Originally Posted by
Concentric
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.
AGMA 6123-B06 table 8 provides planet gear mesh design load factor recommendations based on numbers of planets, gear accuracy grade, application, mounting arrangement/flexibility, pitch line velocity, etc. The EC225 output stage would seem to fall under "application level 3" which recommends using a mesh load design factor of 1.30 for the "heaviest loaded planet". These are load factors used for design/analysis of the gear system, are based on accumulated industry experience, and tend to be conservative. However, obtaining even load sharing when you have a large number of planets (ie >4) and a very lightweight, flexible supporting structure, can be extremely difficult in practice.
Here is a good article on the
issue of planet gear load sharing analysis if you're interested in reading more about it.