Author Topic: Figure 9 eye knot (jamming threshold)  (Read 189 times)

agent_smith

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Figure 9 eye knot (jamming threshold)
« on: November 18, 2018, 02:43:08 PM »
TEST REPORT
Knot: Figure 9 eye knot

Test objective:
To determine the jamming threshold of the F9 and compare it to F8 eye knot (#1047).
A widely held view is that the F9 is less vulnerable to jamming in comparison to the F8.

Type of rope material:
Two different types of ropes were tested; EN892 (dynamic) and EN1891 type A (low elongation).
[ ] Class: EN 1891 type A
[ ] Diameter: 11.1mm (7/16 inch) diameter
[ ] MBS: 35.3kN
[ ] Manufacturer: Bluewater USA

[ ] Class: EN892 (certified as single, half and twin)
[ ] Diameter: 9.1mm
[ ] Manufacturer: Beal (France)
[ ] Model: Joker

Tester: Mark Gommers
Test date: 18 November 2018
Tester classification: Hobbyist / enthusiast

Test rig: (refer to image for details of rig setup)
[ ] Dynafor 5 ton digital tension load cell
[ ] 2 ton lever hoist
[ ] Natural tree anchors in backyard
[ ] Unilateral setup - force generating machine (ie 'lever hoist') located on one side of knot - all force injected from one side
[ ] Lever hoist pumped by hand while observing load cell LCD display

Assumptions:
At the threshold of jamming, it is still possible to untie a knot by hand ? although with considerable effort. Beyond this threshold, it will no longer be possible to untie the knot by hand. Tools will be required.
Maximal jamming state is reached when even the use of tools will not loosen the knot.

Observations:
EN1891 type A rope:
At 3.0kN, the F9 was very difficult to untie but, no tools were required.
At loads up to 5.0kN, the F8 (#1047) was still possible to untie by hand.
Note: Some further tests of #1047 F8 using somewhat older and stiffer Bluewater II rope managed to reach 7.0kN without jamming. In new supple Bluewater II rope, the jamming threshold was found to be 5.0kN. It appears that stiffer rope boosts the jamming threshold by a small margin.

EN892 rope:
At loads up to 4.0kN, the F8 (#1047) did not jam.
However, at 4.0kN some degree of effort was required to loosen and then untie the F8 knot.
The F9 knot was not tested in EN892 dynamic rope.

Conclusion:
The F9 is more vulnerable to jamming than the F8. This contradicts the long held view about F9 eye knot?s alleged resistance to jamming.
The jamming threshold of F9 (in EN1891 type A rope) is 3.0kN.
The jamming threshold of #1047 F8 (in EN1891 type A rope) is 5.0kN.
The jamming threshold of #1047 F8 (in EN892 rope) is above 4.0kN.
It is speculated that the additional twists of rope segments within the F9 core facilitates greater compression ? which promotes jamming.

Another long held view within the rope access and caving communities is that the F9 has a higher MBS yield than the F8.
In this test, MBS yield was not investigated. Based on the vulnerability of the F9 to jamming, it is speculated that the MBS yield may be lower than the F8 (when tested with identical rope).

It is hoped that others will conduct follow up testing to confirm or refute the vulnerability of the F9 to jamming.
It is also hoped that the MBS yield of the F9 can be investigated and compared to the F8 (using EN conforming ropes).
« Last Edit: December 05, 2018, 02:49:51 AM by agent_smith »

Harold Kahl

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Re: Figure 9 eye knot (jamming threshold)
« Reply #1 on: November 19, 2018, 03:18:50 AM »
As I recall, I had a figure 9 jam in my testing too.

agent_smith

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Re: Figure 9 eye knot (jamming threshold)
« Reply #2 on: November 21, 2018, 12:50:41 PM »
FOLLOW UP TEST OF F9 EYE / LOOP KNOT
Date: Nov 21, 2018

Test objective:
I wanted to compare 2 different geometry's to determine if there was any difference in the jamming threshold.
There is a long held belief in the rope access industry that the F9 eye knot has superior jam resistance compared to #1047 F8.
IRATA (a UK based organisation that has massive market share) actively promotes the F9 eye knot as being superior to #1047 F8.
I long suspected that this was another knot myth that was waiting to be busted...

