WojtekSz
Of course sensible. This is the very mechanism by which EASA and FAA (by reference) alerted the operators would (could likely) cause catastrophe. The Splines serve at an intermediate joint. By definition this joint has clearances, else it would not allow rotation. Excessive wear of the Splines, detailed minutely by the regulator (via Rolls), allow distress at the coupling. The loss of radial commitment due scrubbed splines is the mechanism of failure. But the wear is caused by something. No sane team would field a system of extreme importance that had disqualifying levels of failure at an interval of 400 cycles since new. Yet this is exactly what was encountered by dozens of supplied powerplants to Airbus.
The IP aft shaft and fore shaft create a perfect environment for damage through vibration of their Splines. Any vibration here, at the Thrust Bearings, in close proximity to the HP shaft and LP shaft is amplified by the joint itself, with elements of whip, and bind. If this is the case, wear would be rapid, and progressively worse through the passage of time. The more the wear, the faster it wears. At the end of the sequence, the Splines are 'immune' from a 400 cycle inspection. It would seem to be more prudent to lessen the interval for borescope as the engine's life exceeds its first inspection limit at shop visit. Perhaps it was not a misprint that the "C" Mod was to have its first borescope at 200 cycles, then only 100 after that. So, 300 cycles and then what?? The "D" ?? It cannot happen that way, it is an admission that this engine is being designed on the "fly", with passengers aboard.
The Roller bearings at the end of the IP shaft end up being the area of least expression of wear, their purpose is radial resistance to stress. It is in this dimension that the failure mode at the Drive Arm presents, hence less likelihood for failure here. The Roller bearings run in a smoothe channel, their axial movement appears to be snubbed by their cage. It isn't farfetched to think that since the Axial drift is snubbed at the Ball bearings, no provision to stop the rollers from drifting was supplied. If the vibration at the LP and IP Shaft area is the problem, the Ball Bearings need to resist not only thrust, but radial and also divergent (chaotic) stress. This is obviously of concern. Instead of a conforming stress to the bearings as designed, each ball is subjected to changing stressors, independently of the others. Wear becomes random and irregular, hastening by a great deal the failure of the bearing itself.