Sipralock passes vibration tests

For resistance to vibration, axial-torsional loading, joint fatigue and thermal extremes, a locking thread outperforms standard fasteners in tests at MIT and the Goddard Space Flight Center.

For resistance to vibration, axial-torsional loading, joint fatigue and thermal extremes, a locking thread form outperforms standard fasteners according to tests from top institutions including MIT and the Goddard Space Flight Center.

For engineers combating vibrational loosening, axial-torsional loading, joint fatigue and thermal extremes, conventional fastening methods involving standard 60deg threads have posed design limitations.

Engineers are attacking these problems with a self-locking fastener called Spiralock, the effectiveness of which has been validated in published test studies at institutions including MIT, the Goddard Space Flight Center, Lawrence Livermore National Laboratory and British Aerospace, as well as at companies such as Mack Truck and Dana.

With the standard 60deg thread form, the gap between the crest of the male and female threads can lead to vibration-induced thread loosening.

Stress concentration and fatigue risk at the first few engaged threads are also problematic due to surface contact.

Because the standard thread form is axially loaded, there's an increased probability of shear, especially in soft metals.

And temperature extremes can expand or contract surfaces and materials, potentially compromising joint integrity.

The standard methods used to lock traditional fasteners have their own limitations.

Locking adhesives, for instance, progressively lose effectiveness as temperature rises.

In high volume, their use typically requires a large capital expense to purchase and program robot applicators.

And when reapplication is necessary, cleaning the threads of affected components takes added time and labour before reapplication is possible.

Bolts secured with single-use, drypatch adhesive - activated when the bolts are tightened - can similarly add to assembly, maintenance or warranty costs.

This is because, once used, the bolts must be replaced for any necessary rebuilds or maintenance.

Affected internal threads must also be cleaned before new bolts with drypatch adhesive can be applied, adding to time and labour costs.

To resolve these design problems, testing at leading institutions and corporations has demonstrated the effectiveness of a locking fastener called Spiralock.

Spiralock has re-engineered the standard 60deg thread form, adding a 30deg wedge ramp at the root of the thread which mates with standard 60deg male thread fasteners.

The wedge ramp lets the bolt spin freely relative to female threads until clamp load is applied.

The crests of the standard male thread form are then drawn tightly against the wedge ramp, eliminating radial clearances and creating a continuous spiral line contact along the entire length of the thread engagement.

This continuous line contact spreads the clamp force more evenly over all engaged threads, improving resistance to vibrational loosening, axial-torsional loading, joint fatigue and temperature extremes.

In dynamic and static testing by Goddard Space Flight Center, Spiralock nuts were tested under vibration and static load conditions.

The most severe vibration tests did not loosen the nuts when subjected to high amplitude and sine random testing.

The Tinius Olsen tension machine was used to pull the bolts in tension and calibrate the strain in the bolt for a given load.

Static tests were performed to measure gapping and to determine the friction constant for both the lubricated and unlubricated nuts.

"Preliminary tests in vibration clearly demonstrated that these nuts would not back off under the vibration specifications listed for the space shuttle," states the report.

"Further vibration tests in sine and random were applied that were ten times as strong and ten times as long as the shuttle specifications, and they did not back off".

Vibration testing of Spiralock wire thread inserts by British Aerospace confirmed the resistance to vibration.

Testing was done on an Unbrako fastener vibration machine using M6 x 25mm grade eight bolts with wire inserts in L168 aluminum, and 20 samples of Spiralock wire inserts and standard 60deg wire inserts also known as helicoil screwlock wire thread inserts.

These were tested at 13.6Hz and tightened to 816kg of preload.

Results showed that Spiralock wire inserts yielded consistent vibration resistant performance with an average preload loss of 15%, while the standard wire inserts yielded erratic results, losing from 22% to 95% preload given the same test parameters.

"The results prove that, at one set of experimental conditions, Spiralock has a far greater resistance to transverse vibration and requires a high final removal torque," says the report.

"Under vibration, the ability of the inserts to assist in resisting vibration was clear".

"All 20 Spiralock results consistently gave a 15% preload loss over the first 1000 cycles followed by a 1% loss over the next 5000 cycles - the helicoils, in contrast, were erratic, at best a loss of 22% after 1000 cycles holding to 39% at 6000 cycles and at the worst losing preload very rapidly, a 90% loss in 500 cycles and a 95% loss at 6000 cycles".

