Best rope lengths?

[Peter Burgess]

Must go somewhere! If a rope shrinks by ca 10 percent, then the diameter will only have increased by little more than 3 percent, which I doubt many would notice.

 

[TheBitterEnd]

Really interesting that they seem to go on shrinking - the older they are, the more they have shrunk (if I am reading your figures correctly) - my guess would have been that after a few soakings they would settle down.

 

[cap n chris]

Something of a Fulk Hero, imo. Thanks for the stats - an excellent benchmark reference. 

 

[owd git]

Q, do they shrink in length? or increase in girth (with ingress of 'cave matter' ) not allowing full extension of filaments? :-\
Discuss (at reasonable length, Sic')
Owd Git.

 

[Fulk]

Peter Burgess:

Must go somewhere! If a rope shrinks by ca 10 percent, then the diameter will only have increased by little more than 3 percent, which I doubt many would notice

I make it ~ 5%; however ? IF you treat a rope as a solid but malleable rod, then if you compress its length to 90% of what its original length was, then its girth would increase by ~ 5%. Obviously, ropes aren't solid rods but constructed of fibres, with air space between them, so you can't treat them as solid objects.

So maybe the whole lot just sort of shrivels up, reducing the inter-fibre airspace without much effect on the thickness?

 

[TheBitterEnd]

Not forgetting all that space in the polymer molecules themselves ...

 

[Peter Burgess]

Peter Burgess:

Must go somewhere! If a rope shrinks by ca 10 percent, then the diameter will only have increased by little more than 3 percent, which I doubt many would notice

I make it ~ 5%; however

Well spotted - my mistake.

So maybe the whole lot just sort of shrivels up, reducing the inter-fibre airspace without much effect on the thickness?

Which would make the new rope like a stretched spring that with age becomes shorter, possibly as the fibres become less flexible? And as they become less flexible, they become more prone to failure? This is all pure speculaton!

 

[Fulk]

To expand on my last post: If you had a malleable, ductile metal rod and squashed it longitudinally by 10%, it?s easy to deduce from the formula for the volume of a cylinder that its girth would increase by about 5%.

However, a rope is obviously a different thing entirely. There must be a significant volume of air within the individual fibres that go to make up a rope, so I would imagine that when a rope shrinks, what happens it that the fibres (for whatever reason) are squashed more closely together, the overall volume of nylon being exactly the same, but the volume of ?inter-fibre air? is substantially reduced.

Incidentally, the ?volume? of 100 m of 10 mm rope is ~7.85 litres . . . but it?s not easy to get it into a 20-l tackle sack!

 

[TheBitterEnd]

Just a thought, are the ropes that have shrunk the most, more stretchy? Perhaps Bob Mehew would know if new ropes have a lower elongation than old ropes?

(My thinking is that the individual fibres are stretched during manufacture and then twisted, which may lock in the stretch. Wetting and use then allows this initial in-built stretch to relax causing the rope to shrink. If so the shrunken rope may elongate more).

 

[Bob Mehew]

Just a thought, are the ropes that have shrunk the most, more stretchy? Perhaps Bob Mehew would know if new ropes have a lower elongation than old ropes?

(My thinking is that the individual fibres are stretched during manufacture and then twisted, which may lock in the stretch. Wetting and use then allows this initial in-built stretch to relax causing the rope to shrink. If so the shrunken rope may elongate more).

The simple answer is no I don't.  And I am unsure what I would be measuring. 

Let's start with a simple feature of rope.  Manufacturers' usually quote a mass per unit length and a diameter.  By comparison with the density of a block of nylon one can work out voidage in % terms.  My first problem is I find a range of densities (1.02 to 1.18 g/cc).  Doing the calculations gives

Marlow
diameter mm   9 10 10.5 11
density 1.02 16 21 22 22
            1.18 28 32 32 33
Edelrid
diameter mm   9 10 11
            1.02 17 19 19
            1.18 28 30 30
Beal
diameter mm 10 10.5
density   1.02 24 24
            1.18   35    34

(I think the columns are just about OK.)

Secondly if you take a set of circles and try and pack them as close together as possible, then you come up with hexagonal packing (so named because the centres of the circles form a hexagon around the centre circle).  Now hexagonal packing has a voidage of 9%, so you can see that the ropes I quote above have extra voidage.  (This 9% value is independent of circle diameter.)  So the answer to an earlier question as to where does the shrinkage go, is well there is a lot of air / spare space in a rope!

Which takes me onto another simple fact.  The yarns in the sheath are helically coiled around the rope as they progress along the rope.  A fairly rough calculation indicates that the length of yarn per unit length of rope in the sheath is around 1.4, that is 40% longer.   

I am not sure how many of you look at the core of your ropes.  (I will plug one piece of advice of getting the shop to demonstrate to you that your rope has not been sat around for years on their shelves by cutting the sheath to expose the core and the tape whcih quotes the year of manufacture.)  But you will see the manufacturers' some how impart a "crinkle" into the yarns in the core.  One problem is that I think this crinkle is unique to each manufacturer (not that I have done it for all ropes but it is an impression I have gained over a number of years).  So the core is more than a unit length per unit length of rope, I vaguely recollect less than 10% from some simple test.

