Screw vacuum pump

1. 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.

2. A screw vacuum pump according to claim 1 , wherein said male rotor and said female rotor have the numbers of teeth different from each other in the perpendicular-to-axis cross-sectional shapes.

3. A screw vacuum pump according to claim 1 , wherein each of said male rotor and said female rotor has the perpendicular-to-axis cross-sectional shape which is formed so as to gradually increase its area as moving in an axial direction from a discharge side to a suction side.

4. 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

when a lead angle of said male and female rotors on a suction side is set to θ 1 , a lead angle thereof on a discharge side is set to θ 2 , and engagement gaps between said male and female rotors in the perpendicular-to-axis cross-sectional shapes on the suction side and the discharge side are set to L 1 and L 2 , respectively, the cross-sectional shapes are configured such that engagement gaps between said male and female rotors on the suction side and the discharge side satisfy L 1 sin θ 1 =L 2 sin θ 2 =a constant value.

5. A screw vacuum pump comprising a gas working chamber formed by a male rotor, a female rotor, and a stator, said male rotor and said female rotor comprising 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; and gas inlet and gas outlet ports provided at said stator so as to communicate with one end portion and the other end portion of said working chamber, respectively, wherein:

said unequal lead screw of one 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 and said unequal lead screw of the other of said male and female rotor has a perpendicular-to-axis cross-sectional shape that is constant regardless of the change in lead angle,

said male rotor and said female rotor have a gap therebetween in the perpendicular-to-axis cross-sectional shapes which are 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.

6. A screw vacuum pump according to claim 5 , wherein said male rotor and said female rotor have the numbers of teeth different from each other in the perpendicular-to-axis cross-sectional shapes.

7. A screw vacuum pump according to claim 5 , wherein said one of said male rotor and said female rotor is formed into the perpendicular-to-axis cross-sectional shape of is formed so as to gradually increase its area as moving in an axial direction from a discharge side to a suction side.

8. A screw vacuum pump according to claim 5 , wherein when a lead angle of said male and female rotors on a suction side is set to θ 1 , a lead angle thereof on a discharge side is set to θ 2 , and engagement gaps between said male and female rotors in the perpendicular-to-axis cross-sectional shapes on the suction side and the discharge side are set to L 1 and L 2 , respectively, the cross-sectional shapes are configured such that engagement gaps between said male and female rotors on the suction side and the discharge side satisfy L 1 sin θ 1 =L 2 sin θ 2 =a constant value.