Understanding Pneumatic Power: Single-Acting

vs. Double-Acting Cylinders

In the realm of fluid power, whether hydraulic or pneumatic, cylinders are considered to be the "muscles" of the machine. They transform power into linear motion, which can be used to push upwards, pull, lift, or press. If you're developing an entire system or are sourcing components, the most basic option you'll have to choose is between the single-acting cylinder (SAC) and a Double-Acting Cylinder (DAC).

Although they may look similar from the outside, their internal mechanics and applications are vastly different. Let's examine the major differences to help you choose the best one for your project.

1. The Single-Acting Cylinder (SAC)

The Single-Acting Cylinder is considered to be the simplest of actuators. It utilizes compressed air (or fluid) to move the piston in a single direction, and only that one direction. It is usually "out" (extension).

How it Works:

• The Power Stroke: Compressed air is pumped through a single port, propelling it forward.

• "The Return" After the air pressure has been let go, the piston can't remain in place. It is then forced back to its starting point by the force of a mechanical spring or, in certain instances, by gravity or external pressure.

Best For:

Because they are the only airline, they're simpler and use less compressed air. They are great for:

• Positioning and clamping.

• Ejection of pieces onto the conveyor.

• It is a simple "on/off" lifting task where gravity does the work of lifting the load.

2. The Double-Acting Cylinder (DAC)

The name implies that a Double-Acting Cylinder makes use of power to both "push" and "pull." The most commonly used kind of cylinder is used in automation systems for industrial use.

How it Works:

• The extension: The air enters through the port in the rear and pushes the piston forward, while the air in the forward chamber gets exhausted.

• The Retraction. For the purpose of pulling back the piston airflow reverses. Air is pushed into the port in front, which pushes the piston back towards the beginning, while the rear chamber ejects.

Best For:

Because you can manage the force and speed equally in all directions, they are the most reliable of all complex machines. They are perfect for:

• Closing and opening valves or gates that weigh a lot of weight.

• Robotic arms, pick-and-place machines, and pick-and-place equipment.

• All applications that require a long stroke (springs in SACs have length limitations).

Key Differences at a Glance

Which One Should You Choose?

Choose Single-Acting when:

• You are limited in space (SACs are usually shorter).

• Your budget isn't enough.

• The task is easy (like the basic "fail-safe" where the spring is released if it loses power).

Select Double-Acting if:

• You must maintain a consistent acceleration and force in each direction.

• You require a stroke of a longer length (greater than 4 inches).

• It is necessary for the piston to be stopped and/or "dwell" at any point in its journey.

The Verdict

The decision is usually down to the control. If you need just an instant "nudge" or a clamp, the Single-Acting Cylinder is an affordable hero. If you require accuracy, strength, and flexibility, the Double-Acting cylinder is well worth the additional plumbing.