Abstract

This study presents recent research results on the Nut-Factor of coatings applied to high-strength steel threaded fasteners mainly used in the Oil and Gas industry. PTFE coatings are commonly used to protect B7 grade material from corrosion within the industrial marketplace; however, most coatings applied to threaded fasteners do not offer a consistent tightening and they are affected by torquing during assembly. This results in an inconsistent Nut-Factor and accuracy of the provided nut-factor is not only inconsistent but also lower than most published values. The lack of consistency in the engagement produces variations in the load when torque is applied, the same torque will produce different loads depending on the surface conditions of the threads, which changes every time the fastener is re-used. Therefore, the aim of this study is to provide data on the nut-factor of coated threaded fasteners after assembly/disassembly cycles and the use of different lubricants. Torque cycles were applied to threaded fasteners with different coatings, Nickel-Cobalt electroplating, Zinc undercoat with PTFE topcoat and TSA with PTFE topcoat. The nut-factor was calculated before and after commercial lubricants were applied. Nickel-Cobalt electroplating showed an outstanding performance compared to the other tested coatings since it offered the highest nut-factor consistency. This study summarizes the optimal operational conditions for lubricants and materials and how these conditions affect the nut-factor and thread engagement.

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