port (4). In hydraulic systems, the return port is
connected by a line to the reservoir. In pneumatic
systems, the return port is usually open to the
atmosphere.
Pilot-Operated Three-Way Valves
A pilot-operated, poppet-type, three-way
directional control valve is shown in figure 6-29.
Valves of this design are often used in pneumatic
systems. This valve is normally closed and is
forced open by fluid pressure entering the
pilot chamber. The valve contains two poppets
connected to each other by a common stem. The
poppets are connected to diaphragms which hold
them in a centered position.
The movement of the poppet is controlled by
the pressure in the pilot port and the chamber
above the upper diaphragm. When the pilot
chamber is not pressurized, the lower poppet is
seated against the lower valve seat. Fluid can flow
from the supply line through the inlet port and
through the holes in the lower diaphragm to fill
the bottom chamber. This pressure holds the
lower poppet tightly against its seat and blocks
flow from the inlet port through the valve. At the
same time, due to the common stem, the upper
poppet is forced off of its seat. Fluid from the
actuating unit flows through the open passage,
around the stem, and through the exhaust port
to the atmosphere.
When the pilot chamber is pressurized, the
force acting against the diaphragm forces the
poppet down. The upper poppet closes against its
seat, blocking the flow of fluid from the cylinder
to the exhaust port. The lower poppet opens, and
the passage from the supply inlet port to the
cylinder port is open so that the fluid can flow
to the actuating unit.
The valve in figure 6-29 is a normally closed
valve. Normally open valves are similar in design.
When no pressure is applied to the pilot chamber,
the upper poppet is forced off of its seat and the
lower poppet is closed. Fluid is free to flow from
the inlet port through the cylinder to the actuating
unit. When pilot pressure is applied, the poppets
are forced downward, closing the upper poppet
and opening the lower poppet. Fluid can now flow
from the cylinder through the valve and out the
exhaust port to the atmosphere.
FOUR-WAY VALVES
Most actuating devices require system pressure
for operation in either direction. The four-way
directional control valve, which contains four
ports, is used to control the operation of such
devices. The four-way valve is also used in some
systems to control the operation of other valves.
It is one of the most widely used directional
control valves in fluid power systems.
The typical four-way directional control valve
has four ports: a pressure port, a return or exhaust
port, and two cylinder or working ports. The
pressure port is connected to the main system
pressure line and the return line is connected to
the reservoir in hydraulic systems. In pneumatic
systems the return port is usually vented to the
atmosphere. The two cylinder ports are connected
by lines to the actuating units.
Poppet-Type Four-Way Valves
Figure 6-30 shows atypical four-way, poppet-
type directional control valve. This is a manually
operated valve and consists of a group of
conventional spring-loaded poppets. The poppets
are enclosed in a common housing and are
interconnected by ducts to direct the flow of fluid
in the desired direction.
Figure 6-29.—Three-way, poppet-type, normally closed directional control valve (pilot-operated).
6-20
