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The
Direct Operated Solenoid Valve is probably the most
commonly used mechanism in many pneumatic systems. Valves
with this basic design and different materials in the flow
path are commonly used for water, oil and gases (WOG). If
you ever wondered what a WOG valve was (common in bronze
body ball valves), now you know.
The most
common style of direct operated solenoid valves use an iron
armature with an elastomer seal on one or both ends. One end
seals against pressure and if the unit has an exhaust path
the other end seals the exhaust when operated. You don’t
have to be simple minded to appreciate the beauty of this
simple mechanism. As a stand alone valve shown in the
diagram below these are easy to recognize. Where you may not
have recognized this direct operated solenoid valve is as
the solenoid pilot operator for any number of spool and
poppet valves, process valves, etc.
The dynamic
of this mechanism is a classic solenoid with added features
to produce a directional control valve for liquids and
gases. The armature blocks the orifice and alternatively
provides a generous annular ring flow path with a short
stroke. With little dynamic friction and “stopper” seals
this mechanism is prepared for many million trouble free
cycles often without lubrication and adequate filtration.
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A
spring holds the armature against the valve seat
(orifice) in the body to block pressure. The spring
force must be greater than the force of the pressure
times the area.
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To use
the same mechanism at a higher pressure either the
spring must have more force, the area of the orifice
must be smaller or a combination of the two.
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When
energized the solenoid creates electro-magnetic force to
pull the armature deeper into the cavity of the
surrounding coil which compresses the spring.
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The
exhaust path is blocked by the opposite seal on the
armature. At this point pressure will assist the exhaust
closure.
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With
the armature deeper inside the coil cavity the lines of
permeability are increased and the electrical current is
reduced from inrush to holding current.
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The
term “direct” implies that the solenoid valve was
actuated directly as a result of the electrical
current without assistance.
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A
common manual override might be located in the body or
base of the solenoid mechanism to raise the armature
(valve) mechanically with a lever or cam when pushed or
turned.
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The
exhaust path is often threaded, possibly 10-32 UNF
or M5. For some applications introducing pressure
into the exhaust path provides a remote override
effect or alterative pressure input. The threaded
exhaust allows exhausting fluid to be plumbed away
for clean rooms or nasty gases.
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When actuated and in a holding current condition the
majority of these valves are rated for continuous
duty. They may be turned on and left on.
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If
the armature (sticks) does not move into the holding
position and the coil continues to draw “in-rush”
current the excess heat will soon destroy the coil.
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The
nature of an alternating current sign wave presents
an interesting wrinkle as it passes through zero
voltage. Without a remedy the spring will force the
armature out of the coil cavity until the voltage
and current increase again.
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A
shading ring is installed that produces a small
counter current that is 90 degrees out of phase with
the main sign wave. As the main current approaches
zero the counter current from the shading ring will
add enough current to hold the spring compressed.
Many manufactures place a shading ring (coil) in all
of their valves. If an AC coil is used to replace a
DC coil the valve will still work if the coil
matches the electrical signal applied.
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In a
world that is increasingly lean and green, we use valves
that are almost the same except for a standard and low
watt solenoid operation.
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You
know the drill. To go from a 7 watt coil to a 2 watt
coil we must deal with a change in magnetic force.
Swapping the coil spring and orifice will correct the
balance of forces required. Remember there is more to it
than just a coil swap
Tiny to modest sized direct operated solenoid valve provide
pressure to small cylinders and pilot pressure to much
larger valves and devices. This drop dead simple little
beauty is one of my favorites.
For more information: Thomas Kreher started Applied Pneumatic
Controls, Inc., with his wife, Gloria, in 1995. He can be reached at
tom@appliedpneumatic.com.
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