EE WAS ON UNIT #221 (LOADER) TO LIFT THE GRAVITY TAKE UP WEIGHT SO MAINTENANCE COULD CHANGE A RETURN ROLLER. AFTER LIFTING THE WEIGHT HE WENT TO EXIT THE LOADER. AS HE EXITED THE LOADER THE RE TURN ROLLER FELL STRIKING HIM ON THE TOP OF HIS HARD HAT. HIS HARD HAT WAS DRIVEN INTO HIS HEAD.