Blender 2.8 Tutorial: How to Design a Simple Gear Train with Coaxial Input and Output Shafts 

In this tutorial, you will learn how to design a gear train with a given gear ratio (1:12) and specific position of the output shaft relative to the input shaft (input and output axes coincide). A simple python script (shown below) is used to pick the optimal number of teeth for each of the involved gears based on the given constraints. Some neat tricks are used to turn a gear contour to a solid object without creating overlapping faces.
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The python script used in the tutorial to pick the number of teeth of all gears:
import bpy
from math import *
myList = []
for z1 in range(8, 50):
for z2 in range(8, 50):
for z3 in range(8, 50):
for z4 in range(8, 50):
if( z1 + z2 == z3 + z4 and (z2/z1) * (z4 / z3) == 12):
myList.append( [z1, z2, z3, z4] )
for a in myList:
print( a, max(a) )