Bend radius for ropes

[Ron]

kn,

I enjoy your remarks about F8s and I need to spend some time considering your points. Knots are fun, and I have noticed that there are some different ways presented to tie F8s. Mostly I see F8s tied with strands crossing each other that should and could be parallel to each other.

However, the F8 was really not the focus of the knot pull tests I did. I tied all my F8s the same way, either all correct or all incorrect. The single goal was to determine if a loop knot, a F8 in this case, would fail in the knot or where the loop passes over the screw link. In all cases the rope failed somewhere in the knot and appears that the failure occurred nearer the working end of the rope than the eye.

I haven't seen any indications that a 1:1 bend ratio weakens a rope by 50%. And the 50% thing was simply introduced for sake of discussion.

The fact that I used a screw link of a smaller diameter than the rope and the rope showed no signs of stress where it engaged the screw link, almost suggests that the bend radius is much less critical than we might think.

My thought is that the rope flattens where it crosses the 'pin' and the flattening reduces the strain in the outer fibers. I wished I had looked at that when I was doing my latest pull tests.

I also suspect how much the bend ratio effects the rope depends somewhat on the type and construction of the rope. E.g. the less it can flatten, probably the more severe the stress and the lower the breaking point.

How about posting some pics of properly tied F8s?

 

[Ron]

[ QUOTE ]
Thanks Ron, very interesting.


[ QUOTE ]
So placing a loop knot over a biner and forming a 1:1 bend ratio does not weaken the rope by 50% of the rope's strength.

But if you tried to use the same rope and biner as a simple pulley system, the load the rope could support with the 1:1 bend ratio would indeed be half the strength of the rope.

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Have you tested the latter statement? Photos? Info??

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Mahk,

I'm sorry, some how I missed your post and just now saw it. Please accept my apology for being so late with a reply.

There seems to be some misunderstanding about the 50% thing. I have never claimed that and I have repeatedly and consistently submitted that it is not true. I simply took the number and worked with it. I was trying to point out that for a loop knot or doubled rope as in DRT, even if the 50% reduction for a 1:1 ratio were true, it still wouldn't reduce the strength of the rope by 50% because there would still be two strands holding the load.

But, for thorouhgness, and so people wouldn't get the mistaken idea that ALL rope configurations were the same, I offered an example where the rope strength would be reduced by 50% IF the 1:1 bend ratio did reduce the rope's strength by 50%. I did so to be consistent with the comparisons.

 

[Lazarus2]

OK, who opened the can of worms!

Thanks for all the info from the engineers!

I'll try and put a conceptual slant on it.

Seems to me that the 1:1 strength reduction is the same in a loop knot or a line, i.e. a 6000# BS line double stranded in a loop makes for 12000# BS, in a similar way to a basket hitch configuration of a sling. With a 1:1 bend ratio reducing the strength by 50%, that still makes for a 6000# BS in the loop knot, but the knot in the line will break first because at 50% of 6000# as it has a 1:1 bend on a single strand, plus fusion failure etc. Also, if the ends of line are not seperated, a 1:1 bend will reduce that 6000# line to 3000#. Obviously flatter ropes like hollow braids (Tenex) seem to act more like webbing and handle lower diameter bends than stiffer ropes.

Now, I don't think anyone claimed 50% strength reduction as an absolute in knots or 1:1 bends, but as can be seen, there are so many varables in knot strength that, for critical arborist situations, FACTORING a 50% strength loss in the system for 1:1 bends and use of knots is wise - 30% may apply, but figuring 50% is safer. In this context, the fact that the loop knot is a 50% reduction of double strength isn't so important as that loop must be tied or spliced into a single strand (hence long eyes on a cow hitch sling helping improve strength).

I suspect that it is the friction of large static bends that helps gain double strength of spliced slings in a basket hitch and 70% in a spliced loop sling; as the bend gets down to 1:1 the strength loss increases due to the excessive tension and compression of fibres across the bend (of course flat webbing retains higher strengths because a 1:1 bend on webbing is miniscule).

