Pneumatic Rack-and-Pinion Rotary Actuators Rack-and-pinion pneumatic actuators, also called limited rotation cylinders, are rotary actuators used for turning, opening, closing, mixing, oscillating, positioning, steering and many more mechanical functions involving restricted rotation. These actuators are also often used for automation of quarter-turn valves, like ball or butterfly valves. Pneumatic rack-and-pinion actuators convert the energy of compressed air by means of a pneumatic cylinder to an oscillating rotary motion. The clean, dry, and processed gas required by this actuator is provided via a central compressed air station, which usually supports a range of pneumatic devices in a process system. Pneumatic actuators, in comparison to their electric counter parts, are generally more durable, better suited for hazardous environments and less expensive. In addition, they often require less maintenance and provide higher torque in comparison to their size. Function Rack-and-pinion pneumatic actuators can be either single-acting or double-acting. It is also possible for these actuators to provide multiple stops. Single acting vs. double acting In a single-acting actuator, air is only supplied to one side of the piston and is responsible for the movement of the piston in only one direction. The movement of the piston in the opposite direction is performed by a mechanical spring. Single-acting actuators conserve compressed air, but perform work in only one direction. A downside of single-acting cylinders is the inconsistent output force through a full stroke due to the opposing spring force. In a double-acting actuator, air is supplied to chambers on both sides of the pistons. Higher air pressure on one side can drive the pistons to the other side. Double-acting actuators are used when work needs to be performed in both directions. One of the advantages of double-acting cylinders is the constant output force through a full rotation range. The drawbacks of double-acting cylinders are their need for compressed air for movement in both directions and a lack of a defined position in case of a power or pressure failure.