Worm-drive damper (A worm drive is a gear arrangement in which a worm(which is a gear in the form of a screw) meshes with a worm gear (which is similar in appearance to a spur gear). The two elements are also called the worm screw and wormwheel Center knob opens and closes the vent wings. In fully open position the vent wings are completely retracted allowing 100% unrestricted air flow Easy installation and long term durability / 360 rotation / Accepts 4" flex duct. Approximate size: Grille face diameter 5 1/2", Back diameter 4", Side: 2 Easy installation and long term durability / 360 rotation / Accepts 4" flex duct. Approximate size: Grille face diameter 5 1/2", Back diameter 4", Side: 2