Abstract:
A trailer unloading system anchors one or more trailers to a deck, with, or without tractor attached. The sides of the trailers are free to pivot from a top, longitudinal, pivot axis once unlatched from the edge of the trailer floor. The deck tilts the trailers sideways, the doors swing free, and the load discharges. The deck returns to level, the sides latch to support the floor and the trailers are ready to be reloaded.

Description:
RELATED APPLICATIONS 
   This patent application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/428,727 filed Nov. 25, 2002 and entitled TRAILER TIPPING SYSTEM. 

   BACKGROUND 
   1. The Field of the Invention 
   This invention relates generally to the field of trailer dumping systems. More particularly this invention relates to an apparatus that tips or rotates trailers to unload their contents. 
   2. The Background Art 
   Trucks frequently carry cargo comprising comparatively small loose particles such as coal, gravel, or hog fuel (large sections of tree bark burned for heating purposes). Various mechanisms exist to unload such cargo. For example the underside of a vehicle may have a trapdoor that is opened to release the cargo. Such a system requires either that a receiving structure be positioned directly beneath the trapdoor or that the cargo be released while the vehicle is in motion to avoid dragging the wheel assembly through large mounds of material. 
   Such systems have significant disadvantages for cargo that is not comparatively dense. For such cargos, a unitary or trailing vehicle&#39;s volume must be maximized in order to approach the weight bearing capacity of the frame and engine or of a towing vehicle. Providing a trapdoor and an actuator to open and close a trapdoor on the underside of a vehicle takes clear, unsupported space and necessitates larger structural members to compensate for the weakness created by the opening. Furthermore, the size of a trapdoor must necessarily be limited to avoid structural weakness. Therefore, bulky and irregular cargos will be likely to jam when unloading. 
   Other systems provide an opening on top of a vehicle and unload cargo by rotating the entire vehicle such that the cargo falls out the top. However, such systems require a great deal of time and energy to lift and rotate the vehicle through an angle of 140 degrees or more. 
   Still other systems provide doors at the rear of a vehicle and unload the vehicle by raising the front of the vehicle relative to the rear so that the cargo flows out the rear doors. Such systems have the disadvantage of requiring a great deal of space, energy, and power to lift the center of gravity of the vehicle to a sufficient height for the contents of the vehicle to flow out. Furthermore, the size of the opening at the rear of the vehicle is typically limited to the width of the vehicle. Where the cargo is bulky and irregularly shaped, such an opening may cause the cargo to bridge or otherwise jam during unloading. 
   Thus it would be an advancement in the art to provide a system that allows for unloading of bulky, irregularly shaped cargos, such as debris, long objects, and so forth. Such a system would provide a large, clear, unobstructed opening that would not require excessive lifting or tilting of the vehicle to unload the cargo. It would be a further advancement in the art to provide a large opening of substantially the dimension of the cargo hold, facilitating unloading while preserving the structural integrity of the vehicle. 
   BRIEF SUMMARY OF THE INVENTION 
   An invention is disclosed in sufficient detail to enable one of ordinary skill in the art to make and use the invention. An unloading station may be sized to receive a truck and one or more trailers. The unloading station may be actuated hydraulically, or otherwise, to tip the truck and trailers, rotating them about a longitudinal axis. The trailers may have side mounted doors hinged at the top of the trailers. Upon tipping, the contents of the trailers empties under the force of gravity. An anchoring system may maintain the truck and trailers in a stable position during tipping. In some embodiments, hydraulically tensioned chains may secure to the side of the trailers opposite the side mounted doors. 
   Bulky lower support members, especially under a cargo hold, may limit the ability of a trailer to both hold large volumes of cargo and unload cleanly. Accordingly, lower support members may have diminished size and be supported by the doors when the doors are closed and locked into place. During unloading, the doors must open. Therefore, an external support mounted to the unloading station is brought up to support the lower support members of the trailers. In some embodiments a series of plates pivotally mounted on a cross beam may be lifted hydraulically into a position, e.g. with the plates extending horizontally initially, supporting the lower support members. Mounting the plates pivotally may permit them to both rest flat on the unloading station while the trailers are driven onto an unloading station and to seat properly on the lower support members when raised. 
   The doors may latch by lifting upwardly such that catches positioned on the lower edge of the door (e.g. side-opening door) are brought into engagement with receptacles on the lower support members (e.g. under the cargo hold floor) of the trailer. Such a configuration allows the door to act as a shear plate and a lift exerting an upward force on the lower support members. An actuated latching system may lift the door hydraulically or otherwise and lock the door in position. In some embodiments, the latching system may be a linkage having an over-center position, e.g. effectively a four-bar linkage, for example, (e.g. breaking over-center into a stable position). The latching system may be moved to the over-center position to effectively lock the door in a closed position without the need for constant application of force to the linkage. 
   Due to the nature of the bending forces in the loaded trailers, the net force on lower support members near either end of the trailers may be upward. Accordingly, the doors may latch in a manner allowing the doors to exert a net downward force on the lower support members. Pins, or like structures may be inserted, or extended from the lower support members such that the pins, or the like, are positioned just below the lower edge of the door in a position to exert an upward force on the door. In some embodiments, the pins may be hydraulically extended from and retracted through the lower support members. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects and features of the present invention will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only typical embodiments of the invention and are, therefore, not to be considered limiting of its scope, the invention will be described with additional specificity and detail through use of the accompanying drawings in which: 
       FIG. 1  is a perspective view of an unloading station and side-dumping trailers in accordance with the invention; 
       FIG. 2  is a perspective view of an unloading station in accordance with the invention; 
       FIG. 3  is a partial perspective view of an unloading station in accordance with the invention; 
       FIG. 4  is a perspective view of an anchor in accordance with the invention; 
       FIG. 5  is a perspective view of an alternative embodiment of an anchor in accordance with the invention; 
       FIG. 6  is a side view of an unloading station in accordance with the invention; 
       FIG. 7  is a side view of an unloading station and trailers in a tipped position in accordance with the invention; 
       FIG. 8  is a perspective view of a front anchor in accordance with the invention; 
       FIG. 9  is a perspective view of an alternative embodiment of a front anchor in accordance with the invention; 
       FIG. 10  is a cutaway perspective view of a trailer in accordance with the invention; 
       FIG. 11  is a side view of a catch in accordance with the invention; 
       FIG. 12  is a perspective view of an alternative embodiment of a catch in accordance with the invention; 
       FIG. 13  is a cutaway view of a lifting and locking mechanism in accordance with the invention; 
       FIG. 14  an exploded view of a lifting and locking system in accordance with the invention; 
       FIGS. 15A–15B  are side views of a lifting and locking system in accordance with the invention; 
       FIG. 16  is a perspective view of a housing for a lifting and locking system in accordance with the invention; 
       FIG. 17  is a partial perspective view of an end support in accordance with the invention; 
       FIG. 18  is a cutaway perspective view of an end support in accordance with the invention; 
       FIG. 19  is front quarter cutaway perspective view of an external support in accordance with the invention; 
       FIG. 20  is a rear quarter cutaway perspective view of an external support in accordance with the invention; 
       FIG. 21  is cutaway perspective view of an alternative embodiment of an external support in accordance with the invention; and 
       FIG. 22  is a process flow diagram of a method for using a side-dumping trailer and unloading station in accordance with the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   It will be readily understood that the components of the present invention, as generally described and illustrated in the Figures herein, may be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the system and method of the present invention, as represented in  FIGS. 1 through 22 , is not intended to limit the scope of the invention. The scope of the invention is as broad as claimed herein. The illustrations are merely representative of certain, presently contemplated embodiments of the invention. Those embodiments of the invention will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. 
   Those of ordinary skill in the art will, of course, appreciate that various modifications to the details of the Figures may easily be made without departing from the essential characteristics of the invention. Thus, the following description of the Figures is intended only by way of example, and simply illustrates certain presently preferred embodiments consistent with the invention as claimed. 
