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FIELD OF THE INVENTION 
     The present invention relates to an improved plow. 
     DESCRIPTION OF THE PRIOR ART 
     A typical conventional vehicle mounted snowplow has many disadvantages. 
     It is mounted to the front of a truck or other vehicle at multiple attachment points, and it is time consuming to mount and unmount the plow. Also, the hardware for mounting the plow is vehicle specific. Owners of many vehicles cannot obtain a conventional plow to fit their vehicle. 
     The conventional plow is intended to push snow ahead of the vehicle. Effective plowing can be accomplished only when the vehicle is moving forward, and no significant plowing takes place when the vehicle is backing up. Plowing can be slow. 
     If an obstacle or other excessive load is encountered, the conventional plow is designed to pivot or roll over out of the way to avoid damage to the plow or vehicle. This interrupts the plowing action. 
     When the conventional plow is raised for transport from the lower plowing position, the plow mold board tilts or pivots relative to vertical. 
     Known systems for applying a down force in a conventional plow are not fast and flexible enough to accommodate uneven or rough terrain. 
     Although plows have been designed to be pulled behind a vehicle, some of these have been light weight plows and have not been sturdy enough to handle heavy loads. In addition, their light weight has permitted the plow to float rather than doing a good job of cleaning. 
     SUMMARY OF THE INVENTION 
     A primary object of the present invention is to provide an improved plow overcoming the above disadvantages. Specifically, the objects of the invention include providing a plow that is bidirectional and can plow in both directions; that mounts quickly to a hitch receiver of any vehicle without additional mounting or wiring; that is light yet very strong; that does not tilt or pivot when lifted; and that supplies constant down force for effective plowing over rough terrain. 
     In brief, in accordance with the present invention, there is provided a plow assembly for a vehicle having a tubular hitch receiver. The plow assembly includes a laterally extending plow head and a hitch box. The hitch box includes a mounting bar sized to mate with the vehicle hitch receiver. A linkage movably interconnects the plow head and the hitch box. A lift drive system raises and lowers the plow head relative to the hitch box between plowing and transport positions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The present invention together with the above and other objects and advantages may best be understood from the following detailed description of the preferred embodiment of the invention illustrated in the drawings, wherein: 
         FIG. 1  is a simplified diagrammatic side elevational view of a plow of the present invention mounted at the rear of a vehicle, with the plow in its lower operating position; 
         FIG. 2  is a view like  FIG. 1  with the plow in its upper transport position; 
         FIG. 3  is an enlarged isometric view of a preferred embodiment of the plow; 
         FIG. 4  side elevational view of the plow of  FIG. 3 ; and 
         FIG. 5  is an enlarged fragmentary isometric view of the hitch box of the plow. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Having reference now to the drawing, and initially to  FIGS. 1 and 2  there is illustrated in simplified diagrammatic form a plow generally designated as  20  and constructed in accordance with the principles of the present invention. In general, the plow includes a support or hitch box  22 , a plow head  24  and a linkage  26  connected between the hitch box  22  and the plow head  24 . 
     The plow  20  is a self contained stand alone unit that is easily mounted directly to a standard, tubular, two inch square cross section hitch receiver  28  of a vehicle  30 . Although the illustrated vehicle  30  is a sport utility vehicle, a plow in accordance with the present invention may be used with any vehicle having a rear or a front hitch receiver, including trucks, passenger cars, all terrain vehicles, garden tractors and any others. Hitch receivers are usually mounted at the rear of a vehicle, and the plow of the present invention is normally intended to be mounted at the rear of a vehicle. However, the plow of the present invention can be mounted at the front of any vehicle that is supplied with a front mounted hitch receiver. 
     The hitch box  22  includes a generally horizontally projecting hollow tubular mounting bar  32  preferably having a two inch square cross section shape to mate with the standard hitch receiver  28 . The plow  20  is mounted to the vehicle  30  simply by inserting the mounting bar  32  into the hitch receiver  28  and locking the bar in place with a locking pin  34  that is inserted into aligned holes in the hitch receiver  28  and mounting bar  32 . No other mounting or mechanical attaching steps are required for the plow  20  to be mounted on the vehicle and ready for use. 
