An oil seal, often called a grease or dirt seal, is a type of seal utilized to retain grease in a case or housing.

　　Oil seals close spaces between stationary and moving components in mechanical equipment, helping prevent lubricant escape. Preventing harmful containments from entering machinery is another critical function of an oil seal. In a typical application, the oil seal is installed adjacent to a bearing, sealing in or sealing out, as necessary, the various liquids, gases or solids encountered by the particular mechanism.

　　Oil seals are available in numerous lip materials. Popular materials include felt, leather, urethane, nitrile, polyacrylate, ethylene-acrylic or polyacrylic, silicone, fluoro-elastomer, tetrafluoro-ethylene propylene and PTFE. Use of one material over another is application specific, as materials will have different temperature and speed ranges. Oil seals also come in numerous constructions. Of particular note is the number of lips, with single and double lip being the most popular. Many popular styles of oil seals will utilize a spring on the lip to increase sealing capabilities. Some oil seals have a coated metal casing with a “rubber” lip while other seals will be a uniform with the lip material. Available in a wide array of sizes, both inch and metric, most oil seals are interchangeable between manufacturers. Critical dimensions on an oil seal include the inside diameter or bore (the OD of the shaft the seal rides on), the outside diameter (often called the housing bore) and the width. The ID and the OD are the most critical dimensions, as it is the oil seal lip that actually does the sealing.

　　Oil seals are generally the least expensive component in a mechanical system, but they serve to be the most important, as they protect the more expensive components, such as the bearings and gears.