Splice Squeeze Force

[moray]

If we have a uniform taper 8 inches long, only the last inch of the taper would drop below 3 strands thickness. We'll ignore that for the calculation of splice holding ability, and just to be conservative, we'll ignore the second-to-last inch as well. We now have 6 inches of uniform taper, the small end of which is 6 strands thick. We know from the previous experiments that this is plenty thick for the cover to grab it securely when the cover is under tension.

Is it necessary to make some sort of special calculation for the tapered section of the core, or does it behave just like the untapered part? Surprisingly, perhaps, it behaves exactly like the full-diameter core. The reason is the standard treatment of friction is only concerned with force and coefficient of friction, not area. For a given compression force, the size of the core, within broad limits, does not matter. Thus an inch of tapered section will behave just like an inch of untapered core. The analogous problem of the rope around a post is similar: neither the size of the rope nor the size of the post are relevant.

This is good news for the math weary: our problem is now as simple as it's going to get--figure out the numbers for a simple splice with a 14-inch bury of full-diameter rope.

For anyone who makes their own splices this is a valuable piece of info. The taper in the buried core does not weaken the splice, and it is not there just to give a smooth transition from fat splice down to normal rope. All of it, except for the last inch, is a fully functioning load bearing part of the splice.

 

[moray]

A splice that is about to slip apart is analogous to a rope around a post that is about to slip. In the case of the post, the large force towards which the rope is about to move is resisted by a much smaller force at the other end of the rope. The tension grows at a constant rate, some percentage per degree of wrap, from the low- to the high-tension ends.

In the case of the splice, the buried core is analogous to the post, and the cover surrounding the core is analogous to the rope. The tension in the cover varies from a minimum at the throat of the splice to a maximum at the end of the bury. Between these two points it grows at a constant percentage rate per inch. (There is nothing special about using English units for this--any units will do.)

From the numbers derived earlier for squeeze force per inch and coefficient of friction of polyester on itself we can derive the number we want: for each inch of cover between the throat and the bury end the tension in the cover grows by 30%. Over the full length of our 14-inch bury, the tension will increase by a factor of 39. To say it another way, the cover tension, when the splice is about to pull apart, is 39 times as great at the bury end as at the throat.

At the rated strength of the rope, 6000 lbs., the cover tension at the throat will be 154 lbs. For a more ordinary load of 200 lbs., the required throat tension would be only 5 lbs.

To make sure the rope would break before the splice would fail, we need to somehow supply 154 lbs. of cover tension at throat.

Where do we get the 154 lbs. of tension? Normally, of course, the eye is symmetrically loaded, and half the total load is on the cover leg of the eye. This would be 3000 lbs., vastly more than the 154 we need. The only plausible scenario in which the eye itself can't supply the necessary tension is one in which the core leg of the eye has somehow been snagged or impaled on a spike of some kind. In that case, the stitching at the throat supplies the force. If the stitching is strong enough to resist a 154-lb. pull then the splice will hold.

Samson, the manufacturer of Tenex, suggests using a yarn the size of one in the rope for the stitching, and they call for installing at least 12 stitches. One yarn from 3/8" Tenex has a tensile strength over 500 lbs since the rope is comprised of 12 yarns and has a tensile strength of 6000 lbs. Twelve stitches of 500-lb.-test yarn is going to supply vastly more than 154 lbs. of strength at the throat! Personally, I use a yarn of about 70 lbs. test for stitching, but I do take 12 stitches or more. This should give me a very comfortable margin beyond the 154 lbs. needed. Remember, we are talking about the 6000-lb. load needed to break the rope. AND we are talking about the somebody's-bad-dream case of the splice core impaled on a spike. You are more likely to get struck by lightening and bit by a shark in the same day than to have that happen to your rope. Nevertheless, decent stitching has you covered.

 

[moray]

Conclusion

Like many others, I was quite skeptical about the reliability of splices when I first started using them. None of my intuitions about mechanical things seemed to apply, so to hang from a splice far off the ground was pretty much a leap of faith.

No longer. A lot of hands-on experience and numerous (admittedly primitive) experiments have shown me how over-engineered the standard hollow-braid splice really is. If built according to manufacturer's specs, which is easy to do, the splice has a large margin of safety even under the most extreme circumstances. (This statement needs an asterisk. I don't know if a shock-loaded splice would perform the same as one under a steady pull. Maybe I'll try to test this...)

If the information I developed in the course of this project could have been found on a Web page somewhere, I undoubtedly would have done something else instead. I don't mind reinventing the wheel now and then, but this was a lot of work! Anyway, I found what I was looking for and a lot of things I wasn't looking for and had a lot of fun in the bargain. If some splice-averse person out there reads all this and decides to give the hollow-braid splice a try, that will be a bonus.

 

[Adkpk]

I need an eye to eye prusik loop for a foot locking loop I spliced. My lanyard splicing is still hard as a rock. What cord do you think I should use? I am using tenex for my short lanyard and beeline for my longer one. I would like to try one of those splices you used on that amsteel blue.

 

[moray]

Adrpk, I have been using 5/16 Tenex for that purpose--it is fairly small, easy to splice, and seems to work just fine. I actualy use it in the form of a Whoopie sling so I can adjust the total length to suit. There is no prusik in my setup--I use an ascender instead. I tried it with a prusik for awhile but found it too hard to advance the knot.

 

[Adkpk]

I will also need a whoopi sling at some point. I will combine the length of the two when I order it. Thanks and I be back with questions probably when I get the rope.