Abstract: A pair of slotted, sector-shaped centrifugal weights 7, 8 are radially slidable on posts 5, 6 upstanding from a disk 4 mounted on the drive shaft 2 of a reversible electric motor 1. A cup member 11 carrying a pinion gear 13 is rotatably journalled on the shaft, and includes a cylindrical wall 11a surrounding the weights. A reduction gear mechanism 12 transmits the rotation of the pinion gear to a pivotally mounted sector gear 16, to which a linearly movable actuating rod 17 is connected. Alternatively, a plurality of sector-shaped weights 27 may be accomodated in similar shaped cavities between a pair of radially partitioned discs 24a, 24b.