Results 1 to 10 of 13

Hybrid View

  1. #1
    Senior Member
    Join Date
    Feb 2008
    Location
    north carolina USA
    Hammock
    Homemade
    Tarp
    Homemade silnylon
    Insulation
    DIY Down UQ & TQ
    Suspension
    Whoopies or UCR
    Posts
    120
    Images
    38

    Foot end spreader bar failure

    I just finished my DIY bridge hammock and set it up for it's maiden voyage. Unfortunately, my foot end spreader bar broke on the first try. My spreader bar was an Easton aluminum tent pole tube, .433 diameter, single piece 26" long. The sides of the triangle were set at 18". It folded and broke exactly in the middle. I weight 180 lbs. The ridgeline was at 9'4" and the front spreader bar is a (2) piece .625 that is 36" long with sides set at 28".

    I don't remember high school trig, so maybe some of you mathites can figure out where I went wrong.

    Anyhow, I am moving up to .625 diameter easton tubing, setting the sides longer (I am using dynaglide whoopies for the side lengths so adjustment will be a cinch) and using (2) 13" poles with a pole insert in them.

    Happy Trails

  2. #2
    [o]TTeR's Avatar
    Join Date
    Nov 2009
    Location
    Waterloo, ON, Canada
    Hammock
    R-III
    Tarp
    HG 4S Cuben
    Insulation
    Feathers
    Suspension
    Cinch & Webbing
    Posts
    613
    Images
    69
    I'm very interested in a reason for the failure as well! I was planning to place an order with Quest for this exact spreader bar this afternoon, I weigh 203???
    "I was thinking of the immortal words of Socrates, who said, "... I drank what?"
    My Photos My Videos

  3. #3
    RootCause's Avatar
    Join Date
    Apr 2009
    Location
    MN
    Hammock
    WB Traveller, & my DIY gathered-end
    Tarp
    DIY 10x10
    Insulation
    Pads, usually
    Suspension
    Several types
    Posts
    608
    Images
    4
    tpkanu, thanks for sharing this.

    I'm looking at the DIY bridge as well, and Grizz's information specifys the .625OD Easton tubes as well.

    As to the failure mode, if there was no crease or dent in the middle of your tube, then it sounds like a straight structural overloading. "folded and broke exactly in the middle" sounds like a nice even load, too stressful for the material to handle. Since you're already moving to the .625" tubes, I won't bother with the math.

    Sorry that this failed on you, but thanks for hammering home the need for the stouter tubes!

  4. #4
    [o]TTeR's Avatar
    Join Date
    Nov 2009
    Location
    Waterloo, ON, Canada
    Hammock
    R-III
    Tarp
    HG 4S Cuben
    Insulation
    Feathers
    Suspension
    Cinch & Webbing
    Posts
    613
    Images
    69
    On Grizz's Bengal unleashed video, he mentioned the thinner diameter bar for the foot end but I think (Please correct me if this is wrong) suspension lengths (sides of the triangle) needed to be a minimum of 75% of the bar length making 19.5” the min. Is 1.5” enough for structural failure or is it looking like a manufacturing defect? Either way, I'm not take the Quest cart to check-out just yet.
    "I was thinking of the immortal words of Socrates, who said, "... I drank what?"
    My Photos My Videos

  5. #5
    Senior Member hangnout's Avatar
    Join Date
    Nov 2006
    Location
    Monterey, TN
    Hammock
    DIY Bridge
    Tarp
    DIY Tarps
    Insulation
    Leigh's UQ's
    Posts
    1,715
    Images
    122
    One other item to look at is how it is attached to the triangle suspension. If it is not in a direct line that could have contributed to the failure. Using the .625 and the 75% triangle sides should solve the problem.

  6. #6
    Jsaults's Avatar
    Join Date
    Sep 2009
    Location
    St.Albans WV
    Hammock
    HH, CJH NX-250, WBBB 1.7 dbl
    Tarp
    Std, Hex, or WBSF
    Insulation
    Burrows&Incubators
    Suspension
    Poly web w/AHE buc
    Posts
    1,509

    Going from .433 to .625

    raises the rigidity and bend strength exponentially, as I seem to remember from many moons ago in Physics 1. Anyone here care to offer a real analysis? My shade-tree mechanic alarm went off when I saw .433. Just seems a bit too small in dia.

