Minimum bending radius for dynamic movement applications on wearables

Hello !

I’m back with a pretty tough question.
I’m working on a collapsible handfan in which I run cables between the blades to connect the LED. I have 2 ways to do this:

  • Run the cables at the top of the cable along the fabric/paper. This allows to hide them behind a second layer of fabric, but it means the cable folds with it, at a very low radius, which leads utimately to the destruction of the conductor inside (the cables have silicone insulation, which is fine)
  • Run cables freely which allows a much bigger bending radius when closing the fan, but then i have cables hanging freely that are easy to get entangled and it’s ugly

I thought the issue would be the strain relief on the LED strip PCB, but I encase everything in silicone and it works very fine, but this, I have no idea how to solve “in between” other than not allowing the handfan to be closed.

Anyone has recommendation on how to deal with silicone cables bending in a dynamic way (which means movement is frequent and repetitive). I guess people who do wearables are probably confronted to this, but have maybe more space to negotiate bending radius ?
I looked on internet and I see I’d have to go between 10 to 20 times the OD of the wire, which would be 10-20mm (impossible)…

Thank you !

While I don’t have much first-hand experience with this, I did learn a few things coming from a family that runs a wire harness business.

I think the three things you might look for in wire are:

  1. Higher strand count. Most silicon-insulated hobby wire already has a somewhat higher strand count (the first number in a X/Y strand designation), but note you can find wire with up to 168 individual strands in 22 AWG.
  2. A strand construction designation that indicates it’s rope construction with bunch stranded groups. It’s about how they’re twisted during extrusion and stranding, and it can help a bit with repeat bending.
  3. Mil-spec nickel-plated copper alloys (this is is different from tinned strands) are made for aerospace applications that shake a lot. Again, no first hand experience here, but I think you might be able to find some sold specifically for aerospace/automotive that could be good.

Hope this helps. Look forward to seeing a time-lapse video of your repeat-strain rig :slight_smile:

Here is a quick idea depending on how the fans are made.

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Do the cables have to connect the blades at the top? Could they connect the blades near the pivot point so the change in distance between connection points on adjacent blades would be much smaller?

Thank you @jeff , I will look at thes edetails when looking for wire. From what i see, the biggest factor will absolutely to maintain the highest bending radius no matter what.

@jayzonhe3 this is a pretty smart design, but that implies I design and build my own handfans from top to bottom including the frame, where currently i’m only retrofitting existing bamboo handfans with LED.

Indeed, thisis what I used to do, but it makes it very hard to close the fan due to the added thickness… Not as nice.

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So, could you use carefully insulated copper tape strips, or copper sheet to run the power/ground portions of circuit lower down the fan blades past the pivot point, then attach the power wires to each other just below the bottoms of the blades? The signal can be tiny enough to go anywhere without getting in the way, but the power wires would be less obtrusive nearer the pivot. I’ve seen an LED fan made where the blades themselves were PCBs, which gives you a lot more flexibility as to where the wires connect, though you said that was a route you were trying to avoid.

Or possibly, if you were to 3D print your own fan blades, include recesses to run the power wires down to near the pivot for protection and to get them out of the way.

Great point! The power/ground and signal don’t have to be routed together. If you put the power/ground on the left/right of the short end of the blades, then the wires could be wider loops and serve as fancy decorations to the bottom of the fan. Come to think of it, the signal wires could also extend above the long end of the blades. When the fan is open, they straighten out, but when the fan is closed, they become wider loops, avoiding tight corners.

I see what you mean @sorceror and @GeekMomProjects , but cables running free get entangled into people when moving around whith the fan closed for example. That’s why I was going for a full hiding the cables under a second layer of the fan paper.

I guess I have to weight the pros and cons of every build. Still a work in progress ! Thank you

somehow this insane speaker with rotating slip ring reminded me of your quandary.

If somebody has made this insane thing, and it’s still working, then a reliable LED fan must be possible!