UKC

Can you knot a sewn sling ? for example when i want to adjust the length of a personal anchor i can have a big sling with 1 or 2 knots in it so i can clip and adjust the length?

I have seen DMM videos on knotting slings that state it's detrimental to the slings strength.

In reply to Inhambane:
When you knot it to adjust length, it's a good idea to have the sewing join isolated in the dead loop.
Everyone knots slings. The main issue people have is not the weakness imparted by the knot, but the pain of getting the knot out.

In reply to Inhambane:

Any knot in a sling or a rope will weaken it.

You have to use you judgement as to whether it's going to weaken it sufficiently to cause you an issue.

In reply to Inhambane:

Yes, you can put knots in it. Yes, that does reduce the strength.

It's still plenty strong enough for most uses but, with or without knots, you shouldn't ever have slack in the sling if you might fall onto it.

In reply to Inhambane:

I would guess it is better to knot the sling so as to ensure there's no slack than to have an unknotted sling with lots of slack.

In reply to Inhambane:

Knotting a sling to equalise a belay etc. is OK. What you need to avoid is any slack that can cause 'snapping tight' of the sling. And yes, it does weaken the sling slightly. That DMM video does look scary.

I always try to get a bit of the rope between any sling anchor and my harness to absorb dynamically any force.

In reply to zimpara:


Can you explain the reasoning behind this statement?

In reply to Inhambane:

The DMM video pertains to using knotted slings in a static shock loading situation, i.e. In which there is no rope between the falling mass and the anchor. This count happen if you use a sling as a lanyard to clip into abseil anchors for example. In a standard Climbing situation in which the level of load the sling sees is limited by the rope, you won't have a problem at all. Also, ignore the isolate the stitching comment - the stitched part is usually stronger than the rest of the sling.

In reply to Inhambane:

Indeed knotting weakens slings, doubling or tripling them up increases their strength by two or three times, its a no brainer if you have to shorten a sling basket it or triple it up.

In reply to timjones:

Sounds obvious to me, the stitched part is annoying. Getting in the way all the time.

In the future someone will invent the seamless loop sling, and will make a fortune.

In reply to Inhambane:

Thanks guys some good points there

In reply to muppetfilter:

Doubling slings normally reduces the strength considerably, one strand lies over the other and cuts the lower one through.

In reply to Dell:



Skylotec have made a 6mm dyneema cord sling without stitching for a few years now, the CipE.

In reply to Inhambane:


Yes, but you don't have to make it your default procedure. It's not always necessary.

In reply to Inhambane:

Knots can be a right royal pain in the arse to untie if they are loaded (as they normally would be in a belay set up). For that reason I tend to try to avoid it and use the climbing rope instead.

In reply to Inhambane:

what would people use as a personal anchor in sport climbing when you want to clean the top? i have seen videos suggesting using a quick draw from belay loop to bolt and i have seen people use a rope/accessory cord attached to their belay loop with various loops in it.

In reply to Dell:


I find that the important thing is to locate it in the longest loop to ensure that you have space to keep it clear of the knot. Blindly stating that you should locate it in the redundant loop seems like a poor idea.

In reply to Inhambane:

What I get from the DMM video 'Slings at anchors' is:

1) Knots absorb impact forces by slipping.
2) Sliding X is strong but generates high impact forces as it don't slip
3) Overhand knots weakens the slings more than clove hitches
4) Knots on dyneema slings absorb more force than knots on nylon slings as dyneema is more slippy.

In that perspective knotting a sewn sling is not necessarily bad.

In reply to jimtitt:

That's ace.

In reply to jimtitt:
I've been using an Oldham hitch for various purposes for a good few years now, since stumbling across a description of it on here. It's very quick and easy to adjust from a bit less than the full length down to a bit more than half the length of the sling, doesn't tighten up under load and seems pretty stable.