     The plow  20  is bidirectional and can be both pulled and pushed by the vehicle  30 . The plow head  24  includes two opposed mold boards  36  and  38  facing in opposite directions along the path of motion of the vehicle  30 . Mold board  36  faces in, toward the vehicle  30  and mold board  38  faces out, away from the vehicle  30 . As a result, for example, when vehicle  30  moves forward, the plow  20  is pulled and mold board  36  moves snow in the path of the plow. When vehicle  30  moves to the rear, the plow  20  is pushed and the mold board  38  moves snow in the path of the vehicle. 
     The position of the plow head  24  relative to the hitch box  22  can be changed and adjusted by the user. The linkage  26  permits the plow head  24  to be raised and lowered between the lower operating or plowing position of  FIG. 1  and the raised transport position of  FIG. 2 . In the lower position of  FIG. 1  the plow head is positioned on or near the ground or road surface and is continuously biased downward with a resilient force that permits the plow head  24  to lift and move over obstacles, rough terrain or the like. In the upper position of  FIG. 2  the plow head  24  is supported above the ground or road surface so that the plow  20  does not interfere with motion of the vehicle  30  when it is not plowing. 
     The linkage  26  includes an upper link  40  and a lower link  42  configured so that the plow head  24  remains in the same orientation relative to vertical as it is raised and lowered. In the illustrated arrangement the axis of symmetry of the opposed mold boards  38  and  36  is vertical and remains vertical in any position of the plow head  24 . In the drawings the plow head  24  is illustrated in a neutral position with the mold boards  36  and  38  perpendicular the direction of vehicle travel. The linkage  26  is designed to permit the plow head  24  to pivot horizontally in both directions from the neutral position so that plowed snow is moved by the plow head  24  laterally to one side or the other of the vehicle  30 . 
     A pivot driver system  44  is operated by the user to pivot the plow head  24  horizontally. A lift driver system  46  is operated by the user to rise and lower the plow head  24 . Movement of the plow head  24  relative to the hitch box  22  can be effected in any of a variety of ways, for example manually, hydraulically or using electrical power. In a manual system, the drivers  44  and  46  can include manually operated ball and screw linear actuators or the like. In a hydraulic system, the drivers  44  and  46  can include linear hydraulic cylinders operated from a motor pump unit mounted in the hitch box  22 . In an electrical system, the drivers  44  and  46  can include electrically powered rotary winches mounted on the hitch box  22 . 
     In a hydraulic or electrical driver system, electrical power may be provided directly from the battery of the vehicle  30  or, depending upon power requirements, from a cigarette lighter type power receptacle of the vehicle  30 . In many instances it is desirable for the plow to have a dedicated power supply in the form of a battery pack power supply  48  that can be carried within the vehicle  30 . Power supply  48  may include a battery charger energized from a twelve volt dc source of a  120  volt ac source or both. In addition, for convenient operation by the user, a pendant is provided including a handheld or other small control unit  50  located in the vehicle cab. A cable or harness  52  extending between the hitch box  22 , battery pack  48  and control unit  52  includes power and control connections for operation and control of the drivers  44  and  46 . 
     A pair of indicator lights  54  is supported above the plow head  24 . Preferably the indicator lights  54  are located at the outer horizontal ends of the plow head  24  and serve as markers visible to the user of the ends of the plow head. The indicator lights  54  may include turn signal indicators as well as marker lights, and may be connected by a cable  56  to a standard trailer light connector socket  58  of the vehicle  30 . 
     A preferred embodiment of the present invention is seen in more detail in  FIGS. 3-5 . Some of the elements of the plow  20  such as the indicator lights  54  and the cables  52  and  56  are omitted in  FIGS. 3 and 4  for clarity. The hitch box  22  ( FIG. 5 ) includes upper and lower walls  60  and  62 . A pair of vertical walls  64  and  66  are spaced apart to accept between them the mounting bar  32 . Two vertical rows of holes  68  are provided in the walls  64  and  66 . The mounting bar  32  has two mating holes  70 . The mounting bar  32  is placed between the walls  60  and  62  with holes  70  aligned with selected holes  68  at a desired height. The bar  32  is then secured with a pair of fasteners  72  ( FIG. 4 ). The hitch box  22  can be adjustably positioned at a desired height above the ground to position the plow  20  properly with vehicles of various heights. 
