Rotary-actuator; pneumatic device for providing rotary motion to the tooling.

Rotary-actuator; in the context of this web site, though there are rotary actuators powered by other than compressed air, we are talking specifically about pneumatic variety.

The purpose of a rotary-actuator is to provide the user the ability to move the machine tooling in a rotary motion, instead of the linear motion that is provided by typical air cylinders.

With a combination of rotary-actuators and linear actuators then, the machine builder can provide tooling movement similar to that of a human arm, with rotary devices providing the wrist action, grippers to simulate fingers, and linear actuators to provide the reach.

There are two methods of providing rotary motion with a pneumatic rotary-actuator.

One style uses internal vane or vanes, the other provides rotary motion through a rack and pinion linkage.

Both styles of devices will rotate a shaft that protrudes from inside the actuator and to which the arm and end-of-arm tooling is attached. As the shaft rotates, so too will the arm and the tooling.

The vane style uses similar technology and concepts to an air tool or air motor. Compressed air flows into one of two air ports in the actuator’s vane housing via an air line from the control valve. As the compressed air moves into the vane actuator, it tries to flow from high pressure towards atmosphere – and in so doing it rotates the vanes inside the housing and imparts rotary motion to the shaft.







Vane type rotary actuators are usually double acting . Compressed air flows alternately to one of two air ports in the housing, providing rotary motion in one direction when the control valve is shifted one way, and the other when the valve is shifted back and air flows into the other port.

Of significant concern about vane and other types of rotary actuators is the calculation of the moment of intertia for the load in order to select an actuator with sufficient force. This involves calculating the continuous distribution of the mass of the load at a continally varying distance from the axis of the actuator shaft.

For example, a 1 Kg load placed axially to the shaft will require a certain amount of force to rotate it. That same load, placed tangentially from the center of the shaft, will require a much greater force to move and to stop it.







When it comes to determining how big a rotary actuator you need for the application, get your staff engineer involved, or use the charts and guidance available from the manufacturer of the rotary actuator. The mathematic calculations required to determine these details is well beyond this author's capability.

It’s really easy to make a mistake in calculations which will result - at best - in a rotary actuator that won't rotate, and at worst, damage to people or equipment. Be careful!

Vane type rotary actuators are not normally used for greater than 270 degrees of rotation. It’s difficult to manufacture them to provide continuous, unidirectional motion. The vanes themselves provide an internal impediment to providing a full 360 degrees of rotation. Other types of rotary actuators can provide more than 360 degrees of rotation if your application requires it.

There will be applications where a vane type actuator cannot impart sufficient torque or degrees of rotation.

When this occurs, your next available option is to use a rack-and-pinion style of rotary actuator.

Picture a standard double acting air cylinder. Now, remove the piston rod and add another piston inside the cylinder barrel. Between the two pistons attach a rack, and have that rack intersect with a pinion located at the center of the cylinder. As compressed air alernately flows into one air port and then the other, the rack moves back and forth.







The teeth on the rack mesh with the teeth on the pinion gear, imparting a rotational movement to the pinion as the rack moves back and forth. The pinion gear is connected to a shaft that protrudes out the side of the cylinder and voila... you have a rack-and-pinion rotary actuator.

Depending on how long the cylinder barrel and the corresponding rack is, this type of rotary actuator can impart rotation to the shaft, arm and tooling from 0 degrees up to whatever may be required for the application.

When determining what rotary actuator to use for your application, be aware of backlash and how this will affect the operation of your system.

Here are the things you’ll need to know to select and use your rotary actuator: