Author Topic: Another Bend for Dyneema  (Read 5614 times)

NautiKnots

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Another Bend for Dyneema
« on: July 21, 2017, 10:55:20 PM »
In http://igkt.net/sm/index.php?topic=4756.msg39862#msg39862, I showed a bend that does not slip when tied in Dyneema (I used 3mm Amsteel Blue).  I've played a little with variations to see if I could improve the strength, and the knot below seemed to fit the bill.   All I did was add a half-hitch around the standing part before the last tuck.  This version is only slightly more complicated (still much simpler than any other Dyneema bend I've seen) and seems to be stronger.  I don't have any measurements, but in every test I've done, my previous bend broke when this one was only beginning to show signs of strain.

In the photo below, I tied the two legs of the bend in opposite chirality to show both faces of the knot in a single picture.  I don't see any reason why that would make a difference in performance though.  It wouldn't be necessary in real-world application.

I also tried doubling the final tuck instead of hitching around the standing part.  That variation also did not slip, but it broke before this one.

I speculate that the additional half-hitch strengthens the knot for two reasons:
  • The half hitch provides another point of constriction, thereby spreading the load over a larger surface area, and/or
  • The hitch around the standing part straightens the entry to the knot, so it can bear a higher load before breaking.

Does that sound reasonable?

Most of the proposed bends for Dyneema involved additional tucks, trying to make the knot better by adding more crossings -- and those attempts don't generally seem to pan out.  These bends involve tighter nip instead, and they hold.  That gives me some ideas for other candidates -  more compact bends with high nip.

Regards,
Eric

Birchhatchet

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Re: Another Bend for Dyneema
« Reply #1 on: July 22, 2017, 09:28:37 AM »
Hi Eric,
I have used the attached little bend for years in Dyneema rop.

There are also some modification shown under
https://igkt.net/sm/index.php?topic=5564.msg37805#msg37805

Maybe this would be a stimulus for your investigations.

Cheers, Karl

NautiKnots

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Re: Another Bend for Dyneema
« Reply #2 on: July 22, 2017, 03:09:59 PM »
I have used the attached little bend for years in Dyneema rop.
Karl,

I tested your knot in 3mm Amsteel Blue and it slipped under relatively low load.

Regards,
Eric

NautiKnots

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Re: Another Bend for Dyneema
« Reply #3 on: July 22, 2017, 03:50:08 PM »
I've been trying other candidates as well, including two nice, compact bends composed of interlocked Strangle Knots (ABoK 1239).  Ultimately, however, those bends either slipped or broke at lower load than the Angler's Loop based bend.  They were also not any simpler to tie, nor could they be untied after loading.  I think that the success of the "Twang Bend" (and the variation I posted above) is due to the way that each knot is loaded on three legs, not just two.  Tension causes it to draw tighter rather than to slip (or roll).

I also tried "Two Bends" (ABoK 1454) and the Two Bowlines Bend (ABoK 1455).  I can't explain why, but 1454 slipped whereas 1455 held. 

For what it's worth, the Two Bowlines isn't a bad bend for Dyneema, especially if the loops are eye-hitched (square knotted) together as shown below.  I loaded that knot until it broke, and was still able to untie it afterwards -  without needing any tools.  Out of curiosity, which photo do you prefer, the black background or the white?

The "Twang Bend" (as I'm calling it until a better name arises) and it's variations are stronger than the Two Bowlines Bend, but they cannot be untied easily.

Regards,
Eric

« Last Edit: July 22, 2017, 11:16:29 PM by NautiKnots »

SS369

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Re: Another Bend for Dyneema
« Reply #4 on: July 23, 2017, 02:05:46 PM »
To my eyes, the black background allows more details. But, the white background may allow for better printing.

SS

Dan_Lehman

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Re: Another Bend for Dyneema
« Reply #5 on: July 23, 2017, 06:59:08 PM »
I speculate that the additional half-hitch strengthens the knot for two reasons:
  • The half hitch provides another point of constriction, thereby spreading the load over a larger surface area, and/or
  • The hitch around the standing part straightens the entry to the knot, so it can bear a higher load before breaking.

