Free Flow in One Direction

Another way to control the flow of an actuator would be to use a flow control valve. A flow control valve incorporates a needle valve with a check valve. A flow control valve allows us to control the flow of an actuator in one direction, but allows free flow in the other direction.

Flow control valves and needle valves look very similar. There is a schematic symbol on the side of most fluid power components.

Here is a schematic symbol of a flow control valve.

Parker Hannifin Corporation. (1980). Industrial Pneumatic Technology. Parker Hannifin Corp. DOI: 1557690154

A flow control valve works by allowing air to follow two paths. One path, depending on where and which direction you install the flow control valve, will force the air to go through the needle valve portion of the flow control valve. This happens because the check valve portion of the flow control valve forces the air through the needle valve. As the air comes back through the opposite part of the flow control valve, it will go over the check valve and will not pass through the needle valve, thus, allowing the air to free flow. This happens because the air wants to follow the path of least resistance.

Look at the following picture:

Parker Hannifin Corporation. (1980). Industrial Pneumatic Technology. Parker Hannifin Corp. DOI: 1557690154

In the picture above, we want to control the speed of the extending cylinder. As the air enters the left side of the picture, it is blocked by the poppet (check portion) of the flow control valve. It is then forced through the needle valve, which restricts the flow of the air, which controls the speed of the actuator on the extend part of this system. When the cylinder retracts, the air will push the poppet (check valve) portion of the flow control valve against the spring, and the air will free flow out of the flow control valve without going through the needle valve portion.

See the following picture:

Parker Hannifin Corporation. (1980). Industrial Pneumatic Technology. Parker Hannifin Corp. DOI: 1557690154

As you can see, it is absolutely important that you install a flow control valve correctly. Not doing so will cause the system to operate incorrectly.

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