Jean M. Hoffman
Senior Editor
With a NASA Centennial Challenge
purse of $200,000 up for
grabs at the Elevator:2010 Strong
Tether contest in 2006, one would
likely assume that the parts for
building the competition’s tetherpull
machine would have been
finalized months in advance. But
that wasn’t the case, says Jay Blum,
a Southwest branch manager for
Pacific Integrated Handling, a
Bosh Rexroth Corp. distributor
located in Tempe, Ariz. Blum was
on the receiving end of a call for a
last-minute order from Ben Shelef,
aerospace engineer and cofounder
and leader of the executive crew of
the Spaceward Foundation, the
event’s host.
The Spaceward Foundation
team based in Mountain View,
Calif., was on its way to the competition
in Albuquerque and had
not yet finalized the project parts
list. “When they called for assistance,
they told us they would be
passing through Phoenix on their
way to the competition and would
appreciate any help we could give
them,” said Blum. “We were able to
have the aluminum extrusions machined
and kitted in less then 24 hr
to accommodate their trip through
Phoenix.
“Aluminum structural framing
met our performance criteria (loads
are less than 1 ton) while letting us
assemble the tether-pull machine
quickly and easily,” says Shelef.
The extruded aluminum and bolttogether
connections were also
recycled when we overhauled the
unit for the ’07 challenge. “We also
wanted to give it a more compact
footprint than its predecessor and make it easier to see which tether
‘gives’ more,” Shelef says. It’s also
safer. The mechanism now runs
vertically rather than horizontally.
This orients the strains towards the
floor and ceiling and away from the
audience. And finally, the aluminum
assembly gives the audience a
clear view of the competition.
Tether-pull machine
During the competitions, two
competing tethers connect to the
pull machine end to end. Both are
stretched using a hydraulic piston
until the tether breaks. “The pulling
device is designed to run a comparative
test between two tether
samples,” says Shelef. “One of the
decisions made early in the program was that we wanted to make
tether testing an exciting business
to watch, so we created a head-tohead
strength competition.”
For this purpose Shelef and his
team designed what is essentially
a tug-of-war machine. The tetherpull
machine constrains two tether
samples at one of their ends. It pulls
on the other ends with equal force
using a single hydraulic actuator
and a pivot arm. As the force rises,
one side will break first. The remaining
tether, along with the pull
mechanism, lunges the other way
and clearly indicates a winner.
The tether machine is a rectangular
box about 12-ft long and
18-in. high on each side. Using bolttogether
connectors from Bosch Rexroth Linear Motion and Assembly
Technologies, Buchanan,
Mich., the tether pull structure easily
assembles without special tools
or skills. The extruded aluminum framing looks clean
and attractive without
painting or other finishing.
The team could
also choose from numerous
accessories to
extend the machine
beyond a simple frame
and base to a complete
multifunctional
structure. And, every
aluminum structuralframing
component is
reusable, which made
it simple for the team
to make changes as
designs developed.
Building the BOM
The bill of materials
(BOM) for the tetherpull
machine included
two of the Bos c h
Rexroth 45 S er ies
aluminum extrusions
along with a range of
connectors and accessories.
At the heart of
the aluminum structural-
framing system
is a T-slot design: Insert
a T-bolt into a
T-slot and tighten.
No welding or grinding
is needed. Frames
assemble quickly and
easily using a few simple
hand tools. T-bolts
have an alignment
mark on the end of the
threads to indicate position in the
T-slot. The mark set perpendicular
to the T-slot shows that the T-bolt
is locked into position.
The high-strength extruded
aluminum 45 Series profiles used
in the contest have a natural-color,
anodized surface that’s scratch and
corrosion resistant. The machined
45 45-mm (1.77 1.77-in.),
profile is a general-purpose,
thicker-walled profile for strong,
rugged performance. It features
four 10-mm T-slots. The 45
90-mm (1.77 3.54-in.), profile is
heavy duty. It features six 10-mm
T-slots.
Gussets are die cast and sized to fit the profile series. They feature
elongated holes to ease positioning
along the aluminum extrusion
and alignment tabs to ensure
quick positioning in the profile’s
T-slot. Alignment tabs can be easily
removed with a flat-tip screw
driver for cross connection of profiles.
For the tug-of-war machine,
Spaceward chose 45 45 gussets
with zinc-plated steel fasteners and
optional polyamide 6 cover caps
to spruce up its looks. For designs
that connect two profiles with
60-mm side dimensions, there are
offset blocks that help align gussets
with the profile’s outer edge.
