QC Lab: Ultimate Strength Testing of the Gear You've Always Wondered AboutWednesday, May 9, 2018
Thanks to all the folks that submitted ideas for this QC Lab post. We had some great ideas, some ideas we’d already done, some whacky ideas, and some sketchy ones. Instead of diving deep into one particular topic we decided to do some quick and dirty quick hits, where we’ll break some gear and talk about it without getting into too much detail. I solicited one of our crack crew of Quality Engineers, as well as our in-house photo wizard, and we spent a few hours breaking stuff in our trusty tensile tester.
In typical non-PHD level thesis style, we’re going with n=1. As in, ONE data point for each test. Even though I always give a disclaimer that this is just informational, for discussion purposes, it never fails that I get feedback from mathematicians, scientists and engineers out there claiming this data is not statistically relevant due to the small sample size. I get it. If those folks want to do a full-on scientific study on any climbing gear, that’d be awesome as the more information out there that educates new and old climbers alike, the better.
We decided to look at some of the basics: Stoppers, hexes, cams, slings, and belay loops. We looked at their ultimate strength, where they broke and why. When thinking about the ultimate strength of climbing gear, it’s always good to consider the loads you usually see in the field under typical climbing circumstances, though it’s important to remember that there can be situations where loads can exceed these typical situations. Our friends over at Petzl just published a short article with some great information about real world loads. It’s worth the read to get some context while thinking about how strong gear is, or needs to be: Forces At Work in a Real Fall
TESTING STOPPERS AND HEXES
Different size stoppers and hexes often have different strength ratings. This is because of the different size cable used, as well as the different dimensions of the stopper causing the cable to make tighter or more gentle bends. Someone asked where stoppers break, so we broke a few. We used a standard stopper jig and hex fixtures which provide a perfect stopper placement, and just used a carabiner on the other end to represent real world use.
#1 Stopper – rating – 2kN
Broke at 3.3kN – cable at nut.
This very small diameter cable makes a tight radius at the nut end of the stopper— so it’s no surprise it broke there. These super small stoppers are typically used for aid climbing only, as it doesn’t take much to generate a 2kN load.
#5 Stopper – rating – 6kN
Broke at 8.3kN – cable at nut.
Once again, the cable breaks at the nut in this case. These loads aren’t that common but are possible in the field.
#11 Stopper – rating – 10kN
Broke at 11.2kN – cable at carabiner.
With the larger cable and not-so-tight bend at the nut, the failure mode shifted to the tighter cable bend at the carabiner—once again breaking above the rating, and at loads unlikely to be seen in the field in normal climbing scenarios.
#4 Hex – rating 10kN
Broke at 11.4kN – cable at carabiner.
Similar to the stopper above.
#6 Hex – rating 10kN
Broke at 11.9kN – cable at carabiner.
Hexes are slung with cable these days, so makes sense that a hex with the same size cable as a stopper would break at about the same load.
#6 Hex – threaded with 6mm cord
Broke at 7.4kN – cord was cut at the hex.
One of our manufacturing engineers was digging through her gear and found some old hexes slung with cord. Back in the day when I started climbing, you actually bought JUST the hex—it didn’t come with cable, cord, nothing. You would get some 6mm and sling it yourself with a double fisherman’s. So that’s what we did—just to compare strength-wise to the “new school” way of using cable. We didn’t want to break Taylor’s vintage gear so we cut the cable off a new hex and re-slung it with 6mm. Not surprisingly it was significantly weaker than if cable was used—the cord ended up being cut by the edge of the holes in the hex.
Not only does using cable on a hex make it stronger, but it also makes it easier to place, much more functional, and easier to remove because of the relative stiffness of the cable. Trying to place a slung hex is tough as it’s as floppy as a wet noodle.
Different size cams have different strength ratings. And different styles of cams can have different strength ratings and break in different ways. Most people have never seen a cam tested to ultimate strength so we decided to break a few in different orientations.
#1 Camalot – rating 14kN – tested at 50% retracted
Broke at 15.5kN – cable at thumb loop.
