Pneumatic cylinders (sometimes known as air cylinders) are mechanical devices which produce force, often in combination with movement, and are powered by compressed gas (typically air).

To perform their function, pneumatic cylinders impart a force by converting the potential energy of compressed gas into kinetic energy. This is achieved by the compressed gas being able to expand, without external energy input, which itself occurs due to the pressure gradient established by the compressed gas being at a greater pressure than the atmospheric pressure. This air expansion forces a piston to move in the desired direction.

Operation

  General:
Once actuated, compressed air enters into the tube at one end of the piston and, hence, imparts force on the piston. Consequently, the piston becomes displaced (moved) by the compressed air expanding in an attempt to reach atmospheric pressure.

  Specialized functions:
Depending upon the design of the system, pneumatic cylinders can operate in a variety of ways. Examples include having the ability to perform multiple strokes without the need for intermediate intervention, to perform a full stroke with intermediate stopping points, to be adjusted so as to control the amount of extension and/or retraction of the piston rod once actuated.

  Fail safe mechanisms:
Pneumatic systems are often found in settings where even rare and brief system failure is unacceptable. In such situations locks can sometimes serve as a safety mechanism in case of loss of air supply (or its pressure falling) and, thus, remedy or abate any damage arising in such a situation.

  Types:
Although pneumatic cylinders will vary in appearance, size and function, they generally fall into one of the specific categories shown below. However there are also numerous other types of pneumatic cylinder available, many of which are designed to fulfill specific and specialised functions.

  Single acting cylinders:
Single acting cylinders (SAC) use the force imparted by air to move in one direction (usually out), and a spring to return to the "home" position

  Double acting cylinders:
Double Acting Cylinders (DAC) use the force of air to move in both extend and retract strokes. They have two ports to allow air in, one for outstroke and one for instroke.

  Other types:
Although SACs and DACs are the most common types of pneumatic cylinder, the following types are not particularly rare:

Rotary air cylinders: actuators that use air to impart a rotary motion
Rodless air cylinders: actuators that use a mechanical or magnetic coupling to impart force, typically to a table or other body that moves along the length of the cylinder body, but does not extend beyond it.

   Sizes:
Air cylinders are available in a variety of sizes and can typically range from a small 2.5mm air cylinder, which might be used for picking up a small transistor or other electronic component, to 400mm diameter air cylinders which would impart enough force to lift a car. Some pneumatic cylinders reach 1000mm in diameter, and are used in place of hydraulic cylinders for special circumstances where leaking hydraulic oil could impose an extreme hazard.