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Forces applied to slings?

Stu

Active member
Take a sling. When loaded with a force of x you'd expect half x on each strand, yes? Double the sling up (so four strands). Apply force x and you'd expect 1/4 x on each strand, yes?

If this thinking is correct (?) will doubling a sling make it twice as strong i.e. if it breaks at max. x when loaded as a single loop of sling, will it be able to bear 2x when doubled? I'm thinking it won't.
 
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NZcaver

Guest
Theoretically, yes - I believe a doubled sling should be twice as strong. Of course it will depend on the strength of the tied or sewn connection too.

I use a common "double-up" method for anchoring tube tape/webbing to trees, rocks etc, especially for rescue loads. It's called a wrap-3, pull-2 - and is made exactly like it sounds. Three loops around the anchor, tie the ends together at the front of the anchor, and pull the other 2 loops out to clip into. Not only do you have double or more the strength of a single loop, but the knot has very little load on it due to it's placement and the surface friction of the anchor. Makes it really easy to untie, too! :D
 
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Dave H

Guest
stu said:
Take a sling. When loaded with a force of x you'd expect half x on each strand, yes?
Depends on the angle between the two strands...
If the two strands are parallel then 50% of the load is taken on each strand.
The greater the angle between the two strands the greater the load. I don't have the actual figures to hand at the moment, but it very quickly gets to the point where the load down each strand is equal to the load on the rig (maybe 90degrees). This is why the arms of a Y hang shouldn't be at an obtuse angle.
 

cap n chris

Well-known member
With 100kg load suspended on sling between two anchors:

Angle / load on EACH anchor

90 / 70kg
120 / 100kg
160 / 300kg
 
E

epik

Guest
stu said:
If this thinking is correct (?) will doubling a sling make it twice as strong i.e. if it breaks at max. x when loaded as a single loop of sling, will it be able to bear 2x when doubled? I'm thinking it won't.

The sling at any point will still be just as strong (ignoring the fact knots reduce its strength as does tight turns as in feeding through bolts etc) as its quoted strength i.e. 12kN etc just the system you have created makes reaching that force less likely! Also remember your system is only as strong as its weakest part so if you double it up enough your system will be stronger than the bolt which will fail first :)
 
M

MSD

Guest
The other thing to consider is that with the system you describe, the sling will only take 4 times the load if the load is exactly equally divided between all four strands. Obviously it will tend to self-equalise, but friction around the crabs and between the different strands (especially if they are squashed together) will mean that it won't completely equalise. On the other hand it will also stretch a bit if heavily loaded and the stretching will help to equalise the load. I don't have any concrete evidence, but I would guess that a "stretchier" sling tied from climbing rope would equalise better than a spectra or dyneema sling.

In summary....if you make sure that it is pretty well equalised before you load it, I think you should approach 4 times the strength, but not quite.

My next question is....who cares? Assuming that the slings you use for safety-critical applications are 22kN (CE standard stuff in other words), why do you want more strength? If weight is a concern, dyneema or kevlar slings are this strong and extremely lightweight with little bulk.

Mark
 

Stu

Active member
Dave H said:
stu said:
Take a sling. When loaded with a force of x you'd expect half x on each strand, yes?
Depends on the angle between the two strands...
If the two strands are parallel then 50% of the load is taken on each strand.
The greater the angle between the two strands the greater the load. I don't have the actual figures to hand at the moment, but it very quickly gets to the point where the load down each strand is equal to the load on the rig (maybe 90degrees). This is why the arms of a Y hang shouldn't be at an obtuse angle.

I should have made the parameters clearer i.e. I knew about angles, weakest links etc and had purposely left them out of the equation; should have said so.

I was only interested in the physics I suppose.
 

nickwilliams

Active member
stu said:
Take a sling. When loaded with a force of x you'd expect half x on each strand, yes?

[Rummages in memory for long forgotten 'A' level physics knowledge...]

You've not laid out quite all of the parameters, but turning this into a mechanics problem, let's assume the belay is frictionless, the sling is a continuous loop (aka a 'roundsling') which has no mass and is in-elastic, and the sling is simply hung over the belay. In this case the tension in each side of the sling must be equal or the sling would rotate around the belay, so the tension in each side is half the weight which is hung on it (= x/2). This is also the common sense answer.

stu said:
Double the sling up (so four strands). Apply force x and you'd expect 1/4 x on each strand, yes?