   Referring to  FIGS. 1 and 2 , in one embodiment an apparatus  10  may include an unloading station  12  having a forward end  14  and a rearward end  16 . A longitudinal direction  18  may be defined as substantially parallel to a line connecting the forward end  14  and the rearward end  16 . A transverse direction  20  may be defined as substantially parallel to the line of action of gravitational forces when the unloading station  12  is not tipped. A lateral direction  22  may be defined as substantially perpendicular to the longitudinal direction  18  and the vertical direction  20 . Rotational directions  24 ,  26 ,  28  may be defined as rotation about any axis parallel to the longitudinal direction  18 , vertical direction  20 , and lateral direction  22 , respectively. 
   A truck  30  may tow a lead trailer  32 . A truck  30  may also tow a pup trailer  34  secured to the lead trailer  32 . The truck  30  and trailers  32 ,  34  may be positioned on the unloading station  12 . The truck  30  and trailers  32 ,  34  may be driven onto the unloading station  12 , hoisted onto the unloading station  12 , or otherwise positioned on the unloading station  12 . 
   The unloading station may then be actuated to unload the trailers  32 ,  34 . In some embodiments, the truck  30  or tractor  30  may be positioned on the unloading station  12  during unloading. In others, the truck  30  may simply draw the trailers  32 ,  34  onto the unloading station  12 , detach from the trailers  32 ,  34 , and drive off the unloading station  12 . In still other embodiments, the trailers  32 ,  34  may be detached from the truck  30  and hoisted or towed onto the unloading station  12 . The trailers  32 ,  34  may have doors  36  to facilitate unloading. 
   The tops  37  of the trailers  32 ,  34  are typically removable and may be embodied as tarps or like structures for preventing the cargo from escaping. The tops  37  are typically removed to facilitate loading of the trailers  32 ,  34  from above. 
   Referring to  FIGS. 1 and 2 , an unloading station  12  may have a bearing surface  50  sized to support the a trailer  32 , trailers  32 ,  34 , or truck  30  and trailers  32 ,  34 . The bearing surface  50  may be made of steel or other sufficiently strong structural material. In some embodiments the bearing surface  50  is a sheet  52 , or sheets  52  of steel. The sheet  52  may be supported by undergirding members  54  or the like to increase its stiffness. A wheel guide  56  may positioned on or near the bearing surface  50  and may resist sliding of the wheels of the trailers  32 ,  34  or truck  30  in the lateral direction  22 . 
   In some embodiments of the present invention, the doors  36  may provide structural strength to trailers  32 ,  34  when closed. Accordingly, an unloading station  12  may include an external support to relieve stresses in the trailers  32 ,  34  when the doors  36  are opened during unloading. In embodiments of loading stations  12  simultaneously accommodating both trailers  32 ,  34 , multiple external supports may be provided to independently support each trailer  32 ,  34 . 
   In some embodiments, the external supports are belly supports  60  positioned to support the underside of a trailer  32 ,  34 . The belly support  60  may be hydraulically actuated such that it may be positioned flush with the sheet  52  while the trailers  32 ,  34  are drawn onto the loading station  12 , and then raised to support the trailers  32 ,  34  during unloading. The belly support  60  may extend through apertures  61  in the sheets  52 . 
   The unloading station  12  may be actuated to unload the trailers  32 ,  34  by rotating the bearing surface  50  in a rotational direction  24 . Accordingly various anchoring mechanisms may be used to prevent trailers  32 ,  34  from falling or dislodging during rotation. Anchors  62  may therefore engage the trailers  32 ,  34  and the truck  30  to prevent tipping or sliding relative to the bearing surface  50 . In some embodiments, the anchors  62  may be positioned along one lateral side of the trailers  32 ,  34  opposite the doors  36 . In some embodiments, the trailers  32 ,  34  may have doors  36  on both lateral sides, accordingly the position of the anchors  62  may be defined as being opposite the doors  36  that are opened for unloading purposes. 
   Referring to both  FIGS. 1 and 2 , fixed restraints  64  may prevent dislodgement or rollover of the trailers  32 ,  34  or truck  30  if the anchors  62  fail or are not engaged properly. The fixed restraints  64  are typically arranged such that no human intervention is required for their operation. For example, the fixed restraints  64  may be embodied as towers  66   a – 66   c . The towers  66   a – 66   c  may be positioned such that they do not interfere with the opening of the doors  36 . 
   Typical embodiments of the trailers  32 ,  34  have doors  36  extending along substantially the entire lateral side of the trailers  32 ,  34 . Accordingly, a tower  66   a  may be positioned near the rearward end  68  of a pup trailer  34 . A tower  66   b  may be positioned between the lead trailer  32  and the pup trailer  34 . A tower  66   c  may be positioned near the forward end  70  of the lead trailer  32 . A tower  66   a – 66   c  may have a cross member  74 . The towers  66   a – 66   c  are typically sufficiently narrow such that an opening door  36  will not contact the towers  66   a – 66   c  or be obstructed thereby. The cross member  74  may extend in the longitudinal direction  18  such that the top of a tipping trailer  32 ,  34  would catch on the cross member  74 . The cross members  74  are typically positioned vertically such that they do not interfere with opening doors  36 . In some embodiments, a cable may be strung across the cross members  74  for added security. In still other embodiments, cable may be strung between the towers  66   a – 66   c  at a height providing security against falling for people working on the bearing surface  50 . 
   Referring to  FIG. 3 , the wheel guide  56  may include a rail  80  secured to the bearing surface  50 . In some embodiments the wheel guide  56  may include two rails  80  both positioned to resist motion of wheels in the same lateral direction  22 . Thus, for example, one rail  80  may exert forces on an inward facing surface of a wheel on one side of a vehicle while another rail  80  exerts forces on the outside surface of a wheel on the other side of a vehicle. Alternatively, embodiments having two rails  80  may position the rails  80  such that each engages the inside surface of the wheels. In which case, one of the rails  80  serves to guide the truck  30  during positioning on the unloading station  12  and does not serve to restrain the truck  30  or trailers  32 ,  34  during unloading. The rail  80  may extend a sufficient distance  84  above the bearing surface  50  such that lateral forces may not readily cause a tire to deflect and slide over the rail  80 . The larger the distance  84 , the greater the extent to which a tire must deflect to slide over the rail  80 . 
   A walkway  86  may secure to the side of the bearing surface  50 . In some emboidments the walkway  86  may be a portion of the sheet  52  extending away from the anchors  62 . In the illustrated embodiment, the walkway  86  comprises a separate sheet  88  supported by undergirding  90 . Providing a separate walkway  86  having a separate sheet  88  and undergirding  90  may provide the significant advantage of requiring less massive structural members inasmuch as the walkway will only need to bear the weight of people walking thereon. Also, the span under the trailer need not be so extended. A walkway may include an outer railing  92  to prevent falls. Additional posts  94  may be provided between the walkway  86  and the bearing surface  50 . The posts  94  may secure to a cable  100  or the like preventing operators from approaching to close to the trailers  32 ,  34  and truck  30  during the unloading process. Hooks, or the like, forming part of the anchor  62  may also hook to the cable  100  when not in use in order to present the anchoring hooks in a convenient position for subsequent use. 
   An anchor  62  may be embodied as a fastener  110  secured to a line  112 . The line  112  may have a length such that the fastener  110  need only secure to a trailer  32 ,  34  in order to adequately anchor the trailer  32 ,  34 . Alternatively, the line  112  may secure to a tensioning system that tightens the chain during the unloading process. The line  112  may be embodied as a strap, cable, chain, cord, bar or any structure suitable for withstanding tensile forces. 
   Likewise, a fastener  110  may be embodied as any structure capable of securing one structure to another in a manner capable of resisting forces tending to separate them. Accordingly, the fastener  110  may be embodied as a hook, loop of material, or an eyebolt for fitting over a hook secured to a trailer  32 ,  34 . The fastener  110  may alternatively be any structure sized to fit into a receptacle and resist movement in at least one direction after insertion, or any receptacle secureable to a line  112  and capable of receiving another structure and resist removal thereof. 
