Slings at anchors
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.
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.
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.
