Overhung fan?
If the drive end bearing radial load is too light, rollers can skid and cause surface damage that can cascade into big trouble.
Specific and fairly unusual kinds of vibration monitoring can sort-of detect roller skidding pretty early.
3DDave said "Adding vibration and thermal monitors at all the bearings may be the only defense against unexpected shutdowns ....."
You need them. Temp sensors touching the outer race are way better for early problem detection than temp sensors in the oil sump or even the housing outer surface.
I'd expect that kind of monitoring with fast acting real time displays up in the control room would likely have set off alerts and alarms long before the problem was terminal.
"despite satisfactory oil level and condition. "
How is satisfactory oil level determined ?
If oil condition means oil analysis, oil analysis would have to be done very frequently to catch the beginning of this problem.
Like maybe every day.
Is oil pumped to each bearing?
Or is the large sump in the common housing filled up so the oil level reaches the lowest rollers?
Or some other completely different design?
Looks to me like that bearing got mighty hot.
Looks like the inner race cylindrical bores are clamp against shaft shoulders.
Axial expansion must be considered, and allowed. In simpler systems one bearing's outer race would be axially "fixed" and the other would have axial clearance between the outer race faces and the housing shoulders or cover pilot. The large housing that has both outer bearing seats and the cover pilots need features controlled to provide proper axial location and allow thermal expansion.
The lower housing half appears to have a shoulder on the inboard edge of the failed (drive end) bearing.