   Referring to  FIG. 4 , a tensioning system  114  may include an actuator  116  that may be powered to provide tension and relieve tension in the chain  112 . The actuator  116  may be any actuating system known in the art and capable of applying tension to a line  112 . The actuator  116  may secure to a mounting bracket  118  secured to the undergirding  54 . Alternatively, the mounting bracket  118  may mount directly on the sheet  52 . In still other embodiments, the actuator may secure directly to the undergirding  54  or to the sheet  52 . The mounting bracket  118  may include two side pieces  120  both secured to the undergirding  54  and spaced apart from one another. A lever arm  122  may secure to the mounting bracket  118 . An end  124  of the lever arm  122  may secure to the mounting bracket  118  by means of a pivot  126 , such as a bolt  127 , rod  127 , or other similarly performing structure. 
   Another end  128  of the lever arm  122  spaced apart from the end  124  may secure to the line  112  by means of a fastener  130 , such as a bolt, weld, pin, or the like. In the illustrated embodiment, the fastener  130  maybe a link  132  of the line  112  bolted to the lever arm  122 . 
   The actuator  116  may be embodied as a cylinder  134 , such as a hydraulic cylinder and piston  134 , secured to the mounting bracket  118 . The cylinder  134  may be mounted to the mounting bracket  118  by means of a pivot  136 , such as a bolt  137 , pin  137 , rod  137 , or the like. The piston of the hydraulic cylinder  134  sliding within the cylinder  134  may have one end secured to the lever arm  122 . The piston of the hydraulic cylinder  134  may secure to the lever arm  122  by means of a pivot  138 , such as a bolt  139 , pin  139 , rod  139 , or the like. The point of attachment of the piston of the hydraulic cylinder  134  to the lever arm  122  may vary depending on the capacity of the piston and cylinder  134  to provide force and displacement in tensioning the line  112 . The piston, and ram or rod associated therewith, of the hydraulic cylinder  134  may secure to the lever arm  122  at a position such that when the rod or ram extending from the piston head of the hydraulic cylinder  134  is at or below its maximum extension from the cylinder  134 , the end  128  of the lever arm  122  has tensioned the line  112  enough to adequately anchor a trailer  32 ,  34 . 
   Various equivalent embodiments of the tensioning system  114  described are possible, for example the cylinder  134  may secure to the lever arm  122  with the piston ram or of the cylinder  134  secured to the mounting bracket  118 . In addition, a tensioning system  114  may simply have a line  112  coupled directly within an actuator  116  capable of tensioning the line  112 . 
   In some embodiments, an agitator (e.g. vibrator, etc.) is interposed between the point of securement of the tensioning system  114  and the point of attachment of the fastener  110  to the trailer  32 ,  34 . An agitator may shake loose the contents of a trailer  32 ,  34  and ensure that unloading occurs successfully. In alternative embodiments the agitator  140  is interposed between the fastener  110  and the line  112 . Alternatively, the agitator may be interposed between the line  112  and the lever arm  122 , between the piston  138  and the lever arm  122 , or between the cylinder  134  and the mounting bracket  118 . 
   Referring to  FIG. 5 , in some embodiments of a tensioning system  114 , multiple anchors  62  may be tensioned by the same actuators  116 . For example, the lines  112  of multiple anchors  62  may secure near the free end  150  of a frame  152 . The lines  112  may secure to standoffs  154  extending upwardly from a point near the free end  150  of the frame  152  in order to position the lines  112  convenient for securement of the fasteners  110  to the trailers  32 ,  34 . 
   A secured end  156  may secure to the undergirding  54 , or the like, by means of a pivot  158 . The pivot  158  may be embodied as a rod  160  passing through apertures  162  formed in the frame  152 , with the ends of the rod  160  secured to flanges  163  secured to the undergirding  54 , or other structure comprising the unloading station  12 . One or more hydraulic cylinders  134  and pistons  138  may cause the frame to pivot about the pivot  158 . 
   Referring to  FIG. 6 , An unloading station  12  may include a plurality of support assemblies  168  spaced apart longitudinally along and beneath the bearing surface  50 . A support assembly  168  may include a base  170  that may rest on a flat but otherwise unprepared surface, a specially prepared foundation, or on a trailer. The base  170  may secure to a foundation by means of bolts or the like. In embodiments resting on a trailer, the base  170  may secure to the bed of the trailer. The base  170  or trailer may include selectively positionable outriggers, such as might be provided for large trailer-mounted construction cranes, to provide stability during the unloading process. Likewise, ramps or the like may be provided to allow trailers  32 ,  34  to be raised from the ground to the bearing surface  50 . 
   The base  170  may include beams  172  extending in the lateral direction  22 . A vertical support  174  may secure to the beams  172  and extend upwardly therefrom. The vertical support  174  may raise the bearing surface  50  to provide a space for lifting hardware. A portion of the bearing surface  50  may pivotally secure to the vertical supports  174 . 
   A vertical support  174  may be embodied as any suitable supporting structure. In the illustrated embodiment the vertical support  174  is a section of a square beam  176 . A strut  177  may secure to the vertical support  174  and base  170  to reinforce the vertical support  174 . Typically, the bearing surface  50  pivotally secures to the vertical supports  174  proximate an edge of the bearing surface  50 . Accordingly, a pivot  178  may secure the vertical support  174  to the bearing surface  50 . A pivot  178  may include any means known in the art for pivotally securing structural members. 
   In some embodiments, the pivot  178  may be embodied as a shaft  180  extending through apertures formed in both the vertical support  174  and the bearing surface  50 . In some embodiments, the vertical support  174  may include a tongue  182  fitting into a groove  184  secured to the undergirding  54 . A shaft  180  may then extend through apertures  185  extending through the tongue  182  and groove  184 . 
   An unloading station  12  may include a lifting actuator  186  for tipping the bearing surface  50  and trailers  32 ,  34  resting thereon. It will be understood that although  FIG. 6  illustrates one actuator  186 , other actuators  186  may be distributed along the unloading station  12 , spaced apart in the longitudinal direction  18  in a configuration substantially equivalent or even identical to that shown. Typically, each support assembly  168  will have an actuator  186  associated therewith. Furthermore, any mechanism or method known in the art for lifting and tipping, or both, may serve as an actuator  186  for tipping the bearing surface  50 . 
   An actuator  186  may be embodied as a hydraulic cylinder  188  and piston  190 . The piston  190  may secure near the edge of the bearing surface  50  opposite the point or location of attachment of the vertical support  174  to the bearing surface  50 . The piston  190  typically pivotally secures to the bearing surface  50  by means of a pivot  191 , such as a shaft  192  extending through an aperture  194  in the undergirding  54  of the bearing surface  50 . The cylinder  188  may pivotally secure to the base  170  by means of a pivot  196 , such as a shaft  198  extending through an aperture  200  in the base  170 . Alternatively, flanges  202  may extend from the base and have the apertures  200  formed therein to receive a shaft  198 . 
   In some embodiments, the cylinder  188  may have a length  206  such that when the piston  190  is withdrawn within the cylinder  188 , the cylinder  188  supports the bearing surface  50  against downward movement. Alternatively, a stop  208  may maintain the bearing surface  50  in a horizontal orientation during positioning of the trailers  32 ,  34  on the bearing surface  50 . The stop  208  may be embodied as a square beam  210  extending upwardly from the base  170 . The top of the beam  210  may secure to a plate  212  to distribute forces from the beam  210  to the bearing surface  50 . 
   Referring to  FIG. 6 , The hydraulic cylinder  188  and piston  190  may tip the bearing surface  50  as illustrated. The bearing surface  50  typically rotates through an angle sufficient that gravitational forces are enabled to draw the cargo from the trailers  32 ,  34 . A rotation of approximately 50 degrees has proven adequate. If the cargo is not wood debris or similarly long, stiff, binding material 40 degrees suffice, and sometimes  30 . 
