In industrial plants, pneumatic actuators open and close valves thousands or even tens of thousands of times every day. They have no motor and are not electrically driven, so what makes them move?

The answer is compressed air.

1. The principle in one sentence

The operating principle of a pneumatic actuator can be summarized as a simple sequence:

Compressed air enters the cylinder → air pushes a piston or diaphragm to generate thrust → a mechanical mechanism converts this thrust into linear or rotary motion → the motion is transmitted to the valve stem → the valve opens or closes.

The entire process converts pneumatic pressure energy into mechanical energy, with energy conversion efficiency reaching more than 70%.

2. The operating process in three steps

  • Step 1: Supply pneumatic energy

Compressed air, typically at 0.4-0.7MPa, first passes through a pneumatic three-piece service unit: a filter to remove impurities and moisture, a regulator to stabilize pressure, and a lubricator to add lubricating oil to the air. After this treatment, a solenoid valve controls its entry into the cylinder.

According to Pascal's law (P=F/A), air pressure acts on the effective area of the piston or diaphragm to produce axial thrust.

For example, air at 0.6MPa acting on a piston 80mm in diameter produces approximately 3kN of theoretical thrust, enough to drive a large-diameter valve.

  • Step 2: Convert mechanical motion

Different actuator types use different conversion mechanisms:

Piston type: compressed air pushes a piston, usually made of aluminum alloy or stainless steel, in a straight line inside the cylinder. Sealing rings on the piston, commonly made of polytetrafluoroethylene, prevent air leakage. The hard-chrome-plated piston rod transmits linear displacement to the valve, with stroke accuracy controllable within ±0.5mm.

Diaphragm type: a rubber-fabric composite diaphragm deforms under air pressure and moves a pushrod. This design has a relatively short stroke, generally less than 100mm, but responds quickly, in less than 0.5 seconds, making it suitable for modulating service requiring a fast response.

Rack-and-pinion type: this is the mainstream design used in HES AT (DRSC) series actuators. Compressed air pushes two pistons outward in a straight line. The piston racks rotate the central pinion shaft, converting linear motion into 90-degree rotation to open or close a ball or butterfly valve.

  • Step 3: Control the motion

Single-acting type (spring-return): air entering one side pushes the piston to perform work, while a return spring is installed on the other side. When the air supply is interrupted or the signal is lost, the spring automatically pushes the piston back to its starting position for fail-safe protection. The design is simple, but the spring affects output force, making the two directions asymmetric.

Double-acting type: compressed air can enter either side of the piston. A directional valve switches the air supply direction to move the piston both ways. Balanced thrust and precise positioning make this type suitable for applications with high control accuracy requirements.

3. A brief guide to thrust calculation

Calculating actuator output force is an essential step in practical sizing:

F = P x A x η

F: output force (N); P: operating pressure (Pa); A: effective piston area (m²); η: mechanical efficiency, typically 85%-95%.

This formula provides an initial indication of whether an actuator size is suitable for the available air supply and the valve's torque requirements.

4. Additional notes: efficiency characteristics of different types

Different actuator types have distinct energy conversion characteristics:

Rack-and-pinion: compact, efficient and long-lasting; the most widely used actuator type.

Scotch-yoke: output torque varies with rotation angle and is highest at the end of the stroke, making it suitable for large-diameter ball valves.

Diaphragm: fast response, suitable for modulating service, but with a short stroke.

Piston: high thrust and a long stroke, suitable for linear-motion valves such as gate and globe valves.

Although pneumatic actuator operation appears simple, it depends on close coordination between pneumatic transmission, mechanical conversion, sealing technology and other elements. Understanding these basics supports more confident equipment selection, installation, commissioning and routine maintenance.

Zhejiang HES Automatic Instrument & Valve Co., Ltd.

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