Flow Controls

Flow Controls are devices that are used when you need to control the speed of an air cylinder or a pneumatic rotary actuator.

Flow controls can control the speed of actuators on either the extend stroke, the retract stroke, or in both directions.

Not A Needle Valve

I recommend that you do not purchase a needle valve to try and provide speed control for an air cylinder.

A needle valve will work somewhat in controlling the speed of an air cylinder. It is not your best option.

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 the problem.

This means that in order to get the cylinder speed slowed to your desired cycle rate, in using the needle valve you have limited the inrush of compressed air into the cylinder too.

Needle Valve Issues

In larger bore air cylinders the compressed air flowing into the cylinder through a needle valve will be limited. The restricted compressed air flow may not be fast enough for that air cylinder to be able to build up sufficient force inside the air cylinder to move the piston smoothly.

Even with restricted compressed air inrush, pressure will 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 - for the pressure to build sufficiently again, before it can move.

It takes longer to read about it that it does to actually happen, but the result for some air cylinders is that the use of a needle valve to reduce the flow of air to slow the cylinder, can create a choppy piston / rod movement. An undesirable result, to say the least.

The potential is there for this to happen for every cylinder you try to control with a needle valve type flow control.

The Right Choice

The most effective choice for controlling air cylinder speed is the cylinder flow control.

The flow control might be similar in appearance to a needle valve but it does not work the same way.

The proper flow control contains an internal by-pass through which the air can flow unimpeded.

From the diagram you can see the bypass, which has an integral check valve in it.

The cylinder flow control reduces the flow on compressed air only in one direction, and that is as the air is flowing out of the air cylinder.

Since only the exhausting compressed air is affected by throttling with the flow control, the inrush of air is at full volume. The result is that the cylinder piston will want to move very quickly as full flow of compressed air enters the cylinder.

In order for the piston to move very quickly as the inrush tries to pressure the piston to the other end of the cylinder, the air that is in that other end of the air cylinder must exhaust just as quickly. The cylinder flow control contains a check valve to ensure that the out flowing air can't bypass the throttled air path, and as a result, the cylinder speed can be adjusted to suit the application without negatively affecting the piston movement.

When the valve controlling the cylinder shifts, the air flow reverses, and now air is flowing into the cylinder through the same cylinder flow control through which it has just exhausted. Now the air tries to bypass the restriction, succeeds in blowing the check ball off its seat, and now has a free and open path to pressurize the cylinder almost instantly, to the level necessary to move the piston relatively smoothly.

At the other end (assuming that you wish to control the cylinder speed in both directions) the out flowing air travels through another cylinder flow control, the air tries to bypass the needle but cannot as the ball is seated on the check, and the cylinder speed in that direction is controlled by reducing the flow of the exhaust air, reducing the speed of the air cylinder by throttling only the exhaust air flow.

Different Flow Controls

Cylinder flow controls come in all shapes and sizes. Some are designed to be in-line between the valve and the cylinder port. Others are built to screw right into the cylinder port. Some come with their own quick fittings already installed. Other cylinder flow controls screw into the exhaust ports of the air valve.

If the air valve is a 4 ported x 2 way, that means that it has a common exhaust for both cylinder ports. Therefore, an exhaust flow control installed in the single exhaust port will control the speed of the cylinder in both directions. This means that you will have to be satisfied with the speed on the cylinder in both directions, as your only speed control option.

Also, 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.

Ensure that you install the cylinder flow control in the correct direction. There will usually be a schematic on the side of the device to show the correct plumbing. Remember, for correct speed control of your air cylinder and for best cylinder performance, you want to control the speed of the compressed air coming out of the cylinder port, not the air flowing in.