The oil supply to the check valves can be made available by gravity if an oil tank or reservoir is positioned above the check valve inlet. Another method is to provide oil under a low charge pressure to the relief valve with a positive displacement gear pump. A check valve (See Fig. 9) usually consists of a ball and seat positioned between two ports. As a directional control, it has a free flow and no flow direction. Flow through the seat will push the ball away, flow to the ball pushes the ball against the seat so pressure forces it to seal the passage.
POSITIVE DISPLACEMENT PUMPS
Resistance to the flow of oil produces pressure necessary for the system to function. If the amount of resistance could not be controlled, it could become so high that the weakest component containing the oil would burst, or the energy source would be stalled. Every hydraulic circuit that utilizes a positive displacement pump therefore requires a relief valve to protect against excessive pressure. If a load cylinder were stalled or reaches the end of its stroke, for example, an alternate path must be provided for the pump output. The relief valve allows the oil to be returned to the system supply or reservoir.
A simple relief valve (Fig. 12. ) is similar to the check valve in construction. The component that acts as a valve may be a round ball, or a poppet that has a smooth concentric surface to act upon a concentric seat area. The important part of the valve is the relief spring. It determines the pressure of the system by the force it applies to the valve. This force is called the cracking pressure.
On most relief valves the cracking pressure can be adjusted to some degree with screw type mechanisms, or by placing adjusting shims above the spring. We must caution that raising the relief pressure should not be attempted without proper test equipment to gauge the hydraulic system, nor should it be changed if other pump or actuator components are failing to perform at their rated capacities.
[Source: Simplicity publication, Hydraulic Systems Training Information, #840172, Principles and Operation of Tractor Hydraulic Systems]