Forces on Hammock stand guylines/stakes...

**kohburn** · 10-16-2007, 14:10

Originally Posted by 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!

following that formula the .75 od .62 id round 7075 tube 84" long would have a buckling strength of 126 lb. not including safety margin.

a 1" od tube would be 250 lb

figuring it the area moment of inertia for the square tubing is more complicated.

**Rapt** · 10-16-2007, 15:06

Sounds about right...

**SthrnAMP** · 10-16-2007, 19:00

Man and i thought there was a lot of math as and electrician. Thanks for the explanations and equations. I am far from making my own hammock hanger...i don;t think my homeowners association would appreciate it. But the equations will help when i finally try ot set up my first hang. (FINALLY ordering my first HH this week...yall make me feel so juvenile.) thanks though for the info,

**warbonnetguy** · 10-17-2007, 23:04

got my tubes today, they seem plenty wide and thick. they fit together, but they are really tight. 9 thousandsth of an inch difference. they need to be a little looser. does anybody have any ideas besides alot of time and a piece of steel wool?

i decided on (3) 2' sections per pole. a little heavier, but who cares, this thing is gonna be way lighter and much more protable than most other stands, it will be perfect for car camping in kansas. (note, hopefully i will never have to camp in kansas)

**Rapt** · 10-18-2007, 06:52

I'm guessing they tend to stick when you aren't perfectly aligned putting together or taking apart.

Use a strip of FINE emery cloth, go slow it'll take off quite a bit and you don't want to take off too much. Cut the cloth into a strip as wide as you need and run it back and forth around the pole. Hole it stable and it'll be pretty easy.

If you have a lathe that you can chuck the tube in then you can do it the easy way.. Be careful not to get tangled if you're using emery cloth.

**warbonnetguy** · 10-18-2007, 07:25

is emery cloth avail at home depot next to the sandpaper? should i use fine or coarse grit emery cloth? (is that even an option?) never seen the stuff before.

**Rapt** · 10-18-2007, 07:29

Home Depot does carry it, you want fine... probably as fine as you can get, at least 150-200 grit... Its basically sandpaper with a cloth back that's flexible. Its often sold in plumbing supply places as a roll in 1" width.

HD may have it with the copper tubing and fittings/solder etc....

**kohburn** · 10-18-2007, 08:05

some lube never hurt either

**kohburn** · 10-18-2007, 14:17

i knew i saw it somewhere...

Ft Adams Fall Camporee, October, 2006

Here is a photo of the HH setup on Ft Adams (Newport RI) center field during the Fall Camporee Oct 13th. We ended up using some 6' T-bar fence posts, light chain and large tent pegs to use the HHs on the open field. Setup worked pretty good and now we have 4 sets for any future car camping event we do with the scouts. If we had one large tree we could set them up in a star pattern using the tree and one post for each hammock.
The fence post kits for each hammock uses two each of the following parts 1 6' Tbar fence post, 15' of #3 chain, 1 shackle, 4 quick links and 2 large tent pegs. We drilled a 1/4" hole 1 1/2" from the top of the post, mounted the shackle, used the quick links to connect and adjust the chain to the shackle and the tent pegs. The trick was to balance the hammock rope/chain angle and tension so that the post sees only compression vs bending loads. With all the right angles we could have used a much thinner (and lighter) fence post. I hope to modify the design to work with 1" EMT (electrical metal tube). The chain and quick links replace an earlier prototype using ropes as the chains and quick links were easily adjustable and didn't stretch (and entering the hammock didn't bend the posts).

**Rapt** · 10-18-2007, 15:19

Neat pic...

Things to note...

Steel is 3 times as "stiff" as Aluminum.

T bars are a fairly stiff shape.

As long as the tie out and the hammock attach at the same point there is only compression on the pole no matter what angle its placed at, assuming the pole isn't anchored down or driven into the ground significantly. All the angle does is vary the level and direction of force on the stake by taking a differing portion of the hammock tension load.

The conduit (electrical tubing) is thin walled. But MIGHT be stiff enough calculations are as above with the round columns under compression... Steel is stiffer than aluminum for the same dimensions and stronger, so its possible to use thinner walls. But thinner walls are more prone to denting when handling... Its also heavier for a given thickness...