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CROSS REFERENCE TO RELATED APPLICATIONS 
     (Not Applicable) 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     (Not Applicable) 
     FIELD OF THE INVENTION 
     This invention relates to earth moving equipment and more particularly to a new and improved surface leveling system capable of leveling earth in two directions. 
     BACKGROUND OF THE INVENTION 
     Leveling systems such as scrapers or graders are commonly used for roughly leveling relatively large areas of ground and are well known in the art. These leveling systems are commonly pushed or pulled by bulldozers, tractors, or other similar equipment. These leveling systems have also been known to be self-propelled. The wide variety of available motive sources are able to move the leveling systems in many different directions. However, current leveling systems are not capable of using this ability. 
     Current leveling systems are able to effectively grade earth in, for example, a forward direction, but not in a reverse direction. One common reason why these systems are not used in two directions is that the hitches used for connecting the earth-grading tool to the motive source have generally been designed for use in only one direction. An example of a hitch well known in the art that only works well in one direction is a three point hitch. Although a three point hitch works well pulling an earth grading tool, the hitch does not work well pushing the tool because the force of the earth on the tool tends to push the tool upwards. 
     As a result of the directional limitation of current leveling systems, these systems must execute complex maneuvering to properly level the ground. An analogy to the difficulties with current leveling systems is to imagine driving into and out of a driveway with a car that does not have reverse. The maneuver is simple when the car can travel in two directions. However, the maneuver is very complex when effective travel is limited to only one direction. Additionally, as is well known in the art, leveling systems must sometimes grade surfaces several times before the surfaces are level. Without having the ability to effectively grade earth in both directions, current leveling systems must completely turn around before a surface can be regraded. This excess maneuvering wastes motion, and therefore, wastes time. 
     Laser beam level control systems are commonly used with leveling systems to define a precise reference level. With such systems, a laser beam reference level is used to enable the leveling systems to create a uniformly level surface that cannot be easily achieved using conventional level control systems. The effectiveness of the laser beam level control systems are also limited by leveling systems that can only grade earth in one direction. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide an earth leveling system capable of being pushed or towed by a motion source. 
     It is another object of this invention to provide an earth leveling system that levels earth in at least two directions. 
     It is yet another object of this invention to provide an earth leveling system having a multi-link hitch that can be pushed. 
     It is still another object of this invention to provide an earth leveling system that combines the advantages of a laser beam level control system with bi-directional leveling. 
     In accordance with the invention, these and other objects are accomplished by providing a earth leveling system comprising a chassis having a hitch, a grader frame, and a forward frame having a caster wheel to support the chassis. The hitch comprises a rear support having means for attachment to a motive unit and a plurality of mechanical links substantially parallel to one another. One end of the links are pivotably attached to the rear support and the opposing end to a middle support fixedly connected to the box grader frame. The grader frame supports a box grader having a bi-directional earth grading tool. The grader frame is also pivotably attached to the forward frame. Extending above the grader frame is a torque member fixedly attached to the grader frame. 
     Also included is a damping assembly having upper and lower distal ends and disposed between the rear and middle supports. The upper distal end is pivotably connected to an upper link adjacent the middle support, and the lower distal end is pivotably connected to a lower bar on the rear support. 
     A hydraulic piston is pivotably connected to the forward frame and to the torque member whereby extending the hydraulic piston raises the grader frame and the earth grading tool relative to ground. A control unit attached to the chassis and hydraulically connected to the hydraulic piston controls the extension and retraction of the hydraulic piston. 
     In an alternative embodiment of the invention, the surface leveling system further includes a laser beam generator remote from the chassis capable of projecting a laser beam parallel to ground at a predetermined distance above ground. Also included is a laser sensor attached to the chassis and fixedly positioned relative to the earth grading tool for receiving the laser beam. The laser sensor transmits position data to the control unit. 
     In another alternative embodiment of the invention, the hitch is a three point hitch, and thus, has two lower links and one upper link. 
     In still another alternative embodiment of the invention, the damping assembly is a shock absorber. The shock absorber preferably also acts as a biasing means for urging the grader frame downward. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     There are shown in the drawings embodiments of the invention that are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown, wherein: 
     FIG. 1 is a perspective view of a surface leveling system according to the invention and a partial perspective view of a motion source. 
     FIG. 2 is a cross section, partially in section, of a hitch. 
     FIG. 3 is a cross section along the lateral axis of a box grader. 
     FIG. 4 is a side view of the surface leveling system with the box grader at ground level. 
     FIG. 5 is a side view of the surface leveling system with the box grader raised. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1,  2  and  3  illustrate a surface leveling system according to the invention. The surface leveling system&#39;s chassis  10  comprises a hitch  12 , a box grader frame  14 , and a forward frame  16 . The box grader frame  14  is pivotably attached to both the hitch  12  and the forward frame  16 . 
     The chassis  10  is connected to a motion source  27  which moves the chassis  10  during the operation of surface leveling. The motion source  27  preferably can push and pull the chassis and can turn in any direction. The motion source  27  preferably has sufficient power to push or pull the weight of the chassis  10  through soft earth, mud, sand or any other medium which is to be leveled. Motion sources  27  capable of such performance are well known in the art of earth moving. 
