Although split-ring seals are most often used for reciprocating motion, they are sometimes applied to hydraulic swivel joints and transmissions. They are compact, relatively inexpensive, and provide long life under harsh operating conditions.
Expanding split-ring seals, or piston rings, are used in compressors, pumps, internal-combustion engines, hydraulic cylinders, and pneumatic cylinders. Contracting split-ring seals, or rod seals, are used in linear actuators where high pressure and temperature, radiation, thermal fatigue, and reliability requirements make packings undesirable. Everything said of piston rings has a counterpart in rod seals.
Both expanding and contracting seals come as one, two, and three-piece seals. Single-ring straight-cut seals are the simplest and most economical, but they also leak the easiest.
Standard step-cut seals are used for unidirectional sealing where leakage is not critical, at bore diameters down to 1 3/8 in. Special high-precision step seals are available for applications where leakage is critical. One-piece or two-piece seals with special joint designs are needed for bidirectional sealing. Three-piece seals are used for very small or very large cylinder diameters.
Garter springs and axial springs can be added to any of the seals to ensure contact at sealing surfaces. Contact forces in both axial and radial directions must be large enough to overcome friction. Other things being equal, radial and axial forces should be about the same.