Tying Knots in Kevlar Cord...

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StreuB1

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I tie a simple overhand knot, assuring my cords are always parallel before pulling tight. I then trim the tag to length and secure with a small zip tie and then trim the ziptie tag flush with flush-side cutters for electronics.
 

manixFan

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Ok, to keep things on the side track, here's a pic of my Singer 107 sewing machine. I've got a 1/4" of kevlar under the foot and while it's a bit of a tight fit, it chews through it like butter. I've also sewn leather with it with no issues. It has an adjustable zigzag so it does pretty well at sewing harnesses. I mostly use it on nylon strap, but I thought I'd try it with some scrap kevlar I had to test it at 1/4". It's using kevlar thread as well.


Tony

sewing-machine.jpg
 

beeblebrox

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I use the most versatile knot available... The Bowline. It is strong, easy to tie and easy to undo if need be...
The tape is to keep it from untying itself when not under tension.

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prfesser

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In this thread we're looking at too many variables: shock cords of different sources, different sizes, different weaves. And in some cases the flyer probably found that Knot A broke or loosened, so Knot B was tried; it worked so the flyer posted the use of Knot B without testing other knots. That's okay but it doesn't provide a lead to an optimum arrangement.

Some enterprising young rocketeer could make a nice engineering research project for university and/or TRA/NAR publication:

"Parting Strength and Location of Break for Aramid Shock Cord Loops".

Use round tubular cords of known strengths, preferably from the same source. An S-type load cell operating under tension, with a block and tackle pulley to provide the tension, would be fairly inexpensive and ought not to be too difficult to set up.

University student(s) who finds this interesting, remember: reproducibility, reproducibility, reproducibility! Multiple runs for each cord+knot combination. Same size of eyebolts for each run (better yet, same eyebolts). Inasmuch as is possible, change variables (other than those being investigated) into constants.

Best -- Terry
 

manixFan

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The Figure 8 stays tight even without a load on the line but can come apart easily. Used a lot in climbing…
The issue isn't the knot coming untied, it's the knot itself. Kevlar and other aramid fibers don't like any kind of sharp bend - they weaken greatly due to stress concentration. When the military did studies in the '70s, they concluded that knots were not a suitable way to secure or terminate aramid ropes. They either potted, spliced, or sewed them. The bowline knot fares poorly with aramid cordage, and likely the figure 8 would as well.

Many climbing and boating sites have also tested knots and come to the same conclusion - aramid ropes and most knots do not get along. Time and again, the splice (fingertrap basically) is by far and away the best way to terminate a rope. I'll link to yet another site that compares knots, with Dyneema, which is similar to an aramid type rope, and polyester. You'll note in the results the Bowline knot fails at 35% of the rated strength of Dyneema, and at 47% of poly, which is consistent with other results I've posted. The conclusions are clear - the splice is by far the best choice.


Tony


Here's the graph of the results – Dyneema is a synthetic fiber rope considered the strongest available, while Marlowbraid is a polyester. While poly is not a good choice for our uses, it shows how much better it fares than something like Dyneema, which is similar to aramid in many respects.

LRResultsDyneemaMarlowbraid.jpg
 

Wild Bill

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thank you

"aramid ropes and most knots do not get along. Time and again, the splice (fingertrap basically) is by far and away the best way to terminate a rope".
My I add if you do not have experience spicing aramid line and have the correct tools find a certified (sail boat rigger) .
thanks
 

rjstone208

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So despite what I've read, apparently Kevlar isn't useful for model rocketry unless you know how to splice or know a boat rigger. So how do you splice Kevlar to an elastic shock cord. Or am I missing something. Give up on knotting them together. Super glue. Oh wait, not supposed to glue Kevlar. I can barely tie a granny knot much less all these other convoluted knots that make my arthritic fingers hurt. What's a good, simple answer?
 

manixFan

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So despite what I've read, apparently Kevlar isn't useful for model rocketry unless you know how to splice or know a boat rigger. So how do you splice Kevlar to an elastic shock cord. Or am I missing something. Give up on knotting them together. Super glue. Oh wait, not supposed to glue Kevlar. I can barely tie a granny knot much less all these other convoluted knots that make my arthritic fingers hurt. What's a good, simple answer?
Flyers have been using Kevlar successfully for years with rockets using a wide variety of knots and other methods. The big issue is understanding how a particular knot works with Kevlar. I used Kevlar for a good 10 years before I really got interested in how to take full advantage of what it offers while not falling prey to its weaknesses. It wasn’t until I suffered a couple of near misses with recovery harnesses made from Kevlar that I really started researching it. Like many others, I generally had such a huge safety margin in the strength rating that knots didn’t matter much. But once I started cutting it closer I learned the hard way.