T-nuts are manufactured with a slightly curved neck
so the nuts can selfalign
in the profile’s
T-slot. The nuts also
feature ridges that bite
through the aluminum
extrusion’s anodized
surface. This
gives the assembly
better vibration resistance
and reduces
the likelihood of electrostatic
discharge
(ESD). Rexroth also
offers polyamide 6
i sol at ion c aps for
8-mm T-nuts that will
isolate electrical devices
such as light fixtures
or outlet strips
from sensitive ESD
components also attached
to the profile.
Connection screws
can assemble five 45
45 or 45 45H profiles.
They don’t block
the profile T-slot so
they can be used for
guarding and enclosure
applications. The
oversized connection
screws have a large
surface contact area
for extreme strength.
Antirotation
T-blocks help keep the
assembly from twisting
at the connection site.
Corner cube kits
connect two or three profiles and
come with black polyamide cover
caps that give the assemblies a
neat, finished appearance.
Make contact
Bosch Rexroth – Linear Motion
and Assembly Technologies,
(800) 739-7684,
www.boschrexroth-us.com/framing
Pacific Integrated Handling,
(888) 305-8789,
pacificintegrated.com
Spaceward Foundation,
(560) 969-2010,
spaceward.org/elevator2010.htm
2008 Elevator: 2010
Strong Tether
contest
The $900,000 purse in the 2008
Elevator: 2010 Strong Tether contest
will go to the first team that
develops a nanocarbon-reinforced
composite tether (cable) that can
beat the current “house tether.”
The innovations necessary to win
the Strong Tether challenge’s headto-
head tug-of-war will help pave
the way toward a tether strong
enough for machines to climb with
payloads of 15 tons from the Earth
surface to geosynchronous orbit.
Another $900,000 will go to
a team that can get its climbing
machine (powered by electricity
generated from solar panels and a
ground-based booster light beam)
up a tether at a minimum speed of
2 m/sec (6.6 fps). A high-performance
prize goes to teams that can
move at 5 m/sec (16.5 fps). (The
2006 climber-bot challenge was
featured in Wireless energy transmission:
New fuel for space missions,
in the March 08, 2006 issue
of Machine Design). Registration
for the 2008 games opens this
month. |
The Cat’s out of the bag and the race is on
During the 2007 Strong Tether challenge, the defending 2006 champion Team Astroaraneae took on, Team Delta-X from MIT, newcomers to the competition. The reigning champions led by Michael Remington of Aerojet
Corp. used the same tether as in ’06, which according to event host, Spaceward Foundation, came close to the
theoretical spec of its core fiber (or so Spaceward estimates, because the true make-up of the tether is confidential).
The MIT contenders, lead by Stephen Steiner, featured a Carbon Nanotube (CNT) tether — the material that
NASA and Spaceward expect to far surpass all other existing materials. The 2-gm CNT was reportedly fresh out
of the furnace.
This gave team Delta-X no time to form it into the required shape of a closed loop. Instead they could only tie
it in a knot, fully knowing that the knot would fail. Predictably, the knot slipped off before the material was even
properly stretched. Next year, Spaceward says, they are looking forward to having two CNT tethers go up against
the house tether with its 3:2 weight advantage. “If the CNT contenders can perform with a specific strength 50%
better than the commercial state-of-the-art, it would be a significant milestone on the way to the Space Elevator.”
But that’s not the end of the story, says MIT’s Steiner. “Recently, Alan Windle at the University of Cambridge
announced the development of 20-GPa yarns derived from nanotube yarns. These materials contain graphitic
hyperfilaments composed of nanotubes, which exhibit strengths comparable to an individual nanotube but over
macroscopic length scales.”
According to Steiner, the MIT team has been working on the production of these materials for some time. Independently,
he says, they developed the same processing technique as the Cambridge researchers, but with their
own twist. In fact, he says, they are not only producing yarns, but also ribbons. Steiner feels cautiously optimistic
about the possibilty of a space elevator. “However,” he says, “these recent results from our team and the Cambridge
researchers have strengthened MIT’s resolve to push this technology to the limit. I think it is quite possible that we
could see 70-GPa yarns within a decade.” The impact of CNT advancements, he says, is more profound than just
the space elevator. “Nanocomp materials are going to change the way we think about strength.” To keep abreast of
team Delta-X’s race for the 2008 prize check out Stiener’s blog postings at teamdeltax.com |