Patent Publication Number: US-2010124479-A1

Title: Lift assembly for a vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Patent Application having Ser. No. 61/115,142 filed on Nov. 17, 2008, the entire disclosure of which is expressly incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to lifting devices and more specifically to detachable platform lift assemblies adapted for connection to a vehicle such as a pickup truck through the vehicle&#39;s tubular trailer hitch receiver, and which can be installed and used by an individual without altering or modifying the vehicle. 
     2. Description of the Related Art 
     Various vehicle-mounted platform lifting devices have been in use for years. Typically, these devices include a platform that is raised and lowered vertically immediately behind a truck cargo bed between ground level and a desired height, which typically corresponds to the cargo bed floor level. 
     The main problem with many such conventional lifting devices is that the vehicle has to be permanently modified for product use. For example, the bumper may require removal. Alternatively or additionally, the lift assembly may require extensive structural interconnection to the truck such that it becomes significantly inconvenient to use it on a temporary basis. Another problem with such conventional lifting devices is that a user usually cannot install the device himself or remove it when he is finished using it. While these devices may be suitable for the particular, dedicated purpose, they generally are unsuitable for most users who do not wish to expend considerable time and effort, or to modify their vehicles, undertaking the permanent lift assembly installation normally entailed. 
     Another problem with some other conventional lift assemblies for use with pickup trucks is that they operate between the ground and the rear edge of the truck&#39;s lowered OEM tailgate, which has severely limited loading capacity compared to the cargo bed itself. OEM tailgates are not designed to accommodate loads greater than a few hundred pounds. Often the tailgate suspension cables, which extend between the vertical rear bed rails and the opposed side edges of the tailgate, are the weak link, and have been know to break under a tailgate load of only a couple of hundred pounds. Further, many conventional lift assemblies for pickup trucks are limited in capacity to only about 500 pounds. Most lift assemblies capable of handling significantly heavier loads are of the type requiring vehicle modification and/or extensive structural interconnection, as discussed above. 
     Other prior platform lift assemblies, particularly those light-duty versions that raise the platform to the edge of the truck&#39;s lowered tailgate, are attached to the rear of the truck through its trailer hitch receiver, which is typically located below the bumper and fixed to the vehicle frame. A problem with such prior lift assemblies is that they often substantially encroach into the truck&#39;s departure angle. Referring to  FIG. 1 , the departure angle θ is an indication of how a vehicle can drive off a steep hill or ramp without contacting underside elements aft of its rear wheels. When looking at a truck profile from the side, an imaginary straight line extends from the contact patch of the rear tire to the lowest hanging point of the truck behind its rear wheels. The angle between this line and the ground is the departure angle θ. Vehicles with a high departure angle θ can negotiate steep ramps without inflicting damage to these low-hanging elements. The departure angle is generally a function of vehicle ground clearance behind the rear wheels and the vehicle&#39;s longitudinal overhang distance between the rear axle and the lowest hanging points behind it. Referring to  FIG. 1 , the trailer hitch  20  inserted into the depicted truck&#39;s trailer hitch receiver  22  defines its departure angle θ T  which is clearly less than the departure angle would be with the receiver alone. Typically, most receiver-mounted lift assemblies according to the prior art will, compared to a trailer hitch  20 , further decreases the departure angle θ. It is desirable to provide a lift assembly that, when installed, avoids reducing a vehicle&#39;s departure angle as much as possible, and preferably to no less than the departure angle θ T  defined by a trailer hitch that might otherwise be inserted into the receiver. 
     There is a need for a high capacity lift assembly that avoids the load being supported by the truck&#39;s OEM tailgate, can be quickly and easily installed by an individual without vehicle modification, and does not reduce the departure angle to an undesirable level. 
     SUMMARY OF THE INVENTION 
     The present invention provides a new portable hydraulic platform lift assembly construction wherein the same can be utilized for allowing an individual to be able to use a high capacity, hydraulically controlled lift on a vehicle without having to alter or modify the vehicle, and to allow total portability of lift assembly. Further, the inventive lift assembly is easily installed to the vehicle&#39;s trailer hitch receiver by the individual user, and preserves the vehicle departure angle to a significant amount relative to prior art lift devices. 
     The general purpose of the present invention, which will be described in greater detail below, is to provide a new portable hydraulic platform lift that has many of the advantages of the lifting devices mentioned heretofore and many novel features that result in a new portable hydraulic platform lift which is not anticipated, rendered obvious, suggested, or even implied by any of the prior art lifting devices, either alone or in any combination thereof. 
     Certain embodiments of the present invention include an interchangeable platform attached to a frame including a four-bar linkage that is actuated by a hydraulic system that includes a control unit and a cylinder and piston assembly. The four-bar linkage controls platform motion and allows for loading heavy objects from ground level up to the bed level of a pick-up truck. A general use platform for most loads to rest upon for lifting is interchangeable with a platform of a different size and/or configuration that is adapted for other, special types of loads (e.g., recreational vehicles) and uses. The lift assembly structure includes an elongate tongue member that is inserted into and secured to the receiver much as a trailer hitch would be, to attach the unit to the vehicle. The frame is connected to the tongue member through a pivoting connection that permits the lift assembly to be rotated into an orientation for being stowed while attached to the vehicle, in which the departure angle is increased. The platform is rotatable between a substantially horizontal position in which the lift assembly is normally operated and a substantially vertical position in which the platform serves as tailgate when the lift assembly is stowed. 
     There has thus been outlined, rather broadly, certain features of an embodiment of the invention in order that the detailed description thereof may be better understood, and in order that the present contribution to the art may be better appreciated. Additional or alternative features of embodiments of the invention are described in further detail below. 
     In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. 
     A primary object of the present invention is to provide a portable platform lift assembly that will overcome at least one of the shortcomings of prior art devices. 
     Another object of the present invention is to provide a portable platform lift assembly for allowing an individual to be able to use a high capacity, controlled lift on a vehicle without having to alter or modify the vehicle, and to allow easy installation and total portability of the lift assembly. 
     Another object is to provide a portable platform lift assembly that can be used to lift heavy objects from ground level at the rear of a vehicle by an individual user in an automated operation. 
     Another object is to provide a portable platform lift assembly that can be fully and easily installed and removed by an individual. 
     Another object is to provide a portable platform lift assembly that will not require any alteration or modification to the host vehicle, or extensive interconnection thereto, for the device to function. 
     Another object is to provide a portable platform lift assembly that has a unique motion that enables the device to operate around a vehicle bumper and allow direct access to a pickup truck&#39;s cargo bed floor, without encountering the OEM tailgate, which is load limiting. 
     Other objects and advantages of the present invention will become apparent to the reader and it is intended that these objects and advantages are within the scope of the present invention. 
     The present invention provides a lift assembly adapted for use with a vehicle having a trailer hitch receiver, the lift assembly including a tongue member having a longitudinal axis and adapted for slidable insertion along the longitudinal axis into the receiver, the lift assembly having lowered and elevated states when the tongue member is in the receiver. A frame is connected to the tongue member and includes at least one four-bar linkage comprising rigid first, second, third, and fourth members, each linkage member having first and second pivot points, the frame defining first, second, third, and fourth substantially parallel pivot axes that are substantially horizontal when the tongue member is in the receiver. The first and second linkage members are pivotably attached to each other at their respective first pivot points through which extends the first pivot axis; the second and third linkage members are pivotably attached to each other at their respective second pivot points through which extends the second pivot axis; the third and fourth linkage members are pivotably attached to each other at their respective first pivot points through which extends the third pivot axis; and the fourth and first linkage members are pivotably attached to each other at their respective second pivot points through which extends the fourth pivot axis. A platform is connected to the frame and has a substantially planar load surface, the first linkage member and the platform moveable together between the lift assembly lowered and elevated states. When the lift assembly is in its elevated state and with no ground-supported portion thereof extending therefrom, the frame has first and second orientations relative to the tongue member. The vertical distance between the tongue member longitudinal axis and the lowermost point of the lift assembly is substantially greater in the first orientation than in the second orientation, a vehicle departure angle defined by the lift assembly increased in the second orientation relative to the first orientation. 
     The present invention also provides a lift assembly adapted for use with a vehicle having a cargo area defining a substantially horizontal floor above ground level and a trailer hitch receiver located vertically between the cargo area floor and ground level, the lift assembly including a tongue member having a longitudinal axis and adapted for slidable insertion along the longitudinal axis into the receiver. A frame is attached to the tongue member and includes a pair of four-bar linkages, the linkages disposed on opposite sides of the tongue member longitudinal axis. Each of the linkages includes rigid first, second, third, and fourth members, with each linkage member having first and second pivot points. The frame has first, second, third, and fourth substantially parallel pivot axes extending between the pair of four-bar linkages, the pivot axes substantially horizontal when the tongue member is in the receiver. Respective to each of the pair of four-bar linkages, the first and second members are pivotably attached to each other at their respective first pivot points through which extends the first pivot axis; the second and third members are pivotably attached to each other at their respective second pivot points through which extends the second pivot axis; the third and fourth members are pivotably attached to each other at their respective first pivot points through which extends the third pivot axis; and the fourth and first members are pivotably attached to each other at their respective second pivot points through which extends the fourth pivot axis. A platform is connected to the frame, has a substantially planar load surface that defines a front edge and is substantially horizontally positioned. The first linkage members and the platform are moved together when the tongue member is in the receiver between a lift assembly lowered state in which the load surface is located at a height between that of the tongue member longitudinal axis and ground level and a lift assembly elevated state in which the horizontally positioned load surface is generally parallel with the cargo area floor and the load surface front edge is proximate to the cargo area floor, the platform having arcuate movement between the lift assembly lowered and elevated states and generally horizontal movement relative to the cargo bed as the lift assembly is moved toward and out of the elevated state. 
     To accomplish the above and related objects, the invention may be embodied in a form illustrated in the accompanying drawings, attention being called to the fact, however, that the drawings are illustrative only, and that changes may be made in the specific construction illustrated. Moreover, it is to be noted that the accompanying drawings are not necessarily drawn to scale or to the same scale. In particular, the scale of some of the elements of the drawings may be exaggerated to emphasize characteristics of the elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various other objects, features and attendant advantages of the present invention will become fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the several views, and wherein: 
         FIG. 1  is a partial side view of a pickup truck having a trailer hitch receiver, to is installed a trailer hitch in accordance with the prior art; 
         FIG. 2  is a partial side view of the pickup truck of  FIG. 1 , its tailgate removed, to which is installed a lift assembly according to an embodiment of the present invention; 
         FIG. 3  is a partial perspective side view of the installed lift assembly shown in  FIG. 2  in an elevated state, with the vehicle shown in phantom lines; 
         FIG. 4  is a rear perspective view of the installed lift assembly shown in  FIG. 2  in a lowered state, with the vehicle shown in phantom lines; 
         FIG. 5  is a side view of the installed lift assembly shown in  FIG. 2  in an elevated state, with the vehicle shown in phantom lines; 
         FIG. 6  is a side view of the installed lift assembly shown in  FIG. 2  in a state intermediate its lowered and elevated states, with the vehicle shown in phantom lines; 
         FIG. 7  is a side view of the installed lift assembly shown in  FIG. 2  in its lowered state, with the vehicle, and the lift assembly in different states intermediate its elevated and lowered states, shown in phantom lines; 
         FIG. 8  is a side view of the lift assembly shown in  FIG. 2  in a stowed configuration, with the vehicle shown in phantom lines; 
         FIG. 9  is an enlarged partial perspective view of one of the ground-engaging stabilizing feet of the lift assembly; and 
         FIG. 10  is a partial perspective side view of the installed lift assembly in an elevated state similar to  FIG. 5 , showing an alternative platform. 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and may herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. Elements shown in more than one Figure that may be similarly configured have been indicated using the same reference numerals. 
     DETAILED DESCRIPTION 
     Turning now to the drawings, in which similar reference numerals denote similar elements throughout the several views, a lift assembly embodiment according to the present invention is illustrated. The shown portable platform lift assembly  30  generally includes a removable, interchangeable platform  32  that is hydraulically actuated and allows for loading heavy objects (not shown) from ground level  34  up to the cargo bed floor level  36  of a pick-up truck  38 . The truck&#39;s tailgate (not shown) is removed prior to installation of the lift assembly  30 , which brings its platform  32  to a position substantially parallel with and adjacent to the end of the truck&#39;s cargo bed floor  46 . The platform  32  has a substantially planar load surface  40  on which the objects of the load to be lifted or lowered are positioned. The platform  32  includes tie-down brackets  48  to which the load may be strapped to secure it during lift operation. 
     Movement of the lift assembly  30  between its elevated  42  and lowered  44  states, wherein the horizontally positioned platform  32  is respectively positioned adjacent to and substantially parallel with the cargo bed floor  46 , and substantially at ground level  34 , is controlled by a pair of four-bar linkages  50  partially defining an articulating frame of the lift assembly. The four-bar linkages  50   a ,  50   b  are substantially identical, laterally spaced, and operate in parallel. They each include four rigid linkage members  52 ,  54 ,  56 ,  58  each having first  60  and second  62  pivot points at which they are interconnected, as described further below. Four substantially parallel pivot axes  64 ,  66 ,  68 ,  70  extend laterally across the lift assembly  30  between the pair of four-bar linkages  50 , and extend through the interconnected pivot points  60 ,  62  of the linkage members. Between the pair of linkages  50   a ,  50   b , two members  68   a ,  68   b  are fixed together through the lift assembly frame  72 , and during lift operation are fixed relative to the vehicle  38 ; and two other members  52   a ,  52   b  are fixed together to support the platform  32 . The remaining four linkage members  54   a ,  54   b ,  58   a ,  58   b  of the pair of four-bar linkages  50   a ,  50   b  are swinging linkage members that move the platform-supporting members  52   a ,  52   b  and the platform  32  through an arcuate path relative to the vehicle  38  between the lift assembly elevated and lowered states  42 ,  44 . 
     In a preferred embodiment, the pivot points  60 ,  62  of each four-bar linkage  50  in the lift assembly  30  are arranged to define a trapezoid, the two swinging linkage members  54 ,  58  interconnecting the vehicle-fixed third member  56  and the platform-supporting first member  52 . The forward-most fourth member  58  which is shown disposed closest to the truck bumper  74  in the elevated state  42 , never drops below the rearward-most second member  54  which is shown disposed furthest from the truck bumper  74  in the elevated state  42 . 
