Originally Posted by
riff_raff
It is indeed true that the bearings used in most helicopter gearboxes are manufactured by a vendor like SKF or Timken. But the bearings are made to a detailed specification supplied by the OEM.
The particular type of planet gear bearing shown in the example above is quite common with helicopter gearbox epicyclic stages. But one problem with these epicyclic stages that use more than 3 planet gears is their capability to evenly share loads between the planet gears. With the example of the EC225 output stage that used 8 planet gears, the load mismatch between the planet gears/bearings could be up to +/-25%. And the fatigue analysis for these planet gears/bearings would need to take this into account.
Would that +/-25% be for a brand new matched set or a mixture (after an overhaul) of new and ‘on-condition’ gears that may have used up some of their wear and fatigue lives?
In any single revolution, ovality of the sun gear or of the ring gear may affect the sharing of load over the set of planet gears. Could there be a numerical phenomenon whereby the number of gear teeth, the number of planets and the initial assembly position of the gears with respect to their individual maximum ovality axes could combine at the same position and repeat with more frequency (and reduced fatigue life) than say a similar module with 9 planets and corresponding differences in teeth numbers? It may sound a bit far fetched but it might explain how a gearbox could pass all the inspection checks at major overhaul yet fail relatively soon afterwards if that one gear repeatedly takes a higher share of loading than the others.
It shouldn’t be as difficult as cracking the Enigma code machine (another cluster of cogs) to work out if this can happen.
I still think the primary ‘cause’ will be found to be a manufacturing process producing an irregularity in the polycrystalline material but the design margins that should prevent this leading to failure do not appear to have been wide enough to compensate for manufacturing or operating margins.