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  1. #1
    Senior Member DemostiX's Avatar
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    Minimizing weight by balancing loads on 2 legs

    Under influence of Sgt Rock's recent thread on pushing the limits:

    Is a loopie* sling the minimum-weight constrictor? It would seem so because the there is no cordage wasted on a bury for an eye.

    There are always two legs always carrying the load, so the cord strength can be halved*, and only enough tail is required to keep the loop from coming apart at full length, or, if desired, to form a half-hitch.

    What is wrong with this analysis?

    ***Another citation to come.

    *In a loopie, the bury is inserted from the other direction.

    **In a whoopie sling, most of the time the load is being carried on two legs. In some destructive tests the weak point has been in the length of cord between splice-and-bury for the eye and bury / sleeve. In other words that length we try to minimize where where just one cord carries the load.

    ***Another HF member has recently put forward this suggestion in another context. No, they would be two: Turnerminator , who appears here later, and gd___ both of whom led me here to wonder afresh about alternatives to current standard practice.
    Last edited by DemostiX; 03-29-2012 at 14:25. Reason: clarification of buries --credit to Turnerminator and gd___

  2. #2
    GrizzlyAdams's Avatar
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    and the two shall be as one

    I've seen your question or assertion about two legs splitting the load before and hoped a mechanical engineer could weigh in. Where's Knotty or Rapt when needed??? (or others whose identities I don't know...)

    The question / observation I have for said engineer is that you have two legs...up to the point where they join---top of the loop, over the webbing or whatever. What is the strain on that join point? The cord system is only as strong as the weakest point.
    Grizz
    (alias ProfessorHammock on youtube)

  3. #3
    Senior Member DemostiX's Avatar
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    A world turned upside down

    Yes, lets have a real engineer here, or I may have to read papers on testing protocols from the Cordage Institute and discern what the angels do at the point of the pin--break faith, or split up and share the load -- and how the sharpness of the anvils used in breakage strength testing affects the measurements of breaking strength.

    But, the load splitting is arithmetic, Professor. They won't let my daughter stay in the undergrad engineering school at Champagne Banana if she does other than choose that answer on the program pre-test. (Q 34. Correct answer for undergraduates is "A". [50-50 load sharing] At issue is if "E" [none of the above] is the correct answer for the same course taught to the graduate students, who are sooner to be in the real world.)

    Worse, I'll join in Sgt Rock's fun and propose how everyone who felt safe with Dynaglide can split the load between two legs of 1.8mm 550lb breaking strength Zing-It loopies and be more secure than with a Dynaglide whoopie sling. (Will DemostiX do anything to restore Sampson's monopoly on HF member business?)

  4. #4
    GrizzlyAdams's Avatar
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    Quote Originally Posted by DemostiX View Post
    But, the load splitting is arithmetic, Professor.
    And the fact that at one point in this loop there is not two cords but one bearing the load is elementary observation. So let's have someone who actually knows what he or she is talking about join the conversation. If I was interested enough to find out the answer I would do the research.
    Grizz
    (alias ProfessorHammock on youtube)

  5. #5
    turnerminator's Avatar
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    A rough guide to the breaking strength of doubled braid can be gleaned from Opie's destructive testing of the Nacrabiner, I think.

    That came out at 175% of the lines rating with a diamond knot, and that broke at the knot if I remember correctly.

    FWIW, my loopies are showing much less wear than my whoopies and the loopies have done twice as many hangs. (well over 100 with the loopies, 50 or so on the whoopies)

    I'm interested in what comes out of this by someone who knows.

  6. #6
    GrizzlyAdams's Avatar
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    Dug out an old physics text and looked up "pulley", hoping there might be something there that sheds some light on the situation.

    There was. A diagram with a massless frictionless pulley (you gotta pay extra for those), and two masses m1 and m2 suspended from the ends of a string looped over the pulley.

    Equations given for acceleration of the masses and tension on the pulley. Plug in m = m1=m2, an approximation to the situation at hand, and the equation for tension
    T = g x 2 x m x m /(m + m)
    (g is the gravitational constant)
    gives T = g x m

    e.g., tension on the string is seemingly constant throughout and
    is equal to the split load. The top of the loop isn't a weak point.

