UKC

Is it safe to tie knots in dyneema slings? For example, if you use a sling to equalise a belay, can you tie a figure of eight knot to provide a loop to clip a krab through at the equalised point rather than using a self-equalising set-up which can shock-load the anchors if one of them fails? Or could any slippage in the knot if it tightens as it is loaded raise the temperature of the dyneema enough to weaken it to the point of failure? I'd be interested to know what people think about this,

Stuart

In reply to stuartf: You can tie a figure of eight at the equalisation point, but IMHO it is much better to use the 'sliding self equalising knot' with an overhand knot tied about 3" either side of the take off point. That way you get the best of both worlds: equalisation and the mimimum of shock loading should 1 anchor fail.
The only way I could see there being anything like enough heat generated in a sliding knot to melt the sling would be if you were using a long sling with out the knots on either side of the take off point, and even then I would have thought the bigger risk would be of your second anchor failing.

In reply to stuartf:

when i use slings, use an overhand to equalise them with no problems

also, The Mountain Skills Handbook shows various methods of equalising slings with knots and makes no mention of any risk when using Dyneema and all photos showing concepts show Dyneema slings

all a bit anaecdotal, but I certainly don't have a concern over this

In reply to D Berry:

The questioner he is asking about whether just the closeing and tightening of the knot is a problem. This is probably an issue you and other lay people will not have considered.

Dynema melts at an alarmingly low temperature. Several experts have demonstrated dynema failing just from the rapid tightening of the knot rather than wholesale slipage. The manufacturerers at this time refuse to comment on the findings.

In reply to H:


Several expert authors, mentioning no names and I am not familiar with the book you mentions, have simply regurgitated old information in their manuals. This is disappointing since many of those authors have the resources and facilities (test rigs etc) to re-write the book as it were. I am not saying that the practices described in most of the manuals are wrong but rather that they are not as right as they could be.
Book purchasers are simply handing over their money for old ideas dressed up in shiny new photo's and print.

In reply to stuartf:

Another point, figure of 8 knots invert very easily in the event of cross loading of the knot. i.e. when on of the anchors fails. Use an overhad knot as they have the tendancy to tighten rather than invert. An inverting Figure of eight would generate sling melting heat as it traveled along the length of the sling. It may also change the geometry of the anchor loading.

On a slightly different note. I recently had to cut a big sling, what do people think about rejoining the ends with a tape knot as you would a nylon sling?