    Jim

  7. #7
    Brian's Avatar
    Join Date
    Nov 2006
    Location
    Boston, MA
    Hammock
    WBBB 1.1 Dbl
    Tarp
    MacCat Deluxe Sil
    Insulation
    Homemade Down qilt
    Posts
    373
    Quote Originally Posted by Jsaults View Post
    raises the rigidity and bend strength exponentially, as I seem to remember from many moons ago in Physics 1. Anyone here care to offer a real analysis? My shade-tree mechanic alarm went off when I saw .433. Just seems a bit too small in dia.

    Jim

    Yep, you're correct. With a clean fracture right in the middle of the tube it sounds like a buckling failure, and increasing the thickness of the tubing exponentially increases the buckling strength as you are increasing the radius of inertia of the bar. It isn't a linear relationship but rather a quartic one with respect to the inner and outer radii. Assuming everything else constant, the thicker pole (.625", .038" thickness) will be ~390% stronger than the thinner (.433", .030" thickness) for buckling loads.

    Force-wise, for 7000 series aluminum, the thicker pole will buckle at 460 lbf with the thinner pole buckling at 117 lbf (which is well within reason for an adult getting into a hammock!).
    Last edited by Brian; 03-01-2011 at 14:59.
    Brian MacMillin
    www.OutdoorEquipmentSupplier.com
    Home of the MacCat and OES 4-Season Hammock Tarps

  8. #8
    Senior Member bigbamaguy's Avatar
    Join Date
    Jan 2009
    Location
    NE Alabama
    Hammock
    Clark NX-200
    Tarp
    Clark XL-Fly
    Insulation
    KAQ NR UQ/AHE KAQ
    Suspension
    Strap/cinch buckle
    Posts
    2,426
    Quote Originally Posted by Brian View Post
    Yep, you're correct. With a clean fracture right in the middle of the tube it sounds like a buckling failure, and increasing the thickness of the tubing exponentially increases the buckling strength as you are increasing the radius of inertia of the bar. It isn't a linear relationship but rather a quartic one with respect to the inner and outer radii. Assuming everything else constant, the thicker pole (.625", .038" thickness) will be ~390% stronger than the thinner (.433", .030" thickness) for buckling loads.

    Force-wise, for 7000 series aluminum, the thicker pole will buckle at 460 lbf with the thinner pole buckling at 117 lbf (which is well within reason for an adult getting into a hammock!).
    And the Mathletes have spoken...........
    Par Si Vis Pace Para Bellum

  9. #9
    Brian's Avatar
    Join Date
    Nov 2006
    Location
    Boston, MA
    Hammock
    WBBB 1.1 Dbl
    Tarp
    MacCat Deluxe Sil
    Insulation
    Homemade Down qilt
    Posts
    373
    Quote Originally Posted by bigbamaguy View Post
    And the Mathletes have spoken...........
    Nah, I'm no mathlete. Just a bored engineering student wanting to feel like his degree is worth something!
    Brian MacMillin
    www.OutdoorEquipmentSupplier.com
    Home of the MacCat and OES 4-Season Hammock Tarps

  10. #10
    GrizzlyAdams's Avatar
    Join Date
    Apr 2007
    Location
    Illinois
    Hammock
    DIY Bridge, v0.n, where n is large
    Tarp
    depends on season
    Insulation
    DIY UQ
    Posts
    4,628
    Images
    564
    FWIW, I do use the thinner tube on my own rig, I'm closer to 200 than to 180 (sad to say) but also I lengthen the suspension triangle from 19.5" minimum (for a 26" spread) to 24". That reduces the compression on the tube by 20% or more, if I've done my sums correctly.

    In addition to the other factors offered, it might have something to do with just _where_ you plunked yourself down. The closer you are to that end, the more of your weight it's going to try and take.
    Grizz
    (alias ProfessorHammock on youtube)

Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •