There are two kinds of seal in vacuum device, one is that the seal is divided into a container filled with liquid medium and the external vacuum space. In this case, the main role of the seal is to prevent leakage of liquid medium into the vacuum space. Another condition is that the sealing part of the vacuum chamber and the separation of the external cavity, it is to maintain the role of the vacuum degree. This situation is more complicated, because the surrounding gas medium, even a slight leakage into the vacuum chamber, it will reduce the degree of vacuum. Vacuum equipment may be damaged if it is leaking into the corrosive gas. In many cases, the vacuum degree and the vacuum space are clean, and the quality of the various process under vacuum conditions has a decisive influence on the quality of the process. Mechanical seals used in vacuum devices have no special structure. For the first case, it is usually a good metal bellows seal with good air tightness. The second case is to use a double mechanical seal, if the use of a single end face seal, it is bound to exist in the seal face can not be lubricated.