temporary fixing of dyneema

[markxengineerin]

I am building a new standing rig from 11mm dux dyneema. This involves putting 3-4000 pounds of force on the lines after splicing. To anchor the un-used end while pulling on it with a come-along, I have wrapped it many times around a tree.


It works ok, but will not work when I get towards the end of the job and don't have enough line left to make all the wraps. What's the best way to grab it?

I don't really want to tie any knot, because it will mess up the set of the line, and influence my measurements and calculations for lengths. I don't want to do temporary splices, because I am slow and would end up un-doing them all, plus would also likely influence measurements.

The only idea I have is to use cable clamps on the dyneemaas one would clamp a cable. I have enough extra line that the clamped part could be sacrificial/thrown away. But would 3 or 4 clamps hold 4000 pounds?


I could also trust my calculations enough to splice the terminators in at each end, and use those as the anchor points, but I'm not yet confident enough in the length change when pre-loaded. I much prefer to splice one end, pre-load, then re-measure how much line I need at the other end before putting in the 2nd splice.

Thanks for any ideas, and also, ignore the van- it seemed to be moving or close to it at 1000 pounds, definitely will need 2 trees to get to 4000.

[StuM]

Originally Posted by markxengineerin

I could also trust my calculations enough to splice the terminators in at each end, and use those as the anchor points, but I'm not yet confident enough in the length change when pre-loaded.

Can you pre and post measure the other lines to a get a stretch factor that you can trust?

[wsmurdoch]

I have tried grabbing 12mm UHMWPE with a cheap ascender. It will hold the rope to over 1500 lb measured with a Dillon dynameter, but it does deform the line which flows into the "works". I would not recommend it for your use although a better device may behave better. https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1

[deblen]

One of these may work.
Used by power,tel & cable cos.
https://www.google.com/search?source...EQjJkEegQIIhAC

[thinwater]

Originally Posted by markxengineerin

.... I don't really want to tie any knot, because it will mess up the set of the line, and influence my measurements and calculations for lengths. I don't want to do temporary splices, because I am slow and would end up un-doing them all, plus would also likely influence measurements.

Just pull the tail through, like the adjustable end of a whoopee sling. It won't harm the line and takes only seconds. Used in rigging all the time. Logging too.

https://www.animatedknots.com/whoopie-sling-knot

[Manateeman]

What about one of those metal braid Chinese finger trap things electricians use to pull cables. They look like the cover of a double braid with a loop on the end.
Maybe it will just pull on the cover but it won’t crush the line at one point.
We hope you own the tree.
Mark

[markxengineerin]

It is my tree, and I'm being very nice to it, even wrapped it in aluminum flashing then towels ; )

Thanks for all the ideas- the whoopee sling idea seems like the easiest to try first. Is it safe to say this could damage the line, and that section should be thrown out after? I wonder how much bury is going to be needed, and if I have enough. The reason I assume it could damage the line is just that this could be one of the reasons more complicated splices exist in the first place. Or maybe it has more to do with dynamic loading?

Re stretch factor: I am developing it as I go, but by doing both ends at once, I double my inaccuracy. I'm trying to hit lengths +/- about an inch in the worst case, since I'm using turnbuckles, want to have some adjustment left over for long term creep, and have to account for all the other small inaccuracies in the project, which are many (measurements, new chainplates, new terminations on mast, and the fact that the previous rig I measured might not have been tuned ideally)

[Breaking Waves]

There are several suppliers of dyneema rigging compression terminals (like https://www.oceanchandlery.com/blue-...-terminal.html), which are strong and use up minimal rope length. They do cost real money.

Otherwise, any of a number of splices, with a relatively short bury will do it. A 35x bury will typically hold full strength under a static steady state load (like what you are proposing with the come along). This is not recommended for 'real' splices because it can slip under dynamic loading - but again your application should not experience that. With 11mm dyneema, your 4000lbs is only like what 40% strength, so in theory you could use an even smaller length bury - I have not tested it but 30x would almost certainly hold and x25 might (but I have never tested that). On super short burys I would put some lock stitching in the bury.

[markxengineerin]

Thanks, the blue wave fittings look cool and would be handy to have around in an emergency, as they advertise, but for some reason they skipped 11mm, the size I have. I check if I have enough for a 35x bury and start there.

[Breaking Waves]

^^ I believe are a couple other suppliers/manufacturers of Dyneema compression fittings - one of the others might have 11mm. Google will find them. I have only used the blue wave so have no experience/recommendation among the rest.

some other random thoughts:

Since you are not looking for 100% strength, but really only 40% strength. A knot, like a water bowline, would actually do the job. You would have to cut it off afterward, but that would use less rope than even a 35x bury splice.

