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SHUTTLE VALVE
In certain fluid power systems, the supply of fluid
to a subsystem must be from more than one source
to meet system requirements. In some systems
an emergency system is provided as a source
of pressure in the event of normal system failure.
The emergency system will usually actuate only
essential components.
The main purpose of the shuttle valve is to isolate
the normal system from an alternate or emergency
system. It is small and simple; yet, it is
a very important component.
Figure 6-27 is a cutaway view of a typical shuttle
valve. The housing contains three ports— normal system inlet, alternate or
emergency system inlet, and outlet.
A shuttle valve used to operate
more than one actuating unit may contain additional
unit outlet ports. Enclosed in the housing
is a sliding part called the shuttle. Its purpose
is to seal off either one or the other inlet ports.
There is a shuttle seat at each inlet port.
Figure 6-27.—Shuttle valve.
When a shuttle valve is in the normal operation
position, fluid has a free flow from the normal
system inlet port, through the valve, and out
through the outlet port to the actuating unit. The
shuttle is seated against the alternate system inlet
port and held there by normal system pressure
and by the shuttle valve spring. The shuttle
remains in this position until the alternate system
is activated. This action directs fluid under pressure
from the alternate system to the shuttle valve
and forces the shuttle from the alternate system
inlet port to the normal system inlet port. Fluid
from the alternate system then has a free flow
to the outlet port, but is prevented from entering
the normal system by the shuttle, which seals
off the normal system port.
The shuttle may be one of four types: (1) sliding
plunger, (2) spring-loaded piston, (3) spring-loaded
ball, or (4) spring-loaded poppet. In
shuttle valves that are designed with a spring, the
shuttle is normally held against the alternate system
inlet port by the spring.
TWO-WAY VALVES
The term two-way indicates
that the valve contains and
controls two functional flow control ports-an
inlet and an outlet. A two-way, sliding spool
directional control valve is shown in figure 6-23.
As the spool is moved back and forth, it either
allows fluid to flow through the valve or prevents
flow. In the open position, the fluid enters
the inlet port, flows around the shaft of the
spool, and through the outlet port. The spool cannot
move back and forth by difference of 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).
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.
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