By a rewquest of Taif Shams. this tutorial deal with gears constraint
Step 1: Pitch circles
In each of gears, you must draw pitch circle. they will be used to set constraints.
Pitch circle is an imaginary circle that intersects the teeth of a gear at the actual points where
the teeth mesh with another gear: the pitch circles of two enmeshed gears are tangent.
Select pitch circles, Mate , mechanical mates, gear.
Write the ratio between picth diameters.
That is all.