Monday, July 28, 2014

Rack and Pinion Pneumatic Actuator Basics

A rack and pinion gear.
(Courtesy of  Wikipedia)
Rack & Pinion actuators are designed for operating quarter-turn valves such as butterfly, plug, and ball valves or for actuating industrial or commercial dampers.

The rotational movement of a rack and pinion actuator is accomplished via linear motion and two gears. A circular gear, referred to a “pinion” engages the teeth of a linear gear “bar” referred to as the “rack”.

In a pneumatic actuator, pistons are attached to the rack. As air or spring power is applied the to piston, the rack is “pushed” inward or “pulled” outward. This dual direction linear movement is transferred to the rotary pinion gear providing bi-directional rotation.


rack and pinion actuator
Rack & Pinion actuator internals.
Note vertical pinion gear and its
engagement with linear rack gear.

Pneumatic actuators have cylinders with pistons and springs that provide the linear movement. When one side of the piston is pressurized with air, gas or oil, the pinion bearing turns in one direction. When the air, gas or oil from the pressure side is vented, a spring (spring-return actuators) may be used to rotate the pinion gear in the opposite direction. A “double acting” actuator does not use springs, instead using the air, gas or oil supply on the opposing side of the piston to turn the pinion gear in the opposite direction.

rack and pinion actuator
Internal view showing springs
used to rotate pinion in
opposite direction.
Pneumatic pneumatic rack and pinion actuators are compact and save space. They are reliable, durable and provide a good life cycle. Mechanical wear of the heads and seals are their primary disadvantage.

Most actuators are designed for 100-degree travel with clockwise and counterclockwise travel adjustment for open and closed positions. World standard ISO mounting pad are commonly available to provide ease and flexibility in direct valve installation.


NAMUR mounting dimensions on actuator pneumatic port connections and on actuator accessory holes and drive shaft are also common design features to make adding pilot valves and accessories more convenient.

Feel free to contact CTi Controltech at www.cti-ct.com or 925-208-4250 with any questions you may have about valve actuation.