Among the common types of pneumatic actuators, the scotch-yoke design holds an indispensable position in large-diameter valves and high differential-pressure applications because of its distinctive torque output.

How does it differ from the widely used rack-and-pinion design, such as the AT series? Why is it said to deliver “the most force at the critical moment”? This article provides a full introduction to scotch-yoke pneumatic actuators.

1. What Is a Scotch-Yoke Pneumatic Actuator?

A scotch-yoke pneumatic actuator converts the piston's linear motion into rotary motion through a crank-and-slider mechanism. It is suitable for on/off isolation or modulating control of 90-degree valves, including ball valves, butterfly valves and plug valves.

In modern industrial automation, scotch-yoke actuators are widely used in petrochemicals, environmental protection, water supply and drainage, power generation and other industries. They are particularly suited to large valve systems requiring precise control.

2. Main Components

A scotch-yoke pneumatic actuator consists mainly of the following key components:

  1. Housing (cylinder body): Usually made from aluminium alloy or steel with an anti-corrosion surface treatment, it offers good strength and corrosion resistance. Threaded connections or flanges on the housing connect it to the valve.
  2. Scotch-yoke mechanism (core component): This is the main feature that distinguishes this actuator from other types. It converts linear piston motion into rotation of the output shaft. As air pressure changes, the yoke moves inside the actuator, converting pneumatic energy into mechanical energy.
  3. Cylinder: The actuator's source of driving force. It is usually cylindrical and contains a piston. Air ports at both ends connect to the air supply.
  4. Piston: Moves axially inside the cylinder in response to changes in air pressure. As pressure increases, the piston pushes the scotch-yoke mechanism and operates the valve. Depending on the design, actuators can use a single-cylinder or twin-cylinder piston arrangement.
  5. Spring-return unit: Single-acting scotch-yoke actuators have a spring-return unit to return automatically when the air supply fails. The spring provides return force according to the piston position, delivering fail-safe protection.
  6. Transmission components: Connected to the yoke, these transfer actuator motion to the valve and typically connect to components such as the stem, gears and couplings.
  7. Valve connection: Flanges, bolts and other connectors link the actuator to the valve's operating components and transmit rotary motion.

3. Operating Principle

Scotch-yoke pneumatic actuators operate through the combined action of an air pressure differential and a crank-and-slider mechanism:

When compressed air enters one end of the cylinder, pressure moves the piston linearly inside it. Through the yoke, or crank-and-slider mechanism, the piston converts linear motion into output-shaft rotation, usually 90 degrees, to open or close a ball valve or butterfly valve.

Single-acting version: Air power opens the valve, and spring return closes it.

Double-acting version: Air power drives both opening and closing, with a directional valve switching the air supply direction.

4. The Main Difference Between Scotch-Yoke and Rack-and-Pinion Designs: The Torque Curve

This is the most distinctive feature of the scotch-yoke design.

A rack-and-pinion actuator provides relatively steady torque throughout its rotation, with little variation.

A scotch-yoke actuator's torque varies with the rotation angle: it is lower at mid-stroke and reaches its maximum at both ends of travel, when the valve starts opening or finishes closing.

This characteristic closely matches the torque requirements of ball and butterfly valves: they need the greatest torque at starting and closing, and less torque in the middle. Scotch-yoke actuators can therefore match the valve load more efficiently and provide better value.

Zhejiang HES Automatic Instrument & Valve Co., Ltd.

Founded in 1989 · Over thirty years focused on pneumatic actuators