Author Topic: A Strong Loop Knot (AKA a double)  (Read 4502 times)

DerekSmith

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A Strong Loop Knot (AKA a double)
« on: November 04, 2007, 12:57:47 AM »
I am always on the lookout for knots which do their job without significantly weakening the cord they are tied in.  Generally however, the really strong knots tend to come with quite a tedious production overhead.  The Plaited Double is an example of a strong knot which takes a bit of tying.

Recently, I was experimenting with methods of producing load shedding 'twists' and came up with a method of producing what promises to be a very strong 'double' or loop knot that can be tied in a matter of seconds.

Here is what it looks like:-
   

The SP enters essentially just a twist while the loop wraps back up around the outside of the twist.  Load exchange from SP to loop is very gradual through the loop/wrap assembly.

To tie it, take the end and double ca 2ft.  Hold this double about 18" from the end loop and double it again.  Put your index finger through the doubled loop and hold the four strands between fingers and thumb of the left hand.  Rotate the index finger to 'wind up' the four cords into a twist.  When sufficient twist has been wound, take the loop strands and wind them back down over the twist in an opposite direction.  When you reach the end of the twist there will be a small loop which you had your index finger through to wind the twist.  Pass the loop through this twist termination.  Pull on the loop and the lines to tighten the twists.

I will put it into the list of knots to be tested, but I expect it to perform very well, even against the plaited double.

Has anyone seen this type of structure documented before?

Derek

Dan_Lehman

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Re: A Strong Loop Knot (AKA a double)
« Reply #1 on: November 04, 2007, 03:24:11 AM »
I've fiddled the same & similar things, and have "documented" them, personally
(and have a bunch of knot-in-rope records awaiting such pen-to-paper recording,
partly on account of the bother they are to illustrate; partly because I'm not all
so enthusiastic about them).  It's a somewhat natural direction to go, given
certain premises, and so on.  Most recently, I put in the additional criterion
of having the loopknot be able to be tied AFTER sizing the eye--one of the
endearing qualities of the Bwl (unlike most other LKs)--; not so sure if that one, either.

Let me note something I've noticed in these and various other knots, when
tied in braided cord/rope:  the material shows considerable torsion--one
can see the strands going one direction (Z or S) all arched and obviously
bearning no tension, with the opposite ones drawn tight.  I'm not sure how
much a like effect carries to the core (depends on the nature of the core,
for starters), but it's hard to speculate that the knot's really strong if half of
the sheath is not on duty!

I think that the Spider Hitch of anglers is similar.  ("think" because the final
state of angling knots is seldom illustrated well, and I've not played around
much with the fine fiddly stuff.)

The interest in knots that don't weaken the line much might be found to
have little real significance in the real world; anglers seem to have much
of it, but one can wonder at how successfully their variously tied knots realize
the expected strength.  Again, I refer you to Sport Fishing mag.'s Doug Olander's
recent testing of various knots in various fish lines.  (His reports are available
on-line.)  But there is this appeal of finding solutions to an at least imagined
issue that can be put to a seemingly objective test.  (I find myself aiming for
the Strength Target all the time!)

--dl*
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DerekSmith

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Re: A Strong Loop Knot (AKA a double)
« Reply #2 on: November 04, 2007, 07:34:12 PM »
I've fiddled the same & similar things, and have "documented" them, personally
(and have a bunch of knot-in-rope records awaiting such pen-to-paper recording,
partly on account of the bother they are to illustrate; partly because I'm not all
so enthusiastic about them).  It's a somewhat natural direction to go, given
certain premises, and so on.  Most recently, I put in the additional criterion
of having the loopknot be able to be tied AFTER sizing the eye--one of the
endearing qualities of the Bwl (unlike most other LKs)--; not so sure if that one, either.

Let me note something I've noticed in these and various other knots, when
tied in braided cord/rope:  the material shows considerable torsion--one
can see the strands going one direction (Z or S) all arched and obviously
bearning no tension, with the opposite ones drawn tight.  I'm not sure how
much a like effect carries to the core (depends on the nature of the core,
for starters), but it's hard to speculate that the knot's really strong if half of
the sheath is not on duty!

I think that the Spider Hitch of anglers is similar.  ("think" because the final
state of angling knots is seldom illustrated well, and I've not played around
much with the fine fiddly stuff.)

