HOIST WENT DOWN AT 4:30 PM. CALLED MAINTENANCE & ELECTRICIAN AND THEY FOUND A SLOW DOWN CAM WORN TO THE POINT THAT FLATTENED BRASS WAS STICKING OUT FAR ENOUGH TO CONTACT THE SWITCH WHICH SHUT THE HOIST DOWN. REPAIRS WERE MADE AND HOIST RETURNED TO SERVICE 6:45PM AND MSHA WAS CALLED.