Fasteners from Spiralock Corp., Madison Heights, Mich. (spiralock.com), just made a 2-billion mile journey through space. The company's threaded, hold-tight fasteners were used on both the NASA Cassini orbiter and the Huygens probe which successfully parachuted to the frozen surface of Titan, Saturn's largest moon. The fasteners made sure several cavities on both spacecraft remain sealed and vacuumtight despite a seven year journey through space with all the associated temperature extremes, shock, vibrations, and the 100-mph/sec deceleration upon arrival..
Conventional fasteners, such as adhesives, nylon rings, deformed threads, and other traditional methods of preventing joints from loosening, couldn't handle the vibration and high temperatures of launch. "We needed the most reliable locking engagement threads," says Dan Harpold, a NASA scientist. "Screws had to remain tight, and there would be no opportunity for retightening them. With conventional threads, however, screws loosen and back out under testing."
The Spiralock fastener has a 30° ramp cut at the root of the female thread. Under clamp load, the crests of the threads on standard male bolts are drawn tightly against the ramp. This eliminates side motions that lead to loosening and it distributes joint load on all engaged threads, a claim supported by research conducted at the Massachusetts Institute of Technology.
In other studies, the proportion of the load carried by the first engaged thread was significantly lower than on standard threads, which reduces possible bolt failures. The thread also handles thermal expansion and contraction without slippage. NASA conducted a series of about 12 high-temperature "bake outs," where screws and matching internal threads were heated from room temperature to 300°C to simulate temperature-induced thread loosening.
"But once torqued down properly, the screws stayed put in the threads, which helped us meet our flight schedule," says Harpold. "To date, not one has come loose."