Finger Trap Technique

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Something I learned by accident is that kevlar greatly increases in friction when it's wet; I accidently cut some pieces of steel with it. It did not need the slurry of ground carbide, lol.
 
I figured I’d ask my question to this thread to keep things in one place.

I also finger trap my Kevlar lines but how much to finger trap has always been a mystery. I know for slippery (low-friction) webbing you should finger trap more. I use 1/4” Kevlar webbing from Giant Leap and it’s not coated and not slippery.

I finger trap 5-6” of line which equates to a ~20:1 engagement factor for 1/4” webbing. There are a few bar tacks sewn in for good measure. This works for my application but I’ve never load tested it.

Are there any rules for finger trap engagement?

IMG_1504.jpeg
 
Kip_Daugirdas said:
I figured I’d ask my question to this thread to keep things in one place.

I also finger trap my Kevlar lines but how much to finger trap has always been a mystery. I know for slippery (low-friction) webbing you should finger trap more. I use 1/4” Kevlar webbing from Giant Leap and it’s not coated and not slippery.

I finger trap 5-6” of line which equates to a ~20:1 engagement factor for 1/4” webbing. There are a few bar tacks sewn in for good measure. This works for my application but I’ve never load tested it.

Are there any rules for finger trap engagement?

View attachment 611753
Click to expand...
Kip,

There are some rules regarding the buried length – typically the buried end should be 47 times the diameter of the cord. The buried end also must be tapered. In all my research of splicing Kevlar and similar high-modulus cord, sewing across the cord will basically negate the benefit of the splice. The first sewn line will act as a stress riser and it seems most likely the cord will fail at that point. There are ways to lock slippery cord in place without sewing. When I first started researching how to secure Kevlar, the navy was testing it for mooring and other types of lines, and the standard was a sewn loop, which turned out to be far weaker than a spliced loop or other formed loop.

I am slowly transitioning to Dyneema, which is a material used in sailing, and incredibly, has a higher tensile strength than Kevlar. However, it is not heat resistant like Kevlar, so it is best used for CO2 ejection systems or where the cord can be adequately protected from heat and bp ejection charges. If you google 'how to splice dyneema', you'll find instructions that you can directly apply to Kevlar spices. The most important takeaway is that the buried end must be tapered – both Kevlar and Dyneema don't like any kind of sharp angle, including one caused by a blunt buried end.

An example of a non-sewn loop for a loop is a Brummell eye splice, as shown here:

https://atlanticbraids.com/dyneema-brummel-eye-splice-1-end/
FWIW, I've made fids from both aluminum and brass tubing. I've used a variety of methods to form a point at one end and then slightly flare the open end. (The point needs to be very smooth so as to not catch any fibers.) And once you start using non-sewn loops, the next thing you need to investigate are soft shackles, they are far superior to quicklicks.


Tony

example of a soft shackle:
https://shop.marlowropes.com/en-gb/soft-shackle-wss
 
47X the diameter!? That seems like a lot.

I agree that the bar tacks are running the wrong direction. And having 3 essentially negates the finger trap in that location.

I plan to have these redone with a couple zig-zag stitches near the loop running length-wise a couple inches. I also plan to increase the buried length to 9” and will test the lines to failure. The reason for the 9” is because some of my harnesses are quite short (shortest being 18”).
 
1)
I round off the length to 50x diameter (easy to remember). It sounds like a lot, but is not once you do it a few times. (Plus the taper shortens this length.)
.1in => 5 inches
(Emma kits 700# or 1000#)

.125in => 6.25 in
(1/8" Tubular Kevlar or LOC 3/16 Tubular Nylon)

.25in => 12.5in
(1/4" Tubular Kevlar)

2) The "trapped end" MUST be tapered. Again, to make it easy to remember use 3x diameter, with 3 steps...ie. if you have 12 strand 1/8" (the buried length is 6.25inch to end from above.) To taper, mark at 3/8, 3/4, and 1-1/8. Start at the 1-1/8" mark and cut and remove 3 strands. Then at 3/4" cut and remove 3 more. Then at 3/8 cut and remove 3 more. (Note: if the bundles are fraying apart [and they most likely will be at some point on these small diameters], it's ok to just trim an estimated amount. The goal is an even taper.

