By the time the Cassini-Huygens spacecraft entered Saturn''s orbit in July 2004, it had endured -- for the first time in human history -- not only the vibration, shock, and temperature extremes of a Titan IV rocket launch, but also a 7 year journey from Earth across more than 750 million miles of space. The Huygens probe''s journey included diving into the murky atmosphere of Titan, Saturn''s largest moon, measuring atmospheric composition all the way down to Titan''s frigid surface.
To achieve accurate atmospheric measurement of both Saturn, from the Cassini orbiter, and Titan, from the Huygens probe, several hundred bolts had to maintain vacuum-tight sealed cavities for the duration of their mission, with no thread loosening or stripping.
Whether the goal is to work on a construction site, get to the next sand dune, or travel to Saturn, operators depend on quality vehicles that perform as expected time after time. Yet for all today''s high tech components, simple fasteners and thread forms must still hold vehicles together reliably and functionally. When components loosen, rattle, or fail, the problem can often be traced back to the lowly threaded fastener, a technology basically unchanged for centuries.
To keep threaded fasteners tight, traditional techniques include split washers, prevailing torque nuts, deformed threads, nylon plugs, and chemical bonds. But these approaches not only have limited effectiveness against vehicular vibration and shock but also can significantly add to total costs through increased warranty and service repair, more complex assembly, and a general lack of reusability. These traditional fastening methods, in fact, are manufacturing "Band-Aids" that fail to get to the heart of the problem -- the thread itself.
Solving Fastening Problems with a New Thread Design
Through the physics of the thread itself, Madison Heights, MI-based Spiralock Corp has introduced a new thread form designed to address the fastener loosening and stripping problem caused by vehicular vibration, shock, and temperature extremes. The secret to Spiralock''s thread form is a 30 deg "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. This causes thread contact forces to be applied at approximately 60 deg from the bolt axis, rather than 30 deg away as in a standard thread form. The mechanical advantage -- the angular relationship between the unique 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.
Research studies at both MIT and the University of Michigan confirm that the percentage of the load carried by each engaged thread produced with a Spiralock tap is much more uniform than with conventional 600 thread forms. More importantly, the studies show that the percentage of load on the first engaged thread produced with a Spiralock tap is significantly lower. As a result, Spiralock thread forms eliminate intense concentration at the first engaged thread, thereby reducing bolt failures and improving product performance.
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. This dramatically increases the holding power of any standard male fastener, without excessive torque or messy friction additives.
Threads Withstand Blast-Off Vibration and Temperature
NASA was one of the first to appreciate the advantages of the new thread when designing the main engines of the Shuttle orbiter. Each of the three main engines develops 400,000 lb of thrust and terrific vibration. But the Space Agency also wanted a 15-cycle reuse capability per fastener.
Under its own test, NASA determined that the fasteners in Spiralock-threaded holes did not back off or loosen when subjected to 10 times shuttle-specified vibrations, and they stayed that way 10 times longer than called for. As far as its 15-cycle reuse capability was concerned, NASA tests found the Spiralock-thread fasteners delivered 50 uses, with no loss of clamping power. To this day, every shuttle engine carries no fewer than 757 Spiralock fasteners.
For atmospheric measurement of Saturn and Titan on the current Cassini-Huygens mission, NASA used the Spiralock internal thread form to resist vibration and temperature-induced thread loosening on mass spectrometer instrumentation. In the orbiter and probe, several hundred fasteners had to maintain vacuum-tight sealed cavities from final assembly and testing through launch, until the end of the 7 year mission.
"To survive the vibration and high temperatures of launch, we required the most reliable locking engagement thread," said 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 "bakeouts," where screws and their matching internal thread forms were heated from room temperature to 3000°C to simulate temperature-induced thread loosening.
"The Spiralock thread form retained a tight seal at 3000°C," 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."