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Old 16th Jul 2017, 09:29
  #50 (permalink)  
Concentric
 
Join Date: Jun 2016
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Originally Posted by etudiant
Surely the way to resolve this issue is by running the power train under sustained stress.

The 225 gear box has a graceless failure mechanism which was unexpected.
That should be reproducible, given all the clues we have. Running the system full out for 6 months would be ample to hit all the life limits afaik.
What has been learned from such efforts or has nothing such been attempted?
Whilst some testing is appropriate and I would hope has been undertaken, I think just running the power train under sustained stress alone might not be the way to replicate the conditions that led to either of the 2 planet gear/bearing premature failures occurring. There are probably many H225 and L2 MGBs that have reached their full service life limit on planet gears without incident.

What you might have to start with is introducing a deliberate surface imperfection, then run the bearing under typical service conditions and examine it at periodical intervals to monitor the growth of any cracks and/or spalling. All that will tell you of course is the rate of growth, not what caused the process to begin by creating the initial imperfection.

This paper describes such a test programme by a well known bearing manufacturer. It also describes the different spalling growth behaviour of ball bearings to roller bearings. The planet gear spherical bearings would approximate to the ball bearing behaviour. Unfortunately the paper only tests a setup where the indentation exceeds the Hertzian width in the rolling direction. It would be interesting to find any testing where the indentation is smaller than say the Hertzian width at typical mid-flight loading.

In my opinion the key area to be investigated is how micro-pits came to be created on the outer raceway. Was it corrosion due to moisture? Was it indentation from fine (or not so fine) metallic particles suspended in the lubricant? Was it a failure of the lubricant to provide or maintain an elastohydrodynamic film between roller and raceway? Was it related to long-term storage rather than operation?

You are correct that the 225 gearbox has a graceless failure mechanism. Part of that is due to the architecture, which Airbus may be unable to change due to certification limitations.
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