A finite element analysis report prepared for Lawrence Livermore National Laboratory compared a spacecraft node with strut threaded connection using 10-32 UNF to Spiralock female thread.

In analysing axial thread load distribution, separate analysis was done to compare the load distribution for each thread of a Spiralock with a UNF.

The boundary condition on both nuts was changed to constrain the face of the nut in the axial direction, but free to move in the radial direction.

"The fine mesh of the Spiralock threads with more detail in the contacting surfaces was used because it can capture the local deformations and plastic flow at the contact points," says the report.

"The first thread of the Spiralock nut supports 12.5% of the total load at the assembly preload of 786kg and no thread supports less than 8% of the total load".

The report goes on to compare loading between the Spiralock and UNF standard nuts.

"The first two threads of the UNF nut each supports 21% of the total load - and the last three threads each support 4% or less of the total load," says the report.

"It is clear that at 786kg, the first few UNF threads are highly stressed".

"Each of the ten Spiralock threads has a high local stress at the points of contact".

"The Spiralock thread form clearly distributes the load more uniformly throughout all of the threads than the UNF thread form does".

"The thread bearing stresses in the Spiralock threads are concentrated at the sharp points of contact where there is localised yielding in compression".

"These high local compressive stresses do not cause a strength problem, but the local yielding allows a more uniform load distribution on each thread".

A report by the Massachusetts Institute of Technology for Chrysler similarly studies vibration resistance and stress distribution in threaded fasteners.

It compares the Spiralock thread form (on the nut) with the standard 60deg thread form, both in combination with a standard 60deg thread form bolt, on two counts; the load and stress distribution on the threads, and the resistance to relative rotation between the nut and the bolt.

According to the report, calculations show the total bolt load is more evenly distributed over the engaged threads for Spiralock than form than the 60deg thread form.

The maximum stresses at the root of the bolt thread are of the same order of magnitude in both cases, and the movement required for relative rotation is significantly higher for Spiralock.

"The load distribution is significantly more uniform in the case of the Spiralock thread form than in the standard 60deg thread form," says the report.

"The first engaged thread carries only 18% of the load in the case of Spiralock, versus 34.1% in the case of the standard 60deg thread form".

Regarding stress loading, the report says: "The stresses in the case of the Spiralock thread form calculate out lower than those in the case of the regular thread form".

"This is in spite of the fact that the load goes on the end of the bolt rather than being distributed over it".

"The reduction can be attributed to the more uniform load distribution".

Some manufacturers have also weighed in on the issue.

In design testing to boost reliability, performance and assembly effectiveness, Dana compared the clamp force retaining ability of locking adhesive with that of Spiralock locking fasteners.

In an impact-durability test to examine torsional fatigue, at least 3000 back and forth cycles at maximum vehicle torque load were carried out.

"It was the equivalent of putting the vehicle in neutral and flooring the gas, first forward, then reverse, thousands of times," said Frank Metelues, Design Engineer at Dana.

A Dynamometer Test also simulated extreme use over the life of a vehicle with varied torque speeds.

"In both tests, Spiralock locking fasteners demonstrated 15 to 20% better clamp retention than traditional locking adhesives," said Metelues.

"The bolts using Spiralock fasteners did not back out".

"The design distributes load more evenly than traditional threads which minimises thread yielding and deformation, while the wedge ramp helps prevent torque and axial loads from backing the bolts out".

"And unlike adhesive, whose locking effectiveness degrades at higher temperature, the design exhibits significant temperature resistance".

Spiralock's vibration resistance characteristics are not influenced by extreme temperature changes as long as they are within the engineered working limits of the materials used.

Todd Werner, Design Engineer at Mack Trucks, found benefits for high-temperature diesel engine applications.

"During a particular engine durability test, the fasteners were exposed to temperatures as high as 700C, which is hotter than normal operating temperatures," said Werner.

"The engine was then rapidly cooled every 12 minutes for 3000 hours".

"Upon inspection every 250 hours, the Spiralock fasteners maintained joint integrity without losing torque for 15,000 cycles".

"After their adoption, none have failed in the field to my knowledge".

"They're not only self-locking but also re-usable during service without damage to the nut or stud".

"The Spiralock fasteners are now used on every Mack turbocharger mount across our vocational truck line and on the EGR valve mount on our highway truck line".

Production changeovers to Spiralock fasteners are typically quick and seamless, usually requiring an exchange of traditional nuts, wire inserts or drilling out and re-tapping existing parts stock that have standard tapped holes.

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