The Alumavent 4000 Series standard performance, unsealed steel control damper is specially engineered with a rugged one-piece frame that locks together without bolts, screws or rivets that can vibrate loose. Triple-vee blade design provides maximum strenght. With its non-corrosive bearings, lock together frame and shake proof concealed linkage the 4000 series is very low maintenance and is an excellent choice for use in most low to medium velocity and pressure commercial applications. It is also ideal as a manual balancing damper.