R: pitch diameter of the fixed sun gear
r: pitch diameter of the planetary gear
k = R/r = 3
Loci of various points on the planetary gear are shown.
The red is for a point on the pitch circle of the planetary gear.
It is a special hypocycloid: deltoid. The two other loci are hypotrochoid.
1 cycle of the mechanism corresponds 1 revolution of the input crank.