Explain What these Spool Valve Symbols Represent?

Control valves used in pneumatic and hydraulic fluid power systems often take the form of a spool mechanism, and the fluid power schematic symbols for these spool valves are quite unlike that of valve symbols in P&IDs and loop diagrams:

Solenoid Valve Spool Valve Symbol

Explain what these spool valve symbols represent, and how they are to be interpreted. Also, comment on the construction of a spool valve.


The symbols show each valve’s “normal” (unactuated) position. To understand what happens when the valve is actuated, you must visualize the boxes sliding over into alignment with the inlet/outlet pipes, like this:

Valve Symbol in Normal Position

The same holds for the reversing valve, except that it has three positions:

Explain What these Spool Valve Represent

Share your answers with us through the below comments section.

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Credits: Tony R. Kuphaldt

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