PURPOSE:To improve efficiency of a Root's blower, by arranging the relief clearance of a rotor by the specific method. CONSTITUTION:The secondary relief clearance of a rotor is determined corresponding to a crossing angle formed between the normal at a point on the periphery of a base circle of the rotor and the line connecting this point with the center of the rotor. For example, when the center line of twin-type both rotors and the major or minor axis of the rotor form an angle PSI of 0 deg., 90 deg., the normal (n) at a point on the periphery of the rotor crosses at an angle beta of 0 deg. with the line connecting this point with the center of the rotor, and the both rotors move almost not approaching together even if one of the rotors is built being dislocated with a phase error delta. However, the rotors move rapidly approaching at the time of angle where PSI=45 deg., thus the secondary relief clearance of the rotor is determined on the basis of a condition at this time.
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