Jeff Randall said:
My tests have shown a pretty consistent 26-28% reduction with a Scaffold Knot testing on 6mm and 8mm cord. We use the knot a lot for rope climbing systems since it keeps a carabiner oriented and from cross loading. Click to expand…
Jeff Randall
Tom, best I can tell the sheath and core failed at the same time. Would probably need to do some high speed video to really see what happens. About 10′ of rope is what I’ve been using but that’s cutting it close. I cut a 10′ piece off of the 300′ I recently bought from you to do this test. I would prefer 12′ to make it a bit easier. Not sure how I can test the core separate from the sheath. My thinking is carefully cut the sheath around the core and then do a pull test. Then simply take the sheath off the core to do a sheath pull test. Your thoughts?
ratagonia
First off, I am happy to supply samples for testing, rather than shortening perfectly good ropes.
We get returns from time to time when people take a brand new rope, set it up and rappel the same rap many many times. Which squeegees the sheath down the core, and make a loose sheath section past where the core stops. I am thinking of cutting one of those and pulling the core out of the sheath… 12 feet could be quite challenging! Let me see what I can do.
Tom
Bootboy
Once you get it started, its really not that bad to extract the core. Once you have a couple inches of core exposed, melt it and use a C-clamp to affix it to a workbench, proceed working sheath down to free end.
Jeff Randall
And just for grins I went back and broke the core shot rope again. It was too short to wrap around the drums or knot so I used prusiks. It broke at 6.94 kN in the core shot, which is slightly lower than the first break above the knot. I suspect the first break weakened it. Here’s a pic of the break:
Jeff Randall
Decided to do some actual unknotted rope strength tests, so here are a couple of tests on two different canyon ropes – both are new and never used.
The first was on Sterling’s CanyonPrime. Sterling’s stated specs are: 8.5mm, polyester sheath over polyester core, 1.65% elongation @ 300 Lbs., an MBS of 17 kN (3821 Lbs) and a weight of 3.75 Lbs per 100’. Test was conducted by wrapping each end of the rope around a 4” diameter pipe (4 wraps each end, then tying off) then slow pulling until failure. The Sterling CanyonPrime failed about 12 inches below the top 4” diameter drum at 19.28 kN (4334 Lbs) – 13.5% above the stated MBS.
Second rope test was on Imlay’s Canyon Fire. Imaly’s stated specs are: 8.3mm, polyester sheath over polyester core, 1.2% elongation @ 390 Lbs., an MBS of 18.2 kN (4100 Lbs) and a weight of 3 Lbs, 13 Oz. per 100’. Test was conducted by wrapping each end of the rope around a 4” diameter pipe then pulling until failure (same as Sterling test above). The Imlay Canyon Fire failed about 10 inches below the top 4” diameter drum at 19.16 kN (4326 Lbs) – 5.5% above the stated MBS.
Plan on testing the Atwood 8mm Grand rope this week. Of course it’s Dyneema core so higher MBS.
Pic of Imlay rope after break:
Sterling CanyonPrime:
ratagonia
Awesome, thanks.
Did the sheath and core break at the same time?
How much rope do you need to do the test using the drums?
I have a piece of rope that has a loose sheath, am thinking of sending it your way to test the sheath and core separately.
Tom
Craig
Is anyone else afraid of that Scaffold Hitch? I have used it to tie my cow’s tail biners so they won’t flip around or cross load, but it is easy to tie incorrectly. I would probably have to cut it off if I found one in a canyon. I don’t like bowlines either so maybe the problem is me.
Jeff Randall
Not afraid of the Scaffold at all. Like any knot it can be tied wrong but it’s very easy to tie correctly.
ratagonia
Hi Jeff –
I agree. You and I have different objectives. Your method is good for producing results that meet your needs. Keep up the good work.
Tom
Jeff Randall
Completely agree on 3 sigma. 99.73% will break at or above the stated MBS. My point is, in the real world, nothing stays the same. We may be doing a rescue using Carabiner A or prussic cord B today and then do the same type of rescue using someone else’s carabiner or prussic cord that is slightly different tomorrow. We may have a knot in the lab that reduces the strength of the rope by 26.5% but we get in the real world and run that rope through a high directional, or the rope gets wet or a knot undresses due to load, now that reduction percentage changes. What we are looking for when testing gear are safe averages to figure a safety factor from. So, I will accept the manufacturer’s 3 sigma MBS because most likely it is (as you state) lower than the actual Minimum Break Strength. So, figuring from that low side gives me more of a safety margin in the real world. If I want to sit behind a desk and do exact calculations on a very specific section of rope, then I will agree with you, but that does me no good in the real world because things change. So, the bottom line is I will accept a manufacturer’s stated MBS and do calculations from them to come up with what works in the field, or at least the safety factor I’m happy to play on.
Jeff Randall
Well, I pretty much trust the manufacturer’s stated MBS and go from that. Not sure why they would lie and not test it with the product liability being an issue.
ratagonia
I am not suggesting they lie. Manufacturers publish a RATING on an item. The actual Mean Breaking Strength / Minimum Breaking Strength / 3 sigma breaking strength is likely to be somewhat to quite a bit higher than the rating. If you want to know the strength of the item you have in your hand, you will need to break it yourself (or have access to the manufacturer’s batch testing data and know the batch your piece comes from).
Tom