Originally Posted by
Concentric
Interesting article, thank you. The 225 planet carrier certainly looks like it has low enough stiffness to share the load well over 8 planets.
Just compare it against the industrial planet carrier on the cover photo of the article (pdf download version).
One significant drawback of making the pins and carrier 'flexible' is that it puts a greater share of planet load on the upper bearing race. That effect on the raceways could easily outweigh any benefit to the gears of having carrier/pin flexibility especially as the bearing load distribution becomes more unequal as torque increases.
Doubtless this will all have been factored into the original fatigue design(?). It should be evident in higher occurrence of spalling in the upper races than lower ones.
Concentric,
Load sharing is always a problem with any type of gear drive where torque is split into several parallel paths. The most recent example being problems with the torque-split arrangement used on the new CH-53K main rotor gearbox. The CH-53K MRGB uses a set of torsionally flexible quill shafts with spline teeth at each end that provide a very precise vernier-type angular index adjustment at assembly. In theory, this adjustment feature should compensate for any manufacturing tolerance errors and produce the required load distribution between the parallel gear paths. In practice, it proved more difficult than Sikorsky's gearbox designers anticipated.
Load distribution between epicyclic planet gears has also been a chronic problem with large wind turbine transmissions.
Here is an interesting tech paper from Timken that describes their flexible planet pin concept.