Sewing Eyelets on Nylon and Polyester (PET) 6mm Cords for Break Tests
Pics by Harlan W S
Aug 11, 2023
First, heed this warning: If you try to sew rope you will eventually DIE! Leave rope-sewing to elite outdoor-gear purveyors, who have good lawyers!
This note is really the background for an on-going study on the effect
of knots on force dissipation in ropes. In those tests, the cord is
pulled to breaking, which invariably occurs in one of the knots. It is
obvious that to find the breaking strength and force dissipation
behavior of the in-line knots, the end attachments of the cord (from
which it is pulled) must be stronger than any inline knots.
So in the tests REPORTED BELOW, we verify that we have a way to pull
end-to-end on unknotted rope, and recover the reported rope strength
So
how do you make attachments at the ends of a rope section, so that the
rope stength is preserved? Most knots will weaken the rope by 35-50%.
There are a
few knots (such as the modified trilene) that are almost full rope
strength, for nylon and polyester (but not Dyneema); they take up a lot
of rope and must be cut free after the test. There are capstain clamps
and big bollards that involve
wrapping the rope (many times) around a cylinder that is at least 20x
the diameter
of the rope, and they get near rope full strength -- but use up a lot
of rope.
Or, if you really really really know what you are doing, you can sew eyelets at the ends. (WARNING WARNING ROPE-SEWING IS THE REALM OF THE DEVIL! I take no responsibility if you choose to sew rope!)
I've covered how I sew rope elsewhere. My "equipment" is shown in the last frame of this page. Aside from common sense about sewing, you want:
(number of stitches through both strands)*(strength of thread) >
2*(reported strength of rope). For these tests, each termination had at
least 42 stitches.
For this hand-sewing, I use 100 lb test Spectra (UHMWPE) braided
fishing line. One interesting property of braided Spectra: it is easy
to form a cut end into a flat blade with the flats of pliers, and then
very easy to push the flattened end through the eye of
"reasonably-sized" needle. Normally I would suggest polyester or nylon
thread to sew polyester of nylon; but these tests below seem to show
the Spectra is fine, and it is damnably hard to find poly or nylon
thread that is more than 25 lbs breaking strength. The worry is that
the Spectra will cut right through the weaker nylon or poly yarns; and
that probably happens, but only near the breaking strength of the rope,
so who cares?
10-06-2023 NOTE: "Capstan Clamp" test
I made a capstan clamp from two 1.5" (38mm) OD
U-shaped iron pipes, and wrapped bluewater cord around each pipe 4x
(and tied off with half-hitches), then pulled to breaking at 1922 lbs.
In principle, this system should approach the native strength of the
6mm nylon rope. This test gives me confidence the sewing works well, as
the cord with sewn loops actually broke at 1.011x the reported MBS, and
the capstan clamped-rope broke at 0.983x
the MBS. I have no good way to judge the likely uncertainty, so 1.011,
1.000, and 0.983 may be effectively the same number.
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