Foot end spreader bar failure

**tpkanu** · 03-01-2011, 13:18

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

**[o]TTeR** · 03-01-2011, 13:24

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???

**RootCause** · 03-01-2011, 13:44

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!

**[o]TTeR** · 03-01-2011, 14:03

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.

**hangnout** · 03-01-2011, 14:29

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.

**Jsaults** · 03-01-2011, 15:00

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

**Brian** · 03-01-2011, 15:46

Originally Posted by Jsaults

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!).

**bigbamaguy** · 03-01-2011, 16:02

Originally Posted by Brian

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...........

**Brian** · 03-01-2011, 16:04

Originally Posted by bigbamaguy

And the Mathletes have spoken...........

Nah, I'm no mathlete. Just a bored engineering student wanting to feel like his degree is worth something!

**GrizzlyAdams** · 03-01-2011, 16:41

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.