Full definition
An oil seal, also known as a shaft seal, is a critical component in rotating machinery designed to retain lubricants and prevent the ingress of contaminants. These seals are typically radial lip seals that fit into a bore and are engineered to create a tight seal against a rotating shaft. The construction of an oil seal includes a metal case that provides stiffness and durability, an elastomer body that forms the primary sealing mechanism, and a precision-molded sealing lip that exerts controlled interference pressure against the shaft. This pressure is usually in the range of 0.1-0.5 N/mm, which is essential for maintaining the integrity of the seal over time. Furthermore, a garter spring located behind the sealing lip ensures continuous contact force, compensating for wear as the sealing lip comes into contact with the shaft during operation.
The design of oil seals follows standards such as ISO 6194 and DIN 3760/3761, which provide specifications for dimensions and tolerances. For example, a seal designated as TC 40×62×8 indicates it is a type TC (double lip), suitable for sealing and dust protection, with a shaft diameter of 40 mm, a bore diameter of 62 mm, and a width of 8 mm. The choice of lip materials is crucial: NBR is commonly used for standard applications with oil resistance up to 100°C, while FKM is preferred for high-temperature scenarios up to 200°C and environments with aggressive chemicals. PTFE is used in high-speed applications due to its broad chemical resistance but requires a polished shaft for optimal performance.
Shaft surfaces must meet specific requirements to ensure the effectiveness of oil seals. A surface finish of Ra 0.2-0.8 μm and a hardness greater than 45 HRC are typically necessary to prevent premature wear. Additionally, spiral machine marks should be avoided, as they can compromise the sealing capability. The operational speed limits for oil seals vary: NBR seals are generally limited to speeds of 12-15 m/s, while PTFE seals can operate in excess of 20 m/s. The maximum pressure ratings for standard lip seals fall between 0.3-0.5 bar, emphasizing that they are not designed for high-pressure applications. Recognized brands in the industry include NOK, Freudenberg (Simrit), SKF, Trelleborg, and Parker, offering a range of oil seals that meet these stringent specifications.