Hydraulic Spool Valve Design

spool valve
"A radially symmetrical valve, usually actuated by low pressure gas controlled through a pilot valve. In many spool valve based paintgun designs, the spool, or core of the valve, also functions as the bolt."

hydraulic
Of or relating to the science of hydraulics
of or relating to the study of hydraulics; "hydraulic engineer"
(of cement) Hardening under water
moved or operated or effected by liquid (water or oil); "hydraulic erosion"; "hydraulic brakes"
(hydraulically) in a hydraulic manner; "the block is then tested hydraulically to its full design test pressure on each stream separately"
Denoting, relating to, or operated by a liquid moving in a confined space under pressure

design
Decide upon the look and functioning of (a building, garment, or other object), typically by making a detailed drawing of it
Do or plan (something) with a specific purpose or intention in mind
the act of working out the form of something (as by making a sketch or outline or plan); "he contributed to the design of a new instrument"
plan: make or work out a plan for; devise; "They contrived to murder their boss"; "design a new sales strategy"; "plan an attack"
an arrangement scheme; "the awkward design of the keyboard made operation difficult"; "it was an excellent design for living"; "a plan for seating guests"