The following article is courtesy of Black Diamond Equipment.
Slings and quickdraws (note: the webbing part of an actual quickdraw is often referred to as a dogbone) come in many different
shapes, sizes, widths and lengths and are usually made from nylon, an ultra-high-molecular-weight polyethylene or some combination of the two. Dynex
is the BD name for the polyethylene that we use. Other common brand names are Spectra and Dyneema. For argument's sake, they're all pretty darn similar,
so I'll just go with Dynex. Some are sewn together using traditional (or not-so-traditional) bartacks, some are knotted together, and some use a Chinese-finger-trap-type
attachment along with sewing.
The main differences between nylon and Dynex as it pertains to use in typical climbing situations:
- Nylon stretches a bit (ie. absorbs energy) whereas Dynex doesn't
- Dynex doesn't absorb water as much as nylon
- Dynex has a lower melting temperature (~145 °C) than nylon (~245 °C)
- Nylon holds a knot, Dynex does not
Pretty basic stuff here-the meat of the CE standard for slings is they must meet at least 22kN when new. No durability requirements, no cyclic requirements,
no wear requirements, etc.
On one hand it seems the general feeling is that metal products should last forever (which of course they don't) whereas there appears to be some unwritten
understanding that fabric type products do actually wear out. Even with that said, however, I have certainly seen some people whipping and rapping
off some pretty sketchy draws and slings out there.
Over the years I've been removing worn out fixed gear from cliffs I frequent and testing it. The takeaway from all of them—gear doesn't last forever.
Old slings, especially those fixed on sport routes and rap anchors can have a high sketch-factor—but why?
HOW & WHY SLINGS WEAR WITH TIME
Looking at all the slings and quickdraws we've tested over the years, I'd say there are two main factors that cause a reduction in strength:
Now I wish I could say that we've done some high-tech super study on this stuff, but we haven't. We've tested old fixed gear (as stated above), and also
helped a high school student do a science fair project a few years ago on the UV effects on slings.
His findings basically showed that nylon and Dynex have a reduction in strength when exposed to UV. Interestingly, his study showed that the drop in strength
was similar between the two materials, and also it appeared that the major drop in strength (~16%) occurred during the first 10 weeks of exposure,
whereas additional exposure resulted in only very incremental decreases in strength. Anecdotal testing from the field (cited above) doesn't exactly
correlate to these findings, however, we must remember that there are other factors involved in real world use (falls, abrasion, etc.).
We decided to try to compile a bit more data. BD athlete and tech rep Sam Elias stripped a bunch of old mank gear from Rifle, where they were installing
steel perma-draws. He brought the gear to BD and we went to town with our tensile tester (remember-when new, slings need to meet 22 kN ultimate strength
RESULTS (DOGBONES STRIPPED FROM RIFLE)
The numbers represent lbf when failure occured.
Yowsa—these were all found on popular routes that people, including myself, pitch off of all the time. They all were significantly less than the
requirement when new, and a few (790 and 810 lb) were well into the conceivable load range, even for a soft-catch sport climbing fall.
Moral of the story—don't just blindly trust fixed gear.
AND EVEN MORE TESTING
We decided to do a few experiments here in the lab by setting up a few tests where we would compare new slings vs. old slings.
We did our best to find representative 'old and well used' samples of what we had new, including:
- 10 mm Dynex dogbone
- 14 mm Dynex dogbone
- Variwidth nylon dogbone
- 18 mm nylon runner
- 10 mm Dynex runner
- ½" tubular webbing
- 1" tubular webbing
DISCLAIMER - these 'old and well-used' slings are non-scientific, totally random, not controlled, not a similar amount of exposure, wear, falls, etc.-just
a bunch of old stuff that a few of us had lying around at home.
The tests we schemed up were:
NEW VS. OLD - ULTIMATE STRENGTH
Basic stuff here—just to get a direct comparison of if and how much weaker old and worn slings can become
NEW SLINGS - 500 CYCLES ON OUR MAKE-SHIFT ABRASION TESTER, THEN TESTED TO ULTIMATE STRENGTH.