The interest in knots that don't weaken the line much might be found to
have little real significance in the real world; anglers seem to have much
of it, but one can wonder at how successfully their variously tied knots realize
the expected strength.  Again, I refer you to Sport Fishing mag.'s Doug Olander's
recent testing of various knots in various fish lines.  (His reports are available
on-line.)  But there is this appeal of finding solutions to an at least imagined
issue that can be put to a seemingly objective test.  (I find myself aiming for
the Strength Target all the time!)

--dl*
====

Hi Dan,

I agree, this is a natural direction to go, but only if you are in a mind to hunt for strength in a design.  It seems we had that in common when we arrived at this design (and doubtless others have before us), and likewise, this little beastie has sat on my desk now for a couple of weeks waiting for me to find time to document and publish it.  However, this is one knot I will definately not be creating a line drawing for, nor will I attempt to calculate the Overs Index for it -- it will just have to go into that end OI slot of OI:(Lots and Lots)

Yes it is similar to the Spider, in fact it was the Spider I was experimenting on when I saw the limitations and 're-engineered' it to a more idealised structure -- sort of an 'evolution' of the Spider.  For those unfamiliar with the Spider take a look at the Leadertec depiction of it.

http://www.leadertec.com/tipsandtechniques/fishing_knots/knots_spider.html


I accord with your comment re 'torsion'.  I have an as yet unsubstantiated belief that torsion plays a big part in weakening fibers inside a knot, both by overtensioning and by over compression.  The cord I prefer to use for experimentation is R20C, it is a relatively stiff, cored, 16 plait polyester braid.  It is very resistant to twist and will kink badly rather than allow a twist to 'bed in' to a knot.  For experimental work I very much favor a cord which is unforgiving about twist and will constantly remind me of the need to add or extract twist in order to keep a knot torsion neutral

In making this knot it is very easy to incorporate torsion and I am sure that this would be to the detriment of the final performance.  However, if the tyer is conscious of the importance of managing twist, then it is relatively easy to 'wind up' this knot keeping all the cords torsion neutral and when you do this, the knot does not exhibit those arched/rigid torsion points you have identified.  However, saying that torsion can be neutralised when tying a knot is a bit academic if it relies on the tyer being conscious of the importance of managing torsion in the first place.  I have seen so many knots with torsion kinks in them that I think it is the norm for tyers not to give a moments thought to twist in the cord or the knot.  In that respect, any knot which cannot (by structure) manage its own torsion should be considered to be weak or at risk and this little structure might well be thought of as the Queen of Kinks.

Re your comment but it's hard to speculate that the knot's really strong if half of the sheath is not on duty!  I put this 'knot' under extreme load and had some very surprising results.  I have noticed that I am now able to detect when this cord is just starting to fail by the occurence of a series of high pitched pings or crackles as individual fibers part and the highly tensioned sample resonates to these tiny fractures.  I took a sample of this knot to this 'crackle point' and photographed the result.

The first thing to note is that this knot absorbed a huge amount of energy as it approached its breaking point.  The knot was loaded near 2 kN and then the wound core started to compress and continued to do so over an extension of ca 0.1m -- so this little knot stored roughly 200 Joules at full load by compressing the coils of the central twist.  The coils can be thought of as the crumple zone of the knot.  Not much value in static loading but very important in absorbing the force of a dynamic load and to put 200 Joules into perspective, its the kinetic energy contained in a 1kg mass when it hits the deck after having been chucked off the top of a 20m tall building -- and all that energy was absorbed in this one little knot made out of 2mm polyester braid.

The shock for me came when I saw where the cord was beginning to fail.  Here are two images of the knot just prior to failure.  The first shows just how compressed the coils have become and the second is a closeup of the top outer shoulder, where the loop strands, having passed several times around the core, turn to become two of the four core strands.  If you look carefully you can see the frayed fibers and the badly distorted cord structure.

 

If anyone would like to study higher resolution pictures, let me know and I will email them to you.

You never know, this little knot might turn out to be a valuable 'shock absorber' loop for static climbing ropes ??

Dan_Lehman

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Re: A Strong Loop Knot (AKA a double)
« Reply #3 on: November 04, 2007, 09:04:11 PM »
Are you saying that the knot broke in the EYE LEGS?
--for that is what appears to be frayed, and which show great
torsion (they are squared nearly into diamond cross section!).

