Abstract:
An implement/vehicle hitch system for coupling implements to a utility vehicle includes two hydraulic cylinders for adjusting the position of the implement. The hydraulic cylinders are connected to a pair of motor/pump units which cause the hydraulic cylinders to operate in a synchronous manner.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to a hitch for movably coupling an implement to a utility vehicle, and in particular, to a hitch having a pair of hydraulic cylinders for controlling the position of the implement with respect to the utility vehicle.  
         BACKGROUND OF THE INVENTION  
         [0002]    Implement hitches are known for coupling implements to agricultural and industrial vehicles. For example, a three point hitch may be attached to the rear of an agricultural tractor. Such a hitch normally includes two lower arms and an upper arm whose free ends are equipped with catch hooks or other fastening means for fastening attached implements. Usually each of the lower arms is connected through lift struts with a common lift shaft, that can be rotated hydraulically. This results in a rigid mechanical connection between the two lower arms and an enforced synchronization of the lower arms.  
           [0003]    U.S. Pat. No. 6,253,859 shows a hitch, in which each lower draft link can be raised and lowered individually by a separate lift cylinder, instead of a lift strut and a lift shaft. In such a hitch, which is often used with front power lifts, the synchronization of the two lower arms must be guaranteed by an active control of the hydraulic cylinders. This requires additional power consumption and a corresponding control system.  
         SUMMARY OF THE INVENTION  
         [0004]    An object of the present invention is to provide a hitch with separate lift cylinders which are synchronously operated without the application of additional energy sources and without additional control systems.  
           [0005]    This and other objects are achieved by the present invention wherein an implement attachment interface or hitch couples an implement to a utility vehicle. The hitch includes a pair of hydraulic cylinders which operate to control the position of the implement. The hydraulic cylinders are communicated with a gear-type flow divider or motor/pump unit which synchronizes the operation of the cylinders without requiring an additional power source.  
           [0006]    The motor/pump unit synchronizes the cylinders even during floating operation, such as when the hydraulic cylinders are unpressurized. Floating operation can be used during ground breaking or during plowing with a plow. Alternatively, the lift cylinders can be connected with a conventional depth control system to control implement depth.  
           [0007]    The invention assures the synchronous operation of the hydraulic cylinders in the floating position without mechanical connections. No common lift shaft or other mechanical connecting means between the coupling elements is required. An additional hydraulic pump is not required during floating operation to maintain a constant side-to-side orientation of the implement. Defects that could occur from an active control of the lift cylinders, can be avoided.  
           [0008]    It is possible to connect only the piston end chambers or only the rod end chambers of the two hydraulic cylinders with the motor/pump unit. However, preferably, the hydraulic cylinders are double acting hydraulic cylinders and the two piston end chambers and the rod end chambers are connected to the rotors of the motor/pump unit.  
           [0009]    Preferably, the motor/pump unit includes at least two rotors arranged on a common shaft. The rotors are pairs of gears that are configured as pump/motor units. Each rotor is connected on the one hand with the piston end chamber of one of the two hydraulic cylinders and on the other hand with a reservoir, such as the fluid reservoir for the operating hydraulic fluid of the utility vehicle.  
           [0010]    Preferably, the motor/pump unit includes at least two rotors arranged on a common shaft, each of which is connected on the one hand with the rod end chamber of one of the two hydraulic cylinders and on the other hand with a reservoir.  
           [0011]    Alternatively, the motor/pump unit includes four rotors arranged on a common shaft. Two of these rotors have a larger intake capacity and are connected with the piston end chamber of one of the two hydraulic cylinders. Two of the rotors have a smaller intake capacity and are connected with the rod end chamber of one of the two hydraulic cylinders. All rotors are connected on the other hand with a reservoir.  
           [0012]    In a further preferred embodiment, the motor/pump unit includes at least two rotors arranged on a common shaft of which a first rotor is connected with the piston end chamber of a first hydraulic cylinder and a second rotor is connected with the rod end chamber of a second hydraulic cylinder and both rotors, on the other hand, are connected with a reservoir.  
           [0013]    Two motor/pump units may be provided where a first motor/pump unit includes two rotors arranged on a common first shaft, of which a first rotor is connected with the piston end chamber of a first hydraulic cylinder and a second rotor is connected with the rod end chamber of a second hydraulic cylinder. A second motor/pump unit also includes two rotors arranged on a second common shaft of which a first rotor is connected with the rod end chamber of the first hydraulic cylinder and a second rotor is connected with the piston end chamber of the second hydraulic cylinder. All rotors are connected, on the other hand, with a reservoir.  
           [0014]    Preferably, the intake capacity of the rotors conforms with each of the associated hydraulic cylinder chambers. If, for example, the two piston chambers of a double acting hydraulic cylinder are connected with a rotor of a motor/pump unit that include two rotors, then the synchronous operation of the hydraulic cylinders is guaranteed when the intake capacity of the first rotor is proportional to the effective piston surface of the associated first cylinder volume, and simultaneously the intake capacity of the second rotor is proportional to the effective piston surface of the associated second cylinder chamber.  
