An air cylinder is usually a cylindrical or tube shaped device that contains a shaft, plunger, and rod. There are other shapes and sizes, but the tube shaped air cylinder is the most common.

The rod within the air cylinder fits into the shaft and is capable of moving freely in and out. The plunger of the air cylinder is attached to the end of the rod and cannot be seen. The purpose of the plunger is to take the brunt of the air pressure flowing within the air cylinder. The wider, flat surface of the plunger provides a wall against which air pressure pushes.

There are two main types of air cylinders: single acting and double acting. A single acting air cylinder pushes out as a spring returns the rod into the shaft. A double acting air cylinder moves the rod both in and out due to the air pressure on one end, which creates a vacuum on the other end. The vacuum pulls the rod back inside the shaft.

Air cylinders have many uses. They are found in cars, houses, hospitals, and businesses. The usage of an air cylinder depends on its rating. An air cylinder is rated based on three things: pressure, bore, and stroke.

Pressure is the determination of the safest pounds per square inch (PSI) load the air cylinder can handle. Bore is the air cylinder's diameter on the inside of the shaft. Finally, stroke is the distance or length the rod can move inside the shaft without snapping or bending the plunger inside.

An air cylinder is used to control the pressure of the air brakes on an 18-wheeler. An air cylinder is also used to help a car's engine push the pistons, which in turn help turn the crankshaft, which makes the car's tires spin. An air cylinder can also help close certain types of doors. Many of these actions take place without being seen, so they can be difficult for a novice to understand.

The most common example of an air cylinder is the tube shaped device that helps a storm door open and close itself. As a person pushes the storm door open, the rod inside the air cylinder's shaft extends and the cylinder uses an inflow of air to help push the rod along. When the door is released, the air flows backwards out of the air cylinder. The rod distends, moving back into the shaft, and the door closes. This is a perfect example of a double acting air cylinder.