A pin on the piston rod slides in a slot in a yoke keyed to the output shaft. The geometry gives high torque at both ends of the stroke and less in the middle, which matches quarter-turn valves that need breakaway torque to unseat and torque to reseat. Rack and pinion actuators, the other common type, give nearly constant torque and are cheaper in small sizes.
Scotch yoke on that 16 inch ball valve gives us plenty of breakaway, the old rack and pinion kept stalling at the seat.
Also heard as
- scotch yoke actuator
- SY actuator
Related terms
Actuator
The powered device that opens, closes or positions a valve using air, hydraulic oil or an electric motor.
Double-acting
Describes an actuator that uses air or oil pressure to drive in both directions, with no return spring.
Spring-return
Describes a single-acting actuator that uses air to move one way and a spring to move back and fail safe.
Ball valve
A quarter-turn valve that opens and closes by rotating a ball with a bore through it.