Hang on, I'll just get a quick photo sorted out....
http://www.deepsoup.f2s.com/UKC/oldham.jpg
^ This thing.

I only use it for bodyweight(ish), wouldn't want to expose it to a shock load or use it as part of a belay. Obviously it will reduce the strength of the sling, I'm guessing by a bit less than an overhand knot would but I don't really know.

I'd be very interested to know if anyone has done any testing to see how strong it actually is and how it fails.
(Jim? If anyone knows, I bet it would be you.)

In reply to Stefan Jacobsen:

Point 4 is the exact opposite! Dyneema has a lower melting point and the heat generated by the pressure of the know tightening is sufficient to cause far greater weakness in knoted Dyneema than knotted Nylon.

In reply to Inhambane:

Not exactly sure what you mean by clean the top. I clip into the belay with a sling, quickdraw or chain of quickdraws then re-thread to lower off without untying so the rope is always protecting me and can't be dropped.

Edit: knotting slings weakens them and loaded knots can be very hard to get out of thin tape. Knotted sings are easily strong enough to be safe in any normal application.
jk

In reply to jimtitt:

Hi Jim , have you got any references for this. I cant find anything in any of my industrial rigging manuals.

In reply to deepsoup:








That´ s quite cool really, wonder I´ ve never come across it in all my nautical days. More attractive an idea than most.
No idea how strong it´ d be though but I´ d guess pretty good. All the Dyneema stuff I test seems to break at much the same level anyway no matter what knot I put in it, between 10 and 12kN depending on the age. Strongest knot seems to be the bowline on a bight as an end termination loop though.
Got to test some stuff this week so might see what gash slings I´ ve got around.

In reply to jimtitt:

I can't quite believe that this statement is correct...

In reply to brianjcooper:

Short lengths of dynamic rope don't do as much as we think with regards to energy absorption in a fall.
It is in fact the tightening of a slack knot - that fig 8 you can't get undone after a fall...

The British Caving Association has recently conducted research into caver's cowstails (lanyards made from dynamic) where this has been found.

Of course dynamic is always going to be better than static, but don't expect too much from a <<40% [UIAA-101 / EN-892] elongation of a 30cm length between you and the belay, keep that knot at your harness neat and slack.

Harness and body deformation also plays a huge factor - us humans are nothing like those steel test masses used in the standards.

In reply to beardy mike:
Yes, in way you are right. For FF1 with clove hitches, the impact force is higher with dyneema. But for FF2 with clove hitches the impact force is lower with dyneema. Also, if one anchor point fails the impact force using dyneema is 4.9 kN and nylon 7.1 kN.
The picture is not clear cut when comparing the two species of slings.

In reply to Tradical:


Yes, and there is friction between the climber and the rock making a fall less serious than a free fall.

In reply to Stefan Jacobsen:

Can I ask where you get these figures from?

In reply to beardy mike:

http://dmmclimbing.com/knowledge/slings-at-anchors/

In reply to Stefan Jacobsen:
That video is all very interesting bar one fairly obvious point. How often do you set up a belay like they have purely with slings and clip directly into your harness? And then of those times, how often do you then move about above the belay? I can honestly say I never do this. The saliant point as I stated in my first post was that if you have rope involved within the belay, whether that is tieing directly into the anchors or tieing into a strong point belay, i.e. using the rope to isolate your mass from the belay, you have an effective shock absorber. The rope is far more effective than any combination of knots in your slings to the point at which the sling strength starts to become irrelevant...

PS just realised I posted my first comment as Casa Alfredino which is my commercial profile...

In reply to muppetfilter:


Just something I´ ve noticed over the years testing stuff. Other people know as well, that´ s why DMM make their doubled slings on the dragon cams like they do.

In reply to SuperLee1985:


Kinda hard to know what you find incorrect There is a quote feature available.

