Understanding your Equipment


Slings and Daisychains

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.

An example:

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

  • Only use undamaged webbing slings that have a known history (age, number of climbing tours, number of falls etc.).
  • If several strength-reducing factors occur at the same time, the strength reduction of the sling can become critical.
  • Tying knots into slings during belaying should be avoided as far as possible (if you need to extend a sling, you can also do this with a karabiner)
  • Slings must never lie on top of each other
  • It is important to ensure that no rope is in frictional contact with the sling as this can cause the sling to burn through
  • If slings have to be knotted, always tie the knot into both strands of the sling.  Use a strop bend. If your belaying technique is based on a series construction, polyamide slings are the preferred choice since they are less weakened by the knots
  • Due to the considerable reduction in strength caused by knots, Dyneema® slings are less suitable for belaying purposes

    A summary of this sling-on-sling connection testing is: 
    1.  Joining two slings together using a hitch reduces the ultimate strength of the material up to and over 50%.   Only a nylon-on-nylon connection with a hitch seems to result in a reasonable amount of strength loss compared to other sling materials. 
    2.  In general, the narrower the material used, the greater the reduction in strength when joined together with any hitch.   
    3.  In general, materials of different width joined together with a hitch results in a significant reduction of strength. 
    4.  If you must link two runners together, a karabiner is stronger.  If you must use a girth hitch then put the girth hitch on the web that has higher strength or use a strop-hitch. 
    5.  Better yet - use a longer runner altogether or a thick nylon daisy-chain for harness connections.

For more information please visit the following sites from which I have drawn most of this material.



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copyright: david bruton