What is for certain, is that tight bends and knots will have a noticeable effect on cycles to failure, no matter what the material, especially a moving bend like a rope through a karabiner or webbing through a buckle.

I suspect the smaller 3:1 bend cited for spliced eyes, is for the taper angle to the splice and to improve cycles to failure.

Perhaps a better way to explain it would be, a 1:1 bend can reduce the BS on a 'configured system' by 30-50%?

 

[Ron]

[ QUOTE ]
...Perhaps a better way to explain it would be, a 1:1 bend can reduce the BS on a 'configured system' by 30-50%?

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I edited this to add this right from the start:
I dont' think this is a can of worms at all. I think it's an excellent subject, and a well behaved and worthwhile discussion. This subject has some impact on all of us that trust our limb (pun intended) and life to rope(s).

I also think that we now see there are some tests that need to be done to examine the effects of bend radius on rope.

Ok, end of edit.

The point is, I think, we simply don't know. I have yet to see one study that would support that the bend radius effects the strength of rope and/or by how much.

We do know that an eye splice retains near 100% of the rope's strength. What does that mean? Does that mean the eye splice retains near 100% of the strength of the rope if it's not placed on a 1:1 ratio? No, retained strength is never specified with a minimum bend radius nor is one implied.

Though by no means conclusive, that does infer that the bend radius may not reduce the single strand strength of the rope at all. Specifically, if a rope is rated for 6000 lbs and an eye splice reduces the strength to 95%, or 5700 lbs, and we place the eye over a 1:1 bend ratio, I know of know study that indicates the eye would break where it goes over the bend or lower than 5700 lbs.

What is amazing to me, is to think that we've been climbing on rope for at least a 100 years, and we still haven't, (or don't know of) tested the effect of a rope over different size bend ratios.

 

[Lazarus2]

It is a myth that splices retain 100% of rope strength - maybe 3 ply but not braided.

And I agree it is amazing that this isn't widely available information, but I think you might find that if you contact rope manufacturers, they have this kind of data from years of testing - this is how they know what bend ratio to recommend. And probably why they test rope strngth over large (releative) diameter capstans. The fixed eye is no different, its just a higher configured strength e.g basket hitch.

As for the eye reduction of strength, the best way to test would be to fold a rope eye and eye sling over a 1:1 bend and attach the eyes to the winch. The 1:1 bend should be the weakest point (the eyes bends won't be, because now there are 4 legs of rope supporting the load). If this sling was rated at 6000#, and a fixed bend of 1:1 gives no strength loss, configured like this should give around 12000# strength. So, a dynamometer should tell at what rating it failed, and then a percentage of strength loss worked out accordingly. If the Break load is 12000# then the fixed eye bend radius reduction at 1:1 is irrelevent to strength. If it fails at 6000# then the 1:1 bend does account for a 50% (or whatever) strength loss.

I have some testing planned in a few weeks - I'll run this to see what we get.

 

[Ron]

Nobody here said eye splices retain 100% of the rope strength, I said near 100% and used an example of 95%. So how much strength do you say the retain?

I have been very disappointed in contacting rope manufacturers about rope data. I doubt seriously they could give you bend radius info, but I will call Samson and New England this afternoon and ask them about bend ratios and the strength of eye splices in various rope.

What you suggested does sound like a good way to test the bend radius effect.

 

[Lazarus2]

There are a lot of variables in splices as well as knots - i've bust a 3ply 1" polypro dead eye sling tied with a timber hitch - 5 tucks in the splice and it pulled through. I've inspected several double braid dead eye slings during company equipment inspections where the splice has pulled. Many manufacturers rate double braid splices as only 90% of BS. I have heard of some splices that climbers use only rated to 30%. someone will chime in with more experience in that department I'm sure.

I suppose it depends upon what you call a failure.

Thanks for all your efforts with the tests Ron, its been very interesting. I'm sure the rope manufacturers (particularly the sailing market) has the data, its just a question of will they give it to us?

