193 Prevailing Torque  Locking Feature in   Threaded Fasteners using Anaerobic Adhesive  

Alan Hernandez * and D aniel P. Hess* 

Abstract  

This paper presents results from tests to assess the use of anaerobic adhesive for providing a prevailing  torque locking feature in threaded fasteners. Test procedures are developed and tests are performed on  three fastener materials , four anaerobic adhesives, and both unseated assembly conditions.  Five to ten  samples ar e tested for each combination. Tests for i nitial use, r euse without additional adhesive, and reuse  with additional ad hesive are performed for all sampl es. A 48-hour cure time was used for al l initial use and  reuse tests. Test data are presented as removal torque versus removal angle with the specification required  prevailin g torque range added for performance assessment. Percent specification pass rates for the all  combinations of fastener material , adhesi ve, and assembly condition are tabulated and reveal use of  anaerobic adhesive as a prevailing torque locki ng feature is viable. Although not every possible fastener  material and anaerobic adhesive combination provides prevailing torque values within specification, any combination can be assessed using the test procedures presented. Reuse without additional anaerobic  adhesive generally provides some prevailing torque, and in s ome cases within specification.  Reuse with  additional adhesive often provides comparable removal torque data as in initial use.  

Introduction and B ackground    Prevailing torque locking features and anaerobic adhesives are used to provide secondary  locking in threaded  fasteners.  The aerospace industry has embraced prevailing torque locking in fasteners, in part, due to the  ease in which the feature can be  validated during installation.  In addition, the industry has well -defined  standards and specifications for this type of locking feature [1- 6].   Anaerobic adhesives provide excellent locking and can maintain preload in fasteners provided sufficient curing occurs, but this is not always guaranteed. Although validation for this type of locking is possible [7], it  is not yet in widespread use in practice.    Recent application of anaerobic adhesive for repair of worn prevailing torque locking in inserts on the Space Shuttle windshield [8, 9] has  given ris e to the premise of this work. Namely, use of adhesive for providing  prevailing torque type locking feature, and quantifying its performance using the well -defined prevailing torque   standards and specifications. No published research on this conc ept currently exists.     Use of anaerobic adhesive for providing prevailing torque locking actually relaxes the usual expectation of adhesive to maintain preload to the common expectation for prevailing torque locking features which is to prevent disassembly.  

This paper  presents results from tests performed to assess the use of anaerobic adhesive to provide a  prevailin g torque type locking feature. This work provides much needed test data supporting the use of  adhesive in providing prevailing torque locking for repair as well as on fasteners without secondary locking.   

Performance of prevailing torque locking fasteners in reuse is required in qualification specifications.  However, these requirements are often stated in terms of reuse in unseated r ather than seated conditions   and this distinction is sometimes missed. This issue has been recently examined through a series of tests                                               * University of South Florida, Tampa, FL  

Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016