International Guild of Knot Tyers Forum
General => Knotting Concepts & Explorations => Topic started by: agent_smith on November 20, 2018, 12:15:16 AM
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As the title suggests, this is the place to discuss the theory behind designing and setting up a knot test rig.
I have offered up 3 different configurations to start discussion...
In the first image ('onesided') - the knot specimen and rope will s-t-r-e-t-c-h to the right (in the direction toward the force generating machine). In this setup, force is is injected and originates from one side. This appears to be the default setup for most testers.
With regard to the bilateral setup - I believe Alan Lee uses this type of rig? (Need confirmation).
With regard to the injection of force originating from the middle (in between the knot specimens) - although this configuration is possible - i have never seen it in professional labs or with hobbyist/enthusiast testers.
I make no comment about the validity of the test rigs.
The idea behind using duplicate knots - one at each end termination - is thought to be originated by Dan Lehman.
Dan Lehman's idea was to have a 'survivor' knot specimen (one always breaks, leaving a 'survivor' knot to examine). It is highly unlikely that both knot specimens will fail at the same instant in time.
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@ Mark,
In the first image ('onesided') - the knot specimen and rope will s-t-r-e-t-c-h to the right (in the direction toward the force generating machine). In this setup, force is is injected and originates from one side. This appears to be the default setup for most testers.
Incorrect, as the load points are driven apart, the specimen is stretched between them. As the rate of extension is so low, the loading from inertial acceleration is negligible.
I would also suggest a modification to the following diagram :-
(http://igkt.net/sm/index.php?action=dlattach;topic=6332.0;attach=23663;image)
Instead of loading the loops onto a floating load point, reverse the orientation - fix the bar to the rigid LHS load point and loop the midline over the pin of the floating RHS load point. This will give improved stability to the placement and retention of the two test pieces.
Derek
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As the rate of extension is so low, the loading from inertial acceleration is negligible.
So, might a UIAA drop-test dynamic loading be of
a different matter, then? (I think it was of that that I'd
seem some indication of a difference; but whatever, I
saw certainly only a small sample, agreed.)
--dl*
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As the title suggests, this is the place to discuss the theory behind designing and setting up a knot test rig.
I have offered up 3 different configurations to start discussion...
In the first image ('onesided') - the knot specimen and rope will s-t-r-e-t-c-h ...
For any variety of reasons,
I'm willing to go with Derek's dismissal of concern
re this #1 set-up being biased; at the very least,
it might be so at a so-small/negligible level as to
be totally irrelevant. Maybe in a drop test, not so!?
THIS set-up (#1) is most
material,
time,
and machine/force-generating
efficient. For S # of specimens (each w/2 knots)
you run S # of tests
and get 2xS number of results :
i.e., breaks of S knots,
and survival of those various breaking forces
by the other S knots.
IMO, this is best.
It might not be the charm of statistical purists,
but I think that the commonly seen test-FIVE
specimens regimen falls also well shy of giving
much of a high confidence statistical record,
and as I've previously said, having a biased-LOW
strength value is probably to the better.
(And, e.g., if one is comparing knot-vs-knot
--relative rather than absolute strengths--,
the bias will be there for all candidates,
just as arbitrarily multiplying the values
by some constant could be.)
The idea behind using duplicate knots --one at each end termination--
is thought to be originated by Dan Lehman.
Dan Lehman's idea was to have a 'survivor' knot specimen
(one always breaks, leaving a 'survivor' knot to examine).
And to have a more sure, biased-lower strength mean.
Indeed, I'm very happy that my few tested
eye knots in 5/16" 12-strand HMPE were with
surviving knots (all knots having been *marked*
with embedded gold,pink, white threads so as
to gauge locations in the SPart & tails)!!
In one test (per knot), what was I going to do
with a single (even two) break value? But with
a single survivor & a broken one, well, I can
believe that there could be variance even here,
in terms of the *where-it-broke* evidence,
still, getting SOMEthing to examine,
irrespective of the value,
IMO is a considerable value.
--and i.p. I so much would NOT have preferred
the same knot, even if 2per-specimen, to have
been tested, so as to make a set of values for
statistical manipulation.
--dl*
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per Derek:
Incorrect, as the load points are driven apart, the specimen is stretched between them. As the rate of extension is so low, the loading from inertial acceleration is negligible.
There is no 'incorrect' about anything...the images of knot test rigs are simply offered up as examples of possible rig design.
There are lots of different design combinations...and I simply offered up 3.
So to use the word 'incorrect' appears to demonstrate that you come to to this forum with an altogether different mindset and agenda.
With regard to the ["low"]rate of extension - I am somewhat curious as to how you arrived at such a statement. It appears to be framed as a statement of fact.