 

[Ron]

Sherrill Tree's catalog states their splices are good UP TO 90% of the rope's strength. That doesn't give one a warm, cozy feeling of confidence in splices. UP TO can mean just about anything, but I expect that's all the commitment we're gonna get from anyone.

Some have mentioned that certain knots retain a very high percentage of rope strength. I have several ropes with eye splices, and I have climbed on them and they didn't budge a fraction of an inch. But for some reason, I'm more comfortable trusting my life to a knot or hitch than I am a splice. Don't know why that is; I guess it's one of those psychological things.

I couldn't get Samson's website up to get their number. I'll try again shortly.

 

[Ron]

This is interesting, from a Samson manual in .pdf format found at:

http://wescovan.com/catalogs/arborist.pdf

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Fixed PIN Termination Diameter:
The diameter on fixed pin termination should be at least 3 times the diameter –i.e., the bending radius for 1/2" rope should be 1-1/2".

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and,

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Sheave Diameter and Sizes:
Any sharp bend in a rope, under load, decreases its strength substantially – and may cause premature damage or failure. Many rope users are surprised to learn that a simple overhand knot (a series of sharp bends) reduces rope strength by almost 50%. In sizing the radius of Bitts, Fairleads, and Chocks, for best performance – the following guidelines are offered:

Where a rope bends more than 10 degrees around its bitts or chocks, or, for that matter, is bending across any surface – the diameter of that surface should not be less than 3 times the diameter of the rope. Another way of saying it is that – the diameter of the surface should be at least 3 times the rope diameter. A 4/1 ratio (or larger) would be better yet – as durability of the rope increases substantially as the diameter of the surface over which it is worked increases.

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Everybody adheres to this advice, right? You're not clipping/tying your 1/2" rope directly to a biner are you?

And surely you're not using a running bowline to lower limbs, that'd violate all the rules.

 

[knudeNoggin]

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Nobody here said eye splices retain 100% of the rope strength

[/ QUOTE ]
Although I just read such a claim by a retailer for a particular parallel-fibre rope, and I think that that's not unique.
Another angle, for which I just looked but didn't find support for, is that the Cordage Institute at least at one time had defined rope strength to be spliced strength (they made some bit of a promoted fuss re this in trying to show how much better their system was than those used overseas that might use forumlas and other data).
Indeed, a Sept. 2000? Practical Sailor article echoed this, as I recall--won't now go chase that for check of date, etc.--, and then carried a supposed confirming article in response to one in Sail by Brion Toss in which he raised alarms about the weakness of knots in hi-mod cordage. Funny thing was that in some of PS's test specimens their spliced eyes slipped out at less force than a Bowline was holding!!

Ron, re the Fig.8, your lower pic has it fine, though loose/unset; from that start then comes the issue/question of which end should be loaded, and as I indicated, there might be some further shaping -- by careful setting -- of the knot to make either loading optimal. And LoaL has yet another version.

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I have been very disappointed in contacting rope manufacturers about rope data.

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Ditto. A few years back Bob Thrun reported that he'd heard that there'd be some testing done by Samson of arborist ropes -- heard, IIRC, at the Baltimore TCI conference (that year that certain arbonuts ran rampant over nearby nautical history w/KevinZ ) . But Samson was hardly forthcoming with their results, and I sort of gave up my pursuit. --heckuva way to further understanding, eh?

We should bear in mind, though, that re strength of knots/rope, it's not all a matter of what might be shown in some lab testing under prescribed new-rope-rating test methods. Useage might well treat different structures in ways that testing won't show. One can read of differences in relative knot strengths with changes to load application (gradual vs. sudden), and for some applications where a knot is semi-permanent factors such as abrasion figure much (seeing a Fisherman's Knot in much marine cordage shows its advantages in abrasion resistance and alignment of ends with line of tension, e.g.). Still, after all these years, shouldn't we know things such as which side of a Sheet Bend (an asymmetric union of bight and loop) breaks?
And there are many other simple cases which nevertheless are wholely ignored in the treatment of knots (e.g. the "girth hitching" of tape slings! --it's appalling how no one notices how different the actual structures are, as though since any of them might result under that knot name it doesn't matter!?).