Does that sound reasonable?

I liks your thinking about what might make a difference
& why, vs. just throwing out different knots & hoping ... .

Have you tried the fisherman's knot extension that I suggested
in msg.#168 (current latest) in the other thread?  My thinking
on that is that putting the tail to be nipped hard by the
SPart might give all of the other places of "nip" some
help to hold, and that this hard nipping doesn't put
a sharp deflection in the SPart, so ... hopes for strength.

(But skinny HMPE stuff sure seems to defeat a lot of knots!)

--dl*
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NautiKnots

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Re: Another Bend for Dyneema
« Reply #6 on: July 23, 2017, 09:07:27 PM »
Have you tried the fisherman's knot extension that I suggested
in msg.#168 (current latest) in the other thread? 
Dan,

I tried following your instructions, but depending on how I make the final tuck (through the center of the overhand knot alongside the standing part, or through a crown of the overhand knot so the running end does not touch the standing part), I get two different knots.  I tried both, and each one slipped under relatively low load.

Dyneema does confound our regular notions on knots.  Take the Bowline (ABoK 1010), for example.  Conventional wisdom says:
  • the Bowline is stronger than the Perfection Loop, and
  • a Bowline breaks at the apex of the nipping loop, not at the entrance to the collar.
My tests with Dyneema, however, yield the opposite results:
  • when tied opposite each other, the Bowline breaks before the Perfection (Angler's) Loop, and
  • the Bowline breaks at the entrance to the collar.<edit> I believe I misremembered my test results, and I can't recreate them, so I retract this statement.

I speculate that the best bends for Dyneema, will have two characteristics:
  • each working end needs to be nipped as close to a standing part as possible (what you're trying for), and
  • if you consider a symmetric bend to consist of two halves - each one tied about the other - then tightening one half must constrict both itself and the other.  That is, when pulled, each end must tend to draw up both halves.

Bends made from conjoined loop knots have this second property, and I'm looking for ways to incorporate that into more compact knots.  Of course, I could be completely wrong.  After all, ABoK 1454 should be a good bend according to these properties, but I can't explain why it slipped when 1455 did not.

Does that make sense?
Eric
« Last Edit: July 25, 2017, 01:35:31 PM by NautiKnots »

NautiKnots

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Re: Another Bend for Dyneema
« Reply #7 on: July 24, 2017, 04:58:23 AM »
Well, I spent the day evolving more compact bends for Dyneema (it's my birthday and I'll tie if I want to) and after discarding numerous attempts to create new knots with the above characteristics, I decided to try to shrink the "Twang Bend" instead.  So, I removed the legs from the middle of the knot, and joined the two overhand knots together directly.  I tried a few variations (each time I tried to tie a bend, it came out differently) and settled on two viable candidates.

The first begins with a 2-strand Matthew Walker Knot form and finishes with the same final hitch as the Angler's / Perfection Loop on each end.  The second ties two overhand knots together slightly offset, and again finishes both ends like the Angler's Loop. 

I tied them inline with 3mm Amsteel Blue and tightened them up on my bench winch.  Although the two bends look very similar in flat form, they draw up into quite different geometries under load.  The first takes on a diamond form with the tails opposite each other.  The second becomes barrel-shaped with the tails exiting the same side.  See the photos below of the two bends after loading.

Both bends had some tail migration as they drew up, but then settled and held position thereafter.  I put as much force as I could on my winch, taking 6 wraps of 7/16" (11mm) double-braid polyester (all that would fit) around the drum in order to tail the load.  Just when I thought I couldn't winch any harder, one of the end loops (not either of the knots) broke.  That's the first time an end failed in all my testing of Dyneema bends.  Every other bend I've tested has broken first.

These are both definitely the strongest bends for Dyneema that I've tried.  If you look closely at the first photo below, you'll see that some of the fibers in the knot have ruptured, but those that remain still hold.  The second photo doesn't show any breakage.  Both bends exhibit high strain on their central crossings.

