Hi there ZZ56, I sometimes drill a small indent in the shaft for the grub screw to lock into. Do not use red locktite as you will not get the grub screw loose again. Blue locktite will do the trick. When you drill the indent the drill must be small enough not to damage the treads where the grubscrew goes, better yet mark the spot,remove the coupling & then drill the indent. You will find some motor shafts are hardened & you may not be able to drill an indent, in which case you may be able to file or grind a flat on the shaft. Mick B.