There are two shorter splice than the bury - (1) the multiple Brummel - a like 5x Brummel (with no bury) is about 80% strength is pretty easy to make when you have good access to both ends but that's not really the case in your application, and (2) a 'tuck splice' is 100% strength and can be quite short (use perhaps just a little more rope as a knot) and is easier to do in bigger rope (your 11mm is a useful size for it) but it takes a bit more work than the straight bury splice (you need to do a series of tucks with 6 strand pairs)

[AndyOz]

Originally Posted by Manateeman

What about one of those metal braid Chinese finger trap things electricians use to pull cables. They look like the cover of a double braid with a loop on the end.
Maybe it will just pull on the cover but it won’t crush the line at one point.
We hope you own the tree.
Mark

I like this idea, but maybe the braid won't take enough strain.

Would be worth trying to simply insert (with a fid) the tail up the middle of a shorter length of the 11mm dyneema (perhaps taper it also), a couple stitches near the top of the bury to make sure it won't slip. It shouldn't damage any of the long dyneema (except the tapered part, of course), easy to undo etc. Of course, you then have the problem of how to secure the other end of the short dyneema piece...

[Benz]

To figure out the rope shrinkage, you can form the eye without splicing--just fold the rope over and tape the bury down, put that eye on a zero mark, and put a tape mark at six or eight feet away. Then make the splice, pull it with your tree, put the eye on the zero mark, and see where the tape mark has ended up.
If you make all your splices the same, this will give you an accurate idea of shrinkage ("Deduction"). You can then account for that in your calculations, build both splices, and pull both ends at the same time.
If you prefer to continue doing each at a time, then you can tie an overhand loop in the leftover far end, and put a rod or fid through the body of the knot so that you can get it undone later. The larger diameter the pin, the easier it'll be to slip out of the knot and get it undone (within reason).

[thinwater]

Originally Posted by markxengineerin

It is my tree, and I'm being very nice to it, even wrapped it in aluminum flashing then towels ; )

Thanks for all the ideas- the whoopee sling idea seems like the easiest to try first. Is it safe to say this could damage the line, and that section should be thrown out after? I wonder how much bury is going to be needed, and if I have enough. The reason I assume it could damage the line is just that this could be one of the reasons more complicated splices exist in the first place. Or maybe it has more to do with dynamic loading?

Re stretch factor: I am developing it as I go, but by doing both ends at once, I double my inaccuracy. I'm trying to hit lengths +/- about an inch in the worst case, since I'm using turnbuckles, want to have some adjustment left over for long term creep, and have to account for all the other small inaccuracies in the project, which are many (measurements, new chainplates, new terminations on mast, and the fact that the previous rig I measured might not have been tuned ideally)

No, it will not damamge the braid unless you take it above the WLL (about 1/4 of the line strength in this case). Bury should be >50:1. It is about 25% weaker than a bury splice, but more than enough for this and very predictable.




You can also just do a quick buy splice and skip the lock stitching. A tight wrap of tape will be enough compression to get it to lock up and grab. I've don this many times for test set-ups.

[markxengineerin]

To figure out the rope shrinkage, you can form the eye without splicing--just fold the rope over and tape the bury down, put that eye on a zero mark, and put a tape mark at six or eight feet away. Then make the splice, pull it with your tree, put the eye on the zero mark, and see where the tape mark has ended up.

I'm doing something similar to this, but not getting consistent "shrink" numbers so far, have to keep working on it this weekend.

In other problems, I'm trying to calibrate a wire tension gauge I got really cheap on ebay, used by cell phone tower companies, while I do this work.

If it works, I could use it to check tension on the boat. I don't know why this gauge wouldn't work just as well for dyneema, but I'm getting a different calibration when I measure two different lengths of the same dyneema.

The load cell I'm using seems good- goes back to zero when I let off tension, as does this gauge. It's almost like the dyneema doesn't have consistent properties. I am going to try measuring the diameter in a few different positions under tension to see if it's round.

[Benz]

Originally Posted by markxengineerin

I'm doing something similar to this, but not getting consistent "shrink" numbers so far, have to keep working on it this weekend.

In other problems, I'm trying to calibrate a wire tension gauge I got really cheap on ebay, used by cell phone tower companies, while I do this work.

If it works, I could use it to check tension on the boat. I don't know why this gauge wouldn't work just as well for dyneema, but I'm getting a different calibration when I measure two different lengths of the same dyneema.

The load cell I'm using seems good- goes back to zero when I let off tension, as does this gauge. It's almost like the dyneema doesn't have consistent properties. I am going to try measuring the diameter in a few different positions under tension to see if it's round.

I was under the impression that tension gauges for wire don't work with dyneema or carbon shrouds. I have seen guys modify a Loos gauge for use with dyneema, but it took careful calibration, and was different than wire.

In order for your deductions to come out consistent, you have to pull each splice as hard as every other: don't know if you can do that with van, unless you have a second person to eyeball a load cell.