The image of the 'lever hoist' is simply a metaphor for a hand operated force generating machine. I have access to a few different force generating machines - some can pull at slow speeds while some can pull at faster rates.
In test rigs with fixed anchorage points at each end - the s-t-r-e-t-c-h-i-n-g of the rope/knot can be significant (particularly with EN892 dynamic rope) - and you can reach the limit of travel (stroke) of the force generating machine before the desired target load has been reached (which forces a reset and a re-start after making adjustments to increase the travel/stroke).
In purpose built test labs with expensive test beds and rams - the travel/stroke of the ram is accurately known - and can be generous. In the case of enthusiast/hobbyist test rigs, because it is all 'homebrew' and done on a limited budget, everything tends to be more constrained and limited.
per Derek:
Instead of loading the loops onto a floating load point, reverse the orientation - fix the bar to the rigid LHS load point and loop the midline over the pin of the floating RHS load point. This will give improved stability to the placement and retention of the two test pieces.
It is simply an offered up image...it isn't intended to be definitive or to be a statement of absolute design perfection - its simply one possible design (of many).
I can only draw images of rig combinations at a certain pace in the limited free time I have - and I can't offer up every conceivable combination.
Further to that concept, no matter how many combinations of test rig designs I draw and upload - there will always be ways to improve upon them.
With regard to the concept of a U turn test rig - a significant issue is the fact that the forces required to reach the target load will all be doubled - and this changes the entire dynamics of a homebrew system. To reach such loads - particularly if testing EN1891 and EN892 ropes - means injecting a significant amount of force, and risks escalate accordingly.
I personally prefer a rig design where a single linear rope length is tensioned. Injection of force via a hand operated machine is less arduous and the forces are significantly lower.
I also prefer a single force generating machine to inject force. I am not an advocate of a dual injection setup where you have 2 force generating machines (one at each end).
Having said that, it would be interesting to compare data from a dual injection rig (where you have 2 force generating machines - one at each end) against the data from a single injection rig (where you have only one force generating machine located on one side).
As far as I am aware, all test rigs and published knot test results all use a single injection rig (where the force generating machine is located on one side).
The only exception that I have become aware of recently is an IGKT member who does use a dual injection rig - with a force generating machine located at each end.
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Here is yet another possible test rig configuration.
For a hobbyist/enthusiast tester using homebrew equipment - I would recommend a length of standard galvanised scaffold tubing (48.3mm diameter) - which is very strong.
Scaff tubing is universally available and very cheap.
In this test rig setup, there is only one (1) force generating machine that is injecting force into the system.
This force generating machine is installed on one side.
I personally find it easier to link the force generating machine directly to the load cell - so it is easy to keep an eye on the scale.
Particularly if you are working alone - it is hard to both hand crank a lever and watch the scale at the same time.
There are lots of different combinations for test rigs - this is simply another offering.
There is no 'correct' or 'incorrect' test rig design per se.
Rather, its a question of budget, constraints of working alone (if you have no assistant), safety, and finding anchor points that wont break or deform.
For hobbyist/enthusiast testers who are building a homebrew rig to operate in their 'backyard' - limited finances and the desired force to be reached will impact upon decisions...
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Here is another affordable homebrew rig by our Guild member Dmitry.
https://www.youtube.com/watch?v=IOxaUiuVxAA
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per Derek:
Incorrect, as the load points are driven apart, the specimen is stretched between them. As the rate of extension is so low, the loading from inertial acceleration is negligible.
There is no 'incorrect' about anything...the images of knot test rigs are simply offered up as examples of possible rig design.
There are lots of different design combinations...and I simply offered up 3.
So to use the word 'incorrect' appears to demonstrate that you come to to this forum with an altogether different mindset and agenda.
Hi Mark,
It is said that a picture paints a thousand words, and this is certainly the case for your excellent diagrams, for which I thank you for your time and efforts. This topic would surely be far more cluttered with lengthy (and perhaps misleading) wordy descriptions were it not for the time you have spent generating the images for us. For this we are in your debt.
And of course, your statement is totally correct - there is no 'incorrect' about anything pertaining to these diagrams, they are simply designs for consideration and comment - to suggest otherwise would be stupid of me and I hope that I am not (yet) that stupid. No, my statement was directed not at the diagrams, but at your accompanying description. I think this would have been obvious to most readers, so I will not dwell further on it - at least other readers have been appraised of the fact that unless inertia is a significant element, then 'direction of injection of force' is a figment of imagination and the experimental biases you have reported have some other cause (perhaps pin diameter...).
