Screw rotor machine having means for axially biasing at least one of the rotors

A helical screw rotor machine for compressing or expanding a pressure medium, said helical screw rotor machine comprising:

• a housing in which at least one rotor provided with trunnions is enclosed in a working space that includes an inlet port and an outlet port, wherein the working space is delimited by a low pressure end-section, a high pressure end-section and a barrel section extending between said end-sections, wherein the trunnions extend into bearings disposed in the end-sections, and wherein at least one of the trunnions extends through an associated end-section and presents an axially projected thrust surface in a delimited chamber which contains means for creating a force that acts axially on said thrust surface;
• and
• a casing closely fitted around said at least one trunnion, wherein the casing has a generally circular-cylindrical outer surface and is freely disposed in the chamber and has an outer end which is closed by a bottom wall having a center hole in a center portion, and wherein the casing is rotatably mounted on and axially displaceable along said at least one trunnion through a given distance between a first axial position in which the bottom wall is spaced from an end wall of the chamber and a second axial position in which the bottom wall is in abutment with said end wall;
• and
• a supply channel provided with a valve and extending from a pressure medium source, wherein said supply channel is connected to an opening in the end wall of the chamber opposite the center hole of the bottom wall of the casing for controlled delivery of the pressure medium into the casing via the center hole for moving the casing from said first axial position to said second axial position while creating an overpressure within the chamber.