in machinery, a device that prevents the passage of fluids along a rotating shaft. Seals are necessary when a shaft extends from a housing (enclosure) containing oil, such as a pump or a gear box.

A common type of shaft seal consists of an elastomer (elastic rubberlike) ring bonded to a metallic ring that is a press (tight) fit in the hole in the housing through which the shaft extends. The sealing is done by a lip on the elastomer ring that is pressed snugly around the shaft by a helically wound garter spring. When properly designed and installed, the lip rides on a film of lubricant about 0.0001 inch (0.0025 millimetre) thick. If the film gets too thick, fluid leaks; if it is too thin, the lip gets hot, and the seal may fail. Leather, synthetic rubber, and silicones are among the materials used for the sealing ring.