Tusi couple (Cardan) gear set

thingiverse

This is a simple demonstration of a [Tusi couple](https://en.wikipedia.org/wiki/Tusi_couple) with a two-gear set. Perhaps more commonly known as a specific case of a [Cardan gear](https://en.wikipedia.org/wiki/Gerolamo_Cardano#Other_contributions), as the inner gear moves inside the outer gear, any point (tooth) will trace a perfectly straight line across the outer gear's diameter. All parts can be printed together. The peg should fit snugly in the inner gear's center hole without requiring any adhesive. To really show off the effect, put a visible marking on one or more of the inner gear's teeth. You can also mark spots on the outer gear where this marked inner tooth makes contact to demonstrate that the contact points remain the same rotation after rotation.