     A torque tube  80  extends laterally between and is fixed to stub arms  82   a ,  82   b  of the fourth linkage members  58   a ,  58   b . Relative to their first pivot points  60  at which the respective third  56  and fourth  58  members are pivotably attached together, the fourth member&#39;s stub arm  82  extends short distance in a direction away from the fourth member&#39;s second pivot point  62  at which it is connected to the platform-supporting first linkage member  52 . The torque tube  80  is secured to the end of each stub arm  82  and fixed against rotation relative to the fourth members  58 . The torque tube  80  is constructed in any desired shape and of any desired material, such as steel. A relatively greater amount of force acting on one  50   a ,  50   b  of the pair of four-bar linkages  50  is partially distributed to the other linkage  50   a ,  50   b  through the torque tube  80 , which promotes simultaneous, smooth, stabilized, equivalent movement of the pair of four-bar linkages. 
     The frame  72  is connected to a tongue member  84  having an elongate structural beam  86  that is longitudinally inserted into and fixed to the trailer hitch receiver  22  of the truck  38 . The tongue member  84 , once in the receiver  22 , supports the lift assembly  30  and the object on the platform  32  that is being lifted or lowered. The frame  72  has a pair of laterally spaced adjustable feet  90   a ,  90   b  connected thereto that are selectively rotated between a laterally extending stowed position and an operating position in which they extend downwardly to engage the ground  34 , which supports and stabilizes the lift assembly  30  during operation. The feet  90  (one of which is shown in  FIG. 9 ) are each provided with an adjusting mechanism  92  that allows its base  94  to drop vertically downward into engagement with the ground  34 , after which the base  94  is secured against upward travel relative to the frame  72  during lift assembly operation. 
     Each foot  90  has a sleeve  96  pivotably attached to one of the laterally opposite ends of the frame&#39;s rearmost structure beam  100 , and is lockable into a horizontally extending stowed position and the downwardly extending operating position shown in the various Figures. The sleeve  96  is provided with a screw  102  having a large, hand-turnable knob  14 . A shaft  106  is telescopically engaged with the sleeve  96  and its exposed terminal end  108  is attached to the ground-engaging base  94 . The foot shaft  106  is limited in its telescopic travel relative to the sleeve  96  by a longitudinally extending slot  110  provided in the shaft  106 , in which the end or tip of the screw  102  is disposed. The end of the screw  12  engages the opposite end walls  112 ,  114  of the slot  110  at the longitudinal limits of shaft&#39;s telescopic travel inside of the sleeve  96 . The slot  110  has a planar floor surface  116  that is angled relative to the shaft surface  118  in which the slot  110  is formed, with the slot  110  having a shallow end  120  near the foot base  94  and an opposite deep end  122 . Once the foot  90  is in its downwardly extending operating position, the screw  102  is then adjusted to allow the foot shaft  106  to drop downwardly under gravity to a ground-engaging position in which the base  94  contacts the ground  34  or an intermediate structure, wherein the lift assembly  30  will be ground-supported. The screw  102  is then brought into engagement with the slot floor  116 . Upward telescopic travel of the foot shaft  106  in the sleeve  96  is prevented by the abutting engagement of the tip of the screw  102  with the inclined floor surface  116 . To permit the foot  90  to telescopically contract, the screw  102  is backed away from the slot floor  116 . The compressive abutting engagement of the screw tip and the slot floor also inhibits the foot&#39;s telescopic extension, but backing the screw  102  slightly out of its engagement with the slot floor  116  will allow the foot  90  to then freely extend to the telescopic limit at which the slot wall  114  at the deep end  122  abuts the screw  102 . The two feet  90 ,  90   b  are independently adjustable, and may be extended to different lengths as conditions warrant. It is preferred, however, that the ground  34  on which the truck is parked  38  and the feet  90  engage during lift operations be level and firm. 
     The tongue member  84  is used to attach and remove the lift assembly  30  from the hitch receiver  22  of the truck  38 . Its elongated beam portion  86  has a longitudinal axis  130  along which it is inserted into and withdrawn from the receiver  22 . Once the tongue member  84  is in the receiver  22 , a pin  132  is laterally extended through aligned holes in the receiver  22  and tongue beam portion  86  to lock their relative position along the now horizontal longitudinal axis  130 , which extends fore and aft relative to the vehicle  38 . A threaded pin  132  may be used in a threaded hole in the tongue member  86  beam to further stabilize the tongue member  84  against side-to-side rocking or wobbling within the hitch receiver  22  as necessary. Alternatively, a specially designed, tapered pin  132  may be inserted into the aligned holes of the receiver  22  and the tongue member beam  86 , and tightened to secure the lift assembly  30  to the receiver  22 . 