Too often we do things that work and think that we’re doing it right, but we’re really just getting lucky. I’m trying to be more right than lucky as I push the upper limits of my skills.


Tony

PS: if you’re connecting Kevlar to elastic, I don’t think Kevlar is going to be your main area of concern. Any knot will be far stronger than the elastic.
 
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DarthMuffin

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I'm not skilled enough to do splices, so I do knots. Yes, they weaken the cord but with kevlar cord it's easy to go up a size or two and more than compensate for this on a rocket. I did some searching to find knots that retain a substantial amount of strength and use these two primarily. The first one for tying to an anchor point, the second for making a loop for a parachute, etc. They both do not come undone or slip and look nice. https://www.animatedknots.com/uni-knot https://www.animatedknots.com/alpine-butterfly-loop-knot
 

DES

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So despite what I've read, apparently Kevlar isn't useful for model rocketry unless you know how to splice or know a boat rigger. So how do you splice Kevlar to an elastic shock cord. Or am I missing something. Give up on knotting them together. Super glue. Oh wait, not supposed to glue Kevlar. I can barely tie a granny knot much less all these other convoluted knots that make my arthritic fingers hurt. What's a good, simple answer?
Fishline type notes similar to https://www.animatedknots.com/albright-knot work for joining dissimilar cords. Like a sheet-bend, with a lot more wraps. The kelvar will be much stronger than the elastic cord, so any loss of knot strength probably inconsequential.

FYI - animatedknots.com is an awesome source of knot tying instructions, lots of instructions on splicing too.

1642275451618.png
 

manixFan

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Fishline type notes similar to https://www.animatedknots.com/albright-knot work for joining dissimilar cords. Like a sheet-bend, with a lot more wraps. The kelvar will be much stronger than the elastic cord, so any loss of knot strength probably inconsequential.

FYI - animatedknots.com is an awesome source of knot tying instructions, lots of instructions on splicing too.

View attachment 499818
That looks like a great knot for that purpose. Wrapped knots seem to be very well suited for kevlar.


Tony
 
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If you read the Marlow ropes knot analysis it says after all the tests that the knot strength varies with the type and diameter of the rope.
Generalisations for knots are useful indicators but not necessarily the answer for a specific rope or cord.
Marlow ropes cover more specifically sheathed ropes. The outer sheath provides wear or sunlight protection for the inner structural element. The inner structural element may be simply parallel or very loosely woven strands or a fully woven interior structural core. So now you’re trying to get a knot in a rope system not just a single material. The outer sheath tends to allow the inner core to continue to slip even after the knot is tied. Sheathed parachute cord is an example of this most readily available.
We tend to use uncovered braided Kevlar. At least I do.
Another variable now it’s not covered. Many knots are based on fishing. They tend to use a monofilament which is slippery when wetted.
As Terry pointed out, there are a lot more variables at play here than the Marlow ropes manual or the fishing knots handbook. We need to remember why we’re using Kevlar in the first place. For me this is about it’s ability to survive a hot ejection charge And have sufficient strength to survive. As long as the knot you use has this allowed for, the one you use does not matter.
Personally I use the bowline. It has to date never failed me. I take into account the strength reduction and live with that decision.
The simple answer in all rocketry is test what you do and then do what you‘ve tested. Anything else is just an untested hypothesis.
Fly high.
Norm
 

DES

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One question I have is ultimate strength, vs working strength. A typical small dia kelvar cord I use has a maximum breaking load limit of 550#, and a recommended working load limit of 110#. So a factor of safety of 5. A working load limit of 110# is plenty for a smaller rocket, and likely exceeds the strength of attachment to the motor mount, the 1.25-inch paper body tube, etc.

So heres the question - a knot in the cord clearly reduces the ultimate strength and thus the breaking load limit. But is that accounted for in the factor of safety used to arrive at the working load limit?

I believe it is, meaning that 110# is still a safe working load for the cord, even when knotted.
 
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One question I have is ultimate strength, vs working strength. A typical small dia kelvar cord I use has a maximum breaking load limit of 550#, and a recommended working load limit of 110#. So a factor of safety of 5. A working load limit of 110# is plenty for a smaller rocket, and likely exceeds the strength of attachment to the motor mount, the 1.25-inch paper body tube, etc.

