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Dumbbell Cube
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Two 3x3x3s connected with a complex mechanism. Every turned side is followed by one side of the other cube.

Dumbbell Cube is a challenge for intellectual weight lifters. This puzzle consists of two Rubik's Cubes that are connected through an intricate gear system. If you turn a face on one cube, the identically colored face at the other cube will follow. However, the connections between the two cubes have no symmetry. This makes solving this puzzle extremely difficult.

A lot of work has gone into the design of the gear train. The connecting rod has seven layers in total. The inner layer connects the two spherical cores. The outer layer connects the two opposite white faces of the two cubes. The other five layers have gears to connect specific faces. For logistical reasons, I dyed the five sets of gears red, orange, yellow, green and blue, matching with the colors of the faces. This way, I could keep track which gear drives which face.

The gears are so-called "double chevron gears", as invented by car maker Andre CitroŽn. This gear design has the advantage that the gears do not slip sideways and that they can handle a lot of force. They also generate less noise than regular spur gears because of their continuous meshing.
The gear trains work really well. They never jam or touch adjacent gears.

Oskar considered the first prototype (with the already choosen name "Barbell Cube") a failure because of to much friction. The Dumbbell Cube is more satisfying design.

The number of permutations for this puzzle is halve of the square of the number for the Rubiks Cube:
Size(Dumbbell Cube) = Size(Rubiks Cube)^2 /2
This can be explained as follows: The permutations of a Rubiks Cube can be divided into odd and even ones, regarding the sum of face orientations. Due to the connection the permutations of both 3x3x3s have to be odd or even. Regarding the remaining aspects the two permutation halves are independent.

There are several other variants of the Dumbbell Cube thinkable. The concept rules that the directly opposite inner two faces have to move connected. All other connections can be designed freely.
Without symmetric variants there are 22 different configurations. Three of them are worthless since the second cube will always be solved after the first is solved.
Edge length: 60 mm
Distance: 120 mm
Weight: 280 grams

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Contributors

Thank you to the following people for their assistance in helping collect the information on this page: Andreas Nortmann, Lawrence Cuthbert.

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