I have a 15mmø shaft that sheared off at a roll-pin hole. The shaft is connected to the input drive with a shear-bolt that did not bread or even bend. In looking at the fractured edge there is a definite difference in the appearance of the two sides of the hole. It also looks like the metal yielded and twisted at the fracture based on the taper in the hole on both ends. Any thoughts on why this shaft would fail and not the shear bolt?