Type TRE are seals with a completely rubber covered outer diameter and a secondary dust lip which protects the other lip against dust and other fine contaminants. This type of radial oil seal is recommended for use in polluted environments. Radial Oil Seals are also known as Rotary Shaft Seals, consisting of a rubber sealing lip, metal case and a spiraled tensioning spring Their function is to keep oil and grease within the system dirt and dust out.