Yes, this is correct. Again, all the tensions must balance or the sling will move, so the force due to the weight x is evenly distributed and the tension = x/4.

stu said:
If this thinking is correct (?) will doubling a sling make it twice as strong i.e. if it breaks at max. x when loaded as a single loop of sling, will it be able to bear 2x when doubled? I'm thinking it won't.

You are making life difficult for yourself by a poor use of terminology. The 'strength' of the sling does not change, what changes is the tension in each of the 'legs' and hence the relationship between that tension and the ultimate strength (breaking strain) of the sling. If the tension is only x/2, and x is the breaking strain, then yes, doubling the sling increases the 'strength' of the system by a factor of 2.

The physics of this are the same as those which apply to a hoist system using pulleys - use a single pulley at the top with the rope slung over it and lift the weight directly and the tension in the rope is equal to the weight. Add another pulley and fasten one end of the rope to the ceiling with the weight attached to a pulley in the middle and the tension required to lift the weight is now only a half the weight.

In fact, complexities like friction (especially between any two parts of the sling which come into contact with one another) are very significant and cannot be ignored so the answer is never as simple as it appears in the abstract, but the information given as standard with any CE marked lifting slings might help to give a good feel for the performance of the system in any given configuration. See http://www.tonybeal.com/lift.htm#round

Nick.
 
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hoehlenforscher

Guest
Not sure if this is correct (or indeed relevant), just thinking out loud. It has been assumed that each side of the sling bears half the load which i see must be correct, but surely (ignoring a small degree of friction) the point where it passes through the fixtures (maillons, krabs P hangers whatever) on either end of the system then it is bearing all the load on a single point at either end. Is this not more critical in terms of system weakness? Would it not be more beneficial to take a few turns around the fixtures either end to increase friction in stead of shortening the sling since as already pointed out the sides of the sling are far less likely to break having only 1/2 the load????.
 
M

MSD

Guest
hoehlenforscher said:
Not sure if this is correct (or indeed relevant), just thinking out loud. It has been assumed that each side of the sling bears half the load which i see must be correct, but surely (ignoring a small degree of friction) the point where it passes through the fixtures (maillons, krabs P hangers whatever) on either end of the system then it is bearing all the load on a single point at either end. Is this not more critical in terms of system weakness? Would it not be more beneficial to take a few turns around the fixtures either end to increase friction in stead of shortening the sling since as already pointed out the sides of the sling are far less likely to break having only 1/2 the load????.

No, quite the reverse. The loss of strength from bending a tape over a krab is not that much. But if you take several turns around the crab at each end, you might not get the load equally shared between the two sides of the tape (due to friction as described above). Therefore your suggestion is likley to make the system weaker, not stronger.

I still don't know why we are having this discussion since slings are very strong. Anybody had one break?

Mark
 

Peter Burgess

New member
I still don't know why we are having this discussion since slings are very strong.

It's the increasing obsession with eliminating risk at any price. Risk cannot be eliminated, only reduced to an acceptable level, and the definition of what is acceptable is purely subjective.

When the world comes to its senses, it will realise that risk is a part of everyday life, and money and effort is better spent in enjoying life rather than spending the whole time worrying excessively about what might happen. [pompous smiley] :wink:
 

Brains

Well-known member
Sort of yes, rigged a doubled rope sling and due to friction it wasnt quite equalised. Under load, as far as I can tell, the frition caused the rope to melt part way through. No injuries, just no need for laxatives for a day or two :oops: (In my climbing days before I became a caver and learnt proper ropework... Should this last bit be on the "scrap" thread? :twisted: )
 

Stu

Active member
Peter Burgess said:
I still don't know why we are having this discussion since slings are very strong.

It's the increasing obsession with eliminating risk at any price. Risk cannot be eliminated, only reduced to an acceptable level, and the definition of what is acceptable is purely subjective.

When the world comes to its senses, it will realise that risk is a part of everyday life, and money and effort is better spent in enjoying life rather than spending the whole time worrying excessively about what might happen.

Hang on! My question has nothing to do with slings breaking or paranoia of any sort!! I've been actively involved in the outdoors for 30 years and profesionally for about twelve. Stop making assumptions. It was a question about the physics I was interested in: simple! In the abscence of any sort of smiley I presume you meant to be pompous? :evil:

Edit! Actually re-reading your post I'm even angrier at your arrogance! Granted you answered a question posed by MSD (whose opinion I normally value) but FFS who do you think you are to presume something of me. At the risk of getting mod'ded or the thread pulled, sorry I can't help myself... but Pete; you're a prize prick!
 