As for my 'mindset and agenda', clearly my 'mindset', or should I say 'knowledge and experience' is different to yours, hence our disparity in comments. My agenda however is to share facts, knowledge and perception, both with and from my fellow posters. Any other assumed alternative or hidden agenda is fictional imagination or has some other basis.
Derek
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per Derek:
then 'direction of injection of force' is a figment of imagination
The only figments of imagination is your apparent inability to read properly - and to stop and consider the meaning I had constructed.
Instead, your fertile imagination took and giant leap and assumed that my use of the phrase 'injection of force' to mean something else.
Due to this fixation on a phrase (like a dog fixated on a bone) - you constructed a meaning that drove your innate desire to insult and ridicule at the slightest whiff of something perceived to be wrong.
Had you paused for a microsecond before typing the defamatory and insulting words on your keyboard - you might have understood that 'injection of force' had nothing to do with 'direction'.
The concept of direction is a figment of your imagination.
I had intended the meaning of 'injection of force' to refer to a force generation machine being installed on one side.
In this instance, the force generating machine was a hand operated winch.
With regard to the concept of 'injection of force' - again, it was your imagination that attached a 'direction' to it.
In my mind, I was simply pointing out a design where the force generating machine is installed on one side - and that is the source of force - which is then 'injected' into the test rig by manually operation.
I also pointed out that in my particular test rig setup - the rope and knot s-t-r-e-t-c-h-e-s to the right (toward the force generating machine).
This is a physical observation - because what happens is that as the rope stretches - you can run out of travel/stroke in the hand winch - which could then force a reset and reconfiguration in order to gain more stroke/travel distance. In other words, all test rigs of this type have a travel limit. I call that limit the travel/stroke of the system. If you don't have sufficient travel/stroke, you can run out of room to pull the knot. This is not a violation of Newtons Laws....rather, it is a simple observation of reality - in that the knot and rope s-t-r-e-t-c-h-e-s... and while it stretches, you start to run out of of travel/stroke.
In your fertile imagination - you likely read roos post which tried to cry foul of a violation of Newtons Laws of physics or some imagined 'impossibility' of design.
He made the same mistake - in that he did not read properly - and leapt on an opportunity to cry foul of some imagined misunderstanding of the laws of physics. He then submitted a bunch of diagrams in support of his imagined proposition that I did not under physics and that my phrase 'injection of force' (which did not have a direction in its meaning) prompted him into action.
The reality is that all I was pointing out was the location of the force generating machine - nothing beyond that. It was simply to say... "Heh, just use one force generating machine and install it on one side. Please don't install two force generating machines (one at each end)."
The attachment of the word 'direction' to my 'injection of force' was your own creation and fertile imagination.
Now - there are test rig setups where you can have two (2) force generating machines - one at each end - and both 'injecting' force into the system.
This is in contrast to my system where I had only one (1) force generating machine - which was installed on one side only. I think of it as the 'point of origin of injection of force' - because it is installed on one side and is hand operated and generates force. Yes - as this force is 'injected' - Newtons Laws kick in and an equal and opposite balancing force is applied by the other side. But that is not what I was describing...I was simply describing the location of the force generating machine - ie, that it was installed on one side.
Now, it is possible that you will read the above and try to apply some imagined meaning to support yet another opportunity to cry foul of a violation of Newtons Laws of physics. And if you do - it will all be from yet another imagined viewpoint.
If I were in your shoes - I would be feeling rather embarrassed about changing the meaning of my words by attaching the word 'direction' to 'injection of force'.
The defamatory and insulting remarks would actually apply to yourself - for not being able to read properly.
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The reality is that all I was pointing out was the location of the force generating machine - nothing beyond that. It was simply to say... "Heh, just use one force generating machine and install it on one side. Please don't install two force generating machines (one at each end)."
And just what is the problem with having two "force generating machines"? You even went so far as to put a thumbs-down icon on such a setup in your Figure 9 thread.
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And just what is the problem with having two "force generating machines"? You even went so far as to put a thumbs-down icon on such a setup in your Figure 9 thread.
I have to admire your willingness to soldier on...!
I'll give you the courtesy of a considered reply... (please read very carefully)...
There are two parts to you question as follows:
1) the first part implies that some form of 'problem' exists; and
2) the second part implies that I went to some lengths to emphasize that there is a 'problem'.
To be clear - its about finding ways to replicate/duplicate test results.
When I tested the F9 eye knot - I used one (1) force generating machine which was installed on one side. The point of origin of injection of force was from the a single hand-operated lever hoist which was installed on the right hand side.
If you install two (2) force generating machines - and 'inject' force bilaterally (ie have two winches both pulling from opposite sides) - this would constitute an entirely different physical setup compared to what I had.
My point being... if you test something in a different way - your results may be different due to the design of your test rig.
