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Thursday 16 August 2018

Tensegrity Prisms etc.

Diagrams from Hugh Kenner's Geodesic Math....

show a few tensegrity prisms with 3, 4 and 5 sides: not all that clearly!
My 1st prism based on Kenner's instuctions, 9" timber struts with string 2/3 times as long.... and
The same thing with my printed version from

printed rod ends with fibreglass rod. 

Setting up a model.

Peeling the model off the tin


Rod size

Model hole size, when you print, the whole model can be adjusted in size up or down to suit rods from about 7 - 10 mm.

The prisms can be imagined in a rolled-out form, it ends up a bit like a rope ladder without ropes joining rungs on the diagonals...

and then put into diagram form on 2d cad, the progression A to F shows fewer and fewer joining parts as the same rubber bands take the place of more tension members.
The struts started to look oversize on this model, so..
I made a different rod end.  This shows setting up a model on an empty plastic turps bottle....

with this result.
After a bit of tweaking, it looked like this,

and this,
and finally this!


For about a year and a half, I have been experimenting with 3d models and shapes, and somewhere along the line came across a reference to the late Hugh Kenner's book "Geodesic Math and How to Use It" (link is to google books).  My supervisor at Monash uni, Mark Richardson had incorporated geodesics and tensegrity into some designs for 3d printers and velomobiles, so even before hearing of Kenner's book I had some vague knowledge of geodesics and tensegrity, and their inventor, Buckminster Fuller.

So I thought I should know more, and lashed out and bought a copy (web link , but it may be possible to find pdf versions free online) and then put it away for a while, until my friend George came to visit to pick up a bike, and we somehow got talking about geodesics and I dug it out and showed it to him and said "I really should read it", and so I started.

But really the book calls for involvement, and on page 6 says the reader should start building models of the things. And so I did, but really, it was a bit tricky and fiddly using the recommended timber dowel and string, and I thought I could do better, so I sussed out some old hollow fibreglass tent poles from the shed, and measured them, and then designed some 3d printed rod ends to suit the diameter.  The resulting 3d printable file helps make tensegrity prisms but needs rods and rubber bands to complete them. The stl file is free and can be found free through Cgtrader here . Once you make a prism using the models, you realise that you can use fewer and fewer rubber bands to come up with the same result.

So results including an attempt to make a sort of system of reducing the number of rubber bands are shown above.  Eventually I started making models where the rods were clashing in the middle, so decided to redesign, this time using (back to the shed, which yes, has enough spare junk to build a moonrocket) 3mm or thereabouts solid fibreglass rod. The bottom 5 photos show the results, including (ta-da!) the wonderful application of holding a beer can.

This topic hasn't been exhausted by a long way, firstly, there are 166 more gobbledigook-filled pages of Mr. Kenner's book to plough through with models to build, and secondly, I have already diverged off with some rod-end inspired designs.  More next time, part 2 is here.


Steve Nurse

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