Kevlar Cord instead of eye-bolts?

cls

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Yeah, no glue on knots. But I put a dot of white glue on the top of the cut ends to prevent fraying.
 

thzero

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Lots of good advice and specifics in this thread. There are a few rules to live by, axioms if you will, that if followed will result in consistent successes…
  1. Kevlar does abrade and hence weaken, so inspect it. If it goes through a bulkhead make ensure it can be replaced.

  2. Never use glue on a Kevlar between then attachment point and the load, it makes it brittle and prone to breaking. This has happened to enough rockets that the occurrence seems more than anecdotal.

  3. Knots generally weaken Kevlar. Stitch it instead. It does take some time, but IMO its sort of fun and affords you some attachment options you would not have otherwise.


  4. Harness length is the great redeemer nothing more reliably reduces shock load than the longest harness you can get into the rocket.

So 2... what type of "glue"; thats a pretty broad statement. I'm no chemical engineer, but to make the material brittle it would be reasonable that the "glue" would need to impact the chemistry of the material. Certainly gluing kevlar to a tube, which is common technique seen in plenty of kits and builds, does create a potential weak point or at least a point where abrasion and potential weakness could arise. Got any studies on what type of "glue" makes Kevlar, or similar materials brittle?

Oh point 3. Its not just Kevlar, its pretty much any rope that is knotted is weaker; different knots have different strength ratings. But where are the studies that show that sewing is superior to knots? After all the forces are now acting on smaller, and potentially weaker threads, and there is not nearly the number of them combined together as in the base material. There is probably a good pattern for sewing too that provides the most strength with this technique. Got any studies, papers, etc. on this?

Thanks
 

Jowayen

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I lock-braid all my Kevlar then bury the ends inside. Near 100% strength retention. No sewing. No knots. No glue. The best knot properly tied retain only 40%-80% of strength. If you don't like D-rings, use a soft shackle. But my favorite rockets don't have any Kevlar. (see post #19)
 

jahall4

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So 2... what type of "glue"; thats a pretty broad statement. I'm no chemical engineer, but to make the material brittle it would be reasonable that the "glue" would need to impact the chemistry of the material. Certainly gluing kevlar to a tube, which is common technique seen in plenty of kits and builds, does create a potential weak point or at least a point where abrasion and potential weakness could arise. Got any studies on what type of "glue" makes Kevlar, or similar materials brittle?

Oh point 3. Its not just Kevlar, its pretty much any rope that is knotted is weaker; different knots have different strength ratings. But where are the studies that show that sewing is superior to knots? After all the forces are now acting on smaller, and potentially weaker threads, and there is not nearly the number of them combined together as in the base material. There is probably a good pattern for sewing too that provides the most strength with this technique. Got any studies, papers, etc. on this?

Thanks

I'm confident "studies" have been done. I have seen some informal test and as I have already mentioned this has happened to enough rockets that the occurrence seems more than anecdotal. BTW, you are making some assumptions. Do some research, talk with others, and watch the video, it discuses a common pattern and how the number of stitches is determined.
 

thzero

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I'm confident "studies" have been done. I have seen some informal test and as I have already mentioned this has happened to enough rockets that the occurrence seems more than anecdotal. BTW, you are making some assumptions. Do some research, talk with others, and watch the video, it discuses a common pattern and how the number of stitches is determined.

