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According to
Wikipedia, "A rack and pinion is a type of linear actuator that comprises a pair of gears which convert rotational motion into linear motion. A circular gear called "the pinion" engages teeth on a linear "gear" bar called "the rack"; rotational motion applied to the pinion causes the rack to move, thereby translating the rotational motion of the pinion into the linear motion of the rack."
When applied as a pneumatic valve actuator, opposing racks are attached to pistons in a cylinder. When one side is pressurized, the pinion bearing turns in one direction. When air or spring tension is applied to the opposite side, the pinion bearing turns in the other direction, thus allowing rotational motion in both directions.
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| Transparent actuator showing opposing pistons (ROTEX) |
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| Pinion bearing and rack with piston and seal (ROTEX) |
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| Housing which forms outer pneumatic cylinder walls (ROTEX) |
Advantages of pneumatic rack and pinion actuators are compactness and space savings, reliability, durability and life cycle. Disadvantages are gear and bearing wear, piston seal fatigue and stem seal leakage.
Here is a great article on rack and pinion actuators from Valve Magazine.
