This circuit is designed as shown in Figure 10.2.
When the four-way valve is in its spring-centered position, the cylinder is hydraulically locked. Also the pump is loaded back to the tank at atmospheric pressure.
When the four-way valve is actuated into the flow path configuration of the left envelope, the cylinder is extended against its force load (Fload) as oil flows from port P through port A. The oil at the rod end of the cylinder is free to flow back into the reservoir through the four-way valve from port B through port T. The cylinder will not extend if the oil in the rod end is not allowed to flow back to the reservoir.
When the four-way valve is de-activated, the spring-centered envelope prevails, and the cylinder is once again hydraulically locked.
When the four-way valve is actuated in the right envelope configuration, the cylinder retracts, as oil flows from port P through port B. Oil in the blank end is allowed to flow back to the reservoir from port A through port T of the four-way valve. At the end of the stroke, there is no system demand for oil. Therefore the pump flow goes through the relief valve at its set pressure, unless the four-way valve is de-activated. In any event, the system is protected from cylinder overloads.
The check valve prevents the load from retracting the cylinder, while it is being extended using the left envelope flow path configuration.