PERSON WAS STANDING ON SHIELD BASE AT TAILGATE END OF SHEAR. THE RANGING ARM ON SHEAR WAS BEING USED TO LIFT TAIL DRIVE SPROCKET COVER WITH 5/16"" CHAIN. WHILE LIFTING COVER THE CHAIN BROKE AND STRUCK PERSON ON HARD HAT. HIS HAT WAS KNOCKED DOWN AND THE BILL OF HAT CUT HIS NOSE.