   During rotation, the center of gravity  220  of the trailers  32 ,  34  and bearing surface  50  may cross over the pivot  178 . As a result, gravity will tend to urge the bearing surface  50  to continue rotating. It will then be necessary for the actuator  186  to change the direction of application of any lifting force and instead resist further rotation of the bearing surface  50 . In some embodiments, hydraulic fluid directed toward the cylinder  188  may pass through an over-center valve, to direct hydraulic fluid toward an opposite face of the piston  190 , resisting extension of the piston  190  from the cylinder  188 , rather than promoting it, as soon as the center of gravity  220  crosses over the pivot  178 . 
   Referring to  FIG. 8 , a front anchor  228  may secure a truck  30  (e.g. tractor  30 ) against tipping during the unloading process. In some embodiments a front anchor  228  may secure to the wheel guide  56  near the forward end  14  of the tipping station  12 . Alternatively, the front anchor  228  may secure directly to the bearing surface  50 . In some uses of the apparatus  10 , snow and ice may be present, even prevalent. Accordingly, securement of the front anchor  228  to the wheel guide  56  may raise the front anchor  228  above snow and ice on the bearing surface  50  to promote accessibility. 
   A front anchor  228  may be embodied as a comealong  230  (e.g. boomer, winch, ratchet, etc.). Any apparatus known in the art may serve for increasing tension in a mechanism securing one structure to another. For example, any embodiment of a comealong  230  known in the art may be used. In some embodiments of a front anchor  228 , the comealong  230 , or like apparatus, may remain secured to a fastener  232  secured to the wheel guide  56  or bearing surface  50 . In typical embodiments, the fastener  232  will permit the comealong  230  to rotate relative to the bearing surface  50  in order to move the comealong  230  out of the way of the truck  30  and trailers  32 ,  34  as they are driven off the unloading station  12 . Accordingly, the fastener  232  may be a ‘U’ bolt  234  fitting through an eye  236  formed in the comealong  230 . A second eye  238  may selectively secure to a hook, post, or other such structure, formed on the front bumper or frame of the truck  30 . 
   Referring to  FIG. 9 , given the icy conditions that may exist on an unloading station  12 , a comealong  230  that remains secured to the bearing surface  50  may become encrusted or buried in snow and ice. Accordingly, a comealong  230  may remain secured to the front bumper, frame, or other structure, of the truck  30 . A receptacle  240  may be provided to provide a securement point for the comealong  230 . In some embodiments, an aperture  241  may be formed in the sheet  52 . 
   The receptacle  240  may be embodied as a hook  242 , post  242 , rod  242 , or like structure capable of securement to a comealong  230  or the like, extending into the aperture  241  and positioned flush with the sheet  52  or below the sheet  52 . Such an embodiment permits the receptacle  240  to be self-clearing. For example, the aperture  241  may be made sufficiently large that it will not easily clog with snow. Thus, snow and ice will not hinder access to the receptacle  240 . 
   Referring to  FIGS. 10 and 11 , a trailer  32 ,  34  may have several significant functional features. In typical embodiments in accordance with the present invention, both the lead trailer  32  and the pup trailer  34  function similarly. Accordingly  FIG. 11  illustrates the pup trailer  34  but the functional features referred to below may belong to both the lead trailer  32  and the pup trailer  34 . 
   For certain cargos that include a plurality of large irregularly shaped masses, it may be particularly important to remove structures from a trailer  32 ,  34  that may interfere with unloading thereof. Furthermore, in cargos consisting of both large and small bodies, material may tend to collect in areas (e.g. blind corners) of the trailer that are not positioned such that the material is drawn out when the trailer is tipped. Accordingly, support for the floor  246  of the trailers  32 ,  34  may be provided by upper beams  248   a ,  248   b . Structural members in the floor  246  may then be configured to avoid interference with unloading cargo while maximizing volume. 
   For example, the cross members  250  in the floor  246  may extend in a lateral direction  22 . For less dense cargo, placement of the cross members  250  above the floor  246  may be desirable to maximize the volume available to hold cargo. Lateral extension of the cross members  250  will enable cargo to slide along the cross members  250  during tipping, rather than be obstructed by them as might be the case for longitudinally extending structural members. 
   However, laterally extending cross members  250  do not resist bending in the transverse plane extending in the longitudinal direction  22 . Accordingly, longitudinal supports  252   a ,  252   b  may extend along the sides of a trailer  32 ,  34  and provide support against bending. However, longitudinal supports  252   a ,  252   b  may be limited in bulk to avoid interfering with the unloading of cargo. Inasmuch as stiffness against bending would require a large vertical dimension, if longitudinal supports  252   a ,  252   b  were independently capable of bearing loads they would have a large vertical dimension. A large vertical dimension would either create a large lip above the floor hindering unloading of cargo, or would require that the floor  246  be raised thereabove, thereby reducing the volume of a trailer  32 ,  34 . 
   However, the longitudinal support  252   b  is not positioned near the door  36  and its vertical dimension may be large enough for the longitudinal support  252   b  to independently bear bending loads, inasmuch as no cargo will need to pass thereover during unloading. In certain embodiments in accordance with the current invention, doors  36  may be provided on either side of a trailer  35 , in which case both longitudinal supports  252   a ,  252   b  may be shortened vertically to facilitate unloading. 
   The longitudinal supports  252   a ,  252   b  and floor  245 , and cross members  250  may be constructed such that without support from the upper beam  248   a  they would fail or otherwise not function properly. For example, longitudinal supports  252   a ,  252   b  and floor  245 , and cross members  250  may be made such that unsupported one or more of them they would fail prematurely due to fatigue, excessive bending, excessive yielding, or other form of failure. 
   A longitudinal support  252   a ,  252   b  is typically configured as a beam extending along the length of the trailer  32 ,  34 . For example, longitudinal support  252   b  may be a beam  254   b  extending along the length of the trailer  32 ,  34 . A longitudinal support  252   a  may be embodied by a similarly configured beam  254   a . A ramp  256  may extend from the floor  246  to the top of the beam  254   a  to facilitate unloading of cargo. 
   A door  36  may transfer loads between the floor  246  and an upper beam  248   b  to limit bending deflections of the longitudinal support  252   a . The upper beams  248   a ,  248   b  may be increased in size to bear such loads because they are not in a position to interfere as significantly with unloading. Furthermore, the door  36  provides structural support by bearing shear forces and by distributing the larger bending moments developed at the center of the trailer  32 ,  34  across the entire upper beam  248   a . A latching system  260  may then enable latching of the door  36  in a manner allowing the lower edge portion of door  36  to exert an upward force on the longitudinal support  252   a . A latching system  260  may be any system known in the art to provide such functionality. 
   A latching system  260  may comprise a plurality of catches  262  spaced apart from one another along the longitudinal support  252   a . The distance between the catches  262  may depend on the expected loads at a particular point along the longitudinal support  252   a , with the catches being spaced closer together in comparatively highly loaded portions of the longitudinal support  252   a . Alternatively, the catches  262  may each be separated from one another by the same distance. 
   Corner supports  264  positioned at the four corners of the trailer  32 ,  34  may support the upper beams  248   a ,  248   b . The corner supports  264  are typically dimensioned to bear the weight of the door  36  as well as the loads transferred to the door  36  from the floor  246 . The rearward end  266  of the trailer  32 ,  34  may rest on a wheel assembly  268 . The forward end  270  of the trailer  32 ,  34  may be supported proximate a point  272  by a hitching system of the truck  30 . In embodiments of the trailers  32 ,  34  implemented as a “B-Train” trailer system, the forward end  270  of the pup trailer  34  rests on a hitch assembly forming part of the lead trailer  32 . The forward end  270  of a “B-Train” lead trailer  32  rests on a hitch assembly secured to the truck  30 . 
