Depending on the type used, flow controls can control the speed of actuators on either the extend stroke, the retract stroke, or in both directions.
Yes, a needle valve controls the flow of air both into and out of the air cylinder. The compressed air flows through the needle at the same speed regardless of the direction of air flow. And therein lies part of the problem with using needle valves to try and control cylinder speed.
In order to get the cylinder speed slowed to your desired cycle rate by throttling the exhaust with a needle valve, since compressed air has to follow that same path when the cylinder reverses, you now have throttled the inrush of compressed air into the cylinder too.
With restricted compressed air inrush due to the needle valve throttling the in flowing air, pressure can sometimes build up in the cylinder to the point that the piston moves. But then, as it does, the resulting larger volume inside the cylinder momentarily lowers the air pressure behind the piston. The cylinder piston has to wait - yes, sometimes only a fraction of a second but it still hesitates or jerks - for the pressure to build sufficiently again, before it can move again.
It takes longer to read about it than it does to actually happen. Yet the result for some air cylinders is that the use of a needle valve in both lines to reduce the flow of air to slow the cylinder, can create a choppy piston and rod movement. An undesirable result when the intent was to control the rod movement.
The potential is there for this to happen for every cylinder you try to control with a needle valve type flow control.
The flow control might be similar in appearance to a needle valve but it does not work the same way. A properly constructed flow control contains an internal by-pass through which the air can flow unimpeded when flowing into the air cylinder.
In the diagram you can see the compressed air bypass. It has an integral check valve in it that same bypass.
The bypass and check valve allows the cylinder flow control to reduce the flow on compressed air only in one direction. The ideal installation path is that the air flowing out of the air cylinder is throttled.
Since only the exhausting compressed air is throttling with the flow control, the inrush of air in the other air line flows at full volume. The result is that the cylinder piston will move very quickly from the stopped location as a full flow of compressed air enters the cylinder.
At the same time that is happening the air that is in the other end of the air cylinder must exhaust out the other line, and through the flow control in that now exhaust air line. The check valve ensures that the out flowing air can't bypass the throttled air path, and as a result, the cylinder speed can be adjusted to slow the cylinder to suit the application without negatively affecting how quickly the piston reacts or the available force of the air cylinder.
When the valve controlling the cylinder shifts, the air flow to the air cylinder reverses, and now air is flowing into the cylinder through what was just the exhaust line. Now the inrushing compressed air tries to bypass the restriction, succeeds in blowing the check ball off its seat, and the air now has a free and open path to pressurize the cylinder providing relatively instant, full force, cylinder rod movement.
At the same time, and assuming that you wish to control the cylinder speed in both directions, the out flowing air travels through the other cylinder flow control. The exhausting compressed air tries to flow to the patch of least resistance, tries to bypass the throttle in the flow contro, cannot as the ball is seated on the check by that moving air, and the cylinder piston speed is controlled in the reverse direction as well.
Other cylinder flow controls screw into the exhaust ports of the air valve instead of the air cylinder ports.
Be cautious with valve exhaust flow controls. If the air valve is a 4 ported x 2 way it has only one common exhaust for both cylinder ports.
Therefore, an exhaust flow control installed in the single exhaust port will control the speed of the cylinder, and provide the same rod speed, in both extend and retract. If this doesn't suit the application, you will need to select a different form of flow control. .
Another issue with valve exhaust port flow controls is if the air line from the cylinder to the valve is long, it is possible that the cylinder will have already completed its stroke before the exhaust cylinder flow control will have had time to throttle the exhaust flow of air and slow the cylinder.
There will typically be a schematic on the side of the flow control to show the correct plumbing orientation.
The best cylinder performance will be achieved if the speed of the compressed air exiting the cylinder port is restricted, but not the compressed air flowing into the cylinder. That is achieved by using flow controls, not needle valves.