3) Locking stitches for the trap go at the mid point between the "thick" end of the taper, and the loop. ONLY 3 STITCHES down, turn 90 degrees and 3 STITCHES back. Then 90 degree to start point. Tie ends together. The lock stitches are only to prevent "gradual slipping" of the tail during cyclical loading. At this mid point there is almost no shear load on the lock stitches. [If you stitch the full length, or put the lock stitches in the wrong place, the trap can't stretch to squeeze like it should, and the lock stitches carry shear load.]
 
So in short, you open the eye by scrunching the outer braid to free up some length of the inner braid, allowing the big knobby knot on the end to pass through.
 
This is a how to do a locking splice by Marlow, which is a leader in Dyneema:



Here's a good video showing how to make soft a shackle, with a test showing the breaking strength:



And why tapering the cord is important:



Yes, it is a rabbit hole, and there isn't an end to it.


Tony
 
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One of the easiest things I’ve done in rocketry. Thanks for the tips guys!

Coat hanger and a dowel make a great finger trap tool.

Though I’m still not sold on the 50:1 length to diameter ratio. That has to be based on low friction cord.

IMG_1507.jpeg

Center stitch with size 69 Kevlar thread.
IMG_1508.jpeg
 
Info for soft shackle. I just made this one today out of Emma Kites 700# Kevlar. [Test to see if would work, and if my numb fingers could do it. ]

Lengths NEED to be "tweaked" slightly, but I would use it for a MPR.

It took 18" of raw material, and weighs 1.3 grams. I loaded to 200# lbs to seat the splices. I will test the assembly later and post results.

Soft shackle:
20231026_091653.jpg

Soft shackle, Loop OPEN:
20231026_091753.jpg

Soft shackle, with 1/8 kevlar, and 507# swivel, (still open):
20231026_091838.jpg

Soft shackle, loop over ball, and loop closed:
20231026_092059.jpg

Let me know if this helped, or you have any questions.
 
Tested the assembly as shown in post #43 to failure. The hole thru the fixed end of the swivel failed, by tearing thru at 526 pounds of force.
20231026_161930.jpg

I'll find a heavier duty link and test again to see what the next weekest link is.... to be continued...

[EDIT: Just for the keen eyed, I had taken the assembly apart between the photo in post 43, and testing. When I reassembled it I "flipped" the swivel.]
 
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I am curious about the stitches on bury splices. As I understand the mechanism of the splice, and the reason for the stitches, it seems like putting a single stitch loop through it, right up where you start to taper the end, would serve the purpose and would be much tidier than my poor attempts at sewing. Maybe a second stitch near it as a backup if the first came loose. Is there a reason all the online sources want a line of stitches?
 
JoePfeiffer said:
I am curious about the stitches on bury splices. As I understand the mechanism of the splice, and the reason for the stitches, it seems like putting a single stitch loop through it, right up where you start to taper the end, would serve the purpose and would be much tidier than my poor attempts at sewing. Maybe a second stitch near it as a backup if the first came loose. Is there a reason all the online sources want a line of stitches?
Click to expand...
I first started using splices for 38mm and 54mm MD rockets, where the size and weight of quicklinks was an issue. I didn't know about soft shackles at that point, so I would form the splice loop in place, around whatever it was I was connecting the cord to. I didn't use any kind of locking stitches, since I would need to undo the splice if I wanted to separate the pieces. Following the 47X rule, I never had a splice fail due to the tail pulling out. The only failure I can recall having was right at the end of an un-tapered buried end, which is how I learned about the importance of tapering the buried end. I've used the same method on all sizes of rockets, all the way up to my Level 3 project.

The way I look at it is these methods are being used by climbers whose lives literally depend on the safety and strength of their connections. I also went back and read a lot of the original studies by the military when Kevlar and similar materials were new and being considered as replacements for both traditional rope and steel cables. From my reading, the point of the lock stitches being in a line with the cord is to allow them to move with the cord and prevent a stress riser across the cord. Kevlar actually has a lot of properties that make it less than ideal for our uses, but it is so easy to create load safety margins that it is well worth the effort to do it correctly. If industry, military, sailors, and climbers have decided these are best practices, that seems like a good reason to follow their guidelines.

The other thing I learned in all that research is that knots are the enemy of Kevlar and similar materials, to be avoided at all costs.


Tony
 
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David Schwantz said:
To OP, show us the 2 rows of stitching and how to do them, please.
Click to expand...
The OP hasn't been on TRF since 2019 ... I will make a make a set of photo instructions tomorrow.

