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You are an expert at summarizing long articles. Proceed to summarize the following text: 
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a 371 of International application no. PCT/US2013/058878, filed Sep. 10, 2013, which claims priority to U.S. provisional application, Serial No. 61/702,548, filed Sep. 18, 2012, the contents of which are hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates generally to commercial cargo van-style vehicles, and, more particularly, to a ladder adapted to be clamped to a rear access door of such vehicles to allow convenient access by a work person to a roof area of the vehicle. 
         [0004]    2. Discussion of the Prior Art 
         [0005]    Various automotive vehicle manufacturers offer cargo vans for carrying supplies and tools to a work site. Such vans have a raised roof sufficiently high to allow an adult worker to stand upright within the vehicle&#39;s box and a pair of rear access doors that are hinged to the vehicle&#39;s sidewalls along rear edges thereof and which, when swung closed, meet and latch at a midline of the vehicle&#39;s rear end. Frequently, such vans will be equipped with a roof rack on which items, such as extension ladders, step ladders and other tools, are carried. Due to the height of the roof, typically about six to seven feet above the ground, it is difficult to reach the rooftop for placement and removal of objects intended for rooftop storage. 
         [0006]    To solve this problem, the present invention provides a ladder that is generally universally attachable to one of the rear doors of the vehicle whereby a worker can safely climb onto the vehicle&#39;s roof 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention comprises a ladder, preferably fabricated from aluminum, having a pair of elongated side rails held in parallel, spaced-apart relation by a plurality of transversely extending longitudinally spaced apart rungs and where the side rails are appropriately bent along their length to better conform to the contour profile of a vehicle door on which the ladder is adapted to be mounted. Adjustable brackets are attached to the upper and lower ends of the ladder side rails with clips for clamping to the top and bottom edge portions of one of the vehicle&#39;s rear doors to thereby rigidly mount the ladder to the vehicle door. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0008]    The foregoing features, objects and advantages of the invention will become apparent to those skilled in the art from the following detailed description of a preferred embodiment, especially when considered in conjunction with the accompanying drawings in which like numerals in the several views referred to corresponding parts. 
           [0009]      FIG. 1  is a perspective rear view of a typical prior art cargo van on which the present invention finds use; 
           [0010]      FIG. 2  is a perspective view of a van door mountable ladder comprising a preferred embodiment of the present invention; 
           [0011]      FIG. 3  is an exploded detailed view of the clamping structure for engaging a top edge of a van door; 
           [0012]      FIG. 4  is an exploded detailed view of the clamping structure for engaging a bottom edge of a van door; and 
           [0013]      FIG. 5  is an end view of the tubular mounting bracket shown in  FIG. 3 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0014]    This description of the preferred embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description of this invention. In the description, relative terms such as “lower”, “upper”, “horizontal”, “vertical”, “above”, “below”, “up”, “down”, “top” and “bottom” as well as derivatives thereof (e.g., “horizontally”, “downwardly”, “upwardly”, etc.) should be construed to refer to the orientation as then described or as shown in the drawings under discussion. These relative terms are for convenience of description and do not require that the apparatus be constructed or operated in a particular orientation. Terms such as “connected”, “connecting”, “attached”, “attaching”, “join” and “joining” are used interchangeably and refer to one structure or surface being secured to another structure or surface or integrally fabricated in one piece, unless expressively described otherwise. 
         [0015]      FIG. 1  illustrates a rear view of a conventional cargo van with which the present invention finds use. It illustrates the van with the rear doors open and, as can be seen by the view, the roof of the van is sufficiently elevated relative to its floor so that a workman may stand erect within the van&#39;s box. The van is indicated generally by numeral  10  and has a roof structure  12 , sidewalls  14  and  16 , and with left and right rear doors  18  and  20  hinged to the sidewalls  16  and  14 , respectively. 
         [0016]    Looking closely at the bottoms of the doors  18  and  20  in  FIG. 1 , an edge portion  22  of the left door&#39;s sheet metal front panel extends down a short distance below the inside rear bottom edge of the door. This can also be seen at  24  on the right rear door  20 . While somewhat difficult to see in the view of  FIG. 1 , the top edge of the door&#39;s front panel also extends slightly above the inner rear door panel, as at  26  and  28 . Also, by viewing the door  20 , it can be seen that the sheet metal front panel of that door is contoured such that approximately the lower half thereof is generally vertically oriented and then the upper approximately half curves such that when the door are closed, that upper portion slopes in a forward direction. 
         [0017]      FIG. 2  is a perspective view of the ladder comprising a preferred embodiment of the present invention. It is seen to comprise first and second longitudinally extending aluminum rails  30  and  32  that are held in parallel, spaced-apart relation by a plurality of transversely extending, longitudinally spaced-apart rungs  34 . Attached to the upper end portion of each of the rails  30  and  32  is a pair of upper end door-mounting bracket devices  36  and, likewise, affixed to the lower end of the rails  30  and  32  are door-bottom mounting bracket devices  38 . 
