Author: Dr. Harold Hess
Phone: (913) 214-9911
Permanently locking fasteners that deform the threads are often used in critical applications where loosening of the fasteners is unacceptable. The problem, however, is that for maintenance these fasteners must be drilled out or cut off for removal, and they require specialized tools for installation or removal. Fasteners relying on nylon inserts do not have a consistent preload (clamping force) for a given degree of torque, and they cannot be used in high temperature applications. Fasteners employing mating washers rely on friction or deformation of the substrate by the washer, so if there is any loss of preload, the fasteners will loosen. Mechanically locking fasteners employing safety wire or castellated nuts with Cotter pins are labor intensive for installation and removal.
A new currently available technology is a fastener that permanently locks; yet it is reversible and reusable. When a standard 6-point socket is applied to the nut, the nut is free spinning, so an accurate preload can be applied. When the socket is removed, it permanently locks. The locking mechanism incorporates engagement of teeth, so it does not rely on friction. The threads are not deformed, so it can also be used repeatedly.
The requirement for a self-locking nut in aerospace (NASM 1312-7) is to withstand 30,000 cycles of vibration. These fasteners survived 300,000 cycles. Following vibration testing the fatigued parts underwent tensile testing, where they proved to be Type I bolts and Class II nuts. They have been tested for reuse up to 250 times. Preload accuracy for a given degree of torque is approximately 5.7%, which is comparable to fasteners that deform the thread.
A primary benefit of the technology is that it saves time and money in installation and maintenance. No specialized tools are required, as only a standard 6-point socket is utilized. As the fastener permanently locks (through teeth engagement), less maintenance checks are required to ensure that the fastener remains tightly engaged. The fastener does not rely on friction or thread deformation for its locking feature, so it can be removed and reapplied numerous times. The teeth of the fastener are not engaged during installation or removal, so there is minimal wear of the teeth over time. As the nut is free-spinning with socket engagement, an accurate preload can be applied. This is essential when clamping composites. All components of the fastener are manufactured out of metal, so they are also ideal in high temperature applications.
The fastener technology is currently available in both a locking nut – washer – bolt combination and in a locking threaded bolt. It can be provided in numerous materials such as Stainless A-286, Inconel 718, and Titanium in sizes from ¼” and greater.