Ongoing efforts to increase fuel efficiency, tactical mobility, and payload capacity in aerospace design are driving creative efforts to reduce mass through the use of lightweight materials such as composites, aluminum, and plastics. Use of these materials, however, can create compromises, as well as finding safe and reliable ways to fasten assemblies that assure joint integrity under the severe conditions of shock, vibration and thermal cycling common in aerospace applications.
Whereas threading directly into ductile aluminum is common, most composites are too brittle to be tapped. Shear strength and concentration of forces need to be carefully reviewed to confirm specific loads and vibration environments can be tolerated. Structural joints in particular rely on high strength to develop sufficient pretension in the joints used to assemble them.
In most structural joints using lightweight materials, the parent material may need to be reinforced through the use of a wire insert or an ultrasonic insert; the implementation of inserts allows higher joint tension and extended reusability when compared to a tapped hole directly in the softer materials.
There are a variety of thread reinforcement options used in aerospace applications: wire thread inserts for aluminum or soft materials, potted blind inserts for composites, and ultrasonic inserts or molded inserts for plastics. However, the majority of these inserts still fall short of addressing the limitations and potential for vibration-induced thread loosening that is inherent in the standard 60° thread form.
The answer may be a patented 30-degree wedge ramp design from Spiralock. The self-locking technology is being applied to application specific inserts in various materials with the added benefit of providing an easy conversion from traditional 60-degree “vee” threads to a 30-degree wedge ramp design.
The 30-degree 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.
This wedge ramp design has also been produced on wire thread inserts to offer the same vibration resistance and reusability while bringing higher strength and clamp load capability to softer materials such as aluminum.
The wire inserts are available in standard sizes from #2-56 through 7/16-20 in tanged or tang-free Drive Notch™ and M3 through M16 in tanged only. The tang-free Drive Notch™ wire inserts conform to NAS1130 dimensionally, however, the internal thread form has the Spiralock thread profile.
NASA was among the first to take advantage of this design on the main engines of the Shuttle orbiter. Each of the three main engines developed 400,000 lb of thrust and terrific vibration. But the Space Agency also wanted a 15-cycle reuse capability per fastener. Under its own test, NASA determined the fasteners in Spiralock-threaded holes did not back off or loosen when subjected to ten times shuttle-specified vibrations, and they stayed that way ten times longer than called for. NASA tests found that these fasteners delivered 50 uses with no loss of clamp load. Every shuttle carried no fewer than 757 Spiralock fasteners.
Filed Under: Aerospace + defense, Design World articles