Threads get a grip on shock, vibration

Jan. 8, 2004
Unconventional fasteners seal interplanetary probe
Spiralock's 30° wedge-ramp female thread securely connects standard male fasteners.
Photoelastic comparison of conventional (left) and wedge-ramp threads shows the latter exhibits more-uniform loading of each engaged thread.

Jim Gillis
Madison Tech Center
Spiralock Corp.
Madison Heights, Mich.

When the Cassini-Huygens spacecraft goes into orbit around Saturn next July, it will have endured not only the vibration, shock, and temperature extremes of a Titan IV rocket launch, but also the seven-year, 750 million mile journey from Earth. Later, the Huygens probe will dive into the murky atmosphere of Titan, Saturn's largest moon, measuring atmospheric composition all the way down to the frigid surface. Despite the harsh conditions, several hundred fasteners must maintain a vacuum-tight seal or else the mission is doomed.

To meet this challenge, NASA engineers turned to an unconventional internal thread design that has helped manufacturers combat thread loosening and stripping in a wide array of tough earthbound applications.

A thread form developed by Spiralock addresses loosening and stripping caused by vibration, shock, and temperature change. It differs from that of conventional fasteners in that a 30° "wedge" ramp is cut at the root of the female thread.

The ramp lets the fastener spin freely until clamp load is applied. At that point, crests of the standard male thread are drawn tightly against the ramp, eliminating radial clearances and creating a continuous spiral line of contact along the entire length of engagement.

Thread contact forces are thus applied at approximately 60° from the bolt axis, rather than 30° away as in standard threads. The mechanical advantage -- the angular relationship between the ramp and male thread -- restricts bolt and screw movement. This eliminates transverse motion that loosens threads under vibration. It also spreads the clamp force more evenly over all engaged threads, reducing the risk of fatigue failure and increasing joint integrity. This means the thread increases the holding power of any standard male fastener, without excessive torque or messy friction additives.

Research at the Massachusetts Institute of Technology and University of Michigan confirm the percentage of load carried by each engaged thread produced with a Spiralock tap is much more uniform than with conventional 60° threads.

More importantly, the studies show the percentage of load on the first engaged thread is significantly lower. Eliminating load concentration on the first engaged thread reduces failures and improves overall performance.

Extensive laboratory tests comparing three types of female threads -- a standard nut, prevailing-torque locknut, and Spiralock nut -- also demonstrated the latter better resists vibration loosening. Spiralock threads can be removed and retightened with no appreciable loss of holding power. And they do not need add-on locking components such as lockwashers, thread adhesives, crimping, or inserts.

For the Cassini-Huygens mission, NASA chose Spiralock internal threads to resist vibration and temperature-induced thread loosening on mass spectrometer instrumentation. Several hundred fasteners in the Cassini orbiter and Huygens probe had to maintain vacuum-tight seals from final assembly and testing through launch to the end of the seven-year mission.

"To survive the vibration and high temperatures of launch, we needed the most-reliable locking-engagement thread," says NASA scientist Dan Harpold. Screws had to remain tight and there would be no opportunity for retightening. With conventional threads, screws tend to loosen and back out under testing, he adds. These tests included a series of high-temperature "bake outs," where screws and matching internal threads were heated to simulate temperature-induced thread loosening.

"The Spiralock thread retained a tight seal at 300°C," says Harpold. "Once torqued properly, the screws stayed in the threads, which helped us meet our flight schedule. To date, not one has come loose."

The threads are also used on fasteners for ATVs, motorcycles, skid loaders, trucks, buses, and many other applications.

Make Contact:

Spiralock Corp., Madison Tech Center, Box 71629, Madison Heights, MI 48071(800)

Traditional limitations

For all the high-tech componentry they carry, today's vehicles must still hold together reliably with relatively simple fasteners. When components loosen or fail, the problems often trace back to a lowly threaded fastener.

In these cases, vibration, shock, or temperature is often the culprit. Loose fasteners drive up warranty and service costs, impair quality and safety, and raise liability issues. Fastener assembly costs are also a concern, as well as field-servicing when the fasteners must be removed and reinstalled.

To keep threaded fasteners tight, engineers have traditionally used split washers, prevailing-torque nuts, deformed threads, nylon plugs, and chemical bonds. But these approaches are only partially effective against vibration and shock, and 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 are, in fact, manufacturing "Band-Aids" that fail to get to the heart of the problem -- the thread itself.

Threaded fasteners inherently need some clearance to permit easy assembly of male and female threads. Design and manufacturing tolerances must also allow for a variety of unavoidable problems such as drill, tap, and die wear that cause hole-size and thread-finish variations. As a result, engaged threads have clearance between the crest and root of each male and female member.

Unfortunately, radial clearances between traditional male and female 60° V-shape threads permit relative sideways or lateral movement under shock, vibration, or transverse loading. Sideways movement in conventional threaded holes reduces locking friction between the thread flanks. As this happens, the tension (load force) in the male fastener generates self-loosening rotational movement. Moreover, conventional 60° V-threads put most of the clamp load on the first engaged thread. This lets subsequent male threads virtually "float" within the female threads, and can lead to stripping or shearing problems.

Tests show the first two threads carry as much as 80% of the load. That is because applied loads stretch the male fastener between the head and first engaged thread. As clamp load increases, the second thread takes some of the load. In many cases, however, the force required to transfer a significant amount of the load to the third and fourth threads begins to shear or strip the first thread.

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