Worm 1 rotates about fixed axis A and meshes with worm wheel 5 which rotates about axis B-B. Worm wheels 5, 6 and 7 are keyed to shaft 8 which runs in the bearings of yoke 4. The yoke turns freely about axis A. When yoke 4 is turned, either worm wheel 6 meshes with worm 3, or wheel 7 meshes with worm 2. When engaged by their corresponding wheel, worms 2 and 3 rotate about fixed axes C and D. Rotation can be transmitted from either worm wheel 6 or 7 to worm 3 or 2 only if the worms have sufficiently large lead angles.
$2821$WG,ML$