QC Lab: Extending a Cam SlingWednesday, September 7, 2016
When I rack for a multi-pitch trad climb, I rack like this:
- Cams (with a biner on each cam), set of Stoppers on my harness
- Quickdraws and a few spare biners on my harness
- Over-the-shoulder runners with one biner on each—over my shoulder (well duh...)
If I need to place a cam, I grab one, place it, clip it and go.
If I need to place a cam and extend the sling, I place a cam—then either use one of my quickdraws and clip through the sling like this:
Or grab a shoulder sling and extend all the way like this:
Not rocket science.
(FYI: if I place a Stopper I either use a quickdraw, a shoulder sling with the biner that's on it PLUS a spare biner, or a quickdraw AND a shoulder sling for extra length.)
So a while ago when I was down in the desert climbing a tower with a buddy, and I came up to several cam placements like this—I was confused.
I saw him fiddling at all of these cam placements and was wondering what in the world was going on up there—he's taking forever farting around with gear—maybe it's because he's so strong he doesn't get it—but for me, I need to place the piece and keep moving before I flame out. So as I'm seconding his pitches and having to deal with this unfamiliar conglomeration of slings on slings on cams, etc—I'm wondering:
- Why is he doing this? and
- How much does this affect the strength?
- And, I am getting pumped out of my mind.
I mean nylon on nylon or Spectra on Spectra, etc—sounds like bad juju to me—and I'm not talking about girth hitching anything here—just looping it throughâ€¦
I get to the belay and ask my partner what's up with the method—and then it all clicks—ahhhh "old school alpinist," light is right, save a biner, etc, etc—and old habits are hard to break. Regardless, I got back from the weekend, explained the situation to the crew in the QA lab and we proceeded to do a few quick tests.
We slung some 8 mm Spectra through a typical cam sling and did a few pulls in the tensile tester and a few drops in the drop tower. We compared the results to a cam sling only.
Here are the results:
Average: 15.8 kN
Actual historical average for cam sling ONLY: 25.5 kN
Therefore sling on sling method provided results 61.8% of historical average—or another way to look at it, it reduced the strength of the cam sling by almost 40%.
Drop Tests (note: these are NOT UIAA drop tests)
In both test configurations (i.e., cam sling only and cam sling threaded with 8 mm Spectra), the rope broke after more than six successive Factor Two drops (80 kg mass) with peak loads of over 10kN. The cam sling or sling-on-sling method was NEVER the failure mode in drop test scenarios.
The sling-on-sling method of extending a cam sling does save the use of a biner but in my opinion is cumbersome for both the leader and the second. It also appears to reduce the ultimate strength of the system, however, in most cases not so much as to be the weakest link in a real-world climbing situation.
When you're extending a cam sling, use a biner and make everyone's life a bit easier. If you're a super light-and-fast type of guy, the sling-on-sling method works, but know that it does weaken the system. Also note that none of the tests we performed took into account possible wearing from rubbing and friction—possibly even reducing the overall strength of the connection even more.
Kolin Powick (KP) is a Mechanical Engineer hailing from Calgary, Canada. He has nearly 20 years of experience in the engineering field and has been Black Diamond's Director of Global Quality since 2002. Kolin oversees the testing of all of Black Diamond's gear from the prototype phase through continual final production random sample testing.