The first one is probably easier to tie, as it begins as a well-known knot, but I like the looks of the second one better.  Neither is overly complicated.  Both are compact.  Neither one can be untied after loading.  Overall, I'm happy with my "birthday bends" - they're a nice present to myself.

I'll post pictures of the flat forms tomorrow.

Regards,
Eric

NautiKnots

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Re: Another Bend for Dyneema
« Reply #8 on: July 24, 2017, 06:41:14 PM »
Below are front and rear views of the two bends in flat form.  The top two pictures are of the first bend (the one that draws up into a diamond shape).  The bottom two are of the second bend (barrel shape).

The photos make the knots look more complicated than they are.  Both are simply a twin overhand bend with a half-hitch finish (angler's loop style) on each end.  The trick is that not just any overhand bend will do.  These two knots retain the 3 bearing leg structure of the "Twang Bend" internally, and exhibit similar security.  Overhand bends that don't have a load-sharing structure - even ones with good constriction (and I've tried several) - don't seem to hold in Dyneema, even with the final nipped half-hitches.

I'll post tying instructions soon.

Regards,
Eric

NautiKnots

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Dyneema Bend 1 Instructions
« Reply #9 on: July 24, 2017, 09:06:38 PM »
OK, here are instructions for tying the first of the two more compact bends for Dyneema:
  • Tie the "Twofold Overhand Bend" (ABoK 1426) loosely.  If drawn up, this bend has the same form as the 2-strand Matthew Walker's Knot.  Don't tighten it though.  Instead, arrange the Overhand Knots as shown below.  Leave plenty of tail on the working ends.
  • Finish one tail like the Angler's Loop (a.k.a. "Perfection Loop").  Pass the working end behind the opposite standing end, through the crown of the overhand knot, and tuck it back under itself.
  • Repeat Step 2 with the other working end.
  • Dress the bend.  Pull each standing end and working end repeatedly to remove slack from the knot.  The tighter you make the bend now, the less tail it will swallow when drawing up under load.  This bend retains its flat form, becoming diamond-shaped when tight.
I don't have a name for this bend.  Any suggestions?

Regards,
Eric

NautiKnots

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Dyneema Bend 2 Instructions
« Reply #10 on: July 24, 2017, 09:23:01 PM »
Here is how to tie the second more compact bend for Dyneema:
  • Tie two interlocked overhand knots per the first photo below.  As far as I know, this particular arrangement of overhand knots does not have an ABoK number.  If I'm mistaken about that, please let me know the number and I'll correct this post.  Be sure this step is correct.  Any error here will yield a different final knot.  Again, leave sufficient length on the working ends.
  • (Not shown) Finish one end like the Angler's/Perfection Loop.  Pass a working end behind the adjacent standing end, through the crown of the overhand knot, and tuck it under itself.
  • Repeat step 2 with the other working end.
  • Dress the bend.  Pull each standing end and working end repeatedly to remove slack from the knot.  The tighter you make the bend now, the less tail it will swallow when drawing up under load.  This bend twists when tightened, taking a barrel-shape form with both working ends exiting from one side of the bend.
I think this is a more attractive, and possibly stronger form than the bend above.  The fourth photo below shows the crown of the bend.

I don't have a name for this bend either.  Any suggestions?

Regards,
Eric

NautiKnots

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Re: Another Bend for Dyneema
« Reply #11 on: July 24, 2017, 10:47:21 PM »
Here is one of my rejected candidates.  It began as ABoK 1031, tied as a bend instead of a loop.  With the ends finished with the same half-hitches as the others, it became a very handsome bend with a herringbone look.  I had high hopes for this knot, but unfortunately it did not hold in Dyneema.  At moderate load, the bend rolled off its tails.  In hindsight, I can see how the interlocked overhand knots do not have sufficient "counter-pull", which is why I think it failed.

Perhaps this bend can find a home as a decorative knot, since it doesn't appear to be very practical.  It might make a pretty start to a wide knotted bar.  Right now, I'm thinking of calling it a "Herring Bend", both for the herringbone pattern, and because it was a "red herring" in my search for Dyneema bends.

