All About Pneumatic Actuators

Pneumatic actuators use air power to produce rotary and linear motion. In the broadest sense, an air cylinder is a pneumatic actuator. For the purpose of this article, pneumatic actuators that are intended to produce rotary motion for operating valves, for automating manufacturing, for clamping work, etc. are the primary focus. Their use in linear applications such as valve operating and positioning is also discussed. Air cylinders, in general, are not covered. For information on other types of actuators, please see our related Actuators Buyers Guide.

Pneumatic rotary actuators
Pneumatic rotary actuators rely on a variety of mechanisms to produce rotary motion. Two common styles use pistons or diaphragms. In one style, the piston moves a rack past a gear pinion, rotating the pinion to produce a limited range of rotation at the output shaft. Another style uses a scotch yoke, a well-known kinematic link, to produce up to 90° of rotation of the output shaft. A third method uses no pistons or diaphragms but instead employs one or two vanes that are pressurized to produce rotary motion directly within a round housing.

Rack-and-pinion styles use at least one, and sometimes two or four, cylinders to drive the rack(s) past the pinion. The pinion rotates in response, driving the output shaft. A rack-and-pinion actuator will continue to revolve the pinion until it reaches the end of the stroke, although modulation is possible. In many instances, the pistons in the cylinders will work against large coil springs which provide the valve with the capacity to return to a safe position during a power interruption. Diaphragms are sometimes used in place of pistons but the operating principle remains the same. The number of revolutions a rack-and-pinion actuator can make is limited only by the length of the rack.

Scotch-yoke actuators are usually limited to 90° of rotation and see applications in quarter-turn valves. Pneumatic vane actuators can produce rotations in excess of 360°.

Pneumatic actuators work with on/off valves such as ball valves. They can be adapted to work with control valves as well. Control valves necessarily need to be positioned at intermediate locations in order to regulate fluid flow. Modulating pneumatic actuators are available for this task. Typically, a 3-15 psi industry-standard air pressure signal is sent to a modulating positioner at the valve, which adjusts (in the case of a quarter-turn valve) the valve stem anywhere between 0 and 90° depending on downstream flow measurements. Electro-pneumatic positioners do the same thing using electrical signals.

Motion-control applications using pneumatic rotary actuators generally fall into the rack-and-pinion or vane styles. Often, double-acting rack-and-pinion arrangements are used. Multi-position actuators, with three, four, or five stops, are often used for sequential assembly operations. Rotary actuators may also be used for indexing, stepping, and pick-and-place motions.

Pneumatic linear actuators
Pneumatic linear actuators are used on rising-stem valves to directly operate the gate, globe, etc. Two types are normally used, the diaphragm and the piston. Diaphragm styles are popular as their wide surface areas can produce tremendous force with moderate air pressure. The diaphragm is a rubber membrane whose rim is sealed to the outer casing of the actuator. Air pressure displaces the diaphragm up or down against spring pressure depending on whether the actuator is designed to fail open or fail closed. Stroke lengths are generally shorter than piston valves where strokes are dependent only on the length of the cylinder, not the amount of stretch the diaphragm can tolerate. Piston style actuators can be sized to produce an appropriate actuating force based on the pressure of available air and can be manufactured in double-acting and spring-return styles. Some linear actuators use the familiar air springs in place of diaphragms.

As globe valves are common for control applications, modulating linear actuators are available. Pneumatic modulating valves are particularly effective because their speed is adjustable by throttling the airflow. Thus, a pneumatic actuator can speed quickly toward a setpoint but slow as it reaches it, eliminating overshoot. Unlike electric actuators, pneumatic actuators can run continuously.

Combination rotary/linear actuators
The ability to provide both linear travel and limited rotation is handy in certain applications such as work holding. Clamps can be raised and swung clear of a workpiece to allow its removal and then pressurized and reengaged once a new workpiece is positioned.

Applications
Air-powered actuators are a proven method of obtaining rotary and linear motion. For valve actuation, both on/off and control, air provides a reliable, safe, economical method. Discounting the cost of furnishing compressed air itself, air actuators are usually more cost-effective than electric actuators up to a certain diameter valve. As valves become larger, the economics begin tilting more in favor of electric actuators.

Electrical valve actuators are being employed with greater frequency. Still, pneumatic actuators have their proponents due to their simplicity, ruggedness, and ease with which they are made fail-safe. They can move faster than electric actuators and are not subject to duty cycles. Efforts to make traditional pneumatic and hydraulic actuators more compatible with distributed control systems are ongoing.

Rotary actuators are used in many motion-control systems to operate, for instance, pick-and-place handlers or clamps. Linear air cylinders have long been a staple of automation and continue to be so despite the influx of sophisticated stepper- and servo-motor driven electrical actuators. While their primary application is in point-to-point moves, cylinders equipped with feedback are enabling servo-pneumatic actuators to handle positioning applications that are too fine for ordinary cylinders yet too coarse for electrical actuators. Such devices can provide high forces in small packages and can operate continuously without heat buildup. Part of their development has been in figuring out control algorithms that can account for the compressibility of air, which has always made air cylinders better for point-to-point motions.

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