Some may conceptualise this as potentially introducing a bias into the results.
Physical observations of my test rig shows that the knot and rope s-t-r-e-t-c-h-e-s to the right (in the direction of the force generating machine). As the rope stretches, you begin running out of room to keep pulling. I refer to this as the travel/stroke limit of the test rig.
If you have two force generating machines, you introduce new variables as follows:
1. Simultaneous operation of both force generating machines... (ie both injecting force from their point of origin simultaneously).
2. Reciprocal operation... one machine pulls first, then stops. Then, the opposite machine pulls and then stops. Cycle repeats (pull from right, then pull from left, then pull from right, etc).
3. Sequential operation.... one machine pulls 'x' distance of its travel/stroke limit then stops. Then, the opposite machine pulls 'x' distance of its travel/stroke limit. In this case, the rope s-t-r-e-t-c-h-e-s first in one direction, then... it s-t-r-e-t-c-h-e-s back in the opposite direction.
We need to keep in mind that the knot core compresses as force is applied. You have different rope segments moving within the core at different rates due to knot geometry and friction.
Friction results in heat which in turn can cause melting.
As force is 'injected' into the system, a good % is converted to heat which causes melting.
The rope also acts like a spring (Hookes law and also modulus)...it isn't perfectly stiff and non yielding...with EN1891 ropes having a higher modulus than EN892 ropes.
With all the variables introduced in a dual injection test rig (ie a 2 force machines both pulling at each opposite end) - it should be obvious that there are potentially a lot of biases that can be introduced and hence, the test results may be different.
I presume that satisfies both parts of your question?
Edits: typos corrected
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3. Sequential operation.... one machine pulls 'x' distance of its travel/stroke limit then stops. Then, the opposite machine pulls 'x' distance of its travel/stroke limit. In this case, the rope s-t-r-e-t-c-h-e-s first in one direction, then... it s-t-r-e-t-c-h-e-s back in the opposite direction.
If you put a camera on the right side, glued to the tip of the rope, the rope always looks like it is stretching away from the camera regardless of which mechanism operates. If you put a camera on the left side, glued of the tip of the rope, the rope always looks like it is stretching away from the camera regardless of which mechanism operates.
It really makes no difference. Where you happen to stand as you observe stretch is no less arbitrary than the cameras. The forces on both ends are the same. Rope stretch will always be a function of force. The rope doesn't keep track of which side might be moving slowly relative to some arbitrary viewpoint.
I think your experiences with rope jamming on one side can easily be chalked up to mere chance.
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per roo:
It really makes no difference... The forces on both ends are the same. Rope stretch will always be a function of force. The rope doesn't keep track of which side might be moving slowly relative to some arbitrary viewpoint.
Are you posting this information for your own gratification or is there some other intent behind it?
I have seen a dog fixated on a bone - it is funny to watch.
Once they see the bone - they cant let go of the image in their mind.
In a similar way, I see you as being fixated on playing your Newtons laws of physics card.
I actually prefer Einsteins understanding of force, gravity and accelerating reference frames.
You are fixated on arguing obvious Newtonian laws which bare no resemblance to what I have described.
I am happy for you to retain your fixation and continuing struggling to argue something entirely different to my original contention....I wish you well.
I think there is another purpose behind this...and you are driven to that end.
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snip...
I think your experiences with rope jamming on one side can easily be chalked up to mere chance.
You could well be right Roo, the tiny number of results available are still within the scope of flipping five heads on the trot.
But I felt it worth noting the bias to Mark, in case it is a real artefact produced by his test rig. Certainly it was not important enough to justify Mark backing himself into a hole and throwing out invective to cover his embarrassment.
Anyway, I think I have perceived a mechanism which could potentially cause this effect - it is down to vibration / shock loads.
If we take a pair of knots, in line, up to the point where the slip point is exceeded and cord starts to flow through / within the knot, after a while the flow stops and the friction returns to static mode, that is, it requires a higher load to exceed the incrementally increased slip point. If at this point we inject a small 'jar' into the system so that a small pressure pulse is added to the static tension, then, as the pressure pulse flows into the test cord and on into the first knot, if the pulse is sufficient, it might cause the overall tension to exceed the slip point and allow cordage to be drawn from the knot (incrementally tightening it). The energy in the pulse would be absorbed by the work done in moving the cord, so the pulse would not travel further and reach the second knot.
Conceptually, this mechanism could produce the results we see in Marks data set, but it would require the test rig to include some means of producing small pressure pulses frequently / regularly. I believe the Come-along type of tensioning device Mark shows in his diagrams can do exactly this, as every stroke, there is a small jar as the return stroke is held by the non return ratchet. Perhaps vigorous actuation of the lever handle is sufficient to generate sufficiently energetic pulses, able to produce the observed anomaly.
