Oil-free screw compressor and method of manufacture

A method of manufacturing an oil-free screw compressor which includes a rotor casing, male and female rotors which have spiral profiles meshing with each other, bearing means for rotatably supporting said rotors in said rotor casing, driving means for driving one of said rotors, and timing gears through which the rotation of said one rotor is transmitted to the other rotor in a timed relationship, the compressor being operable without any cooling oil supplied to said rotors, the method comprising the steps of:

• preparing a semi-finished metallic female rotor which has a spiral profile, and a semi-finished metallic male rotor which has a spiral profile, and also preparing said rotor casing, said bearing means, said driving means and said timing gears;
• machining the surfaces of said semi-finished rotors into predetermined configurations;
• forming a metallic coating film containing particles of a grinding material on the surface of said semi-finished female rotor;
• forming, on the surface of said semi-finished male rotor, a coating film of a material softer than the metallic coating film on said female rotor;
• mounting the thus coated rotors on said bearing means so that said rotors are assembled in said rotor casing with a substantially constant spacing between the axes of said semi-finished rotors;
• mounting said timing gears on said rotors so as to drivingly connect said rotors to each other;
• and
• driving said rotors by said driving means while restraining a back lash in the rotating direction by said timing gears and applying a compression load to said rotors so that the coating film on said female rotor grinds and generates the coating film on the surface of said male rotor, thereby completing profiling of said male rotor and establishing a desired minimum gap between both rotors, and
• wherein said semi-finished male rotor has a deddendum circle diameter and a first pitch circle diameter less than said deddendum circle diameter, and said semi-finished female rotor has an addendum circle diameter and a second pitch circle diameter greater than the addendum circle diameter of said semi-finished female rotor, whereby relative slip motion takes place over entire regions of both semi-finished rotors when said semi-finished rotors are rotated in meshing engagement with each other.

An oil-free screw compressor comprising:

• a rotor casing;
• male and female rotors both housed in said rotor casing and having spiral profiles meshing with each other;
• bearing means rotatably supporting said rotors in said rotor casing;
• driving means for driving one of said rotors;
• timing gears mounted on said rotors so that the rotation of said one rotor is transmitted to the other rotor in a timed relationship;
• the compressor being of the type that is operated without any lubricating oil supplied to said rotors;
• a metallic coating film formed on a peripheral surface of one of said rotors, said metallic coating film containing particles of a grinding material;
• the other of said two rotors having an outer peripheral surface generated by said metallic coating film on said one rotor;
• said rotors being arranged in said rotor casing such that a desired minimum gap is established between said two rotors, and
• wherein the other rotor has a deddendum circle diameter and a first pitch circle diameter less than said deddendum circle diameter and said one rotor has an addendum circle diameter and a second pitch circle diameter greater than the addendum circle diameter of said one rotor.