Pneumatic seals are any of a class of seals used in applications including rotary or reciprocating motions. They are often used in pneumatic cylinders and valves. Pneumatic seals do not work well under high pressure. Therefore, they are generally used in areas of lower pressure.
Pneumatic seals require minimal lubrication when exposed to air in order to create a tight seal. Pneumatic seals may also be exposed to high operating speeds at which the pressure is not high. Piston seals, rod seals, flange packings, u-cups, and vee-cups are a few sealing designs that take advantage of pneumatic seals. The difference between pneumatic seals and hydraulic seals is pressure. Pneumatic seals typically have a pressure range of 1 to 150 pound-force per square inch (psi), while hydraulic pressure can reach greater than 10,000 psi.
Sometimes, composite seals are used as pneumatic seals. Composite seals are seals composed of two or three different materials. Therefore, pneumatic seals are often found in products requiring one seal for many parts. A PTFE ring and an elastomer ring are often used in this situation. NASA uses this technology in their rocket systems. Composite pneumatic seals are also used in the diesel engines for trucks.
A pneumatic seal's sealing orientation can be internal as with a rod seal, external as with a piston, symmetrical, or axial. With internal pneumatic seals, a housing bore surrounds the seal and the sealing lip touches the shaft. This seal requires very little lubricant.
With external piston pneumatic seals, the seal surrounds a shaft and the sealing lip touches the housing bore. This system requires more lubrication. Symmetric pneumatic seals are the same on both sides, and axial pneumatic seals fit axially against the housing. In both cases, however, the seals are used externally and require more lubrication.
Rotary applications need only one pneumatic seal. This seal is considered to be single acting because it can seal in one axial direction while the application is moving. On the other hand, a reciprocating application requires two pneumatic seals, or double acting seals. In this case, one seal is needed for each of the directions. Double acting pneumatic seals are more complicated than single acting ones.