Test rig setup:
As per attached image

Rope material:
Type: EN1891 type A
Manufacturer: Bluewater USA
Diameter: 11.1mm
MBS: 35.3kN

Assumptions:
I started with the assumption that attention to symmetry and placing the SPart on the outside curve might raise the jamming threshold.
Jamming threshold is defined as the load at which the knot is still possible to untie by hand (without use of tools). Beyond this load - jamming occurs.
Maximal jamming occurs when it is not possible to untie the knot - even with the use of tools.

Observations:
The F9 tied with attention to geometry and tracing the SPart on the outer curve actually jammed at 3.0kN. Significant efforts to loosen and untie the knot (no tools) failed. It was necessary to use a hammer to loosen the knot.

The F9 tied with the SPart tracing the inner curve was right on the threshold of jamming. It was possible to untie, but only after significant effort and time.

As a comparison, #1047 F8 (using the exact same rope) was easily untied after being subjected to 3.0kN.


Conclusion:

The F9 is more vulnerable to jamming compared to #1047 F8.
This is in direct contradiction to a widely held belief that the F9 eye knot is superior to #1047 F8 on account of jam resistance.
The jamming threshold of #1047 F8 is approximately double that of the F9 eye knot.

NOTE: MBS yield was not investigated.
There is a long held belief in the rope access industry that the F9 eye knot has a higher MBS yield than #1047 F8.
EDIT NOTE: In a limited test series of five (5) breaks conducted Nov 25, 2018, the F9 eye knot achieved higher yields than the F8 (see separate test report).
MBS yield is not by itself a determinant of knot efficiency or superiority. Give that no operator would be able to reach the MBS yield of a knot in the field, of more relevance is jam resistance.

Peer review follow up testing:
It is hoped that others will conduct follow up testing to either confirm or refute my conclusions.
It is preferable that testers use EN1891 type A rope (11.1mm / 7/16 inch)....however, if funds are tight or access to EN1891 rope is not possible, an alternative is to make a trip to your local yachting outfitter and purchase 'double braid' rope.
« Last Edit: November 26, 2018, 03:54:52 AM by agent_smith »

agent_smith

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Re: Figure 9 eye knot (jamming threshold)
« Reply #3 on: December 05, 2018, 02:54:26 AM »
TEST REPORT

Test details:
As per attached images.

Commentary:
This is a follow-up to my original report to add a further data set.
I wanted to use an unmatched pair of knots to see if this might impact upon the results.
In the F8 v F9 test, the F9 still jammed at a lower threshold compared to F8.

I make no comment about any propensity of a knot closest to the force generating machine to compress and be harder to untie relative to the opposite side.
I'll leave it to others who may wish to investigate and report on this phenomena...

Conclusion:
The F9 is more vulnerable to jamming relative to the #1047 F8.
This new round of tests confirms the F9 jamming threshold at 3.0kN.
In comparison, the jamming threshold of #1047 F8 is 5.0kN (in new, supple Bluewater II EN1891 type A rope).

I encourage others to conduct further follow-up tests to see if my results can be reproduced.
« Last Edit: December 05, 2018, 03:00:32 AM by agent_smith »

Dan_Lehman

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Re: Figure 9 eye knot (jamming threshold)
« Reply #4 on: December 05, 2018, 08:22:28 PM »
I wanted to use an unmatched pair of knots to see if this might impact upon the results.
In the F8 v F9 test, the F9 still jammed at a lower threshold compared to F8.

Annnd, in such a "mis-matched" case, you have clearly
the same force handled by the competitors (whatever
the guage/device might say)!

Thanks,
(-;

agent_smith

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Re: Figure 9 eye knot (jamming threshold)
« Reply #5 on: December 06, 2018, 01:50:54 AM »
Quote
Annnd, in such a "mis-matched" case

Not sure about the annnd bit.
Also, I'm fairly sure I didn't use the phrase 'mis-matched'.
I think I wrote 'unmatched' - rather than 'miss-matched' (miss-matched suggests something is amiss or incorrect, which was not the case).

Dan, I simply wanted to try a few different combinations to get a few more data points and to see if anything unusual would show up.
It comes under the general title of 'experimentation'.

Other than that, maybe you could consider undertaking some of your own tests and publish the results herein?