Electric Butterfly Valve

Pneumatic valves use compressed air flowing into and exhausting from an actuator to open and close the valve. The type of pneumatic actuator required depends on the actuator valve. Valves requiring linear motion (gate, globe, or diaphragm valves) typically require a pneumatic diaphragm actuator. Quarter-turn valves (ball and butterfly valves) typically require either a rack-and-pinion air actuator or a scotch yoke air actuator. We’ll discuss the two types of quarter-turn air actuators in greater detail.

Scotch yoke actuators are composed of a piston directly connected to a slotted yoke. A pin on the rotating shaft rides in the yoke slot, converting the linear motion to rotational motion in much the same way a crankshaft converts the linear motion of a piston connecting rod. Scotch yoke actuators have a nonlinear torque profile, with the highest torque at the beginning and end of the stroke. Scotch yoke construction is readily scalable, making this type of actuator suitable for large valves requiring high torque.

Rack-and-pinion actuators use a piston to drive a toothed shaft (the rack) which is geared to the actuator output drive (the pinion). They are typically used in lower torque applications, offer more precise control, and provide even torque throughout the stroke.