Abstract:
The invention is a unitary cantilever liftgate that attaches to the vehicle body instead of the vehicle chassis. The liftgate may be fully assembled and tested prior to installation on the vehicle body. This greatly reduces installation time while improving performance and reliability. Various features such as steps, bumpers and brackets are integrated into the liftgate. Further, the liftgate is preferably configured so that multiple units can be stacked together for shipping and ease of unloading and loading. The stackability of the liftgates simplifies storing as well, by removing the necessity of storing several components separately.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to liftgates used to bring loads to and from ground level and the level of a vehicle&#39;s payload area. 
   2. Description of the Related Art 
   Liftgates are typically mounted at the rear of trucks, truck trailers and the like to bring payloads from ground level up to the bed of the truck or from the bed to the ground. In some applications, it is desirable to configure the liftgate so that it folds away when not in use. Examples of such designs are the cantilever Tuk-A-Way® liftgates available from Maxon (Santa Fe Springs, Calif.), assignee of the current invention. Typically, these liftgates employ parallelogram linkages to maintain the liftgate in a horizontal plane through the lifting range. The liftgate is attached to the parallelogram linkages by pivot members, which allow the liftgate to be placed in a generally vertical position when in the lowered position. When in the vertical position, operation of the lifting mechanism rotates the liftgate into an inverted, stowed position beneath the vehicle body. 
   Prior art Tuk type cantilever liftgates present various challenges having to do with their shipping and installation. All prior art Tuk type cantilever liftgates mount to the vehicle chassis. Thus, these liftgates are susceptible to misalignment with the vehicle body. When misaligned, operation of the liftgate can exert differential force on the vehicle body and truck body, tending to separate them. Further, typical cantilever liftgates, when shipped from the factory, are partially disassembled and have awkward shapes, requiring substantial packaging. Further, proper installation necessitates that several of the components must be precisely aligned and then securely mounted to the vehicle. Moreover, the liftgate must be fitted to the vehicle only after the truck body has been attached to the chassis. Consequently, proper installation of prior art liftgates involve significant time and expense. 
   Accordingly, what has been needed is a Tuk type cantilever liftgate design configured for easy shipping that simplifies installation while improving overall strength and reliability. This invention satisfies these and other needs. 
   SUMMARY OF THE INVENTION 
   The system of the invention is a pre-assembled, unitary Tuk type cantilever liftgate comprising an opposing pair of side plates, an extension plate extending between the side plates, a hydraulically driven lift frame pivotally attached to the side plates and a liftgate platform rotatably attached to the lift frame wherein the liftgate is configured to be secured only to a vehicle body. In preferred embodiments, the side plates and extension plate are configured so that liftgate system can be attached to the vehicle body by bolts, welding or the like. The side plates may further comprise steps, brackets and bumpers. Also preferably, the lift frame may comprise a lift frame tube configured to function as an underride guard. The unitary liftgate may also be configured to allow multiple units to be stacked together to facilitate shipping and storage. 
   The invention also comprises the unitary liftgate system secured to a vehicle body. This lift gate may be fully assembled and tested prior to being secured to the vehicle body. Further, since the liftgate is secured to the vehicle body, it can be attached prior to the body being secured to the vehicle chassis. The features of the invention allow the liftgate to be installed in significantly less time than prior art cantilever liftgates. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The aforementioned advantages of the invention, as well as additional advantages thereof, will be more fully understood in light of the detailed description of the invention below when taken in conjunction with the accompanying drawings in which: 
       FIG. 1  is an elevational view of the inventive liftgate shown with the liftgate platform in the stowed position; 
       FIG. 2  is an elevational view of the liftgate platform lowered to the vertical position; 
       FIG. 3  is a side view of the liftgate platform in the lowered position with the liftgate platform extended; 
       FIG. 4  is an elevational view of the liftgate platform in the raised position with the liftgate platform extended; 
       FIG. 5  is a side view of the liftgate in the raised position with the liftgate platform extended; 
       FIG. 6  is an elevational view showing the liftgate platform secured to a portion of the vehicle body. 
       FIG. 7  is an elevational view showing a plurality of inventive unitary, fully assembled liftgates stacked together to facilitate shipping, for example. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1-7  show the cantilever liftgate system  10  of the invention. The system comprises a framework  12  having side plates  14 , a trunnion tube  16  extending between side plates  14  and an extension plate  18 , also extending between side plates  14 . The structural members of liftgate  10  are preferably formed from steel, but other metals or composite materials can be suitable depending upon the intended application. The liftgate also comprises a lift frame  20  with a lift frame tube  22  extending between an opposing pair of parallelogram linkages  24  each having upper arms  26  and lower arms  28  with and proximal pivot members  30  and distal pivot members  32 . Proximal pivot members  30  are attached to trunnion tube  16  thus securing lift frame  20  to framework  12 . Preferably, trunnion tube  16  comprises integral mounts  34  for attaching proximal pivot members  30 . Trunnion tube  16  also comprises cylinder tower  36 . Hydraulic cylinder  38  is pivotally attached at either end to tower  36  and lift frame tube  22  to drive lift frame  20 . Distal pivot members  32  are secured to liftgate platform  40  so that, together with proximal pivot members  30 , upper and lower arms  26  and  28  form parallelogram linkages  24 . The linkages  24  are configured to maintain the orientation of distal pivot members  32  to proximal pivot members  30  as upper arms  26  and lower arms  28  are raised and lowered. Retraction of hydraulic cylinder  38  drives lift frame tube  22 , rotating upper arms  26  and lower arms  28  about proximal pivot members  30  to lower liftgate platform  40 . Conversely, extension of hydraulic cylinder  38  raises liftgate platform  40 . 
