A
solenoid is a device that converts energy into linear motion. This
energy may come from an electromagnetic field, a pneumatic (air-powered)
chamber, or a hydraulic (fluid-filled) cylinder. These devices are
commonly found in electric bell assemblies, automotive starter systems,
industrial air hammers, and many other machines that rely on a sudden
burst of power to move a specific part.
In
order to understand the underlying principle, a person can examine a
typical pinball machine. At the beginning of play, a steel ball rests
on a rubber-tipped plunger that is held in place by a compression
spring, which means it has no energy to move the ball when at rest. The
player’s hand provides additional energy as the plunger assembly is
pulled back. Upon release, the spring forces almost all of the plunger
pin’s kinetic energy on a small area of the steel ball. The ball is
flung into the playing field and the pinball game begins. This manual
plunger is a rudimentary example of a solenoid.
The
difficulty with using manual pinball plungers on other machines is that
someone must constantly pull the spring back and release the energy by
hand. An improved solenoid would provide its own means of pulling back
on the pin and releasing it under control. This is the principle behind
a simple electric one, in which a metallic cylinder acts as the
“plunger.”
A
compression spring holds this metal pin partially out of an
electromagnetic housing. When power from a battery or electric
generator flows around the electromagnet, the metal pin or cylinder is
magnetically drawn inside the housing, much like the player’s hand pulls
the plunger back in the pinball example. When the electric current
stops, the pin is released and the compression spring sends it forward
with significant force. The pin may strike the inside of a bell or
forcefully eject a part from a molding machine. Many electronic
machines contain numerous solenoids.
Other
types depend on compressed air for their power. A single piston may be
placed in an airtight cylinder connected to a source of
highly-compressed air. A strong internal spring may hold the piston in
place until the air pressure has reached a predetermined level and then
the piston is released. The compressed air is allowed to escape as the
piston drives forward.
Because
the energy released by a solenoid can be concentrated, pneumatic ones
are popular for heavy tools and machining applications which require
substantial power. A jackhammer is a good example of this type in
action. The central piston is driven by air into the concrete, then the
recoil of the hammer returns the piston to its original
position.
An
even more powerful solenoid uses hydraulics as its source of power.
The piston or pin is seated in a cylinder filled with a hydraulic
fluid. As this hydraulic fluid fills the cylinder, everything is pushed
forward, including the piston or pin. As the piston travels towards a
piece of metal or other target, the fluid buildup becomes very resistant
to compression, and the piston will concentrate all of the cylinder’s
energy on whatever it encounters, even the heaviest titanium.
When
the solenoid has released all of its energy, the hydraulic fluid drains
out of the chamber and the piston is drawn back to its original
position. This action can take place in a matter of seconds. This type
is so powerful that it is generally used only for the heaviest
projects. Wave pools use them to release the giant stoppers at the
bottom of their holding tanks. Aircraft manufacturers use this type to
bend titanium and other heavy metals.