Abstract: An actuating mechanism for imparting oscillatory motion to an output shaft journalled for pivotal movement about a fixed pivot axis. The actuating mechanism is adapted to be driven by a reversible motor means and comprises a power shaft carrying a drive worm, a worm gear in mesh with the drive worm and coupled to a drive shaft, and an articulated crank and link means coupling the drive shaft to the output shaft for translating rotation of the drive shaft into oscillation of the output shaft through an arc of predetermined angular size.The drive shaft is journalled in a slide means. A guide means supports the slide means for shiftable movement in a direction parallel to the longitudinal axis of the power shaft between first and second positions, the slide means being shiftable from one position to the other upon the direction of rotation of the motor being reversed.