Single Acting
            Air Cylinders

Single acting air cylinders; what are they and why do we need them?

Contrary to what many have believed for years, compressed air is not cheap. It really is quite expensive to convert electrical energy into compressed air.

No Load Cylinder

If you have an air cylinder that is cycling in one direction under no load, or under low load, you can reduce your compressed air consumption and energy expense by using a single acting air cylinder rather than a more conventional double acting air cylinder.

A single acting air cylinder only uses compressed air to move the piston and rod in one direction while 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.

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 with the rod extended is complete, the valve shifts, and rather than using compressed air to retract the rod, the weight of the load on the rod is often sufficient to drive the rod into the retract position, without use of compressed air.

In this case, the single acting air cylinder operates without the need for internal spring as well, 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 format, or one air port 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 ingests air as the piston moves back. Otherwise, the cylinder would alternately compress air in the blind end, or pull a vacuum inside itself, not conducive to the smooth operation of the cylinder, for sure.

Any Cylinder 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, the load that is being moved by the single acting cylinder will either cause the rod to retract or extend 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 the ingress of contaminants as the cylinder cycles.

You can get single acting air cylinders in NFPA footprint These will be air cylinders with a bore size from 1.5" and up.

Often, single acting air cylinders are used in high cycle, low load applications, applications where the use of aluminum non-repairable style cylinders is the norm. These cylinders are either North American manufacturer standard, or of European ISO-6432 standard.

Selecting Single Acting

When you are selecting a single acting air cylinder, remember that if you want the rod to be extended when the cylinder has no compressed air flowing to it, you want a spring-extend unit, and if you want the rod to be retracted when there's no air flowing to the cylinder, you would select a spring-retract.

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.