1994 ford club wagon van question?

the serpentine belt on my van keeps coming off and getting cut up by the fan blade. ive replace it 3 times and the same thing keeps happening. it only fits on way and i followed the diagram evertime. The only thing i noticed was that the fan pulley doesnt have grooves for the belt and where the belt sits it looks as the pulley has worn down allowing the belt to come off??... anybody that can help please input. thanks alot