         [0018]    With continued reference to  FIG. 2 , it will be noted that the rails  30  and  32  rise generally vertically over a length identified by bracket  40  and then bend slightly in a forward direction, thus, following the contours of the exterior of the doors  18  and  20 . 
         [0019]    Turning next to  FIG. 3 , there is shown an exploded detailed view of the upper door-edge mounting bracket devices  36 . Bolted or otherwise affixed to the upper end portions of the ladder rails  30  and  32  are mounting plates, as at  42 . More particularly, bolts, as at  44 , extend through washers  46  into selected ones of a series of regularly spaced threaded bores  48  that are formed through the rails  30  and  32  such that the placement of the mounting brackets  42  is adjustable over a limited range along the length dimension of the rails. A somewhat tubular clamp member  50  is adapted to be affixed to each of the bracket-mounting plates  42  by bolts  52  that pass through bores  54  and  56  in the legs of the tubular clamp  50  and the mounting bracket  42 , respectively. 
         [0020]      FIG. 5  is an end view of the extrusion comprising the tubular mounting bracket  50  and it is seen to include an inwardly protruding key  58 , which is adapted to mate with a keyway groove  60  formed in an insert rod  62  adapted to fit into the tubular opening of the bracket  50 . In this fashion, the degree of extension of the rod  62  from the tubular bracket  50  can be adjusted and then fixed when the bolts  52  are tightened into the nuts  64  which serve to compress the legs  66  of the bracket member  50  together and reduce the I.D. of the tubular portion thereof. Bolted to the outer end of the rod  62  is a clip  68  which is arranged to engage the upper portion  26  of the vehicle&#39;s door as seen in  FIG. 1 . 
         [0021]    Referring next to  FIG. 4 , there is shown a similar arrangement for securing the bottoms of the ladder rails  30  and  32  to the lower edge  22  or  24  on the vehicle&#39;s rear door  18  or  20 . As in the attachment arrangement shown in  FIG. 3 , the lower ladder attachment bracket assemblies include an attachment plate  70  of a somewhat different shape attached to each of the ladder rails  30  and  32  by bolts  72  that pass through washers and slotted apertures  74  into selected ones of the threaded bores  75  formed in the ladder rails. The mounting plates  70  provide a means whereby a tubular mounting bracket  76  can be attached by bolts, as at  78 , that extend through apertures in the legs of the tubular mounting bracket  76  and through apertures, as at  80 , formed in the mounting plate to be secured by nuts, as at  82 . The tubular mounting bracket  76  has an end view like that shown in  FIG. 5  including a key  58  for cooperating with a keyway or groove  84  formed in a rod  86 . Again, the extent to which the rod  84  projects out from the tubular mounting bracket  76  is slideably adjustable and then can be rigidly secured upon tightening of the bolts  78  into the nuts  82 . 
         [0022]    Affixed to the outer end of the rod  86  is a clip member  88  which fastens to the rod  86  by means of a bolt  90  that is arranged to pass through aligned apertures in the clip  88  and the rod  86 . A nut  92 , when tightened on to the bolt  90 , establishes a rigid connection with the end of the rod  86 . 
         [0023]    In assembling a ladder on to the door  18  or  20  of the van, the ladder will be suspended from the upper edge of the door  26  or  28  by the upper door-edge mounting bracket devices  36  illustrated in  FIG. 3  with the clips  68  fitted over and engaging the upper edges  26  or  28  of door. Then, the rods  86  of the lower door-edge ladder attachments will be inserted into the tubular brackets  76  and raised therein until the clips  88  fit behind and engage the lower edge  22  or  24  of the vehicle&#39;s rear doors. At this point, all bolts as at  90 ,  78 , and  52  will be tightened to rigidly clamp the ladder to a selected one of the vehicle&#39;s rear door. The upper and lower ladder attachment brackets are designed so as not to mar the vehicle&#39;s doors in any way while yet rigidly securing the mounting ladder to the vehicle at a location so that it can be easily climbed to reach the exterior roof area of the cargo van. 
         [0024]    This invention has been described herein in considerable detail in order to comply with the patent statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use such specialized components as are required. However, it is to be understood that the invention can be carried out by specifically different equipment and devices, and that various modifications, both as to the equipment and operating procedures, can be accomplished without departing from the scope of the invention itself.

Summary:
A ladder specially designed for attachment to a rear door of a cargo van to facilitate the ability of a person to climb onto the vehicle&#39;s roof. The ladder has two parallel rails joined by a plurality of transversely extending rungs. Affixed to the upper and lower ends of the ladder rungs are length adjustable brackets supporting door engaging clips designed to fit about the upper and lower edges of a van door without interfering with the ability of the door to open and close. The ladder rails are preferably bent to conform to the contour of the van door.