I do not have access to equipment to test this possible source of testing bias, and I doubt that Mark is of a mind to swallow his pride and come and play ball, so if anyone has access to suitable equipment, perhaps we could set up some trials to test the hypothesis.
My own 10 ton hydraulic test rig has rigid actuators between both anchor pins. I have tried injecting small shock waves by giving small hammer blows to one anchor pin, however, although I 'think' I am seeing an effect, the shock wave is also progressing through the solid actuators and reaching both anchor pins, while Marks 'frame' is made up of his garden between his anchor trees, eliminating any chance of a shock wave reaching the other end of the test knots.
Thoughts anyone?
Derek
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I have seen, in my own ad hoc testing, and in rope use the "chattering/ratcheting effect" as a knot tightens or even with something such as a truckers hitch at the "pulley" while snugging down. Friction builds to a point, the rope stops momentarily and releases to further tighten.
Regarding the rig design: Although the two inline test specimens ultimately feel the same load, I think the one closest to the force maker sees it first, thus reacting first.
IMO, using anything that does not impart a steady pull rate will probably impart a cyclic loading. And with smooth pull rates you may not even see the ratcheting effect.
That said, a come-along could more realistically be resembling real world use.
I believe that this is the mechanism the most times causes breakage due to the final frictional heat build up in synthetic rope. It can be accentuated by foreign materials.
SS
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I will reiterate that although I was/am-yet? on the side of
thinking that evidence suggests a difference,
IMO it's not something to rise to great significance
and worth much worry. (Might there be enough
extant test reporting to show a *hint*of bias?)
((Admitting that to "feel the force (first)" requires
there to be some resistance to the injected movement
which resistance is *feeling* it ... on & on & on.))
More significant, IMO, is the use of TWO knots
per test, simultaneously loaded. I admit that there
are *impure* aspects to this vis-a-vis statistics,
but again I'll assert that their significance is small
--or that a conservative/more-2BSure lower value
is better to have in mind(!)-- and not to be worried about
(and esp. as stat.s would want prohibitively costly
testing).
Better is to have a way to extract good general
values and information about how/where knots
break.
(E.g., EvansS had some tests of HMPE in end-2-end
knots where he reached some strong force (maybe
still well shy of 50% tensile), the knot held A WHILE,
then ... slipped down to 50# force or so!!? I'm thinking
like SS369 that perhaps there was such heating and
melting --low-threshold for HMPE!-- that its surfaces
at least in the knot became fluid and ... that
won't hold well!
What if he could repeat the test but pour icewater
or ... do it in sub-0^F temps?! That might show
a different result?! (HMPE does pretty well, cold
--better than PP, e.g..)
Heck, even if testing were done NOT TO RUPTURE,
but to say about 40-50% tensile --where likely a
knot had taken a highly compressed form unlikely
to change much, further (well, this would be something
we'd be able to check), just seeing what our dressed
& set knot had become, geometry-wise, would be
helpful insight. "OHhhhhhh, so THAT explains it!"
could well be a reaction. --seeing so much torsion
in the SPart, say, or a thought-to-be nice gradual
curve hauled dead straight into a hard turn.
... and so on. AND presumably such insight would
not be something in need of repeat testing PER
ROPE TYPE, but only across various ropes.
E.g., Mark did some side-by-side presentations
of images of BWII under 3? increasing loads.
That tells much! --and his rope lives to show again.
Thanks,
--dl*
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A key concept that needs to be reminded is that very few knot testers report details of their test rig design and on which side knots have been either breaking or jamming.
It is an area that is largely devoid of data.
Testers simply report their data - and it is just assumed that readers somehow understand the design of their test rig and what actually transpired.
Those testers that use video - in these instances we can catch a glimpse of their test rig - but again, details are normally not given - you just have to try to understand it all from the video footage.
It does appear that the default setup is to have force 'injected' from one side - that is, the force generating machine is installed on one side of the rig and it 'injects' (or introduces) force from one side.
Now this is not in conflict with Newtons laws - I am merely describing physical design.
In certified test labs, the test rig can be horizontal or vertically mounted - and normally the force generating machine is in the form of a ram.
The ram is installed on one side - that is, it is actuated from one side against a fixed anchor point (one anchor point is fixed in place and does not change its location). The opposite anchor moves - in order to induce tension which s-t-r-e-t-c-h-e-s the test article. All test rigs have a travel/stroke limit.
I haven't seen professional, certified test rigs which have 2 rams installed - each one actuating and 'injecting' force from opposite sides simultaneously.
Note: Please read carefully...I did not say this is 'bad' - I'm simply saying that I have never seen it.
The same goes for the word 'inject' - its simply a physical description of where the force generating machine is installed - and the point-of-origin of force. In my personal case, my force generating machine is a 'lever hoist' - which is hand operated from one side of the test rig.
As Scott has alluded to, each knot will behave differently when subjected to load - and different rope segments within the core will move at different rates - no doubt determined by its geometry. Although many like to assume that force is transmitted instantaneously from point to point in a test rig - the fact that there is a knot which compresses and has moving segments may confound that assumption. Now I used the word 'may' - I didn't say 'will' or 'does'.
I have a feeling that force is not transmitted instantaneously from point-to-point in a test rig...there is a 'point-of-origin' (ie the force generating machine) - and force must pass through the knot which itself has segments moving at different rates. There may be a millisecond delay as the knot absorbs initial force and rope segments are ratcheted out. It may be a millisecond or it may be more or less (I don't know). No one has actually investigated this.
This is one of the reasons why I prefer to test identical knots against each other (like against like)...rather than 2 different knots against each other.
I think there is very little data about actual test rig design and how this might affect test results.
The point being...if every tester uses a different design rig with different test parameters - it may be difficult to build a consistent database.
This may hint at why there is such a huge disparity in test results around the world (aside from the fact that everyone uses different ropes/cords and rarely reports on exactly how a knot specimen is tied).
As stated, none of this has really been explored or discussed in any detail before - its all just been 'assumed'.
And the phenomena of a knot breaking on one side of the test rig or the other...this hasn't been properly explored. I don't know of any knot tester who even bothers to report which side of the rig the knots broke on (or if its random). It appears to be rarely reported (if ever).
And if a tester does report a little more detail - it potentially opens pandoras box...so maybe that's another reason why some testers prefer to only report the results and not go into details about the test rig design and other observations (eg on which side the knots broke or jammed).
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Should you also be differentiating between "Strain generating machines" and "Force generating machines"?
What's being described in this setup is a strain generator. When the knot slips, you are likely losing some load in the system.
That is very different from a real-world situation where you have someone hanging off the end of the rope. If the knot slips in that system, load is not released.
Is this worth considering? I suspect that it's much easier to build a test rig with a ratchet than find enough heavy weights!
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I suspect that it's much easier to build a test rig with a ratchet than find enough heavy weights!
Thanks for your post GlasafMT.
I think you had already answered your own question with your last sentence..
The manual handling of a series of weights would create a number of problems - not to mention back strain from all the lifting.
Also, such a test rig would have to have a number of weights - which would need to be added sequentially - and each time a weight is manually handled and added, there would be a spike in the load as it is attached and then released from your grasp.
It isn't practicable to work like this.
Also, you would have to purchase a number of weights - likely to be circular disk weights that body builders use on a bench press machine (these are insanely expensive to purchase).
Then there is the storage of all those individual weights...
You mentioned that if a "knot slips, load is not released"...while a knot is 'slipping' - something causes it to slip and this usually means load. If zero load, then no slip.
I might also point out that the knots I test don't 'slip' - they just reach their yield point and then fail. It is not a case of 'slipping', rather, the knot compresses more and more and tries to resist the injected force. Once the force reaches a certain threshold - the knot yields (you can hear snapping/popping sound as this yield point is reached).
The ideal test rig is to have a permanent bench setup that uses a hydraulic ram that can reach 5 tons or more force.
Attached to the ram would be a tension load cell that polls at 50 cycles a second to capture data - that can be linked to a laptop computer.
The Rock Exotica 'Enforcer' load cell is a marvelous gadget but, it only goes to 20kN (about 2 metric tons).
I wish Rock Exotica would release a newer version that can reach at least 40kN (about 4 metric tons).
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Derogatory comment by Derek:
Certainly it was not important enough to justify Mark backing himself into a hole and throwing out invective to cover his embarrassment.
This type of comment is an example of a fertile imagination actuated by malice.
The real embarrassment ought to lie with Derek - for failing to read English properly.
I assume that Derek speaks and reads English as a first language - and if this is true - it only adds to the level of puzzlement over his inability to read properly.
I never attached the word 'direction' to injection of force - instead; merely describing a physical design where the force generating machine is installed on one side.
Derogatory comment by Derek:
and I doubt that Mark is of a mind to swallow his pride and come and play ball,
This type of comment is another example of a fertile imagination actuated by malice.
The person who should be 'swallowing his pride' is Derek - for failing to properly read and understand his native language (English?) - and to reach false conclusions based on a false premise.
The concept I had advanced being; 'injection of force' - was directionless. It was simply to describe a physical design where a force generating machine was installed on one side of the test rig. This was to distinguish it from a design where you have two (2) force generating machines - one at each end - both 'injecting' force simultaneously.
And further to that, I made no comment about a dual force generating machine setup (ie each one pulling at each end simultaneously) being 'bad' or 'no good' - merely hoping to encourage repeat testers to use a similar test rig setup to mine - so as to remove potential for bias in the tests results (compared to mine).
I had surmised that as force is 'injected' into the test rig - the knot begins to respond according to its geometry. Different rope segments will move at different rates within the knot core. I also surmise that there could be millisecond delays in the response of the knot to load - and that maybe end-point to end-point transmission of force is not instantaneous or precisely equal on account or ratcheting of the knot segments and heat buildup and heat loss by direct radiation and by convection. In other words, it seems possible that some energy pumped into the system is converted to heat and possibly vibration. I don't have all of the answers and I doubt anyone else does either - all I can do is report what I observe.
One way to measure potential losses is to use two (2) load cells - one installed at each end. The load cells would have to be very accurately calibrated and poll at 50hz - with data captured in real time on a laptop. If there is disagreement between the load cells and in millisecond delays - even by the tiniest amount - that would point to something worth following up on with a battery of further tests. I would say that this sort of real-time precision measurement would be beyond the capability of the hobbyist/enthusiast tester.
...
I have noticed that the people who make stuff up and seem willing to sling insults are also the same people who spend more time slinging insults than doing practical testing and publishing their results for others to see.
I also note that it is rare for a knot tester to go into any fine detail about their test rig and to report on observations about which side their knots jammed or broke on. Its simply never reported.
And yet, if you do go the extra mile and report these observations - you risk drawing negative attention for doing so.
I can understand why many on this forum choose to remain silent and not bother to report on their own tests..I mean, why bother when you have people who make stuff up, sling insults and construct meanings from their own imagination...?
It all seems not worth the effort.
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To All;
Healthy debate, intelligent banter, staying on topic and positive contributions are what we should stay doing. I feel that there is a situation that is degrading here and personalities are equaling knots under strain.
So, lets leave the invective, etc. out of here and put back the ideas and discussions.
What I have used in the past has been a ratcheting come a long that employs a cable, winch like. I know there has got to be too many built in loses in my testing set up. And I don't have a load cell either. I use the stated MBS of the rope's manufacturer as the breaking benchmark. And, sincerely, that matters little because I am trying to cause a break. I use the tug of war method between test specimens, sometimes with X vs Y different knots. Example; Standard bowline vs modified bowline or vs F8, etc.
My anchor points are generally a large tree and my truck's trailer hitch. I can adjust the distances that way. I loop a heavy duty, industrial sling ( way beyond the strength needed) around the tree with a screw shackle that is near 3/4 inch in diameter to the test sample. From there the other end of the sample, two eye knots, generally 16 inches overall length, some locations marked with sharpie, to the come a long which is connected to my truck via large chain. I take out all the slack by driving, lock the vehicle down well then continue by adding load at the rig. This allows me to watch the test fairly close and I can even record, stills and video.
Not scientific in the least ( nor very safe), I know, but my testing is for me, to verify certain ideas I have before I offer them to anyone.
I would like to thank you Mark for what you bring to our table and I personally do understand how much you invest, life and otherwise.
Same goes out to Alan Lee.
Thank you both!
So, please, no more sword slapping, by anyone.
SS
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The amazing (and surprising) power of the shock wave -- relevant perhaps to the proposal that small shock waves induced by the Strain inducer ratchet action could be leading to a bias in the jamming observed in the knot nearest to the Strain inducer.
https://www.facebook.com/5min.crafts/videos/708318142644107/?t=41 (https://www.facebook.com/5min.crafts/videos/708318142644107/?t=41)
Derek
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Ahh, I see now.
Maybe edit your post with some explaining of the relevance.
I have personally noticed the incidence of tensile resonance. I suspect and believe it is from the grabbing and releasing of forces in the tight part(s) of the knot.
And I am of the opinion that the knot closest to the tensioning mechanism receives it first.
First come, first served...
SS
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Ahh, I see now.
Maybe edit your post with some explaining of the relevance.
I have personally noticed the incidence of tensile resonance. I suspect and believe it is from the grabbing and releasing of forces in the tight part(s) of the knot.
And I am of the opinion that the knot closest to the tensioning mechanism receives it first.
First come, first served...
SS
RE 'post with some explaining' - Will do Scott,
Re 'First come, first served' - as the shock wave enters the first knot, there is a localised Stress wave and the knot, verging on responding to an incremental increase in load, is tipped over its static frictional resistance threshold and tightens (slips internally). This movement (force x distance) is work and the energy to induce it is taken from the shock wave, cancelling it out, thereby preventing it from getting to the second knot.
