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Why does a thread/rope swirled like this: enter image description here

become more strong than just the group of single threads without swirling?

Is it because of more contact between them and therefore more friction? (I don't know the appropriate word for this process, I called it "swirling")

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    $\begingroup$ What gives you the idea that this makes the group of threads stronger? If you tell us where you learned this, it might help someone to tailor a better answer to you $\endgroup$ – Jim May 16 '14 at 13:44
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We can perhaps note that in the "real" world, you will probably not stress all the thread the same way, and it may that at some point one thread will deal with too much a tension, it will then break.

The friction between the threads will allow for the tension to be more equally shared between all the threads (the best case being an equal repartition of the tension between all thread).

The more you entangle them together, the better the friction and the sharing of the tension.

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it does not make the collection of threads stronger. if one takes thread or string and wraps it around and between two pegs in a loop, and then wraps the length of the "rope" formed in this way with more string one has created a Selvagee Strap, an old school longshoreman's or wagoners rig for lifting heavy weights on a crane. The distribution of the load stress throughout the bundle allows it to support a much larger load than the same number of individual threads. The wrapping prevents damage to the outer fibres from the lifted item, important for long term use but not part of the strength of the strap. The other advantage is that the strap does not induce rotation under loading the way that regular rope would. The original method of rubbing the threads into a bundle has the same effect as making a Selvagee Strap as long as the rubbing is not so hard as to damage the fibres.

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So let's say that a single one of these threads, perfectly constructed, has a strength of 1. So if you have three of them tied off to the same object, you should expect to get a strength of 3.

But threads aren't perfect. At some point, every thread is stronger than other parts, and weaker than other parts. If you have three separate threads lifting a load of 3, then they can fail one-by-one at those weak spots.

But if you roll them together, they bind. So in most cases, the weak spot on one thread is beside a non-weak spot on another. It may even be beside a strong spot on another, so that the total strength along the length is roughly constant.

So if you have 1000 threads woven in a rope, in the worst case you might have a spot with a strength of, say, 950. Whereas if you have 1000 separate threads, you have strength 999, then 998, and so on.

In fact, there are some ways to make ropes where they actually get stronger as individual threads fail. The trick is that they are woven so that normally bear the loads on an angle, but as the threads fail the remaining ones straighten out and provide more strength. IIRC you have to cut something like 20% of the threads before they begin to weaken. Unfortunately, I can't recall the name of them, although I learned about them as part of a proposal to make a high-altitude balloon-borne quasi-permanent platform.

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