As for shock absorbing, I'm skeptical of relying on knots for this.  Dave Merchant
of "Life on a Line" has reported that some more complex knots such as the Fig.9
show weakness on shock loading, he conjectures from frictional heating.

As for torsion neutralizing, I think it can be a matter of anticipation--of putting
in a counter-torsion based on expected torsion, not merely having the
knot neutral at the start, for the elongation during loading torques the material.

--dl*
====

DerekSmith

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Re: A Strong Loop Knot (AKA a double)
« Reply #4 on: November 06, 2007, 05:59:32 PM »
@Dan,

Yes that is exactly what you are seeing, all the action was concentrated at the SP end.

This knot was severely damaged by the time it started to yield and yet it was all focused in the one spot.  I do not doubt for one moment though that the SParts were equally damaged, but it was a new thing to see the outer radii of the eye legs deforming and yielding in this manner.

I think it will be worth while pitting this against some of the stronger knots when formal testing commences.

Derek

Phil_The_Rope

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Re: A Strong Loop Knot (AKA a double)
« Reply #5 on: November 06, 2007, 09:20:16 PM »
I'm going to have to start doing some homework and trawl through all my books and past issues of "Knotting Matters"!

From an amateur's point of view, I dare not make any bold statements about the strength and stability of knots/bends/hitches etc., but I do recall from my early days of learning basic knots that in order to maintain as much strength as possible one should attempt to "loop" or "tie" rope or cord as parallell as possible to the original lay of the rope/cord. This, presumably, is why splicing is so popular?

My apologies for "dumbing down" the debate, but I find it incredibly difficult to follow all the long discussions!

Is it possible to produce a paper on the subject, starting from first principles? Maybe one's already been written?

There again, maybe I should stick to making bellropes? To my knowledge, none of them has ever collapsed under the strain of being pulled!

Fascinating stuff, guys - I'll do my best to follow your arguments (whoops - I mean "discussions").

Phil

Brian_Grimley

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Re: A Strong Loop Knot (AKA a double)
« Reply #6 on: November 06, 2007, 11:26:54 PM »

There again, maybe I should stick to making bellropes? To my knowledge, none of them has ever collapsed under the strain of being pulled!


Well, speaking of bellropes breaking ... The extra length of bellrope left when lashing the bellrope around the spokes of a bell's wheel, shown, for example here: http://bp1.blogger.com/_lh70hFYWpkU/RsGACgpk39I/AAAAAAAAAOE/G7qVHdm9AAI/s1600-h/IMGP1364.JPG , is left to allow for splicing when the bellrope breaks from wear. The wear occurs where the bellrope passes through the wheel and where the bellrope passes through a pulley or a fairlead that arranges the bellropes for the ringers a floor or so below.

I know, I know, Phil, you meant a different type of bellrope!  ;D :) Neat, though, huh!?  ;D

Phil_The_Rope

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Re: A Strong Loop Knot (AKA a double)
« Reply #7 on: November 06, 2007, 11:54:06 PM »
Point taken Brian!

Actually, when I mentioned my interest in knot tying to a friend recently, it turns our he's a bell-ringer. He asked about teaching bell ringers to splice, so that's another video project on the cards ... What, you didn't follow the "Video on the web" thread? Shame on you!

Just goes to show that there is scope for filming knot tying, or splicing in this case, for teaching purposes. There are obviously still (thankfully) folks who need to work with rope, and my friend was quite concerned that he was the only one in his bell ringing "gang" (what do you call a group of bell ringers?) who could splice.

Apologies to Derek and Dan - we've detracted from the main discussion here. Funny though, isn't it, how a discussion can reveal other applications where the IGKT could help?

Phil


Brian_Grimley

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Re: A Strong Loop Knot (AKA a double)
« Reply #8 on: November 07, 2007, 03:38:34 AM »
Phil, let me add my apologies to Derek and Dan for distracting from the main topic.

Oh, by the way, if you have added "splicing a bell rope" to your video projects, here is the type of short splice suggested by the The Worcestershire & Districts Change Ringing Association: http://www.wdcra.org.uk/study/splice.htm . I think that it is the "Rigger's Short Splice", ABOK #2648 (I might stand to be corrected). The Worcestershire & Districts Change Ringing Association also shows a long splice: http://www.wdcra.org.uk/study/long.htm . I may have posted this site some time ago but, I couldn't find it so I thought I would post it again.

If you have a chance, I would be very interested in knowing which splice your friend uses. Thanks in advance.