A pneumatic actuator is an important actuating mechanism in automation control systems; it converts pneumatic energy into mechanical motion through compressed air to achieve the opening, closing or regulating of a valve. Its types can be divided from multiple angles, among which the most common and the one that most affects safety design is the division into single-acting and double-acting by mode of action. This article sorts out the types clearly and then explains the differences and selection points.
1. The 4 Classification Dimensions of Pneumatic Actuators
-
By mode of action: single-acting (spring-return), double-acting - look at where the valve goes after air loss, which determines safety
-
By form of motion: rotary, linear - look at whether the valve rotates or moves in a straight line
-
By structural form: rack-and-pinion, scotch yoke, diaphragm type - look at the torque magnitude and installation space
-
By control method: purely pneumatic, electro-pneumatic control - look at whether the site uses air signals or electrical signals
The other dimensions determine "whether it fits and whether it moves"; the mode of action determines "whether it is safe when something goes wrong".
2. By Mode of Action: Single-Acting and Double-Acting
Single-acting pneumatic actuators adopt a spring-return structure: compressed air pushes the piston or scotch-yoke mechanism to drive the valve open, and when the air supply stops or the pressure disappears the spring automatically returns, closing the valve (it can also be designed as air-to-close - fail-open, i.e. it opens when the air supply is lost - according to the process). Their features are an automatic return function, relatively high safety and a relatively simple structure, so they are often used for emergency shut-off valves, safety valves or applications with high safety requirements. Note that their output force must deduct the spring counteracting force, so under the same pressure it is smaller than that of a double-acting actuator.
Double-acting pneumatic actuators require the drive of an air supply for both opening and closing: compressed air enters different chambers of the cylinder respectively to achieve opening and closing, and the reverse action is made by the solenoid valve changing direction. Their features are stable output torque, smooth action and no need for a spring-return structure, and they are suitable for frequent opening and closing or applications requiring relatively large torque.
Common point: the opening action of both is usually driven by the air supply, and precise control is achieved by controlling the magnitude and direction of the air pressure.
3. The 6 Key Differences Between Single-Acting and Double-Acting
-
Reset method: single-acting relies on the spring; double-acting relies on air supply reversal
-
Number of air ports: single-acting admits air on one side, generally 1 air inlet; double-acting admits air on both sides, air ports A and B
-
Behavior on air loss: single-acting automatically returns to the preset safe position; double-acting stops at the current position (holds position)
-
Output force (torque): single-acting must deduct the spring counteracting force and is smaller; double-acting has balanced thrust and is larger
-
Air consumption: single-acting is smaller; double-acting is larger
-
Typical applications: single-acting for fail-safe and emergency shut-off; double-acting for frequent opening and closing, large torque and regulating
"Behavior on air loss" is the dividing line: whether it returns to the safe position by itself or stays where it is after air loss - this is the most essential difference between the two structures and the starting point of fail-safe interlock design.
4. Two Other Dimensions: Form of Motion and Structural Form
In terms of form of motion, a rotary actuator outputs rotary motion (commonly 0-90°) and is used for rotary valves such as ball valves and butterfly valves; a linear actuator outputs linear displacement and is used for gate valves, globe valves and control valve plugs. First look at what motion the valve needs, then determine the actuator form.
In terms of structural form, the rack-and-pinion type has a compact structure and strong versatility and is the most widely used category among rotary types; the scotch-yoke type outputs greater torque under the same cylinder bore and suits large-diameter, high-torque operating conditions; the diaphragm type has a simple structure and a relatively short stroke and is mostly used in regulating applications where the thrust is not large. Different structures also mean different installation heights and footprints, so retrofit projects should verify this in advance.
5. Selection Points and Order of Judgment
Five factors should be considered in selection:
-
Safety factor: reserve a certain safety factor to ensure the valve can still be reliably driven under complex operating conditions
-
Actuator size: choose the size according to the on-site installation conditions to ensure convenient installation and maintenance
-
Operating environment and conditions: valve type and torque requirements, working pressure and temperature, whether an emergency closing function is required
-
Control method and accessories: whether only on-off is needed or continuous regulation is required; limit feedback, positioner, solenoid valve and filter regulator should all be determined during selection
-
Air supply conditions: whether the pressure is stable and whether the air is dry and clean directly determine the stability of long-term operation
The order of judgment can follow this path: first determine the behavior on air loss (choose single-acting if fail-safe is required) → then calculate the torque and leave a safety factor (choose double-acting if single-acting is not enough) → then determine the form of motion and structural form → next configure the control method and accessories → finally verify the air supply and environment (temperature, corrosion, explosion protection, maintenance space).
6. 4 Common Misconceptions and Recap of Key Points
Misconception 1: Compare prices first and check the operating conditions later. The price difference often lies in the specifications and accessories, and choosing too small a size costs more in later replacement.
Misconception 2: Believing that double-acting is better than single-acting. The two are not a matter of better or worse. Operating conditions where safety comes first must choose single-acting, while operating conditions where thrust comes first are suited to double-acting.
Misconception 3: Believing that double-acting actuators need not consider safety. A double-acting actuator holds position when air is lost, but the current position is not necessarily the safe position; when the process requires a return to a specific position on failure, an air-controlled position-holding valve or a spring-return accessory needs to be added.
Misconception 4: Only buying the right main unit and pairing the accessories at random. The solenoid valve bore, positioner matching and limit switch position all affect performance, and accessories should be considered together with the main unit as a set.
Recap of key points: pneumatic actuators are divided by mode of action into single-acting and double-acting, by form of motion into rotary and linear, and by structural form into rack-and-pinion, scotch yoke and diaphragm type. Single-acting relies on spring return, returns to the safe position on air loss and has a smaller output force; double-acting relies on air supply reversal, holds position on air loss and has greater torque, and the opening action of both is usually driven by the air supply. In selection, first determine the behavior on air loss, then calculate the torque and leave a safety factor, then determine the form of motion and structure, and finally pair the accessories and verify the site - the three items of safety factor, installation size and operating conditions cannot be omitted.
Conclusion
The question of types is essentially "by which dimension to choose": the mode of action determines safety, the form of motion determines whether it can be used, the structural form determines torque and space, and the control method determines how it connects to the system. Follow these four threads through, and combine them with the safety factor, installation size and on-site operating conditions, and selection has a clear basis. If you need selection calculation and accessory configuration support for pneumatic actuators, please feel free to contact our service team.
Zhejiang Huaershi Auto-control Instrument Valve Co., Ltd.
Since 1989 · 30+ years focused on pneumatic actuators and pneumatic valves
Website: