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  1. #21
    MDSH's Avatar
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    Ayup

    Quote Originally Posted by cvlngnir View Post
    OK, I'm new to hammocking but not to engineering. SGT ROCK's theror is flawed I hate to say. The tension in you webbing decreases dramatically with the length of contact with tree trunk. Try this:

    Wrap the webbing only 180 degrees around the trunk and have someone (or yourself if you like pain) sit in the hammock. Unless it is a really big tree you won't be able to win the tug of war.

    Now wrap it another 360 degrees even without overlapping the wraps and sit down. You will be very close to supporting your own weight if you can't already.

    This just shows that the tension transferred from the strap to the bark of the tree fairly quickly, so the extra wrap does little for you.

    So the strap is putting a lot of tension (compression on the bark) very early so you want to have as much width of strap to distribute it as much as possible as soon as possible.
    Exactly. And one edge of a hard strap digging in, given the load at 30*, is not easy on the tree whereas a softer fabric will grab more surfaces as the first few weaves at the top of the strap stretch.
    Last edited by MDSH; 08-27-2012 at 11:55. Reason: clarity

  2. #22
    Senior Member SGT Rock's Avatar
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    Pictures of the test in case anyone was interested.
    Attached Images Attached Images
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  3. #23
    Senior Member SoundMan's Avatar
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    Interesting post- I am no math wiz, however, in my line of work we suspend tons of equipment over peoples head. We use wire rope, shackles, & spansets to go around steel building supports to "fly" this equipment. Any rigger that wrapped a piece of wire rope around a beam multiple times would be fired immediately. We use one loop with the wire rope/beam cushioned with a burlap bag to prevent wear.
    As to the size of the webbing- it seems the sewing on the loop of a 1/2" piece would have less load rating versus a 1" piece. The rigging is only as strong as it's weakest link.
    Thoughts???
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  4. #24
    MDSH's Avatar
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    Quote Originally Posted by SoundMan View Post
    Interesting post- I am no math wiz, however, in my line of work we suspend tons of equipment over peoples head. We use wire rope, shackles, & spansets to go around steel building supports to "fly" this equipment. Any rigger that wrapped a piece of wire rope around a beam multiple times would be fired immediately. We use one loop with the wire rope/beam cushioned with a burlap bag to prevent wear.
    As to the size of the webbing- it seems the sewing on the loop of a 1/2" piece would have less load rating versus a 1" piece. The rigging is only as strong as it's weakest link.
    Thoughts???
    Excellent point, SoundMan. On my multiple wraps of cord there are no sewn points to fail.

  5. #25
    Senior Member SGT Rock's Avatar
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    Quote Originally Posted by SoundMan View Post
    Interesting post- I am no math wiz, however, in my line of work we suspend tons of equipment over peoples head. We use wire rope, shackles, & spansets to go around steel building supports to "fly" this equipment. Any rigger that wrapped a piece of wire rope around a beam multiple times would be fired immediately. We use one loop with the wire rope/beam cushioned with a burlap bag to prevent wear.
    I'm no rigger so I can only guess why. But I would think that the reason for that is they are not concerned with hoop stress (this is what I have learned is the technical term for what we are discussing) but are worried more on friction wear. Having multiple wraps of steel cord around a beam would have a chance for stretch which would me abrasion and possible failure. One loop would be less so. And then they pad it for extra protection. But that is only a guess.
    As to the size of the webbing- it seems the sewing on the loop of a 1/2" piece would have less load rating versus a 1" piece. The rigging is only as strong as it's weakest link.
    Thoughts???
    That has been another big concern for me on this. I've had double stitched X box sewn patterns fail on me on 1" straps, so I've gone to using bar tacks. I usually space them about 1/8" apart and do one stitch per 200 pounds of desired strength, I've never had one of these fail. With a half width strap I may go with one par for every 100 pounds.
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  6. #26
    Senior Member SGT Rock's Avatar
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    Quote Originally Posted by cvlngnir View Post
    OK, I'm new to hammocking but not to engineering. SGT ROCK's theror is flawed I hate to say. The tension in you webbing decreases dramatically with the length of contact with tree trunk. Try this:
    I know there is something to this. I'm just not sure how to calculate that in.
    Wrap the webbing only 180 degrees around the trunk and have someone (or yourself if you like pain) sit in the hammock. Unless it is a really big tree you won't be able to win the tug of war.

