Worm driving a servo actuator with spring return and rotary valve employing same

A motorized servo actuator has an output shaft rotated by a sector gear driven by a worm. The worm has a co-axially disposed torsion spring which is wound as the worm rotates. The worm is driven by a motor and gear train. In the event of motor failure during rotation of the sector gear in a given direction from an initial position, the wound torsion spring has sufficient stored energy to rotate the worm to return the sector gear to the initial position.