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Control of Single-Acting Cylinder. Figure 331. The 3-way directional valve used here has a
solenoid actuator and spring return on its spool.
The single-acting cylinder extends by air pressure
and retracts by spring force.
When the solenoid is energized (top figure), the
valve spool moves downward. Air is connected
to the cylinder and the exhaust connection on the
valve is blocked. The cylinder extends,
compressing the return spring.
When the valve solenoid is de-energized, the
valve spool moves upward. Inlet air is blocked,
while the cylinder is vented to atmosphere
through the valve ports. The internal spring
causes the cylinder piston to retract.
Open triangles at valve Ports P and E show entry
and exit points for compressed air to and from
the valve.
Air Motor Control. Figure 3-35. A 3-way
directional valve may be used instead of the shutoff valve of Figure 3-11 to start and stop a nonreversing air motor, with the advantage that when
stopped, both motor ports are vented to
atmosphere. A reversing air motor would require
a 4-way valve.
A solenoid 3-way valve is shown here. Other
types of actuators could be used if appropriate.
When the solenoid is de-energized (top view),
inlet air pressure is cut off, and the motor is in a
free floating condition with both ports open.
With solenoid energized, air pressure is
connected to the motor.
If this circuit is used to control hydraulic motors
it should be limited to low power applications
because the incoming oil supply is stopped. On
high power hydraulic motor applications some
means should be provided to unload the oil
supply. One good method is to use a 4-way rather
than a 3-way valve to control the motor.
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SHUTTLE VALVE (DOUBLE CHECK)
Shuttle Valve. Figure 3-54. This valve is
officially termed a "Double Check Valve With
Cross Bleed". It has two inlets and one outlet.
Since no official port markings have been
assigned, the letters "P1" and "P2" are used for
the two inlets, and "A" for the outlet. This is the
same marking used for a 3-way, 2-pressure valve,
which also has two inlets. The purpose of a
shuttle valve is to permit flow into Outlet A from
either P1 or P2 (but not both at the same time)
while keeping the two inlets isolated from one
another. The valve will pass flow in the reverse
direction to either P1 or P2, depending on the
position of the shuttle at that moment.
The floating poppet which is called the "shuttle"
is free to move back and forth, closing off the
inlet which has the lower pressure and opening
the inlet which has the higher pressure. A
pressure difference of 1 PSI or less between the
two inlets is usually sufficient to shift the shuttle.
Shuttle Valve Application. Figure 3-55. If Valve 1 is actuated, air coming through this valve enters Port P1 of
the shuttle valve, pushes the shuttle over to block Port P2, and moves into the cylinder to extend it. Shuttle action
prevents loss of air to atmosphere through Valve 2 exhaust. When Valve 1 is released, air flows back through the
shuttle valve and vents to atmosphere through the exhaust of Valve 1. The cylinder can be operated in like
manner with Valve 2. Directional Valves 1 and 2 are 3-way directional type as described on Page 100.
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FLOW CONTROL VALVES
Flow Control Valves. Figures 3-63, 3-64, 3-65. Usually
built from brass or steel bar stock and designed for
installation in cylinder or fluid motor connecting lines to
control the rate of fluid flow, hence the travel speed of the
cylinder or motor. Flow is controlled in one direction only,
flowing toward the right. Fluid flowing toward the left can
pass in free flow through the internal check valve. Two flow
control valves are used to control speed individually in each
direction of a double-acting cylinder or reversible fluid
motor. Their use in controlling single-acting rams is shown
in Figure 2-17. Graphic Symbol. Figure 3-63 is the ANSI
(American National Standards Institute) graphic symbol for
use on circuit drawings. Also shown are the NFPA
(National Fluid Power Association) port markings, "P" for
controlled flow inlet, "F" for free flow inlet. Flow control
valves are usually marked with an arrow which shows
CONTROLLED flow direction. They must be installed
correctly to work properly, and if necessary their free-flow
direction can be determined by applying an air hose to each
port while the needle valve is screwed all the way in. Mouth
pressure should not be used because most check valves
crack at 3 PSI or higher, and a person usually cannot
develop enough mouth pressure to open them.
Pressure Compensated Flow Control Valves. A more
precise flow control for hydraulic cylinders, usually those
operating at 1000 PSI and over, which has a variable orifice
which automatically adjusts to keep the flow rate constant
through the valve under changing load conditions. They are
not suitable for air or air-over-oil applications.
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DUAL EXHAUST AIR VALVES
Four-way valves built for compressed air service
usually have two exhaust connections, one from
each end of the spool (see diagram below). These
valves are called "dual exhaust valves". Exhaust
Port EA exhausts cylinder Port A, and exhaust
Port EB exhausts cylinder Port B. Examples of
nomenclature are shown below and in Figure 4-6
on the opposite page.
The purpose in bringing out both internal exhaust
passages is two-fold: valves can be manufactured
more economically, and more important, these
exhaust connections can be used separately to
restrict cylinder speed in each direction of travel.
This is illustrated in Figure 4-6 on the opposite
page. and in Figure 4-44 on Page 149.
FIGURE 4-5. Dual exhaust air valve connected to a double-acting air cylinder. The graphic symbol is
constructed in the same way as related on Pages 128 and 129 for a single exhaust valve. The actuators shown
above or those shown on Pages 138 to 140 may be used on either end of the valve symbol.
