News, views & analysis blogs by: Karim Shah





Subject

Karim Shah :
For engineers combating vibrational loosening, axial-torsional loading, joint fatigue, and thermal extremes, conventional fastening methods involving standard 60 degree threads have posed design limitations for too long and proven solutions, backed by test data, have been relatively nonexistent.


Article:

Karim Shah :
New Test Data Offers Resolution for
Toughest Engineering Challenges
For resistance to vibration, axial-torsional loading, joint fatigue, and thermal extremes,
a unique locking thread form outperforms standard fasteners according to tests from
top institutions including MIT, the Goddard Space Flight Center, and others

For engineers combating vibrational loosening, axial-torsional loading, joint fatigue, and thermal extremes, conventional fastening methods involving standard 60 degree threads have posed design limitations for too long and proven solutions, backed by test data, have been relatively nonexistent.

Fortunately today, engineers are successfully attacking these problems with an innovative self-locking fastener called Spiralock, whose effectiveness has been validated in published test studies at leading institutions including MIT, the Goddard Space Flight Center, Lawrence Livermore National Laboratory, and British Aerospace, as well as at noted corporations such as Mack Truck and Dana Corporation.

Resolving Design Limitations
With the standard 60 degree 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 is also a problem due to substantial 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 re-application is necessary, cleaning the threads of affected components takes added time and labor before re-application 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 labor costs.

To resolve these design challenges, engineers often look to solid test data for demonstrated effectiveness of products that claim to ease their engineering pains. Fortunately, testing at a wide range of leading institutions and corporations has demonstrated the effectiveness of an innovative locking fastener called Spiralock. Spiralock Corporation has re-engineered the standard 60-degree thread form, adding a unique 30-degree wedge ramp at the root of the thread which mates with standard 60-degree male thread fasteners.

The wedge ramp allows the bolt to 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.


Resistance to Vibration
In dynamic and static testing by Goddard Space Flight Center, Spiralock nuts (stainless alloy A-286 and alloy steel) were tested under vibration and static load conditions. The most severe vibration tests (Sine: 24.7 Hz - 2G and Random 20-400 Hz - 2 G RMS) did not loosen the nuts when subjected to both 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: Naval Warfare Division confirmed the locking fastener´s resistance to vibration. Testing was done on an Unbrako Fastener Vibration Machine using M6 x 25mm grade 8 bolts with wire inserts in L168 aluminum, and 20 samples of both Spiralock wire inserts and standard (60 degree UN thread) wire inserts also known as Helicoil Screwlock wire thread inserts. These were tested at 13.6 Hz and tightened to 1800 pounds 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,´ states 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 Helicoil, 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.´

Resistance to Axial-Torsional Loading
A Finite Element Analysis report prepared for Lawrence Livermore National Laboratory compared a spacecraft node to strut threaded connection using 10-32 UNF to Spiralock female thread. In analyzing axial thread load distribution, separate analysis was done to compare the load distribution for each thread of a Spiralock and 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,´ states the report. ´The first thread of the Spiralock nut supports 12.5% of the total load at the assembly preload of 1733 lb 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,´ states the report. ´It is clear that at 1733 lb, the first few UNF threads are highly stressed and threads 7-10 are below 10 ksi.´

In contrast, the report states, ´Each of the ten Spiralock threads has a high local stress at the points of contact. At the 1733 lb assembly preload, the effective stress away from the points of contact ranges from 10-18 ksi and extends completely to the bottom of the nut. The Spiralock thread form clearly distributes the load more uniformly throughout all of the threads than the UNF thread form does.´

The report explains how the Spiralock design allows for more uniform load distribution on each thread: ´The thread bearing stresses in the Spiralock threads are concentrated at the sharp points of contact where there is localized 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.´

Resistance to Joint Fatigue
A report by the Massachusetts Institute of Technology for Chrysler Corporation similarly studies vibration resistance and stress distribution in threaded fasteners. It compares the Spiralock thread form (on the nut) and the standard 60 degree thread form, both in combination with a standard 60 degree 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 60 degree 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 60 degree thread form,´ states 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 60 degree thread form. The last thread carried 9.4% vs. 3.1%.´

Regarding stress loading, the report states, ´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.´

While vital institutional research has been done comparing Spiralock´s effectiveness with traditional fasteners, some of American´s most reknowned manufacturers have also weighed in on the issue.

In proactive design testing to boost reliability, performance, and assembly effectiveness, for instance, Dana Corporation compared the clamp force retaining ability of locking adhesive with that of Spiralock locking fasteners. Dana Corporation is a leading supplier of axle, driveshaft, engine, frame, chassis, and transmission technologies to vehicle OEMs with $9.1 billion in worldwide sales last year.

In Dana Corporation´s exceptionally rigorous Impact-Durability Test designed to examine torsional fatigue, a minimum of 3,000 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, a design engineer at Dana Corporation.

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

´In both tests, Spiralock locking fasteners demonstrated 15 to 20 percent better clamp retention than traditional locking adhesives,´ states Metelues.

´The bolts using Spiralock fasteners did not back out,´ adds Metelues. ´The design distributes load more evenly than traditional threads which minimizes 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.´

Resistance to Thermal Stress
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, a design engineer at Mack Trucks, Inc., one of North America´s largest producers of heavy trucks, provides evidence of the Spiralock locking fastener´s resistance to extreme temperature and of its re-usability. He found special benefits for high-temperature diesel engine applications.

´During a particular engine durability test, the fasteners were exposed to temperatures as high as 1300? F, which is hotter than normal operating temperatures,´ says Werner. ´The engine was then rapidly cooled every twelve minutes for 3,000 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,´ continues Werner. ´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 just an exchange of traditional nuts, wire inserts or simply drilling out and re-tapping existing parts stock that have unreliable standard tapped holes.

Spiralock has been tested in thousands of applications and is used to solve design challenges in wide range of industries including automotive, heavy truck, aerospace/military, medical, agriculture, construction, rail, oil drilling, and food processing.

For detailed test data, including comparative graphic loading characteristics or photoelastic analysis/load vector comparison animation, visit Spiralock at www.spiralock.com; email slinfo@spiralock.com; call (800) 521-2688; fax (248) 543-1403; or write to Spiralock at Madison Tech Center, PO Box 71629, Madison Heights, MI 48071.

































































































































































































































































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