Spool valves can be positioned manually, or by springs, or by pressure, and in many applications have lands of different diameters.
In this example, the different diameters result in a larger face area on land 1 than on land 2.
This is known as a differential area and when fluid under pressure enters at port B, this differential area creates a differential force.
This moves the valve to the left since the force acting on land 1 is greater than the force on land 2. Alternatively, the valve could be moved by admitting fluid at port C.
In both cases the valve movement allows previously blocked fluid at D to flow between lands 3 and 4 to exhaust at E or to pass to some other part of the system. If fluid pressure is removed from C the valve will remain stationary.
Even if fluid is admitted at port A, no movement can occur as the areas on lands 2 and 3 are equal and both generate an equal force.
The valve can only be moved back to its original position by introducing spring pressure or fluid pressure on the left hand side of land 1.
