A spool valve steers oil by sliding a shaped shaft through a machined bore. Its wide lands seal some ports while its thin necks open others. Watch how moving the spool connects the pump, the tank and the two work ports.
A spool valve is the traffic controller of a hydraulic system. Instead of a flap or a ball, it uses a sliding shaft — the spool — whose shape opens and closes paths as it moves. One valve can send oil to a cylinder, bring it back, or hold the cylinder still.
From the outside a spool valve is just a small block with hoses. The whole clever part — a shaft that seals and opens ports by moving only a few millimetres — is completely hidden, so a cross-section is the only way to see why it works.
The pump keeps oil under pressure at port P. In the valve's neutral position the spool's lands block that pressure from reaching the work ports, so the oil simply cannot get through.
An electric signal energises the solenoid, whose plunger pushes the spool along its bore. The spool only has to move a few millimetres for its necks to line up with different ports.
Each wide land is a moving seal and each thin neck is a moving window. As the spool slides, a neck arrives next to P and lets the pressurised oil flow out to one work port, while a land seals the other.
With the necks lined up, oil travels from P to work port A, and the returning oil from port B is opened through to tank at T. Reversing the spool swaps the connections, so the cylinder moves the other way.
When the solenoid is switched off, the return spring pushes the spool back to neutral. The valve therefore fails safe: with no signal, no oil is sent to the work ports and the cylinder stays put.
A spool is a shaft with wide lands and thin necks. Sliding it along a machined bore lines the necks up with different ports, so the land between two ports seals the path while a neck opens it. Moving the spool switches which ports are connected.
Each land acts as a moving seal and each neck as a moving window. With three or more lands one valve can connect the pump to one work port while connecting the other work port to tank, all in the same stroke.
P is pressure in from the pump, T is return to the tank, and A and B are the two work ports that run out to the cylinder or motor. The spool decides which of them are joined at any moment.
It is the return spring. When the solenoid is switched off the spring pushes the spool back to its neutral position, so the valve fails safe to the state where no oil is sent to the work ports.
Oil is thin and under high pressure, so even a tiny clearance would leak past a land. The spool is ground to fit its bore with a clearance of only a few thousandths of a millimetre to keep the sealing tight.