Don't use a needle valve to control your air cylinder speed
Needle VALVESMany 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.
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