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I just e-visited www.ropecord.com , e-home of the Cordage Institute. They offer for free (PDF) download a document on criteria for rope inspection and a few other things; these should be worthhwile.

Re relevance of testing as under discussion here, let me quote from a quite pricey book, <u>Handbook of fibre rope technology</u> :

12.9.3 Nylon abrasion: a pathological study of a mistaken choice
The third investigation did not require any analytical study, but it demonstrates the importance of choosing the right rope for the purpose -- and also an interesting specimen for research on fibre failure by the fibre fracture group at UMIST. Getting what seems tobe a better deal can lead to trouble. A three-strand polyester rope supplied by a good ropemaker had given excellent service to tie the skirt of a hovercraft to the vessel. Then a rope was returned to the ropemaker with a complaint. It was badly abraded and had almost broken where it was tied to an eyelet. [THINK 1:1 DIA ! ] Examination showed that it was not their rope and it was not polyester. A purchasing agent had found a cheaper rope, which looked much the same and had much the same strength -- but it was a nylon rope, unsuited to stand up to repeated tensioning and flexing particularly over the eyelet in a wet environment.

And, hmmm, at the end of the 12.9.2 "A loss under tow", also re nylon ropes, I see the concluding remark that "Placing soft eyes round a small shackle pin with a D/d ration of about 0.8:1 (i.e. pin diameter less than rope diameter) would reduce the strength by a further 33%." That might be a comment to be understood as for cyclic loading, not one-time pull. And I believe that "the ropes were made up into grommets with soft eyes" means that essentially one had doubled material (with "eyes" maybe seized into shape fore each end), and the reduction thus is of roughly double the rope strength.

*kN*

 

[Ron]

[ QUOTE ]
Nobody here said eye splices retain 100% of the rope strength...

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One of the things that does bother me about nylon rope is the effect wettness has on it. Info I've seen states that water, i.e. a wet nylon rope, reduces the strength of the rope by 15%. Granted that isn't much, but things add up. If the rope is 5 years old, at 2% per year, plus being wet, plus a knot with 75% strength retention comes up to a total of 10% for age + 15% for wet + 25% for the knot = 50%.

At least in a polyester rope the wet wouldn't effect the strength, so it would have about 65% rating under the same conditions.

Edit:
Just to add: A rope that's 10 years old, assuming a 2% loss in strength per year plus another 10% loss in strength from use (wear and tear, and 10% for a 10 year old rope might be a bit low), 15% from being wet, and 25% loss from the knot, yields a rope strength loss of 20% (2% x 10 years) + 10% (use) + 15% (wet) + 25% (knot strength loss) = 70% strength loss. So if the new rope strength is rated at 5400 lbs (minimum ANSI) the accumulated strength losses gives a final rope strength of 1620 lbs!
end of Edit..

I have mixed thoughts about nylon and abrasion. Cavers use nylon ropes in the most abrasive environments there is and I don't see any indication that they are moving toward polyester. But who knows, maybe next year, or the next, the cavers will be claiming the benefits of polyester. I won't be holding my breath on that, there's some "we've always used nylon" pride in play, just like polyester is 'the best' arborist rope.

 

[Ron]

New England Rope just returned my call. I asked specifically about the eye splice strength AND the effect of clipping the eye into a biner.

He said they rate their eye splices at 90% rope strength. He said that when the eye is clipped into a biner, you never see the eye break at the biner, it always breaks on the other side of the eye in splice somewhere. He explained that the reason for this is because even though the rope is weakened by the bend, there are two strands of rope supporting the load. Does this sound familiar?

He said the same thing applies to a knot. When they test knots, he said the rope never breaks at the bend or in the loop, and I swear he said this with absolutely no prompting by me, he said it breaks where the rope enters the knot. His words, not mine.