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Figure 4-6. A needle valve may be screwed into each exhaust outlet connection, EA and EB, of a dual exhaust
air valve to obtain individual speed control in both directions of cylinder movement. A needle valve in EA
restricts air coming from cylinder Port A and reduces speed of retraction without affecting extension speed. A
needle valve in EB restricts air coming from cylinder Port B and reduces speed of extension. Refer to pictorial
cutaway on opposite page for path of air flow through 4-way valve.
POPULAR
4-WAY
COMPRESSED AIR
VALVES
use as 2-way, 3-way, or 5-way valves. However,
certain other brands, if used this way, would have
their packings unseated or washed out. Always
consult the valve manufacturers literature, if it is
available, before using a 4-way valve for any
other type service.
FOR
Graphic diagrams under this heading are
primarily intended to show end actuators for
compressed air valves. Flow arrows inside the
valve blocks are those for dual exhaust 4-way
valves which are the most common type for
compressed air. Actuator symbols are equally
valid for single exhaust valves. This list shows
actuator types most popular but does not cover
all variations offered by some manufacturers.
Four-way valves of some brands may be reconnected or may have unused ports plugged for
These actuator symbols were taken from
Specification ANSI Y32.10-1967. A copy of this
specification may be purchased from the National
Fluid Power Association, P.O. Box 49,
Thiensville, Wisconsin 53092.
GENERAL PURPOSE symbol may be used on one or both ends
of valve symbol. It actuates the spool in both directions. It may be
used if exact type of actuator is not known or not yet selected. It can
purposely be used to avoid limiting a valve to a certain type of
actuator, or when the actuator is optional.
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momentary impulse of electric current, and will
stay shifted until returned by an actuator on the
opposite end (solenoid, pilot signal, manual,
etc.).
SOLENOID VALVES. To keep the spool in
shifted position, current must be maintained on
the electric solenoid of all valves which have
spring centered or spring returned spools. Valves
which do not have spool springs will shift on a
• Single solenoid, 2-position, spring return. Valve spool
springs back to original position when solenoid is deenergized. Dual exhaust model shown.
• Single solenoid, 2-position, no springs. Spool is
returned by a pilot pressure signal from another valve
after solenoid has been de-energized.
• Double solenoid, two position action. Valve shifts and
remains shifted when one solenoid or the other is
momentarily energized.
• Double solenoid, 3-position spool, spring centered.
Porting in center neutral depends on spool selected.
Push button or palm button actuators are not
practical on valves larger than 1/4 inch. They
have no lever advantage, and the high shifting
force causes operator fatigue. Palm button valves
may use either the actuator symbol shown here or
the one shown on Page 107. Symbols shown are
for dual exhaust valves. See Page 130 for symbol
construction.
MANUALLY OPERATED VALVES include
all valves in which the spool is shifted manually
by an operator. Manual lever actuators are used
on valve sizes to 1 inch. For higher flow, a 1/4"
manual lever valve may be used to control a
larger pilot-operated main valve.
• Two-position spool stays in shifted position, held by
packing friction, when manual lever or button is released
by the operator.
• Two-position spool springs back to original position when
operator releases lever.
• Three-position spool spring centered. Porting in center
neutral depends on spool type selected. Dual exhaust
shown.
• Three-position spool held by center position or 3-position
click detent. Spool stays in position to which shifted.
Porting in center neutral depends on spool type selected.
PILOT-OPERATED VALVES
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Includes all valves where the main spool is
shifted by application of pressure to the end of
the spool,
either directly on the spool area or through a
piston, diaphragm, or cylinder. A pilot-operated
main valve is controlled from a remote point by
an operator using a miniature 3-way valve to
pressurize and vent the pilot lines.
• Two-position, double pilot, no spring. Spool stays either
position when pilot pressure is vented.
• Two-position, spring return. Spool returns to original
position when pilot pressure is vented.
• Three position, spring centered. Spool goes to center
when both pilot connections are vented.
MECHANICALLY ACTUATED VALVES
Mechanically actuated valves of small size can
be used to pilot operate larger valves. Care
should be exercised in mounting these valves in a
position where they will not be damaged if the
machine member should accidentally over travel.
These symbols may be used for any valve in
which the spool is directly actuated by a traveling
member of the machine, and includes cam roller,
remote clevis, and stem actuated types.
• Cam or mechanically operated, two-position spring
returned spool. Dual exhaust type shown.
• Cam or mechanically operated, two-position with
spool return by remote pilot pressure signal from
another source. Dual exhaust model shown.
FOOT OPERATED VALVES
Includes pedal valves which are operated with toe only, and treadle valves which can be operated with
toe in one direction and heel in the other. Foot operated valves are furnished only in small sizes, but can
be used to pilot operate larger valves.
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• Pedal actuated (treadle optional), 2-position spool is
returned to original position by spring force when
pedal is released.
• Pedal actuated (treadle optional). 2-position spool
returns to original position when a remote pressure
signal is received from another source.
• Treadle actuated 2-position spool held in each valving
position by packing friction when treadle is at rest. Not
suitable for pedal actuation.
• Treadle actuated 3-position spring centered spool. Not
suitable for pedal actuation. Closed center spool
shown.
• Treadle actuated 3-position spool with click detents.
Not suitable for pedal actuation. Closed center spool
shown.
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