This January, after a 7-year, 2-billion-mile journey strapped to the side of the Cassini orbiter, the Huygens probe decelerated 12,000 mph in less than 2 minutes before parachuting to the frozen surface of Titan, the largest of Saturn's moons. On the way, it took various atmospheric measurements after which it photographed the surface.

Throughout this process, as well as during initial launch back at on Earth, both the probe and orbiter endured tremendous shocks, vibrations and temperature extremes. This posed a serious threat to, among other components, the vacuum-sealed cavities on each spacecraft containing mass spectrometer instrumentation.

NASA, which built the orbiter, and the European Space Agency, which built Huygens, solved this problem by using dozens of threaded fasteners from Spiralock Corp. (Madison Heights, MI). Specifically, the two spacecraft incorporated the Spirlock thread form into the threaded holes surrounding the sealed components. This thread form includes a 30-degree "wedge ramp" at the root of the female thread. Under clamp load, the crests of the threads on any standard male bolt are drawn tightly against this feature, effectively locking the assembly.

Previously, NASA had used Spiralock thread forms when building the main engines for the space shuttle. Although each of the three main engines develops 400,000 pounds of thrust and a terrific amount of vibration, the space agency wanted threaded fasteners that could be reused up to 15 times.

Using its own tests, NASA determined that the fasteners in Spiralock-threaded holes did not back off or loosen when subjected to 10 times the shuttle-specified vibrations. They also stayed tight 10 times longer than called for and could be removed and reattached 50 times with no loss in clamping power. To this day, every shuttle engine carries no fewer than 757 Spiralock fasteners.

"To survive the vibration and high temperatures of launch, we required the most reliable locking engagement thread," says Dan Harpold, a NASA engineer who contributed to the Huygens project. "Screws had to remain tight without opportunity for retightening. With conventional threading, however, screws loosened up and backed out under testing."

Among the tests carried out were a series of high-temperature "bake outs," in which screws and their matching internal thread forms were heated from room temperature to 300 C to simulate temperature-induced thread loosening.

"The Spiralock thread form retained a tight seal," says Harpold. "Once torqued down properly, the screws stayed put in the threads, which helped us meet our flight schedule. To date, not one has come loose that I'm aware of."

In building the sealed cavities, NASA also considered using adhesives, deformed threads, nylon rings and prevailing torque nuts. However, each of these methods fell short when faced with the extreme thermal expansion and contraction to which the spacecraft are submitted.

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