The scotch-yoke actuator uses a piston, which moves a linear push rod that in turn engages a pivoting lever arm to provide rotation. A rack & pinion actuator uses opposing pistons with gears to engage a pinion gear shaft. The practical difference however is in size, power, torque curve and ease of adding peripherals.
|Rack & Pinion|
Rack & pinion actuators tend to be more compact, have standardized mounting patterns and have an output torque range suitable for small to medium sized valves. They include standard bolting and coupling patterns to directly attach to a valve, solenoid, limit switch or positioner.
Scotch-yoke actuators, for the most part, are much larger than rack & pinion actuators and consequently provide much higher torque output. These actuators are most often used on large quarter turn valves and require careful attention to the mounting requirements because of their power.
Both styles come in either "direct acting" or "spring return" versions. Direct acting actuators use the air supply to move the actuator in both directs (open and close). Spring return actuators, as the name implies, uses springs to move the actuator back to its "resting" state.