A beauty red Camalot, tested in a textbook placement. In normal climbing situations, you’re very unlikely to ever see a load in that range—if so, there are likely bigger things to worry about.
A lot of folks wonder why we have the double thick sling on our Camalots and this is why. I’ve talked about this in a previous post: QC Lab: Reslinging Camalots and C3s The double thick webbing distributes the load and prevents the cable from pinching and cutting the sling. It also prevents the cable from getting tweaked under normal falls, and ultimately makes it stronger.
#1 UL Camalot – rating 12kN – tested at 50% retracted
Broke at 18kN – cable at pin.
The UL Camalots break in a different place than C4 Camalots. The design is such that the super strong dynex core goes around the tight radius pin at the head of the UL Camalot, and this is where it fails when tested to destruction.
#1 Camalot – rating 12kN – tested in Umbrella (Passive) at 50% spacing
Broke at 15.2 kN – cam lobe destruction.
I’ve been climbing over 25 years and never placed a cam in umbrella, but if I did—it’d be plenty strong. The dual axle cams not only provide a burly strong cam in passive placements, but also provide greater cam range.
#1 Camalot – rating 12kN – testing in Umbrella (Passive) at tight spacing
Broke at 15.4kN – cable at thumb loop.
We tested another one, with slightly tighter spacing in the passive placement. The result was a typical failure mode of the cable breaking at the thumb loop. Of course, if you were to place a cam in umbrella, the tighter the spacing, the better.
TESTING SLINGS – Steel vs Nylon vs Dynex
Back in the day people would knot webbing to use as slings. Nowadays, sewn slings are the norm—usually nylon or Dyneema/Spectra/Dynex (for argument’s sake, all the same stuff). We pull-tested a few to show the ultimate strength, the difference in stretch, and how they break. We also made a “steel cable sling” for comparison. We tested using 12mm pins to avoid potentially breaking carabiners during the test.
3/16” steel cable double swaged – 60cm Broke at 27.1kN – cable at pin.
We broke this just for reference and comparison—strength and stretch.
18mm Nylon Runner – sewn – 60cm – rating – 22kN
Broke at 27.2 kN – webbing at pin.
This Nylon runner stretched WAY more than the steel cable.
10mm Dynex – sewn – 60cm – rating 22kN
Broke at 27.4kN – webbing at pin.
People say Dynex is just as strong as steel. In this case, that is correct. This 60cm sewn 10mm Dynex sling weighs about 19 grams and broke at over 27kN. For comparison, the 3/16” steel cable weighed 154 grams and broke at about the same strength—just over 27kN; and the 60cm sewn 18mm nylon sling weighed 36 grams and also broke at just over 27kN.
Strength to Weight
|3/16" steel cable loop||154g||27.1kN|
|18mm Nylon runner||37g||27.2kN|
|10mm Dynex Runner||20g||27.4kN|
Amount of Stretch
You can see by looking at the graph that the steel cable didn’t stretch much before it broke at over 27kN—just over an inch. The Dynex by comparison stretched almost four times as much as the steel, and broke at about the same load. So yes, Dynex stretches more than steel. But then look at the Nylon—it stretched twice as much as the Dynex, and about eight times as much as the steel, and broke at the same load.
This is why you want to use the right tool for the job. Use Dynex slings when weight really matters and you’re willing to accept the consequences of higher loads because of not much stretch. Use nylon when weight isn’t as critical and when you’re wanting the system to absorb a bit more of the energy from the load, like on sketchy gear placements.
TESTING SLINGS – Suspect Slings
18mm Nylon Runner with pin-hole – sewn – 60cm – rating – 22kN Broke at 19.4kN – at hole.