           [0015]    Since it may be difficult to make the volumes comply precisely to each other, during floating operation negative pressures may occur in the system. These may have a negative effect on the efficiency and the properties of the hydraulic fluid and lead to cavitation in the system. Therefore, it is advantageous for a reliable operation to provide check valves that permit a flow of fluid from the reservoir to the hydraulic cylinders, as long as the pressure in a hydraulic cylinder chamber drops below a pre-determined pressure.  
           [0016]    The hitch is preferably a three-point implement hitch in which the lower arms are not connected over lift struts to a lift shaft, but are connected through a hydraulic cylinder each configured as a lift cylinder in order to raise and lower them. During floating operation the hydraulic cylinders are separated from the pressure supply, that is used for the raising and lowering of the hydraulic cylinders, and connected with flow dividers which synchronize the cylinders.  
           [0017]    This hitch or implement interface can be attached to the rear of a tractor, or it can also be used as a front power lift, and is preferable to previous arrangements in which the lower arms of the front power lift are rigidly connected to each other in order to attain synchronization. In this case, however, an inclined position in which the two transverse elements occupy different positions in height, was not possible.  
           [0018]    In one embodiment, the hitch includes hydraulic cylinders arranged in the form of a hexapod, such as described in German patent application DE 199 51 840.8, which is incorporated by reference herein. In this case, at least two of the six hydraulic cylinders of the hexapod are connected as described above with a motor/pump unit. Preferably, the two hydraulic cylinders of the hexapod which extend generally in the vertical direction and therefore absorb vertical forces, are connected to one or two motor/pump units.  
           [0019]    In order to move the implement to a desired position, the piston end chambers and/or rod end chambers of the hydraulic cylinders are connected via a control valve with a pressure source. The control valve permits a switch between a floating operational mode, in which the motor/pump units are effective, and an operating position in which the hydraulic cylinders can be connected to a hydraulic pressure source. The valve arrangement may be used to control and flow of hydraulic fluid to and from the hydraulic cylinders. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a schematic view of an implement attachment interface or hitch according to the present invention, with two lower draft links and an upper link or arm.  
         [0021]    [0021]FIG. 2 is a schematic view of an alternative hitch assembly with hydraulic cylinders in a hexapod arrangement.  
         [0022]    [0022]FIG. 3 is a schematic diagram of a hydraulic control circuit for the implement attachment interface of FIGS. 1 and 2.  
         [0023]    [0023]FIG. 4 is a schematic diagram of an alternative hydraulic control circuit. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]    As seen in FIG. 1, a hitch includes two lower arms or draft links  10 ,  12  and an upper arm or link  14  with ends  16 ,  18 ,  20  for pivotally coupling to an agricultural tractor (not shown), and with free ends having conventional couplers  22 ,  24  or a catch hook  26  for engaging an implement (not shown). Preferably, the upper arm  14  includes and integral hydraulically operated upper arm cylinder (not shown) for varying the length of the upper arm  14 , so that an implement can be pivoted about a generally horizontal axis which is transverse to the fore and aft axis of the vehicle. Hydraulic cylinder  30 ,  32  each are coupled between a corresponding one of the draft links  10 ,  12  and fixed pivot supports  25  and  27 .  
         [0025]    The cylinders  30 ,  32  are double acting cylinders, each having a piston end chamber and a rod end chamber. The piston end chamber and a rod end chamber are connected via hydraulic hoses  34 ,  36 ,  38 ,  40  with a first and a second motor/pump unit  42 ,  44 . The first motor/pump unit  42  includes a first gear-type rotor  50  which is connected by a first shaft  52  for rotation with a second gear-type rotor  54 . Similarly, the second motor/pump unit  44  includes a third gear-type rotor  56  which is connected by a second shaft  58  for rotation with a fourth gear-type rotor  60 . The rotors  50 ,  54 ,  56 ,  60  operate as both hydraulic pumps and motors. If one of the two rotors of a motor/pump unit is driven by fluid pressure then that rotor acts as a motor and drives, via the corresponding shaft, the other rotor, which acts as a pump. If the pressure relationships in the hydraulic circuits are reversed, then the two rotors operate in the opposite way.  
         [0026]    The rod end chamber of a second hydraulic cylinder  32  is connected by a first hydraulic hose  34  to the first rotor  50  of the first motor/pump unit  42 . The piston end chamber of a first hydraulic cylinder  30  is connected by a second hydraulic hose  36  to the second rotor  54  of the first motor/pump unit  42 . The rod end chamber of the first hydraulic cylinder  30  is connected by a third hydraulic hose  38  to the third rotor  56  of the second motor/pump unit  44 . The piston end chamber of the second hydraulic cylinder  32  is connected by a fourth hydraulic hose  40  to the fourth rotor  60  of the second motor/pump unit  44 . Each of the rotors  50 ,  54 ,  56 ,  60  is also connected with a reservoir  62 .  
