The strength of gears are generally calculated by considering the bending strength and surface durability. In the case of using gears under severe conditions, the scoring resistance is also considered.
This section introduces the formulas for strength calculation, based on excerpts from JGMA (Japan Gear Manufacturers Association’s Standards). For detailed information, please refer to the following JGMA Standards:

JGMA Standards
JGMA 401-01 : 1974 Bending Strength Formulas for Spur Gears and Helical Gears
JGMA 402-01 : 1975 Surface Durability Formulas for Spur Gears and Helical Gears
JGMA 403-01 :1976 Bending Strength Formulas for Bevel Gears
JGMA 404-01 : 1977 Surface Durability Formulas for Bevel Gears
JGMA 405-01 : 1978 The Strength Formulas for Worm Gears
Japan Gear Manufacturers Association
Kikai Shinko Kaikan Room No.208, 5-8, Shiba Koen
3-chome, Minato-ku, Tokyo Tel 03(3431)1871 or 1872

Related links:
Strength and Durability of Gears – A page of The ABC’s of Gears / Basic Guide – B