forces set up within the cylinder, since the forces
there are equal. As indicated by the arrows against
the pistons of the spool, the same pressure acts
on equal areas on their inside surfaces. In the
closed position, one of the pistons of the spool
simply blocks the inlet port, thus preventing flow
through the valve.
A number of features common to most sliding
spool valves are shown in figure 6-23. The small
ports at either end of the valve housing provide
a path for any fluid that leaks past the spool to
flow to the reservoir. This prevents pressure from
building up against the ends of the pistons, which
would hinder the movement of the spool. When
spool valves become worn, they may lose balance
because of greater leakage on one side of the spool
than on the other. In that event, the spool would
tend to stick when it is moved back and forth.
Small grooves are therefore machined around the
sliding surface of the piston; and in hydraulic
valves, leaking liquid will encircle the pistons and
keep the contacting surfaces lubricated and
centered.
THREE-WAY VALVES
Three-way valves contain a pressure port, a
cylinder port, and a return or exhaust port. The
three-way directional control valve is designed to
operate an actuating unit in one direction; it
permits either the load on the actuating unit or
a spring to return the unit to its original position.
Cam-Operated Three-Way Valves
Figure 6-28 shows the operation of a cam-
operated, three-way, poppet-type directional
control valve. View A shows fluid under pressure
forcing the piston outward against a load. The
upper poppet (2) is unseated by the inside cam
(5), permitting fluid to flow from the line (3) into
the cylinder to actuate the piston. The lower
poppet (1) is seated, sealing off the flow into the
return line (4). As the force of the pressurized fluid
extends the piston rod, it also compresses the
spring in the cylinder.
View B shows the valve with the control
handle turned to the opposite position. In this
position, the upper poppet (2) is seated, blocking
the flow of fluid from the pressure line (3). The
lower poppet (1) is unseated by the outside cam
(6). This releases the pressure in the cylinder and
allows the spring to expand, which forces the
piston rod to retract. The fluid from the cylinder
flows through the control valve and out the return
Figure 6-28.—Three-way, poppet-type directional control
valve (cam-operated).
6-19