   Liftgate platform  40  is configured to be raised into either the load lifting position shown in  FIGS. 4 and 5  or the stowed position shown in  FIGS. 1 and 6 . Specifically, liftgate platform  40  is rotatably attached to distal pivot members  32  with motion limiting stops  44  that prevent rotation of liftgate platform  40  away from upper and lower arms  26  and  28  beyond the horizontal plane. This keeps liftgate platform  40  in a plane parallel with the ground and vehicle body  46  to facilitate loading and unloading. Alternatively, liftgate platform  40  may be rotated at distal pivot members  32  toward upper and lower arms  26  and  28  to the generally vertical position shown in FIG.  2 . In this position, actuation of hydraulic cylinder  38  raises and inverts liftgate platform  40  until it is located in its stowed position shown in  FIGS. 1 and 6 . As used herein, the term cantilever refers to liftgates having a platform supported at one end only. For example, platform  40  is supported by the pivotal attachment to distal pivot members  32 . 
   In a preferred embodiment, liftgate platform  40  is equipped with a liftgate platform extension  48  attached by hinge  50 , creating a bifold configuration. Hinge  50  allows the liftgate platform extension to fold against liftgate platform  40  in the direction of the system  10 , but prevents liftgate platform extension  48  from rotating beyond the plane of liftgate platform  40 , thus providing a suitable lifting platform. Similarly, when liftgate platform  40  is placed into the vertical position shown in  FIG. 2 , liftgate platform extension  48  folds back against liftgate platform  40 , providing a more compact profile for storage. In other embodiments, one or more additional platform extensions may be provided creating a trifold configuration, for example. 
   Framework  12  preferably comprises a variety of additional features to simplify manufacture and to increase the utility of the liftgate system  10 . For example, side plates  14  may comprise formed steps  51 , pump bracket  52  for mounting pump  54  to power hydraulic cylinder  38 , dock bumpers  56  and vehicle light brackets  58 . Generally, these features are integral with the framework  12  but they can also be separate members that are suitably attached to the frame. Also preferably, lift frame tube  22  is configured to function as an underride guard to minimize injury in the event of a rear end collision. Integrating underride protection into the existing liftgate system removes the need for an additional, dedicated horizontal member. 
     FIG. 6  shows framework  12  secured to vehicle body  46 . Generally, extension plate  18  is secured to the rear lower horizontal frame member  60  of the vehicle and side plates  14  are secured to the rear lower horizontal frame member  60  and sub-structure cross members  62 . Preferably, framework  12  is secured to the vehicle by bolts  64 , but welding or other conventional means of attachment are also suitable. Since all the liftgate components are attached to the framework  12 , only the framework  12  needs to be attached to the vehicle and no alignment or adjustment of the lifting mechanism will be necessary. Further, the unitary construction of the liftgate allows it to be attached to a wide variety of truck types, since only the framework  12  need be secured. Typically, the only adjustment necessary is to level the extension plate  18  with the truck bed  66 . This is easily accomplished by the use of shims  68  between side plates  14  and cross members  62 . In short, this invention allows the integrated liftgate system  10  to be fully pre-assembled requiring only final bolt-on attachment to a vehicle body with little or no adjustment. 
   As an alternative embodiment framework  12  can be preattached to lower horizontal frame member  60  at the factory and shipped as a unitary lift assembly. In such an instance the framework  12 , including lower horizontal frame member  60 , would be attached on site to the rear of the vehicle by attaching sub-structure cross members  62 . 
   The integrated, unitary and compact nature of the invention also facilitates shipping, packaging and storing. Framework  12  with the liftgate platform  40  in the stowed position presents a compact, box-like profile that may be stored or shipped with virtually no packaging. Further, multiple systems may be stacked and bolted together for shipment, avoiding or minimizing the need to use pallets, banding material, or other packaging means. For example, as shown in  FIG. 7 , the bottom profile of the side plates  14  is configured to nest with the top profile of side plates  14  and extension plate  18 . This allows a stack of multiple liftgate units to be very stable and allows them to be easily secured together, by bolts or other suitable means. 
   Providing a unitary cantilever liftgate that attaches to the vehicle body rather than the vehicle chassis offers numerous benefits. First, the liftgate may be completely assembled and fully tested at the factory prior to being delivered for installment. This greatly improves quality control and simplifies installation. The liftgate need only be secured to the vehicle body by welding, bolting or other means, dramatically reducing the installation time required. Where prior art cantilever liftgates typically require 6 to 8 hours to install, the unitary liftgates of the invention can be installed in about 1 hour. Further, since the liftgates of the invention secure to the vehicle body, the lift platform will always be in alignment with the body. Prior art liftgate designs, since they attach to the vehicle chassis, have lift platforms that may become misaligned with the vehicle body. Indeed, when the misalignment becomes severe, operation of the liftgate can exert a differential force on the vehicle body, causing it to separate from the vehicle chassis. In contrast, the inventive liftgates completely avoid such problems. 
   Described herein is a preferred embodiment, however, one skilled in the art that pertains to the present invention will understand that there are equivalent alternative embodiments.