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  1. #1
    Senior Member Jsaults's Avatar
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    Bridge hammocks for Heavy Hangers

    Sigh. Who said it is only an addiction if you are trying to quit?

    I have a HH and a CJH. Now I am thinking about bridge hammocks........
    Spoke to Jacks R Better about the 250# weight limit on their model. The limit is due to the tension applied to the spreader bars.

    I wa wondering if there are any other Heavy Hangers (AKA "Big Guys") out there who have DIY'ed a bridge out of Industrial Strength Materials? Winter is coming, and I am looking for a sewing and an engineering project.

    Jim

  2. #2
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    Tension / Compression

    Hi,

    Sorry if this is too nitpicking but being careful with how you describe the problem helps identify the possible solutions.

    I am assuming that the spreader bars are the first point of failure as stated in your email. Therefore, if the spreader bars are re-designed, the other components will be able to withstand the increased weight load. Of course, you may end up having to improve the design in other areas after the spreader bars are no longer the first point of failure.

    The spreader bars are in compression and are held in place by notches in the spreader bars that mate with the rings.

    Buckling of the spreader bars is the failure mode we are trying to prevent.

    The spreader bars are hollow to reduce weight. The hollow bars make the spreaders less resistant to buckling.

    This is why you should not hang items from the spreader bars NOR use the spreader bar to pull yourself up, the spreader bars can not withstand a lot of lateral force while in compression, they will buckle much more easily when there are lateral loads present.

    The easiest ways to make the spreader bars more resistant to buckling is to:

    - decrease the spreader bar length (means a smalller circumference bridge so probably not a realisitic choice)

    - change the material (from alumimum to steel)

    - increase the wall thickness (taken to its extreme, use a solid round bar of aluminum and machine the notches)

    - I don't think you can get a more secure connection then the notch/ring set-up but I did want to note that the end connections play a part in low much compression load will cause buckling.

    I would test the final set-up with at least ~ 1.5 times the expected load.

    I hope this helps,
    TJM
    Last edited by tjm; 09-29-2010 at 13:21.

  3. #3
    Senior Member Ramblinrev's Avatar
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    A lot depends on what you are willing to put up with. I used 3' lengths of 3/4" hardwood dowel. I put a 2.5" bolt through the dowel and each end and placed the dowel through a 1.5" welded steel ring. Not light weight, compact or easy to transport by any stretch. But it works quite nicely and my wife doesn't go far from the car anyway.

    I used 3/4 polypro webbing for the edges. The thin rope cuts my legs and I don't like it. Again not a UL solution but then my wife don't go UL as it is.
    I may be slow... But I sure am gimpy.

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  4. #4
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    Further reading

    http://en.wikipedia.org/wiki/Buckling

    http://www.engineersedge.com/column_...lumn_ideal.htm

    With a little care, you could use these equations to decide between an aluminum tube with 0.xxx" wall thickness and a solid wood dowel of x.xx" diameter.

    Actually, someone with a little time and a good handle on STATICS (most mech eng grads), could figure out what the compression loads on the spreader bars will be based on your weight load and then figure out the what spreader bars designs can withstand those loads with an appropraote safety margin.

    If I take a stab at it, I will post it for you here.

    good luck,
    TJM
    Last edited by tjm; 09-29-2010 at 22:32.

  5. #5
    Senior Member bigbamaguy's Avatar
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    Jsaults:
    I am glad you asked this question..............I have been meaning to send a PM to Grizz and ask him some of the same question that you have. I thought about the hardwood dowel but then I want to ask the question about a bamboo spreader bar. Was an Idea several months ago but it got lost in Life. I will be following this with interest.
    Par Si Vis Pace Para Bellum

  6. #6
    gargoyle's Avatar
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    I'd stay away from wood and bamboo. Growth patterns, wood density, knots, moisture content....to many variables.

    Personally, up the wall thickness of your alum. spreader bar and you should be good.
    Of course it will be heavier to pack.

    I sat in a bridge for a while and I weigh 330. Easton alum spreader bars.
    To be honest it wasn't all that comfy. And the narrowness seemed confining, hard to lay sideways in.
    Getting in and out seemed more difficult also.
    I can get plenty flat in my gathered ends. And sleep on my side.
    Ambulo tua ambulo.

  7. #7
    Senior Member Jsaults's Avatar
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    Yep, the spreader bars

    appear to be the weak point of this design.

    I am not an engineer, but I like to think like one. While wooden dowels of sufficient size will adequately resist the tension loads, I wonder if it might make more sense to simply increase the diameter and wall thickness of an aluminum tube to obtain the strength needed? After all, the compression and tension forces experienced when a tube bends are borne primarily by the outer surfaces - and a solid rod wastes material weight for a small increase in strength. I seem to remember that the main wing chord of a 747 is a massive tube, and not a rod.

    A number of kayak paddle manufacturers have made paddles using vaulting pole stock. Somewhare I have a link to a source of carbon fiber tube, and also a sourc of CF "sock" that can be used to make DIY CF/epoxy tubes. Now, those would be light and stiff.

    Of course, I would hate to have one snap so close to my head!

    Another question rattling around in my brain is the relative comfort for someone who is broad-shouldered. While I normally sleep on my side, I would like to occasionally lay on my back for a spell (reading and such). In some of Griz's videos it appears that the shoulder width is snug. If the fabric is widened for more shoulder comfort, the bar would have to be longer, requiring it to be stiffer, which would require larger diameter, etc, etc.

    Wow. This is makinf my brain hurt.

    Jim

  8. #8
    Senior Member dblhmmck's Avatar
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    which JRB poles?

    Spoke to Jacks R Better about the 250# weight limit on their model. The limit is due to the tension applied to the spreader bars.
    Just to clarify; are you talking about the older aluminum spreaders that were 7.2 ounces, or the heavier anodized ones that weigh 8.6?
    "Better living through Hammockry"

  9. #9
    Senior Member Jsaults's Avatar
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    Hmmm. I don't know which ones.

    I spoke to them only a couple of months ago, so whatever is current.

    Jim


    Edit: Ah-Hah! I see now that the weight limit has been increased to 275# by changing to beefier spreader bars. The plot thickens! Maybe after I slim down I should buy myself a present..........
    Last edited by Jsaults; 09-29-2010 at 14:39.

  10. #10
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    Enjoying this thread:

    The spreader bars are in compression, not tension.

    Thinking about it more, I agree that wood and bamboo has many too many variables to be reliable.

    A solid bar would not be wasting material in this application since we would be increasing the moment of inertia (resistance to bending) which is a function of the cross sectional shape. That said, a solid bar is probably overkill.

    Ask JRB for the specs of the old spreader bar vs. the new spreader bar what weight limit improvement that gave them. Hell, those guys have been awesome to deal with; they probably have a very good idea of the increase in wall thickness you will need to get to the weight limit you are trying to achieve.

    Another question rattling around in my brain is the relative comfort for someone who is broad-shouldered.

    I use a BA insulated aircore pad [20"x72"] in the pad sleeve. To me, the pad significantly changes and improves the shape of the bridge from a round U to a flat l_l.

    I don't think I would like a bridge hammock that didn't have a pad sleeve with a pad in it. In fact, I am think of buying a wider pad to take more advantage of the 26" pad sleeve.

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