The little screw is in the belt track on the "ring" that keeps coming off. Remove the belt from the "ring" and place the ring back on the motor shaft, then use a small screw driver to tighten the little screw. If the little screw is missing, you will have to replace it. Then put the belt back one. If it is too tight to put on easily, loosen the motor mount screw and replace the belt, then retighten the motor mount screw, with the belt not too tight, just firm.