Abstract:
A device for mechanically establishing the lowest point a three-point tractor hitch can sink to. The device comprises a modified chain which is attached at both ends to the towed implement near where the lift arms are attached to the implement. The chain&#39;s middle portion is draped over and adjustably attached to the top link, with the result that as the lift arms are lowered the chain draws taut and prevents the lift arms from sinking any lower.

Description:
BACKGROUND 
     1. Field of Invention 
     This invention relates to the field of tractor hitches. More specifically, the invention comprises a device which adjustably sets the lowest point of travel for a conventional three-point agricultural hitch. The device is useful for maintaining an agricultural implement at the desired lowest point of travel without putting pressure on the hydraulic pump commonly used to adjust the height of such hitches. 
     2. Description of Prior Art 
     Three-point tractor hitches have been in common use for several decades. U.S. Pat. No. 4,071,105 to Von Allworden (1978) is a typical example. Similar configurations are disclosed in U.S. Pat. No. 2,685,160 to Kuhary et.al. (1954), U.S. Pat. No. 2,828,968 to Engler (1958), and U.S. Pat. No. 2,462,588 to Wondra (1947). The basic three-point configuration has changed little since its inception. 
     Turning to FIG. 1, the prior art three-point hitch will be explained. Tractor  10  has a pair of rocker arms  12 , which rotate on a common shaft as hydraulic pressure is applied. When rocker arms  12  rotate upward, they pull lift links  16  with them. Lift links  16 , in turn, pull lift arms  14  up with them. Lift arms  14  are pivotally connected to the underside of tractor  10  (illustrated in other views). 
     Many different agricultural implements may be attached to the hitching system. In the particular example illustrated in FIG. 1, mower  22  is attached. Mower  22  has A-frame  20  rigidly attached to its forward portion. A-frame  20  has two lift pins  24  rigidly attached thereto. Lift arms  14  have holes sized to slip over and engage lift pins  24 . 
     Top link  18  is pivotally attached to tractor  10  as shown. The opposite end of top link  18  is pivotally attached to the upper portion of A-frame  24 . The purpose of top link  18  is to keep mower  22  roughly level as it is raised and lowered (via the well-known operation of a 4-bar mechanism). Those skilled in the art will easily realize how the illustrated device functions. Rocker arms  12  are forcibly rotated by hydraulic power. If they are raised, then mower  22  is raised. Likewise, when they drop, mower  22  drops. Top link  18  is typically just a steel bar with an adjustment mechanism used to vary its length. The particular top link  18  illustrated in FIG. 1 is a new version, incorporating features of the present invention. However, it also provides the normal functions of a conventional top link. 
     Operator controls (not shown) are provided to allow the driver to adjust the height of the implement. It is very important to maintain a fixed height off the ground during many agricultural operations. The importance of this is obvious for mowing. It is also very important if the towed implement is a spraying rig—such as the type used for fertilizer and pesticides. The prior art operator controls often have an adjustable lower stop. The operator can set this stop to provide the correct height for the activity he is conducting. The height is hopefully maintained by hydraulic pressure holding rocker arms  12  in the set position. One must realize, however, that most agricultural implements are quite heavy. 
     In actual practice, this method of height maintenance has a serious shortcoming. The hydraulic pressure provided to position rocker arms  12  is supplied by a pump powered off the tractor&#39;s engine. The pressure is regulated by one or more control valves—usually of the proportional type. As the control valves wear with age, they often leak. This fact means that the height adjustment will creep downward. Over twenty or thirty minutes of operation, the operator will look back and realize that the implement has sunk six inches lower. He must then raise and lower the implement to reset the system. On older systems, he may have to reset the height every two or three minutes. 
     Even with new pumps and valves, height variations occur. On a cold morning, the hydraulic fluid circulating in the system starts with a much higher viscosity. As the fluid warms, its viscosity diminishes. Thus, an initial height adjustment will tend to sink as the system comes up to normal operating temperature. Significant differences in ambient temperature can cause the same phenomenon. A height setting made on a cold day will not bring the implement to the same position on a hot day. 
     The known devices for setting the height of an agricultural implement are therefore limited in that they: 
     1. Do not maintain a fixed position above the ground, instead letting the implement creep downward; 
     2. Do not maintain a fixed position as the hydraulic system heats up under cold conditions; and 
     3. Do not determine the same position under varying ambient conditions. 
     OBJECTS AND ADVANTAGES 
     Accordingly, several objects and advantages of the present invention are: 
     1. To maintain the implement in a fixed position above the ground; 
     2. To maintain said fixed position despite varying viscosity of the hydraulic fluid in the system; and 
     3. To maintain said fixed position despite varying ambient conditions. 
    
