Patent ID: 7226280
Filing Date: 2007-06-05
Classification: F04C

Abstract:
1. A Roots vacuum pump comprising a casing having an inlet port and an outlet port both formed therein and a pair of three-lobed rotors provided in the casing to be rotatable so that the inlet and outlet ports are prevented from communicating with each other, whereupon air is sucked through the inlet port and the sucked air is discharged through the outlet port, wherein the inlet port is located at a position n spaced by a positive displacement angle of 120° in one direction from a center of each rotational axis relative to an imaginary line m connecting the centers of the rotational axes of the respective rotors, and the outlet port is located at a position o spaced by a positive displacement angle of 120° in a direction opposite the direction of the inlet port from the center of each rotational axis relative to the imaginary line m, wherein outside air or cooling air feed port is provided at a position t on a peripheral wall of the casing obtained by returning by 90° from the position o to the inlet port side so that two closed spaces are defined by adjacent lobes of the rotors and an inner peripheral wall surface of the casing at the inlet and outlet port sides immediately after suction of air respectively, wherein the casing has a plurality of discharge grooves which are formed in a region of the inner peripheral wall surface so as to communicate with the outlet port, said region ranging from the position o to a position u obtained by returning by 45° from the position o to the inlet port side, the discharge grooves having a total volume ranging from 2% to 5% of a volume of one of the closed spaces.