a damper has a plurality of blades movable between an open and a closed position in response to an operating / actuating means attached to the blades . a shaft rotatable about its axis has lever arm means attached to it and to said blades , to open and close said damper in response to rotation of said shaft for 180 degrees of travel . connector arm means are pivotally connected to crank arms and pivotally connected to said blades at pivot point locations such that the pivot points of said pivotal connections are over center of the plane of the axis of said shaft when the blades are in the open and closed positions . limit switches are provided to automatically stop the travel of the blades at fully open and fully closed positions .