Exactly my thought about your post, assumptions which is why I asked for studies, etc. since you were posting these as 'rules' with caveat of them be anecdotal. But ok, nothing to share, so I or anyone else will have to go our own research for sure.
 

jahall4

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Not at all. If you will read, you will find that what I have shared is what others much smarter and more experienced than I have learned over decades. Engineering, common sense, experience, does not require "studies" to be correct. You’re welcome to learn it on your own w/o taking the advice of others, but it is costly and can be a safety concern. You may find that you are not allowed to fly at section and prefecture launches because rocket construction and configuration are not considered sound.
 

thzero

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You are not allowed to fly at section and prefecture launches because rocket construction and configuration are not considered sound.
As long as the rocket meets the NAR and Tripoli standards there should not be any issues; but as always the RSO controls the field within the NAR/Tripoli guidelines as appropriate.
 

jahall4

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As long as the rocket meets the NAR and Tripoli standards there should not be any issues; but as always the RSO controls the field within the NAR/Tripoli guidelines as appropriate.
You betcha, and they are going to rely on accepted engineering practices, common sense, and their own experience to determine whether the guidelines are being met as appropriate.
 

Off Grid Gecko

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Aside from standards and testing...I will say that learning a little bit about sewing taught me a lot about just how strong stitches can actually be. I have threads that are mostly made for heavy duty applications due to my life circumstances and cotton in not very often the material I reach for. I have some specialized nylon threads that are near impossibly to break with your bare hands and you are likely to cut your finger against the thread trying.

Stitches themselves are interesting from a materials standpoint. They weave between the other fibers and put a lot of thread behind a tiny joint. Even just straight stitch two pieces of fabric together with a running stitch. It's not going to be super strong but you might be surprised how well it actually will hold, because it looks like it'll hold nothing at all.

On the glue point, I think it has some merit. Stiff fibers next to loose fibers, think those little bits of fiberglass that like to stick out and poke you in the fingers when you lay down mat fiber. Or even reinforcing fibers in cured concrete if you've had the pleasure. Wherever it starts to move is a haven for tearing and abrasion just based on circumstances.
At least for the most part. Kevlar sheets (or even heavy canvas) bedded in epoxy can stop pistol rounds after only a handful of layers. It's really application specific, but I think in our examples, you don't want it stiff here and floppy over there. Either all stiff or all floppy. Just my observations and years of watching stuff break. Soak part of a cotton shirt in epoxy resin, then watch where the fabric starts to abraid as it gets tugged and pulled and run through the washing machine.

Cool cool. Lots of experience in this thread already, thank you all so much for posting. Making me thing about a lot of different things.
 

jahall4

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Aside from standards and testing...I will say that learning a little bit about sewing taught me a lot about just how strong stitches can actually be. ... Stitches themselves are interesting from a materials standpoint. They weave between the other fibers and put a lot of thread behind a tiny joint. Even just straight stitch two pieces of fabric together with a running stitch. It's not going to be super strong but you might be surprised how well it actually will hold, because it looks like it'll hold nothing at all.
Me too! The hand stitching I have seen and performed myself was done using Kevlar thread and a thread count whose aggregate strength exceed the rating of the Kevlar harness it was used in. As a practical matter this is probably unnecessary, but I suspect the reasoning is simply for the harness to fail before the stitching can possibly.
 

G_T

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Glue on Kevlar at stress points -

The problem is not turning the Kevlar into a composite. Kevlar works fine for composites, for tension loads.

The problem is essentially that one is creating a transition between a composite (Kevlar fibers in a matrix of glue) and a fiber structure (the rest of the Kevlar line). The composite portion becomes rigid, but the line will flex. If stress is applied at an angle then only a portion of the line encounters the stress. It can tear like paper. Pulled uniformly, paper has good tensile strength. But pull at an edge and it tears easily. Same deal here.

That 500# rated line doesn't hold but a fraction of that if pulled at an angle relative to a glued section. Kevlar is a low stretch material which makes that more problematic. So flat out avoid that situation. Or overbuild like crazy and inspect, inspect, inspect... and hope you spot the problem before it lets go.

Recall how a knot weakens a line? Glue is worse.