Derek
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Should you also be differentiating between "Strain generating machines" and "Force generating machines"?
What's being described in this setup is a strain generator. When the knot slips, you are likely losing some load in the system.
That is very different from a real-world situation where you have someone hanging off the end of the rope. If the knot slips in that system, load is not released.
Is this worth considering? I suspect that it's much easier to build a test rig with a ratchet than find enough heavy weights!
Glasa, you are of course totally correct, we are using Strain generators, the strained cordage and associated knots respond by generating a reactant Stress force measured on the load cell device.
And again, I totally agree, in a gravitationally loaded system, as the knot slips the load remains constant. But, just as we can use gravity to load a knot to a certain (constant) value, so with a Strain inducing system, we can induce strain until the requisite stress is achieved and the results can be observed. For example, with my Instron screw driven system, I am able to program in a Stress force and record the resultant Strain as the test subject slips or distorts.
Oh, and welcome to the Forum / Topic...
Derek
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@ Mark,
I might also point out that the knots I test don't 'slip' - they just reach their yield point and then fail. It is not a case of 'slipping', rather, the knot compresses more and more and tries to resist the injected force.
A simple observation of the amount of 'slippage' that occurs within a knot may be made by sewing a small 'witness' thread close to the points the cordage enters the knot. You will be able to observe the 'witness' marks moving away from the knot as slippage occurs inside the knot and cord is drawn out from it. Some, poor, knots also 'slip' by distortion and consequential migration along the cord. In low cF cordage, the great majority of known knots 'slip' badly.
All knots slip under load.
Derek
NB - Dan taught me this simple but valuable technique several years ago when I set up my 10 ton hydraulic test rig to run knot comparison tests.
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per Derek:
All knots slip under load.]
Although off-topic and not directly related knot test rig design - I completely reject this statement.
However, my rejection originates from the strict English dictionary definition of 'slip'.
It is possible that Derek intends and has constructed a narrow meaning to the word 'slip'.
For example, I have conducted load testing on the Zeppelin bend and I observed no 'slippage' (by my definition of the word). What I did observe is response to load which caused the following:
[ ] core compression
[ ] extrusion of rope from the core
[ ] stretching
These events continue until core compression (collapse) is complete and the elastic limit of the knot is reached...at which point the knot reaches it yield point.
It is possible that Derek may choose to construct a meaning where core compression, extrusion of rope from the core and stretching is held to be 'slippage'.
However, the ordinary definition of 'slip' or 'to slip' could be taken to mean:
^to let loose from a restraining leash or grasp
^to cause to slip open : release, undo or slip from a lock
^to let go of
^to disengage from (an anchor)
In the context of knotting,'to slip' implies some form of insecurity. And this 'slippage' therefore indicates tail length reduction as the knot allegedly 'slips'.
In the case of the Zeppelin bend, I don't see tail length reduction - because it isn't 'slipping' (by my definition).
I see the same phenomena in #1425A Riggers bend and #1411 F8 bend. Any tail length reduction is simply a consequence of core compression - the tails partially draw into the core (but only a small amount) - until core compression inhibits any further movement and at which point the elastic limit of the rope is reached and fracture is imminent. This tail draw-in is not 'slippage' (in the ordinary sense).
Due to the complexities of the English language, it is likely that some will construct a meaning where tail draw-in due to core compression and extrusion of rope outwards from the core is defined to be 'slippage'. I would disagree with such a construction.
Edit note: Fixed quote function
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the tails partially draw into the core (but only a small amount) - until core compression inhibits any further movement
With apologies for prolonging an off topic excursion Mark, could I ask how you see compression within a knot causes an otherwise unloaded tail to 'draw into the core'. Or do you see it as some other mechanism that causes the tail to tail to be drawn into the core?
Derek
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Derek; start a new topic post titled:
"Proposition that all knots slip under load"
Debate your proposition there.
I would advise that you carefully define the word 'slip' - before embarking on expanding your theory.
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@ Mark,
Thanks for your suggestion and your advice, I will consider both.
Meantime, I will take it that you have no rational answer to my question, which is good, because I could not think of one either.
Derek
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Meantime, I will take it that you have no rational answer to my question, which is good, because I could not think of one either.
Completely incorrect.
Your question required no formal response - because I had already outlined my position with regard to 'slip' and 'slipping'.
And as previously explained - I take the ordinary meaning of 'slip' and 'slipping' to mean something entirely different your notional understanding of that word (in the context of knots).
If you wish to debate your proposition that all knots 'slip' when subjected to load - you should open a new topic.