Screw rotor machine with spring and fluid biased balancing pistons

A screw rotor machine comprising a housing structure including a barrel portion having intersecting bores with coplanar axes forming a working space extending longitudinally of the barrel portion which has a high pressure end wall, the structure having a low pressure port communicating with one end of the working space the major portion of which port is located at one side of the plane of the axes of the intersecting bores and a high pressure port communicating with the other and high pressure end of the working space the major portion of which port is located at the oppsoite side of said plane, and male and female rotors rotatably mounted in the respective bores and having meshing helical lands and grooves with an effective wrap angle of less than 360.degree., the lands of the male rotor having substantially convexly curved flanks and intervening grooves the major portions of which are outside the pitch circle of the male rotor and the lands of the female rotor having substantially concavely curved flanks and intervening grooves the major portions of which lie inside the pitch circle of the female rotor, the lands and grooves of the rotors intermeshing to form with confronting portions of said housing structure chevron-shaped closed chambers each comprising a portion of the male rotor groove and a portion of the communicating female rotor groove, the chambers being defined at their base ends by the high pressure end wall of the working space and at their apex ends by the places of intermesh between the lands of the rotors, characterized by

• a plurality of stationary cavities in the housing structure;
• a plurality of substantially non-rotatable balancing pistons slideably mounted to respective ones of said stationary cavities in the housing structure and each balancing piston being associated with a respective rotor;
• pressure fluid means coupled to said stationary cavities for biassing said balancing pistons so as to impose a biassing force on their respective rotors to bias the respective rotors in a direction towards the high pressure end wall of the housing structure, at least a part of the said end wall comrpising a thrust bearing surface for the rotor;
• at least one spring mechanically biassing at least one of said balancing pistons in the same direction as said pressure fluid means;
• and
• a plurality of axial bearings, each connecting a respective rotor to its associated balancing piston, said bearings each including a non-rotatable part coupled to a respective balancing piston and supporting sealing means slideably engaging the inner wall of the respective stationary cavity.