Input: blue crank.
It is an external epicyclic gear mechanism of two satellite gears.
Zf/Zs = 3
Zf: tooth number of stationary grey gear.
Zs: tooth number of green gear.
Tooth number of yellow gear is arbitrary.
Points on the green gear crank trace various lines.
The red is an equilateral triangle of rounded vertices.when the gear of internal teeth is not preferable.