So heres the question - a knot in the cord clearly reduces the ultimate strength and thus the breaking load limit. But is that accounted for in the factor of safety used to arrive at the working load limit?

I believe it is, meaning that 110# is still a safe working load for the cord, even when knotted.
You'd need to test your specific rope. If you are using kevlar kite cord, there are a few different weaves used. Some are woven a bit too solidly for me( does the cord feel hard) . Each weave will give different results. The rope diameter to knot minimum radius is your "weak point" Of course that's a generalisation.... :)
The strength of a knot and/or rope also depends on the rate at which it is loaded. Just to add to the variables. In rocketry this would be close to shock loading, mostly. (generalising) :)
 
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manixFan

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If you read the Marlow ropes knot analysis it says after all the tests that the knot strength varies with the type and diameter of the rope.
Generalisations for knots are useful indicators but not necessarily the answer for a specific rope or cord.
Marlow ropes cover more specifically sheathed ropes. The outer sheath provides wear or sunlight protection for the inner structural element. The inner structural element may be simply parallel or very loosely woven strands or a fully woven interior structural core. So now you’re trying to get a knot in a rope system not just a single material. The outer sheath tends to allow the inner core to continue to slip even after the knot is tied. Sheathed parachute cord is an example of this most readily available.
We tend to use uncovered braided Kevlar. At least I do.
Another variable now it’s not covered. Many knots are based on fishing. They tend to use a monofilament which is slippery when wetted.
As Terry pointed out, there are a lot more variables at play here than the Marlow ropes manual or the fishing knots handbook. We need to remember why we’re using Kevlar in the first place. For me this is about it’s ability to survive a hot ejection charge And have sufficient strength to survive. As long as the knot you use has this allowed for, the one you use does not matter.
Personally I use the bowline. It has to date never failed me. I take into account the strength reduction and live with that decision.
The simple answer in all rocketry is test what you do and then do what you‘ve tested. Anything else is just an untested hypothesis.
Fly high.
Norm
All important points. In different 'knot threads' I've linked to many different studies on knot strength, and typically they are either done for fishing or climbing. The same knot can perform very differently depending on the type of cord used. It's interesting that rjstone208's very first post is about being frustrated about what does and does not work with kevlar. It is confusing. And in spite of nearly all of us doing things we aren't supposed to do with kevlar, we've done just fine. As you point out, if your safety margin is high enough a bowline knot can work just fine. But I think most folks would be surprised at the loss of strength caused by that knot.

In past threads I've linked to a number of military studies on aramid fiber ropes that started me on my journey to try and use kevlar (and other aramid fibers) correctly. I had experienced a couple of recovery failures with it and wanted to avoid further issues, and was pretty surprised with what I learned. In many ways kevlar is really not very well suited for our uses, but we blunder along just fine for the most part, mainly because folks use 200# cord for a 1lb rocket, or 2000# cord for a 20lb rocket. A lot of safety margin can cover a lot of mistakes.


Tony
 
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One of the main things that causes a knot to break is the minimum bend radius for any specific cord diameter. This is where the multiple wrap knots get their strength from. The multiple wraps increase friction on the rope so that when the rope gets locked off back on itself, there is minimal load on the lock point.
It would be useful if clubs had a knot tying for rocketry day.

I'm currently weaving my own kevlar cord. The weave has a spring to it. Something commercial cord doesn't. Saw a cord making machine on youtube. Couldn't resist it. Took me 2 days to make 7 metres so not cost effective and the neighbours are probably fed up with the noise. :)
Norm
 

DarthMuffin

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I'm currently weaving my own kevlar cord. The weave has a spring to it. Something commercial cord doesn't. Saw a cord making machine on youtube. Couldn't resist it. Took me 2 days to make 7 metres so not cost effective and the neighbours are probably fed up with the noise. :)
Norm
That's really cool! Getting OT here but I'd love to see another post with details of your setup and process.
 

Gus

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For woven Kevlar in the 28-100# range, used on model rockets, and lightweight competition rockets, tying knots and securing the knot with a drop of CA works just fine.
 
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That's really cool! Getting OT here but I'd love to see another post with details of your setup and process.
You asked for it.....
3mm Kevlar Rope making.
My new Kumicreator. It creates friendship bracelets. I'm using a 9 thread weave. 8 Thread was a bit boxy. The max you can get on a spool is 3 metres of thread. Joining new threads in is a PITA.

Play it loud........(not when you're in bed next to your wife. Mine finds that really annoying. Your's probably will find it annoying too. Let's say no more on the subject.)
1642381525854.png

8 thread weave
 

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