Abstract: Expansible chamber apparatus for a positive displacement, high volume, low friction, reversible rotary pump for gases and/or liquids. Two or more chambers are provided, defined in part by one or more small abutments that move radially in a slot to switch or redirect fluid flow with minimal loss of fluid mechanical energy. A complementary pair of chambers, formed by a single groove, is radially divided by a concentric land ring and is longitudinally segmented by the abutment, which seals the chambers against reverse leakage. The abutment is slightly smaller in radial dimension than the groove and is floated to avoid hard contact with the inner and outer land ring surfaces of the groove by a balancing of the Bernoulli effects that develop. The apparatus can also be operated as a fluid compressor, as a motor and in other applications, in single cycle or multiple cycle operation. The pump provides substantially non-pulsating fluid flow in one or more stages.