Chaotic Double Pendulum

I have always loved chaotic pendulums. I've seen many types of them. Some have 2 moving parts, some have 3, even more are possible.

I saw a version on YouTube that I liked very much. It like all others I've seen are one sided, offset from a wall or stand. I wanted to make one that was supported from both sides, to make it symmetrical and freestanding. A version that works as a sculpture even when static.

I designed this using Rhino. I've included the model in STEP format which can be imported into other CAD programs, should you desire to customize the design. Visual changes can be made as long as the sizes (and therefore weights) remain close to the same.

Important NOTE:

The hardware used is expensive. I used parts from McMaster-Carr because they have CAD files for all the parts they sell to use in your design. The hardware required comes to $150. The 4 ball bearings are $10 each. It would definitely be possible to modify the design using less expensive screws and bearings. Simple sleeve bushings would almost certainly work as well as the ball bearings. The only modification required to the main parts would be to change some hole sizes.

Less important note:

You will notice there are small differences between the drawings and the final object. I made some minor changes to the design after I completed the assembly to make a few things better. If in any doubt, go by the CAD model images.

Hardware, sourced from McMaster-Carr

www.mcmaster.com:

6381K412 Sleeve Bearing, 4

6381K415 Sleeve Bearing, 2

6381K416 Sleeve Bearing, 2

6384K52 Ball Bearing, 4

90015A340 Truss Head Screw, 2

90015A343 Truss Head Screw , 1

90015A349 Truss Head Screw, 2

90410A149 Thread-Cutting Screws, 4

99637A199 Binding Barrels, 2

99637A212 Binding Barrels, 1

99637A221 Binding Barrels, 2

Any combination of lead weights, large fishing weights or similar.

Wood glue and super glue

Wood finish of your choice

These parts can be made from nearly anything you choose. And they can be cut using any method that works. But I designed this with laser cutting and/or CNC in mind. I used a vendor to cut the parts, sendcutsend.com. They used laser cutting for the 1/4" thick parts and CNC for the 1/2" parts. That is why they look different in the photos. I liked the mix of clean machined edges and laser cut burned edges.

The wood parts are birch plywood. The plastic is clear acrylic, approximately .25" thick (sendcutsend.com offers .214).

Cutting the parts by hand with power tools is possible, but the holes would need to be drilled very precisely.

Included are drawings for all parts, in DXF and PDF formats.

Some parts are stacked and glued together. The 4 1/4" thick pieces that are the weighted end. And the 18 1/4" pieces that make up the "feet". These could be made of fewer parts of greater thickness but this is what worked for me.

You may notice a difference in the feet in the photos. I changed the design in a few areas after I built this. The drawings reflect the new assembly.

Glue all of the base parts together. I used a thin coat of wood glue plus a few drops of super glue. The moisture in the wood glue cures the super glue super fast.

Press the ball bearings and sleeves into place in the arm pieces. It will be a tight fit.

I used walnut oil. I keep it in the refrigerator because it is convenient. Walnut oil is one of the very few oils that harden on their own without additives, as nearly all finishes do.

Keep the finish from getting into the bearings.

Weight needs to be added to the end of the arm and under the base. In the arm use lead fishing weights. Don't worry about how much you use; it can be changed later.

Weight needs to be added under the base. Anything can be used. I glued 4 3 ounce fishing weights to the base. A steel plate could be used.

Assemble all of the parts. There is no particular order this needs to be done it. It may take a few attempts.

Try it out. Experience the chaos.

You may have an amount of weight in the end of the arm that works well on the first try. If not, just remove the weight holder and adjust. If the weights are loose, you'll have a kinetic rattle. If you would like it quiet, just put some caulking in with the weights.

Final thoughts.

Again, it might be well worth it to take the time to modify this to use less expensive hardware. I may do that myself if I make another one. I also would plan on using brass hardware just for appearance.

If you got this far in these instructions, thanks for your time!