Fundamental Concepts

Air pressure and cylinder motion

When air is compressed, it exerts a force on the sides of its container. The amount of force exerted depends upon the amount of surface area available, and as such, the force of air is given as in units of pressure, typically pounds per square inch (PSI). Pneumatic pistons take advantage of this fact by using compressed air to do work. To extend a piston, a solenoid opens a valve, allowing compressed air to flow into the back of the piston. The compressed air pushes on the base of this piston rod, causing it to move outward. At the same time, the air on the other side of the piston rod is released into the atmosphere, because otherwise it would become compressed and attempt to push the piston in the other direction. To retract the piston, the opposite steps are taken.

The cross-section of a pneumatic cylinder. As compressed air flows into the back of the piston chamber, it pushes the piston rod out. When compressed air flows into the front, it moves the piston rod in.

Calculating piston force

FRC allows pneumatic systems to employ pressures up to 60 PSI. This is quite strong - pistons operating under 60 PSI are capable of lifting an entire robot. As such, it is important to consider piston strength during design. The force exerted by a piston will be equal to the force exerted on the end of its rod by air pressure. Therefore, to determine piston force, one multiplies air pressure by surface area of the cylinder rod.

As an example, consider a piston with a bore of 1" (meaning the circular base of the rod has a 1" diameter) acting at 60 PSI. The surface area of the rod base is equal to πr², and r is 1/2", so surface area is approximately 0.785 in². Thus, the force from the piston is 47.12 lb, which equals 0.785 in² * 60 lb/in².

Note that the surface area of the piston rod is slightly greater on the back side than the front, because the rod extends out to the front. Therefore, the piston will not be able to provide as much force on the instroke. To calculate the surface area of the front, it is necessary to subtract the area the rod removes from the base from the area of the base.

A cross-section CAD model of a pneumatic cylinder.