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TriGears by Oskar van Deventer, and Bram Cohen via Jorn...

TriGears is a special puzzle. There are three gears set so that turning one gear turns the other two. If the three gears were in a flat plane they would, of course, jam. Oskar had the gears bevelled and set them at 60 degrees so that they all mesh in the middle. This apparently has no practical value and would not be a puzzle if Oskar had not made the teeth of the gears of varying width and of varying spacing. The object of the puzzle is to place the three gears in their holder such that they spin freely and do not jam. There is only one way to do this (see the solution). All other assemblies quickly jam. Once solved, the puzzle becomes a toy in that it is very satisfying to keep spinning the gears.

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