One example of a robot using gears would be www.instructables.com/id/Connect-robotic-wheels-to-your-old-laptop/. Step 15 shows close-up photos of the gearing assembly that's used to drive the wheels on this robot.
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There are quite a few robot projects on the site, many of which will include gears, cams or both. Try searching for "robot" or "gear robot", and see if any of the results appeal to you.