In order to test my "counter-pull" speculation, I tied other two-overhand-knot structures that looked like poor candidates (including the Reef Knot and Thief Knot), finishing them off with the same half-hitches.  They all slipped under relatively low load.

So, I guess the message here is that beginnings are important.  The two (I said "more compact" previously, but "less bulky" might be a better term) bends above are the most secure ones I have yet evolved.

I hope that was interesting,
Eric

Dan_Lehman

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Re: Another Bend for Dyneema
« Reply #12 on: July 25, 2017, 12:53:19 AM »
when tied opposite each other,
 the Bowline breaks before the Perfection (Angler's) Loop, and
the Bowline breaks at the entrance to the collar.

!?  Sorry for being w/limited attention here,
but "tied opposite each other" means interlocked eyes,
or rather the "twin" structure where between nubs run
the would-be-eye-legs of each and are *shared* by the
knots (like Ashley's "twin bowlines" but you had one be
the perfection loop).

As for where the break came,
hmmm, the collar would have to be *dang* tight to
cause rupture at entry --might be a matter of being
hard to discern!?

Note that bowlines w/eyes around a metal "pin" have
slipped.

Thanks,
--dl*
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NautiKnots

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Re: Another Bend for Dyneema
« Reply #13 on: July 25, 2017, 03:20:14 AM »

!?  Sorry for being w/limited attention here,
but "tied opposite each other" means interlocked eyes,
or rather the "twin" structure where between nubs run
the would-be-eye-legs of each and are *shared* by the
knots (like Ashley's "twin bowlines" but you had one be
the perfection loop).
I've attached a photo showing what I mean by tying the Bowline opposite the Angler's / Perfection Loop.  I've done destruction tests on this exact configuration and its equivalent (Angler vs. Bowline) in 3mm Amsteel Blue at least a dozen times.  Not once has the Angler's Loop failed.  Every time, the Bowline has either:
  • broken,
  • slipped and then broken, or
  • slipped and come apart.
Therefore, I'm convinced that the Angler's / Perfection Loop is both stronger and more secure than the Bowline when tied in Amsteel Blue (Dyneema SK75).  That is the inverse of previously published results in other materials.

Quote
As for where the break came,
hmmm, the collar would have to be *dang* tight to
cause rupture at entry --might be a matter of being
hard to discern!?

I retract my statement about breaking at the collar.  I looked at broken Dyneema Bowlines after destruction testing and recall having seen both ruptured nipping turns, and intact nipping turns.  Unfortunately, I have not kept the broken knots for review, and my attempts to recreate the results today have failed.  All my Bowlines slipped.  Only one broke (after slipping) and it was so entangled in the other Bowline I tied it to (which had also slipped) that I couldn't make out for sure where it broke.  I think it was at the nipping loop.

So, what I really meant to say is that tension transfers through knots in Dyneema differently than other materials and in ways that are sometimes contrary to prior tests and explanations.  My second example is flawed, and probably mistaken.  I stand by the first one, though.

Regards,
Eric

Dan_Lehman

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Re: Another Bend for Dyneema
« Reply #14 on: July 25, 2017, 11:20:07 PM »
So, what I really meant to say is that tension transfers through knots in Dyneema differently than other materials and in ways that are sometimes contrary to prior tests and explanations.

Indeed !!  By firstly being sooo much less frictive,
so force isn't *shared* with adjacent parts,
and then by being so INelastic, so there isn't
some elongating at knot entry point AND THEN ...,
but force goes deep immediately!?  <argh!>

BTW. you might try a bowline where the fundamental
"nipping turn" is in fact a "cloverhand" --i.e., its an mis-tyed
clove hitch form (with the crossing of ends on the opposite
side of each other, which ... is just an overhand oriented to LOOK
like a clove hitch.  This seems to make a nice base, per SPart
curvature and all; maybe it will help?!

Thanks,
--dl*
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