Scotch-Yoke Piston Actuators

Assured Automation’s P Series pneumatic scotch-yoke actuators are used on quarter turn (90°) ball valves, plug valves, dampers and resilient seated butterfly valves. The features of these pneumatic scotch-yoke actuators include:

• High duty cycle life, up to 1,000,000 cycles
• Double acting and spring return models
• DIN/ISO 5211, DIN 3337 (F03 to F16)
• NAMUR solenoid and limit switch mounting
• Diamond and square output drive on all units
• Permanent lubrication
• Corrosion resistant coating
• Visual position indicator (Open/Closed)

Scotch-Yoke Piston Actuator Primer

Scotch-Yoke Piston Actuators are classified as linear-to-rotary actuators. A rotary actuator may be defined as a mechanism that converts linear force into quarter-turn torque and is suitable for most valves but is most effective on those of larger sizes.

Valve Forces

The product of a rotary actuator is torque, and torque is defined by the formula:

Torque = force x distance
Where:  force=piston force, defined as (p x A)     p=air pressure     A=piston area
distance =The distance from the center of rotation to where the force is applied (moment arm) making Torque=p x A x distance

Force is given in pounds and distance in inches, the product, torque, is given in inch-pounds. This may be converted to foot-pounds by dividing by 12.

To increase the amount of torque produced by a rotary actuator it is necessary to increase the air pressure (p) to the cylinder, increase the piston area (A), and/or increase the moment arm dimension (d). For this reason, a typical torque chart for a pneumatic actuator will show increasing torque values as air pressure increases.

Current pneumatic rotary actuator designs include scotch-yoke, vane, and rack and pinion.

Scotch-Yoke Piston Actuator Mechanics

The typical scotch yoke mechanism consists of a piston, connecting shaft and a rotary pin.

When air pressure is applied to the piston, a linear force is generated (pxA). The force is applied to the yoke, through the power transfer pin, at a distance (d) from the center of the output shaft. As the yoke moves in a rotary fashion, torque is generated at the output shaft to provide the turning force required to cycle a valve.

The amount of torque a scotch-yoke actuator will produce is based on all of the factors of the torque formula (T=p x A x d), but for a given piston size and air pressure it is the length of the yoke (d) that is the main determining factor.

To measure the output torque of a scotch-yoke actuator it is necessary, with p and A constant, to measure the distance (d) from the point of applied power to the center of rotation at all points in the piston stroke.

Torque Greatest at Beginning and End of Piston Stroke

At both extremes of travel (open and close), d is greatest. At the midpoint of travel, d is at its shortest dimension. Applying these various d dimensions to the torque formula produces the following output torque curve over 90 degrees of rotation.

This is the characteristic output torque curve of a scotch-yoke actuator as it strokes from open to close (0 to 90). A torque chart for a double-acting scotch-yoke actuator will give two values for sizing: breakaway, the highest value; and running, the lowest.

To generate more torque from a scotch-yoke actuator, p A and/or d must be increased. For a given air pressure (p) and piston area (A), the yoke dimension (d) must be increased.

When designing such a torque enhancement, it is necessary to consider that as dimension d increases so does the piston stroking distance and therefore the overall length of the actuator.

Power Transfer Pin

One other design factor to consider with scotch-yoke actuators is the power transfer pin, or roller. Made of round stock, the pin applies piston force to the inside wall of the yoke with a single line contact.

This is an extremely high stress area and, as the unit operates for an extended period, is where most of the lost motion expected in the design occurs. The scotch-yoke actuator is available in a variety of sizes, materials and configurations.

Assured Automation’s P Series pneumatic scotch-yoke actuators offer a simple and cost effective solution for your valve automation needs. For use on quarter turn (90°) ball valves, plug valves, dampers and resilient seated butterfly valves.

For assistance in sizing and specifying valve actuators call one of Assured Automation’s highly trained sales associates at 1-800-899-0553 or email us at info@assuredautomation.com