Thursday, May 11, 2017

Rack and Pinion Actuator - Double Acting vs. Single Acting

pneumatic valve actuator
Pneumatic rack and pinion valve actuator
Courtesy Emerson - Hytork
Automating industrial valve operation requires numerous considerations in selecting the correct power source, drive type, torque range, and much more. The widest range of possible operation conditions should be anticipated and accommodated by the actuator selection to assure safe and effective valve operation under normal and adverse conditions.

The use of compressed air or gas as the energy source for valve positioning has been in use for many years and remains popular to this day. Among the perceived advantages of this energy source are the ability to store it in pressurized vessels for emergency short term use and the absence of any potential ignition source, as may be the case with electric powered actuators.

A rack and pinion valve actuator delivers a linear torque output throughout its full range of travel. The movement of a piston causes movement of the rack. The rack is toothed, and drives the pinion, converting linear movement of the rack into rotational movement of the pinion. The pinion is connected to the valve shaft, providing re-positioning of the valve. Adjustable stops, part of the actuator, limit the travel of the valve trim.

spring return and double acting valve actuator diagrams
Double acting pneumatic rack and pinion actuator (left) on its inward stroke. Spring return actuator (right) on its
outward or air powered stroke  (Illustrations courtesy of Emerson - Hytork) 

There are two common configurations of rack and pneumatic pinion actuators. A double acting actuator has provisions for delivering or exhausting air from both sides of the piston. Small control valves coordinate the delivery and removal of pressurized air or gas to drive the pistons inward or outward, producing torque in a clockwise or counterclockwise direction. Its operation could also be described as "air to open, air to close".

The single acting version of the pneumatic rack and pinion actuator provides air driven movement in only one direction. In this case, reversing the direction of travel is accomplished with a spring installed within the chamber on one side of the pistons. The spring powered movement provides a fail safe positioning of the valve in the case of control air pressure loss. This actuator provides "air to open, spring to close" operation, although, in some cases the fail safe position can be changed.

This is the simple version. Share your process control challenges with a valve expert, combining your own process knowledge and experience with their product application expertise to develop effective solutions.