The Cassini-Huygens spacecraft uses a Spiralock thread form to survive the vibration and high temperatures of a rocket launch, as well as the demands of a seven-year mission.

When the Cassini-Huygens spacecraft entered Saturn’s orbit this summer, it had to be able to handle the vibration, shock, and temperature extremes of a rocket launch. In addition, the Huygens probe has to dive into the atmosphere of Titan, Saturn’s largest moon, to measure atmospheric composition all the way down to the frigid surface. For the Cassini orbiter and Huygens probe to be successful, several hundred bolts must maintain vacuum-tight sealed cavities for the entire seven-year mission, with no thread loosening or stripping.

Madison Heights, Mich.-based Spiralock Corp. had the answer: A new thread form with a 30° “wedge” ramp cut at the root of the female thread. Under clamp load, the crests of the threads on any standard bolt are drawn tightly against the wedge ramp. Thread contact forces are therefore applied at approximately 60° from the bolt axis, rather than 30° as in a standard thread form. The angular relationship between the 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 the engaged threads.

The secret to Spiralock’s thread form is a 30° wedge ramp cut at the root of the female thread. Under clamp load, thread contact forces are applied at approximately 60° from the bolt axis, rather than 30° as in a standard thread form. The angular relationship between the wedge ramp and the male thread restricts bolt or screw movement.

The wedge ramp lets the fastener 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.

For atmospheric measurement of Saturn and Titan on the Cassini-Huygens mission, NASA used the Spiralock internal thread form to resist vibration and temperature-induced thread loosening on mass spectrometer instrumentation.

“To survive the vibration and high temperatures of launch, we needed the most reliable locking engagement thread,” said Dan Harpold, a NASA scientist who worked on the project. “Screws had to remain tight because there is no 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,” 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.”

Lasers go industrial

Ladar generates millions of points to form what is called a point cloud, such as this 3D image of a forest. NIST is developing methods based on Ladar and other sensor information for autonomous vehicle navigation.

Lasers, already used for everything from price scanning at supermarkets to eye surgery, could change largescale manufacturing, remote sensing, defense, and construction industries. A new report from the National Institute of Standards and Technology (NIST) predicts “tremendous” applications for laser scanning devices known as Ladars (Laser Detection and Ranging) and pushes for the creation of next-generation Ladar — a coffee-cup-size device with millimeter accuracy.

Industry has used Ladar, which creates 3D images of areas and objects, since the late 1970s. But advances in microchip lasers, optics, microelectromechanical systems, and computers have increased Ladar’s data-acquisition speed, range accuracy, and reliability, as well as reduced its size and costs. Ladars are now used to generate topographic images, survey the depths of large bodies of water, and as 3D documentation of construction when building plans aren’t available. Manufacturers are also beginning to use Ladar to recreate critical machine components from single examples.

NIST is testing Ladar for navigating unoccupied military vehicles, which could lead to collision-avoidance advances for civilian automobiles. To encourage industrial use, NIST is working to develop test objects for Ladar performance standards to instill confidence in laserscanning readings and system comparisons.

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