For one of my introductory engineering courses, I and two other people have to design and build a robot capable of foward motion over various terrains using two servo motors (cyan). The catch is that the servo motors only output 180 degrees of motion, leaving us to try and figure out how to convert this into circular motion. The design I came up looks like a tank and uses gears (red) and ratcheting treads (yellow) to keep the robot moving foward on both cycles of the servo motion. The wheels attached to the gears (transparent dark orange) have to be semicircles or else they would coincide. I can get away with with that because the motors only rotate them 180 degrees anyway. There's more about it at http://www.creativemechatron ics.com

I modeled the design in Rhino and then exported for a pretty POV-Ray rendering to give my class a visualization of our project before we build it. The tread and the wheels coincide which is why the wheels bleed through the tread. I didn't bother taking the time to slightly offset the tread.