilever arm  119  provides means for attachment of cantilever arm  119  to platform  12 . 
         [0051]    An actuator  22  is pivotally mounted to stringers  114 ,  116  adjacent to their forward ends by rod  13  extending through pivot mount  121  and stringers  114  and  116 . Actuator  22  is a threaded screw  126 , which may be operated by an electric motor  122 , or by a pneumatic pump  172 , or by an electric hydraulic pump  191 . Truck battery  150  is used to operate these devices. The free end of screw  126  is pivotally connected at pivotal connection  138  to first link  130 . First link  130  is pivotally connected to stringers  114  and  116  at pivotal connection  134 . The other end of first link  103  is pivotally connected to second link  132  at pivotal connection  139 . First link  30  is pivotally connected to rod  34  which extends between stringers  14 , 16 . Second link  32  is pivotally connected to cantilever dogleg L-shaped arm  19  at  36 . 
         [0052]    First transverse bar  140  and second transverse  142 , which extend across stringers  114  and  116  have supports  143 , which support rollers  144  and  146 . 
         [0053]    As shown in  FIGS. 10 through 12 , when threaded screw  126  and arm tube  128  are operated by electric motor  122 , pivot mount  121  pivotally secures electrically operated screw  126  and arm tube  128  by means of rod  131 , which extends through stringer  114  through pivot mount  121  and through stringer  116 . Pivot mount  121  is rigidly affixed to gear box  124 . Electric motor  122  is operatively connected to gear box  124 , which rotates screw coupler  127  and in turn imparts rotation motion to screw  126 . Arm tube  128  is threadably attached to screw  126  by arm bushing  129  whereby the rotational motion imparted to screw  126  extends or retracts arm tube  128  along the length of screw  126 , thus lengthening and shortening arm tube actuates movement of cantilever dogleg L-shaped arm  119  through its entire motion from fully extended to fully retracted provided the motion necessary to move platform  12  from fully loaded to sitting on the ground (see  FIGS. 3 and 4 ). 
         [0054]    First link  130  is rotatably attached at its base end on rod  134 , which extends between stringers  114  and  116  and rotatably supports first link  130  therebetween. Second link  132  is pivotally attached along the length of inner segment  119   a  of cantilever arm  119 . Rod  136  passes through box sides of inner segment  119   a . Second link  132  extends between gusset members  147 , which are spaced apart along the inside of the angle formed by inner segment  119 A and intermediate segment  119   b  of cantilever arm  119 . Second end  133  of second link  132  is rotatably attached to first link  130  by pivot  139 . Pivot  139  extends between two upper distal ends of first link  130 . Second end  125  of arm tube  128  is rotably attached to pivot  138  along the length of first link  130  spaced below pivot  139 . Hole  137  in distal end  125  of arm tube  128  to receive pivot  138 . 
         [0055]    The linkage system is arranged in the form of a tetragon having a base or bottom member made up of a portion of stringers  114  and  116 , and having two upright members, first link  130  and the lower portion of dogleg inner segment  119   a  of cantilever arm  119 , and having an upper member second link  132 . These linkage members and the tetragon they form have non-parallel opposing sides which interact to transmit for force exerted by screw  126  and arm tube  128  to dog leg L-shaped cantilever arm  119  to extend or retract the arm to unload or load platform  12  as desired. The tetragon constantly changes shape as cantilever arm  119  moves through its range of motion from fully retracted to fully extended. Threaded screw  126  and arm tube  128  are exterior to the tetragon and extend forwardly therefrom. At their forward end, threaded screw  126  and arm tube  128  are pivotally attached to stringers  114  and  116  by rod  131 . 
         [0056]    The schematic of the electrical system, shown in  FIG. 12 , shows battery  150 , which is the battery of pickup truck  2 , connected through fuse box  151  to junction box  152 . Pendant  153  for remote operation of hoist  110  is also attached to junction box  152 , as well as limit switches  154  and  155 . Limit switch  154  stops motion in the fully retracted position and limit switch  155  is positioned to limit motion in the fully extended position. From junction box  152 , electric motor  122  is powered and gear box  124  is driven to impart rotation to screw  126  to extend and retract arm tube  128  as desired as previously described. 
         [0057]    As shown in  FIGS. 13 through 15 , the pneumatic drive system is shown comprising hoist structure  110  as previously described and replacing the electric drive with the pneumatic motor  170 , the electric pneumatic pump  172  and air lines  173  and  174 , and, motor mount  175 , which supports pivot mount  121 . Pivot mount  122  allows screw  126  and arm tube  128  to pivot to accommodate the movement of first link  130  and second link  132  and dogleg L-shaped cantilever arm  119  as needed while screw  126  is rotated to extend or retract arm tube  128  as desired. 
         [0058]    Electric pneumatic pump  172  is operatively connected to junction box  152 , which is otherwise connected as previously described, including battery  150 , pendant  153  and fuse box  152 , limit switches  154  and  155 . Electric pneumatic pump  172  is connected by airlines  173 ,  174  to pneumatic motor or drive  170 . 
         [0059]    As shown in  FIGS. 16 through 18 , hydraulic motor  190  drives electric hydraulic pump  191  with junction box  152  operatively connected therewith as previously described, having hydraulic fluid reservoir  194  operatively connected therewith and hydraulic lines  192  and  193  carrying hydraulic fluid to and from hydraulic drive  190 . Hydraulic drive  190  imparts rotation to screw coupler  127  thereby imparting motion to arm tube  128 . 
         [0060]    It will be understood that modifications can be made in the embodiments of the hoist illustrated and described herein without departing from the scope and purview of the invention.