Cassette oil seal is composed of two or more than two components to form a closed cavity. Its shape is like a box, a number of auxiliary sealing lips combine smartly in the interior of the cavity, forming a multiple relative independent closed spaces, so it is also named combination oil seal. In terms of structures, it is divided into two kinds: open cassette oil seal and closed cassette oil seal. The oil seal can be separated naturally, which is called open combination oil seal. Oil seal which body can not be separated naturally is called closed combination oil seal.