Abstract
The endurance test of the gears designed for very high loading capacity showed some types of failures, that are very characteristic to such gears : Big influence of edge contact at tooth side and tip on the failure was observed. It is because relatively small amount of crowning and tip relief was given to lead form and profile form of those gears respectively in comparison with that of usual gears to make the load distribution over whole tooth Hank flat. Material condition and pitting cracks observed on the surface near side boundary of contacting area of tooth flank suggested that a cause of such failure is the repeated go and back contact-slipping between tooth side edge and mating tooth flank surface. The trochoidal interference appeared near tooth root of the test helical gears and plastic flow and white layer were found there. Chipping was observed at edges of tooth of smaller-face-width gear and micro cracks initiated from the chipping. Fatigue fracture proceeded from those cracks into the subsurface of gear tooth and led to tooth breakage. Most of those failures are not well recognized by gear designers until now and there is not yet any method to predict load carrying capacity of gears concerning such failure modes.
