Originally Posted by
riff_raff
Good point about the planet gear teeth being subject to a full reverse bending load cycle twice per rotation. The EC225 MGB epicyclic output stage has a fixed ring gear, sun gear input, and planet carrier output. The planet carrier is coupled to the main rotor mast, so the number of planet gear tooth bending load cycles per rotor rev would be the no. of ring gear teeth divided by the no. of planet gear teeth multiplied by two. However, I would point out that the gears used in helicopter MRGB's are typically designed for unlimited fatigue life in tooth bending. Since root fracture of a gear tooth due to bending fatigue is a failure mode you never want to occur.
Your later comment about how the magnitude of load cycles and when they occur during the component's lifespan can have different effects on fatigue life is also relevant. Here is a good explanation of the
Goodman relation.
I take it this means bent in one direction at the sun followed by bent in the other at the ring once per rev?