   Anchor receptacles  274  may distribute along the trailer  32 ,  34  for engagement with the fasteners  110  of the anchors  62 . In the illustrated embodiment, the anchor receptacles are rings  276  positioned within cavities  277  formed in the beam  254   b.    
   Referring specifically to  FIG. 11 , while still referring generally to  FIG. 10 , a catch  262  may include a receptacle  278  and a fastener  280 . In the illustrated embodiment, the receptacle is embodied as a pocket  282  secured to the longitudinal support  252   a  by means of welds, bolts, or the like. The fastener  280  may be embodied as a hook  284  secured to the door  36  by means of welds, bolts, or the like. 
   The pocket  282  may secure to the ramp  256  forming part of the longitudinal support  252   a . The ramp  256  may secure to the top of the longitudinal beam  254   a  and descend gradually to the floor  246 . The slope  288  of the ramp  176  may be chosen to promote the unloading of cargo. A slope  288  of approximately 25 degrees has been found to be adequate. 
   The pocket  274  may include a rear plate  290  formed at approximately the same slope  288  to promote unloading of cargo. The pocket  282  may include a front plate  292  oriented to retain a hook  284  In the illustrated embodiment, the front plate  292  is oriented substantially vertically. The front plate  292  may have an inner edge  294  that is chamfered, or otherwise formed to facilitate insertion of the hook  284 . The front plate  292  may secure directly to the rear plate  290 . Alternatively, a top plate  296  may secure to the rear plate  290  and the front  292  to maintain the front plate  292  spaced laterally apart from the rear plate  290 . A side plate  293 , or side plates  293 , may secure on either side of the rear plate  290 , front plate  292 , and top plate  296  to increase rigidity of the pocket  282  and to inhibit longitudinal movement of the hook  284 . Of course, the pocket  282  may be any structure capable of retaining the hook  284 , which does not unduly inhibit unloading of cargo. 
   A door  36  may be formed with a lip  306  extending inwardly in the lateral direction  22 . The hook  284  may comprise a plate, or other structure, fitted to the contour of the lip  306  with a prong  310  extending upwardly therefrom. The prong  310  may include a chamfered edge  312  to facilitate insertion of the prong into the pocket  282 . The hook  284  and the lower edge  314  of the door  36  may be formed to rest flush with the beam  254   a.    
   In some embodiments the chamfered edge  312  of the hook  284  and the chamfered edge  294  of the front plate  292  may allow the door  36  to be opened at one end and allowing the force of the cargo to force the hook  284  to slide out of engagement with the pocket  282 . 
   Referring to  FIG. 12 , while continuing to refer to  FIG. 10 , an alternative embodiment of a catch  262 , a fastener  280  may secure to the longitudinal support  252   a , and a receptacle  278  may secure to the door  36 . For example, a receptacle  278  may be a bar  316  positioned to extend between two side brackets  318   a ,  318   b  welded, or otherwise secured, to the door  36 . The fastener  280  may be a hook  284  secured to the ramp  256  and oriented to receive the bar  316  when the bar  316  is brought from below the hook  284  into engagement with the hook  284 . 
   Referring to  FIG. 13 , the trailers  32 ,  34  may include a lifting mechanism  330  that lifts the door  36  and brings the fastener  280  into engagement with the receptacle  278 . A locking mechanism  332  may maintain the fastener  280  in engagement with the receptacle  278 . Typically a locking mechanism  332  will maintain an upward force on the door  36 . The locking mechanism  332  may be positioned at any point on the door  36  allowing the door  36  to be fixed relative to the corner supports  264 , the upper beam  248   a , or the longitudinal support  252   a . Accordingly, the locking mechanism  332  may be any locking system capable of fixing the position of the door  36  relative to the corner supports  264 , the upper beam  248   a , or the longitudinal support  252   a  and having sufficient strength to bear loads such as the weight of the door  36  and loads transferred from the floor  246  to the door  36 . 
   In the illustrated embodiment, the lifting mechanism  330  is a hydraulic cylinder  338  and piston  340 . The piston  340  may raise a shaft  342  extending along the upper beam  248 . The piston  340  may secured to an arm  344  near the end  346  of the arm  344 . The other end  348  of the arm  344  pivotally secures to a structural member of the trailer  32 ,  34 , such as the upper beam  248  or corner support  264 . The shaft  342  secures to the arm  344  at a position suitable to transfer force adequately between the piston  340  and the shaft  342 . The position of the point of securement of the shaft  342  to the arm  344  may also be chosen to ensure that the range of motion of the piston  340  translates into an adequate translation of the shaft  342  to bring the fastener  280  into engagement with the receptacle  278 . In the illustrated embodiment, the shaft  342  secures to the arm  344  between the ends  346 , 348 . The shaft  342  typically secures rotatably to the arm  344 , for example, by passing through an aperture  350  sized to allow rotation. 
   The shaft  342  may secure directly to the door  36  to lift the door  36 . Alternatively, in the illustrated embodiment, the shaft  342  may secure to a linkage  352  serving to transfer a lifting motion from the shaft  342  to the door  36 . In the illustrated embodiment the linkage  352  may also function as the locking mechanism  332 . 
   The lifting mechanism  330  may include two sets of hydraulic cylinders  338  and pistons  340  positioned near either end  266 ,  270  of the trailer  32 ,  34 . A locking mechanism  332  may likewise include two linkages  352  positioned near either end  266 ,  270  of the trailers  32 ,  34 . The configurations of the hydraulic cylinder  338 , piston  340 , and linkage  352  at one end  266  of the trailers  32 ,  34  may substantially mirror the configuration of the hydraulic cylinder  338 , piston  340 , and linkage  352  at the end  270  of the trailers  32 ,  34 . 
   Referring to  FIG. 14 , the linkage  352  may be any linkage lockable in an over-center position. For example, the linkage  352  may include the illustrated links to accomplish its locking function. The linkage  352  may engage a bushing  360  secured to the door  36  by means of side plates  362  welded, or otherwise secured, to the door  36 . A bearing plate  264  may secure to the bushing  360  along one of its edges by means of welds, or other fasteners. The bearing plate  264  may also secure to the side plates  362  along its edges. In some embodiments, the bushing  360  may be welded directly to the door  36 . Alternatively, the side plates  362  may have apertures serving the function of the bushing  360 . Of course, various other structures are equivalent and may provide an aperture for receiving a shaft or the like. 
   Guide plates  366   a ,  366   b  may be positioned on either side of the bushing  360 . Bearing cross plates  368   a  and  368   b  may secure to the guide plates  366   a , 366   b  by welds, or other securement mechanisms, and maintain the guide plates  366   a ,  366   b  in a fixed position relative to one another. The guide plates  366   a ,  366   b  also secure to the upper beam  248   a  by means of welds, or other securement means. The guide plates  366   a ,  366   b  may be planar or any other shape providing sufficient structural strength. Likewise, a single plate  366   a  may be used. 
   The guide plates  366   a ,  366   b  may include a guide  370 . The bushing  360  will typically be directed by the guide  370  during lifting and locking of the door  36 . The guide  370  may be a groove, rail, or any such structure, engaging the bushing  360 , its equivalent structure, or the side plates  362  in a manner allowing the guide  370  to direct the movement of the bushing  360 , or its equivalent structure. The guide  370  is typically oriented at an angle  372  such that lifting the door  36  also brings the door  36  closer to the body of the trailer  32 ,  34 . The angle  372  may facilitate locking of the linkage  352  as discussed below. In the illustrated embodiment, the guide  370  is an elongate aperture  374 . 
   A pivot  378  may also be provided on the guide plates  366   a ,  366   b . The pivot  378  is any structure that secures to a second structure and enables relative rotation of the second structure as well as the transfer of loads between the pivot  378  and the second structure. Accordingly, the pivot  378  may be a post, or like structure, secured or formed on a guide plate  366   a ,  366   b  for insertion into an aperture on a second structure. In the illustrated embodiment, the pivot  378  is embodied as apertures  380  formed in the guide plates  366   a ,  366   b  for receiving a shaft, or the like. 