This is an interesting one, as we weren’t expecting it. I literally grabbed this sling from my office, not knowing what it was, where it was from, its history, etc. While putting it in the tensile tester, we notice it had a small, but visible pin hole in it—maybe 1mm in diameter. We figured it’d be fine, as long as we get to 22kN—the point was to have a comparison of how much the nylon stretched compared to steel and dynex. We were a little surprised when it broke at just over 19kN, with a noticeably more audible BANG—startling a few folks having a discussion not too far from the tensile tester. So, a few takeaways:
- Never blindly trust anything from KP’s office
- It’s always a good idea to inspect your gear
- Even the most unsuspecting things that appear very minor, may have an effect on strength, durability, etc.
- If you notice something suspect before testing, take a photo prior to testing—we didn’t in this case. Bummer.
18mm Nylon Runner – with hole pierced on purpose – sewn – rating – 22kN
Broke at 16.7 kN – webbing at hole
Based on the above test, we tried to recreate it by purposely stabbing a hole through the webbing. I would say this hole was slightly bigger, but not as clean as the previous test. By this point, given our intention, it wasn’t surprising that this sling failed to meet the rating. Once again, check your gear. If it’s suspect—best to retire it.
TESTING SLINGS - Knotted
Sewn slings need to meet 22kN in order to be certified to the CE standard. Some folks still buy webbing and make their own slings, and sometimes you may be in the field and find yourself in a situation where you have to cut a sling in order to tie it around a tree or chockstone to bail. So, we tested a few knotted slings.
18mm Nylon Runner, cut and tied with a water knot
Broke at 20.1kN – broke at knot
It’s not surprising that this failed at a lower load than if sewn, and also not surprising that this failed at the knot. In most cases, the knot is the weak point.
10mm Dynex tied with a water knot
Slipped at 7.7kN
There’s actually a reason you can’t buy Dynex/Dyneema/Spectra from a spool and this is it. This material is very slippery and doesn’t hold a knot. Don’t knot your Dynex slings.
10mm Dynex tied with a water knot
Slipped at 7.8kN
We tied and tested another sample just to be sure. Once again, don’t knot a Dynex sling.
Random Flat webbing – tied with a water knot
Broke at 13.9kN
There are many kinds of webbing out there, and it’s not always obvious if it is “strong” or not. Once again, I grabbed a piece of webbing, literally off the floor of my office. I have no idea where it came from, or what it’s from—it could be some webbing used in a backpack or a belt for some ski pants for all I know. We tied it with a water knot and it withstood almost 14kN. Not bad, but nowhere near where a piece of 18mm nylon knotted or sewn sling would hold. Don’t just blindly trust webbing that “looks like” climbing webbing.
TESTING BELAY LOOPS
Belay loops are burly strong. One of the first QC Labs we ever did was about belay loops: QC Lab: Strength of Worn Belay Loops
As far as the CE standard for harnesses go, there is no belay loop specific test, rather the belay loop is in the full harness system during testing and must withstand 15kN.
Most Black Diamond belay loops are constructed with a protective layer of webbing that protects the structural bar-tacks from abrasion—so when testing a belay loop to destruction, it’s actually the protective layer that peels away first. It’s pretty cool to see, so we thought we’d break a few.
Belay loop – rating – 15kN
Sample 1 - Broke at 22.9kN – at tack
Sample 2 – Broke at 21.9kN – at tack
The protective covering starts to crackle and pop and break at lower loads, but the ultimate strength of a belay loop far exceeds any load you’d see in the field under typical use.
- • Small stoppers aren’t as strong as big stoppers – the cable breaks.
- • Don’t sling hexes—they’re too hard to place, and the cord cuts at a lower load.
- • C4s break at the thumb loop—the double thick sling helps avoid it from being cut.
- • UL Camalots break at the rivet near the head.
- • Dual axle cams are strong even when placed like an umbrella.
- • Dynex actually is as strong as steel.
- • Nylon stretches more than Dynex.
- • Small holes and nicks can weaken slings. Check your gear.
- • Knotted Nylon is OK.
- • Don’t knot Dynex, it’ll slip at relatively low loads.
- • Even though webbing may “look” like burly webbing, it may not be.
- • Belay loops are burly strong.
- • Don’t trust anything from KP’s office.
Be safe out there,