Self-deployable octa photos now available

[Spencer W Hunter]

New public-domain pictures of my self-deployable octas are now up in
the Tensegrity Blocks directory at:

http://www.u.arizona.edu/~shunter/tblock.html

Under "Self-deployable octahedra," here's a description of the models:

On the left is the tendon-equator octa, with an elastic band running
down the middle to effect self-deployment, consisting of eight hinged
struts and four equatorial tendons. It was inspired by Tim Tyler's
compression ring, the illustration of which is linked from the
Tensegrity Blocks directory.

In the middle is my first attempt at Le Ricolais' octahedron,
consisting of six struts joined at the center by lashed paper-roll
connectors, two inelastic-tendon depolarized triangles, and six
elastic tensioning tendons to form a collapsible and self-deployable
anti-prism in the spirit of Wang's cable-strut modules.

After noticing how similar the strut configuration was to Piñero's
pantograph (whose patent illustration is also linked from the
Tensegrity Blocks directory), I rebuilt the octa as a pantegrity,
shown on the right, consisting of three double-length struts joined at
the center by an elastic band, along with the depolarized triangles of
inelastic tendons and six elastic tensioning tendons.

An effort was made to make the struts in the first two models and the
tendons in all three models the same length, with the struts being
shorter than the tendons.

Under "Close-up, pantegrity octa," I show a closer view of the model
on the right, where it can clearly be seen that the struts are
double-length and are bound at the center.  This is a kind of "poor
man's pantograph," since in a real pantograph, the struts would
actually be connected to each other.  One might even argue that it's a
tensegrity, since the structural integrity of the three struts by
themselves rely on the tension of the elastic band that binds them
together.

It has occurred to me that, rather than being the "entirely new hybrid
structure" as I had described it, the pantegrity has actually been
with us for uncounted centuries.  If one looks at Kenneth Snelson's
original X-module at:

http://www.kennethsnelson.net/new_structure/images/page14_03.jpg

...it can be seen as the tensegrity most of us know as the "diamond
kite."  If the tensioning kite skin is removed, the remaining struts
have no integrity and fall apart.  Almost anyone who has built and
flown such a kite will quickly attest, though, that an essential
component is missing--that the struts are usually lashed together at
the center.  This has two important effects: first, the connection
stabilizes the central node during flight; and second, the connection
effectively doubles the strength of each strut, since they mutually
support each other at the center.  Without the tensioning kite skin,
the struts still remain together as they are lashed; i.e., they
comprise a crude pantograph.
--
Spencer Hunter, Tucson, AZ
gopher://www.u.arizona.edu:80/hGET%20/%7Eshunter