Importance

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Abstract

In a Roots vacuum pump, an inlet port is located at a position n spaced by a positive displacement angle of 120.degree. in one direction from a center of each rotational axis relative to an imaginary line m connecting rotor axes. An outlet port is located at a position o opposite to the inlet port relative to the line. An air feed port is formed at a position t on a casing wall obtained by returning by 90.degree. from the position o to the inlet port side so that two closed spaces are defined by adjacent rotor lobes and a casing inner wall at both port sides immediately after air suction respectively. The casing has discharge grooves in an area of the inner wall so as to communicate with the outlet port. The area ranges from the position o to a position u obtained by returning by 45.degree. from the position o to the inlet port side. The discharge grooves have a total volume ranging from 2% to 5% of a volume of one of the closed spaces.

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First Claim