Slings at anchors

23 September, 2013

In a previous video we compared the impact forces generated using nylon and Dyneema® (Dynatec) slings with a dynamic load. It clearly highlighted the importance of ensuring there is no slack in a system using slings. As an example, a 85 kg mass free-falling just 60 cm on to a 60 cm Dyneema sling (fall-factor 1), with an overhand knot in it, generated enough force to break the sling.

Extending this previous theme we've looked at using nylon and Dyneema® slings in four different belay set-ups:

  • Equalised with an overhand knot at the balance point.
  • What is commonly known as 'self-equalising' or sliding X.
  • Clove hitch at anchor and clip-in points.
  • Overhand at anchor and clip-in points.
In addition, we simulated a marginal placement ripping by using 2 mm cord at one of the anchor points. In this scenario the 'self-equalising' arrangement placed the greatest shock-loading on the remaining anchor point and isn't advised with natural or hand-placed anchors for that reason.

Carrying out a final test replacing the slings with 8.2 mm rope for a worst-case scenario with a fall-factor two clearly showed - as expected - that utilising the shock absorbing properties of your rope dramatically lessens the impact forces on the anchors and is the best option. This could be by either clipping the anchors directly with the rope or into a central point in the system. For a full explanation and examination of the results watch the video.

Sling Set-up Material FF1 (Force kN) FF2 (Force kN)
Overhand Knots Nylon 8.8 11.1
  Dyneema 11
sling broke
10.4
sling broke
Clove Hitches Nylon 9 13.1
sling cut a bit
  Dyneema 10.2
a bit of melting
10.8
slippage of hitches
'Self-Equalising' (Sliding X) Nylon 11.5 19.7
  Dyneema 16.1 27
Equalised with Overhand Knot Nylon 10.8 15.5
  Dyneema 12.5 21.7
sling broke
Clove Hitches + Anchor Point Fail Nylon 5.5 -
  Dyneema 5.5 -
'Self-Equalising' (Sliding X) + Anchor Point Fail Nylon 10.2 -
  Dyneema 14.9 -
Equalised with Overhand Knot + Anchor Point Fail Nylon 7.1 -
  Dyneema 4.9 -
Dynamic Rope (Pair of 8.5mm tied off with clove hitches) Dynamic Rope - 7.6

The results show how very high forces can easily be generated using slings to attach to anchor points if there is slack in the system. Clearly, it's important to be aware of this if for example, you are moving around at a stance while clipped into an anchor using a sling or rigging a multi-pitch abseil. For perspective, most leader falls are between 4 – 7 kNs. Forces above 10kN may cause internal injuries – 10kN equates to 1 metric tonne.

The forces quoted in the results table are at the clip-in point, and in all cases except that of the simulated anchor point failure, would be divided between the two anchor points.

Self-tensioning system
Note: While the 'sliding-x' may be referred to in some climbing text-books as self-equalising, the forces on the anchor points will only actually be equal if both the angles between the direction of the load and each of the slings are identical (see diag). Strictly speaking it would be better described as self-tensioning.

Related Knowledge Articles: Why is Knotting Dyneema Tape Generally Considered a Bad Idea and How to Break Nylon and Dyneema® Slings.