Patent ID: 8425212

Claim:
A positive displacement flowmeter, comprising: a pair of helical gears in a casing thereof; wherein the flowmeter measures flow using rotations of the helical gears; wherein the pair of helical gears employ an oval pitch curve having two or less leaves as their reference rack tooth profile; wherein a moving radius of the oval pitch curve is expressed by ρ=a/(1−b cos nθ) (where “ρ” is the moving radius that is a distance from the center of rotation to the oval pitch curve, “a” is a similarity factor, “b” is a degree of flatness, “n” (n≦2) is number of leaves, and “θ” is an argument); wherein when the number of leaves n is n=2, the similarity factor “a”, the degree of flatness “b”, and a distance “g 0 ” are determined such that a tooth height ratio h is π m/4 (where “m” represents “tooth profile module”) in θ 0 +α 0 −tan −1 |−(1 −b cos nθ 0 )/ nb sin nθ 0 |=0 a=S 0 (1 −b cos nθ 0 )/sin θ 0 g 0 =a cos θ 0 /(1 −b cos nθ 0 ) h=a /(1 −b )− g 0 where a cutter pressure angle at a pitch point is “α 0 ”, an argument at the pitch point is “θ 0 ”, a ¼ reference pitch is “S 0 ”, a distance between a pole of the oval pitch curve and a pitch line is “g 0 ”, and the tooth height ratio is “h”.