I looked at that power point you referenced. The failure modes are different in the ppt vs the paper. In the paper, the mode of failure was primarily in the transition from the bury to the "clear" line due to uneven stressing on the fibers at that location...the eye held though. That indicates that the loops on either end were strong enough to shift the weak point somewhere else in the rope, IE causing a failure in the rope itself as a function of the best-case splicing or end loop construction, not due to the rope being wrapped around something too narrow. the ratio for D was at least 3:1, indicating a useful method for retaining strength of the sling.
The ppt test where D = 1:1 or 2:1 yielded a different failure mode where the rope failed at the point it was wrapped around something, indicating that the weakest point was at the bend around a pin, not at the splice point.
I don't think the two sources contradict each other. The failure modes were different in what seems to be a clear result of a function of size D.
What we can say with some confidence is that the tighter the bend, the more the rope is prone to failure at the point of attachment, vs overloading the min (useful) breaking strength of the rope itself.
on another note I went to ace hardware over lunch to see a 3/8" quick link, since 3/8" is a little more than 3:1 for 7/64ths, and the size and weight made me realize why you're fighting 3:1 or 4:1 so much lol!
Anyone got any ideas on a carabiner or quick link style of attachment that's 3/8" in diameter that's made of aluminum or something lighter than steel? There's no way I'm using 2 3/8" quick links!
The Camp Nano seems to be a favorite. The width on the large end of mine avg. 0.32" and the small ends avg. 0.27".
Originally Posted by mrjaw14
For a small weight penalty, my BD Neutrino's measure ~0.366" at both ends.
I haven't figured out where they're getting all the data for slide 10. I read through the testing methods and only found mention of eye dimensions and not pin size, could have missed something though. ASTM says the eye length should be twice the diameter of the pin and ISO says 6x the diameter of the rope.
Please don't say "made you look".....
Cordage Institute CI 1500-02
"5.2.7 - For break testing of ropes with spliced eyes, the diameters of the test machine pins shall be at least twice the nominal diameter of the ropes to be tested."
ASTM D4268 - 93
"23.1.3 - The holding and pulling ends of the testing machine shall have pins or posts whose diameters are no less than one and one half times the diameter of the rope being tested when using eye splice terminations"
I wasn't able to make much sense out of the ISO standard.
Thanks! I guess I was skimming too quickly.
Originally Posted by gmcttr
Here's a different samson publication with some interesting info in it
and another - look at 35 and 37. all our questions are answered there:
Last edited by mrjaw14; 08-28-2013 at 14:25.
The following is an email I sent to Samson Rope and the reply.
For a single leg, eye each end rope (dogbone), the limit Samson recommends for attaining full strength is a minimum pin diameter at least twice the diameter of the rope and and eye length at least 3 times the pin diameter.
Sent: Tuesday, August 27, 2013 1:05 PM
Subject: Amsteel Blue Strength Testing Methods
I am a member of Hammock Forums.Net.
We use 7/64” and 1/8” Amsteel Blue in our hammock suspensions in the form of whoopie slings, continuous loops, etc.
Hardware in the form of hooks, carabiners etc. are frequently used to connect to the Amsteel Blue.
We are currently having a discussion about the minimum diameter a connector can have that will still maintain the minimum breaking strength of the Amsteel Blue and are unable to find firm answers online.
The technical paper on your website, “The Effect of Bending on the Tensile Strength of Statically Loaded Synthetic Ropes” leads me to believe that minimum strength is tested/determined using a section of Amsteel Blue with an eye splice on both ends and connected to the testing machine by “pins” having a D/d (pin diameter/rope diameter) of 2.
If this is correct, I assume a smaller pin diameter would result in a lower minimum breaking strength.
The most important information we would like to have is what is the minimum diameter a connecting device can be and still maintain the rated minimum breaking strength of the Amsteel Blue.
Any information you might provide on this subject would be greatly appreciated. Thank you for your consideration.
You are correct that we have completed a significant amount of testing on many pin diameters with various sizes of AmSteel Blue ropes. Our standard break testing procedure is to fabricate eye-each-end slings with eye lengths of at least 3 times the diameter of the pins used. As you stated, these pins are always at least twice the diameter of the rope. This will apply the same for a whoopee sling, with the standard reduction in strength due to this alternate splice method.
However, things get a bit more complicated if you are talking about the strength of endless loop, or grommet, slings. All tension members experience a decrease in strength when loaded over a bend due to the uneven loading of the top and bottom sections in the bend. As the diameter of the bend decreases, the load on the outer portion of a rope increases.
Testing has shown this reduction in strength no longer has a significant effect on AmSteel Blue at D/d ratios greater than 20, where “D” is the diameter of the bend and “d” the diameter of the rope. This means that at very large D/d ratios, a grommet sling has a breaking strength twice that as a single part of line as there are two legs of the sling contributing strength. At lower D/d ratios however, the contribution of both legs is limited by the strength loss due to the bend. This does not impact eye-each-end slings on pins with D/d ratios as low as 2 however since there are two legs supporting the eye and a single leg holding the entire load in the center of the rope.
In short, the limit we recommend for attaining full strength of a single leg sling are pin diameters at least twice the diameter of the rope. For grommet slings, the strength will be greater than published minimum strength of single leg slings as long as you maintain this same ratio, but will not approach the full two leg strength until your D/d ratio is above 20.
Hopefully this answers at least part of your question. Please let me know if I can help with any further clarification.
While some companies tend to ignore the little guy, Samson's personnel have responded quickly. I'm impressed.
Additional correspondence with Samson....
"Sent: Wednesday, August 28, 2013 1:44 PM
To: Dustin ____
Subject: RE: Amsteel Blue Strength Testing Methods
This information is very helpful and answers my original question. The help is greatly appreciated.
I have come up with two additional questions though.
1 - Stepping outside of recommended usage, what percentage of published minimum breaking strength is attained when the D/d ratio is only 1 for the two smallest diameters of Amsteel Blue?
2 - What percentage of published minimum breaking strength is attained when two ropes of the same diameter are joined by end eyes spliced through each other (not cow hitched)? Does the rope to rope connection simply act as an eye on a pin with a D/d of 1?
Thanks for all your help,
It has been observed in testing of AmSteel Blue with spliced eyes on small diameter pins, providing a D/d ratio of 1, that the breaking strength is not significantly affected. Samples tested broke above MBS, sometimes breaking in the base of splice while often cutting through the back of the eye.
Extensive testing of matching diameter AmSteel Blue lines spliced eye-to-eye has not been conducted. However, based on observations from similar testing (varied rope diameter, small diameter pins, etc…) we expect not to see a significant loss in strength in this configuration.
My second question was asked with soft shackles and similar connections in mind.
gmcttr, thanks for your work with the correspondence!!! Based on the info you received I'm not understanding why a cow hitch derates the rope so much. How is it any different than a pin with a 1:1? My original thinking was the tight bend is what weakens it.
Thanks for posting this, this is a bit of a game changer with regards to the info previously available.
With regard to the cow hitch, it's probably the way the rope fibers are stressed that unevenly distribute the load. It's probable that the two flexable ropes flatten each other out and both sides loose strength being bent so severely. it's possible that it's actually less than 1:1 at that point. He did say that as D gets larger, more strength is retained.