High-cycle gear tooth bending fatigue lives of spur gears under fully-reversed and fully-released loading conditions are compared in this experimental study. The experimental methodology described in an earlier publication, (Hong et al, 2020), is employed to perform two sets of rotating, gear tooth bending fatigue tests. Statistical analyses are performed to regress stress versus life (S-N) curves under both loading conditions. These curves indicate that a gear under fully-reversed loads has a shorter bending life at the same maximum tooth root stress as a gear under fully-released loads. Various planetary gear set kinematic conditions with different stationary members are considered to determine the equivalent number of tooth loading cycles per revolution of the sun gear. They are combined with established S-N curves under both loading conditions to determine the ratios of allowable maximum tooth root stresses amongst the gear components of a P-planet gear set such that each gear in the set has the same bending fatigue life. A “stress-balanced” gear set designed to these stress ratios is expected to have the same bending fatigue life for its sun, ring and planet gears, ensuring that the planetary gear set life is the longest.

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