Of all the ways to classify pneumatic actuators, the most important and fundamental is by action type: single-acting or double-acting. What is the difference? How does each work? When should you choose one or the other? This article explains them together.
1. What Is a Single-Acting Pneumatic Actuator?
A single-acting pneumatic actuator is also called a spring-return actuator. It contains a set of return springs, and compressed air enters the cylinder from only one side to move the piston in one direction.
Operating principle: When compressed air enters through the inlet, it pushes the piston and compresses the springs. The rack on the piston rotates the pinion shaft, opening or closing the valve. When the pneumatic signal is interrupted or the solenoid valve loses power, spring force pushes the piston back to its original position and automatically returns the valve to its preset safe state.
- The process can be expressed in a simple sequence:
Air supplied → springs compressed → valve operates
Air lost → springs return → valve returns to the safe position
Thrust calculation: The opposing spring force must be subtracted from a single-acting actuator's output force, so at the same air supply pressure its output is lower than that of a double-acting actuator. Spring force increases as compression increases, making the output force lowest at the end of the stroke.
2. What Is a Double-Acting Pneumatic Actuator?
A double-acting pneumatic actuator has no return springs. Compressed air can be supplied to either side of the piston. A directional control valve switches the air supply direction to move the piston both ways.
Operating principle: When compressed air enters the left side of the cylinder through port A, it moves the piston to the right, opening or closing the valve, while port B exhausts. To reverse the motion, the solenoid valve changes over and compressed air enters the right side through port B, pushing the piston left.
Thrust characteristics: Double-acting actuators generate thrust in both directions, with balanced output. At the same air supply pressure, their output torque is greater than that of single-acting actuators. Thrust is calculated as F = P × A × η, where P is working pressure, A is piston area and η is mechanical efficiency, typically 85-95%.
3. How Do You Choose?
- When to choose single-acting (spring-return)
Processes with demanding safety requirements, where the valve must automatically return to a safe position after a loss of electricity or air.
Applications with a less stable air supply pressure.
Safety interlock applications such as emergency shutdown systems (ESD) and fire and gas systems.
Simple control logic requiring only on/off operation.
- When to choose double-acting
Valves requiring more torque than a single-acting actuator can provide.
Applications with limited air supply pressure, where double-acting operation makes full use of pneumatic force.
Applications that do not require fail-safe return, or where position holding is provided by other means.
Modulating service using a valve positioner for higher control accuracy.
4. Common Misconceptions
Misconception 1: Double-acting is better than single-acting.
Neither is inherently better; they suit different applications. Where safety takes priority, single-acting must be selected; where thrust takes priority, double-acting is suitable.
Misconception 2: Adding a lock-up valve lets a single-acting actuator work as a double-acting actuator.
A lock-up valve can only maintain pressure in the pneumatic circuit; it cannot make the actuator air-powered in both directions. Structurally, a single-acting actuator is supplied with air on one side and cannot replace a double-acting design.
Misconception 3: Safety does not need to be considered with double-acting actuators.
A double-acting actuator stays in its current position after air loss, but that is not necessarily a safe position. If the process requires a return to a particular position on failure, additional equipment such as a pneumatically operated lock-up valve or a spring-return accessory is needed.
Understanding the differences between single-acting and double-acting designs helps you make an informed selection based on actual operating requirements.
Zhejiang HES Automatic Instrument & Valve Co., Ltd.
Founded in 1989 · Over thirty years focused on pneumatic actuators
Products include the AT and HG series and a full range of single-acting and double-acting actuators.