Direct Verification of Threaded Fastener Locking Compounds and Adhesives
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TECHNICAL ARTICLE—PEER-REVIEWED
Direct Verification of Threaded Fastener Locking Compounds and Adhesives R. Hunter • D. P. Hess
Submitted: 8 October 2012 / Published online: 6 November 2012 ASM International 2012
Abstract This article explores the feasibility of a method to directly verify that thread lock compound or adhesive has cured sufficiently to provide secondary locking. The application of a torque after assembly and cure time was found to provide a viable verification test. Fastener material and coatings were found to significantly affect cure. For example, medium-strength anaerobic compound used with fasteners made of inactive material such as stainless steel had curing issues which prevented determination of a useful verification test torque. However, modifications such as higher-strength compound or primer can be introduced making this method viable even for fasteners made with inactive materials and coatings. This article outlines a process to implement this method in practice. This requires sample tests with representative product to determine a test verification torque. This process is particularly useful in identifying curing and locking performance issues, and provides guidance for modification so that the method can be successfully implemented in practice. Test results show that application of verification test torque with or without standard vibration test exposure does not degrade the locking performance. Keywords Fastener Thread lock compound Loosening Thread adhesive Prevailing torque Secondary locking Verification method Locknut
R. Hunter D. P. Hess (&) Department of Mechanical Engineering, University of South Florida, 4202 E. Fowler Avenue, ENB 118, Tampa, FL 33620-5350, USA e-mail: [email protected]
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Introduction and Background Threaded fasteners remain as widely used components in machinery and structures. Fasteners literally hold systems together. The friction in the thread, bolt head, and nut face provide an inherent resistance to (or locking against) loosening moments induced from the joint preload itself and external loads. In many cases, a secondary level of resistance to loosening is needed and required. This is introduced using one of a number of possible locking features including the following: • • •
Mechanical features such as lock wire or cotter pin with castle nut; Prevailing torque devices such as lock nuts with distorted threads or nylon strips; and Adhesives such as anaerobic compounds and epoxies.
These are generally referred to as secondary locking features and are required in many applications and industries. These components have qualification specifications specific to application or industry. An important aspect of the assembly process with a secondary locking feature is a means of verifying the locking feature during or after assembly. Mechanical features such as lock wire and cotter pins can be visually verified after assembly. Prevailing torque in lock nuts can be verified with a torque measurement during assembly. Locking from fastener adhesiv
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