chapter  4
Basic Hydraulic Valves and Circuits
Pages 13

Hydraulic valves for the control of systems, vehicles, or equipment usually are one or a combination of these three types: directional, pressure control, or flow control.

Physically, any valve is a controllable restriction, whether the working element be a flapper, ball, diaphragm, globe-disk, swing-disk, gate, needle, plug, poppet, shearball, shear-plug, shear-plate, butterfly, pinch tube, or spool.