Patent ID: 7392643

Claim:
A lawn mower comprising: a cutter housing; a motive power source mounted on the cutter housing, the motive power source having a rotationally driven output shaft; a lower cutter blade attached rotatably inside the cutter housing and substantially horizontally to an end of the output shaft of the motive power source; an upper cutter blade attached rotatably inside the cutter housing and substantially horizontally to the output shaft above the lower cutter blade; and means for mounting the lower and upper cutter blades to the output shaft to enable angular adjustment of the two blades relative to each other about the output shaft, the means for mounting including arc-shaped holes formed in the upper cutter blade, fixing holes formed in the lower cutter blade and aligned with respective ones of the arc-shaped holes, and bolts extending through respective pairs of aligned arc-shaped holes and fixing holes to fasten the upper and lower cutter blades together; wherein the lower cutter blade has lower blade edge parts formed at leading edges thereof in the rotation direction for cutting grass and air-lifting parts formed at regions thereof behind the lower blade edge parts for creating a swirl current and a lift current, the upper cutter blade has upper blade edge parts formed at leading edges thereof in the rotation direction for cutting grass and straight clearance parts formed at regions thereof behind the upper blade edge parts to avoid interference with the air-lifting parts, is free of air-lifting parts at trailing edges thereof, and is set approximately at the same height as rear edges of the air-lifting parts of the lower cutter blade, the upper blade edge parts are advanced in phase and positioned forward a prescribed phase angle in the rotation direction with respect to respective lower blade edge parts, and the straight clearance parts of the upper cutter blade converge toward the respective upper blade edge parts so that the width of the upper cutter blade progressively reduces toward opposite ends thereof, the straight clearance parts intersecting with respective ones of the lower blade edge parts of the lower cutter blade when viewed in plan view.