 

[RogerBerry]

They aren't big in the arboring world because they make mostly dynamic rope, but the guys at Bluewater seem to love testing their stuff and have been pretty free with test info. Scott is the guy that does most of the testing. www.bluewaterropes.com, 770-834-7515.

 

[Lazarus2]

[ QUOTE ]
New England Rope just returned my call. I asked specifically about the eye splice strength AND the effect of clipping the eye into a biner.

He said they rate their eye splices at 90% rope strength. He said that when the eye is clipped into a biner, you never see the eye break at the biner, it always breaks on the other side of the eye in splice somewhere. He explained that the reason for this is because even though the rope is weakened by the bend, there are two strands of rope supporting the load. Does this sound familiar?

He said the same thing applies to a knot. When they test knots, he said the rope never breaks at the bend or in the loop, and I swear he said this with absolutely no prompting by me, he said it breaks where the rope enters the knot. His words, not mine.

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Ron, I don't think anyone disputed this statement at all???

Until tests are done, perhaps in a way I suggested, we won't know how much effect a 1:1 bend has on a fixed eye.

I don't think the cumulative strength loss in ropes works quite in the way you have suggested.

 

[TheTreeSpyder]

Nylon soaks up water more than other synthetics; even from the air on opening the bag. this absorption is like 5x that of polyester. The swelling gives more internal frictions i believe; standing against the strength. A Natural rope fiber like manila, can take on enough water, to double it's weight, but some rate an increase in strength.

In Natural fibers, the elasticity/dynamic absorbtion qualities aren't altered wet. Nylon is like 4x the dampening when dry(couldn't resist); has the strength loss wet;But, usually carries an increased rating in elasticity wet.

i'd suspect NE's .9 eye splice rating is a conservative CYA one after set, new.

Though, the tight bight is of concern in the eye, for the more leveraged tension fibres/disallowed compression fibers; it is still a double leg device to the loading. Also, even though we relax/widen that for less leveraging by these factors; that increases the angle type leveraging of each leg, as multipliers of the half load to each leg.


Orrrrrrrrrrrrrrrrr sometin'like dat

 

[Ron]

Ron:[ QUOTE ]
New England Rope just returned my call. I asked specifically about the eye splice strength AND the effect of clipping the eye into a biner.

He said they rate their eye splices at 90% rope strength. He said that when the eye is clipped into a biner, you never see the eye break at the biner, it always breaks on the other side of the eye in splice somewhere. He explained that the reason for this is because even though the rope is weakened by the bend, there are two strands of rope supporting the load. Does this sound familiar?

He said the same thing applies to a knot. When they test knots, he said the rope never breaks at the bend or in the loop, and I swear he said this with absolutely no prompting by me, he said it breaks where the rope enters the knot. His words, not mine.

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Lazarus2:[ QUOTE ]
Ron, I don't think anyone disputed this statement at all???


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Do you not remember saying this, and I quote,
.
Lazarus2: "I think you are barking up the wrong tree - I see what you are saying about the two legs of rope, but thats not how it works in my experience. A 6000# rope with a knot tied to a 'biner (did I just say 'biner'?) errr... a krab, should only be expected to hold around a 3000# load."
.
I'd call that directly, clearly disputing.
.
.
[ QUOTE ]
I don't think the cumulative strength loss in ropes works quite in the way you have suggested.

[/ QUOTE ]
.
To quote Mahk, "Have you tested the latter statement? Photos? Info??"

 

[Ron]

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...i'd suspect NE's .9 eye splice rating is a conservative CYA one...

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Oh, I agree completely. They would be quoting the minimum for the reason you stated - CYA. That means eyes probably average higher breaking strengths than 90%.

[ QUOTE ]
Though, the tight bight is of concern in the eye, for the more leveraged tension fibres/disallowed compression fibers; it is still a double leg device to the loading. Also, even though we relax/widen that for less leveraging by these factors; that increases the angle type leveraging of each leg, as multipliers of the half load to each leg.