   The linkage  352  may include a driven link  386   a , or driven links  386   a ,  386   b , which secure to the shaft  342  and are rotated thereby as the shaft  342  is lifted by the piston  340 . The driven links  386   a ,  386   b  may rigidly secure to the shaft  342 . Alternatively, the driven links  386   a ,  386   b  may rotatably secure to the shaft  342 , and the arm  344  may fixedly, or rotatably, secure to the shaft  342 . In the illustrated embodiment, the driven links  386   a ,  386   b  include a surface  388  adapted to receive the shaft  342 . For example, the surface  388  maybe curved to receive a round shaft  342 . In the illustrated embodiment, the shaft  342  fixedly secures to the driven links  386   a ,  386   b  by welds, or any other suitable securement mechanism. 
   The driven links  386   a ,  386   b  each secure to the pivot  378  of the guide plates  366   a ,  366   b . In the illustrated embodiment, the driven links  386  include an aperture  390 . The apertures  390  may fit over a post or like structure formed on the guide plates  366   a ,  366   b . In the illustrated embodiment, a shaft  392  passes through the apertures  390  of the driven links  386   a ,  386   b  and through the apertures  380  of the guide plates  366   a ,  366   b.    
   Various means may serve to retain the shaft  392  in engagement with the apertures  380 ,  390 . In the illustrated embodiment, a retaining bracket  394  having an aperture  396  extending around the shaft  392  may serve this purpose. The bracket  394  may be welded, or otherwise fixedly secured, to the end  398  of the shaft  392 . Alternatively, the bracket may have no aperture  396  and simply be welded, bolted, riveted, or otherwise fastened to the end  398  of the shaft  392 . The bracket  394  may also fasten to the driven link  386   a  by means of a bolt  400 , or other such fastener mechanism, extending through both the bracket  394  and the driven link  386   a  and being secured with a nut  402 . The end  404  of the shaft  392  may also include a lock structure for preventing removal thereof. In the illustrated embodiment a snap ring  406  prevents removal of the shaft  392 . Of course, various other structures for retaining the shaft  392  are possible, such as set screws, keys, lynch pins, or the like. 
   A bushing  408  may insert through the apertures  380  in the guide plates  366   a ,  366   b . The bushing  408  may be sized to extend through the apertures  380  during operation of the linkage  352 . The shaft  392  may then extend through the bushing  408  without contacting the guide plates  366   a ,  366   b.    
   Lifting links  420   a ,  420   b  may pivotally secure to the bushing  360 , or its equivalent. The lifting links  420   a ,  420   b  directly exert a lifting force on the door  36 . Any lifting structure known in the art for pivotally securing one structure to another may be used. For example, the lifting links  420   a ,  420   b  may pivotally secure to posts extending from the side plates  362 . In the illustrated embodiment, the lifting links  420   a ,  420   b  secure to the bushing  360  by a shaft  422  passing through the bushing  360  and apertures  424  formed in the lifting links  420   a ,  420   b.    
   In some embodiments, bushings  426  may insert into the apertures  424  with the shaft  422  extending through the bushings  426 . The bushings  426   a , 426   b  may be welded or otherwise secured to the lifting links  420   a ,  420   b . For example, slip rings, set screws, or the like, may prevent unintended removal of the bushings  426   a , 426   b  from the lifting links  420   a ,  420   b . The lifting links  420   a ,  420   b  may secure to the bushings  426   a , 426   b  such that the bushings  426   a , 426   b  maintain the lifting links  420   a ,  420   b  separated from the guide plates  366   a ,  366   b . Separation may be necessary to provide room for the driven links  386   a ,  386   b , which may be positioned between the lifting links  420   a ,  420   b.    
   The bushings  426   a ,  426   b  secure to the shaft  422  by any appropriate connector structure, such as bolts, snap rings, set screws, or the like. The bushings  426   a , 426   b  may secure either fixedly or pivotally to the shaft  422 . In the illustrated embodiment, a snap ring  430  secures to the shaft  422  near the bushing  426   b . The bushing  426   a  is secured to the shaft  422  by a bolt  432  extending through both the bushing  426   b  and the shaft  422  and held in place by a nut  434 . 
   The lifting links  420   a ,  420   b  also secure to the driven links  386   a ,  386   b  in order that the driven links  386   a , 386   b  may either lift or lower the lifting links  420   a ,  420   b , depending on the direction of rotation. The lifting links  420   a ,  420   b  may pivotally secure to the driven links  386   a ,  386   b  by any securement structure for pivotally securing one member to another. For example, a lifting link  420   a ,  420   b  may be bolted to a driven link  386   a ,  386   b , respectively. Alternatively, either a lifting link  420   a ,  420   b  or driven link  386   a ,  386   b  may have a post, or like structure, engaging an aperture, or like structure, on a driven link  386   a ,  386   b  or lifting link  420   a ,  420   b , respectively. 
   In the illustrated embodiment, a shaft  440  extends through apertures  444  in the driven links  386   a ,  386   b  and through apertures  442  in the lifting links  420   a ,  420   b . In some embodiments, bushings  446   a , 446   b  may extend through the apertures  444  in the lifting links  420   a ,  420   b . The bushings  446   a ,  446   b  may be fixed relative to the lifting links  420   a ,  420   b  by means of welds, or any other securement means preventing the bushings  446   a , 446   b  from disengaging from the lifting links  420   a ,  420   b . In some embodiments, the lifting links  420   a ,  420   b  may secure to the bushings  446   a , 446   b  such that the lifting links  420   a ,  420   b  are separated from the driven links  386   a , 386   b . Separating the lifting links  420   a ,  420   b  and the driven links  386   a , 286   b  may reduce or prevent interference between the lifting links  420   a ,  420   b  and the ends of the shaft  392  or the bolt  400 . The bushings  446   a , 446   b  typically do not extend into the apertures  444  in the driven links  386   a , 386   b.    
   The bushings  446   a , 446   b  secure to the shaft  440  by any suitable securement structure. In the illustrated embodiment, a snap ring  450  prevents removal of a bushing  446   b  from the shaft  440 . A bolt  452  may extend through a bushing  446   a  and the shaft  440  and be held in place by a nut  454 . Of course, various fasteners may accomplish this retaining function, such as lynch pins, set screws, keys, or the like. 
   In some embodiments, a bushing  456  may extend between and through the apertures  444  in the driven links  386   a ,  386   b . The bushing  456  may be fixed to the driven links  386   a ,  386   b  by welds, fasteners, or the like. Alternatively, the bushings  446   a ,  446   b  on either side may keep the bushing  456  positioned within the apertures  444 . 
   The linkage  352  includes a number of parts rotatably secured to one another. Accordingly, the weight of the door  36  secured to the linkage  352  may tend to rotate the linkage into unsafe positions or into positions placing undue strain on parts of the linkage  352 . Accordingly, a stop  458  may serve to resist or prevent the linkage  352  moving into such positions. In one embodiment, a stop  458  is a plate  460 , or other such structure  4460 , secured to the bushing  456  and to the shaft  342 , by a weld, fastener, or the like. The plate  412  may interfere with the guideplates  366   a ,  366   b  or the cross plates  368   a ,  368   b  and thereby limit the range of motion of the shaft  342  and driven links  386 . 
   Referring to  FIGS. 15A and 15B , a linkage  352  may move through positions such as those illustrated. It will be understood that the range of movement of the linkage  352  is dependent on the dimensions of the driven links  386   a ,  386   b , lifting links  420   a ,  420   b  and the guide plates  366   a ,  366   b  apparent in the side views of  FIGS. 15A and 15B . Accordingly, several variations in the securement structures of the various links connecting to one another, and the configuration or disposition of the links not affecting the dimensions apparent in  FIGS. 15A and 15B , may occur without materially altering the functionality of the linkage  352 . Likewise, the kinematic behavior of the linkage  352 , including the existence of an over-center position may exist through variations in the dimensions of the members of the linkage  352  illustrated in  FIGS. 15A and 15B . 
