The purpose of a rotary-actuator is to move something, typically the machine tooling, in a rotary motion. Linear motion is provided by typical air cylinders, yet sometimes there is the need to rotate an item with compressed air.
Using a combination of rotary-actuators and linear actuators 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.
One style uses internal vane or vanes, the other provides rotary motion through a rack and pinion linkage.
Both styles of rotary actuators will rotate a shaft that protrudes from inside the actuator, and to which the actuator arm and the end-of-arm tooling is attached. As the shaft rotates, so too will the arm and the tooling.
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 rotary motion in the other direction when the valve is shifted back and air flows into the other port.
It is important to calculate the moment of inertia for the load the rotary actuator is intended to move, in order to select an actuator with sufficient force. This involves calculating the distribution of the mass of the load as it moves at a continually 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, I recommend that you get your staff engineer involved, or use the charts and guidance that is available from the actual manufacturer of the rotary actuator. Much support material for sizing actuators can now be found on line, from the manufacturers website.
It is easy to make a mistake in calculations which will result - at best - in a rotary actuator that will not rotate, and at worst, damage to people or equipment. So please be careful!
Other types of rotary actuators can provide more than 360 degrees of rotation if your application requires it.
There will also be applications where a vane type actuator cannot, due to size limitations, be strong enough to provide sufficient torque or degrees of rotation. When this occurs, your next available option is to use a rack-and-pinion style of rotary actuator.
The teeth on the rack mesh with the teeth on the pinion gear, imparting a rotational movement to the pinion as the rack moves. 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. The longer the cylinder barrel, the more the rotation of the shaft.