Author Topic: KNOT TEST RIG DESIGN AND SETUP  (Read 639 times)

agent_smith

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KNOT TEST RIG DESIGN AND SETUP
« on: November 20, 2018, 12:15:16 AM »
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.
« Last Edit: November 20, 2018, 05:28:53 AM by agent_smith »

DerekSmith

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Re: KNOT TEST RIG DESIGN AND SETUP
« Reply #1 on: November 20, 2018, 04:04:43 PM »
@ Mark,

Quote
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 :-



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

Dan_Lehman

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Re: KNOT TEST RIG DESIGN AND SETUP
« Reply #2 on: November 20, 2018, 10:42:27 PM »
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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Dan_Lehman

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Re: KNOT TEST RIG DESIGN AND SETUP
« Reply #3 on: November 20, 2018, 11:00:23 PM »
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.)

Quote
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*
====

agent_smith

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Re: KNOT TEST RIG DESIGN AND SETUP
« Reply #4 on: November 21, 2018, 02:15:03 AM »
per Derek:
Quote
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:
Quote
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.

agent_smith

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Re: KNOT TEST RIG DESIGN AND SETUP
« Reply #5 on: November 21, 2018, 06:06:04 AM »
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...

alanleeknots

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Re: KNOT TEST RIG DESIGN AND SETUP
« Reply #6 on: November 21, 2018, 09:11:01 AM »
Here is another affordable homebrew rig by our Guild member  Dmitry.
 https://www.youtube.com/watch?v=IOxaUiuVxAA

DerekSmith

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Re: KNOT TEST RIG DESIGN AND SETUP
« Reply #7 on: November 22, 2018, 09:40:29 AM »
per Derek:
Quote
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

agent_smith

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Re: KNOT TEST RIG DESIGN AND SETUP
« Reply #8 on: November 23, 2018, 01:43:59 AM »
per Derek:
Quote
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.

roo

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Re: KNOT TEST RIG DESIGN AND SETUP
« Reply #9 on: November 23, 2018, 01:58:35 AM »
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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agent_smith

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Re: KNOT TEST RIG DESIGN AND SETUP
« Reply #10 on: November 23, 2018, 02:45:10 AM »
Quote
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
« Last Edit: November 23, 2018, 02:49:14 AM by agent_smith »

roo

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Re: KNOT TEST RIG DESIGN AND SETUP
« Reply #11 on: November 23, 2018, 04:35:41 AM »
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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agent_smith

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Re: KNOT TEST RIG DESIGN AND SETUP
« Reply #12 on: November 23, 2018, 04:57:05 AM »
per roo:
Quote
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.
« Last Edit: November 23, 2018, 05:21:46 AM by agent_smith »

DerekSmith

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Re: KNOT TEST RIG DESIGN AND SETUP
« Reply #13 on: November 24, 2018, 06:40:23 PM »
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

SS369

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Re: KNOT TEST RIG DESIGN AND SETUP
« Reply #14 on: November 24, 2018, 07:18:37 PM »
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