A pneumatic actuator can sound like a black box: compressed air goes in, and the valve moves. But what is actually inside it, and what does each component do?

This article opens up the pneumatic actuator for a closer look.

1. The basic principle in one sentence

The operating principle can be understood as a simple chain:

Compressed air enters the actuator → air pushes a piston or diaphragm → the piston or diaphragm moves the valve stem → the stem operates the valve, opening or closing it.

The core of the process is converting the pressure energy of air into mechanical motion.

2. The 8 main components of a pneumatic actuator

Regardless of size or model, a complete pneumatic actuator typically includes the following core components:

  1. Housing (cylinder body): the actuator's enclosure and pressure-containing component. Usually made of high-strength aluminum alloy, stainless steel or carbon steel with an anticorrosion surface treatment. Its interior is precision-machined so the piston or diaphragm moves smoothly while the housing withstands compressed-air pressure.
  2. Piston: the primary force-producing component in linear-motion, or reciprocating, pneumatic actuators. Compressed air entering the cylinder pushes the piston in a straight line. Sealing rings, commonly made of polytetrafluoroethylene or nitrile rubber, prevent air from leaking between the two sides of the piston.
  3. Diaphragm: in a diaphragm pneumatic actuator, the diaphragm performs the piston's function. Usually made of a rubber-fabric composite, it deforms under air pressure and moves a pushrod. This design has a shorter stroke but fast response and good sealing, making it especially suitable for modulating service.
  4. Spring (return spring): in a single-acting, spring-return pneumatic actuator, the spring provides the safety function. During normal air supply, air pushes the piston and compresses the spring. If the air supply is interrupted or the signal is lost, spring force returns the piston to its starting position, automatically returning the valve to its preset safe state in an air-to-open or air-to-close configuration.
  5. Valve stem (output shaft): the key transmission component connecting the actuator and valve. It transmits piston or diaphragm movement to the valve's closure element to open or close the valve. The stem surface is usually hard-chrome-plated to improve wear resistance and service life.
  6. Control ports (air inlet/outlet): the passages through which compressed air enters and leaves the actuator. They typically include inlet and outlet ports, A and B. A solenoid valve controls the direction of airflow to open and close the valve.
  7. Valve positioner (optional accessory): the actuator's brain. It receives a standard signal from the control system, such as 4-20mA or 0-10V, and adjusts air pressure to control valve opening precisely, providing linear adjustment from 0-100%. Actuators fitted with a valve positioner can achieve control accuracy within ±0.5%.
  8. Other accessories: limit switches detect open/closed valve status and provide feedback to the control system; solenoid valves switch pneumatic circuits for remote control; filter regulators (pneumatic three-piece service units) clean compressed air and stabilize pressure; handwheel mechanisms allow manual operation when power and air are unavailable.

3. How the two transmission designs work

Linear actuator: compressed air enters the cylinder through a control port and moves the piston in a straight line. The piston rod connects directly to the valve stem, driving linear valve motion, as in gate and globe valves. Linear actuators offer a simple structure and high thrust for valves that require linear displacement.

Quarter-turn actuator: this is the mainstream design used in HES AT series actuators. Compressed air pushes two pistons outward in a straight line. Their racks rotate the central pinion shaft, converting linear movement into 90-degree rotation to open or close a ball or butterfly valve. The design is compact, efficient and long-lasting.

4. Key selection tips

Understanding internal actuator components helps make more accurate choices during routine sizing:

Check sealing performance: seal material directly affects actuator life and reliability. High-temperature or corrosive-media applications require special seal materials.

Check stroke and torque: diaphragm, piston and rack-and-pinion actuators have different output characteristics and must be matched to the valve type and operating conditions.

Check accessories: selecting the valve positioner, solenoid valve, limit switches and other accessories is equally important, as these directly affect control accuracy and the degree of automation.

A pneumatic actuator may look complicated, but its components are easy to understand when examined individually. Knowing each component's role helps you select equipment more appropriately and locate faults more quickly.

Zhejiang HES Automatic Instrument & Valve Co., Ltd.

Established in 1989 · Focused on pneumatic actuators for more than three decades