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  1. #21
    GrizzlyAdams's Avatar
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    respect for your elders

    Quote Originally Posted by hammock engineer View Post
    Hey now show some respect.
    hey, another Atkins-Knofler fan!

    http://www.hammockforums.net/gallery...loadfile&i=929

    Grizz the obsequious

  2. #22
    New Member hitchman's Avatar
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    Quote Originally Posted by hammock engineer View Post
    Hey now show some respect.

    Hitchman, you have a good looking product. I broke and got dropped using 550 lbs rated paracord. It broke in the middle and not at the knot. It was at a decent angle, 30-45 deg. For me it is nothing rating below 1000lbs.
    Engineer, the rating on ropes has a totally different meaning than the rating on most products. If you read rope manufacturers specs they tell you what the test breaking strength of brand new rope is. Then they recommend you to use it at a maximum of 20-25% of that and use your judgement to be even more conservative when loss of life (or sleep) is at stake. Clearly the cord you used was degraded. Not too long ago a well known climber fell to his death because he was using a worn out harness that broke. Those things are designed with a factor of safety of 10 (or something close to that, I'm improvising a little).
    When you look at equipment for rope handling (pulleys, etc) you will find that the safe working load (SWL) is much less than what the rope sized for it can take. In general it is close to 25% of the average rope strength. The point I'm trying to make is that using a 550lb rope is equivalent to using a device rated for 137lbs (SWL).

  3. #23
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    Quote Originally Posted by hitchman View Post
    Engineer, the rating on ropes has a totally different meaning than the rating on most products. If you read rope manufacturers specs they tell you what the test breaking strength of brand new rope is. Then they recommend you to use it at a maximum of 20-25% of that and use your judgement to be even more conservative when loss of life (or sleep) is at stake. Clearly the cord you used was degraded. Not too long ago a well known climber fell to his death because he was using a worn out harness that broke. Those things are designed with a factor of safety of 10 (or something close to that, I'm improvising a little).
    When you look at equipment for rope handling (pulleys, etc) you will find that the safe working load (SWL) is much less than what the rope sized for it can take. In general it is close to 25% of the average rope strength. The point I'm trying to make is that using a 550lb rope is equivalent to using a device rated for 137lbs (SWL).

    I am not an expert or even well read on this, so don't take this as me arguing with you.

    When I broke paracord (twice) I cut a fresh piece off the roll and tied it on the hammock. Both times it broke the first time I used it.

    If I was using a rope rated to 550 lbs, shouldn't it hold on the first use at least that?

    From what little I understand on pulleys and racketing straps, isn't the rating governed by the pulley or racket itself and not the rope?

    In the end though I am probibly the most conservative person here in terms of material strengh. I am using 7/54 armsteel sprectra rating around 1400lbs, rings rating over 2000 lbs, biner's rated over 2000 lbs, and webbing rated over 1400 lbs. In the end I think the hitches I use to attach the hammock support ropes to the hammock and rings are the weakest link.
    Is that too much to ask? Girls with frikkin' lasers on their heads?
    The hanger formly known as "hammock engineer".

  4. #24
    New Member hitchman's Avatar
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    I'm learning as much from this conversation as anybody so your views are welcome. I don't see htis as an argument.
    Anyway, I'm surprised that the 550lbs rated cord would break. Under the conditions you describe it doesn't seem like it had anymore than half that load. I would confront the manufacturer with that fact.
    All the ratings you mention are probably breaking strength, and I can tell you that the Hitchcraft Monster tie failed at 1800lbs in tests, so that puts it in the same ball park. I just think that when you see a pulley, for instance, rated at 500lbs (SWL) and you use it with a 2000lbs cord, that doesn't necesarily mean the pulley is the weakest link.

  5. #25
    Senior Member TeeDee's Avatar
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    hitchman - I think you are overlooking the horizontal forces. So far you have considered only the vertical force, the weight, and seem to feel that the horizontal forces are not worth mentioning. But most, maybe all, here that use a structural ridgeline, pull that ridgeline very tight when hanging. So the horizontal forces are not insignificant.

    I personally pulled a brand new hook of 1/4" diameter steel out into a straight line the first time I hung a hammock with a structural ridgeline. Pulled the ridgeline tight, got in the hammock, was just relaxing and got dropped to the ground. But it took a hell of a lot more than my weight to pull that 1/4" steel hook into a straight line. So I think you are not really considering all of the forces at play here.

  6. #26
    Senior Member blackbishop351's Avatar
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    Quote Originally Posted by TeeDee View Post
    hitchman - I think you are overlooking the horizontal forces. So far you have considered only the vertical force, the weight, and seem to feel that the horizontal forces are not worth mentioning. But most, maybe all, here that use a structural ridgeline, pull that ridgeline very tight when hanging. So the horizontal forces are not insignificant.

    I personally pulled a brand new hook of 1/4" diameter steel out into a straight line the first time I hung a hammock with a structural ridgeline. Pulled the ridgeline tight, got in the hammock, was just relaxing and got dropped to the ground. But it took a hell of a lot more than my weight to pull that 1/4" steel hook into a straight line. So I think you are not really considering all of the forces at play here.
    He isn't diagramming a hammock with a structural ridgeline, and he's correct as to the suspension forces for one without a ridge.
    "Physics is the only true science. All else is stamp collecting." - J. J. Thompson

  7. #27
    GrizzlyAdams's Avatar
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    Quote Originally Posted by TeeDee View Post
    hitchman - I think you are overlooking the horizontal forces. So far you have considered only the vertical force, the weight, and seem to feel that the horizontal forces are not worth mentioning. But most, maybe all, here that use a structural ridgeline, pull that ridgeline very tight when hanging. So the horizontal forces are not insignificant.
    Question for the engineering crowd here. Suppose when tightening up the ridgeline we pull it at P lbs. Then a vertical weight W plunks down in the hammock, forming an angle theta between the rope and the horizontal. What's the formula for the stress on the rope? Thanks.

    Grizz

  8. #28
    Senior Member blackbishop351's Avatar
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    The same as if you were considering a hammock without a ridgeline - just consider the hammock/ridge/user system as a solid mass.

    That's only considering the suspension between the ridgeline attachment point and the tree, though...between the hammock and the attachment point gets more interesting. Then you get to figure the tension in the ridgeline, too. I'm not sure how it works out right off the top of my head...I'll play with it when I get home from work later, though.
    "Physics is the only true science. All else is stamp collecting." - J. J. Thompson

  9. #29
    Senior Member TeeDee's Avatar
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    Quote Originally Posted by blackbishop351 View Post
    He isn't diagramming a hammock with a structural ridgeline, and he's correct as to the suspension forces for one without a ridge.
    Agreed, but he's said he's using a Hennessy, and the Hitchcraft site depicts a Hennessy being hung. Also, I believe that Hammock Engineer uses a structural ridgeline. Thus, I still do not think he is considering all the forces at play. If he were, then that steel hook would still be unchanged.
    Last edited by TeeDee; 06-01-2007 at 17:45.

  10. #30
    Senior Member Just Jeff's Avatar
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    The force vector on the rope is the hypoteneus of a triangle.

    h = (.5 x user weight) / sin(support angle)
    “Republics are created by the virtue, public spirit, and intelligence of the citizens. They fall when the wise are banished from the public councils because they dare to be honest, and the profligate are rewarded because they flatter the people, in order to betray them.” ~Judge Joseph Story

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