|Basic concept of|
rack and pinion gear.
Pneumatic rack and pinion actuators utilize air pressure as the motive force which changes the position of a valve. A rack and pinion actuator is comprised of two opposing pistons, each with its own gear (referred to as the "rack"). The two piston racks are set against a round pinion gear. As pressure increases against one side of each piston, each rack moves linearly against the opposite sides of the pinion gear causing rotational movement. This rotational movement is used to open and close a valve. See the animation above (provided by Wikipedia) below for a visual understanding.
This short video introduces the basic parts and operation of rack and pinion valve actuators to anyone unfamiliar with the device.
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