Author Topic: Knot Test Report: #1425A derived Riggers X bend (resistance to jamming) 3/3  (Read 4918 times)

agent_smith

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Part 3 of 3

To rule out any bias caused by my test rig, I inverted (flipped) both the the orientation of the lever hoist and reversed the chirality of the knot.

After inverting the test rig and changing to Z/Z chirality, I am confident in stating that the collar on the same side as the force generating machine (ie lever hoist) is most vulnerable to jamming. In all instances where I was eventually able to loosen jammed knots with tools, I could only succeed by starting to loosen the knot on the side opposite from the force generating machine.
« Last Edit: August 20, 2018, 01:28:53 AM by agent_smith »

Dan_Lehman

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Part 3 of 3
 I am confident in stating that the collar on the same side as the force generat[ion]
 is more vulnerable to jamming.
In all instances where I was eventually able to loosen jammed knots with tools,
I could only succeed by starting to loosen the knot on the side opposite from the force generating machine.
Very interesting.  I guess it makes sense in that it is
on the force-generation side that the knot (anchored
on the opposite end) is *urged* to move, and this
requires the drawn force-side SPart to deliver ... into
the knot, and it gets a head start on tightening?

IIRC, there were some Tom Moyer test results that
showed a similar bias on what broke (maybe it was
fig.8 eye knots on both ends?).

And would it be aggravated by loading rate --i.e.,
a lesser force generated in a drop test giving the
same level of jamming as a greater slow-pull force?
(And maybe a greater bias, to boot : i.e., that in this
conjecture the forced parts get tighter quicker AND
the opposite members get tighter slower (than in the
slower-loading case) ?! )


Btw, I've tried to avoid "taking sides" in this
discussion, seeing it not so much per side but of the
force-generating SPart (and whichever side it goes to).
(In a squaReef knot, e.g., the *collar* is *before*
the respective SPart and not *behind* as in #1425.)

Thanks,
--dl*
====

agent_smith

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Quote
it makes sense in that it is
on the force-generation side that the knot (anchored
on the opposite end) is *urged* to move, and this
requires the drawn force-side SPart to deliver ... into
the knot, and it gets a head start on tightening?

You may be correct in this assumption.
I believe that other testers should investigate this further - so we can see if this is an isolated occurrence with my ropes and my test rig or, if it is a common occurrence when the force generating machine is on one side.

A thought... I do wonder if its possible to use two (2) force generating machines - one on each side of the knot - both pulling and injecting force into the knot structure simultaneously (a sort of mirrored test rig)?

However, such a test rig is beyond my hobbyist/enthusiast capabilities...

But, it would be very interesting to get some test results with a mirrored test rig.

Harold Kahl

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Very interesting.  I guess it makes sense in that it is
on the force-generation side that the knot (anchored
on the opposite end) is *urged* to move, and this
requires the drawn force-side SPart to deliver ... into
the knot, and it gets a head start on tightening?
No, it really doesn't make sense. This is basic physics. The only way one side of the knot could experience a different tension than the other side would be if the knot had some considerable mass and was being accelerated at a considerable rate. The knot is not accelerating, and barely even moves.

I think you can see that if the rope were not anchored on the other side, it would be impossible to apply any force to the knot with the tensioning device.

agent_smith

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per Harold:
Quote
The knot is not accelerating, and barely even moves.

In a strict sense, acceleration occurs in reference frames. And Einstein said that you cant distinguish between gravity and acceleration within their own reference frames (I think the equivalence principle?). The knot is a mass concentration and 'tries' to unravel as force is applied. Your comment - I think - would be true if there was no knot - just a uniform length of rope. However, there is a knot (in the approximate middle of the rope) and it tries to pull apart as soon as load is applied. The segments of rope that are extruded from the knot might be 'accelerating' according to a reference frame?

I think what Dan is suggesting is that rope is being drawn out (or extruded) from the knot as load is applied.
The injection of force is from one side only (ie it is asymmetric).

The rope is not rigid - it is flexible and stretches as load is applied - and there is a mass concentration caused by the knot.

Given that force is causing the rope to stretch in one direction (either left or right), it does appear that rope is being drawn out initially from the side of the knot that is adjacent to the force generating machine.

I look at the issue this way; one side of the knot is definitely tighter and more compressed than the opposite side. Something must be causing this, right?

EDIT NOTE:
Several grammar corrections have been made to clarify meaning (and also to correct typos).
« Last Edit: August 22, 2018, 02:18:35 PM by agent_smith »

DerekSmith

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Quote
I look at the issue this way; one side of the knot is definitely tighter and more compressed than the opposite side. Something must be causing this, right?

