Soft Links Vs Mallions

[Ian Ball]

screw link every time 

You won't be saying that at my age when your fingers don't work as well as they used to!

  spanner to the rescue.

 

[Mike Hopley]

Also of note, there are other non-locking carabiners in the 20-25 g range that may cost less than the Edelrid Nineteen G, which is currently the lightest on the market if I'm not mistaken.

I think the 19g is a bit too small for rigging. For another ~3 grams, you can get something more practical (Camp Nano 22).

In their "current" book, the French Caving School even mention rigging with carabiners that are not full strength -- i.e. strong accessory carabiners like this one. This now seems a little mad, given that we have full-strength carabiners that are lighter and cost about the same.

 

[andrewmcleod]

If you were feeling really ballsy, you could rig on DMM XSRE snapgates, rated to 4kN and only 8g

It's probably worth remembering though that rope is about 50g a metre so you can save more by making sure you have the right length ropes... Even making loops larger than they need to be will add extra weight.

 

[JB]

Interesting topic. Looking at how you tie those things in and faff about trying to tighten elastic o-rings over them they?re not for me. Cold hands and gloves could put a stop to your trip I would think.

Like lots of people i?m sure I?m after ease of use, versatility and strength so prefer to use full strength screwgate karabiners (if you end up with two people on a rope for some reason proper karabiners reassuring).

 

[Ian Ball]

7kN was I believe the 1 inch tubular sling, others failed at lower loading, but it didn't say whether drop tests (I assume it was) or steadily increasing loads...

From Aussie website, who makes his money doing tests for rope access:
https://www.ropelab.com.au/category/free/articles/

Nice website 

 

[Tseralo]

I know quite a few people who believe that a (say) 7kN load during a drop test is more likely to break something than a steady 7kN load, but I have yet to hear what I consider to be a convincing explanation of why that should be.

I remember at school bending bars of McCowan's Highland Toffee slowly (and eventually stretching them out), but you could still snap them if you moved your hand quickly enough. Are the two issues related perchance? As in there's a threshold velocity that suddenly overcomes structural stability?

A big difference is heat, at least in rope if you do a drop test you can melt knots especially ones that are not set well as it tightens suddenly. At speed ropes rub on each other without having time to dissipate the heat where as a slow pull would set the knot and even though the same amount of friction/energy is generated it will dissipate slower.

The same issue could be seen if you were testing a descender etc.

I would guess the same could happen in other materials at a more molecular level? but i'm not a physicist and am happy to be corrected.

 

[Chocolate fireguard]

I know quite a few people who believe that a (say) 7kN load during a drop test is more likely to break something than a steady 7kN load, but I have yet to hear what I consider to be a convincing explanation of why that should be.

I remember at school bending bars of McCowan's Highland Toffee slowly (and eventually stretching them out), but you could still snap them if you moved your hand quickly enough. Are the two issues related perchance? As in there's a threshold velocity that suddenly overcomes structural stability?

Sorry, I just saw this post.

I think what you?re describing is called strain rate dependence.

Over the last few years, whenever I remember, I have been looking for information on it and nylon, but without much success.

It seems that many (most?) materials exhibit it, but not significantly under the conditions of temperature and strain rate components are designed for. I have found a few mentions that some physical properties of polymers are sensitive to strain rates, but no actual figures that can be applied to ropes.

Strain is the change in a dimension (length in our case) divided by the original dimension.  The average strain rate is that strain divided by the time taken.

A drop test to break on a new 1m length of caving rope will produce a rope stretch of perhaps 30% and take perhaps 200 milliseconds as the knots tighten. That gives a strain rate of 0.3/0.2, or 1.5 per second.

So we are talking about strain rates of ?a few per second? for a dynamic test and possibly ?a few hundredths per second? for a slow stretch. This difference of a couple of orders of magnitude is important for a toffee bar or a piece of silly putty, but I haven?t found anybody who has done work on nylon at these rates.

Bearing in mind that the break happens in the knot, that a knot will convert several hundred Joules of energy to heat and that after a drop test it is not remarkable to find  the broken end of rope showing signs of melting (and sometimes stuck to another part of the sample), it would surprise me if the time for the heat to spread throughout the knot during a test was not important.

Not much of this would apply to a sewn sling failing over an edge.

Any info on strain rate measurements on nylon will be gratefully received.