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Torque Cord
A lightweight rope that passes on torque without winding on itself | |
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.
[link]
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The problem is that you just can't get very high
torque
resistance in a flexible cord. |
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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. |
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If the cord is stretched, and is fairly flexible, then
the torque will go into twist. |
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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. |
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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. |
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But a cord which is kept very taut isn't much use
as a cord - you might as well use a single rod. |
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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? |
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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. |
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//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.// |
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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'. |
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If this rope of first order helices is long relative to
its overall width, then it will throw itself into
second-order helices. |
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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. |
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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. |
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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. |
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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. |
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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. |
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That's kind of what I'm getting at, only made from links
instead of braided wire. |
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[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. |
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I'm thinking a certain shape and profile of chain links that minimises the reduction in length and maximises the increased stiffness. |
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The two are mutually exclusive. What you sacrifice in
length reduction you gain in strength. |
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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. |
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//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. |
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//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. |
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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). |
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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. |
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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. |
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Realistically, you'd do better if you engineered out of high-
strength plastics and designed it like the stringer of an oil
drill. |
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What do you do with the torque when it gets
to the bottom of the tether? |
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It might be better to transfer the energy
through the tether by means of a low-density
incompressible fluid. |
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Or generate electricity at the altitude. |
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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. |
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You can get something like 8kW per kilogram of
generator. |
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Concentrating on this idea, and reading your
discussions, I just thought of something. |
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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. |
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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... |
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Ropes of this sort can be useful in many fields besides
tethered kite generators. |
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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". |
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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). |
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// 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. |
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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. |
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Of course, that means you need an air compressor up there. Unless you just use a giant funnel to compress the air directly. |
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[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. |
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