The extended size range of today’s pneumatic rack-and-pinion valve actuators that can now automate up to 32-inch butterfly valves, and up to 14-inch ball valves, offers an advantageous alternative to the large-size, scotch-yoke actuator mechanism that has been popular in industry for quite some time.
Advantages of the rack-and-pinion actuator compared to the scotch mechanism include stability, accuracy and a smaller footprint, as detailed below.
Output torque of a rack-and-pinion actuator is more stable. During operation, of the two types of rack and pinions, the torque for a double-acting, or DA, mechanism is constant at any rotation angle, while torque for the spring-return, or SR, version is basically linear.
On the other hand, scotch-yoke torque output changes with rotation angle and is neither linear nor stable.
Besides having a long cycle life, the rack-and-pinion design delivers more accurate control at any specified degree. Because of the multiple gear sets in a rack-and-pinion actuator, force is effectively transferred from linear to rotary. With the one-set structure of the scotch-yoke design, mechanical wearing will be evidenced on the connection surface after several thousand cycles at high torques, which in turn can cause output-torque and rotation-angle issues.
Comparing unit size differences, the balanced design of the rack-and-pinion actuator represents a smaller package than the scotch-yoke design, which hangs out on one side of the valve, and typically needs to be supported with additional hardware and related cost as the size and weight of the actuator increases.
Bob Donnelly is Vice President of Marketing for Flo-Tite Valves and Controls. He is also available to answer questions on applications for quarter-turn ball and butterfly valves, as well as valve automation issues under Processing’s new Ask the Valve Expert column.