     The hitch receiver  22  is a structural vehicle member that is used to interface with a class III trailer hitch of an automotive type application, the size and shape of which is emulated by the inserted beam  86  of the tongue member  84 . However, it should be appreciated that the tongue member beam  86  may alternatively be sized to fit other types of hitch receivers  22 , such as those for a class I or class II hitch, or the like. The interconnection of the tongue member  84  and the vehicle receiver  22  attaches the platform lift assembly  22  to the vehicle  38  in a quick connect/disconnect fashion. The tongue member  84  is constructed of any desired material, but preferably steel, and its beam  86  preferably has a solid, substantially square cross section of the size to appropriately fit the particular class of hitch receiver  22  the vehicle has. It is envisioned that relative to the same lift assembly  30 , its tongue member  84 , or only its beam portion  86 , appropriate for one class of hitch may be interchangeable with that for another class of hitch. The tongue member  84 , once locked longitudinally relative to the receiver  22 , positions the remaining portions of the lift assembly  30  away from the truck  38  such that the intended relative positioning of the lift assembly  30  and the truck  38  is assured. 
     The tongue member  84  is pivotably attached to the structural beams  100  of the frame  72 , which are fixed to the third linkage members  56   a ,  56   b . The tongue member  84  also helps tie the structural beams  100  of the frame  72  together, and the connection between the tongue member  84  and the frame&#39;s structural beams  100  provides weight bearing support for the lift assembly  30  relative to the vehicle  38  to which it is installed. 
     The portion(s) of the platform  32  defining the generally planar load surface  40  is preferably formed of metal or other suitable material, with variable dimensions. The platform  32  is formed to any desired dimension based on the intended use, and may be foldable about hinge  134  as shown in the Figures, with the hinged, foldable portions  32   a ,  32   b  each defining a portion  40   a ,  40   b  of the load surface  40 . The planar portion(s)  40   a ,  40   b  of the platform  32  defining the load surface  40  may be made from any desired materials such as steel or aluminum plate (e.g., commercially available tread plate), expanded metal, or other suitable materials. The platform  32  is constructed with a tubular understructure  136  which is secured to and supports its planar portions. The tubular understructure  136  is releasably connected through a hinge  138  to the platform-supporting first linkage members  52   a ,  52   b  which are themselves fixed together, which allows the platform  32  to be rotated about 90 degrees between substantially horizontal  140  and vertical  142  positions. When in its substantially horizontal position  140 , the platform  32  is supported along the parallel first members  52   a ,  52   b  of the pair of four-bar linkages  50   a ,  50   b  and the lift assembly  30  is operational for movement between its lowered  44  and elevated  42  states. When in its substantially vertical position  142 , with the lift assembly  30  in its elevated state  42  and stowed, the platform  32  serves as a tailgate that forms a rear wall of the truck&#39;s cargo bed. The planar load surface portions  40   a ,  40   b  of the platform  32  may be perforated or formed from heavy wire mesh, thereby allowing the passage of air there-through when serving as a tailgate to reduce wind resistance when the truck  38  is being driven with the stowed lift assembly  30  installed. 
     The linkage members  52 ,  54 ,  56 ,  58  define an arcuate travel path of platform movement between the lift assembly elevated  42  and lowered  44  states, and maintains in those states  42 ,  44  and in all intermediate states therebetween, a substantially horizontal orientation of the platform-supporting members  52   a ,  52   b  of the pair of four-bar linkages  50   a ,  50   b . The rigid members of each four-bar linkage  50  are constructed from any desired material, preferably steel, and are each shaped or configured, and interconnected with each other to not only maintain a substantially horizontal platform orientation during operation, but to accommodate and not encroach the spaces occupied by OEM rear bumpers  74  and other hardpoints typically found in pickup trucks  38  that have had their tailgates removed. Thus, the platform lift assembly  30  accommodates the existing environment typical of most pickup trucks  38 , and avoids impact to that environment. In the depicted embodiment, the swinging second and fourth members  54 ,  58  of each four-bar linkage  50  are shown as being substantially straight between their respective pivot points  60 ,  62 . Alternatively, the linkage members  52 ,  54 ,  56 ,  58  may be curved or otherwise formed to provide a path through which the lift assembly  30  can move between its operational end states, and be stowed on-board the vehicle  38 , while providing desired clearances to vehicle hardpoints without requiring modification of the vehicle  38 . 
     Thus, the lift assembly  30  includes a tongue member  84  having a longitudinal axis  130  and adapted for slidable insertion along the longitudinal axis  130  into the vehicle&#39;s trailer hitch receiver  22 . The frame  72  is pivotably attached to the tongue member  84  and includes a pair of four-bar linkages  50 , the linkages  50   a ,  50   b  disposed on opposite sides of the longitudinal axis  130 . Each of the linkages  50  includes rigid first  52 , second  54 , third  56 , and fourth  58  linkage members, with each of these linkage member&#39;s having first  60  and second  62  pivot points. 
