A gear mechanism that was originally made to help illustrate an engineering textbook problem which stated:

Gear B (central purple gear) roates at twice the angular velocity of arm ABC (blue gear that holds the 2 outer purple gears), both rotate in the same direction. It asked to find the location of the instantaneous center of rotation of the two outer gears.

I decided to model the problem to illustrate it and there is some interesting motions that occur with this combination. It doesn't serve any functionality but it is interesting to look at and analyze its motion. The purpose of the two colors is that one color (purple) represents the 3 gears the problem describes, the second color (light blue) is the necessary, extra gears required to achieve the motion and velocities described in the problem.

There are only 2 gear sizes:

All small gears are 30mm pitch diameter.

All big gears are 60mm pitch diameter.

In addition you will need:

4 x M3x16mm screws

2 x M3x30mm screws

M3 washers (one between every connection to allow rotation)

Assembly can be a little tricky since at some point you will need to slide a washer between gears with everything else assembled and then secure it into place by sliding the screw through it.

The two longer screws go: one through gear ABC from behind, and the other through the driving gear from the front.

I hope you find this mechanism interesting and let me know if you have any issues printing it.