Patent ID: 7637726

Claim:
A screw vacuum pump comprising a gas working chamber formed by a male rotor, a female rotor and a stator, wherein: said male rotor and said female rotor each comprises unequal lead screws engaging each other and having a lead angle that continuously changes with the advance of helix, said stator receiving therein both rotors, gas inlet and gas outlet ports are provided at said stator so as to communicate with one end portion and the other end portion of said gas working chamber, respectively, each of said unequal lead screws of said male rotor and said female rotor has a perpendicular-to-axis cross-sectional shape that changes following a continuous change in lead angle with the advance of helix, said male rotor and said female rotor have a gap therebetween in the perpendicular-to-axis cross-sectional shapes, said gap being formed so as to be gradually narrowed as moving from a discharge side to a suction side, and both of said male rotor and said female rotor have the perpendicular-to-axis cross-sectional shapes changed in accordance with the change in lead angle of said male and female rotors so as to make an engagement gap constant to reduce a conductance of an engagement portion, thereby suppressing back diffusion and largely improving a compression ratio.