     The frame  72  has first  64 , second  66 , third  68 , and fourth  70  substantially parallel pivot axes extending between the pair of four-bar linkages  50   a ,  50   b , and that are substantially horizontal when the tongue member  84  is in the receiver  22 . Respective to each of the pair of four-bar linkages  50 , the first  52  and second  54  members are pivotably attached to each other, preferably through a bolted connection, at their respective first pivot points  60  through which extends the first pivot axis  64 . Likewise, the second  54  and third  56  members are pivotably attached to each other at their respective second pivot points  62  through which extends the second pivot axis  66 . Likewise, the third  56  and fourth  58  members are pivotably attached to each other at their respective first pivot points  60  through which extends the third pivot axis  68 . And likewise, the fourth  58  and first  52  members are pivotably attached to each other at their respective second pivot points  62  through which extends the fourth pivot axis  70 . 
     The platform  32  is connected to the frame  72  through the first members  52   a ,  52   b  of linkages  50   a ,  50   b , and has a substantially planar load surface  40  defining a front edge  144 . The load surface  40  has a substantially horizontal operating position  140 , and the first linkage members  52  and the horizontally positioned platform  32  move together between a lowered state  44  in which the load surface  40  is located at a height between that of the tongue member longitudinal axis  130  and ground level  34 , and an elevated state  42  in which the horizontally positioned load surface  40  is generally parallel with the cargo area floor level  36  and the load surface front edge  144  is proximate to the cargo area floor  46 , the platform  32  having arcuate movement between these lift assembly states  42 ,  44 . The horizontal orientation of the platform  32  is maintained throughout the lifting and lowering movement due in part to the fourth pivot axis  70  being vertically higher than the first pivot axis  64 , which causes the platform  32  to prescribe a noncircular arc about the third pivot axis  68  that facilitates the horizontally positioned platform  32  also having a generally horizontal movement relative to the cargo bed of the truck  38  as the lift assembly  30  is moved toward and out of the elevated state  42 . This allows the platform  32  to be positioned proximate to the rear edge  146  of the cargo area floor  46  without contacting the truck&#39;s OEM rear bumper  74 . Notably, referring to  FIGS. 3 and 5 , in the elevated state  42 , the third and fourth pivot axes  68 ,  70  are substantially vertically aligned with the front edge  144  of the load surface  40  positioned forward of axes  68 ,  70  and above the bumper  74 . 
     The rear edge  146  of the cargo area floor  46  can be defined as being located at a first location  150  in a direction axially along the tongue member longitudinal axis  130 . In the elevated state  42 , the front edge  144  of the platform load surface  40  is preferably located at a second location  152  along that axis  130  within about six inches and rearward of the first location  150 . Owing to the platform&#39;s path of movement having a component along the longitudinal axis  130  rather than being solely vertically, which allows the platform  32  to be brought close to the rear edge  146  of the cargo bed floor  46  without necessitating removal or modification of the rear bumper  74 , the front edge  144  of the load surface  40  in the lowered state  44  can be defined as being at a third location  154  along the longitudinal axis  130  that is at a substantially greater distance than the second location  152  is from the first location  150 . 
     The platform movement is designed to move around the rear bumper  74  in an arc, which results in the load surface front edge  144  having various locations along the longitudinal axis  130  at various lift assembly states  160  intermediate its lowered  44  and elevated  42  states. In the described embodiment, the load surface front edge  144  has fourth location  156  that is at a slightly greater distance than the third location  154  is from the first location  150 , and this fourth location  156  occurs when the second and fourth swinging linkage members  54 ,  58  are substantially horizontal, as shown in phantom lines in  FIG. 7 . For illustrative clarity, imaginary straight line  158  parallel to longitudinal axis  130  is provided in  FIG. 7 , on which are indicated the above-described first location  150  corresponding to the cargo bed floor rear edge  146 , and the second  152 , third  154  and fourth  156  locations corresponding to the platform load surface front edge  144 , along the axial direction of longitudinal axis  130 . 
     A hydraulic system  170  actuates the four-bar linkage mechanism  50  of the lift assembly  30 . The hydraulic system  170  consists of a pump (not shown) and a hydraulic cylinder and piston assembly  172  (hereinafter also referred to as hydraulic cylinder  172 ) attached to each of the pair of four-bar linkages  50 . The hydraulic system  170  is used to controllably induce and prevent motion of the four-bar linkages  50   a ,  50   b . Optionally, additional hydraulic cylinders  172  may be added to the hydraulic system  170  and connected to the pair of linkages  50  to increase the lifting power of the lift assembly  30 . The corresponding third members  56   a ,  56   b  of the pair of four-bar linkages  50   a ,  50   b , located opposite the first linkage members  52   a ,  52   b  along which the horizontally positioned platform  32  is supported, are fixed relative to the tongue member  84  during operation of the lift assembly  30 . These third linkage members  56   a ,  56   b  are both fixed to a central portion of frame  72  that extends laterally between the pair of four-bar linkages  50   a ,  50   b , and thus are fixed to each other. The central portion of frame  72  includes a plurality of structural beams  100  that support the weight of the lift assembly  30  and the lifted object. The structural beams  100  of the frame  72  also tie the platform  32  and linkage members  52 ,  54 ,  56 ,  58  to the tongue member  84 . The rigid structural beams  100  are constructed of suitable materials, preferably steel, and are shaped and configured to add additional strength to the lift assembly  30 . The structural beams  100  are also used to add additional components and options to the lift assembly  30  if later desired. 
