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Torque Cord

A lightweight rope that passes on torque without winding on itself
  [vote for,

Used for tethering high altitude kite wind generators and passing the power from them to ground.

A Kevlar type rope, which when rope end is twisted, the torque is passed onward, without the rope winding and deforming from its elongated shape.

Some halfbaked ideas on how to achieve this: The rope "envelope" is, a wound thread with a more "stiff" internal section, having straight fibers, or fibers wound in contrary direction. (This of course will work only winding one way).

Perhaps also when the torque is applied to the rope, that section of the rope, receiving the torque, hardens, losing its plasticity, and behaving as solid rod. As torque is passed down, different sections of the rope become rigid, while when losing torque the rigidity is released and the rope becomes elastic again.

A last solution is to have a chain of rods, with the links locking into postion when torque is applied.

pashute, Nov 03 2012


       The problem is that you just can't get very high torque resistance in a flexible cord.   

       The reason has to do with the interplay between twist and writhe. Twist is the rotation of the cord around its central axis (as you'd expect). Writhe is the tendency of the cord to throw itself into a helix.   

       If the cord is stretched, and is fairly flexible, then the torque will go into twist.   

       However, if the cord is stiff against twisting, and if it is not stretched tight, then the torque will cause it to writhe instead. Simplest example is a garden hose: when you apply torque, it doesn't twist much but it does writhe, forming broad coils.   

       So, suppose we create a cord which is very stiff against twist (for instance, a series of metal segments linked by universal joints which can't twist. It will only transit torque well if you can stop it writhing, and the only way to stop it writhing is to keep it very taut.   

       But a cord which is kept very taut isn't much use as a cord - you might as well use a single rod.   

       One solution is to encase the flexible cord in a rigid sheath, to prevent it writhing. But if the sheath is rigid, again, what's the point of having a cord inside it?
MaxwellBuchanan, Nov 03 2012

       The last solution is the most feasible; a 'rope' of braided and interlocked chains will twist and deform (shorten) only a certain amount before the links are compressed as far as they'll go. The problem then is how to monitor and regulate torque along the entire length of the rope. If too much torque is placed unevenly on the rope (and the possiblity of this increases dramatically the longer you try to make the rope), the links in that section will bind and shortly thereafter will fail altogether. Once a single link snaps, the torque upon the entire line will concentrate at that point and the rope will break unless the torque is immediately released. Even you do that, releasing it too fast will shock the line, which will also cause the rope to snap at the weak point.
Alterother, Nov 03 2012

       //a 'rope' of braided and interlocked chains will twist and deform (shorten) only a certain amount before the links are compressed as far as they'll go.//   

       Not quite. What will happen is that higher levels of writhe will be produced. As you start applying torque, the 'rope' will first twist by a certain amount (maybe not much). Then it will throw itself into a first-order helix. This will reach a limit once the successive turns of the helix are touching eachother, or are otherwise 'locked'. At this point, it can transmit more torque, but that's because it has effectively become a much wider 'rope'.   

       If this rope of first order helices is long relative to its overall width, then it will throw itself into second-order helices.   

       This iteration can continue ad infinitum. At each stage, the 'rope' will take a certain (and progressively greater) amount of torque before throwing itself into helices which, when fully formed, result in a new, fatter effective rope.   

       If you've ever had a rubber-band-powered model plane, you can see the rubber first twisting, then forming regions of first-order helices which grow until they are contiguous. Then if you continue winding, the rubber will form areas of second- order helices.
MaxwellBuchanan, Nov 03 2012

       True; that's a far more clear and eloquent explanation of what I meant when I said that torque regulation at the point of full compression is necessary to prevent binding.   

       I'm assuming, for the sake of simplicity, a taut line without any contact points between the generator and the receiver, and envisioning a layered interlocking braided chain, sort of like a snake made entirely of chain mail, constructed with each layer braided in a perpendicular orientation to the adjacent layers so that winding clockwise will immediately start link compression widdershins, or vice-versa. Obviously assymetrical loading (even that caused by the weight of the line itself) and line shock will be huge prohibitive factors in the design.   

       I'm approaching this as a rigger, not as a physicist, and what we're trying to do here is exactly what riggers try to avoid.
Alterother, Nov 03 2012

       That's kind of what I'm getting at, only made from links instead of braided wire.
Alterother, Nov 03 2012

       [MB] is it possible to engineer a chain such that it forms into a much stiffer first order helix when torque is applied? That way, it can be a flexible cord, but can stiffen a lot when it needs to bear torque, and can instantly unstiffen again when the torque is relaxed.   

