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Many industrial machines using compressed air as an energy
source, use air cylinders or other pneumatic actuators to do the actual work.
Compressed
air is 'explosive' as it moves from high pressure to low pressure on it's way
back to atmosphere. That means when the air valve shifts and air flows to the
cylinder, the cylinder piston and rod moves extremely quickly. A high speed cylinder
rod may not be best for your application, and you will want to reduce the speed
and the impact.
One easy method of controlling the speed of an air cylinder
is installing flow controls in the air lines between the valve to the air
cylinder, in the cylinder ports themselves, or even in the exhaust ports of the
air valve, though the latter is least desirable.
In the 'valve exhaust' type flow control the controls
themselves may be far enough away from the cylinder that the piston and rod may
have traveled the full stroke before the exhaust flow control can start
back-pressuring the line to slow the cylinder. Cylinder control reaction time
is always negatively affected depending on the distance from the cylinder to
the flow control.
Some
folks opt for needle valves to throttle the flow of air into and out of the air
cylinder, thereby reducing it's speed. The problem with using a needle valve to
control the speed of an air cylinder is that it throttles the compressed air
flow equally in both directions.
If you are using a larger cylinder, by throttling the air
into the cylinder, you are actually preventing the smooth cylinder stroke
desired. As air pressure builds in the cylinder it will reach the point where
it overcomes the friction of the piston and rod seals and the piston will start
to move. As it moves, it increases the volume of the air space inside the
cylinder behind itself. The piston moves toward one end of the cylinder
creating a larger area behind than in front of it. This is an area into which
air has to flow quickly to ensure that the piston keeps moving. If the air
inrush cannot keep up with the increasing cavity size there will be
insufficient pressure to keep the piston moving, and it will stop. So too will
your piston rod, and whatever tooling you have installed on the end of it.
The
rule of thumb for using a flow control to reduce and smooth air cylinder piston
travel is to only throttle the exhaust air from the cylinder. The air flowing
into the cylinder port should never be reduced.
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How is this accomplished?
Use a "cylinder flow control". This is a device
that may not look any different from the needle valve. Inside, however, there
is a "needle bypass" which allows the air to bypass the needle which
is throttling the air achieving full flow of air unidirectionally.
The "free flow" of compressed air through the
cylinder flow control allows the unit, when it's installed in the correct
orientation, to provide full and un-throttled inrush of air to the cylinder,
yet, when the valve has shifted and the double-acting air cylinder has
reversed, the air that is now flowing out of the cylinder is throttled to the
level necessary to achieve the cylinder speed desired.
There will be a second cylinder flow control on the other
line too, and this works exactly the same way.
As a result, air flow into the cylinder ports at either end
of the cylinder is totally unencumbered providing high-force piston movement.
The piston is trying to go fast and at full power, but since the exhaust flow
of air is impeded by the cylinder flow control, the piston moves with full
power, but at a controlled and desired rate.
Most cylinder flow controls will have a schematic on the
side showing the flow paths to ensure that they are installed correctly.
Some cylinder flow controls are equipped with sealant coated
male threads for screwing into the cylinder port, and with an
"instant" type fitting into which the cylinder air line can be
quickly fitted, features that save time and money.
Bill Wade is a former sales representative, sales
manager, marketing manager and president of a number of companies that use and
sell compressed air, along with other equipment and supplies. His sales
agency currently represents a select group of companies. Mr. Wade writes about
understanding compressed air and how it's used at http://www.about-air-compressors.com.
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