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Why Splicing?

Splices are enormously strong when they are under tension, they create friction that is spread out over a large section of material. When they are not under tension though, there is not nearly as much friction, and the parts of rope could possibly move. So many splices have some stitches that hold them together when they are slack. The stitches do not contribute the strength, and when new they are not supposed to be very tight (they shouldn't interfere very much with the fibers aligning in response to stress.) Some splices are secured by whippings (many wraps of small twine around the outside of the cord) and some are stable on their own and don't need sitches. 

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Why Are There Stitches?

Most ropelite products start with 12-strand braided rope that is either dyneema or polyester. 12-strand rope is the most efficient way to get strength form a fiber, giving better strength for the weight than kernmantle rope or webbing. Deep bury splices retain 90-100% of that strength, and the SnakeSplice retains about 86-90%. This is better than sewn connections, which generally retain 80%, and knots, which weaken cordage a lot. 

 

Splicing has another benefit though, in that the splice is smoother and more flexible than sewn terminations or knots. A spliced loop has no bumps or anything that will catch as it is pulled through hardware.  You can tie knots in the spliced part. Nothing gets in the way of wrapping the device up to stow on your harness. 

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Technora and Polyester For Friction Hitches

The way the friction hitch performs will be dependent on the material used to tie the hitch and on the characteristics of the host rope. People use friction hitches for a variety of tasks; ascending systems, as progress-capture when hauling, as rope grab for a moving pulley, as a belay to catch a fall, as a source of friction when lowering or rappelling. At RopeLite I make hitch cords out of both polyester technora. 

 

Polyester

 

Polyester tends to have a long useful life, because it doesn’t lose much strength from being flexed back and forth. It does get stiffer over time, so you tend to retire old hitch cords because they are too stiff. Polyester preforms well with knots.Polyester is available in a wide variety of cordage, including single and double braids for marine use as well as some accessory cord for climbing use. It melts at about 482 degrees Farenheight 250 degrees Centegrade.

 

Aramids (Technora, Kevlar, Nomex)

 

These are high-tenacity fibers that are very strong and very low stretch. The lack of stretch means aramid cords  lose a lot of strength when tied in knots and hitches. Aramids are very hard to cut, thus they are useful for the sheaths of cordage to protect from accidental cutting. Aramids are also very heat-resistant, never melting, they will start to burn at 932 degrees. The disadvantage of aramids are related to how they lose strength from being flexed back and forth. If you peruse some of the testing resources below you will see that sometimes rope made from technora has lost 50% of it’s strength in a 500-cycle test, where nylon and polyester have lost only 20%. It is also worth noting that these worn-out aramid samples do not appear worn-out. There is much discussion in the rope-rescue world about when to retire products made with aramid, and definitely no consensus. 

 

Cordage made from aramids comes in many varieties. Some cords have an aramid sheath over a core made of something else, some have an aramid core underneath a polyester or nylon sheath. There are single and double braid 100% technora cords. Splieceable material of an appropriate size is difficult to source though.

 

Advantages and Disadvantages 

 

Polyester cords work well for most friction hitch jobs. For ascending a rope, performing a load transfer, attaching a pulley, etc. these cords work well, they last a long time, and they are easy to inspect. You can estimate the strength loss from how many fibers are broken. Long hitches with these fibers (think VT hitches or dog-and-tail) have the potential to hold the rope with more strength than a knot in the rope. 

 

These cords will tend to melt in particular circumstances though. Lots of testing has been done with prussik hitches, made from 8mm nylon cord, catching a falling mass of 200kg. This test inevitably involves significant glazing of the cord. Likewise, if you try to rappel using only a friction hitch, arborist style, a polyester one will get very hot and would certainly melt eventually. These applications are where Aramids shine, putting up with this abuse very well. Most of the hitch-cords used by arborists are double-braid construction with aramid in the sheath, for this  reason. 

 

Devices made from aramids are wonderfully strong when they are new, but they lose strength, probably more quickly than you expect, and it’s really hard to know how strong they are after they have been in use for a while. 

 

Conclusion

 

Here at RopeLite, I make loops and eye-t0-eye cords out of polyester and technora single braid, and loops made from polyester. I provide information on the strength and performance they show when new. The polyester products probably have a longer useful life for some users. The Technora cords begin life very strong. The hitch cords made from mixed fibers make lot of sense for many applications, and I use some of them myself. I don’t sell them because there is nothing I can offer that is better or different than what is already on the market. 

 

References:

 

https://www.cmcpro.com/is-my-escape-line-still-safe-after/

 

https://user.xmission.com/~tmoyer/testing/High_Strength_Cord.pdf

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Why is the tail sticking out of the SnakeSplice?

The SnakeSplice is developed and used to keep the splice as short as possible. I could bury the tail and make it dissappear, but then the splice would be longer and either the usable part of the cord would be shorter, or the product would be longer overall. I use deep bury splices wherever I can and use the SnakeSplice specifially to save space. 

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