A screw is adapted to cooperate with a casing and at least one pinion-wheel for compressing or expanding a fluid within the space located between two screw threads, the casing and a tooth of the pinion-wheel. The casing and the crests of the threads carry two cooperating surfaces, and at least part of...http://www.google.com/patents/US4470777?utm_source=gb-gplus-sharePatent US4470777 - Volumetric machine with screw and pinion-wheels