    
     DRAWING FIGURES 
     FIG. 1 is an isometric view, showing a tractor with an attached mower. 
     FIG. 2 is a close-up view of the three-point hitch shown in FIG.  1 . 
     FIG. 3 is an isometric view of the novel top link used in the present invention. 
     FIG. 4 is an isometric view of the height maintaining link used in the present invention. 
     FIG. 5 is a side elevation view, showing the tractor and mower with the mower in a lowered position. 
     FIG. 6 is a side elevation view, showing the tractor and mower with the mower in a raised position. 
     FIG. 7 is an isometric view showing the rear of the tractor with the mower in a lowered position. 
     FIG. 8 is an isometric view showing the rear of the tractor with the mower in a raised position. 
     FIG. 9 is an isometric view showing the rear of the tractor with the chain assembly shifted to a new position. 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 Reference Numerals in Drawings 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 10 
                 tractor 
               
               
                 12 
                 rocker arm 
               
               
                 14 
                 lift arm 
               
               
                 16 
                 lift link 
               
               
                 18 
                 top link 
               
               
                 20 
                 A-frame 
               
               
                 22 
                 mower 
               
               
                 24 
                 lift pin 
               
               
                 26 
                 top pin 
               
               
                 28 
                 top link pivot 
               
               
                 30 
                 height maintaining link 
               
               
                 32 
                 lower attach point 
               
               
                 34 
                 adjustment bracket 
               
               
                 36 
                 adjustment notches 
               
               
                 38 
                 mounting pin hole 
               
               
                 40 
                 lower attach bracket 
               
               
                 42 
                 chain 
               
               
                 44 
                 top link strap 
               
               
                 46 
                 lift arm pivot 
               
               
                 48 
                 gearbox 
               
               
                 50 
                 PTO shaft 
               
               
                   
               
             
          
         
       
     
    
    
     DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates tractor  10  with mower  22  attached to its three-point hitch, the operation of which has been explained previously. FIG. 2 provides a closer view of the novel aspects of the invention. Height maintaining link  30  is attached to mower  22  at two lower attach points  32 . The middle portion of height maintaining link  30  passes over top link  18 . Top link  18  is conventional, with the exception of adjustment bracket  34 , which is provided to adjustably attach height maintaining link  30  to top link  18 . 
     FIG. 3 illustrates top link  18  in greater detail. Top link  18  has two mounting pin holes  38 , which allow it to be attached to the tractor and the implement, as in the prior art. The forward portion of top link  18  is attached to tractor  10  at top link pivot  28  (shown in FIG.  2 ). The rear portion of top link  18  is attached to A-frame  20  at top pin  26  (also in FIG.  2 ). Adjustment bracket  34  is comprised of two steel plates welded to the sides of top link  18  as shown. Adjustment bracket  34  has a plurality of adjustment notches  36 , which allow an operator to alter the point at which height maintaining link  30  is attached to top link  18 . 
     FIG. 4 illustrates height maintaining link  30  in greater detail. Lower attach brackets  40  are provided to enable the user to easily attach height maintaining link  30  to lower attach points  32 . Lower attach brackets  40  are provided with through holes so that conventional bolts may be used for attachment. Height maintaining link  30  also has top link strap  44 , which is designed to engage adjustment notches  36  of adjustment bracket  34 . In between lower attach brackets  40  and top link strap  44  are two lengths of chain  42 . A sturdy cable could be substituted for chain  42 . In fact, height maintaining link  30  could be made as a solid piece. However, the use of chain  42  has been found particularly effective, as will be explained in the following. 
     FIGS. 5 and 6 illustrate the operation of the invention. The tires of tractor  10  are shown removed so that the reader may fully see the interaction of the present invention with the prior art three-point hitch. In FIG. 5, mower  22  is in a lowered position, appropriate for cutting operations. In FIG. 6, mower  22  has been raised by the previously explained operation of rocker arms  12 . As also previously explained, the operation of top link  18  has tended to keep mower  22  roughly level. 
     Those skilled in the art will realize that tractor  10 , lift arms  14 , and top link  18  comprise a classic 4-bar linkage. By observing the positional differences between FIG.  5  and FIG. 6, the reader may easily comprehend the operation of height maintaining link  30 . In FIG. 5, height maintaining link  30  is pulled into one of the adjustment notches  36  on top link  18 . It is placed under tension and it prevents lift arms  14  from going any lower. In FIG. 6, lift arms  14  have been raised considerably. The reader will note that height maintaining link  30  is lifted well out of adjustment notches  36 . In reality, of course, the chain portion of height maintaining link  30  will go slack and height maintaining link  30  will remain in its original adjustment notch  36 , held in place by its own weight. However, by illustrating height maintaining link  30  as rigid, the reader will easily comprehend that lifting the implement, as shown in FIG. 6, removes tension from height maintaining link  30 . Likewise, lowering the implement to the position shown in FIG. 5 places height maintaining link  30  in tension, thereby preventing the implement from going any lower. 
     FIGS. 7 and 8 illustrate the identical positions of mower  22 , seen from the rear. FIG. 7 represents the lowered position. The reader will observe that top link strap  44  is securely engaged within adjustment bracket  34  of top link  18 . Chains  42  are under tension and mower  22  cannot go any lower. FIG. 8 represents the raised position. Height maintaining link  30  is once again illustrated as rigid, with the result that it appears to have lifted well out of adjustment bracket  34 . The reader can visually appreciate the fact that it is no longer under tension and does not prevent the raising of the implement. In reality, top link strap  44  would remain engaged within adjustment bracket  34  by its own weight, and chains  42  would go slack. 
     FIG. 8 is also useful for illustrating how the operator can easily adjust the height set by height maintaining link  30 . When mower  22  is raised as shown, the operator can reach back from the seated position, grasp top link strap  44 , and lift it to the position shown. Turning now to FIG. 9, the operator can pull top link strap  44  forward and place it in a more forward adjustment notch  36 . This alteration will result in a higher minimum height for mower  22 . As shown in FIG. 9, mower  22  can only descend slightly before chains  42  come into tension and arrest its movement downward. 
     In the preferred embodiment, top link  18  and height maintaining link  30  are made of mild steel. It would be possible to make these items from many different materials. However, as strength is more important than weight savings in agricultural operations, steel is a good choice. 
     Those skilled in the art will realize that height maintaining link  30  could be made from a single length of chain. The chain could be attached on either end at lower attach points  32 , and then passed over top link  18 . The more complex version of height maintaining link  30 , as presented, is a refinement of this basic concept. The incorporation of top link strap  44  makes the device function more smoothly. 
     Likewise, those skilled in the art will realize that a single length of chain could simply be attached to one point of the implement and one point of the top link. While this alternative embodiment is within the present invention, it has practical limitations that should be appreciated. Turning back to FIG. 9, this limitation will be explained. PTO shaft  50  is located on the rear of tractor  10 . The purpose of this shaft is to provide rotary power to towed implements. Mower  22  receives this rotary power in gearbox  48 . Ordinarily, there is a drive shaft (“PTO shaft”) connecting PTO shaft  50  and gearbox  48 . For purposes of visual simplicity, this drive shaft has not been shown in the figures. However, the reader may easily observe that the preferred embodiment of height maintaining link  30  avoids interfering with this drive shaft. A single length of chain would have difficulty achieving the same result. 
     SUMMARY, RAMIFICATIONS, AND SCOPE 
     Accordingly, the reader will appreciate that the proposed invention maintains the height of a towed implement on a conventional three-point hitch. The invention has further advantages in that it maintains the fixed height despite varying viscosity of the hydraulic fluid in the system, and it maintains the fixed height despite varying ambient conditions. 
     Although the preceding description contains significant detail, it should not be construed as limiting the scope of the invention but rather as providing illustrations of the preferred embodiment of the invention. For example, many different types of tensile members could be substituted in place of chains  42 , many different adjustment methods could be provided on top link  18 , etc. Thus, the scope of the invention should be fixed by the following claims, rather than by the examples given.