This would allow us to see two things:
- how the abrasion affects the strength when compared to when not abraded
- how the different materials and material widths are affected by abrasion
NEW VS. OLD - 1000 CYCLES TO 5KN (1124LBF) - THEN TESTED TO ULTIMATE STRENGTH
The idea here was to simulate 1000 falls of 5kN (a bit harsh but realistic) to both new and old slings, and compare to the ultimate strength of a new sling
NEW VS. OLD - CYCLED AT 11KN (2473 LBF) TO FAILURE
This was a curiosity test. Exposing the slings to half their rated strength repeatedly until they broke. Not realistic in the climbing world, but maybe
it would tell us something.
RESULTS & OBSERVATIONS (COMPARATIVE LAB TESTING)
NEW VS. USED - ULTIMATE STRENGTH
Once again—no surprises—the older gear is weaker and sometimes significantly weaker than when new.
NEW SLINGS - 500 CYCLES ON OUR MAKE-SHIFT ABRASION TESTER - THEN TESTED TO ULTIMATE STRENGTH
No surprise here—after beating up the sling for a bit on a rough edge, the ultimate strength is reduced.
Interesting that a few of the samples (both nylon and Dynex runners) didn't even make it through the abrasion portion of the test. This is possibly due
to the fact that the webbing was more likely to twist and turn and be cut on an edge instead of having the stiffness of a dogbone which forced the
abrasion to be consistently on across the width.
NEW VS. USED - 1000 CYCLES TO 5 KN (1124 LBF) - THEN TESTED TO ULTIMATE STRENGTH
Comparing after the 1000 cycles ultimate strength to no cycles then ultimate strength tested - it appears that the 1000 cycles to 5 kN had a very minimal
effect on ultimate strength. Comparing the new vs. old samples, once again, the older worn slings had lower results. Two samples did not even endure
the 1000 cycles.
NEW VS. OLD - CYCLED AT 11 KN (2473 LBF) TO FAILURE
This is an unrealistic test as it pertains to the real climbing
world, and gave us some results that are all over the map.
* Obviously this is all super limited data (ONE data point per test). In order to gain a more complete understanding, many more samples and controlled
comparative samples should be tested.
As stated earlier, it seems that more climbers are willing to retire a piece of webbing material (or harness or rope) much quicker than a piece of metal
gear-and this is a good thing. Our findings from testing old gear retrieved from the field definitely show a great reduction in strength of webbing
products, sometimes to the point where field failure is a real possibility. I've definitely heard of several quickdraw or sling failures in the field
(even a few in gyms) where the nylon draws are continually rubbing against the abrasive texture of the wall. Luckily there is usually another draw
not too far below, so the climbers have come away unscathed short of their hearts being thrust into their throats momentarily.
Our makeshift lab tests also show the same. Webbing doesn't last forever, whether it's nylon or ultra-high-molecular-weight polyethylene. Abrasion and
cyclic loading reduce the strength of webbing, and as the high school student science fair project showed, so does exposure to sunlight and the elements.
Now does nylon or Dynex fare better in real world weathering and use scenarios? Most literature you find states that Dynex is more resistant to cutting
and therefore should do better in abrasion. Though nothing is conclusive, my findings from super non-controlled experiments by testing samples retrieved
from the field actually show that nylon outperformed Dynex in ultimate strength after what is believed to be similar exposure to the elements, including
One of the biggest takeaways if you ask me is that nylon stretches a bit, while Dynex/Dyneema/Spectra does not. It's important to realize when that makes
a difference and adjust gear selection accordingly.
Climb safe out there,
Kolin Powick (KP) is a mechanical engineer hailing from Calgary, Canada. He has over 20 years of experience in the engineering field and served as Black Diamond’s Director of Quality for over 11 years. He is currently their Climbing Category Director. If you have a technical question for KP, please email him at firstname.lastname@example.org and he will TRY to respond.
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