Patent ID: 7497672

Claim:
A screw pump comprising: an upper housing having an inlet port for allowing fluid to be drawn therethrough into a rotor housing, wherein an outlet port is formed in the rotor housing for allowing the fluid to be delivered through the outlet port out of the rotor housing; and a pair of screw rotors rotatably disposed in the rotor housing in engagement with each other, an end face of the rotor adjacent to the inlet port being provided with an inlet opening, each rotor having a first portion whose lead angle changes, wherein the first portions and the rotor housing cooperate to form an inlet space which is in communication with the inlet port through the inlet openings for allowing the fluid to be drawn into the inlet space and whose volume is variable in accordance with the rotation of the rotors, wherein during the rotation of the rotors, the communication between the inlet space and the inlet port is blocked by the first portions and the rotor housing thereby to form a closed pump space adjacent to the inlet space, wherein when the communication between the inlet space and the inlet port is blocked to form the closed pump space, a position of the rotors is defined as a starting position of one turn of the rotors, wherein the inlet space changes its volume thereof and the volume of the inlet space becomes the maximum in the range from the starting position to less than one turn of the rotors, wherein volume of the closed pump space is set smaller than the maximum volume of the inlet space by setting the lead angle of the first portions, wherein a closure member is provided which covers at least a part of the inlet openings, and wherein the closure member closes the inlet space when the volume of the inlet space exceeds the volume of the closed pump space.