KNOT TEST RIG DESIGN AND SETUP

Knotting Concepts & Explorations
21

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

22

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

Derek

23

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

24

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

25

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

26

@ 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.

27

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

28

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

29

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.

30

@ 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

31
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.