Vacuum pump with plural labyrinth seal portions

Claim 1

A vacuum pump having a casing provided with an inlet port and an outlet port, a plurality of pump stages provided in said casing, a driving means connected to a shaft of said pump stages, bearings supporting said shaft, and a means for supplying a lubricating oil to said bearings, a gas sucked from said inlet port being discharged from said outlet port, comprising a narrow gap sealing means formed by the portion of said shaft which is between said pump stages and the upper bearing and a stationary member opposed to said portion of said shaft, and comprising a first seal portion for circulating a part of a discharge gas from said pump stages to the rear surface of said pump final impeller, a second seal portion for supplying a purge gas to said first seal portion, and a third seal portion for supplying said purge gas toward said bearing; a means for supplying a part of said discharge gas to said first seal portion; and a means for supplying said purge gas to said second and third seal portions, and wherein said means for supplying a part of said discharge gas to said first seal portion comprises an annular groove for discharge gas, which is provided in said stationary member, and a discharge gas passage formed in said stationary member and communicated with said annular groove so that said discharge gas flows circularly from the first seal portion to the discharge gas passage via said rear surface of said pump final impeller, said means for supplying said purge gas to said second and third seal portions comprising an annular groove for purge gas, which is provided in said stationary member, and a purge gas passage communicated with said annular groove for purge gas and used to supply said purge gas from the ouside to the same annular groove.