Input: blue crank to which is fixed a small worm (1 start, lead: t1, pitch diameter: D1).
Output: large worm, 2 starts, lead: t2 = 2.t1, pitch diameter: D2 = 2.D1
The output moves t2 in 1 rev. of the input.
The small worm can be replaced with a cylinder of circular grooves or with a rack.