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Fasteners hold tight 750 million miles away from home

Feb. 1, 2004
A special internal thread form combats vehicle vibration, shock, and temperature extremes found on Earth as well as on the frigid surface of Titan, Saturn's largest moon.

Fasteners using a special internal thread form developed by Spiralock ensure the Cassini-Huygens spacecraft withstands the vibration and temperature extremes associated with a 7-yr, 750-million-mile mission to Saturn. Artwork courtesy of NASA.

A special internal thread form combats vehicle vibration, shock, and temperature extremes found on Earth as well as on the frigid surface of Titan, Saturn's largest moon.

Introduced by Madison Heights, Mich.-based Spiralock Corp., this thread form uses a 30° "wedge" ramp cut at the roots of female threads. Under clamp load, the crests of the threads on any standard bolt are drawn tightly against the wedge ramp, applying thread-contact forces at approximately 60° from the bolt axis, rather than 30° away as in a standard thread form. The mechanical advantage — the angular relationship between the special wedge ramp and the male thread — restricts bolt or screw movement.

The wedge ramp not only eliminates the transverse motion that causes loosening under vibration but also distributes the loads of the threaded joint throughout all the engaged threads. In fact, research studies at both the Massachusetts Institute of Technology and the University of Michigan confirm that the percentage of the load carried by each engaged thread is more uniform than with conventional thread forms. In addition, the percentage of load on the first engaged thread produced with a Spiralock tap is significantly lower. Hence, Spiralock thread forms eliminate intense concentration at the first engaged thread, reducing bolt failures.

The wedge ramp allows the fastener to spin freely until clamp load is applied. At that point, the crests of the standard male thread form are drawn tightly against the wedge ramp, eliminating radial clearances and creating a continuous spiral line of contact along the entire length of thread engagement. This spreads the clamp force more evenly over all engaged threads, reducing fatigue failure and increasing the integrity of the threaded joint. The result increases the holding power of any standard male fastener, without excessive torque or messy friction additives.

The Cassini-Huygens spacecraft, which is scheduled to enter Saturn's orbit in July, employs several hundred bolts using the Spiralock thread form. These fasteners must maintain vacuum-tight sealed cavities, with no thread loosening or stripping, from final assembly and testing through launch, until the end of the seven-year mission. "To survive the vibration and high temperatures of launch, we required the most reliable locking engagement thread," says Dan Harpold, a NASA scientist who worked on the 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 about 12 high-temperature "bake outs," where 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 at 300° C," remarks 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."