Forces on Hammock stand guylines/stakes...

[warbonnetguy]

this is something i can do myself ? i think i will have a look. much thanks.

[Rapt]

First I suspect you will be able to get away with 6" but like I said I'd start longer and work your way down to it. Say maybe start at 8" and see if you can get down to 5" without failures. I'd set it up then swing about etc...

As far as anodizing goes its a bit of a process. I run a Mil-spec Hardcoat (TypeIII) only one colour (grey-black) anodizing system in my garage.

The basic process is
1)Complete degrease,
2)Chemical etc/strip of oxides,
3)Attach to electrode in a Sulfuric Acid bath (part must be immersed) as must both electrodes.
4)Apply desired current per square inch for desired time to get needed thickness
5)Remove part
6)Dye anodizing if desired
7)Seal anodizing if desired

Go here for sources of materials and information including kits.

There's a fair bit of trial and error usually to get it working the way you want.

[kohburn]

i skimmed the second page but i didn't see any mention so far. Was there an original assumption that the pole had to be vertical? if you angle the top of the pole away from the hammack you will increase the compression force on the pole and reduce the tension on the stake.

[Rapt]

Yes it was assuming the pole was vertical. The reason was that if you angle the pole then you need a longer pole for the same hang height. Also increased length and increased compression also means a thicker pole to prevent buckling.

Both of which increases the weight of the system.

[kohburn]

Yes it was assuming the pole was vertical. The reason was that if you angle the pole then you need a longer pole for the same hang height. Also increased length and increased compression also means a thicker pole to prevent buckling.

Both of which increases the weight of the system.

if the hammock line and tie out line(s) are attacked at the same point and the pole is straight the chances of buckling are minimized. then its just a matter of material strength.

I think it was on the HH website where a scout group used the steal t-bar garden fence posts to support their hammocks in an open field. granted those aren't exactly light, but a comparable aluminum or carbon fiber alternative would be pretty simple.

if i wasn't in the middle of building a house i'd have more time to experiment.

[Rapt]

Any pole will buckle if the compressive load is increased beyond the buckling limit even if the force is perfectly aligned. The longer the pole the lower this limit is. In an earlier post on this thread I link to the calculations involved to determine what this limit is.

Of course reality is that its virtually impossible to get the ideal theoretical limit so we need to leave a safety margin when doing these things. Also we can't always rely on people to do the "right" thing with a design. That is they may tie to the middle of the pole rather than the top adding bending as well as compression to the loads.

I hear you on the house thing... I'm in the middle of renovations myself...

[kohburn]

lets take for a minute some hypotheticals.

aluminum tubing made from a common alloy used for outdoor purposes due to corrosion resistance. 2024

its compression strength is rated at 60,900psi. lets take a safety margin of ~60% and say its 40,000psi.

now that tube is 1/2" diameter with a 1/16" wall (.063)

the cross sectional area of that tube is .085sqin (rounded down)

that means that 1/2" diameter pole should be able to hold 3400lbs of force if it does not have side loading to worry about.

an 84" (7') tall tube that size would weigh .75lb or 12ounces.


now round tubing is extremely strong as l ong as it remains round. but dent it and it will cave (think about standing ontop of a soda can, it'll hold a 300 pound man or a 50 lb kid but tap the side out of round and you can crush it in your hand)

so square tubing would be stronger. if you want to further increase your safety margin you can go to a .75" square aluminum tube with a 1/16" wall - at 7' it would weigh just under 1.5 lbs. but it would be able to hold a 7125lb compressive load (using the 60% safety margin) and be far more resistant to any damage from abuse.

give it a solid machined tip on the bottom with a rubber foot, a machined cap with a smoothed hole for ropes, and a nice grip in the middle and you have a nice walking staff/ bear spear (or have it break down into two trecking poles)

you can buy a peice of aluminum tubing like this for about 7$ , I have a small cnc milling machine at home to make the tips so i'll try this out soon. i'm convinced that i can make this concept work with the wider tent stakes

[Rapt]

Actually you're wrong about the buckling limit... Because the material doesn't fail in compression like your calculation indicates, unless the tube is "short" in comparison to its diameter... The length to diameter ratio matters.

If you go here and either look at the calculations or do your own, (see posts 638 and 640) you'll see that its considerably lower than the 3400 lbs you come up with for your example... More like about 25 lbs for a 72" pole...

This is because tall slender columns are prone to instability due to the "elastic" nature of the material. It doesn't matter if you manage to achieve the theoretical possibility of zero side loads and zero applied moments. Slender columns buckle well below the compressive failure load of the material.

(BTW I'm a design engineer with more than one university degree, a professional license, and many years experience doing this sort of thing for a living. I haven't had anything fall down on me yet. ) It also shows why a little bit of knowledge can be dangerous, and engineers need to be licensed.

[kohburn]

(BTW I'm a design engineer with more than one university degree, a professional license, and many years experience doing this sort of thing for a living. I haven't had anything fall down on me yet. )

got any formulas for square not round tubing? I'm a bit rusty with this stuff since my job hasn't required calculations for years.

there are other things that will greatly increase buckling resistance other than using a solid rod. filling a hollow tube with a light weight hard material does some great stuff. packing a hollow rod full of a graphite (carbon fiber) with minimal resin content for example. the formulas start getting more complicated and still only get you close without physical tests.

[Rapt]

The critical load (Fcr) is a function of the area moment of inertia (I) of the cross-section.

that is assuming a long slender column, with effective length (Leff)

Fcr= (E*I* Pi^2)/(Leff)^2

Using that you can do the calculations with any material and shape you want. Of course adequate safety factors have not been accounted for.

The details of all this stuff can be found here.

Enjoy!