Modifying the Involute Profile on a Spur Gear

The involute profile may be modified using a linear or parabolic deviation from the true involute profile.  The tooth form may be modified above and below the pitch line.  The modification always removes material.

The length of modification above the pitch line (APL) cannot be greater than the addendum.  The length of modification below the pitch line (BPL) cannot be greater than the dedendum.

The start (nearest the pitch line) of a parabolic modification is tangent to the involute.

For more information on profile modifications search the Internet for the technical publication NASA/TM-2000-210061.