On Friday, April 3, 2015 at 8:48:59 AM UTC-5, Dave Nadler wrote:
On Friday, April 3, 2015 at 9:42:40 AM UTC-4, Steve Leonard wrote:
Well, crankshafts don't like to be loaded at their output location
other than along the axis of rotation. Put a belt reduction drive
on there, and you are applying load perpendicular to that.
Interesting dynamics happen with a two cylinder in-line engine
with this setup. It is not an easy system to design.

Steve Leonard

The failure is the prop hub (receiving end of belt reduction),
not at the crankshaft. Last round was classic fatigue - nice
crystalline structure on broken part.

Same sort of issue. Up and down loading on that shaft due to increasing and decreasing tension because of engine dynamics and the loading going in and out of phase with the prop being in low or high moment of inertia relative to the motion (prop horizontal, low moment of inertia relative to motion created by pushing up and down by the drive belt). Likely source of the fatigue failure. But, as stated before, these are complex systems with lots of interactions. Be interesting to know the crack propagation direction relative to the blades on the prop.