Pneumatic Actuator Selection Guide: Working Principle, Components, and Torque Calculation

Introduction

In modern industrial automation control systems, pneumatic actuators are very common driving devices. Whether in chemical plants, food processing lines, or water treatment equipment, they play a crucial role. This article will help you fully understand the working principle of pneumatic actuators, main components, and how to quickly and accurately select the right pneumatic actuator according to your requirements.

Contents

1. Introduction to Pneumatic Actuators
2. Working Principle of Pneumatic Actuators (Single Acting VS Double Acting)
3. Main Components of Pneumatic Actuators
4. Four-Step Pneumatic Actuator Selection Method + Example
5. FAQ

Keywords: Pneumatic Actuator, Double Acting Pneumatic Actuator, Spring Return Pneumatic Actuator, Working Principle, Actuator Torque

You can watch the following videos for a quick understanding:

1. Introduction to Pneumatic Actuators

A Pneumatic Actuator is an automated actuator device powered by compressed air, converting pneumatic energy into mechanical motion. It is mainly used to drive industrial valves for opening, closing, or flow regulation. Pneumatic actuators are widely used in automated valve equipment such as ball valves, butterfly valves, and control valves. Compared with electric actuators, pneumatic actuators feature fast response speed, simple structure, and high safety, making them widely used in industries such as petroleum, chemical processing, water treatment, and power generation.

2. Working Principle of Pneumatic Actuators (Single Acting VS Double Acting)

Pneumatic actuators operate by introducing compressed air into the cylinder, pushing pistons or gear mechanisms to move, thereby driving the valve stem to rotate or move linearly to achieve valve opening and closing. Pneumatic actuators are mainly divided into single-acting (spring return) and double-acting types.

• Double Acting: Air to open, air to close. Compressed air enters from one side to push the piston and open the valve; air entering from the opposite side pushes the piston back to close the valve.

• Single Acting: This type provides an additional safety function. It uses air pressure to open, while spring force returns the valve to its original position. This means that in the event of an air supply failure, the valve can automatically return to a safe position (fully open or fully closed).

Feature	Double Acting (ATD)	Single Acting (ATS)
Power Source	Dual Air Pressure: Both opening and closing actions are driven by air supply.	Air + Spring: Air pressure drives one direction, while built-in spring groups provide the return force.
Internal Design	Rack and pinion with dual pistons; no springs included.	Rack and pinion with modular pre-loaded safety spring cartridges.
Fail-Safe Mode	Fail-Last-Position: Valve stays in its current position if air supply is lost.	Fail-Safe: Automatically returns to a preset “Safe Position” (Open or Closed) via spring force upon loss of air.
Torque Output	Constant: Provides stable and linear torque output throughout the entire stroke.	Variable: Torque changes during the stroke; air torque decreases as springs are compressed.
Safety Factor	Recommended safety factor: 20% – 30%.	Recommended safety factor: 30% – 50%.
Interchangeability	Same body and end caps as ATS; can be converted by adding springs.	Same body and end caps as ATD; can be converted by removing springs.
Size & Weight	More compact and lightweight.	Larger and heavier due to the space required for spring sets.

3. Main Components of Pneumatic Actuators

A complete pneumatic actuator usually consists of the following core components:

Cylinder Body (25)

The cylinder is the main body of the actuator, used to contain compressed air and drive piston movement.

Common Materials:

• Aluminum Alloy

• Stainless Steel

Piston (8)

The piston reciprocates under air pressure and transfers power to the output shaft.

Output Shaft (22)

A key component connected to the valve, responsible for transmitting actuator power to the valve to achieve valve opening and closing.

End Cap (8)

Seals the air chamber and protects internal components.

Spring Assembly (5)

Mainly used in single-acting actuators to achieve automatic reset during air supply failure.

Seals (7)

Used to ensure the airtightness of the actuator and prevent air leakage.

4. Four-Step Pneumatic Actuator Selection Method + Example

Incorrect actuator selection may result in the valve failing to operate, damage to the actuator, or even accidents. Please refer to the following “Four-Step Selection Method”:

Step 1: Confirm the Required Valve Torque

This is the core step. You need to know the operating torque of the ball valve or butterfly valve under maximum working pressure. It is recommended to add a 20%-30% safety margin during calculation.

Step 2: Determine the Air Supply Pressure

Factory compressed air pressure is usually between 4 bar and 8 bar. The output torque shown in the selection table changes with air supply pressure, so accurate matching is essential.

Step 3: Select the Operating Mode

• Pursuing cost-effectiveness and stable air supply system — choose Double Acting.
• Considering safety protection during air failure (fail open or fail close) — choose Single Acting.

Step 4: Verify Installation Dimensions

Check whether the valve mounting hole pattern complies with the ISO 5211 standard and whether the shaft diameter matches. If not, customized brackets and connecting shafts may be required.

Example

A butterfly valve requires 80 N.m torque including safety margin, with an operating air pressure of 5.5 Bar.

Single Acting Actuator Selection: The torque in the opening state must be > 80 N.m, i.e., Open 0° > 80 N.m.

At the same time, the spring return seat torque must also be > 80 N.m, i.e., Open 0° > 80 N.m.

According to the selection table, select AT-115SK10 at 5.5 Bar.

Both 9 and 10 spring configurations can meet the requirement, but 10 springs are selected because even numbers are standard.

Double Acting Actuator Selection: According to the selection table, select AT-88D at 5.5 Bar, as long as the torque is > 80 N.m.

5. FAQ

Q1: What is the difference between pneumatic actuators and electric actuators?

Pneumatic actuators are driven by compressed air, featuring fast action speed and high safety; electric actuators are driven by motors, offering higher control accuracy but usually at a higher cost.

Q2: How to choose between single-acting and double-acting pneumatic actuators?

If automatic reset during air failure is required, it is recommended to choose a single-acting actuator; if larger torque and frequent operation are required, a double-acting actuator is more suitable.

Q3: Why should safety torque margin be reserved for pneumatic actuators?

Due to pipeline pressure changes, media viscosity variations, and long-term wear, the actual operating torque may increase. Therefore, a safety factor should be reserved.

Q4: What should I do if the pneumatic actuator leaks air?

A: In most cases, the O-ring is aged or the inner wall of the cylinder is worn. It is recommended to regularly check the air source filter to ensure the air entering the actuator is dry and free of impurities.

Q5: Why are single-acting actuators more expensive than double-acting actuators?

A: Because single-acting actuators contain a large number of high-quality springs internally, and the cylinder size usually needs to be larger to offset spring resistance.