A clock mechanism with a full set of gears, synchronized to mesh properly and operate the second, minute, and hour hand.
Pydrivers are used for maintaining proper gear relationships, both in speed ratios and phase (for gears to mesh properly).
Be sure to enable "Auto Run Python Scripts" in user preferences.
There is an empty object named "Speed", that rotates at a constant angular velocity, and the gear for the second-hand is driven by this object, etc.
For you mechanical engineers, I already know that the gears are not necessarily the proper shape for meshing, and that they don't mesh perfectly.
I get it.
Thanks to Michel J. Anders (varkenvarken) for the gear script (add_mesh_gear.py).
Here is the video: