Originally Posted by
noooby
Well said Concentric!
As a nobody who used to overhaul a lot of AH epicyclics, I find it difficult to believe that AH seem to think the gear will spall before the inner race. The inner race has the load concentrated in one area, whereas the gear is rotating, spreading the load.
If the gear really does spall before the inner race and then this spalling has the possibility of inducing a fatigue failure, then there is definitely something wrong with their gear design/metallurgy.
The inner race should be spalling well before anything shows up on the gear, producing lots of nice flakes to set off the appropriate alarm, before anything catastrophic happens.
I'm not saying it doesn't happen, but I never saw an epicyclic gear with spalling. The inner race would lunch itself and contaminate the gear, leaving it with a pitted look, but not spalling on the gear itself.
noooby,
Your point about a sector of the planet gear bearing inner race (which is fixed wrt radial loads) being subject to most fatigue cycles is correct. And normally one would expect this to be the most likely location for contact fatigue spalling to occur. But there can be other problems that would cause the planet gear to suffer the type of radial fracture propagating outward from a race surface spall like this example.
First, it is not possible to pre-load the type of spherical roller bearing used in this case. So there will normally be some amount of radial clearance present. If this radial clearance is excessive, the gear rim will be subject to greater reverse cyclic bending loads than it was designed for. Think of a rotating cylinder subject to opposing local inward radial loads as it gets squeezed between the ring and sun gear. The planet gear race surface will experience a tension/compression load cycle every 180 degrees of rotation, adding to the normal contact stresses. If the fatigue/fracture analysis of the planet gear did not consider a condition with excessive radial clearance in the bearing, this problem would not have been exposed. Normally, the analysis assumes the bearing is manufactured within certain tolerances.