Vibration Loosening.
A Little Blue Loctite on the grub screw and even a little on the motor shaft helps keep things tight and running right.
Be Sure to get the Blue, You don't want the Red Loctite.
Red Loctite requires alot of heat to be removed. The Red holds so strong, you will strip the grub screw before it will loosen.
The Blue is made to be removeable. A good choice.