       I'm thinking a certain shape and profile of chain links that minimises the reduction in length and maximises the increased stiffness.
pocmloc, Nov 03 2012

       The two are mutually exclusive. What you sacrifice in length reduction you gain in strength.   

       If you'll permit me to answer a question addressed to another, this is exactly what I'm trying to describe, only it would have to be made of several layers of braided tesselating chain, otherwise one broken link severs the line and expensive hilarity ensues. M'lud Maxwell, for his part, is patiently explaining to me that I am wrong.
Alterother, Nov 03 2012

       //Maybe something along the lines of a large version of an old-fashioned speedometer cable ?// It (ie the whole thing, jacket and all) will still writhe. You'd do better to have a single cable as thick as the jacket of the speedo cable, in terms of torqueability.   

       //is it possible to engineer a chain such that it forms into a much stiffer first order helix when torque is applied? // Yes, I suppose it is. You'll get a much stiffer chain either when the first order helix jams against itself (ie, it "saturates"), or by limiting the amount of bend at each link so that the thing locks up after a certain amount of writhe.   

       But that doesn't really help, for two reasons. First, the first order helix will be no stiffer in torque (and no more flexible along its axis) than if you'd just built a spiral "rope" out of the same material (ie, the helix-forming is simply part of the process of fabricating the final structure).   

       Second, the rope has to get shorter as it forms the first-order helix. This can only happen if the rope was over-long to begin with, or if you reel in the thing it's attached to.   

       Overall, if you design your rope/chain thing with the intention of having it form a stiffer helix when it's torqued, you might as well have built it as a helix (or, better, a tube of the same diameter) in the first place, and have it take the full torque from the outset.
MaxwellBuchanan, Nov 03 2012

       Realistically, you'd do better if you engineered out of high- strength plastics and designed it like the stringer of an oil drill.
Alterother, Nov 03 2012

       What do you do with the torque when it gets to the bottom of the tether?   

       It might be better to transfer the energy through the tether by means of a low-density incompressible fluid.
8th of 7, Nov 03 2012

       Or generate electricity at the altitude.   

       Generator power densities are improving all the time, in parallel with improvements in motor power densities. The best attainable at the moment is something like 8kW per kg. For any kind of very large wind-catching device, the generator need not be a large part of the weight. Having the generator where it's needed is probably more effective than building a heavy torque cable.   

       You can get something like 8kW per kilogram of generator.
MaxwellBuchanan, Nov 03 2012

       Concentrating on this idea, and reading your discussions, I just thought of something.   

       But first a refresher: The requirements are for a cord (ok, I thought the word for musical chord came from the word for a musical instrument's string) that is flexible when NO torque is applied, and behaves like a rod when torque IS applied.   

       I'm sure it can be achieved with material engineering, but I have a simple solution: multiple cones allow for a fairly flexible rope, but when pulled together by tightening an inner rope, become rigid and behave like a rod...   

       Ropes of this sort can be useful in many fields besides tethered kite generators.   

       And by the way there's a youtube out there of a guy using a rope to turn a ground-based generator from a "turbine kite".
pashute, Nov 03 2012

       How about something similar to the tent pole of a dome tent? Obviously you'd want it less bendy and the joint would have to lock rather than spin, but it would become rigid as it is unrolled... Hmm I guess by the time you get this woking you probably wouldn't have a bungee cord down the center so you'd ust end up with a rigid rod that you snap together as the tubine goes up. Is that similar to oil rig stringers as [Alterother] suggested? (I couldn't seem to find any information on the term stringer in the context of oil drilling).   

       // transfer the energy through the tether by means of a low-density incompressible fluid // How about a commonly available mixure of compressible gas? You'll loose some energy when the compressed air cools, but it can take a one-way trip. I'm pictureing a tube from 2 - 12 inches in diameter made of something like mylar that doesn't stretch much.   

       Of course once you have a pressurized thin-walled tube, youmight be able to use it to transmit some torque as well. Before presurization it would be nice and flexible.
scad mientist, Nov 04 2012

       Of course, that means you need an air compressor up there. Unless you just use a giant funnel to compress the air directly.
spidermother, Nov 04 2012

       [scad] your concept is very similar to my second proposal; a string or stringer is a sectional sleeve through which the drill pipe passes. It's probably a slang term; I picked it up while constructing them in a fabrication shop when the company I worked for was building a mobile deepwater oil rig. There are kinds of strings that bend at the joints to explore for oil beneath the ground; I don't know how they work.
Alterother, Nov 04 2012


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