    Now wrap it another 360 degrees even without overlapping the wraps and sit down. You will be very close to supporting your own weight if you can't already.

    This just shows that the tension transferred from the strap to the bark of the tree fairly quickly, so the extra wrap does little for you.
    But for arguments sake lets say that the friction is also a product of the surface area of the strap. Less width would be less surface area, so more of the weight would get transferred around the strap(s) on a thinner strap wouldn't it?

    So the strap is putting a lot of tension (compression on the bark) very early so you want to have as much width of strap to distribute it as much as possible as soon as possible.
    I haven't tried that experiment yet, but what I have generally assumed until I really started thinking about this was the greatest pressure would actually probably be on the back of the tree, not the sides or front (nearest hammock).
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  7. #27
    Knotty's Avatar
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    Quote Originally Posted by cvlngnir View Post
    OK, I'm new to hammocking but not to engineering. SGT ROCK's theror is flawed I hate to say. The tension in you webbing decreases dramatically with the length of contact with tree trunk. Try this:

    Wrap the webbing only 180 degrees around the trunk and have someone (or yourself if you like pain) sit in the hammock. Unless it is a really big tree you won't be able to win the tug of war.

    Now wrap it another 360 degrees even without overlapping the wraps and sit down. You will be very close to supporting your own weight if you can't already.

    This just shows that the tension transferred from the strap to the bark of the tree fairly quickly, so the extra wrap does little for you.

    So the strap is putting a lot of tension (compression on the bark) very early so you want to have as much width of strap to distribute it as much as possible as soon as possible.
    Very true. Anyone who has ever used a winch on a sailboat is familiar with this. Huge loads can be held by hand if enough wraps are made around the winch.

    With each turn around the winch/tree the load is going down. The distribution of load is something that can be determined using calculus, a skill I no longer possess.
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  8. #28
    Senior Member DuctTape's Avatar
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    Quote Originally Posted by cvlngnir View Post
    OK, I'm new to hammocking but not to engineering. SGT ROCK's theror is flawed I hate to say. The tension in you webbing decreases dramatically with the length of contact with tree trunk. Try this:

    Wrap the webbing only 180 degrees around the trunk and have someone (or yourself if you like pain) sit in the hammock. Unless it is a really big tree you won't be able to win the tug of war.

    Now wrap it another 360 degrees even without overlapping the wraps and sit down. You will be very close to supporting your own weight if you can't already.

    This just shows that the tension transferred from the strap to the bark of the tree fairly quickly, so the extra wrap does little for you.

    So the strap is putting a lot of tension (compression on the bark) very early so you want to have as much width of strap to distribute it as much as possible as soon as possible.
    This is consistent with what I have seen. When one looks at the tree after the straps are removed, sometimes there is some damage. This occurs more often with softer bark trees. One can see the damage is not consistent around the entire circumference but is isolated to specific spots. These spots occur consistently at the initial and final point of contact of the straps and tree.

  9. #29
    MDSH's Avatar
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    The ring I've described transfers the load first to the one turn of 550 that is in direct line with the load. As that one stretches the two wraps on either side begin to take up load, and so on ...

    50' of cord would wrap a 9.5" dia. tree 20 times with a back-side strap width of 3".

    Only one cord could hold a man if the tree was 15.9' in dia.

    The advantage then of a very long cord strap played through a ring is the range of application.

    The disadvantage is spending 30 minutes building it.

  10. #30
    Senior Member SGT Rock's Avatar
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    Quote Originally Posted by DuctTape View Post
    This is consistent with what I have seen. When one looks at the tree after the straps are removed, sometimes there is some damage. This occurs more often with softer bark trees. One can see the damage is not consistent around the entire circumference but is isolated to specific spots. These spots occur consistently at the initial and final point of contact of the straps and tree.
    I've seen this and I think that it's a product of two things: the friction of the tree will put more of the load there because it isn't a perfect world and the load isn't getting transferred the same all around, and the strap system generally constricts around that point so that the full width of the strap isn't there, it is smaller and mostly edge is against the tree like MDSH is talking about. To combat this you can put a sacrificial stick in that point as long as the stick isn't too soft so that the tree strap digs into it and breaks it.
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