Tying Knots in Kevlar Cord...

[bronicabill]

Is there a special way you guys tie knots in kevlar cord? I'm finding that my knots hold while under tension, but seem to want to loosen when tension is removed. Since you're not supposed to put any glue on the knot because it weakens the cord, I'm not sure how to proceed.

Any help would be greatly appreciated! Thanks!!!

 

[RocketScott]

If it's not something you tie/untie frequently you can put heat shrink over the knot to protect it and hold it in place

 

[waltr]

I use a 'figure 8' knot.
https://www.netknots.com/rope_knots/figure-eight
This is a climber's knot that does not come untied on its own but easy to get untied after being weighted.
Works very well on kevlar shock cords.

 

[Huxter]

Great site. I use the Surgeons loop (did not know what the knot was called until now!) Never had any problems w it coming loose. I could use more experience tho. Might try the Triple Surgeons knot next.

https://www.netknots.com/fishing_knots/surgeons-end-loop

 

[Reinhard]

It depends a bit on the Kevlar. The type of tubular Kevlar I'm currently using seems a bit slippery and a bowline knot will come loose but I remember using a different type of Kevlar where a bowline held fine.

For the slippery stuff, I switched to the surgeons loop knot for end loops and haven't had problems since then (but the knot is harder to untie).


Instead of trimming the tag end short, I thread it through the working end, Chinese finger trap style after trying shrink tube in the past (didn't like the stiffness). A blunt splicing needle and a piece of line helps with that.

Threading in helper line


Before pulling tag end in



End result. No loose ends, no stiff sections. (sorry for the blurry picture)



For mid loops I use the alpine butterfly loop.


Reinhard

 

[manixFan]

The bowline knot is terrible for Kevlar. Ideally you would not use any knots, only fingertrap or sewn loops, and then links or soft shackles to secure it. If you need to use a knot, use a wrapped knot like what's used in fishing.

The number one thing to know about kevlar or any aramid cord - it does not like any kind of sharp bend - whether in a knot or while running. (I've had kevlar break right at the edge of a body tube well below its rated strength.) Here's a good chart on knot strength:

https://www.marlowropes.com/sites/default/files/Retained strength Knots v Splices infographic_0.pdf
The above PDF shows how different ropes compare with several knots, including a bowline. The bowline suffers from a large reduction in rope strength, with up to a 80% loss with D2 rope (Dyneema, which is similar to kevlar fiber, which are the numbers shown in red). But with a polyester rope, the loss is only about 50%. Based on that info, a bowline knot is not suitable for something like Technora or Kevlar. Note that splices retain a much higher percentage of the original rope strength.

This Wikipedia entry discusses how aramid requires special handling when it comes to knots:

https://en.wikipedia.org/wiki/Triple_fisherman's_knot
Moral of the story: different types of rope/cordage require different kinds of knots to retain maximum strength. One size knot does not fit all. For Technora, I generally use a splice (fingertrap) but if I have to use a knot, I want to maintain as much of the strength of the cord as possible, while also ensuring the knot is secure with the slippery nature of Technora. I have found 'wrapped' knots, like a clinch or uni knot work well, but require the knot to be snugged up against the part, so they really aren't a loop. The surgeon's knot is basically a wrapped knot which is likely a part of why it works well. This thread shows the Kreh loop knot which is a wrapped knot that also forms a loop.


Tony

(Some of my text is a copy/paste from the linked thread in the last sentence. Technora is very similar to Kevlar - both are almost always coated to make them slippery to avoid damage from abrasion if the run past an edge)

 

[Kelly]

For end loops, I just finger trap it. A couple stitches with Kevlar thread ensures it doesn't come undone.

 

[Dipstick]

The bowline knot is terrible for Kevlar. Ideally you would not use any knots, only fingertrap or sewn loops, and then links or soft shackles to secure it. If you need to use a knot, use a wrapped knot like what's used in fishing.

The number one thing to know about kevlar or any aramid cord - it does not like any kind of sharp bend - whether in a knot or while running. (I've had kevlar break right at the edge of a body tube well below its rated strength.) Here's a good chart on knot strength:

https://www.marlowropes.com/sites/default/files/Retained strength Knots v Splices infographic_0.pdf
The above PDF shows how different ropes compare with several knots, including a bowline. The bowline suffers from a large reduction in rope strength, with up to a 80% loss with D2 rope (Dyneema, which is similar to kevlar fiber, which are the numbers shown in red). But with a polyester rope, the loss is only about 50%. Based on that info, a bowline knot is not suitable for something like Technora or Kevlar. Note that splices retain a much higher percentage of the original rope strength.

This Wikipedia entry discusses how aramid requires special handling when it comes to knots:

https://en.wikipedia.org/wiki/Triple_fisherman's_knot
Moral of the story: different types of rope/cordage require different kinds of knots to retain maximum strength. One size knot does not fit all. For Technora, I generally use a splice (fingertrap) but if I have to use a knot, I want to maintain as much of the strength of the cord as possible, while also ensuring the knot is secure with the slippery nature of Technora. I have found 'wrapped' knots, like a clinch or uni knot work well, but require the knot to be snugged up against the part, so they really aren't a loop. The surgeon's know is basically a wrapped knot which is likely a part of why it works well. This thread shows the kreh loop knot which is a wrapped knot that also forms a loop.


