After the first weekend of building automata kits at the Sunday Streets SF, I decided to make another that was larger and more eye catching.

This version builds on the windmill which used a rubber band to transfer the motion of the crank to the axel. In this case there are two bands and two axels. By twisting the bands in opposite directions as they are connected to the axels, they spin in opposite directions.

At this size, the elasticity of the rubber bands and friction in the axels lead to varying speeds. It seems that the bands can stretch for a bit, building up enough energy to spin the axel, then they spin for a while before losing momentum. At first I was frustrated by this effect, but I could not solve it by using different sized bands. As I showed it around, folks seemed to like the variety… so there you go. It’s a feature, not a bug.

Detail of crank with one band on each side of the middle support.
The axel holding the inner disk is a 4mm pipe. Is goes from the front to the middle support and is driven by the orange band. The axel driving the outer disk is a 2mm rod that goes from front to back, passing through the pipe. Twisting the blue band in opposite direction as the orange reverses the motion of this axel.

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