The British Standard specifies an elongation criterion of not more than 5% based on the extension of a rope when applying a 100kg load to a 3m length already supporting 50kg.  The standard also requires that shrinkage is cited for a rope.  It is measured by first extending a pre conditioned (pre 24 hour 10% humidity then 65% humidity & 20C for 72 hour) rope using a 10kg force and making two marks 1m apart.  Having ensured the ends are sealed, the rope is then soaked in clean water for 24 hours at 15C, removed and re extended with a 10kg force and the extension measured. 

I have not thought about trying to conduct some experiments on this topic but it looks just right for a final year student.

I will go along with water lubricating the yarn but most ropes are spun using an oily liquid to aid spinning so presumably they are even more lubricated than with just water.  (So after the initial wash, the rope then will perform worse when wet than dry.)

My gut feel is that given you can visualise that an individual polymer chain is made of carbon carbon bonds in a "wiggly" orientation which is then stretched in making the bulk polymer by cross linking to other polymer chains, the introduction of water breaks these bonds and allows the polymer to shrink.  Not a large amount because presumably not many of the cross polymer chain bonds are accessible.  (Which makes me wonder if this then links to the problem of nylon degrading by hydrolysis in acidic solutions.  So don't wash your ropes with an acidic soap!)  If you can't visualise it, then sorry, I can't find a simple image to refer you to.     

 

[Fulk]

My gut feel is that given you can visualise that an individual polymer chain is made of carbon carbon bonds in a "wiggly" orientation which is then stretched in making the bulk polymer by cross linking to other polymer chains, the introduction of water breaks these bonds and allows the polymer to shrink.

Is that what happens?

I'd have  thought that the 'wiggly chains' of CH2 groups in nylon are pretty inert. There might be some scope for cross-linking at the amide linkages, though, but on balance I'd be surprised if there was much cross-linking between polymer chains.

 

[Bob Mehew]

See http://en.wikipedia.org/wiki/Nylon for the general chemical formula for nylon.  My gut feeling is that the oxygen and hydrogen atoms interlink using "hydrogen" bonding (see http://en.wikipedia.org/wiki/Hydrogen_bond ).  But I can't say it is what is happening, just a speculation.  Is there an expert on these matters out there?     

 

[owd git]

That would depend on the acceptance of Quantum theory being of use, in what seems to be a matter more easily confused with more easily accessed, and explained, theory
O.G.

 

[Fulk]

It's very likely that there  will be hydrogen-bonding between chains, but this is a very weak form of chemical bond compared with a 'proper' covalent link.

 

[Fulk]

As regards

the problem of nylon degrading by hydrolysis in acidic solutions

, this is a question of acid attacking the actual inter-molecular bonds ? i.e. the amide links ? along the entire length of the polymer chain. In other words, if you soak your (nylon) rope in acid, then the whole lot degrades ? presumably, in an extreme case, to 'mush'. Put a bit of rope in a bucket  (plastic!), leave it overnight, and you end up with mush.

And while quantum theory might have an explanation of hydrogen bonding, then to accept that the latter happens doesn't need an understanding of quantum theory; anyway, I thought that noone ? not even quantum physicists ? actually understand it.

 

[Fulk]

Put a bit of rope in a bucket  (plastic!), leave it overnight, and you end up with mush.

OOps. Meant to say 'Put a bit of rope in a bucket (plastic!) of battery acid, leave it overnight, and you end up with mush.'

 

[Peter Burgess]

Does using a rope encourage it to shorten? If a rope is never used, does it retain its length?

If so, here is another thought. I imagine many of you will know how ribbon used to decorate presents can be made to permanently coil up by running a blade along one side of it. This violent distortion of the polymer causes a permanent curl to the polymer, presumably by inflicting some form of physical damage to it.

Passing a rope through a descender might have a similar effect on the polymer in the rope? Perhaps by inducing a permanent distortion to the fibres, the rope will shorten, effectively increasing the inter-fibre spacing.

Of course if ropes are known to shrink with age regardless of use, this theory is just a load of poop. 

 

[martinr]

That would depend on the acceptance of Quantum theory being of use, in what seems to be a matter more easily confused with more easily accessed, and explained, theory
O.G.

Hmmm.... I think it is necessary to reconcile quantum mechanics and general relativity. That means we need to use......

...........String Theory 

 

[Fulk]

Tempted to tell you to get knotted.

 

[Bob Mehew]

As regards

the problem of nylon degrading by hydrolysis in acidic solutions

, this is a question of acid attacking the actual inter-molecular bonds ? i.e. the amide links ? along the entire length of the polymer chain.
And while quantum theory might have an explanation of hydrogen bonding, then to accept that the latter happens doesn't need an understanding of quantum theory; anyway, I thought that noone ? not even quantum physicists ? actually understand it.

I accept your point; I was focusing to much on the strength reduction when wet which I suspect is due to water breaking the interchain hydrogen bonding.  (I must admit to thinking that the problem with quantum theory was not one of understanding, just of uncertainty.  )

Tempted to tell you to get knotted.