Importance

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Abstract

A pair of co-operating screw rotors include a male rotor and a female rotor each having helically extending lobes and intermediate grooves which intermesh. The male rotor has an external radius R.sub.M and a pitch radius R.sub.MP defining a pitch circle C.sub.MP, and each lobe of the male rotor in a predetermined section has a leading lobe flank and a trailing lobe flank which are substantially convex. The predetermined section is perpendicular to an axis O.sub.M of the male rotor and an axis O.sub.F of the female rotor, and each lobe of the female rotor in the predetermined section has a leading lobe flank and a trailing lobe flank which are substantially concave. Each lobe of the male rotor and each lobe of the female rotor has an asymmetric profile in the predetermined section. Each of the leading flank and the trailing flank of at least one of the male rotor lobes has a male circular arc segment having a center of curvature A.sub.1M, A.sub.2M and a radius of curvature R.sub.1, R.sub.2. The center of curvature A.sub.1M, A.sub.2M is on the pitch circle C.sub.MP, and the radius of curvature R.sub.1, R.sub.2 deviates from a difference between the external radius R.sub.M of the male rotor and the pitch radius R.sub.MP of the male rotor. Each of the leading flank and trailing flank of at least one of the female rotor lobes has a female circular arc segment having a center of curvature A.sub.1F, A.sub.2F and the same radius of curvature R.sub.1, R.sub.2 as the male circular arc segment. The female circular arc segments cooperate with the male circular arc segments at a moment of contact therebetween.

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