Also common in pneumatic systems are valves to control flow. There are fewer available types of flow valves compared to pressure or directional valves, but most circuits apply them to make for easy adjustment to cylinder or motor velocity. Controlling velocity in pneumatic systems is more complex than in a hydraulic system, because pressure differential between the work ports of a cylinder plays a larger part.
Flow control valves for pneumatic systems are quite simple, usually available in two configurations used in two different ways. One configuration is merely a variable restriction, with a screw or knob adjustment to open and close a variable orifice, which is also often referred to as a needle or choke valve. The other type introduces a check valve, which allows free flow in one direction, and restriction in the opposing direction. For whatever reason, this valve has hijacked the name flow control all for itself.
Flow control valves are applied in two different ways; meter in or meter out. Meter in is the method of controlling the rate of airflow as it enters a motor or cylinder. When metering in, a cylinder will move rapidly with high force and efficiency, but the motion of the piston is prone to spongy and unpredictable movement. When metering out, the cylinder velocity is more stable and repeatable, but efficiency and dynamic force are lost to the energy required to push past the flow control. Regardless, most pneumatic applications operate using meter out flow controls, because the disadvantages are easy to overcome by increasing upstream pressure.
A method of increasing cylinder velocity, typically for double acting or spring-return cylinder retraction functions, is to add a quick exhaust valve to the cap side work port. Because cylinders retract faster than they extend as a result of differential air volumes, it is harder to evacuate the cap side air volume without oversized valves or plumbing. A quick exhaust valve vents directly to air from the cap side work port, and massively reduces the backpressure created upon retraction, permitting very rapid piston velocity.