Originally Posted by
JohnDixson
Are the planetaries replaced at overhaul, do they have a fatigue life, or are they on-condition?
Typical design practice with aircraft gearboxes is to design the flight critical power gears for unlimited fatigue life in bending at max continuous torque and something like L2 reliability. The reason for this is to avoid having a gear tooth fail from bending fatigue. The planet gears of a simple epicyclic present an especially difficult situation with regards to tooth bending fatigue, since the teeth are subjected to a full reverse bending load every 180deg of rotation.
However, the gear teeth are not designed for unlimited surface contact fatigue life. Gear tooth surface contact durability is similar to that of rolling element bearings. Surface pitting is the most likely failure mode, which progresses slowly and is easy to detect long before it presents a serious problem. The ability to readily detect surface pitting of gear teeth and bearing surfaces long before it becomes a problem is what allows an on-condition service life approach to be used.