In reply to jimtitt:


your knowledge and opinions on these kind of technical matters is amongst the best there is on this forum;
but that one has seriously surprised me (an a few others by the look of it).

any chance you can reference us to some testing / guidance or whatever.....
this sounds quite counter-intuitive, and could seriously change the way I go about using slings.

Thanks,
W.

In reply to jimtitt:

According to the post on here where I first heard about it, it was (maybe still is) used by the Oldham MRT, hence the name. I could be imagining this bit, but I think perhaps they use(d) it as an adjustable bridle leg when slinging a stretcher. Maybe it was someone on the team who came up with the idea originally.


That's an interesting idea, it's never occurred to me to shorten a sling with a bowline on the bight. I would like to say I'll file that one away for future reference, but the state of my memory is such that if I'm ever in a situation where it'd be really useful I probably won't remember until it's just too late.


Cool. If you do, I'd be very interested to hear what you find. I have occasionally thought about using a big ol' chain hoist I have kicking about to break an old sling between a couple of big trees, but without any reliable way of measuring the tension it seems a bit pointless.

In reply to daWalt:

There´ s some conflicting advice and tests available, mainly because of the way slings are tested. In a test rig they are between two pins and if you double the strands they lie beside each other and you get double the strength. If you use a karabiner the strands bunch together and usually lie at least partly one over the other and the pressure of the outer strand cuts through the inner one causing it to fail. The effect is well known to slackliners who either go to a large diameter spool to reduce the cutting effect or put sacrificial layers between the tape to protect it. It´ s really the same effect as the failure inside a knot.
Don´ t know anyone who has tested it apart from me but sure as hell it occurs, seen it often enough.

In reply to Tradical:

..... but perhaps more than some of us think! I have two dynamic sewn rope lanyards (Beal DynaClip). I use a short (40cm) one a lot, for anchoring at the top of sport climbs, and for various instructing scenarios, e.g. connecting across to a stuck climber, removing top rope rigging from bolts; also for running group abseils at sites where I need a fairly short safety attachment (buildings, abseil towers). For the latter a longer (75cm) one is useful. Some video footage of Beal drop tests is available on You Tube. They're pretty convincing - the amount of extension, even in the shortest lanyard, is quite impressive. It's why I bought them.

In reply to jimtitt:

thanks jim.
I know the general concept that the "crushing" (my terminology) within a knot is what causes the weakness.
esp so for tight knots - where the strand loop is very small - so the force gets concentrated into a smaller space (hope this is making some sense)..
I wouldn't have thought that lapping a sling onto / over itself - over the dia of a typical biner would have that much effect. I thought that it would only be a problem if one strand would rub over the other....

take a sling, double it over between two biners - are we looking at similar strength reduction as you see for a knot?

I'd be tempted to say this should be something that any well known and respected equipment manufacturer with a inclination towards testing of climbing equipment in ways that climbers use them might like to look into this...........

In reply to Tradical:
Interesting point. Cheers for the info. By a 'bit' I was thinking 1.5 to 2.0 metres.

In reply to daWalt:

It´ s variable, depends on the slings and the karabiners. Wide tape slings on karabiners with pronounced rope grooves fare terribly as do one with small rope bearing diameters, same sling between the wide part of two 12mm HMS karabiners probably get near the theoretical doubled rating. most of the ones I´ve seen I´ d guess at 2/3 of the original sling strength. When I was doing line lockers for a slackline company we strugged to get 10kN from 15kN rated tape using 10mm rings.
There´ s no real burning need to investigate really, it´ s been tested for generations by climbers and never been reported as an issue, slings are rated to 22kN when new as we know this is sufficient when they are old and used in the normal ways of climbers. None of my soft goods would anywhere near approach the rated strength as I use them but that´ s normal and how the standard system works.

In reply to jimtitt:

thanks, that's something new for me, and it's all worth knowing.
I guess as long as we're well within the bracket of "stronger than the force you're likely to generate"
I'll keep it in mind if I'm rigging a tyrolean......