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webbing strength

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  • tim garner
    some of you have talked about the strength of webbing (& rope too i belive) being compromised w/ knots. so i`m wondering if webbing being forced to bunch up
    Message 1 of 4 , Oct 30, 2005
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      some of you have talked about the strength of webbing (& rope too i
      belive) being compromised w/ knots. so i`m wondering if webbing being
      forced to bunch up tightly together will cause a weak place too? the
      reason i ask is that i came up w/ a strong, simple, light, extreamly
      easy to use adjuster for the hammock. it`s going to be hard to
      describe but i`m working on getting a digital camera so i can post
      photos. by the way... i have a nice nikon N90, w/ nikon zoom &
      dedecated flash that i can`t aford to buy & develop film for. so i`m
      going to take it to a friend who owns a local camera shop & let him
      sell it, then get a nice digital. any one here interrested in a good
      SLR? thanks...slowhike
    • Dave Womble
      ... Tim, I always thought that rope strength was degraded with tight bends (which knots do) that required the individual fibers to stretch at significantly
      Message 2 of 4 , Oct 31, 2005
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        --- In hammockcamping@yahoogroups.com, "tim garner" <slowhike@y...>
        wrote:
        >
        > some of you have talked about the strength of webbing (& rope too i
        > belive) being compromised w/ knots. so i`m wondering if webbing being
        > forced to bunch up tightly together will cause a weak place too?

        Tim,

        I always thought that rope strength was degraded with tight bends
        (which knots do) that required the individual fibers to stretch at
        significantly different rates. When that happens, the stress is not
        shared equally by all the fibers that make up the rope, with the outter
        fibers at the bend taking on more of the stress and causing them to
        fail first and at a lower stress than a rope without bends... it is
        like all the fibers that make up the rope are not working in unison, or
        parallel anymore. So applying that philosophy with webbing it would
        depend on how it is being reshaped. It seems like webbing is designed
        to handle tight bends along its flat part but not so much along its
        edge. But if you are just gathering webbing and rolling it into a
        tubular position, I wouldn't think you would be degrading its strength
        much, it at all.

        Dave
      • Rick
        Tim, The way I hang a hammock with webbing, even though I use a knot, there is little loss of strength at the tree. This is because there is very little
        Message 3 of 4 , Oct 31, 2005
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          Tim,

          The way I hang a hammock with webbing, even though I use a knot, there
          is little loss of strength at the tree. This is because there is very
          little stress on the knot. Most of the stress is taken up by the wraps
          of webbing around the tree, and only a small portion is taken up by the
          keeper knot at the end. The same is true of a hennessey knot.

          http://www.imrisk.com/hammock/improvedknot.htm

          Your point is well taken especially regarding the loop I make back at
          the hammock. This is the reson that I form that loop by sewing. It can
          be sewn, like Ed Speer shows in his book, by sewing the loop around the
          hammock. However, what I have begun to do is to sew a loop in the end
          of the strap and then bring the end of the strap through the loop to
          make a slip knot that goes around the hammock. With this set-up it is
          easy to remove the strap to wash the hammock. Add a whipped end to the
          hammock, and now I can quickly and easily remove the gathering at the
          end of the hammock to dry it quickly after washing.

          I read what Youngblood wrote about differential stress and agree with
          that too. This is the reason that climbing knots using webbing are made
          carefully without bunching the webbing.

          Risk
          ________________
          Rick Allnutt MD
          Author of "A Wildly Successful 200-Mile Hike"
          http://www.wayahpress.com
          personal hiking website: http://www.imrisk.com

          > --- In hammockcamping@yahoogroups.com, "tim garner" <slowhike@y...>
          > wrote:
          > >
          > > some of you have talked about the strength of webbing (& rope too i
          > > belive) being compromised w/ knots. so i`m wondering if webbing being
          > > forced to bunch up tightly together will cause a weak place too?
          >
          >


          [Non-text portions of this message have been removed]
        • dlfrost_1
          ... not ... outter ... unison, or ... would ... designed ... strength ... Your description of unequal loading inside/outside is correct. But there is an
          Message 4 of 4 , Oct 31, 2005
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            --- In hammockcamping@yahoogroups.com, "Dave Womble" <dpwomble@y...>
            wrote:
            > I always thought that rope strength was degraded with tight bends
            > (which knots do) that required the individual fibers to stretch at
            > significantly different rates. When that happens, the stress is
            not
            > shared equally by all the fibers that make up the rope, with the
            outter
            > fibers at the bend taking on more of the stress and causing them to
            > fail first and at a lower stress than a rope without bends... it is
            > like all the fibers that make up the rope are not working in
            unison, or
            > parallel anymore. So applying that philosophy with webbing it
            would
            > depend on how it is being reshaped. It seems like webbing is
            designed
            > to handle tight bends along its flat part but not so much along its
            > edge. But if you are just gathering webbing and rolling it into a
            > tubular position, I wouldn't think you would be degrading its
            strength
            > much, it at all.

            Your description of unequal loading inside/outside is correct. But
            there is an additional factor: The inner-radius segment of the line
            acts as a static bulk about which the outer segment is stretched in a
            levering action which amplifies the loading much further. If it
            weren't for this levering action rope would have a working load
            rating that's 33% of its maximum (as webbing does) rather than the 10-
            20% used currently. (You can also see why nylon is so superior for
            rope: The memory-stretch allows it to accommodate both the radius
            the levering problems much better than other fibers.)

            When rumpled, webbing only has the partial-loading problem to contend
            with, and so retains much of its strength. For example, a water knot
            loop in webbing retains about 75% of the webbing's total strength.
            When loops are properly sewn into webbing (i.e. X-box and oversewn)
            that figure is over 90%. (I also suggest that loops be 3 inches long
            after sewing so that any rumpling at the tie-off is smoothed out by
            the time it reaches the stitching.)

            Webbing's only weakness is wear. Because all of the fibers are
            exposed, it wears out faster than core-and-sheath ropes. On the
            other hand, you can easily see the wear, so inspection and
            replacement are not problematic.

            Doug Frost
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