         [0027]    The capacities of the rotors  50 ,  54 ,  56 ,  60  conform to the volumes of associated piston end chambers or rod end chambers of the cylinders  30 ,  32 . The rotors  50 ,  56  associated with the two rod end chambers have a smaller volume capacity than the rotors  54 ,  60  associated with the piston end chambers, since the piston end chambers have a larger cross section than the rod end chambers.  
         [0028]    The system shown in FIG. 1 operates as follows. If lower arm  10  is raised by cylinder  30 , then its piston end chamber forces hydraulic fluid into the second rotor  54  of the first motor/pump unit  42 . The second rotor  54  drives the first rotor  50  which forces hydraulic fluid into the rod end chamber of the second hydraulic cylinder  32  which retracts and raises the other lower arm  12 . This assures that both lower arms  10 ,  12  move together. Both motor/pump units  42 ,  44  control the flow into the hydraulic cylinders  30 ,  32  as well as the flow out of the hydraulic cylinders  30 ,  32 .  
         [0029]    Referring now to FIG. 2, an alternate embodiment of the hitch system includes six hydraulic cylinders  74 ,  76 ,  78 ,  80 ,  82 ,  84  in a hexapod arrangement between the vehicle body  70  and a generally V-shaped coupling frame  72 . Such hexapod hitch arrangements are known from German patent application DE 199 51 840.8, which is incorporated herein by reference.  
         [0030]    The hydraulic cylinders  76  and  82  project rearwardly and downwardly from the vehicle body  70 , and normally absorb vertical forces applied to the coupling frame  72 . Cylinders  76  and  82  are double acting hydraulic cylinders and are connected with two motor/pump units  42 ,  44  similar to the connection shown in FIG. 1, so that the system of FIG. 2 operates similar to the system of FIG. 1.  
         [0031]    Referring now to FIG. 3, the same reference numbers are used for similar parts as in FIG. 1. The motor/pump units  42 ,  44  are connected to check valves  90 ,  92 ,  94 ,  96  which are located between reservoir  62  and a corresponding one of hoses  34 ,  36 ,  38 ,  40 . The check valves  90 ,  92 ,  94 ,  96  open upon a predetermined pressure difference between the hydraulic lines  34 ,  36 ,  38 ,  40  and the reservoir  62  and permit a flow of hydraulic fluid from the reservoir  62  into the particular hydraulic line  34 ,  36 ,  38 ,  40  under a negative pressure condition. In this way negative pressures in the cylinders  30 ,  32  and the motor/pump units  42 ,  44  are avoided.  
         [0032]    A valve  98  is located between the cylinders  30 ,  32  and the motor/pump units  42 ,  44 . Valve  98  is urged by a spring to a closed position wherein it closes communication through all four hydraulic lines  34 ,  36 ,  38 ,  40 . The valve  98  can be opened by a solenoid under the control of a control unit  100 .  
         [0033]    Selector valves  102 ,  104  control communication between the piston and rod end chambers of cylinders  30 ,  32  and a hydraulic pump  106  and a reservoir  62 . The two selector valves  102 ,  104  are preferably four-way, three-position valves which can be controlled electrically by the control unit  100 . Valves  102 ,  104  are movable from the closed position shown, to a first deflected position wherein the piston end chamber of the corresponding cylinder  30 ,  32  is pressurized and the rod end chamber is unpressurized, so that the associated cylinder  30 ,  32  extends. Valves  102 ,  104  are also movable to a second position wherein pressure is applied to the rod end chamber of the corresponding cylinder and the piston end chamber is unpressurized, so that cylinders  30 ,  32  retract.  
         [0034]    Operator commands are communicated by a manually operated input unit  108  to the control unit  100 , so that the cylinders  30 ,  32  can be controlled by the selector valves  102 ,  104  or a floating operation can be enabled by closing selector valves  102 ,  104  and opening valve  98  as shown in FIG. 3, to synchronize operation of cylinders  30 ,  32 .  
         [0035]    Referring now to FIG. 4, an alternative motor/pump unit  110  includes four rotors  112 ,  114 ,  116 ,  118  rigidly inter-connected by shaft  128 . The two smaller rotors  112 ,  114  are connected with the rod end chambers  120 ,  122  of cylinders  30 ,  32 , and the larger rotors  116 ,  118  are connected with the piston end chambers  124 ,  126  of cylinders  30 ,  32 . If cylinder  30  is retracted, then fluid flows out of piston end chamber  124  and into rod end chamber  120 . This fluid flow drives rotors  116  and  112  in the same direction. Rotors  116 ,  112  act as motors which drive rotors  114  and  118  which pump fluid out of chamber  126  and into chamber  122  of cylinder  32  so that cylinder  32  moves in synchrony with cylinder  30 .  
         [0036]    While the present invention has been described in conjunction with a specific embodiment, it is understood that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, this invention is intended to embrace all such alternatives, modifications and variations which fall within the spirit and scope of the appended claims.