Journal

Wednesday, August 5, 2009

QC Lab: Sling Strength in 3 Different Anchor Configurations

A few weeks ago a crew of super badass climbers rolled through the QA lab and one question came up more than just a few times: “What is the strongest way to rig an anchor at a belay?”

Now that’s one of the most loaded questions I’ve ever heard, because, of course, there is no real definitive answer. There are so many factors involved, including quality of the placements, quality of the rock or ice, materials available, etc. For the sake of the discussion, however, we narrowed it down to assuming two “perfect” bolt placements and using one equalized sling. My immediate answer was that such a set-up would be plenty strong for most climbing applications no matter which way you slice it, but any time you knot a sling it undoubtedly weakens it.

Remember I’m not a guide and don’t pretend to be one, and I’m not suggesting which anchor equalizing method is better or worse. All I’m providing is some data based on a very few (i.e., one) data point for each scenario.

Testing

My crack crew of QA engineers and I decided to check out the three most common equalizing methods using a single 48” runner: Sliding X, Sliding X with Knots, and Figure 8. Again, I’m not going to get into the merits or negatives of each situation (e.g., shock loading if one anchor placement blows, how “equalized” they actually are, etc). This is just an apples-to-apples strength comparison of the three configurations.

Results

Configuration
Peak Load (lbf/kN)
Failure Point
Sliding X
8000/35.6
none (machine limit)
Sliding X with knots
4760/21.2
webbing @ knot
Figure 8
5272/23.5
webbing @ knot


Sliding X


Sliding X with knots


Figure 8

So what do these numbers mean?

A couple of things to remember:

  • CE-certified slings are rated to 22 kN (4946 lbf)
  • Typical CE-certified carabiners (e.g., lockers, wiregates, bent gates, etc) in closed gate are rated 20 kN minimum (4496 lbf)
  • CE-certified cams are rated 5 kN, but most are over 10 kN

Using a Sliding X anchor, our tensile tester couldn’t even break it. Now that is BURLY. And both configurations with knots were more than 20 kN in ultimate strength. So just as we’ve seen in previous sling-on-sling girth hitch experiments, knotting slings, etc, knots reduce the ultimate strength by anywhere from 40-60% and the failure mode is always at the knot. However, even though that seems like a big reduction in strength (which it is) the bottom line is that the anchor is still plenty strong for most any typical climbing scenario thrown at it.

Climb safe —

KP

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. If you have a technical question for KP, please email him at askkp@bdel.com and he will TRY to respond.

Photos

Recent Talk (1)

  • alex farias
    11 Apr 2012, 6:03PM

    do you have any test with nylon vs/ dinex sling strength?

    saludos desde Chile

  • Philip Grimm
    6 Apr 2012, 9:21AM

    This is great. Every time I'm at the Red some idiot tries to chastise the sliding x because of shock loading if one fails. Wasn't there an article where Kolin explained that is only possible if clipped in directly to the x with a static anchor piece, and using a sliding x on top rope makes shock loading impossible because on the dynamic rope in the system?

  • nacho
    20 Sep 2009, 8:42AM

    Could you test setting it up with a clove hitch to the rope bearing binner? I saw it on a Petzl catalog and seemed like a good option, not self equalizing, but avoids shock load and is easy to set up-remove. I wonder if the clove hitch wont slide and fail in the case of one end getting cut. Also what is the loss of strength on this one?

    Great articles! Keep em coming!

  • knudeNoggin
    12 Sep 2009, 9:19AM

    Not testing for extension is one thing,
    but in the case of the Sliding-X with Limiter Knots,
    you should test the case of loading one end and seeing the limiter knot pocket holding
    -- the other two cases arguably are not much changed by one-leg loading, aside from
    the impact force they'll sustain (and, in the case of the Fig.8, which end is loaded).

    *kN*

  • Rene Sagstad
    5 Aug 2009, 9:33AM

    do not forget the posetiv dynamic effect a knot have when the system is shock loaded.
    best regards Renè

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