Building a tug-of-war machine

Jan. 10, 2008
An aluminum structural-frame assembly lets scientists, engineers, and space enthusiasts test their material might in a race to develop the world’s strongest tether

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,

Pacific Integrated Handling, (888) 305-8789,

Spaceward Foundation, (560) 969-2010,

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

The “tug-ofwar” tether machine lets two samples be tested simultaneously.

Extrawide T-slots and larger center bores support heavier connectors — saving time and money by requiring fewer fasteners.

Bosch Rexroth uses quick-load connectors that fit anywhere along the T-slot, making it easy to add accessories or fixtures anywhere to make a multifunctional structure.

The original tether-pull machine frame was constructed with a horizontal orientation. For the 2007 games, the frame was redesigned to a vertical orientation for reduced friction, greater safety, a more compact footprint and a better visual indicator of the winning tether.

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