225 cleared to fly in UK & Norway

[Concentric]

Originally Posted by etudiant

Would it be pointless to run some of these gearboxes to destruction?
Documenting the failure paths in order to provide some improved understanding of these disasters. Perhaps we might even learn something new.

Whilst the desire to recreate actual operating conditions inside a gearbox as faithfully as possible might at first suggest a whole-gearbox test, if that test were taken to failure it would not only be expensive and potentially hazardous but the inertia of the machinery would likely cause significant secondary and collateral damage that could obscure inspection of failed components, fracture surfaces and detection of certain compounds.

Since it is known exactly which component failed (the 2nd stage planet gear/bearing race) it is more scientific to conduct tests on examples of just that component in isolation and simulate the operating conditions.

A test rig used to examine propagation of rolling contact fatigue cracks in bearing steel is shown in this technical paper. Whilst that test rig is designed for external bearing surface test pieces (because it is easier, for one thing) it should be possible to design a rig to test internal surfaces too although space would be much more restrictive.

Rather than just testing to destruction, such a rig allows tests to be paused and the test piece removed for examination at intervals to measure and chart growth of the defect.

In the above test the focus is mainly on crack initiation at or close to the surface and is a result of cyclic contact stresses, often increased in magnitude (up to 4.76GPa) to accelerate the fatigue process. Whilst lubrication is provided in this test it is considered only at the surface.

There are many other technical papers available which investigate lubrication effects not only at the bearing surface but also with seeping into and pressurization inside a crack. This paper (also available here) compared different lubricant additives with the same synthetic base oil to investigate their effects on lubricant seepage into a developing crack. Not wishing to get too technical, it should be clear from a brief scan of these papers that the effect of a lubricant is far more complex than just some oily stuff that you either have in your gearbox and are OK with, or have lost and are (or very soon will be) in trouble without.

Lubricants work mechanically, hydraulically, thermodynamically and chemically. Lubrication is a science in itself and I find it very surprising that it receives little or no mention in investigation reports to date.

I previously queried whether a dramatic reduction in AH statistics of spalling or pitting circa 2009 may have been due to some change in lubrication/lubricants. Having since read up a little on tribology and the different effects of some lubricant additives on the surface (mostly beneficial) and inside cracks (sometimes detrimental) I would not be at all surprised if the next report investigates this area in considerable depth.