(I did Arborist work in the mid/late 90's. Hanging with chainsaws in trees you learned to trust your rope work. This is bringing back memories. I wouldn't trust my work right now to that level, but for rocket recovery it's plenty good enough.)
 
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So I tested 3 cords to failure today. One with 12”, 9” and 6” of engagement.


***All failed between 2000 and 2100 lbs.

- 9” and 6” failed at the tapered buried end.
-12” failed 2-3” above the buried end. At the normal thickness portion of the cord.

(These cords were all 30” long)

So as I suspected - you do not need 50X engagement. The taper is important and perhaps I need a longer one. Giant Leap claims their 1/4” Kevlar is 3000 lb test but I have never confirmed this. I think it’s probably 2000 lb.

That means the Kevlar could survive a 200-400G deployment based on my rocket masses. A normal deployment sees <10G.

The tapered buried end:
IMG_1517.jpeg

The carnage:
IMG_1527.jpeg
 
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Kip_Daugirdas said:
So I tested 3 cords to failure today. One with 12”, 9” and 6” of engagement. All failed between 2000 and 2100 lbs. All failed at the tapered buried end.

So as I suspected - you do not need 50X engagement. The taper is important and perhaps I need a longer one. Giant Leap claims their 1/4” Kevlar is 3000 lb test but I have never confirmed this.

That means the Kevlar could survive a 200-400G deployment based on my rocket masses. A normal deployment sees <10G.

The tapered buried end:
View attachment 612196

The carnage:
View attachment 612197
Click to expand...
Very cool to see some hard info. However, if that first photo is showing the taper you used, that's way too short according to the manufacturer's directions . The taper should be created as shown in the videos – it will be much more gradual, which is why the longer specified engagement is important – about 30% or so of the tail will be the tapered section. I suspect if that if the taper shown is what you are using, and you redo the test using a more traditional taper, you'll get a noticeably higher breaking strength. That's been my personal experience as well as what I've seen in online testing.

The taper should be gradual enough that you can't see or feel a distinct end to the buried section.

And, since you are on a roll, it would be great to see a test with the loop formed using a simple overhand knot, or another typical knot, to check its breaking strength.

Great stuff, thanks for sharing your results.


Tony
 
manixFan said:
30% or so of the tail will be the tapered section.
Click to expand...
I updated my post. Looking closer at the 12” engagement cord, it failed 2-3” above the buried end. I think the taper is important but maybe a bit over exaggerated in how shallow it needs to be. More testing would be needed to 100% confirm this.

My thumb is in the location of the buried end:
IMG_1528.jpeg
 
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The 50x rule is for a round rope. Not a flat. I've not seen a rule for flat braid engagement in a splice....That's comparing apples and oranges. You cannot explicitly apply the rule for one type of rope to another. So you cannot apply a rule for round to flat, or Dyneema to Kevlar. Dyneema, Ultra High Molecular Weight Polyethylene, is a very low friction substance. It's slippy..... So applying a rule for that to Kevlar will work, but be too long. Whereas applying a rule for kevlar to a UHMWP rope won't work as it will be too short and slip. Round rope does not inherently have a hole in the middle unless you push it back. Flat braid, does inherently have a hole in its natural form as it's already a tight weave.
Marlow ropes have a lot of useful information, but be careful applying their rules across different rope types. Add on to that complexity------rope systems...... Where the inner of the rope is made from one material and the outer is another material. Commonly found in sailing and climbing ropes, where the outer is a braid of a material that protects the inner high-strength material. This gives an outer casing that is intended to slip slightly over the inner.

So test what you're going to do....if what you are doing is not exactly what was specified in the demo video.....
This is why if you start looking at climbing videos and slackline videos and the knot testing and rope system testing they do, it is extensive.......
 
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Another important point and conclusion:

All of the retention stitches that @Tractionengines recommended in an earlier post popped on the 6” engagement cord. I used Size 69 Kevlar thread. I am not sure when this started to occur but we started to hear popping sounds around 1800 lbs on every test. Not sure if these were what we heard or not.

***The 12” and 9” lost some stitches but were mostly intact and I could not pull the buried end out of them.

But if these retention stitches fail. And you have a rough recovery with a lot of cyclical loading/buffeting (loading and unloading), the buried end can start to walk itself out. Maybe 100 cycles and the 6” buried end popped out. I realize that the likelihood is low for most flights but something to keep in mind.

Conclusion: With this limited sample, I would recommend maximizing the buried length particularly for drogue harnesses.

