Screw-type rotary machine having at least one rotor made of a plastics material

Claim 1

A screw-type rotor machine for the compression or expansion of an elastic fluid, wherein a working fluid exerts a certain pressure on lands of rotor members in the machine, comprising: a male rotor and a female rotor, each including helically extending lands and grooves formed between adjacent ones of said lands, wherein the female rotor comprises a metal core shaft and a plastics material having a modulus of elasticity of at most 25,000 N/mm 2 which material is injection molded over said core shaft, and the lands of the female rotor have a mean thickness corresponding to the modulus of elasticity of said plastics material so that the lands of the female rotor can deflect resiliently when operatively engaging the lands of said male rotor, but the lands of said female rotor do not deflect significantly solely as a result of the pressure exerted on said lands by the working fluid, said core shaft including helically extending lands formed to project into the bases of the female rotor land; and wherein said core shaft has concave surfaces formed between adjacent ones of the lands on the core shaft, so that said concave surfaces are spaced form the surface of the grooves formed in the molded plastics material of the female rotor by a distance substantially equal to the mean thickness of the rotor lands.