The 500# rating is a derated value from what the Kevlar should be able to take as a linear pull before breaking, for fresh Kevlar. But it isn't the pull strength that matters, it is the rating for repeated shock loading that matters. For Kevlar that's going to be a small fraction of the tensile rating. Kevlar doesn't handle shock loads very well. It doesn't have the stretch to do so. It fails somewhere around 1.5% to 4% stretch for a single shock, depending on the Kevlar. It takes damage under shock without showing visible evidence of the damage, which can lead to catastrophic failure under lesser shock in the future. It takes damage faster than many other fibers. So, it gets a shorter service life. That has contributed to the decline in Kevlar usage.

Kevlar also doesn't like moisture. When wet its rating should be decreased. Note, if working with Kevlar composites, it wet sands reasonably well, but dry sanding is a nightmare. Actually I should say Kevlar really likes moisture. It quickly absorbs it compared to other comparable fibers.

Kevlar self-abrades. It is both brittle and a little abrasive. It cuts other things, and cuts itself. In some applications wax and various polymers have been tried for fiber lubrication and encapsulation. This can extend the life of a Kevlar line considerably, if self-abrasion is the limiting factor.

Kevlar degrades under UV. Long sunlight exposure is not a great idea. But, probably not very relevant for this thread.

https://www.christinedemerchant.com/rope_material_aramid.html (Kevlar is Dupont trademark for certain Aramid fibers, used variously for things like cables and bulletproof vests) It was just the first reasonable link I came across. There's lots of info out there. Dig in if you want to know more.

BTW, damp plus black powder residue is acidic and likely damages Kevlar if left exposed for a while.

People think of Kevlar as flame-proof. It isn't. Just reasonably resistant. It doesn't like 500F and higher temperatures. BP burns much hotter than that.

Sorry this was probably a bit random. It's late.

Gerald
 
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Several weeks ago, I stumbled across these on Ky and Buddy Michaelson's website:

soft_links.png
You can find them at the bottom of this page: https://the-rocketman.com/hardware/

I bought a couple sets of both sizes to experiment with. I haven't used them yet, but once they are double looped, I can't image them ever coming apart under a shock load.

Regards,
Rick
 

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Donnager

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So 2... what type of "glue"; thats a pretty broad statement. I'm no chemical engineer, but to make the material brittle it would be reasonable that the "glue" would need to impact the chemistry of the material. Certainly gluing kevlar to a tube, which is common technique seen in plenty of kits and builds, does create a potential weak point or at least a point where abrasion and potential weakness could arise. Got any studies on what type of "glue" makes Kevlar, or similar materials brittle?

Oh point 3. Its not just Kevlar, its pretty much any rope that is knotted is weaker; different knots have different strength ratings. But where are the studies that show that sewing is superior to knots? After all the forces are now acting on smaller, and potentially weaker threads, and there is not nearly the number of them combined together as in the base material. There is probably a good pattern for sewing too that provides the most strength with this technique. Got any studies, papers, etc. on this?

Thanks

My experience (and I am a chemical engineer) is that there is very little apparent chemical degradation when you attempt to use glue for a knot "stopper". The glue tends to make a hard point that gives the braided kevlar a fulcrum to separate from.

Think about bending a piece of wire back and forth until it breaks. If you have a hard point to flex against, then you should expect the material to degrade in repeated bending, very close to the hard points. A falling rocket will stress this connection in most every direction and make the joint difficult to maintain.

I have used glue as a knot stopper for lower power stuff. I don't use CA. IF I do use glue, I use white or yellow glue, which still has (a little) flex when cured.

Jon Coker has a pretty good video for sewing recovery harnesses.
 

thzero

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My experience (and I am a chemical engineer) is that there is very little apparent chemical degradation when you attempt to use glue for a knot "stopper". The glue tends to make a hard point that gives the braided kevlar a fulcrum to separate from.

Think about bending a piece of wire back and forth until it breaks. If you have a hard point to flex against, then you should expect the material to degrade in repeated bending, very close to the hard points. A falling rocket will stress this connection in most every direction and make the joint difficult to maintain.

Yeah, that makes sense as I was pointing out, but it applies to much any type of material used for this purpose - some would be more susceptible to it.
 
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