Author Topic: Figure 9 eye knot MBS yield (compared to #1047 F8)  (Read 5685 times)


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Figure 9 eye knot MBS yield (compared to #1047 F8)
« on: November 26, 2018, 12:15:55 AM »
Knot: Figure 9 eye knot

Test objective:
To determine the MBS yield of the F9 and compare it to F8 eye knot (#1047).
A widely held view is that the F9 is 'stronger' in comparison to the F8.

Type of rope material:
Only one type of material was tested...EN564 accessory cord (due to budget limitations).
[ ] Class: EN 564 accessory cord
[ ] Diameter: 5mm (0.19685 inch) diameter
[ ] MBS: 5.2 kN
[ ] Manufacturer: Sterling USA
[ ] QC: Batch A050AS0100  Lot #R6-092507KT (purchased Jan 02, 2009)

F9 and F8 knot geometry tied:
[ ] As per attached images (see below)
(NOTE: The knots shown in the test rig are only placeholder images. The actual knot geometry is shown in the detail images).

Tester: Mark Gommers
Test date: 25 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
[ ] Each test article consisted of a pair of knots tied in one linear length of cord - each knot identically tied at each end.

Assumed that F9 has higher MBS yield than #1047 F8 (although not to a significant degree).
This general assumption based on the curvature of an SPart as it penetrates and then deflects inside a knot core - with sharper deflections/bends resulting in more pronounced stress concentrations.

I had enough cord to conduct 5 tests. As expected, hand cranking the lever hoist resulted in the failure of all knot specimens in the approximate range 3-4kN.
When the yield point was reached, a snapping sound (like breaking a small dry twig of timber) was clearly audible which was followed shortly after by total failure of the cord/knot.
The initial snapping sound was likely a partial internal severance of the core.
Some knots failed on the left side of the test rig, while others failed on the right side of the test rig.
The sample cord tested was 9 years old but, this was consistent across all tests. This was unused accessory cord that I had left over and was able to 'sacrifice' for the sake of testing.

                   F8           F9
Test #1     3.04kN     3.54kN
Test #2     3.74kN     4.90kN
Test #3     2.70kN     4.62kN
Test #4     3.32kN     3.64kN
Test #5     2.92kN     4.46kN

Mean         3.14kN     4.23kN
% of MBS   60%         81%

The F9 has marginally higher MBS yield compared to #1047 F8.
However, strength is not a reliable determinant for one knot being superior to another.
It should be re-emphasized that the specimen cord used for the test was 9 years old and it was only 5.0mm (0.196 inch) diameter. Further testing should be carried out using larger diameter ropes.

Based on my previous testing, I had demonstrated that the F9 is more vulnerable to jamming than the F8.
In EN1891 type A ropes, the F9 has a jamming threshold of 3.0kN.

Given that no user will ever reach the MBS limit of a knot in the field - jamming is a more relevant matter.
During vertical rescue training, ropes may be subjected to loads of up to 280-300kg (approx 2.8kN to 3kN).
Given that the F9 jamming threshold is 3.0kN in EN1891 type 'A' 11.1mm (7/16 inch) rope, this fact alone casts doubt on the viability of this knot in vertical rescue applications or any situations where loading is expected to reach 3kN (approx 300kg).
In comparison, the jamming threshold of F8 is approximately double this figure.
In my view, the F8 is a more efficient eye knot compared to F9.
Efficiency should also take into account the amount of rope required to tie a particular knot, the footprint (volume) of the knot core, complexity in tying, inherent security and jamming threshold.
The F9 requires more rope to tie, and has a larger footprint compared to F8.

The small gains in MBS yield are of little relevance given that the breaking limit of the rope will never be reached with field use.

It is hoped that others will conduct follow up testing to either confirm or refute my conclusions.
« Last Edit: November 27, 2018, 05:03:23 AM by agent_smith »


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Re: Figure 9 eye knot MBS yield (compared to #1047 F8)
« Reply #1 on: November 26, 2018, 10:43:03 PM »
Thank you.

The knots in your 1st image --of test configuration--
appear to be with an *interior* loading (twin part
pulling away from the other), whereas those
in which you highlight the SPart's path are of the
*exterior* loading (twin pressed into its partner).
Lyon Equip.'s testing concluded --on summary
evidence-- that the former (interior) got higher
breaks.  Given the figures you report here for the
Fig.9 eye knot, that would suggest that it does
very well indeed, for strength.

And might jamming be a different result, as well?

What is more surprising in these data is the seemingly
low figure for the fig.8 eye knots --only 60% is quite
under what other tests often show.  For trying to get the
most out of "*exterior* loading, I'd set the knot hard by
loading the tail --so to build more curvature into the twin
parts' initial curve which the exterior part would then
be pressing hard against its twin.  With less severe setting,
there is more direct run to the U-turn around the eye

« Last Edit: August 08, 2019, 01:33:25 AM by Dan_Lehman »