Device and method for purifying exhaust gases from motor vehicles. The wings are mounted on a hub which rotates in response to the flapping of the wings. The first rotation restricting portion restricts rotation of the cam ring by abutting contact with the first projection when the cam ring is rotated in a first direction, and the second rotation restricting portion restricts rotation of the cam ring by abutting contact with the second projection when the cam ring is rotated in a second direction opposite to the first direction. The first curved portion of the elongate shaft defines a first plane.