[/ QUOTE ]
Yep! if the eye is over something that causes significant angles in the two strands of the eye, force multiplication does come into play and I suspect, haven't tested or seen tests about it, that it would be possible for the eye to break under those conditions.

But, to avoid any misunderstanding, a single biner would not create this problem. But if you had the eye over two biners, kinda like a false crotch, that would introduce slight additional force in the eye. Although, I'm not sure why anyone would need to use an eye over two biners.

 

[kevknep]

[ QUOTE ]
...
As for the eye reduction of strength, the best way to test would be to fold a rope eye and eye sling over a 1:1 bend and attach the eyes to the winch. The 1:1 bend should be the weakest point (the eyes bends won't be, because now there are 4 legs of rope supporting the load). If this sling was rated at 6000#, and a fixed bend of 1:1 gives no strength loss, configured like this should give around 12000# strength. So, a dynamometer should tell at what rating it failed, and then a percentage of strength loss worked out accordingly. If the Break load is 12000# then the fixed eye bend radius reduction at 1:1 is irrelevent to strength. If it fails at 6000# then the 1:1 bend does account for a 50% (or whatever) strength loss.

I have some testing planned in a few weeks - I'll run this to see what we get.

[/ QUOTE ]

Wouldn't you need to account for the 2:1 MA that occurs at the bend using this setup. If the dyno is attached on the winch side, a reading of 6000# would infer 3000# per leg, but 6000# at the bend, so 12K# at the dyno would produce 12K# at the bend, or am I mistaken?

In other words, it seems to me for this method to be valid the setup need to be

stationary object(tree)-&gt;dyno-&gt;krab-&gt;sling-&gt;winch

or am I mistaken about the physics, which is a very good possibility?

EDIT: oops, just noticed how old the thread was....

 

[Ron]

I can answer that, but I've probably posted too much already, so maybe I should become a reader only.

Well, just this maybe, the thread may be a bit old by the start date, but it has been and is still, very active.

 

[knudeNoggin]

[ QUOTE ]
[ QUOTE ]
...As for the eye reduction of strength, the best way to test would be to fold a rope eye and eye sling over a 1:1 bend and attach the eyes to the winch. The 1:1 bend should be the weakest point (the eyes bends won't be, because now there are 4 legs of rope supporting the load). If this sling was rated at 6000#, and a fixed bend of 1:1 gives no strength loss, configured like this should give around 12000# strength. ...
I have some testing planned in a few weeks - I'll run this to see what we get.

[/ QUOTE ] Wouldn't you need to account for the 2:1 MA that occurs at the bend using this setup.

[/ QUOTE ]
There's no MA here; the eye-2-eye sling is folded over the sharp turn, and so the material is doubled and the question is whether it can bear double tension with such a bend.

The report I quoted above answers this for that towing grommet, where it was asserted that such a configuration reduces (grommet) strength by 33%. This implies that e.g. for a doubled 6_000# rope the possible 12_000# doubled structure would break at 8_000#; this implies 4_000# per side of the rope going around the turn, and clearly that jointly exceeds the strength of a single 6_000# strand feeding an eye.

But it can be illuminating to see how theory compares to reality of some particular circumstance.

As for cavers &amp; nylon vs. polyester, there's significant difference between the sort of loading from towing through an ocean and caving use! The abrasion referred to in some of the towing is internal, resulting from cyclic loading, and cavers aren't likely to be generating this sort of load.

-----
Laz, FIVE tucks pulled out? --possible biased loading on the tucked side? (maybe, were the eye around a rough surface) --sudden load? There was some testing done of 5/8" PP laid ropes and the quick'n'dirty "compression" (tucked whole rope end through lay) splices; they held 'til rupture of the anchored end (apparently the test configuration was not so great), but slipped out with accelerated loading.
(accident investigative testing after a splice failed and spilled chopped logs on a logger (who violated good sense in moving beneath that load))

*kN*