Forces applied to slings?

[Peter Burgess]

I'm glad that you want to clear things up, Stu.

I can accept that by taking my comment in the wrong way you would get upset.

But I think I have made it quite clear that what I wrote was SOLELY a response to an off the cuff question from MSD, and not in any way a reflection on the reason for the question.

I'm happy to 'shake hands' on this is you can happily withdraw your accusations of me being a) pompous, b) arrogant, c) a prize prick.

I may have strong views and set opinions, but that doesn't necessarily make me any of these things.

Please don't make personal comments about someone you have never met or spoken to.

 

[MSD]

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)).

Hmmm, Why is a rope or sling weaker when it is bent over a small radius?

Several factors come into play:

Because the fibres across the cross-section are no longer loaded equally. The fibres on the outside of the curve must be loaded harder than the ones on the inside?

The tape (rope) rubs against the radius and this friction cuts into the material. That is the dominant failure mechanism when the radius is very small (Chinese kite fighting is all about this!)

The fibres on the "inside" of the curve are in compression across the cross-section of the tape. If you do lots of very nasty mathematics involving Eigen vectors etc. you find that the maximum force on these fibres is increased. This of course works in the opposite sense to the first factor.

So we have quite a complicated situation, but I'm pretty sure of one thing, which is that the cross-section of a tape is better able to resist these effects that the cross-section of a rope.

- because it is wide and thin, the difference in radius between the inside and the outside is very slight

- the contact area between the crab and the material is maximised. This reduces the risk of damage by friction and reduces the compression forces.

So, I think the reduction of strength is less than you imagine. Here is some test data from New Zealand:

Test results
Webbing rating 17.8 kN
Sling rating 25 kN
Open sling, 12 mm radiused bar 17.5 kN
Closed sling, 19 mm shackle 29.2 kN
Closed sling, 10 mm Maillon Rapide 27.8 kN

(An open sling is a single length with two stitched loops). Even over a 10mm radius the closed sling fails at greater than the specification of the sling and teh reduction in strength is very little. I also find it fascinating that 1+1 does not equal two - the closed sling is NOT twice as strong as the open sling (the test results are directly comparable because the open sling always breaks at the stitching, not at the maillon).

A pity there is no data for a 7mm radius, since 7mm maillons are in common use for rigging.

see http://www.aspiring.co.nz/webbing2.htm

Mark

 

[Beer]

I did my Mechanical Engineering dissertation this year on trying to develop a method for detecting damage in quickdraws. Other than the fact they are smaller they seem similar in construction and materials to slings I have used in Caving.

During the course of the project I got to play around with lots of nice expensive servo-hydraulic test machines. I found I could simulate thousands of falls at quite high loads without causing a failure. In fact sinusoidal loading at 90% of the rated strength still doesn't decrease the UTS.

As for the mechanism of failure, I found that the most common method was snapping at the ends. However if a quickdraw were exerted to sustained high static load, as opposed to an impulse from a fall, it would fail in the stitching.

In terms of my project I was really disappointed by how strong these things turned out to be. I'd written a 6000 word literature review on various techniques before I got a chance to get on the equipment (blame the HSE) only to find out that it's a bit pointless as they never fail anyway.

 

[Peter Burgess]

I'd written a 6000 word literature review on various techniques before I got a chance to get on the equipment (blame the HSE) only to find out that it's a bit pointless as they never fail anyway.

Ooooo, I'm tempted.

No, Peter, behave!!

[slaps himself on wrist]

:twisted:

 

[potholer]

Am I correct in thinking that in at least some non-tubular-weave slings, the lengthwise fibres alternate between being at the front and back of the sling?

With a sling of that construction, would strength losses due to bending over a bar be lessened to at least some extent, since there aren't dedicated 'inside' and 'outside' fibres running round the bend?

 

[Stu]

I'd written a 6000 word literature review on various techniques before I got a chance to get on the equipment (blame the HSE) only to find out that it's a bit pointless as they never fail anyway.

Ooooo, I'm tempted.

No, Peter, behave!!

[slaps himself on wrist]

:twisted:

Don't congratulate yourself too soon Peter! I can put you in touch with a couple of people who work on the technical committees of some NGB's. People can find various intriguing ways of knacking kit.