The safety of slings and daisychains is now something that climbers take for granted, given that most are rated at 22K newtons. However it is important to understand a little bit more about the dynamics of slings.
Slings are broadly made from two different types of material, dyneema and polyamide, or often a combination of the two.
Dyneema® slings are predominantly white as the material cannot be dyed. It is extremely strong, lightweight and has minimal stretch.
Polyamide/nylon which is the material used in rope and therefore has some dynamic properties. They are more bulky than a dyneema sling.
Why we should be concerned.
If you are sitting on a ledge level with the ring to which your dyneema daisy chain is attached and you slipped, the resulting fall would have a fall factor of 1. That is to say that you would fall the full-length of the daisy chain.
While Dyneema® has a much greater strength-weight ratio (static load) than nylon, its elasticity is far less. Even a 60 cm fall-factor 1 fall on to an open Dyneema® sling can generate enough impact force (16.7 kN) at the anchor to pull a Wallnut 11 wire (12 kN) apart.
Tying a knot in a Dyneema® sling weakens it even further leading to sling failure in a fall-factor 1 loading on to a 120 cm sling. Knots in a sling mean that the webbing material is subject to smaller deflection radii. This reduces the strength of the sling. Knots in slings (tied into the double strand) reduce the breaking strength by an average of approx. 45%.
Knots in webbing slings should therefore be avoided as far as possible. If you cannot do without a knot in your sling when climbing/mountain climbing, you should use knots that have the least effect on the strength of the sling. Your choice should be polyamide slings, as these are less weakened by knots.
As stated above, most leader falls are between 4 – 7 kNs and forces above 10kN will start to cause internal injuries.
Guidelines for the use of slings
For more information please visit the following sites from which I have
drawn most of this material.
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