   Referring to  FIG. 15A , a door  36  may close as illustrated, with the linkage  352  in an over-center position. The over-center position shown may be characterized as one where the line of action  462  of the force exerted by the door tends to urge the linkage  352  to lift the piston  340  rather than be lifted by it. Another characteristic of the over-center position is that the weight of the door  36  tends to urge the linkage into a stable, closed position against some obstruction or the like, effectively causing the linkage to maintain the door  36  in a closed position. In the illustrated embodiment, the line of action  462  of the weight of the door  36  causes the lifting links  420   a ,  420   b  to pull in a direction  464  on the driven links  386   a ,  386   b  and drive them toward the upper beam  248   a.    
   An alternative characterization of the over-center position is that it is one wherein the weight of the door  36  urges the linkage  352  into a position and wherein the dimensions of the links themselves render kinematically impossible further movement in the direction urged by the weight of the door  36 . The lifting links  420   a ,  420   b  and the driven links  386   a ,  386   b  in the configuration shown in  FIG. 15A  cannot rotate further in direction  466  absent breakage or catastrophic deformation of one of the links  386 . 
   Referring to  FIG. 15B , the arm  344  may be lowered by the lifting mechanism  330 , effectively rotating the lifting links  420   a ,  420   b  out of the over-center position and allowing the shaft  422  to descend the guide  370 , thereby allowing the door  36  to descend. When the linkage  252  is not in the over-center position, the weight of the door exerted in a direction  462  causes the lifting links  420   a ,  420   b  to exert a force in a direction  468 , which tends to rotate the driven links  386   a ,  386   b  in a direction permitting movement of the linkage  352 . 
   Referring to  FIG. 16 , a housing  470  may shield the lifting mechanism  330  from the cargo. Housings  470  exist both ends  266 ,  270  of the trailers  32 ,  34  for embodiments having a lifting mechanism  330  positioned at either end  266 ,  270 . A second housing  472  may extend along the length of each or both of the trailers  32 ,  34  and shield the linkage  352  and shaft  342  from the cargo. 
   Referring to  FIG. 17 , given that the trailers  32 ,  34  are supported near their ends  266 ,  270 , the portions of the floor and lower supports  252   a ,  252   b  near the ends  266 ,  270  will be subject to a net upward force, rather than the net downward force existing throughout the middle of the trailers  32 ,  34 . This phenomenon may be problematic for latching systems that require the door  36  to be brought into engagement with a locking mechanism from below. The problem arises from the fact that an upward deflection of the lower support  252   a  is dynamically equivalent to a downward movement of the door  36 . Such a deflection could therefore disengage the door  36  from the lower support  252   a.    
   Accordingly, a lock  480  may enable the door  36  to exert a downward force on the longitudinal support  252   a  near the ends  266 ,  270  of the trailers  32 ,  34 . The lock  480  may be any suitable structure coupling the door  36  to the longitudinal support  252   a  and allowing the lower support  252   a  to exert an upward force on the door  36 . For example, a lock  480  may be a pin  482 , or pins  482 , extending from the longitudinal support  252   a . The pins  482  must typically remove or retract to allow the door  36  to open, inasmuch as the door typically translates downwardly during opening. The pins  482  maybe removed and inserted manually into apertures  484 , or may be manually extended and retracted. Alternatively, the pins  482  may be actuated hydraulically, pneumatically, or mechanically, to extend and withdraw. A bearing plate  486  may fasten to the door  36  to distribute loads more evenly along the door  36 . 
   Referring to  FIG. 18 , the pins  482  may be driven by actuators  488 , such as hydraulic pistons  490  and cylinders  492 . Alternatively, the actuators  488  may be any suitable pneumatic, electrical, or mechanical actuators or the like. The pins  482  may slide within guides  494  secured to a mounting plate  496 . The mounting plate  496  may secure to the beam  254   a  by bolts, welds, or other fasteners. 
   Referring to  FIG. 19 , in embodiments of the apparatus  10  using trailers  32 ,  34  whose doors  36  provide structural support for the longitudinal supports  252   a , a belly support  60  will need to support the longitudinal supports  252   a . This structural arrangement enables opening the door  36  and resists failure of the floor  246 . 
   A belly support  60  may be any suitable structure for selectively supporting another structure. Accordingly, hydraulic, pneumatic, and mechanical lifts of all kinds may serve as belly supports  60 , provided they are dimensionally compatible with the unloading station  12 . In the illustrated embodiment, the belly support  60  is embodied as lifting pads  500  raised by an actuator  502 . A guide  504  may guide the movement of the lifting pads  500 . 
   Referring to  FIGS. 19 and 20 , in the illustrated embodiment, the pads  500  may be plates  510  secured to standoffs  512 . The standoffs  512  may maintain the pads  500  to the level of the sheet  52 . The pads  500  may connect by means of a pivot  514 , such as a shaft  516 , to the standoffs  512 . Stops  518  may maintain the pads  500  in an orientation suitable for engagement with lower supports  252   a.    
   A stop  518 , for example, may be necessary to ensure that as the plates  510  are brought up against the lower support  252   a . The plates  510  will slide and rotate relative to the lower support  252   a  and seat flat against the lower support  252   a . A stop  518  may be necessary to prevent an edge of a plate  510  from coming into initial engagement with the lower support  252   a , gouging into the lower support  252   a , and thus interfering with rotation of the plate  510 . 
   A stop  518  may simply be a cross member  520  positioned to engage flanges  522  secured to the plate  510  for securing the plate  510  to the standoff  512 . A stop  518  may be positioned such that when the flanges  522  rest against the stop  518 , the plate  510  is positioned to seat substantially flat against the lower support  252   a  when the plate  510  is raised. Multiple pads  500  may secure along a single cross beam  524 . Multiple pads  500  may provide better weight distribution. The standoffs  512  may secure fixedly to the cross beam  524 . In some embodiments, the plates  510  may secure directly to the cross beam  524 . Alternatively, the cross beam  524  may itself engage the lower support  252   a  directly. The ends of the cross beam  524  may rest in seats  526 . 
   Pivotally securing the plates  510  to the standoffs  512  may enable the plates  510  to lie flat on the sheet  52  when the pads  500  are lowered. Inasmuch as the pads  500  may move along an arcuate path between lowered and raised positions, the pads  500  may also rotate slightly. Pivotally securing the pads  500  to the cross beam  524  may permit the plates  510  to shift into a horizontal position upon engagement with the trailers  32 ,  34  and when the plates  510  are lowered to the level of the sheet  52 . 
   An actuator  502  may be, for example, two hydraulic cylinders  528  and pistons  530 . Of course, any suitable actuating structure to accomplish the required motion may be used. The cylinders  528  may secure to mounts  532  secured to the undergirding members  54 . The pistons  532  may pivotally secure to the seats  526 . 
   A guide  504  may be embodied as arms  536  pivotally secured to an undergirding member  54  by pivots  538 . The pivots  538  may be pins  540  extending through brackets  542  secured to the arms  536  and mounting plates  544  secured to an undergirding member  54 . The arms  536  may fixedly secure to the seats  526  by means of welds, bolts, or other fastener structures. 
   An agitator  552  (e.g. accelerator, cyclic shaker, vibrator, etc.) may secure to the crossbeam  524  and vibrate the trailer  32 ,  34  to facilitate unloading of the cargo. The agitator  552  may be any suitable system. For example, a hydraulic, pneumatic, or electric motor  554  having an eccentrically mounted mass turning with a rotating shaft will provide a dynamic oscillation. 
   Dampers  556  may mount between the cross beam  524  and the seats  526 . The damper may shield other components of the belly support  60  from the oscillation of the agitator  552 . In the illustrated embodiment, the dampers  556  are embodied as springs  558  positioned between the cross beam  524  and the seats  526 . Of course, any other suitable damping system or shock absorber may be engineered for this role. For example, a rubber pad, or other such inelastic isolator material may be used. 
