when will it break?

mattbever

mattbever

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the land of the free and the home of the brave
I'm looking for some info on the decrease in rope strength when knotted; running bowline, cow hitch, beneath the prussic of a spider leg?...

and,

Is there any formula or general rules to determine the amount of strength loss due to rope abrasion/damage.

I'm looking for creative ways to convey this info to a group of @ 20, most climb. 2 part talk on rope strength and the weight of tree parts. we will be looking at the green log weight chart.

Thanks!
 
Ya no, I have been wandering the same thing. Years ago I heard that it was suppost to be I think 50 percent from a 180 degree bend. And another 10 percent from the knot.
Now it's just a flat 10 percent of the tensile strength.
But on the climbing section under the thread deicing salt on gear, Tom has a good post, with alot of usefull imformation!
But the different knots would take away different amounts.
 
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I'm looking for some info on the decrease in rope strength when knotted; running bowline, cow hitch, ...

and,

Is there any formula or general rules to determine the amount of strength loss due to rope abrasion/damage.

I'm looking for creative ways to convey this info to a group of @ 20, most climb. 2 part talk on rope strength and the weight of tree parts. we will be looking at the green log weight chart.

Thanks!

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A running bowline should be stronger than a bowline per se. The reason is that in a running bowline the knot only bears about half the load. The weakest point more likely is where the load bearing rope crosses with the eye of the bowline - that forms a kind of girth hitch.

Not sure I've ever seen actual test numbers on the strength of a cow hitch, but it's basically a girth hitch with only one strand bearing the load instead of two strands as would be the case in a true girth hitch. If you look at slings in catalogs (Sherrill, Wesspur, Petzl, etc.) you'll see that a girth hitch is not as strong as a basket hitch but I don't know that I've ever seen actual numbers that express the strength of a girth hitch in terms of the single strand strength of the material it's implemented with, but that shouldn't be difficult to determine if the max load is given for the girth hitch and you know the material and size that is used.

I know what a spider leg is; I know what a prusik is; but I'm not quite sure what you're looking for when you say, "...beneath the prussic of a spider leg?"

There is no formula or hard rules to determine the amount of strength loss in a rope due to damage. The problem here is how to describe (mathematically, etc.) the amount of damage, or a damage factor to plug into the formula if one did exist.
 
There is a book with nothing but testing results in it. It is called " All About Ropes". It is a great reference guid on all diffrent knots hitches and equipment testing.
 
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I know what a spider leg is; I know what a prusik is; but I'm not quite sure what you're looking for when you say, "...beneath the prussic of a spider leg?"

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A spider leg is often attached to the main rigging line with a prussic, I think the question he's asking has to do with strength loss in the main rigging line due to the squeeze of the prussic.
 
Just a side not to begin...Pete Donzelli talked about loosing rope effeciency rather than strength. It makes sense but requires time to explain since the term isn't in common useage. If the rope doesn't break or get weakened then the rope retains it's full strength when the knot is untied. the rest of the rope is just as strong but the connection makes for a loss of effeciency.

Back to normal programming--->

Talking about rope breaking, strength of knots/hitches, slippage is not an exact science. There are too many variables to be exact. We can look at tendencies though. There are knots/hitches that tend to weaken all ropes more.

Getting bogged down in exact numbers is a good way to loose the attention of your group. Show them ways to measure the loads, account for dynamic loading, use of blocks, etc.

Use the building block approach that Don Blair wrote about in Arborist Equipment. Build the chain from the weakest link, the rope, to the strongest, the anchor, whether it's a block/sling or natural crotch.

A very simple way to demo loads is to use a 150# game scale as a dynomometer. Use a small chunk of wood for the load. Drop it from a variety of elevations to show the multiplier on the anchor point. I;ve done this with a 20# fish scale too for smaller groups. Scale everything up or down and run the demo at home a bunch of times so that you have your act down.

Do demo rope stretch use a piece of elastic cord. Use the same piece of wood and drop height. Have the audience watch how far the piece drops...through the roof...but how the load on the anchor point changes, to almost nothing!

Doing demos in 'The Theater of the Mind' has been much more effective than going through all of the bother of a real tree setup. Certainly, the real tree is dramatic, but it is MUCH harder to pull off and be effective.
 
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