Sheared alternator shaft

[GJ Humphrey]

The rear alternator shaft in my 1973 T337G sheared.

Fortunately, no metal seems missing that might have gone into the engine, but we're draining the oil and will check the filter carefully.

The ring gear inside the engine accessory case that drives the alternator shaft appears unharmed.

The rotor turns freely and there seems to be no resistance in the bearings.

The coupling (Contitnental type) did not shear , rather the steel shaft sheared. It's a very hefty shaft about .5 inch in diameter. Does anyone know what could cause the shaft to shear and not the rubber coupling? Again, there is no resistance to turning the rotor.