The report provides a bit more context
https://www.gov.uk/aaib-reports/aaib...-ec175b-g-emeb
Since the LAGB had successfully run on a test cell for 12 hours without any deterioration detected in the alternator pinion bearings, with a 10 kVA alternator installed, it is likely that the installation of the 10 kVA alternator by the operator directly influenced the F1 bearing failure that occurred after 18 minutes of the post-maintenance ground run.
Examination of the failed F1 bearing and alternator pinion revealed that they had been subjected to a compressive axial load during the ground run. This axial load was greater than the ability of the F1 roller bearing to withstand it, leading to the bearing overheating and causing the PEEK bearing cage to melt, which was then extruded from the bearing. The overheating also caused significant wear of the bearing rollers, releasing bearing debris into the MGB oil system and causing discolouration of the MGB oil.
Testing conducted by the manufacturer showed that an excessive quantity of grease within the alternator shaft link cavity can create a significant compressive axial load on the alternator pinion when the alternator is clamped to the LAGB. This is due to compression of the excess grease and air within the sealed shaft link cavity acting as a hydraulic piston. This loading case was unintended and had not been anticipated when the LAGB components and associated AMM maintenance procedures were developed.
The method of attaching the alternator to the LAGB used by the operator’s LAE meant that the compression of the grease and air within the shaft link cavity was up to four times greater than would have been the case if the method specified in the AMM had been followed.
The manufacturer stated that the reason for filling the alternator pinion cavity with sufficient grease to cause it to overflow was to ensure that grease remained within the alternator pinion splined area during the in-service period between overhauls, to ensure lubrication of the splines.
This large quantity of grease, combined with the sealed design of the alternator shaft link cavity once the alternator driveshaft was inserted, created a latent condition in which an unwanted axial load could be introduced into the alternator pinion and F1 roller bearing..