AT 1030 A BEARING LOCKED UP ON USI SHUTTLE CONVEYOR. TO REMOVE BEARING NEEDED TO GET STONE OFF BELT, STARTED TO HOSE THE BELT HE WAS STANDING ON THE TAIL PULLEY GUARD. WHEN HE TURNED THE ON TH E HOSE IT KNOCKED HIM OFF BALANCE AND HE STEPPED OFF THE GUARD BACKWARDS AND TWISTED HIS LEFT KNEE.