https://news.ycombinator.com/item?id=46039952
Wonder if I should braid my wired earphones for storage to prevent tangling. I can keep the cable inside a pouch with the earpieces out but that's not very satisfactory.
My current fascination knitted ropes/cables/cords. These are not the typical ropes that are spun and coiled and held together by friction. These ones made of synthetic fibres look like woven tubes, but the insides aren't hollow. The insides seem packed with more woven tubes.
What I really want to see though, are 3d knitted heavy duty carbon fibre flywheels of optimal shape such that it's under equal radial stress everywhere. The shape is interesting to compute for a solid one.
Regarding where can they be used... they are just batteries with a different form factor. You can put them on the grid to have some inertia and be a place to dump or extract energy spikes. They may be commercially viable if rare earth based batteries become very expensive. At a smaller scale one could use them as a mechanical UPS for a building/datacenter. Maybe even to power golf carts, not sure how well they will steer because of the angular momentum.
Even on the grid, batteries for sub-hour duration storage are cheap, as long as you place them at an already existing AC/DC converter site like a solar plant (or a modern internally-DC datacenter's centralized grid rectifier (AC/DC converter)).
Or even a HVDC transmission line.
Or a sufficiently modern aluminum/zinc smelter. Pretty much anything large enough to bother that has at least a boost-PFC on the input. Because with those you could just put them there, beef up the capacitor a bit or better yet, use a native 3-phase PFC that doesn't have strong 100/120 Hz ripple on that capacitor, and then literally just control the already there input transistors to do your grid jobs. If it was the very cheap low efficiency rectifier approach, it also needs the rectifier upgraded to be controlled, so just use it on the higher efficiency ones before upgrading the others. (It's like 1% efficiency difference on 240V, and 2% on 120V power supplies.)
There is no competition between batteries (low power density, high energy density, low storage cycle efficiency) and flywheels (high power density, low energy density, high storage cycle efficiency).
Flywheels (preferably levitated in vacuum) compete only with supercapacitors and superconducting rings (SMES = Superconducting Magnetic Energy Storage).
Supercapacitors/flywheels/SMES have their high-power applications, for which batteries are not appropriate.
Using them where batteries are the right solution is of course not a good choice.
It's rare that you actually have good reason not just through easily avoidable architectural choices to have dedicated 1~100 second range energy storage. Below that you can just use aluminium wet electrolytic capacitors, and above that you can just use [very] high power lithium ion batteries.
Inherently mechanical systems are different, of course.
That said, I love the idea of specifying and being able to knit in 3D. We just need a brilliant designer to come up with something that would be really great to have knit and can’t be knit with traditional techniques. And like six revs of the hardware for scale, tensioning, yarn size, etc.
Anyway - really cool.
In my other comment I suggested carbon fibre flywheels (for energy storage). A design that stresses the rotor uniformly to near it's breaking point would make a great storage device. If it's possible to add density to the fibres but without compromising strength, even better.
For a solid material with equal strength in all direction the optimal cross section is one with an exponentially decreasing thickness.
To give an intuitive reasoning, the more radially inwards you go there's is more material and velocity on the outside that's straining to break free, so you need larger cross-section to resist that. But now, this extra thickness too has to be supported as you move inwards. One can make this formal as a differential equation and the solution is an exponential profile.
Anyhow, for carbon fibres the optimal geometry will depend on the weave because a fibre has different strength along different directions.
There was also some research for using such flywheels for energy recovery in very heavy vehicles with electric motors, e.g. tanks with a turbo-electric generator, but the use in a vehicle has obvious difficulties. Even if the flywheels are paired, to avoid influencing the mobility of the vehicle, that still causes high internal stresses in the case holding the pair of flywheels when the vehicle rotates, which can lead to fatigue failures.
I chose it as my undergraduate project literally several decades ago.
3D woven ones might be stronger as they might resist laminar separation of circumferential layers more. Going by units, the product of stress and volume has the same units as kinetic energy. So it appears breaking stress and volume might be what limits the stored kinetic energy. This addresses doubt and curiosity raised by one comment (not yours).
Carbon fiber is typically woven in a simple fashion, to keep the strands straight because high tensile strength is the key.
But if it can be shown that knitted structures can preserve the tensile strength, that would be interesting indeed.
Think about the recent Titan submersible failure due to carbon fiber construction. What if instead of sheets of carbon fiber that could delaminate, you had a solid knitted carbon fiber shape? You might be able to demonstrate knitting that has more isotopic strength under both compressive and tensile loads.
That is one of the challenges that must be accepted by the solid knitting community… and maybe find a way that it doesn’t collapse on itself.
In addition to directly creating volumes of knitting, rather than sheets or surfaces, this also reduces constraints on stitch connection, since it can depart from the strict structure of alternating row passes.
The Lowell Offering may be in line with the work presented in this paper.
merelysounds•2mo ago
If you enjoyed this article, you might enjoy looking at the existing knitting machines, many are fascinating and very accessible. There are models powered by a hand crank[1], or with programmable patterns[2], or open source (open hardware).
[1]: https://en.wikipedia.org/wiki/Circular_knitting#/media/File%...
[2]: https://machineknitting.fandom.com/wiki/Silver_F370K
willvarfar•2mo ago
rausr•2mo ago
nicolailolansen•2mo ago
srean•2mo ago
jezzamon•2mo ago
estimator7292•2mo ago
I don't fully understand why, apparently most patterns require manipulating the yarn in a way that simply requires human dexterity?
MengerSponge•2mo ago
ludicrousdispla•2mo ago
>> https://www.changhua-knitting-machine.com/how-to-select-the-...