Single Acting
            Air Cylinders



Single acting air cylinders; what are they and why do we need them? Here is lots of information about these versatile actuators to help understand, select and use them.

Contrary to what many may think, compressed air is not cheap. It really is quite expensive to convert electrical energy into compressed air. Compressed air is one of the highest cost energy sources.

No Load Single Acting Cylinder


An air cylinder that is cycling in one direction under no load, or under low load, can reduce compressed air consumption and energy expense simply by using a single acting air cylinder in the application rather than using a more conventional double acting air cylinder.
Singleacting Air Cylinder

A single acting air cylinder only uses compressed air to move the piston and rod in one direction. The double acting air cylinder uses compressed air to both extend and retract the cylinder rod even if there is no load in one direction.

The compressed air ($$) savings on a high cycle cylinder application can be huge.

Single Acting Cylinders
         Spring Extend or Spring Return?



In a typical application for a single acting air cylinder, it is either the load itself, or an internal spring, that returns or extends the air cylinder rod.

For example, a cylinder in a vertical - rod up - configuration, will use compressed air to extend the rod vertically. When the operation is complete, the valve shifts, and rather than using compressed air to retract the rod, the weight of the load on the rod is sufficient to retract the rod without use of compressed air. In this case, the single acting air cylinder operates without an internal spring as the load provides the force to retract the air cylinder.

In another example, a small stroke and bore single acting air cylinder is used to eject parts from a conveyor belt. Compressed air is used to drive the rod out to impact and, with sufficient force, eject the part. The spring inside the air cylinder is compressed as the rod extends under the force of the compressed air acting on the piston. Then, when the valve shifts, the air is exhausted from the back end, and now the spring has enough force to drive the piston back and retract the cylinder rod.

You can spot a factory-made single acting air cylinder, as it will have only one threaded air fitting port. One on the back end if it is a spring return, or one on the rod end if it is a spring extend single acting air cylinder.

In the other end of the cylinder barrel from the air fitting port will be a small hole. This allows the air that is in the non-ported end to exhaust to atmosphere as the piston moves toward that end. It also allows air back into the cylinder as the piston moves back. The hole is necessary and should not be blocked, Without it, the cylinder piston would alternately compress air in the blind end, or pull a vacuum as the cylinder cycled, reducing its effectiveness.

A Double Acting Cylinder Can Be
         Single Acting


Any double acting air cylinder can be used as single acting air cylinder should the need arise.

Since there is no spring inside a typical double acting cylinder, it can act like a singel acting cylinder if the load extends or retracts the rod when compressed air is not flowing to the cylinder. Typically, that will be with the load in a vertical orientation.

The air port on the end of the cylinder not being plumbed for compressed air is left open, or a filter/muffler can be installed to prevent dust from getting inside as the cylinder cycles.

You can get single acting air cylinders that have an NFPA footprint, typpically with a bore size from 1.5" and up. the link takes you to information about NFPA cylinders on this site.

Single acting air cylinders are either built to the standards of the North American manufacturer, or of European ISO-6432 standard. This link takes you to a page about ISO cylinders on this site.

Selecting A Single Acting Cylinder


When selecting a single acting air cylinder, if the applications needs the rod to be extended when the cylinder has no compressed air flowing to it, it is a spring-extend, and for the rod to be retracted when there's no air flowing to the cylinder, select a spring-retract version.

Here are the things you will want to know to correctly size and select your single acting air cylinder:

  • What bore size? How much force is needed including a 25% excess allowance for friction inside the cylinder and a load safety margin.
  • What stroke? What is the distance you want whatever is attached to the cylinder rod to move? If used in an ejection type application, how far will the rod have to extend to reach the farthest item to be ejected?
  • Will a cylinder cushion provide any benefit?
  • Does the cylinder need a magnet on the piston for position sensing with a proximity switch?
  • How will the cylinder be mounted, and what are the dimensions of those mounts?
  • How will the cylinder rod connect to the work-piece?
  • What is the air port size, so you can be sure you have the correct air line fittings?

Single acting cylinders are available with non-rotating rods, should your end-of-rod tooling need specific orientation. Non-rotating rods are necessary if tooling orientation is important, since in normal operation of an air cylinder with a round rod, over time, the piston and the rod will slowly and continuously rotate.

If you have a question about single acting air cylinders here is the cylinder forum on this website.