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."