ND Vibra-Tite is a special blend of acrylics that is applied to aircraft cable connector backshells to prevent them from loosening.
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During the pre-check of a new electronic control system, flight engineers of the USAF Worldwide Airborne Command detected a cable connector-loosening problem. When connector back-shells loosen, they allow corrosion to develop in the connector, causing shielding degradation - a condition critical to an electronic control system's survivability.
After investigating the problem, USAF engineers determined that a thread-locking compound was the best choice because it would have the least impact on normal maintenance and would subject the connectors to the least potential damage. The compound selected was ND Vibra-Tite, a viscous resin that has good cold-flow properties but dries to the touch in seconds and does not require the parts to be pre-cleaned.
According to ND Industries, Inc., maker of ND Vibra-Tite, the material was selected because the specific blend of acrylic compounds provides a flexible consistency that never gets dry. This allows a fastener coated with it to be easily adjusted, removed, and re-used many times. The material is thick, resilient, and acts as a damping compound that enables the fastener to absorb vibration eliminating the possibility of the fastener ever "walking out" of the thread path.
Vibration testing under varying temperature conditions was performed on three different cables to, first, characterize the loosening of the connectors and, second, to evaluate the performance of the ND Vibra-Tite compound. It was discovered that the connector backshells were produced by two different manufacturers. The defective ones were an old design that had since been improved, and subsequent vibration tests proved that the new design did not loosen under normal operating conditions.
The Vibra-Tite material was tested on special cable vibration shaker test equipment. No loosening occurred in the connector. Both a narrow and wide strip of Vibra-Tite was applied perpendicular to the threads. The wider strip resulted in a higher inductive coupling (reduced shielding effectiveness) than the narrow strip. As a result of this testing, a repair procedure was recommended for the loose backshells that included applying a thin strip of the standard ND Vibra-Tite locking/sealing coating to the threads of the backshells' connectors.
Frank Bokulich
