The latest series of DMM safety videos have encouraged climbers to contact us with various questions about best practice in using equipment and safety systems.
A commonly asked question is why is knotting Dyneema® (Dynatec) tape generally considered a bad idea and why isn't it readily available to buy off the reel in outdoor shops.
Dyneema® is a brand name for an Ultra-High Molecular Weight Polyethylene (UHMWPE) fibre that is manufactured by the Dutch company DSM. There are other versions available with the most common being made by American company Honeywell under the brand name Spectra. It has a high tensile strength, low density plus good resistance to chemicals and U.V. – all of which make it great for climbers, as these characteristics allow it to be made into a strong, light, low volume webbing.
However, it does have a couple of physical characteristics that can cause problems for climbers – a relatively low melting point (~145C) and a low coefficient of friction. The coefficient of friction (0.05 - 0.08) for Dyneema® fibres is far less than that of the Nylon 6 fibres (0.2 - 0.4) which are also commonly used to make climbing slings.
The main reason climbers have been reluctant to use Dyneema® tape to create their own slings is down to its low coefficient of friction that gives Dyneema® a ‘slippery' feel. This causes it to have a poor knot-holding ability in day-to-day use. This can manifest itself in a couple of ways:
- A knotted Dyneema® sling can work itself loose when it's being carried around in a rucksack and very long tails on any knot would most certainly be needed.
- A knot tied in a low friction material such as Dyneema® can unravel itself under load.
Sewn slings remove this inconsistency and the need to regularly check a knotted sling for loosening. In addition, knotting slings reduces the strength of the sling by up to 50%, depending on the material, the knot being used and how it is tied (see How to Break Nylon and Dyneema Slings).
It is worth bearing in mind that Dyneema® webbing used in climbing slings is actually a blend of nylon and Dyneema® fibres. The performance characteristics of the sling are a result of the actual ratios of nylon and Dyneema® used and the structure of the webbing. Thus although generically referred to as ‘Dyneema®’ the different webbings that use Dyneema® or a UHMWPE fibre in their construction can exhibit varying characteristics across a wide range of physical properties including their frictional characteristics and behaviour under load.
However, we thought we'd conduct some slow-pull and dynamic tests to give an idea of how knotted Dyneema® slings do behave and the breaking strengths you could expect. In the slow-pull (1.5 mm/sec) tests we used 8 mm and 16 mm Dyneema® webbing and made a sling using a water knot, a fig-8, double fisherman's and a triple fisherman's in each width. We also looked at the effect of soaking them in water overnight and freezing them down to – 40C.
From the video and the results table you'll see that a triple fisherman's was the strongest knot by a large margin - not far off the strength from using a sewn sling (failure at 26 kN) in a perfect situation. Clearly, in a static pull, knots can hold in this type of material, obviously some far better than others, but there were large inconsistencies in behaviour and that is not ideal in a climbing situation. A bar tacked sewn sling on the other hand will exhibit a very consistent breaking mechanism within a known range of tensile loads.
Interestingly, particularly for the 8 mm tape, the breaking strength increased if the Dyneema® was wet or frozen. This is very likely because the water facilitated heat transfer, reducing heat build up along with a slight lubricating effect. It is possible to see steam coming from some of the wet slings as they are being loaded.
The accompanying video illustrates quite well how in certain knots the ends of the webbing simply pull through, rather than the sling dramatically breaking at the knot.
Outside on the drop tower, in a dynamic situation using an 80 kg weight, a 60 cm sling tied with a triple fisherman's survived a fall-factor 1 - generating 16 kN on the load cell - but failed at 14 kN in a fall-factor 2. This is likely to be because the dynamic loading causes a rapid build up of heat from energy dissipation within the knot.
Using an 8 mm Dyneema® sling tied into the same length as an extended Dragon Cam sling (26 cm) it also failed a fall-factor 2. A sewn sling survived the same test.
Although it is possible to achieve quite high strengths in slings tied together with specific knots, it is very hard to judge how they will behave when loaded, especially as the knot alters over time. Home-made tied Dyneema® slings lack the ‘maintenance free’ and consistent high performance inherent in commercially sewn Dyneema® slings.
Overall, we should stress that we strongly recommend that you do not construct your own Dyneema® slings by tying lengths of loose Dyneema® tape together.
Note: These tests are purely indicative and are in no way statistically rigorous. They only relate to the particular weave, construction and Dyneema®/Nylon blend used in our slings.