3-137.9. DIRECTIONAL CONTROL VALVE - MAINTENANCE INSTRUCTIONS.
The directional control valve is comprised of a rectangular valve body containing a precision fitted sliding
spool in a central, longitudinal bore. Spool lands serve to divide the bore into a series of separate chambers,
and ports in valve body lead into these chambers. The position of the spool determines which ports are open to
each other and which are sealed off from the others. Thus, oil flow is directed from one port to another within
the valve body.
b. PRINCIPLES OF OPERATION
SPRING CENTERED - Spring centered valves are provided with a spring and centering washer at each
end of the spool. The springs and washers center the spool within the valve body, when solenoids are de-
(a) Three cross section views of a valve are shown. The cross sections show locations of the spool lands and
the basic valve block machining. Each cross section is provided to show porting of the valve as the spool is
moved within the valve block. Assume the spool is moved to the left within the valve body. The "P" pressure
port will connect to the "A" cylinder port and the "B" cylinder port will open to the "T" tank port.
(b) If the spool is moved to the right as shown, the "P" pressure port will connect to the "B" cylinder port and
the "A" cylinder port will connect to the "T" tank. When the spool returns to center,(solenoids de-energized),
flow is blocked in all ports. It can be seen that the function of a valve is to direct the flow of system fluid within
a circuit. The valve is actually used to direct flow from the pump to the actuator and from the actuator to the
tank or reservoir.