Pneumatic cylinders are the basis for linear motion throughout most automated assembly systems. Cylinders are used for pushing, pulling, lifting and pressing. They transport parts in and out of fixtures, and they move screwdrivers, welders and other tools to and from the workpiece.

Cylinders come in single- or double-acting versions. In a single-acting cylinder, compressed air moves the piston in one direction only. Movement in the opposite direction is powered by gravity or, more commonly, by an internal spring. In a double-acting cylinder, compressed air moves the piston in both directions.

Each type has its advantages. For example, because of the spring, a single-acting cylinder always returns to its default position if the machine is stopped or air pressure is cut off. "That can be an advantage if you don’t want the cylinder to interfere with tooling at startup," says Walt Hessler, vice president of sales and marketing at PHD Inc. (Fort Wayne, IN). "A double-acting cylinder will stay in whatever position it’s at when the machine stops."

Single-acting cylinders also consume less air and have less expensive control valves. "At a minimum, a double-acting cylinder requires a 5/2 valve—five ports, two positions. You have to control the cylinder to both open and close," says Richard Porqueddu, product management for handling systems at Festo Corp. (Hauppauge, NY). "With a single-acting cylinder, you can use a 3/2 valve, which is a little cheaper and can be controlled with only one output from a controller. If you only need one or two cylinders, the cost savings might not be that relevant, but if you have 50 or 60, the savings become evident."

On the other hand, double-acting cylinders can move heavier loads than comparable single-acting cylinders, and the speed of the stroke in both directions is easier to control. "A single-acting cylinder has limited force capability because it must overcome the force of the return function," says Clayton W. Fryer, product sales champion at IMI Norgren Inc. (Littleton, CO). "Compared with a double-acting cylinder of the same stroke, a single-acting cylinder is typically longer [and] more expensive."

Cylinders also come in single- and double-rod versions. In a single-rod cylinder, the piston rod extends from only one end of the cylinder. In a double-rod cylinder, the rod protrudes from both ends, though it’s still driven by a single piston.

Guided cylinders are basic cylinders with guide rods mounted parallel to the piston rod. With the extra stability, these cylinders can tolerate side forces and carry loads without rotating.

Rodless cylinders differ from basic cylinders in that no piston rod extends outside the body. Instead, the internal piston is connected to an external carriage by a magnetic or mechanical coupling. "When you actuate the piston, the cylinder doesn’t get any longer," explains Hessler. "The carriage just travels back and forth along the tube. That gives you longer travel lengths with less space."

Like guided cylinders, rodless cylinders provide nonrotating motion and are unaffected by side loads.

Cylinders are usually made from aluminum, but they can also be made from stainless steel, brass, plastic and even glass. The environment in which the cylinder will operate will determine if an alternate material is required, says Fryer. For example, a stainless steel cylinder is necessary if the machine will be exposed to caustic cleaning solutions. Cylinders built with special low-friction materials are used in light-load, low-pressure applications to limit stick-slip motion.