The core of pneumatics is the pneumatic cylinder. There are other types of pneumatic devices, but the cylinder is the most commonly used for animation. A pneumatic cylinder consists of an outer tube with a metal rod inside. The cylinder has one or two air connections for the flow of air into and out of the internal air chamber. When air is forced into the cylinder it either pushes the metal rod in or out.
The linear movement of that rod is used for mechanical movement. It can be used to push something or to pull something. The force cylinders can generate make them useful for leveraged props, where the short throw of the cylinder can be used to generate a much large movement through a series of linkages. More about that later.
Two basic factors for a pneumatic cylinder is its length ( or throw) and its diameter. The length of the rod, or more specifically how much it moves when pressure is applied, is a key factor in selecting the right cylinder for an application. The diameter of the tube directly relates to how much force can be generated with a given air pressure. The larger the diameter, the more force that can be generated.
Calculating the force of a pneumatic cylinder is fairly straightforward. To calculate how many pounds of force a cylinder will have, use the following formula:
Air Pressure per Square Inch (PSI) x Area of cylinder = Force
So, 40 PSI in a 1" diameter cylinder would look like this:
40 x .78539 = 31.4 pounds or force
To calculate the area of a circle, use the formula 3.14 x 1/2 diameter x 1/2 diameter (pi x radius^2) or for a simple rough estimate, multiply the diameter by the diameter and the result by .78
Types of Cylinders
Single Rod - a cylinder with just one rod.
Single-acting - a cylinder with one air connection. Air is forced in to push the rod. To return the rod to its original position, remove the air pressure, and either gravity or a spring push it back. This type of cylinder is typically used as a full extension or no extension device, it is not used to move to an "in between" position.
Double-acting - a cylinder with two air connections. Air is forced into one end of the cylinder to move the rod and then air is forced into the other connection to return the rod to the original position. This type of cylinder can also be used to rapidly move the rod (or prop) back and forth. It can also be used to achieve a position between full extension and zero extension.
Spring Return - Single acting cylinders sometimes have a spring return to return the rod to its starting position. It has little force and can move little more than the rod itself in most cases.
Cylinders also have various diameter push rods. Generally, this isn't a big concern, but you may have an application that mates more easily with a certain size rod. The rod has a threaded end for making connections or threading a nut to secure it in place. the only other factor for consideration of the rod diameter is if you are dealing with large forces, then you want a rod capable of supporting that force. A small rod on a 1" cylinder with 150 psi could damage the rod. While you probably won't need such high PSI, it just illustrates the point to choose the proper cylinder for the application.
The air connection fittings to a cylinder also vary. Generally 1/4" connections are ideal unless you need to move high volumes of air, such as in an air cannon. try to stick to one fitting size to simplify your work. If you find a deal on cylinders with a the wrong fitting size, that is fine, you can always work around with extra fittings.
Lastly, connecting your cylinder in place is the final consideration. The rod is threaded and so can be bolted together with a prop or a variety of connectors can be screwed onto it and those attached to the prop. The other end of the cylinder also has a variety of connection options. Some have a nut allowing the cylinder to be bolted in place while other have other types of connectors allowing for pivoting.
Pneumatic cylinders are primarily used in industrial applications and so a great variety of options and features are available that are not relevant to the typical builder of special effects. Most of these features increase the price of the cylinder but have little other bearing on your project and so can be ignored. The bare bones cylinder is what most prop builders will need. A good source for cheap cylinders is Ebay. Finding a very specific cylinder is difficult and you may be forced to pay full price through a retailer, such as Grainger.
Read about How To Set Up a Pneumatic Control for a Prop