The One-One-string Tensegrity

How to levitate a table with a minimal number of passive components : magnets, strings only ?

This can be a challenge. It is, already, as today people compete to bring new designs for tensegrity tables using minimal number of strings...

Here I show you how to achieve a ONE string design - in ONE single segment - to levitate a table.

Building tensegrity tables is on the air today, you already find dozens of Instructables for this. Personally I built the excellent cardboard model designed by JoopB1 https://www.instructables.com/Levitating-Side-Tabl...

A tensegrity table is, roughly, a table "levitating" over 4 or 5 strings : one main string sustains the plateau, and 3 or 4 lateral strings prevent the plateau to shift or rotate away from this fragile equilibrium point... So my point is : what if you suppress the lateral threads and keep only ONE thread ?

I found a good starting point, an incredible one-string table by izzy swan on YouTube :

Brilliant !! Then let's try to improve. In izzy swan's table the string is decomposed in 3 segments. One middle segment for the suspension. One top and one bottom segments, prevent the plateau to shift or rotate away from equilibrium position.

I show you in this instructable how to build a similar design, but using ONE piece of string. If you're skeptical you're right : there is a trick. For the most stressed-ones, here's the walkthrough : In fact my design is quite identical to izzy swan's table (and as first steps you may follow his operating mode). At the end, I put two couples of neodymium magnets on top and bottom segments, and... when I'm ready, I cut the threads, that's all !!

The result is quite stable, but keep in mind that the table is built at a toy size (finding magnets strong enough to build a real table wouldn't be so cheap). It is stable enough to put light objects as you can see above...

Here I give the supplies for the model above. Upsizing implies the bought and manipulation of stronger magnets, use them at your own risks, i.e. for your wallet and your fingers too... For the wood, I used 3mm masonite (hardboard) wastes. If you start from scratch, consider 2 or 3 mm plywood or balsa sheet.

1- masonite 525 x 150 mm x 3mm sheet (x1)

2- neodymium 20 x 10 x 2 mm magnets (x 4)

3- M4 bolts : 15 mm screw and nut (x 8)

4- 10 mm washers for M4 (x16)

5- 0.3 mm nylon thread : 400 mm (x1)

Tools : scissors, saw, 1 and 4 mm drill, sandpaper or a small file, hot-glue gun, cyanoacrylate glue, two clips, pencil, ruler, two vises or four heavy books or any objects to wedge elements during assembly.

OK, at first please download the sketch below for the preparation of the parts. But the overall design is quite trivial from the annotated picture above.

1- Saw !! From the wood sheet, cut the following parts (solid lines in sketch) :

- Two 150x150 mm squares (one for the plateau and one for the foot).

- Two U-shaped arms 150mm height x 40 mm width x 75 mm depth.

- Two 40x50 mm rectangles, from there you get four 40x50 mm triangles.

- Mark on the foot, the plateau and the arms the places for gluing parts together (grey areas in sketch).

2- Drill !! Prepare plateau, arms and washers (refer to indications in sketch) :

- At 7 mm from each corner sides , drill a 4 mm hole (repeat eight times, for each corner of the two arms).

- Drill 2 x 1 mm holes inside eight of the washers (they will block the nylon thread in position). You may gently polish the holes, using sandpaper or a file.

- Drill a 1 mm hole at the centers of plateau and foot (they will ease the positionning for the nylon thread).

3- Time to glue :

- Well... see the sketch, look at the picture, or follow your own fantasy ;-)

4- Pre-assembly:

- Glue four washers centered on the four 4 mm holes of one arm. Repeat on the other arm. Washers must stand on the same side for each arm.

- Thread the screws through the eight 4 mm holes on the two arms, and glue them in position on each washer.

- Thread the nylon string through the washers : now this item is set as a washer braid.

- Now it is time to wedge firmly the foot and the plateau in position such as shown in the picture. For this you may use heavy objects, such as books, etc.

- Thread the washer braid extremities inside the 1 mm holes in the foot and the plateau. Thread the washers on the screws - to ease this step, you may stretch the braid gently.

- Arrange the pieces as in the picture, firmly stretch the string and check for the three intervals between arms to be equals. When finished, block the string in tension using two clips.

- Add 8 nuts on the screws. Maintaining tightly the elements in position, block firmly the screws : here you are for the pre-assembly !

5- The final cut :

- turn the pre-assembly 180 degrees along axis in order to show you now the side opposite to the nylon string (see picture)
- now put the four magnets two by two in position as in the picture, beware of your fingers. Notice that each couple must be oriented in attractive configuration. Again : beware !! At this step magnets are quite unstable and they easily rejoin their friends : even if they are quite small, these are really powerfull and my harm your fingertips if unattended. Wear security gloves if you are not used to this kind of work. The role of the nylon string here is to help you to maintain the assembly in correct position all through this delicate step !!

- when finished, cut both the top and bottom nylon strings ends : here you are...

Now put the table on its foot, and adjust slightly the magnets in position to center properly the equilibrium position for the plateau. Congratulations : you've done it !!

For those who wonder if it is possible to go on and replace the last string by magnets, the answer is... NO ! Hum... at least... theoretically !

So this project could be the minimalist levitating device you can desing using strings and magnets.

If you want to know why suppressing the last string is impossible, please, see the excellent instructable from mikhalchuk, on : https://www.instructables.com/How-to-Build-a-Magn... There you will learn about the Earnshaw theorem. It is an Electromagnetism theorem which demonstrates the impossibility to levitate any combination of magnets over other magnets.

But this theorem is true only for static magnets, i.e. for stationnary magnetic fields. It is not valid for variable magnetic fields encountered in other conditions such as rotating or moving magnets, diamagnetic materials, supraconductors, or actively controlled electromagnets (the basis for magnetic levitating devices).

If you want to improve this design, you should consider moving magnets TMHO. Anyway, it is quite weird to consider that this last piece of string, there, under this table, seems to be replaced only by a more complex device now ;-)