Pneumatic Cylinder (Single Acting) Symbol

Definition: The Pneumatic Cylinder (Single Acting) symbol represents a linear actuator driven by compressed air on the extend stroke only, with a spring providing the return force, depicted in fluid-power diagrams per ISO 1219-1:2012 as a rectangle (cylinder barrel) containing a spring symbol and a piston line, with a single air Port A at the cap end and a projecting rod at the opposite end.

Also known as: single-acting air cylinder, SA pneumatic actuator, spring-return air cylinder, single-port air cylinder.

What the Pneumatic Cylinder (Single Acting) symbol means

The Pneumatic Cylinder (Single Acting) symbol represents an air-powered linear actuator in which compressed air drives the piston in one direction (extension) while an internal spring or gravity returns it when air is vented. The spring is shown inside the cylinder rectangle in the ISO 1219-1 symbol, clearly signalling that the return stroke is passive rather than air-powered.

In pneumatic circuit diagrams the single-acting cylinder symbol marks an actuator that only requires one air connection, simplifying valve selection and tubing layout. It is used wherever the return stroke demands no load-holding force and simplicity or fail-safe spring-return behaviour is desirable — for instance, clamping fixtures that must release when power is lost.

How to identify the Pneumatic Cylinder (Single Acting) symbol

The single-acting pneumatic cylinder is drawn as a horizontal rectangle (the barrel) with a zigzag spring symbol occupying the right portion of the interior, a vertical dashed line (the piston) to the left of the spring, and a thin line (the rod) exiting from the right end through the end cap. A single port connection — Port A — appears at the left (cap) end of the barrel. The presence of the spring inside and only one port clearly distinguishes it from the double-acting variant which shows two ports and no spring.

Function in a circuit

A single-acting pneumatic cylinder converts compressed-air energy into one-directional linear force: when air is applied to Port A, pressure overcomes the spring force and the piston extends, pushing the rod outward. When the directional-control valve vents Port A to atmosphere, the spring pushes the piston back to the retracted (home) position. Usable extend force equals air pressure multiplied by bore area minus the opposing spring pre-load force. Typical operating pressures are 4–8 bar (58–116 psi); strokes are usually shorter than double-acting designs due to the spring.

Standards: IEC vs ANSI

Terminals / pins

Typical values

Operating pressure: 4–8 bar (58–116 psi); bore diameters: 6 mm to 100 mm (typical); strokes: 10 mm to ~300 mm (spring length limits stroke); extend force = (pressure × bore area) − spring pre-load; retract force provided solely by spring (typically 20–150 N depending on spring rate and stroke).

Where the Pneumatic Cylinder (Single Acting) symbol is used

• Safety clamping fixtures that must release (spring retract) when compressed air or electrical power is lost
• Pneumatic grippers and ejectors on injection moulding machines where one-direction force is sufficient
• Valve actuators requiring fail-safe open or fail-safe closed operation driven by spring return
• Simple stamping and marking tools in low-force applications needing a single solenoid valve
• Automotive body-shop clamping jigs where gravity-assist aids the return stroke
• Door-lock actuators where the spring ensures unlock (fail-open) on power failure

Example

In a pneumatic clamping fixture diagram, a 3/2 normally-closed solenoid valve connects to Port A of a single-acting cylinder symbol. When the solenoid energises, air enters Port A, the piston extends against the spring, and the jaw clamps the workpiece. When the solenoid de-energises, the valve vents Port A and the internal spring retracts the piston, releasing the workpiece automatically — a fail-safe open design.

Key facts

• The single-acting pneumatic cylinder symbol per ISO 1219-1:2012 is a rectangle containing a spring and a piston line, with one air port (Port A) at the cap end and a rod extending from the opposite end.
• Only the extend stroke is powered by compressed air; the return stroke is driven by the internal spring, making the cylinder fail-safe to its retracted position when air is vented.
• Usable extend force equals supply pressure multiplied by bore area minus the spring pre-load force; spring pre-load reduces the net output force compared to a double-acting cylinder at the same pressure.
• A single-acting cylinder requires only a 3/2 directional-control valve (3 ports, 2 positions) rather than the 5/2 valve needed for a double-acting cylinder, simplifying the circuit.
• ISO 1219-1:2012 and ANSI/NFPA T3.25.3 define the same symbol; IEC 60617 does not cover fluid-power symbols.
• Stroke length is practically limited by the spring length, so single-acting cylinders are typically used for short strokes (up to ~300 mm).
• Common variants include spring-extend (air returns), diaphragm single-acting cylinders, and bellows actuators — all share the single-port spring-symbol convention.

Frequently asked questions

What does the single-acting pneumatic cylinder symbol look like?

The single-acting pneumatic cylinder symbol is a rectangle with a zigzag spring drawn inside the barrel on the return side, a vertical piston line, and a rod projecting from one end. Only one air port (Port A) is shown at the cap end, which distinguishes it from the double-acting symbol that shows two ports and no spring.

What does the single-acting cylinder symbol mean on a pneumatic diagram?

It means the actuator uses compressed air only for the extend (power) stroke; the spring returns the rod to the home position when air is vented. This signals a simpler valve requirement (3/2 instead of 5/2) and inherent fail-safe retraction if the air supply fails.

Which standard defines the single-acting pneumatic cylinder symbol?

The symbol is defined in ISO 1219-1:2012 (Fluid power systems — Graphical symbols and circuit diagrams). ANSI/NFPA T3.25.3 adopts the identical symbol. IEC 60617 does not cover pneumatic actuator symbols.

What is the difference between single-acting and double-acting cylinder symbols?

The single-acting symbol shows one air port and a spring inside the barrel; the double-acting symbol shows two air ports and no spring. The spring in the single-acting symbol represents the passive return mechanism, while the double-acting symbol's two ports indicate active air-powered control in both directions.

What valve is used with a single-acting pneumatic cylinder?

A 3/2 directional-control valve (3 ports, 2 positions) is standard for a single-acting cylinder. One position connects the air supply to Port A to extend the cylinder; the other position vents Port A to atmosphere and lets the spring retract the piston.

Why is the stroke of a single-acting cylinder typically shorter than a double-acting one?

The internal spring occupies space inside the barrel, physically limiting the available stroke length. As stroke increases, the spring must be longer and heavier, reducing net output force. For strokes beyond approximately 300 mm, a double-acting cylinder is usually chosen instead.

What are the pins on the single-acting pneumatic cylinder symbol?

The single-acting cylinder symbol has two connection points: Port A (the single air inlet at the cap end that drives extension) and Rod (the mechanical output shaft at the opposite end). This corresponds to the Port A and Rod pins shown in the circuit diagram symbol.

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