Rotary positive displacement machine for a compressible working fluid

Rotary positive displacement machine of hypocyclic bevel gearing type for a compressible working fluid, comprising an outer and an inner member provided with intermeshing spiral grooves and intervening lands where the number of grooves in the outer member is larger than that in the inner member with a difference therebetween of one and the wrap angle of each groove in the outer member exceeds 360.degree., said grooves and lands forming continuous sealing lines therebetween to define closed chambers between consecutive sealing lines, said members rolling on each other along pitch cones with coinciding apices, at least one of said members being rotatable around its axis and at least one being mounted for revolving oscillation around the apex point of the pitch cones, the circumscribing envelope of the inner member being shaped as a frustum of a cone, the outer member being shaped as a socket having an inscribing envelope in the form of a frustum of a cone and provided with open ends forming low pressure and high pressure ports for communication with stationary low pressure and high pressure channels, respectively, characterized in that the radial depth of the grooves varies axially along the members and in each transverse plane is equal to twice the eccentricity of the axes of the members and that the pitch angle of the spiral at the pitch cone varies continuously in the axial direction.