 Lead is the axial advance of a helix gear tooth during one complete turn (360°), that is, the "Lead" is the axial travel (length along the axle) for one single complete helical revolution about the pitch diameter of the gear.Lead angle is 90° to the helix angle between the helical tooth face and an equivalent spur tooth face.For the same lead, the "lead angle" is larger for smaller gear diameters.