Gears are not always round. Non-circular gears were sketched by Leonardo da Vinci back in the 15th century. Such gears are designed to convert rotational speed in a nonconstant manner. They also look cool.
This project is a non-circular gears system generator: the central gear in orange has a certain shape, and all the other gears in red are built to accomodate it. All of them have a fixed rotation axis. You can manually add more gears with the left mouse button.