| 摘要 |
PURPOSE:To prevent the air leak between a housing and a rotor by forming the part most remote from the revolution center of the rotor and the closest part into a circular arc form having each revolution center point as center. CONSTITUTION:As for the outer shape of a rotor 20, the part between the cross point (a) with a Y-axis and the point (b) is a small circular arc 22 having a radius (r) with the revolution center point O as center, and the angle theta is nearly equal to the angle alpha. The part between the points (b) and (c) is an envelope curve 24, and the point (c) is on the extension line drawn at the angle of about 45 deg. (beta) for the X axis from the point O. Further, the part between the points (c) and (d) is a circular arc 26 drawn with the radius qR and the position pR (point f) on the X axis similar in case of the Root's type in the prior art. The part between the points (d) and (e) is a large circular arc 28 having a radius of Rz. The angle alpha is within a range of 3-30 deg., and the value L for setting alpha within the range is previously set, and the following relation is generated: L/R<=1.2. When the outer shape of the rotor is formed in such a form, air leak between a housing and the rotor is prevented by the large circular arc 28, and the air leak between the rotors can be prevented by the small circular arc 22, and discharge efficiency can be improved.
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