Obviously, yes.  But the point that Harrold is making is totally valid.

When you apply a force one side of the knot to 'draw the rope out' it can only be drawn out if there is an equal force on the opposite side opposing it.

Consider, you apply a tiny force to one side to move the knot.  The force tries to move the knot, but the knot cannot move because it is fixed on the opposite side, so this tiny force also builds up on the other side.  Now put on another tiny force etc etc.  As you apply force it has to be reflected on both sides equally, all the time.  You can never have a force greater on one side than the other.

The only time this would not be true would be if there is vibration fed into one of the lines and the inertia of the knot prevents it from reaching the opposing line.

But before you go hunting for phantom vibration, could the issue be down to the way the knots are tied. A number of tests could be imagined to try to tease out the reason for the anomaly.

Tie two knots in series and see if the same side collar jams.
Tie two identical copies of the knot and test one of them 'reversed'
...

Whatever is causing this anomaly has to be identified, otherwise how can you know which side is the truth.

Derek

Harold Kahl

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per Harold:
Quote
The knot is not accelerating, and barely even moves.

In a strict sense, acceleration occurs in reference frames. And Einstein said that you cant distinguish between gravity and acceleration within their own reference frames (I think the equivalence principle?).
Right, and from the reference frame of the tensioning device, the tree on the other side of the knot is moving.

agent_smith

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The only way to determine if this is a phenomena isolated only to my test rig or, if it is typical of unilateral test rigs; is for others to repeat the tests.

I use the term 'unilateral' test rig to delineate situations where the force generating machine is on one side and all force is being injected from one side.

In my unilateral test rig, I can confirm that rope stretching occurred in one direction only - and that direction is toward the force generating machine. Rope stretch never occurred in the direction toward the opposite fixed termination anchor point.

In my view, the clues are:
1. The force generating machine is on one side of the test rig
2. Force is being injected from one side.
3. In unilateral test rigs, rope stretch always occurs in the direction toward the force generating machine.

Possible test methods include:
1. Use a bilateral test rig (so force is being injected from both sides simultaneously - a mirrored setup)
2. Tie several end-to-end joining knots within a linear series and apply force from one side (unilateral test rig).
NOTE: Harold's test rig is apparently setup as per item #2 above...however, I am unclear if he observed jamming on only one side of the end-to-end joining knots? Also, Harold did not use EN1891 type A ropes or EN 564 cord (different rope materials would yield potentially different results).
« Last Edit: August 25, 2018, 02:54:03 PM by agent_smith »

Dan_Lehman

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... if the knot had some considerable mass and
 was being accelerated at a considerable rate.
Such as on the end of a whip.

Well, I am physics-ignorant, yes.  But it DOES seem to be
something observed, and at least worth a bit of inspection
on that claim --i.e., if observations really amount to something
looking like a bias in need of explanation vs. more coincidence.
(And in the back of my head I felt some tingling of someone
better educated having remarked at this aspect being of
significance --which is not to claim you're UNimportant!   ;) )

(On the home-test front, I suppose one could gather
some batch of light-weight cordage (string) and do
multiple drop tests --quick'n'dirty, cheap'n'easy.

Fish line would likely give a more material-sure/-consistent
result (over cotton string, say, such as I've salvaged from
rice/malt bags (makes useful whipping, where break strength
is a small matter).

(-;

roo

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Re: Knot Test Report: #1425A derived Riggers X bend (resistance to jamming) 3/3
« Reply #9 on: September 11, 2018, 10:49:58 PM »
Part 3 of 3

To rule out any bias caused by my test rig, I inverted (flipped) both the the orientation of the lever hoist and reversed the chirality of the knot.

After inverting the test rig and changing to Z/Z chirality, I am confident in stating that the collar on the same side as the force generating machine (ie lever hoist) is most vulnerable to jamming. In all instances where I was eventually able to loosen jammed knots with tools, I could only succeed by starting to loosen the knot on the side opposite from the force generating machine.
I haven't seen any updates for a while, so I assume you're done with what you're doing.  In view of the small number of trials, I'm guessing that mere coincidence is the most likely explanation for the phenomenon you're seeing as I doubt the weight of the rope would be enough to skew the equal and opposite force action and reaction much even if you had a vertical rig with unusually long rope ends.

Pictures of your setup might help find problems.

Differences in lubricity of the rope samples or contamination (e.g. oils, greases, etc.) may be another skewing factor.  Vibrations may help with compound sliding as explained in the first footnote here.  Perhaps one side of the knot is seeing more vibration.
« Last Edit: September 13, 2018, 09:55:25 PM by roo »
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