12.3 Normal pressure angle

φn{αn} is the standard or generating pressure angle. For hobbed or rack-generated gears, it is the pressure angle of the tool. For helical gears, φn is measured on the generating pitch cylinder in the normal plane. φn is standardized to minimize tool inventory:




φn (deg.) Application



14.5 Low Noise
17.5
20 General Purpose
22.5
25 High load Capacity



Low pressure angle: Requires more pinion teeth (Np{z1}) to avoid undercut. Gives larger topland for same addendum modification coefficient.

High pressure angle: Allows fewer pinion teeth without undercut. Gives smaller topland for same addendum modification coefficient.