We covered Flickr user yamamo2′s (and his father’s) first high-performance double pendulum build back in 2009. The first version would swing for about 10 minutes, without added energy, after being started. The latest version swings for fully twice that, as the embedded video—all twenty-two minutes of it—thoroughly demonstrates. I haven’t seen any info about the design changes required to achieve this level of performance, unfortunately.
Make: Projects — Double Pendulum
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Wow, that reminds me of the old XSpringies toy from Linux.