     A hydraulic control unit  174 , in which the hydraulic pump (not shown), associated valves (not shown) and an electronic controller having a radio receiver (not shown) to selectively actuate the pump and direction of fluid control, are arranged in a closed, weatherproof module, is affixed to the frame&#39;s structural beams  100 . Hydraulic pumps, valves and related hydraulic control circuits, and electronic controllers and radio receivers, and their operations as described below, are all well known to those of ordinary skill in their respective arts, and are not further discussed herein. 
     Two pairs of hydraulic fluid lines  176  extend from the module  174 , one pair of lines  176  in operable fluid communication with each of the hydraulic cylinders  172 , to direct fluid into and out of chambers (not shown) defined by the opposed sides of the axially moveable piston (not shown) in each hydraulic cylinder  172 , as is well-known to those of ordinary skill in the art, to axially elongate or contract the hydraulic cylinder  172 . Each hydraulic cylinder  172  is pivotably attached at its opposite ends to the third  56  and fourth  58  members of a four-bar linkage  50 . The cylinder assembly  172  is hydraulically forced to elongate during lifting, i.e., to move the lift assembly  30  towards its elevated state  42 ; and is hydraulically forced or controllably allowed to contract during lowering, i.e., to move or facilitate movement of the lift assembly  30  towards its lowered state  44 . The torque tube  80  interconnecting the fourth linkage members  58   a ,  58   b  prevents the effect of “crab walking” often experienced when two hydraulic cylinders are used to move a lift platform through a pair of parallel-acting linkages, by distributing between the two linkages differences in the forces the cylinders exert. 
     An electrical wire harness  178  also extends from the module  174 , and its plug  180  is connected to the standard seven-pin trailer electrical power receptacle  182  of the truck  38  typically located adjacent the receiver  22 . One pole (not shown) of this receptacle  182  provides constant voltage, and is connected to the electrically-driven hydraulic pump located inside the module  174 . Actuation of the hydraulic system  170  to elongate or contract the hydraulic cylinders  172  is responsive to an associated signal received from a user-actuated remote control transmitter  184  which wirelessly communicates radioed control signals to the control module  174  from the user, who is able to stand safely away from the lift assembly  30  during its operation to avoid possible injury and better oversee the lifting and lowering operations. Once the desired platform position is reached, the user ceases transmitting the powering signal, and the pump stops. A valve in the hydraulic system  170  causes the motion of the platform  32  to “lock” and hold the platform  32  in its current position until power is re-applied to further actuate the hydraulic system  170 . 
     As noted above, the tongue member  84  is pivotably attached to the structural beams  100  of the frame  72 . The pivotable connection between the tongue member  84  and the frame  72  is about a fifth pivot axis  190  substantially parallel with the first, second, third, and fourth pivot axes  64 ,  66 ,  68 ,  70  and located closely rearward of the bumper  74 . The tongue member  84  extends further rearward along the direction of its longitudinal axis  130  to a downwardly depending and laterally extending first flange  192 . The tongue member first flange  192  is selectively superposed by a downwardly depending and laterally extending second flange  194  fixed to a structural beam  100  of the frame  72 . The first  192  and second  194  flanges each have a pair of laterally spaced holes  196 , best shown in  FIGS. 3 and 4 . The pairs of holes  196  are respectively aligned when the first and second flanges  192 ,  194  are superposed. A pair of tethered pins  198  is inserted through the aligned holes  196  to rigidly fix the frame  72  to the tongue member  84 , and prevent their relative rotation about the fifth pivot axis  190  during lifting and lowering operations. 
     At the completion of lift assembly operations, the lift assembly  30  is positioned in its elevated state  42 , and the rearmost portion  32   b  of the hinged platform  32  is folded forward about hinge  134  over the frontmost portion  32   a  such that their respective load surface portions  40   b ,  40   a  are interfacing. The platform  32  is then pivoted forward about the hinge  138 , through which the platform  32  is releasably connected to the first linkage members  52 , and into its generally vertical position  142 . The platform  32  may be secured into its vertical position  142 , for example by a latching mechanism (not shown). The feet  90  are then collapsed and rotated laterally inward and locked into their stowed positions. 
     At this point, the lift assembly  30  is now in its elevated state  42  and the frame  72  has a first orientation  200  relative to the tongue member  84 . The user may now wish to remove the lift assembly  30  from the vehicle  38 , which entails disconnecting the electrical harness plug  180  from the vehicle&#39;s electrical receptacle  182 , removing the locking pin  132  that extends through the aligned holes in the receiver  22  and tongue member beam  86 , and pulling the lift assembly  30  rearward as a unit along its longitudinal axis  130  using the handle  202  fixed to the rearmost frame structural beam  100 , to disengage the lift assembly  30  from the receiver  22 . The lift assembly  30  may be pulled rearwardly onto a specially-adapted wheeled dolly (not shown) on which the lift assembly  30  can be rolled away and stored in a space-efficient manner. The truck&#39;s tailgate (not shown) is then reinstalled. It is envisioned that the above-mentioned wheeled dolly may also be adapted for use in moving the objects that are loaded and unloaded using the lift assembly  30 , and that provisions could be made for its storage on the lift assembly frame  72 . 