     Any variation in the hitch structure is acceptable provided the hitch  12  laterally supports the chassis  10  and allows the box grader frame  14  to translate vertically. In the presently preferred embodiment, the hitch  12  is a three-point hitch. An example of an acceptable alternative hitch well known in the art is a four-point hitch. 
     The presently preferred hitch  12  comprises several sub-elements including a rear support  18 , lower links  20  and  22 , an upper link  24 , rear pivot brackets  28  and  30 , and an attachment face  26 . The attachment face  26  is connected to the rear support  18  and is used to attach the hitch  12  to the motion source  27 . Many different means of attaching an attachment face  26  to a motion source  27  are well known in the art of earth moving and any of those attachment means can be used with this invention. 
     Links  20 ,  22 , and  24  of the hitch  12  can pivot relative to the rear support  18  because the links  20 ,  22 , and  24  are pivotably attached to the rear support  18  and the rear pivot brackets  28  and  30 . The upper link  24  is pivotably connected to the upper portion of the rear support  18  with a pivot pin  32 , and each of the lower links  20  and  22  are pivotably connected to the rear pivot brackets  28  and  30  also by using pivots pins  32 . 
     One additional purpose of the hitch  12  is to maintain the height at which surface leveling takes place, particularly when traveling in a forward direction as indicated by the arrow in FIG.  4 . Preventing upward movement of the box grader frame is important because the location of the box grader frame  14  relative to ground determines the height at which surface leveling takes place. The box grader frame  14  tends to move upward during leveling of irregular surfaces, specifically, when high points in the irregular surface are being leveled. 
     To accomplish the purpose of maintaining the height of surface leveling, a damping system is added to the hitch  12 . The damping system absorbs energy transmitted from the box grader frame  14  to the hitch  12  when the box grader frame  14  is being urged upward. This absorption of energy by the damping system helps limit the upward movement of the box grader frame  14 . 
     In the presently preferred embodiment, two shock absorbers  34  are used as a damping system so as to prevent the box grader frame  14  from being urged upward. For example, shock absorber model number 73126, which is available from Gabriel Ride Control, Inc. of Nashville, Tenn., has been found to provide acceptable results. The shock absorbers  34  are preferably mounted so that one distal end is pivotably mounted to the rear support  18  and the other distal end is pivotably mounted to the upper link  24 . Because the rear support  18  is fixed relative to the motive source  27 , the shock absorber will resist pivot movement of the upper link  24  about pivot pin  32 . The coefficient of damping of the shock absorber and geometry of the links is preferably such that movement of the upper link  24  will generally not occur under normal operating conditions when the box grader frame  14  is operated in a forward direction. 
     In addition to use of a damper such as shock absorbers  34 , a bias spring (not shown) can be used to provide a spring biasing force between the stationary rear support  18  and the movable links  20 ,  22 ,  24 . This biasing force can be used to actively urge the box grader frame  14  downward. Thus, this also prevents the box grader frame  14  from being urged upward. 
     The box grader frame comprises lateral members  36  and  38 , a middle support  40 , a cross brace  42 , and grader brackets  44 . The purpose of the box grader frame  14  is to support the box grader  46 . Also, the box grader frame  14  can move vertically relative to the grading surface so as to allow the box grader  46  to grade earth at different heights. In the presently preferred embodiment, the box grader frame  14  is attached to the hitch  12  at three pivot points connecting the links  20 ,  22  and  24  to the grader frame  14 . However, the invention is not limited in this regard. The two lower links  20  and  22  are pivotably connected to grader brackets  44  with pivot pins  32 , and the upper link  24  is pivotably connected to a middle support  40  with a pivot pin  32 . 
     The box grader  46  is adapted to grade earth in both forward and reverse directions. This adaptation includes locating a bi-directional earth grading tool  58  within the box grader  46 . The earth grading tool  58  preferably is located approximately in the center of the box grader  46  so that soil can be accumulated on either side of the grading tool  58  between the end plates  48 ,  50 . Also, the box grader  46  is advantageously formed as shown in FIG. 3 so that both the forward and rearward faces of the earth grading tool  58  can scrape and collect soil. Thus, when the chassis  10  is moving forward, the forward side of the earth grading tool  58  is grading earth and any excess earth is accumulated forward of the earth grading tool  58  in the cavity defined by the end plates  48 ,  50  and the grading tool. Conversely, when the chassis  10  is moving rearward, the rearward side of the earth grading tool  58  is grading earth and any excess earth is accumulated rearward of the earth grading tool  58 . 
     Also disposed between the end plates  48  and  50  are two outer cross beams  54  and  56  and an inner cross beam  52 . Both the inner and outer cross beams  52 ,  54  and  56  are connected to the end plates. Also, the inner cross beam  52  preferably attaches to and supports the earth grading tool  58 . Another feature of the box grader is tool braces  60  and  62  which preferably extend the lateral length of the earth grading tool  58  and disposed adjacent thereto. The tool braces  60  and  62  act to brace the earth grading tool  58 . 
