The angle at which the tooth contact force acts relative to the direction of motion at the pitch point, fixed by the tooth form of the generating rack. Most gearing uses 20 degrees, with 14.5 degrees on many inch-system machine tool gears and 22.5 or 25 degrees used for higher bending strength. A larger pressure angle gives thicker roots and less undercut but more bearing load and lower contact ratio; mating gears must share the same value.
Pressure angle on that old gear is 14 and a half, it'll never run right against a 20 degree pinion even if the DP matches.
Replacement gears for old machine tools are often 14.5 degree; check before ordering a 20 degree stock gear that looks the same.
Also heard as
- PA
- profile angle
- alpha
Related terms
Involute
The curve traced by the end of a taut string unwound from a circle, the shape of almost every gear tooth flank.
Base circle
The circle from which the involute tooth profile is unwound, smaller than the pitch circle.
Contact ratio
The average number of tooth pairs in contact at once, which largely sets how smoothly and quietly a pair runs.
Undercut
A notch cut into the base of small-pinion teeth when the generating tool removes part of the working involute.