A shaft is journalled in a housing for turning movement about an axis; the shaft has a first portion and a second portion. A pendulum is fast with the first portion elongated radially of the axis for swinging movement about the same and a pair of anchor plates are mounted on the second portion axially spaced in parallel planes. The blades each have a first surface both of which face in one axial direction and a second surface which both face in the opposite axial direction. Each of the first surfaces is inclined to and has an edge portion which extends at least substantially radially of the axis and is convexly curved on a small radius in direction axially of the shaft towards the respectively associated second surface. A circumferentially toothed wheel is mounted and biased for rotation in a plane at least substantially normal to the planes of the anchor plates and parallels the axis so that consecutive teeth of the wheel engage first one of the first surfaces to slide along the same to and over the edge portion past the associated second surface while concomitantly imparting to the plates and thereby to the pendulum a swinging pulse and to subsequently abut against the other of the first surfaces where it again slides past the edge portion and second surface while the next-following tooth engages the first-mentioned first surface.