Thoughts:
I would not go below 6” of engagement unless you plan to fully sew the finger-trap with a zig-zag. Clearly the retention stitches start to get loaded particularly in the shorter engagements.

It might make sense to sew the retention stitches with the line taught. This should help with them popping during tensioning/stretch. It may also be helpful to do a zig-zag stitch that follows the weave (particularly for the 1/4” GLR Kevlar).

All in all I feel pretty comfortable with this method and plan to fly MESOS with these harnesses.

IMG_1530.jpeg
 
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Kip_Daugirdas said:
Another important point:

All of the retention stitches that @Tractionengines recommended in an earlier post popped. I used Size 69 Kevlar thread. I am not sure when this started to occur but we started to hear popping sounds around 1800 lbs. Not sure if these were what we heard or not.

But if these fail. And you have a rough recovery with a lot of cyclical loading/buffeting (loading and unloading), the buried end can start to walk itself out. Maybe 100 cycles and the 6” buried end popped out. I realize that the likelihood is low for most flights but something to keep in mind.

View attachment 612237
Click to expand...
@OzHybrid makes excellent points about the different rope types and applying rules from one to the other.

From my understanding, the retention stitches are there to just prevent the buried end from slipping out when it's not under load, and should fail when stressed to prevent them becoming stress risers as the load increases. And also to @OzHybrid points, I've only ever used round braided cord with splices, not flat. But your testing has already shown that a long buried end is important to preventing walk-out under repeated cycling of load/un-loaded stresses. And again, in the many years and flights I have been using this method, I've never had a properly prepared splice fail under recovery.

The way I always look at thing is will I regret doing it one way vs. the other? The consequences of a long taper and buried end are some extra time and material, but I should achieve full strength of the splice; the consequences of a shorter taper and buried and are less time and material, but at the possible cost of lower strength and reliability. To me, that's an easy decision.

In the end, we all need to choose a method we feel comfortable with and have good reason to believe it is up to the task at hand. Seeing the testing and numbers are very useful to determine if this method is acceptable for the particular rocket and expected stresses. I would love to see the testing setup – already this has been an incredibly useful discussion and seeing the results of the tests are fantastic.

Thanks again to @Kip_Daugirdas for taking the time to work up these tests and post the results.


Tony
 
I should acknowledge again, that as @OzHybrid points out, not all cord is the same, specifically in this case flat braided Kevlar vs round. For flat material, sewing loops is an acceptable way to form a loop. Here’s a good example video that shows how the strength for a box x stitch in nylon webbing. A series of bar tacks can also accomplish the same result.

It would be good to have Teddy from @Onebadhawk chime in about sewing flat braided Kevlar. His expertise would be valuable in discussing the differences between flat and round Kevlar.


Tony

While not Kevlar, it’s a useful video in that it shows how the strength of a sewn attachment can be calculated:
 
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Kip_Daugirdas said:
Another important point and conclusion:

All of the retention stitches that @Tractionengines recommended in an earlier post popped on the 6” engagement cord. I used Size 69 Kevlar thread. I am not sure when this started to occur but we started to hear popping sounds around 1800 lbs on every test. Not sure if these were what we heard or not.

***The 12” and 9” lost some stitches but were mostly intact and I could not pull the buried end out of them.

But if these retention stitches fail. And you have a rough recovery with a lot of cyclical loading/buffeting (loading and unloading), the buried end can start to walk itself out. Maybe 100 cycles and the 6” buried end popped out. I realize that the likelihood is low for most flights but something to keep in mind.

Conclusion: With this limited sample, I would recommend maximizing the buried length particularly for drogue harnesses.

Thoughts:
I would not go below 6” of engagement unless you plan to fully sew the finger-trap with a zig-zag. Clearly the retention stitches start to get loaded particularly in the shorter engagements.

It might make sense to sew the retention stitches with the line taught. This should help with them popping during tensioning/stretch. It may also be helpful to do a zig-zag stitch that follows the weave (particularly for the 1/4” GLR Kevlar).

All in all I feel pretty comfortable with this method and plan to fly MESOS with these harnesses.

View attachment 612237
Click to expand...
Kip, what’s your setup for loading and measuring breaking strength? I want to make some of my own soft shackles but would like to proof load them as well. I was thinking of using a come along between trees but don’t know how I want to instrument this yet.
Thanks!
 
Great thread. It is very informative. It might explain quite a few failures I have seen. Thanks for sharing.
 

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