EMPLOYEE WAS REPLACING GUARD ON TAIL PULLEY AFTER REMOVING STONE WHEN IT SPRANG BACK AND STRUCK HIM IN THE FACE, LEFT CHEEK, LACERATING THE SAME. GUARD WAS UNDER SPRING TENSION WHEN CLOSED TO ASSIST OPENING. THE HINGE SPRING MUST HAVE BOUND WHEN OPENED CAUSING IT TO SNAP BACK WHEN HE TOOK HIS HAND OFF THE GUARD TO RE-SECURE IT AFTER CLOSING.