     Alternatively, the user may instead now wish to stow the installed lift assembly  30 , still in its elevated state  42 . Doing this entails the tethered pins  198  then being pulled from the aligned pairs of holes  196 , and rotating the latched-together frame  72  and platform  32  forward about the fifth pivot axis  190  into a second orientation  204  between the frame  72  and the tongue member  84 . After rotating the latched-together frame  72  and platform  32  forward, the lift assembly  30  may be secured into its second orientation  204  by latching together the tongue member  84  and the frame  72 . In the second orientation  204 , the platform  32  is now positioned slightly more vertically upright and substantially closes the rear cargo bed opening outlined by the inside edges of vertical rear bed rails  206  and the cargo area floor  46 , thus serving as a tailgate and providing a rear wall  208  to the truck&#39;s cargo area, as shown in  FIG. 8 . The hydraulic cylinders  172  may then be slightly further extended to bring rubber snubber pads  210  located on the forward sides of the fourth linkage members  58   a ,  58   b  into compressive contact with the truck&#39;s rear bumper  74 , thereby firmly securing the lift assembly  30  into its fully stowed position  212 . The electrical power to the hydraulic control unit  174  may then be disconnected by conventional means such as through a switch (not shown) or by disconnecting its harness to prevent its accidental actuation. 
     With reference to  FIGS. 1 and 2 , once in its above-mentioned first orientation  200  between the frame  72  and the tongue member  84 , wherein the lift assembly  30  is in its elevated state  42  and the feet  90  have been stowed, and there is no ground-supported portion of the lift assembly  30  extending from it, there is a first vertical distance  214  between the tongue member longitudinal axis  130  and the lowermost point  216  of the lift assembly  30  that establishes a first departure angle θ 1  ( FIG. 2 ). Once in its above-mentioned second orientation  204  between the frame  72  and the tongue member  84 , however, there is a second vertical distance  218  between the tongue member longitudinal axis  130  and the lowermost point  216  of the lift assembly  30  that establishes a second departure angle θ 2  that is substantially greater than angle θ 1  ( FIG. 2 ) and which is expected to be no less than angle θ T  established by trailer hitch  20  instead installed in the receiver  22  ( FIG. 1 ). 
     The portable hydraulic platform lift assembly  30  may be configured with a multiple of different capacities, sizes and/or configurations for differing applications and uses. For example, in one embodiment, the lift is configured for an 1100 pound vertical lift, which the above described lift assembly embodiment has achieved with its hydraulic system producing a pressure of about 2200 psig. It should be appreciated that the combined force exerted by the hydraulic cylinders, to vertically lift an 1100 pound load, is greater than 1100 pounds due to the kinematic arrangement through which the hydraulic system and linkage mechanism operates. The hydraulic system can be reconfigured to adapt to different environments and load capacities such as, for example, by varying the size or number of hydraulic cylinders. The design of the linkages  50  can be modified to provide different kinematically arranged travel paths which suit various applications and uses. 
     Additionally, as shown in  FIG. 10 , the general use platform  32  described above can be removed from the first linkage members  52  at its releasable hinged connection  138  and interchanged with another, longer platform  32  more particularly adapted for lifting an all terrain vehicle (ATV)  220  to the cargo bed  46 . It is envisioned at a plurality of different interchangeable platforms  32 , each more particularly suited for a different use or type of load (e.g., for motorcycles, snowmobiles, animal cages, objects on pallets, . . . etc. . . . ) can be made available to users. 
     The herein described portable hydraulic platform lift assembly is intended to be used as a mobile/portable lifting device for lifting heavy objects into the bed of pick-up trucks, SUV&#39;s, or other types of vehicles having a suitable trailer hitch receiver. It provides a trailer hitch-type interface to a receiver-equipped vehicle, which provides a quick means of easy attachment and removal. Once the tongue member of the portable hydraulic platform lift assembly has been secured to receiver of the host vehicle, and the electrical connection from the hydraulic system is made to the vehicle&#39;s electrical system via the trailer electrical power receptacle, the lift assembly is ready for use. 
     As to a further discussion of the manner of usage and operation of the present invention, the same should be apparent from the above description. Accordingly, no further discussion relating to the manner of usage and operation will be provided. With respect to the above description then, it is to be realized that the optimum dimensional relationships for the parts of the invention, to include variations in size, materials, shape, form, function and manner of operation, assembly and use, are deemed readily apparent and obvious to one skilled in the art, and all equivalent relationships to those illustrated in the drawings and described in the specification are intended to be encompassed by the present invention. 
     Therefore, the foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.