Patent ID: 7484943

Claim:
A screw pump comprising: a housing having an inlet port at one end thereof and an outlet port at the other end thereof; and a pair of screw rotors disposed in the housing in engagement with each other, each rotor having a first portion whose lead angle decreases from an end on the rotor adjacent to the inlet port toward the outlet port, wherein the first portion and the housing form an inlet space and a pump space, wherein the inlet space is located at an end of the first portion adjacent to the inlet port and is in communication with the inlet port, wherein the pump space is closed adjacent to the inlet space, wherein winding angle of the first portion has a first region and a second region, wherein the first region is located in a predetermined range from the end on the first portion adjacent to the inlet port toward the outlet port, wherein the second region is located adjacent to the first region, wherein reduction rate of the lead angle of the first portion in the first region is set smaller than reduction rate of the lead angle of the first portion in the second region, and wherein a maximum lead angle of the first portion is set smaller than a maximum lead angle of a changing lead portion corresponding to the first portion whose lead angle decreases at a constant rate in the changing lead portion.