Loopie pull tests--importance of orientation

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moray

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In the arborist catalogs the loopie in use is always shown wrapped around a stem, and the working bight, attached to the hardware, is shown passing through a bight in the sleeve part of the loopie. All these bights and curves add friction to the overall system and help to keep the loopie secure against slippage. The loopie is much less secure in an open loop configuration where the loop is subjected to a straight pull, and least secure when there is no bight formed in the sleeve part of the loopie. Here I report several pull tests on 5/16 inch Tenex Tec loopies. This cord has a rated tensile strength of about 4700 lbs.

The adujsting sleeve of the loopie is much like a splice, and like a splice, the two ends of the sleeve are quite different. One end is like the throat of the splice, and the other end is like the tail. In the attached photo of a loopie in 1/2-inch Tenex Tec, these analogous parts are labelled. The dotted line shows the "eye" that isn't there, and the arrow therefore is pointing at the throat. The point marked "T" is where the tapered end of the bury would be, and is, therefore, the tail. The throat is always easy to recognize because that is where the dead little stub of the cover is to be found--the appendix.

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For the first set of tests, three identical loopies were made, each from 27 inches of rope. The sleeve length before the bury operation was one fid, or 6.75 inches, and about 1.5 inches of appendix was left at the throat. The resulting loopies were long enough to give clean tests in all 3 configurations tested with no wasted rope. In every test, care was taken to milk all slack out of the spliced area just before pulling. In each picture "T" marks the throat end of the sleeve, the spot where the cover ends in a stubby appendix.

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The picture shows the first configuration: the sleeve is straight and both bearings are in unspliced rope. There was no need to hook up my heavy equipment for this test. With a steel screw link at each bearing point, it was easy to pull the loopie apart by hand. I guess it took no more than 5 or 6 lbs.

For the next two tests I resorted to my hydraulic pulling rig. For both tests the bearing points applying load to the loopie were 3/4 inch steel shackle pins. The rope-on-steel friction is part of the whole system, so it is important that it be the same for both tests.
 
For the second test I chose the configuration shown in the picture. Here one of the bearing points is in the sleeve area close to the tail of the "splice". At 1524 lbs the core in the sleeve suddenly slipped about two inches, at which point I stopped the test.

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In this 3rd test the bearing point on the sleeve is near the throat, as shown in the picture. When the tension reached 5940 lbs, there was a loud bang and everything came apart. At first I thought the rope had broken, but no, the core had pulled all the way out of the sleeve.

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Two more tests were performed to test slightly longer buries. In both cases the loopies were long enough that one bearing point was on the sleeve, very near the throat, and the other was in the open on the single-thickness rope. The sleeve lengths, before the cores were inserted, were 8.75 inches and 10 inches. The results:

8.75-- 5826# Broke at shackle pin opposite the throat
10.0-- 6120# Broke at throat

As anyone would have predicted, increasing the sleeve length increases loopie security against slipping. More precisely, the longer the sleeve length between the bearing and the tail, the greater the security. Adding sleeve length between the bearing and the throat should do almost nothing because that section of sleeve should see almost no tension. No tension, no squeeze. No squeeze, no friction between core and cover. No friction, no holding power.

With slightly shorter buries all the loopies slipped. At about 1.3 fids we have apparently crossed the threshold separating the slipping region from the breaking region.
 
Another test was performed to double-check the earlier experiment where the bearing point was located near the tail. Here the sleeve length was increased to 10 inches to improve the loopie's chances of holding. Everything else about the experiment was the same.

Result: the core slipped out at 564 lbs. Since this seemed ridiculously low, and since the rope was undamaged, I repeated the experiment, making quite sure the bearing point was a good inch or so away from the tail. As the tension increased, the core slipped and caught several times before sliding out at 2570 lbs.
 
In this final experiment 4.5-inch-diameter steel bollards were substituted for the shackle pins. Just to be sure we would get breakage rather than slippage, and to make it very easy to position the loopie so that the bearing was near the throat, the sleeve length, before bury, was increased to 12 inches.

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(In the picture, the grey rope is the Tenex, and the bottom leg is broken. The pinkish stable braid is part of the recoil-snubbing system, but did not go into action because a strand of the Tenex remained unbroken.)

Result: the loopie broke at the tail at 7680 lbs. This is an expected weak spot because the cover is distorted at this spot yet it is under maximum tension.

This chart summarizes all the tests:

Test- pin diam (in.) - bury length - bearing location - Max Load- Description

1-- .75-- 6.75--- none ------8-- slipped
2-- .75-- 6.75---- tail ---1524-- slipped
3-- .75-- 6.75--- throat -5940-- slipped
4-- .75-- 8.75--- throat -5826-- broke at pin opposite throat
5-- .75-- 10.00-- throat -6120-- broke at throat
6-- .75-- 10.00-- tail -----564-- slipped
7-- .75-- 10.00-- tail ----2570-- slipped
8-- 4.5-- 12.00-- throat -7680-- broke at tail
 
In summary, the loopie is extremely insecure in a straight pull without a bight in the sleeve. It is still insecure if the bight (load bearing point) is near the tail of the sleeve.

The take-home lessons from all of this seem to be:

1. The loopie is strongest and most secure against slippage when one of the bearing points is on the sleeve.
2. The sleeve bearing should be near the throat, not near the tail.
3. Small-diameter bearings, like the bail of a PortaWrap, are small enough to seriously weaken the loopie.
4. The working loads given by Wesspur and others need to be viewed with some skepticism. The true strength of a given loopie and the correct working load will depend heavily on the size of the bearing pin(s), and this is not a parameter specified in the catalogs.
5. Unlike tree-wrap configurations, the straight-pull configuration is inherently insecure. It requires a very specific setup configuration or the loopie can slip apart far below its rated strength.
 
Moray thanks once again for testing this sort of thing. Love your work.

It is late and I am tired but I am having trouble visualising the most secure method of using this sling. Could you post a photo of how you would use this and the postition in which it produced the best results? I got the worst result thing but your right, even for me half asleep that one was a no-brainer.
 

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