Tensegrity stand for multiple hammocks

**schrochem** · 09-26-2010, 10:17

This reminds me of the universal space frames.
or geodesic domes.

WV · 09-26-2010, 10:36

Originally Posted by dejoha

I'm a little confused about the continous loop --- obviously you've used it successfully, but in my experimenting, when I got in the hammock, the poles I was hanging from pulled together making the top tendon droop down. Adding the second side strut stopped this and kept the poles from pulling inward. I think that's a function of the stretch in the 550 cord.

So, with your continous loop top and bottom, do you create an eye splice at both ends and connect them with a carabiner or nacrabiner? For the top that would be one long line about 33 feet long with eye splices at both ends? And in my case, a bottom line about 18 feet long with fixed eye splices in both ends? Exactly right. Except that with your strut length you could make the top smaller and the bottom bigger and (perhaps) get a structure that doesn't need to be staked down (except to keep the wind from blowing it away). Wrapping a loop through the hole and around the end cap shortens the effective tendon length by about 4.5".

Also, does it matter which way the poles overlap each other?

Yes, somewhat. If you use an asym lie, it's easier to keep your center of gravity closer to the center but still well clear of the nearest strut if the struts rise in a counterclockwise pattern (as seen from above) than a clockwise one. That assumes you lie with your head and shoulders to the left and your feet to the right. If you get the poles clockwise you can still hang from the stand, but it's not optimum. Very perceptive question!

**dejoha** · 09-26-2010, 13:29

Since I got my Tensegrity stand working, we started loading it with hammocks and tarps. My kids like to jump in whatever hammock I happen to be in and so we really started to load it down.

I had one child on his own hammock. He weighs 35 lbs.

I was in the other hammock with my 30 lb 2-year-old. Then my 50 lb son starts to climb in. The total weight in my hammock was 245 lbs, plus 35 lbs in the other hammock for a combined total of 275 lbs.

Within a matter of seconds, the stand collapsed. Luckily I had two kids on top of me so their fall was somewhat cushioned.

The point of failure was not stakes pulling like had happened in previous tests. This time, one of the couplers ripped apart. On closer inspection, the weakness was along the seam. I didn't notice the couplers had seams before. I think I would get longer couplers next time or make sure the couplers' seam is on the side of the pole and not the top? I wonder wether or not that would have any effect though.

Pictures to come soon. I have to get over my sorrow first.

WV · 09-26-2010, 18:50

Too bad, Dejoha. I'm glad there were no injuries. While you're waiting for me to send you some amsteel, I suggest looking for some internal ferrules to use in conjunction with your external couplers.

**dejoha** · 09-26-2010, 19:22

Thanks WV. I think I'll look for some longer sleeves to use as a coupler -- something extruded, probably a bigger pipe. The other modifications I'm thinking of include drilling holes and installing eye bolts for the bottom struts to get the tendons off the ground.

**dejoha** · 09-27-2010, 21:53

Here they are.

This is a photo just before we loaded it up with me and a few kids.

At just under 300 lbs, the coupler failed. The weakness was the seam.

The good news is that we proved the poles will not hit you when the stand fails. But the ground is still unforgiving.

WV · 09-27-2010, 22:16

I looked at my 1.375" couplers and they look exactly like yours - with a seam. Too bad they won't do the job by themselves. Glad nobody was hurt.

**MacEntyre** · 09-28-2010, 06:31

Sure am glad no one was hurt!

...been thinkin' 'bout this... gotta be the fault of the stretchy tendons. The only way the forces on the struts can be exclusively compressive is if the structure is rigid. That is achieved with tendons that do not stretch. Instead of making the structure rigid, your tendons allowed the structure to flex, resulting in a bending moment that burst the union.

My hypothesis is this: The cause wasn't the seam in the coupler. That was the weak point of the strut, but the cause of the failure was the cord used for tendons.

Perhaps Knotty will come along and say something about this.

WV · 09-28-2010, 07:17

I Think Mac is right about this: tendons should not stretch. A lesser factor might be the shims. They may have taken the wobble out of the joint, but did you use enough of them to space them evenly around the couplers so the struts were perfectly straight?

What next, Dejoha? I will send you amsteel as soon as it gets here. How are your splicing skills? I can put locked brummels in the ends and make whoopies for the side tendons if you wish, but we'll need to confer on sizes.

Can you find some conduit that fits easily but snugly inside your struts? A 2' internal ferrule would be a stronger connector. Then shim the external couplers with flashing? Then cable clamps around the couplers? (Maybe skip the cable clamps.)

You're a pioneer, with all the lumps and bumps that precede the victories!

**MacEntyre** · 09-28-2010, 07:28

I've been using these ribbed aluminum mast sections. Here is another source. I'm seeking a wholesale supplier...

Check your local surplus warehouse and see if you can get some!