Single Acting Air Cylinders

Single Acting Air Cylinders

Single Acting Air Cylinders; what are they and why do we need them?

Contrary to what may have been believed for years, compressed air is really quite expensive.

If you have an air cylinder that's cycling in one direction under no load, or under low load, you can reduce your compressed air consumption and money 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 uses compressed air to both extend and retract the cylinder rod.

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

In an application for a single acting air cylinder, it's the load itself and / or a 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 sufficient to drive the rod into the retract position. In this case, the single acting air cylinder operates without the need for internal spring.

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 eject the part. The integral spring inside the air cylinder is compressed as the rod extends under compressed air. When the valve shifts, the air is exhausted from the back end, and now the spring can drive the piston back, thereby retracting the 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's spring return, and one on the rod end if it's a spring extend. In the end of the cylinder barrel opposite from the air fitting port will be a small hole. This allows the air that's in the non-ported end to exhaust to atmosphere as the piston moves toward that end, and to ingest air as the piston moves back. Otherwise, the cylinder would alternately compress air and pull a vacuum inside itself, contrary to smooth operation of the unit.

Any double acting cylinder can be used as single acting, provided that the load that's being moved is oriented in a way that it will either cause the rod to retract or extend when compressed air is not flowing to the cylinder. The fitting air port on the end of the cylinder not being plumbed for compressed air is left open, or a filter/muffler is installed to prevent the ingress of contaminants as the cylinder cycles.

While you can get single acting air cylinders in NFPA footprint, these with a bore size from 1.5" and larger, usually single acting air cylinders are used in high cycle, low load applications, therefore most applications see the use of aluminum non-repairable style. These are either North American manufacturer standard, or of European ISO-6432 ISO standard.

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 cushion provide any benefit?
• Does the cylinder need a magnet on the piston for position sensing?
• How will the cylinder be mounted, and what are the dimensions of those mounts?
• How will the cylinder rod connect to the workpiece?
• 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 application require a specific orientation of the rod tooling. Non-rotating rods are necessary since in normal operation of an air cylinder with a round rod, over time, the piston and the rod will slowly and continuously rotate as the cylinder cycles.