Knots

[Edwin]

Reference to hammock surpension but to knots in general, a puzzle about received wisdom.

Why would a knot halve the breaking strain? Have there been tests? Until the "cord" is turned or twisted into the knot it can't "know" and therefore have a lower breaking strain before it gets to the knot. I have never seen anything break within a knot. So where is the break likely to occur and why? I have tried knotting string and then breaking it and it generally breaks some distance away from the knot, about halway between my hands which is what you might expect.

[Bernie]

I'm sure there are people on here that know more about this than me, but I thought I'd give you my little bit of experience (not knowledge) to help things along.

There have been tests performed on the strength of knots, I suspect particularly climbing knots.

But another factor in breaking strain is the type of chord used, and the design of the knot. Some knots have sharp bends in the bightes, which I suspect will break fibers inside the chord and weaken the rope over time.


Knots weaken the rope in which they are made.[1] When knotted rope is strained to its breaking point, it almost always fails at the knot or close to it, unless it is defective or damaged elsewhere. The bending, crushing, and chafing forces that hold a knot in place also unevenly stress rope fibers and ultimately lead to a reduction in strength. The exact mechanisms that cause the weakening and failure are complex and are the subject of continued study.
Source: http://en.wikipedia.org/wiki/Knots#Strength

My best approach to this is to use chord that far exceeds the required breaking strain.

[Silverback]

http://iopscience.iop.org/1367-2630/9/3/065/fulltext

http://www.josephchansen.com/ropetest.html

[MikeWilkinson]

Nice one Sapper, pretty much sums up what I was going to say. But I'm going to say it anyway.
Not every knot will halve the breaking strain, but will certainly decrease it anywhere from being at 100% down to 30% ish. It mostly depends on how the knot works.

For example a clove hitch is very unlikely to reduce the breaking strain as there is very little in the form of pinch points where the cord enters/exits the knot, there is also very little in the way of twisting or severe bending of the cord meaning that in most cases any loading applied to the cord will still be (roughly) evenly distributed through the cords strands.

However an overhand knot or knot that binds upon itself (evenk hitch for example) pinches the cord where it enters the knot and in the case of some knots acts as a scissors on the cord, resulting in the cord breaking at the point the cord enters the knot.
Severe twisting and bends in a cord can stop the cords fibres/strands from distributing the loading evenly, meaning that one strand may take all/ most of the load, it breaks, shock transfering the load to the next fibre which results in a cascading failure of the cords fibres and snap!!!

The rule of thumb when using knots is to half the working load for safey. Splices are somewhat better and the rule there is to use 70% of the working load.

Pretty much every book on ropes and knots that I own has this information in some form or another in the introductory text. Particularly if they have a section on splicing too.

When you start applying this logic to the knots you use, you soon realise why some knots are better for use natural cordage than others as the less you bind and pinch your home made cordage the more likely it is not to break - makes you think a bit more when trying to bow drill with hand made cordage!

[MikeWilkinson]

My best approach to this is to use chord that far exceeds the required breaking strain.

Couldn't agree with you anymore Bernie, it is one of the main reasons why I deter people from using 550 Paracord in their hammock setups when I see it!! You only need to stick an overhand or figure 8 onto them and it is down to 250lbs breaking strain, add in the 2x -5x Weight multiplication factor for a 15 degree to 5 degree hang and your snapping double and even tripled lengths left right and centre!!!

[Silverback]

Thats why I use a knotless hammock system, thanks to advice ecouraging me to remove the knotted polyprop webbing .....and IF i do have to use any form of securing method for the rope supplied with my hammock the clove hitch will be the knot of choice.

My tarp however will use the evenk and probably a few more knots, but its not keeping my big fat arse off the floor :happy-clapping:

[Edwin]

Interesting information that the line is at its weakest as it enters the knot and is squeezed. Must bow to science but still haven't succeeded myself in breaking a knot actually at the knot or within it.