The maximum rotation of a pneumatic rotary actuator is fixed, says Bill Carlson, vice president of R&I Manufacturing Co. (Thomaston, CT). Most standard models only turn 180 degrees or less, but some models can turn as much as 450 degrees. With some models, the user can adjust the rotation within the maximum limit. Similarly, most standard pneumatic rotaries have only two stopping points, though some have as many as five.

There are two main types of pneumatic rotary actuator: vane and rack-and-pinion. A vane rotary is driven by a simple air motor. "A vane rotary is less expensive than a rack-and-pinion, and it usually needs less maintenance," says Scott Ames, applications engineer at Robohand Inc. (Monroe, CT). "But, it’s not as accurate as a rack-and-pinion. There’s more backlash and a little more friction. If you just want to operate a valve or shift a chute, a vane rotary is fine."

For more precise applications, a rack-and-pinion rotary is necessary. This actuator converts linear motion from a cylinder into rotary motion. The rack is a straight set of gear teeth attached to the cylinder’s piston. The rack is pushed linearly as the piston moves. The rack’s teeth mesh with the circular gear teeth of the pinion, forcing it to rotate. The pinion rotates back to its original position when air pressure is supplied to the opposite side of the cylinder.

The maximum rotation of a rack-and-pinion rotary is limited by its size. "You could make a rack-and-pinion that goes 720 degrees, but you’d have to make the rack really long," says Ames, "because the rack has to keep traveling linearly to keep rotating the pinion."

When specifying a rotary actuator, begin with the payload and its location in relation to the axis of rotation. The latter is particularly important. Though rotaries can operate at hundreds of cycles per minute, the angular momentum of most loads will limit the usable rate. The forces acting on a horizontal rotary with a 1-pound load at its center will be much different than those on a vertical rotary with the same load suspended 10 inches off to one side.

For optimal performance, the load should be kept as light and as close to the axis of rotation as possible. "Rotating the load is not difficult; stopping it is," Ames explains. "If you can decrease the length of the arm hanging off the actuator from 12 to 10 inches, that makes a big difference."

Engineers should also consider the amount of rotation, the cycle time and the dwell time. With that, they can calculate rotation speed and the torque load. For safety, many engineers boost the latter figure by 40 percent to account for load variations, pressure fluctuations and friction. Especially with high cycle rates, it’s wise to limit impact velocity by placing a flow control in the exhausting cylinder, decreasing system pressure, or installing a shock absorber. Impacts that cause vibration can damage the actuator.