    So it would seem that if the load can be transfered immediately to two strands of a whoopie/loopie/x-oopie without being carried first by a single strand, Demostix's observation would hold.

    I still ain't no mechanical engineer though. YMMV.
    Grizz
    (alias ProfessorHammock on youtube)

  7. #7
    WV's Avatar
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    Turnerminator, I'm interested in the loopie sling (but wish it had a different name - I may want to talk about these in public some day). How do you attach to the hammock, larks head around a gathered end? Also, how do you attach to the tree strap, looped over a marlin spike hitch with toggle? Also, if there's roughly half the tension on the constrictor, does that mean it's more prone to slippage? What line do you use, Dynaglide (or the British equivalent)?
    ----------
    Thanks, Grizz. That's why I asked about the loop over the MSH.
    Last edited by WV; 03-29-2012 at 11:31. Reason: slow typing

  8. #8
    AaronAlso's Avatar
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    I feel like this thread title is misleading. As I've read up on the loopie it has become clear that they require more line and hence would not be lighter than a whoopie. Maybe I'm misunderstanding "minimize weight". Although, a dynaglide loopie may be equal to a 7/64" whoopie in loading bearing capacity; how does it compare in mass?
    "The more laws that are written, the more criminals are produced." - "The more laws and order are made prominent, the more thieves and robbers there will be." - Lao Tze

    "Good people do not need laws to tell them to act responsibly, while bad people will find a way around the laws." -Plato

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  9. #9
    GrizzlyAdams's Avatar
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    Quote Originally Posted by WV View Post
    Thanks, Grizz. That's why I asked about the loop over the MSH.
    I think it is a different situation if the loop goes over something with a radius smaller than that of the cord (which does not, at least should not, occur when hanging). The force on that rod or whatever it would be is equal the the sum of the forces on the sides of the loop, and expresses itself as compression on the cord at the point of contact.

    That's why I went looking for a pulley problem, because that's a better expression of the loop over the webbing with a toggle.

    Now I have to go find my lost spherical chicken.
    Grizz
    (alias ProfessorHammock on youtube)

  10. #10
    Senior Member DemostiX's Avatar
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    Quote Originally Posted by turnerminator View Post
    A rough guide to the breaking strength of doubled braid can be gleaned from Opie's destructive testing of the Nacrabiner, I think.

    That came out at 175% of the lines rating with a diamond knot, and that broke at the knot if I remember correctly.

    FWIW, my loopies are showing much less wear than my whoopies and the loopies have done twice as many hangs. (well over 100 with the loopies, 50 or so on the whoopies)

    I'm interested in what comes out of this by someone who knows.
    Serious amateur work on testing breaking strength and weaknesses in the shackle are found in three segments at (HF member?) Allen Edwards' www.l-36.com.

    Briefly, the stopper knot remains the very weak point; and Edward's data are very much worth looking at for understanding the disappointment. The shackle, after all starts with 4 legs carrying the load, but might be no more than 140% the strength of one cord leg. Edwards' understanding is informed by conversations with Brion Toss, rigger and author on cord and knots, who took a couple of Edwards' inventions back with him to NERopes for informal destructive testing. But, we digress........

    On topic here is getting rid of unnecessary parts. The functioning and utility of the constrictor is fair game. In the spirit of Sgt Rock's trim of every unneeded parts into the now-disreputable pink (or gray or orange) slime bucket, I've asked whether the eye-splice of the whoopie can't be done away (in some configurations.)

    Smaller beans here, but many of 1kg scales will count (poorly) grains too, so here goes on UCRs, too:

    Grizz,aka Professor Hammock, notes that the constrictor on the UCR works so progressively the last part of it doesn't work much at all.

    So, why have so much of it, if the provision of that much sleeve is from an inappropriate functional model? Is the half-hitch used to secure the splice sufficient to take the whole adjustable splice to full theoretical breaking strength, say 90% of the cord strength? Or, is the half-hitch just enough to hold the hanger without the UCR slipping? If the UCR is not providing the same strength as a bona fide splice, then the same degree of safety could --but should not be, IMO -- be afforded by an alternative constrictor made of lighter cordage.

    In other words, if the UCR isn't as strong as an alternative cord, it is lighter only because strength has been sacrificed.

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