PURPOSE: To prevent the contact of rotors due to load change from occuring by a method wherein the directions of helix of the concave and convex flutes of the rotors and of the teeth or tooth spaces of timing gears are made in the same direction and a special relationship is established between the helix angles of said flutes and teeth or tooth spaces at the captioned fluid machine equipped with a pair of male-female timing gears.

CONSTITUTION: The directions of helix of the rotor and the timing gear arranged in the same shaft or of the rotor 4 and the gear 9 and of the rotor 5 and the gear 10 are made same. When let the helix angle of the pitch circle diameter of the rotors 4 and 5 and the helix angle of the timing gears 9 and 10 be β1 and β2, the relation between β1 and β2 is regulated to satisfy 0.5β1≤β2≤β1. Owing to the relation as mentioned above, when only the female rotor 5 is moved rightward as indicated with the arrow, the axial movement of the timing gear 10 identical with said axial movement of the female rotor 5 causes the rotation of the timing gear 9 along the direction of the helix of the timing gear 9 and, because the directions of the helix with the helix angle β1 on the pitch circle diameter of the rotors 4 and 5 and of the helix with helix angle β2 of the timing gears 9 and 10, the rotation of the timing gear 9 causes the axial movement of the flute of the male rotor 4, resulting in maintaining the flute clearnce at the initial state all the time. Consequently, even if the axial vibration of the rotor occurs due to the load change during the running of the screw compressor, the flute clearance is not influenced and no fear of the occurrence of the contact of the rotors is resulted.