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I'm interested in making a "tensegrity" coffee table shaped like this: enter image description here That is, two rigid (in my case wooden) frames connected by five prestressed tensioned cables (chains in the image) with a slab on top.

My question is how such a table could be made height-adjustable (manually with a hand crank or automatic, should that make things easier somehow). Naturally the range of height adjustment wouldn't be big, something like 30cm. Having increments (even just at the two extremes) would be fine.

I found this video of a Lego prototype using roller wheels. Such a system of two loops of cables with roller wheels plus a hand crank reel is the only solution I can think of, though quite intricate and I'm unsure how well it would scale.

I'm interested in whether there are other parts or mechanisms that can be used to essentially lengthen or shorten tensioned cables synchronously (note that to raise the top part, the four cables in the corner need to lengthen while the one in the middle needs to shorten).

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Seems like there could be many ways of approaching this somewhat complex design problem.

The simplest thing that comes to mind is having spools that are wound in one direction for the lifting wire(s) and the opposite direction for the tension wires all driven by one crankshaft. Then as one set of spools are taking up the other spools will release.

The question then is what is the ratio between the lifting and the tension wires as the table moves, is it 1:1 or some other relation? That would depend on the tensegrity design; how many pulleys between the spool and the anchored end. Changing the core size on the spools would be a way to control that.

Also keep in mind that the lifting wire and its armature is doing the work of four table legs and then some (to maintain the tension on the control wires). That is going to require some extra strength for those components.

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That's an impressive prototype in the video.

The most important aspect of your table is to avoid a design which can be back-driven. To that end, consider a worm gear drive system.

The video creator calls the drive wheel by the term turnstile, but it's a capstan. Note that there's a solid axle connecting one side to the other. This is a good way to motorize your table without running into side-to-side synchronization problems.

A quick search presents a wide variety of these motors, including some with speed controllers, which would be pretty important for safety reasons.

The above link points to Amazon, image below:

worm gear motor

Amazon also carries stainless wire rope with 368 pound breaking strength that would be flexible enough to match the design in the video. Using flexible wire rope would permit scaling without complications, in my opinion. Use large diameter pulleys to reduce metal fatigue. If you require additional strength, increase diameter of the wire rope and the diameter of the pulleys as well.

Image from linked site:

1/16" wire rope

Note in the image below the cross section of the wire rope. More strands means more flexibility. If you upscale to 1/8" diameter, aim for the highest strand count you can find.

I've used both diameters, the smaller for control cables of an ultralight aircraft, with the larger diameter used for internal wing frame bracing. For optimum strength, use cable thimbles at the attachment points and use appropriate swages and a swaging tool to secure the thimbles. The swages are the small items in the photo of the spool of cable. Tools are priced from under US$20 for the small hand held, wrench compressed version, to hundreds of dollars for powered versions. Cable swaging tool and wire rope crimping tool are suitable search terms.

wire rope cross section

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  • A hand crank with a ratchet such as those commonly found on boat trailers would take care of any backwards slippage without involving motors (and power supply, and control unit...) But either way this doesn't answer the question.
    – rebusB
    Mar 27 at 16:59

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