Hammock Stand Physics Question

[qm1pooh]

Hi ya'll! I've been thinking of making a portable hammock stand and I have a question that I can't quite figure out with my minimal trig and physics knowledge.

I created a sketch based on this awesome diagram that Knotty created. Here is mine:


Now, I know that S (the horizontal force pulling the ends of the uprights towards each other) and T (the tension on the suspension lines) varies depending on the weight in the hammock (W) and the suspension angle.

My question concerns the force pulling down on the eyebolt (the red arrow in my diagram). Is it the weight in the hammock (W), or is it something else? If it is something else, how do I compute it?

The reason I'm asking is I would like to build the stand out of square aluminum tubing and I'm concerned about the eyebolt "egging" out the holes where it runs through the tubing. If it's only the weight in the hammock I don't think it would be a big deal if I put a nut and washer on each side of the bolt to tighten it against the upright, but if the force is huge I may have to use something else.

Thanks in advance for your thoughts.

[qm1pooh]

I guess the diagram didn't post. Let's see if this works:

[WV]

It should be half the weight in the hammock. You could angle the bolt down about 30° or, if you're also going to have a ridgeline under tension, use something less than that.

[qm1pooh]

It should be half the weight in the hammock. You could angle the bolt down about 30° or, if you're also going to have a ridgeline under tension, use something less than that.

Thanks WV! I would never have guessed that the downforce would be half the weight, and angling the eyebolt down to put it more in line with the tension on the suspension lines makes sense.

My HH has a ridgeline, but it doesn't have hardly any tension on it when I'm laying in it. Would that make a difference in the angle of the eyebolt?

[GrizzlyAdams]

Thanks WV! I would never have guessed that the downforce would be half the weight, and angling the eyebolt down to put it more in line with the tension on the suspension lines makes sense.

My HH has a ridgeline, but it doesn't have hardly any tension on it when I'm laying in it. Would that make a difference in the angle of the eyebolt?

A tight ridge-line on the HH would flatten the angle of the suspension line with respect to the horizontal. Basically you want the eyebolt to be in line with the suspension line. If your ridgeline is not taut, it is as if it were not there, force-wise.

[qm1pooh]

Thanks for that insight Grizz! I'm so happy that there are so many on HF who know way more about all of this than I could ever hope to pick up on my own. I can be a bit dense sometimes

Now all I have to work out is what size tubing and what wall thickness will work to keep the uprights from deforming and bending too badly. I'm thinking that may come down to trial and error. Hopefully the bride won't be too shocked at my metal buying budget

[tjm]

I think the best idea is to drill the hole for the eyebolt shank peripendicular to the surface of the square aluminum tubing. That way the washer has full contact with the surface of the square aluminum tubing.

Yes, there will be a torque that presses the eyebolt against bottom of the inside of the tubing and the eyebolt against the top of the outside of the tubing.

If the compression of the washers onto the aluminimum tubing is great enough the washer might take some of the torque load thereby reducing the potential of the tubing to egg [maybe the tubing hole should be slightly larger diamater than the washer ID ?].

If you try to rotate the shank of the eyebolt so that the threads are in-line with the force of the suspension then I could see the edge of the nut being a single point of stress that deforms the tubing. And how do you install a washer with the angle between the stand and the eyebolt shank, you would have to cut part of the washer off

I think you would rather have a failure mode of the shank of the eyebolt egging the holes than having the nut rip through the tubing and land someone on their butts.

Hope that is clear....

Obviously the issue is that the aluminum is a "soft" metal.

[GrizzlyAdams]

I think the best idea is to drill the hole for the eyebolt shank peripendicular to the surface of the square aluminum tubing. That way the washer has full contact with the surface of the square aluminum tubing.

Yes, there will be a torque that presses the eyebolt against bottom of the inside of the tubing and the eyebolt against the top of the outside of the tubing.

If the compression of the washers onto the aluminimum tubing is great enough the washer might take some of the torque load thereby reducing the potential of the tubing to egg [maybe the tubing hole should be slightly larger diamater than the washer ID ?].

If you try to rotate the shank of the eyebolt so that the threads are in-line with the force of the suspension then I could see the edge of the nut being a single point of stress that deforms the tubing. And how do you install a washer with the angle between the stand and the eyebolt shank, you would have to cut part of the washer off

I think you would rather have a failure mode of the shank of the eyebolt egging the holes than having the nut rip through the tubing and land someone on their butts.

Hope that is clear....

Obviously the issue is that the aluminum is a "soft" metal.

good points here. I had been thinking "wood" where one could introduce the angle into the support beam itself. Not reading carefully enough. But it is a different story for aluminum.

[oldgringo]

Pooh, I would find a shop that does Al fabrication...they can advise you on alloys, tube size, wall thickness, etc, and in general, save you the expense of trial and error engineering. They can also weld an eye onto each upright for you.

The aluminum sold by the piece @ Homie's etc. is probably way too soft for what you are contemplating, btw.

[MacEntyre]

Thanks WV! I would never have guessed that the downforce would be half the weight

If this were not true, then a hammock stand would have the capability of amplifying weight.

It's no different than if you stood with each foot on a different box. The vertical force W on each box would be half your weight. Doesn't matter what is inside the box, so to speak. Your foot could be on a trampoline in the box, and the net effect is the same.

Would that make a difference in the angle of the eyebolt?

Not when the thing attached to it can assume whatever angle it wants, because of the eye.

This is a needless post... but I have contributed!