I agree, the difference in face width is not the only factor. But even the difference in Hertzian contact stress I believe may not be the only driving force behind this failure mechanism. Some bearings in wind turbine applications run up to 3 GPa. Normally fatigue damage is actually stress range raised to the power of 3, or higher depending on the S-N curve assumed for the damage calculation, so such a difference in Hertzian stress would produce even more contrasting results than we are seeing here. That approach would seem to apply more to dry rolling contact.
As I understand it the prevention or delay of damage in rolling/sliding contact is highly dependent on the ability to maintain a thin film of lubricant between the contacting surfaces, most significantly for ball or barrel roller bearings with their element of sliding within the Hertzian contact area. Without changing the bearing design or roller/race osculation ratios, the best way for a manufacturer to improve this would be a change of lubricant or introducing certain additives (some of which can be aggressive). That could significantly prolong surface contact fatigue life and delay the onset of spalling; producing an improvement for both FAG and SNR bearings similar to what we see in the reported statistics post 2009.
I am no expert but the profile of the crack running deeper into the material ahead of the travel of the rollers looks uncannily similar to that caused by lubricating fluid seepage into a crack with pressurization and possible entrapment effects. There are several recent research papers available
here and
here.
To put it in layman’s terms, this mechanism is similar to hydraulic fracturing (fracking) of highly compressed rock formations in shale gas recovery. Could it also overcome the residual compressive stress in the carburised raceway, even the higher compression on the FAG bearing?
So my big question is – did AH achieve that dramatic improvement in spalling statistics post-2009 by changing the lubricant to one better able to maintain that thin film but potentially also better able to seep into micro-defects and be pressurized behind the crack tip?
By reducing the degradation process of spalling which, at the time of certification was considered to be benign, could AH have removed a crucial indicator and (unwittingly) simultaneously introduced a lethal failure mechanism that leaves no trace?
I really hope I am wrong.