Input: outer cam (in glass).
Output: inner cam (in blue).
They are coaxial.
The slider (in red glass) slides in a stationary runway.
Yellow pin is fixed to the slider and moves along helical grooves of the outer and inner cams. It is possible to use two yellow pins and sliders arranged symmetrically.
The inner cam consists of six V-shaped portions (N = 6).
The outer cam consists of only one V-shaped portions.
So the output rotates 6 times slower than the input.
Increase N for greater transmission ratio.
The ouput rotary direction may differ from the input one subject to relative position between the input and output shafts at their initial positions.
Weakness of this mechanism: unstable positions when the pin is at the corners of each groove. The output inertia helps to overcome them. So at start position the yellow pin must not be at such unstable positions. Adding a device to avoid that is necessary.