Tuesday, September 21, 2010

Hammock Tensegrity Innovations

David WV and John MacEntyre Allen lead the deployment of tensegrity structures for campers. MacEntyre is inventor and retailer of the MOLLE-based Molly Mac Pack, a human-carryable rig based around a double sided M.O.L.L.E. panel, with shouder straps, hip belt and webbing--as MacEntyre says, "a backpack without the pack." WV has long experience with tensegrities. He describes them as a wound-up spring. he wrote, "Think of a tensegrity as a big spring, because that's what it is. I'm guessing that the tighter you wind it, the more force it takes to make it move (but also the bigger the "Sproingg!" when it breaks.) Thank goodness for spectra. I once made tensegrities using 8' bamboo poles and nylon rope. When rolled down the mountain they would pick up speed and start to bounce. Very impressive." WV also made some cellular tensegrity kites using arrow blanks and 60 lb test spectra rope.

Hammock Morphology

The design focus in an N strut stand to support N hammocks, with a shade tarp over the top and a stove in the center. The structure would be back-carryable, meaning that some of the tendons would be detachable. The design is based on the 3 strut tensegrity tripod. The design is based on the following ratios:
A) Three struts and nine tendons (i.e. - the top and
bottom faces are triangles) The strut/tendon ratio = 1/0.68125
B) Four struts and twelve tendons (i.e. - the top and bottom faces are squares) The strut/tendon ratio = 1/0.6436
C) Five struts and fifteen tendons (i.e. - the top and
bottom faces are pentagons) The strut/tendon ratio = 1/0.60843

With a sufficiently wide base, it should be stable and still allow enough free space to hang the hammock.

Hammock Materials

Struts: steel pipe, as sold for chain link fence, and aluminum poles in 10' length. They are also trying bamboo, and military surplus synthetic sectional segmented antenna poles that are 11.5 feet long and weigh 6.4 lbs each, or carbon fiber (ultra lightweight, just 3 lb., but expensive, about $300 per pound). Aluminum: For example, texastowers.com offers 12' drawn aluminum tubing. In the prototype below the struts are 123". Sectional: WV wasn't sure the sectional rails would support a hammock, but they seem to be OK with his 150 lbs. weight, with no struts bending, not strain at the joints. MacEntyre raised a model on 1 7/8" diameter struts, multi-faceted and not round. Their unions were crimped, and reinforced with an outer sleeve.

Tendons: Amsteel, or whoopee slings. Or Spectra rope. Spectra is ultra-high molecular weight polyethylene (UHMWPE) fibers yield very high strength and cut resistant ropes that can stand-in for steel cables at one tenth their weight.

Fittings: The end fittings have been a challenge. They have used carabiners at the pole ends for mechanical advantage.

Tools: 2-ton "come along" (hand operated ratchet lever winch). The tendons may be fitted with winches ( like a spanish windlass) for tightening.

Attachment to the ground: when there is a small base, some models need to be staked to the ground. Without staking, there is instability when the load shifts. In other words, for a 3-strut hammock stand, all three occupants would need to board and de-board at the same time.
Misc: the shade structure would drain if the struts were asymmetric.

Stacking Two Tensegrity Modules

If I stacked one on top of another, connecting the struts of the top tensegrity to the midpoints of the top tendons on the bottom tensegrity, then both would need to be under considerably more tension to handle the weight of hammocks without deforming. One reason to try this would be to make the base wide enough that it wouldn't need staking at all. There are probably a number of reasons not to try this, but we won't know what they are until we do. Of course, if we use longer struts the base of a single tensegrity could be big enough to make staking unnecessary.

Connecting The Shade Tarp

While the shade is not essential to the tensegrity, it is important as this stand is likely to be deployed in an area without trees, hence the need for shade. MacEntyre is considering a three cornered tarp, quadrilateral on one side and triangular on the other... use bungie to connect the "ridgeline" reinforcement points of the tarp to "my" hammock's stuts, bungie the triangle side's corner to the opposite strut, with the line you describe running down to a single stake, and guy the quadrilateral side's corners down to stakes in the ground. Each additional hammocker does exactly the same thing with an identical tarp. If you are the only hammocker on the stand, keep a line from the top of a strut to the ground, with a bungee prussic on it to pull the tarp, pull the triangle side down through the middle of the tensegrity stand and guy it to a stake in the ground directly beneath the top of the opposite strut.

Hammock Tensegrity Videos

MacEntyre video tour of two 3-strut tensegrity stands with multiple hammocks, uploaded 4 September 2010 to MollyMacPack's YouTube channel.

Assembling a Tensegrity Hammock Stand

The struts are 123" long, tendons are 7/64 amsteel. There was a lot of calculating and then trial and error tweaking to arrive at the lengths of the tendons and how to connect them, but now that's out of the way, and it's not difficult to assemble.

MacEntyre (at) Yahoo (dot) com

Molly Mac Gear will have a tensegrity stand product soon, complete with segmented aluminum struts, color coded tendons, and a modular tarp


1 comment:

  1. This is really exciting, thanks for your efforts!