Fastener Inserts Designed for Lightweight Aerospace Materials Reduce Risks, Costs

Ongoing efforts to increase fuel efficiency, tactical mobility, and payload capacity in aerospace design have driven engineers to find numerous ways to reduce mass through the extensive use of lightweight materials such as composites, aluminum, and plastics. Use of these lightweight materials can create other issues, however, such as finding safe and reliable ways to fasten assemblies that provide complete assurance of joint integrity under the severe conditions of shock, vibration and thermal cycling common in aerospace applications.

Assemblies that combine lightweight materials create other challenges. 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 in order to confirm specific loads and vibration environments can be tolerated.

In most structural joints using lightweight materials, the parent material needs 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 currently being utilized 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.

In traditional fasteners, the radial clearances between traditional male and female 60° “vee” threads can permit relative sideways or lateral movement when shock, vibration, or transverse loading occurs. Stress concentration at the first few engaged threads increases the probability of shear and may lead to fatigue failure. Temperature extremes can also expand or contract surfaces and materials, potentially compromising joint integrity.

As a result, a variety of locking devices from wires and washers to prevailing torque threads as well as chemical and drypatch adhesives are commonly added to prevent loosening. These methods often can have more drawbacks than advantages, though, as they do not always hold up under extreme conditions and may not be reusable. Inserts provide a unique challenge since there is not much material with which to apply a locking feature.

The answer may be in new or redesign ed applications implementing a unique, patented 30-degree wedge ramp design offered by Spiralock (Madison Heights, MI). The self-locking technology – already utilized by aerospace companies such as BAE, Boeing, Honeywell, NASA, Harris, Raytheon, Hamilton Sundstrand, and the US military – is now being applied to a variety of application specific inserts.

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, axialtorsional loading, joint fatigue, and temperature extremes.

In aerospace, wire insert applications have ranged from heat sinks and electronic chassis to satellite connectors, circuit board clamps, and avionics box enclosures. To encompass the widest range of use, 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 appreciate the advantages of the Spiralock thread, when designing 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 the Spiralock-thread fasteners delivered 50 uses with no loss of clamp load. Every shuttle carried no fewer than 757 Spiralock fasteners.

Spiralock Madison Heights, MI 800-521-2688

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