Tony

(Some of my text is a copy/paste from the linked thread in the last sentence. Technora is very similar to Kevlar - both are almost always coated to make them slippery to avoid damage from abrasion if the run past an edge)

Wow, I have used the bowline for a while now after having read here somewhere in TRF that it is a knot that doesn't reduce the strength if Kevlar like a square knot for instance... always something new to learn.

 

[OverTheTop]

I have used bowline knots, heatshrink and small amounts of PVA glue to keep knots in Kevlar from undoing. I guess I just don't put that much stress on the harness as I have not yet had a failure.

I might try finger-trapping next time.

 

[bronicabill]

Someone explain "finger trapping" please. (Sorry to show my ignorance on knots, but there it is!)

 

[manixFan]

Someone explain "finger trapping" please. (Sorry to show my ignorance on knots, but there it is!)

Below is the 'official' way to do it. I don't use a FID, I use a wire instead, but the result is the same. The reason you taper the end of the Kevlar is to prevent a sharp shoulder which puts stress on the kevlar. As I said, it does not like sharp bends, or really any kind of bend. Curves are the answer.




Tony

 

[GAP]

I originally used the figure 8 and bowline knots, but found after a while the knot would start to lose tension. Looked up what knots reduce strength the least and found the Fisherman's Knot and I must say I am a convert! I've never had a failure on personal rockets from the harness failing, but I normally use shock cord that is ridiculously overrated for most rockets (using 7800 lb shock cord for a 30 lb rocket for example).

 

[MrEd]

I use a bowline in just about everything. I wrap the tail end with a few turns of electrical tape to keep it from shaking loose. That makes it very easy to untie if needed. The bowline is extremely efficient and can can be untied without damaging the cord.

In my career have had two memorable rockets come apart under a significant load - one on early deployment and the other was a coupler fold. The sewn loop on 3/8" Kevlar did fail, but the bowline on 1700lb cord held while the Kevlar broke.

 

[JoePfeiffer]

I tie the ends to quick links using either an improved uni or a San Diego jam knot, and cover with adhesive heat shrink tubing.

 

[dr wogz]

I've used plenty of bowlines & figure 8s and I usually add an half hit on teh tag end just to ensure it does hold. (And I do check them periodically) never-undo knots tend to get a sleeve or heat shrink.

I believe teh fisherman's knot is also called the Trilene knot, the knot for monofilament. A twist on that is the Duncan Loop. But, as Manixfan pointed out, these have an extra tight bend in them which may yield unintended results....

 

[OverTheTop]

I wrap the tail end with a few turns of electrical tape to keep it from shaking loose.

I use a cable tie on the loose end.

 

[Joekeyo]

knot that does not come untied on its own but easy to get untied

In sailing school I learned that this was the definition of a good knot.

 

[Sooner Boomer]

I've never had any trouble gluing knots to secure them. Glue *can* be a problem if it causes the materiel (kevlar or nylon) to become brittle. This will cause the line to fail. I use fabric/craft glue, RTV, or very small amounts of contact cement. I avoid CA, epoxy, or other glues that set up hard.

 

[Wild Bill emeritus]

I have been racing sailing boats for 25 years, retired now sail RC boats.
Near tie a knot with synthetic fiber line .
Always splice the line.
If you are not very experienced with splicing Kevlar line find a certified rigger .
sail boat rigger.

good luck

 

[lakeroadster]

I've used this for tying fishing lures for years. So I've, by default, also used it for 100# kevlar..

 

[Wild Bill emeritus]

I will try out the knot.
I like the Mercury capsule .
I was born in the month of June, so I am like the Gemini capsule and I remember both.

thanks

 

[David Schwantz]

To the OP, if you want to mark your loops and send them to me I can sew them for you. A couple of years ago I bought a Phaff 130. It is a 1959 German machine, it will stitch 1/4' leather. I use kevlar thread.

 

[OverTheTop]

it will stitch 1/4' leather

Either an impressive machine or an impressive typo .

 

[jimzcatz]

I was told straight from the man himself Teddy (ONEBADHAWK) to NEVER tie knots or loops in Kevlar. Maybe he will chime in with an explanation!!

 

[tsmith1315]

Either an impressive machine or an impressive typo .

He's probably got a gas engine on it.

 

[David Schwantz]

No typo. Look it up on line. This thing is almost like the Singer that my Mom had. The only part that is not metal is the drive belt. Built like a tank.

 

[OverTheTop]

So it will go through 3" leather?

 

[David Schwantz]

Boy, you must have different math way down there. 1/4" plus 1/4" up here still equals 1/2"

 

[David Schwantz]

He's probably got a gas engine on it.

If I did, I'd put a little turbine on it. Put one on a sled many years ago.

 

[OverTheTop]

Boy, you must have different math way down there. 1/4" plus 1/4" up here still equals 1/2"

The post I referred to said 1/4 foot (which equals 3" down here ).