Flow-controls; Select the correct type for your cylinder

Flow Controls; these are devices that are used when you need to control the speed of the air cylinder or pneumatic rotary actuator, either on extend stroke, retract stroke, or both.

Many folks will opt to purchase a needle valve to provide speed control for their cylinders.

A needle valve will work in controlling the speed of an air cylinder, however, it isn't your best option. You see, a needle valve controls the flow of air both into and out of the air cylinder. The compressed air flows through the reduced orifice in the needle at the same speed regardless of the direction of air flow.

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

Problems with a needle valve

In larger bore air cylinders the compressed air flowing into the cylinder through a needle valve will not be able to build up sufficient force to move the piston smoothly.

Air will build up in the cylinder to a 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 and the cylinder has to wait (sometimes only a fraction of a second) for pressure to build again before it can move. It takes longer to read about it that it does to actually happen, but the result for some cylinders is that using a needle valve to restrict the flow of air does create choppy piston / 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 most effective choice to control air cylinder speed

The most effective choice for controlling air cylinder speed is the cylinder flow control. This device could be similar in appearance to a needle valve, however it contains an internal by-pass through which the air can flow when the needle has reduced the flow. From the simple diagram you can see the bypass, which has an integral check valve in it.

The cylinder flow control is installed so that the air flow that gets reduced by the needle part of the valve is ONLY the exhaust air flow from the cylinder.

As the air pressure in the opposite end of the cylinder increases rapidly the piston will want to move very quickly. In order for this to happen, the air that is in that 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 needle, and as a result, by adjusting the needle to reduce the outflow, the cylinder speed can be adjusted to suit the application.

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. Now the air tries to bypass the needle, succeeds in blowing the check ball off seat, and has a free and open path to bring the pressure in the cylinder almost instantly to the level necessary to move the piston.

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 can't 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.

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 and come with their own quick fittings already installed. Other cylinder flow controls screw into the exhaust ports of the air valve.

Be cautious with this latter type. 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 cylinder flow control installed there will control the speed of the cylinder in both directions. Also, if the air line from the cylinder to the valve is long, it is possible that the cylinder will have already completed it's stroke before the exhaust cylinder flow control will have had time to affect the exhaust flow of air.

One other note, do be careful to 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.







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