Truth is that it's a crappy design. The nut has to be tightened to 50 lbft (70 Nm), otherwise the momentum of the core could twist out the nut. But the shaft is soft, so you can't just wail into it with an impact wrench or it will fret and loosen. A dab of low strength thread locker and a new lock washer wouldn't hurt. Or use a decoupling pulley. But put the JB Weld in the box with your duct tape, it's not going to help anything.