SamT

Moderator
calm down children - I think we will need to put a suit case between you two on the back seat if you carry on like that. :LOL:

Pete - that was off topic - save that one for the scrap thread
Stu - that was a bit knee jerk

to quote pulp fiction.

Everybody - be cool 8)
 

Peter Burgess

New member
Calm down? I wasn't even wound up. My comments were an observation in general terms to a specific question asked.

who do you think you are to presume something of me.

Actually, I think you are presuming something of me.

Stu - ask yourself whether your anger tells us all more about your feelings than it does about my opinions.

I stand by my observations which were addressed at no particular person, and are how I see modern society obsessed with minutiae and ignoring greater problems.

Although off topic, please allow me to just say that I respect the opinions of people who argue reasonably, stay polite, and do not resort to verbal abuse.

I have no comment to make on the physics - others have done this already.

Personal insults are NOT regarded as acceptable behaviour on this forum. It's one of the things that I like about UKCaving. Please can we keep it like that?
 

nickwilliams

Active member
MSD said:
No, quite the reverse. The loss of strength from bending a tape over a krab is not that much.

Be careful when making sweeping statements like this - the reduction can (fairly easily) be significant. I've always used 50% as a rule of thumb number for any bend radius of less than about 25mm (based on similar figures for the strength of knots in ropes of similar size and UTS (Ultimate Tensile Strength)).

Having said this, the UTS should be at least seven times the WLL (Working Load Limit (what used to be known as the SWL)) for a fabric sling, and should be at least 14 times for any fabric sling used for lifting people.

MSD said:
But if you take several turns around the crab at each end, you might not get the load equally shared between the two sides of the tape (due to friction as described above). Therefore your suggestion is likley to make the system weaker, not stronger.

A very important point, and well made.

MSD said:
I still don't know why we are having this discussion since slings are very strong. Anybody had one break?

Well, we are having the discussion because someone asked the question, but probably more to the point, the UTS of a 25mm tape sling is of the same order of magnitude as a 11mm static rope, and we know that these will fail under test conditions which it is (theoretically at least) possible to replicate in a real caving environment (heavy caver, heavily loaded , fall factor > 1) so I don't think it's appropriate to dismiss the whole discussion on the basis that it's a 'spiders from mars' scenario.

Nick.
 

Stu

Active member
I still don't know why we are having this discussion since slings are very strong.

Of course my anger says something about me.... this dismissive little snippet prompted me to assume you as arrogant and I reacted as such. Polite enough?

As for what's acceptable I suggest a quick look through, though of course not in too much detail (!), any FAQ re: netiquette... look up off topic! It'll be close to obscene language!
 

Peter Burgess

New member
Sorry, Stu, what did that last (but one) post have to do with slings? A bit off topic, don't you think? :wink:

Personal insults are NOT regarded as acceptable behaviour on this forum. It's one of the things that I like about UKCaving. Please can we keep it like that?

Obviously not. :cry:

Polite enough?

No. You were very rude, and unnecessarily so.
 

Stu

Active member
Peter Burgess said:
Sorry, Stu, what did that last (but one) post have to do with slings? A bit off topic, don't you think? :wink:

Personal insults are NOT regarded as acceptable behaviour on this forum. It's one of the things that I like about UKCaving. Please can we keep it like that?

Obviously not. :cry:

Polite enough?

No. You were very rude, and unnecessarily so.


Right. Let's try to clear this up.

Assume for a second that people say things (usually) for a reason. What you said re: your monologue about society etc, follows this logic? Yes? The "reason" was in response to my question about the physics of forces on slings. One or two people assumed, through my lack of stating to what purpose my question was intended, to give answers based on an "it's strong and safe so don't worry" theme. All well and good, I knew all that before and stated, hopefully, a clearer need for asking the question later on i.e. I'm curious! That's it.

You followed it up with what I thought was a provocotive piece about how we all worry too much and we don't like to take risks and that we spent too much time worrying about it. Your "we/world" as in modern society was all inclusive, seemingly to ignore the fact of who you were expressing your opinions to! Carrying on, my thought process deduced you were aiming said comments at me. (nice edit by the way!)

What I said was harsh but I don't apologise for the sentiment. Your suggestion that the world is on some HSE fixation to make all things safe is well wide of the mark. That's for another day though.

Sorry for lowering the tone.
 
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