   Referring to  FIG. 21 , an alternative embodiment of a belly support  60  may include two sets of two parallel arms  536  having their ends pivotally secured to the seats  526  by a pivot  564 , such as a pin  566  extending through the arms  536  and the seats  526 . In such an embodiment, the pads  500  will not rotate as they are raised. Accordingly the pivot  514  connecting the stand offs to the cross beam  524  may be omitted. 
   An unloading station  12  and trailers  32 ,  34  may operate using the method of  FIG. 22 . It will be understood that many of the steps in  FIG. 22  may be performed in an order different from that listed or may be performed simultaneously with other steps. Furthermore, each of the steps of  FIG. 26  may include various sub-steps likewise performed in varying orders or simultaneously. 
   A preparing step  580  may include positioning the truck  30  (e.g. tractor, lead truck, etc.) and one or more trailers  32 ,  34  on the unloading station  12 . In some embodiments, the truck  30  may pull the a trailer  32 ,  34  or trailers  32 ,  34  onto the unloading station  12 , disconnect, and drive off the unloading station  12 . In the illustrated embodiment, the truck  30  remains on the unloading station  12 . This may save time inasmuch as the truck  30  does not need to be disconnected, driven off, driven back onto the unloading station  12 , and reconnected. 
   Positioning the truck  30  and one or more trailer  32 ,  34 , on the unloading station  12  may include providing a signal indicating that the unloading station  12  is ready. A driver of a truck  30  may also observe the unloading station  12  to verify that it is clear of ice, snow, debris, or other hazards or obstructions. When driving the vehicle onto the unloading station  12  a driver will typically ensure that the wheel guide  56  is rubbing the inside edge of the appropriate tires as the truck  30  drives onto the unloading station  12 . 
   A truck  30  and trailers  32 ,  34  may be further prepared by deflating air bags normally providing shock absorption in order to stabilize movement. Deflating air bags may allow the truck  30  and trailers  32 ,  34  to sit more rigidly on the unloading station  12 , being less subject to excessive rocking during unloading. During unloading, the engine of the truck  30  is typically shut off and the brakes of the truck  30  and trailers  32 ,  34  are set. The driver will typically exit the truck  30  during unloading of the trailers  32 ,  34 . 
   Anchoring  582  a truck  30  and trailers  32 ,  34  may include securing them to the unloading station  12  such that they do not shift substantially with respect to the deck of the unloading station during unloading. Any anchor system may be used to accomplish this purpose. In one embodiment, a truck  30  may anchor to the unloading station  12  by a comealong  230 , or the like, secured substantially permanently to the front of the truck  30  or to the unloading station  12 . The comealong  230  may secure to both the truck  30  and unloading station  12  and provide tension. After unloading the cargo, the suspension of the truck  30  will be under less load. Accordingly, the suspension of the truck  30  will cause greater tension in the comealong  230  after the trailers  32 ,  34  are unloaded. Therefore, the comealong  230  must be tensioned enough to anchor the truck  30 , but not so much that it cannot be released and provides sufficient travel to disengage after the trailers  32 ,  34  are unloaded. In some embodiments, a sensor or mechanical failsafe device will verify that the front of the truck  30  is anchored and prevent unloading if the truck  30  is not properly anchored. 
   Anchoring  582  the trailers  32 ,  34  may also include engaging the anchors  62  with the trailers  32 ,  34 . Engaging the anchors  62  may include positioning fasteners  110  within anchor receptacles  274  and activating the tensioning system  114  to apply tension to the line  112 . 
   Anchoring  582  the vehicle may include having an operator walk around the truck  30  and trailers  32 ,  34  to verify that the anchors  62  and comealong  230  secure the truck  30  and trailers  32 ,  34  to the unloading station  12 . While verifying anchoring, an operator may verify that the air bags of the truck  30  and trailers  32 ,  34  are properly deflated. 
   A supporting step  584  may bring an external support into contact with the trailers  32 ,  34  to bear loads that are supported by the door  36  when closed. In the illustrated embodiment, the supporting step  584  may include directing hydraulic fluid to the cylinder  528  causing the piston  530  to raise the pads  500  into contact with the underside of the lower support member  252   a . In some embodiments in accordance with the invention, the tensioning system  114  of the anchors  62  is activated after the supporting step  584 . The supporting step  584  may also include having an operator visually verify that the pads  500  are supporting the lower support member  254   a . Meters, deflecting indicator, or the like may install to output an indication of proper location and loading during the supporting  584  of the trailers  32 ,  34 . 
   A disengaging step  586  may include releasing and opening the door  36  such that it no longer supports the floor  246  of the trailer  32 ,  34 . A disengaging step  586  may also include having an operator verify that the doors  36  of both trailers  32 ,  34  have properly opened. 
   An unloading step  588  may include tipping the bearing surface  50  such that the contents of the trailers  32 ,  34  will flow out of the trailers  32 ,  34  to a hopper, conveyor belt, pit, pile, or the like. In the illustrated embodiment, an unloading step  588  may include activating the lifting actuator  186 . During the unloading step  588 , an operator may verify that cargo has unloaded successfully. If the cargo has not exited, the agitator  552  may activate to aid in unloading of the cargo. The agitator  552  may be activated repeatedly until the cargo successfully unloads. Once the cargo is removed, the unloading step  588  may include lowering the bearing surface  50  to a horizontal position. 
   An engaging step  590  may include re-engaging the door  36  with the lower support member  254   a  such that the door  36  again supports the floor  246 . Re-engaging the door  36  may include activating the lifting mechanism  330  to lift the door  36  and activating the locking mechanism  332  to lock the door  36  in position. The engaging step  590  may include having an operator verify that the door  36  has closed. Where the doors  36  do not close properly, an operator may wait until the truck  30  and trailers  32 ,  34  are driven off the unloading station  12 . This may be safer, inasmuch as the doors  36  are typically positioned at the edge of the unloading station  12  over a large drop to a hopper, pit, pile, conveyor belt or the like. Meanwhile, unloaded, the trailers  32 ,  34  can move, even with doors  36  unlocked. 
   A withdrawing step  592  may include deactivating the lifting mechanism is supporting the floor  246  of the trailers  32 ,  34 . For example, one may deactivate the hydraulic cylinder  528  and piston  530 , allowing the belly support  60  to drop, slide, pull or fall away from the longitudinal support member  254   a . A withdrawing step  592  may include having an operator verify that the belly support  60  is positioned flush with the bearing surface  50 . 
   A releasing step  594  may include deactivating the tensioning system  114  of the anchors  62 . In some embodiments, lifting and locking the door  36  may take place after the tensioning system  114  is deactivated. The releasing step  594  may also include removing the fasteners  110  from the anchor receptacles  274 . In some embodiments, the releasing step  594  may include hanging the fastener  110  on the cable  100  extending along the unloading station  12 , or otherwise stowing the fasteners  110  and lines  112 . An operator may also release the tension in the comealong  230  and stow the comealong  230  in embodiments having a comealong  230  substantially permanently secured to the front of the truck  30 . The releasing step  594  may also include having an operator verify that all anchors are disengaged and stowed properly. 
   Removing  596  the vehicle may include reinflating airbags in the suspension system of the trailers  32 ,  34  and the truck  30 . Removing  596  may also include verifying that the doors  36  have locked properly. An operator may then drive the truck  30  off of the unloading station  12 . After driving off the unloading station  12  an operator may take any special measures needed to close the doors  36 , such as clearing jammed pieces of cargo, or the like. 
   The present invention may be embodied in other specific forms without departing from its essential characteristics. The described embodiments are to be considered in all respects only as illustrative, and not restrictive. The scope of the invention is, therefore, indicated by the appended claims, rather than by the foregoing description. All changes within the meaning and range of equivalency of the claims are to be embraced within their scope.