     Optionally attached to the box grader  46  is a sensor support bar  66  and a laser sensor  68  to be used in combination with a laser beam generator  86  and laser beam  88  (shown in FIGS.  4  and  5 ). Using a laser/laser sensor combination with earth grading systems to change the height of grading is well known in the art of earth moving. Although FIG. 1 shows the presence of a laser sensor  68  as part of a laser/laser sensor combination, the invention is not limited to this feature for controlling the height of grading. Other means or methods for controlling the height of grading are well known in the art of earth moving and can be used with the invention. 
     Included on the chassis is a powered means to selectively urge the box grader frame  14  and therefore the box grader  46  upward or downward. Any means of urging the box grader frame  14  is acceptable. Examples include pneumatic pistons, gears powered by a motor, or a system of pulleys. In the presently preferred embodiment, a hydraulic piston  74  is used to urge the box grader frame  14  either upward or downward. The hydraulic piston  74  is preferably attached to a torque bar  64  which extends upward from box grader frame  14 . Extension of the hydraulic piston  74  acts on the torque bar  64  so as to urge the box grader frame  14  upward. This action will be discussed in more detail with the discussion of FIGS. 4 and 5. 
     The forward frame  16  acts to support the box grader frame  14  on the side opposite the hitch  12 . The forward frame  16  also provides a base against which the box grader frame  14  can be urged upward. In the presently preferred embodiment, the forward frame  16  comprises forward lateral bars  76  and  78 , a forward cross bar  80  extending between the forward lateral bars  76  and  78 , and caster wheels  82  and  84 . The hydraulic piston  74  is preferably pivotably attached an opposing end to the forward cross bar  80 . 
     The caster wheels  82  and  84  are preferably attached to forward frame  16  at the distal ends of the forward lateral bars  76  and  78 . The caster wheels can pivot 360° around a vertical axis so that the chassis  10  can be rotated, pushed, or pulled by the motion source  27  in any direction. However, the invention is not limited to caster wheels  82  and  84 . Any feature that both supports the chassis  10  and allows the chassis  10  to move easily in any direction can be used. 
     Although connected to the forward frame  16 , the box grader frame  14  can move vertically relative to the grading surface because the forward lateral bars  76  and  78  of the forward frame  16  are pivotably connected to grader brackets  44  on the box grader frame  14  using pivot pins  32 . 
     A control box  70  contains appropriate solenoid operated hydraulic values (not shown) which are known in the art and are actuated in response to signals received by to the laser sensor  68  or other level controlling means to direct hydraulic fluid to one or the other end of the hydraulic piston  74  using hydraulic hoses (not shown). Power for operation of the solenoid operated hydraulic values and hydraulic fluid is obtained either by connection to appropriate circuits on the motion source  27  or by a separate power source on the chassis  10 . 
     The control box  70  may be disposed anywhere along the chassis  10  provided that it does not interfere with the workings of the chassis  10 . In the presently preferred embodiment, the control box  70  is attached to a control platform  72  that extends from the torque bar  64  to the cross brace  42 . 
     FIGS. 4 and 5 illustrate how extending or retracting the hydraulic piston  74  acts to lift or lower the box grader frame  14  and thus the box grader  46 . FIG. 4 shows the box grader  46  at ground level, and FIG. 5 shows the box grader  46  raised. 
     The action of raising the box grader  46  requires the hydraulic piston  74  to be extended. Extending the hydraulic piston  74  causes the hydraulic piston  74  to push against both the torque bar  64  and the forward frame  16 . The torque bar  64  is pushed upward, along with the box grader frame  14  and box grader  46 , because the ground prevents the forward frame  16  from being pushed downward. The box grader frame  14  can move vertically relative to the forward frame  16  because the box grader frame  14  can pivot about the forward frame  16  at pivot point A. Also, the box grader frame  14  can move vertically relative to the hitch  12  because the box grader frame  14  can pivot about the hitch  12  at pivot points B and C. 
     To lower the box grader  46 , the opposite action must occur such that the hydraulic piston  74  pulls against both the torque bar  64  and forward frame  16 . The torque bar  64  is pulled down, along with the box grader frame  14  and box grader  46 , because the force of gravity keeps the forward frame from being pulled upward. As the hydraulic piston  74  continues to retract, the box grader  46  will continue to lower until the box grader  46  contacts ground. After the box grader  46  reaches this point any additional retraction of the hydraulic piston  74  will cause the forward frame  14  to be raised upwards. The hydraulic piston  74  pulls the forward frame  14  upward because the ground now prevents the box grader  46  from being pulled downward any further.

Summary:
A bi-directional surface leveling system comprises a chassis attachable to a motion source and having a hitch, a grader frame pivotably connected to the hitch and supporting a box grader, and a forward frame pivotably attached to the grader frame and having a caster wheel for supporting the chassis. The box grader holds a bi-directional earth grading tool. Also included is a damping assembly attached to the hitch to dampen upward movement of the grader frame. A powered assembly is pivotably connected at one end to the forward frame and at the opposing end to the grader frame, and a control unit connected to the power assembly selectively extends and retracts the power assembly.