Machine Design

Ramp breaks common thread of fastener failures

A special thread form keeps bolts and nuts tight under vibration by attacking the problem from a different angle.

Aaron Boettcher
Engineering Manager
Spiralock Corp.
Madison Heights, Mich.

A photoelastic image of the Spiralock 30° wedge ramp (right) shows loads are evenly distributed among engaged threads. The first two threads on a conventional fastener (left) may carry as much as 80% of the load, enough to cause shearing or stripping.

Spiralock 1/4-28 tapped holes are used in mounts for AIM-120 AMRAAM airtoair missiles on F-18 Hornet fighter aircraft.

Spiralock threads more evenly distribute loads across engaged threads. Standard 60° threads have a larger surface contact area. This makes the first few engaged threads bear most of the applied load.

Two conventional methods for securing threaded fasteners against preload loss include locking and single-use drypatch adhesives. But thread adhesives are temperature limited and cumbersome to deal with in production and during maintenance. The reason such measures are needed in the first place is the thread form itself. A standard 60° thread form leaves a gap between the crest of the male and female threads. This makes the nut-bolt assembly susceptible to transverse motion and loosening under vibrational loads. Just the first few threads carry most of the axial load, which raises contact stresses, fatigue, and the probability for shear and thread stripping, especially when bolts thread into soft metals.

An alternative thread form called Spiralock from Spiralock Corp., Madison Heights, Mich., does away with thread adhesives and the temperature limitations. It adds a special 30° wedge ramp at the root of the female thread which mates with standard 60° male-thread fasteners. The wedge ramp lets the bolt spin freely relative to the female threads until clamp load is applied. Then, crests of the standard male thread form draw tightly against the wedge ramp, eliminating radial clearances and the transverse motion that causes loosening-under vibration. A continuous-spiral-line contact along the length of the thread engagement more evenly spreads clamp force. Further, the wedge ramp boosts the radial component of joint load compared with standard threads, taking advantage of material hoop strength and lowering the chance of shear failure.

An FEA study conducted by Lawrence Livermore National Laboratory helps quantify the mechanism by which the Spiralock thread form works. It compared the load distribution of a spacecraft node-to-strut threaded connection using standard 10-32 UNF and Spiralock nuts. The boundary condition on both nut types constrains the nut face in the axial direction, but lets it move radially.

At an assembly preload of 1,733 lb, the first thread of the Spiralock nut supports 12.5% of the total load, and no thread supports less than 8%. Each of the 10 threads has a high local stress at the contact points and localized yielding in compression. This gives a more uniform load distribution on each thread. The effective stress away from the contact points ranges from 10 to 18 ksi and extends completely to the bottom of the nut. For comparison, the first two threads of the UNF nut each support 21% of the total load, and each of the last three threads, 4% or less. The first few UNF threads are highly stressed, and threads seven to 10 are below 10 ksi.

A report by Massachusetts Institute of Technology for Daimler-Chrysler also compared load distribution of the Spiralock thread form (on the nut) and a standard 60° thread form, both in combination with a standard bolt. Calculations show that the first engaged thread of the Spiralock nut carries only 18% of the load, versus 34.1% for the standard thread form, and the last thread 9.4% versus 3.1%.

Vibration is a major source of preload loss and fastener failure. Vibration testing by the Goddard Space Flight Center simulates conditions experienced by threaded fasteners on Space Shuttle solid-rocket boosters. A Tinius Olsen tension machine first pulled the bolts in tension to calibrate strain for a given load. Static tests measured friction coefficients for lubricated and unlubricated nuts.

Both Spiralock and standard 1/4-28 nuts (stainless alloy A-286 and alloy steel) underwent a 2-g sinusoidal vibration at 24.7 Hz, and random 20 to 40-Hz vibration at 2-g rms. In both tests, the standard threaded nut began to lose preload within the first minute of testing. The Spiralock nut stayed tight until the bolt failed in fatigue. These vibration tests both are an order of magnitude longer in duration and in severity than spec for the application, states the report.

British Aerospace, Naval Warfare-Div., in the U.K., tested Spiralock wire-thread inserts under transverse vibration. An Unbrako Fastener Vibration Machine tested M6 25-mm Grade-8 bolts preloaded to 1,800 lb in L168 aluminum fitted with Spiralock and standard UNF wire inserts. The machine subjected twenty of each insert type to a 13.6-Hz vibration.

The study found that joints with the Spiralock inserts consistently gave a 15% preload loss over the first 1,000 cycles, followed by a 1% loss over the next 5,000 cycles. Joints with standard inserts, in contrast, at best lost 22% of preload after 1,000 cycles, and 39% at 6,000 cycles. The worst result showed a 90% preload loss in 500 cycles and a 95% loss at 6,000 cycles.

Dana Corp., Toledo, Ohio, a maker of power-transmission components, compared the ability of locking adhesive and Spiralock fasteners to retain clamp force. Dana's Impact-Durability Test torsionally fatigues specimens under a minimum of 3,000 back-and-forth cycles at maximum vehicle torque load. "It is the equivalent of putting the vehicle in neutral and flooring the gas, first forward, then reverse, thousands of times," says Frank Metelues, a design engineer at Dana. A dynamometer test also simulates extreme,use over the life of a vehicle with varied torques and speeds.

"In both tests, Spiralock locking fasteners showed 15 to 20% better clamp retention than locking adhesives," says Metelues. "A more even load distribution afforded by the special thread form minimizes thread yielding and deformation, and helps prevent torque and axial loads from backing out the bolts."

Todd Werner, a design engineer with Mack Trucks Inc., Allentown, Pa., describes an enginedurability test conducted at his company. "The test exposes fasteners to temperatures of 1,300°F, hotter than normal operating conditions," Werner explains. "The engine is rapidly cooled every 12 min for 3,000 hr. Inspection every 250 hr showed the Spiralock fasteners maintain joint integrity for 15,000 cycles without torque loss. The 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."

Spiralock fasteners are reusable and don't damage nuts or studs. Production changeovers typically involve an exchange of conventional nuts, wire inserts, or drilling out and retapping of existing parts.


TAGS: Fasteners
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