A single-acting hydraulic cylinder is usually provided to raise the lower links of a three-point hitch and lowering is accomplished by the weight of the attached implement. In the system illustrated in Figure 7.6, the cylinder rotates a rockshaft; arms attached to the ends of the rockshaft lift the lower links to raise the hitch. A feedback control system is provided as a standard feature of three-point hitches so that the hitch movement will mimic the movement of the hitch control lever. In the system shown in Figure 7.6, moving the position-control handle leftward pulls on the spool of the main control valve, initiating oil flow to the cylinder to raise the hitch. As the hitch raises, a cam on the rockshaft pushes on the spool of the main control valve so that hitch movement stops at a position corresponding to that of the position control handle. Conversely, when the position control handle is moved to the right, the hitch mimics that movement in lowering as the control valve releases oil from the cylinder.
With the first three-point hitches, when a tractor was pulling a heavy-duty tillage implement under varying field conditions, the operator would raise the hitch slightly when the pull became excessive in heavy soils and would again lower the hitch when the pull subsided in lighter soils. Engineers soon modified the three-point hitch controls to accomplish such raising and lowering automatically. In Figure 7.6, a torsion bar is used to sense force in the lower links; as the force increases, twisting of the torsion bar moves a linkage to pull out on the spool of the main control valve and causes the lower links to be lifted. Conversely, declining force in the lower links causes them to be lowered. Just as the position control handle is used to set the desired position of the lower links, the load (draft) control handle is used to set the desired amount of force in them. On some small tractors, draft sensing is accomplished by sensing compressive force in the top link. In recent larger tractors, the torsion bar and mechanical linkage are eliminated. Lower link sensing is accomplished by use of instrumented link pins that sense shear at the lower link points. Through circuitry that is provided, the voltage from the link pins is used to control an electrically actuated hydraulic valve which, in turn, controls raising and lowering of the hitch.
