Villard de Honnecourt's Perpetual Motion Machine
aka the 'Hammerwheel'
In conjunction with the AVISTA sessions on Villard de Honnecourt at the 2008 International Medieval Congress, Steven Walton and George Brooks made a reconstruction of Villard’s Hammerwheel.
The project both demonstrated how the idea could not have worked, but also highlighted what Villard likely was trying to convey when he drew his famous image. For more on this hammerwheel, see Steven A. Walton, "Villard's Perpetuum Mobile," AVISTA Froum Journal 18.1/2 (2008): 24-30.
A video of how the hammerwheel is actually a self-braking wheel, rather than a self-moving (or perpetual motion) wheel.
Article: Steven A. Walton, "Villard's Perpetuum Mobile," AVISTA Forum Journal 18 (2008): 24-30.