     The plow head  24  is of a sturdy yet light weight, hollow, box like construction. The ends of the mold boards  36  and  38  are attached to a pair of end walls or supports  74  by fasteners  76 . A top support wall  78  is attached to the top edges of the mold boards  36  and  38  by fasteners  80 . The lower edges of the mold boards  36  and  38  each support a reversible and replaceable wear bar  82 . A bottom support plate  84  is attached by fasteners  86  to the wear bars  82  and to the bottom edges of the mold boards  36  and  38 . The mold boards  36  and  38  and the wear bars  82  may be made of a flexible material, such as polycarbonate, so that the plow head can flex upon striking an obstruction or the like in order to avoid damage to the plow  20  or vehicle  30 . If desired, parts or all of the interior of the plow head  24  may be filled with rigid foam for added strength. 
     A vertical support post  88  is located at the center of the plow head  24 . The post  88  extends up from a laterally extending support bar  90  located within the plow head  24 , and extends through an upper support bar  92  upwardly to the exterior of the plow head  24 . A pair of support arms  94  extend inwardly (toward vehicle  30 ). The upper and lower links  40  and  42  are similar to one another and are A-frames having single point pivot connections  96  to the inner ends of the support arms  94 . The A frame links  40  and  42  each have two point pivot connections  98  to the hitch box  22 . When the plow head  24  is raised or lowered, the A frame links  40  and  42  maintain the plow head in the same vertical orientation. The plow head  24  pivots horizontally around the support post  88 . 
     The pivot driver system  44  includes an electric winch having a drive motor  100  and a winch drum  102 . The central portion of a winch cable  104  is would on drum  102 , and the ends of the cable  104  are attached to laterally spaced tabs  106  mounted on the support bar  90 . When the motor  100  rotates the drum  102  in opposite directions, the cable  104  is moved to pivot the plow head  24  horizontally in opposite directions. 
     The lift driver system  46  includes an electric winch having a drive motor  108  and a winch drum  110 . The central portion of a winch cable  112  is wound on drum  102 , and the ends of the cable  112  are attached to the ends of upper and lower pivoting levers  114  and  116 . The levers  114  and  116  are pivotally attached respectively to the outer ends of the A frame links  40  and  43  adjacent the single point pivot connections  96 . The opposite ends of levers  114  and  116  are connected to springs  118  and  120 . The springs  118  and  120  are coiled compression springs. However other types of springs such as air springs or extension springs could be used. 
     When the motor  108  is operated to rotate the drum  110  in the counter clockwise direction as viewed in  FIG. 4 , the cable  112  is moved in the direction to tend to rotate the levers  114  and  116  in a counter clockwise direction as viewed in  FIG. 4 . The levers  114  and  116  engage stops  122  and  124  and further rotation is prevented. As a result, further movement of cable  112  lifts the plow head  24  up to a transport position. 
     When the motor  108  is operated to rotate the drum  110  in the opposite direction, clockwise as viewed in  FIG. 4 , the cable  112  is moved in the opposite direction to lower the plow head  24  to the plowing position near or against the ground or road surface. Further clockwise rotation results in clockwise rotation of lever  114 , compressing and loading the spring  118 . Spring  120  is preloaded and extends to take up slack in the cable  112 . Compressed spring  118  applies a continuous resilient downward bias or down force to the plow head  12 . The down force can be several hundred pounds. The plow head can deflect up or down over rough terrain, and the continuous down force results in a consistent good cleaning action. Even though the plow head  24  is very light due to its box like construction, due to the down force, it achieves results equivalent to a much heavier plow. 
     While the present invention has been described with reference to the details of the embodiments of the invention shown in the drawing, these details are not intended to limit the scope of the invention as claimed in the appended claims.

Summary:
A support or hitch box supports a mounting bar in a selected vertical position. The mounting bar is received in a standard hitch receiver of any car, truck or other vehicle. A plow head is a light weight, strong, box like structure with oppositely directed mold boards so that the plow can operate in both forward and reverse directions. A linkage permits the mold board to be raised or lowered relative to the hitch box. The linkage includes upper and lower links to maintain the vertical orientation of the mold board. The plow head is mounted for pivoting in a horizontal plane. A pivot driver is user controlled to pivot the plow head and a lift driver is user operated to raise and lower the plow head. The plow head carries indicator lights. A spring, preloaded by the lift driver, applies a continuous down force to the plow head when it is the lower, plowing position.