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. 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