Session: CT-05-01 Threaded Fasteners

Paper Number: 152224

152224 - K-Factor Reliability on Reuse of Studs With Various Coatings 

Abstract: 

In bolted joint assemblies, accurate torque application is essential to achieving the correct preload, which is critical for joint integrity and long-term performance. The k-factor (or nut factor) is a key element in torque calculations as it directly influences the relationship between the applied torque and the resulting tension, or preload, in the bolt. An incorrect k-factor can lead to inaccurate preload, resulting in either under-tightening or over-tightening. Under-tightening may cause insufficient clamping force, leading to joint loosening, leaks, or fatigue failure. Over-tightening, on the other hand, can yield the bolt, damage the joint components, or over-crush the gasket, potentially leading to catastrophic joint failure.

 

The k-factor is influenced by several factors, including type of lubrication, surface finish, bolt material, coatings, and the condition of the threads. Each of these factors affects the friction present between the threads and under the bolt head or nut, which in turn alters the amount of torque required to generate the desired preload. While the use of new studs is always recommended when assembling flange joints, this is often impractical due to time constraints or the unavailability of new hardware. Understanding the impact of reusing studs, particularly those with different coatings and lubrication, on the k-factor becomes critical for ensuring proper joint performance.

 

This paper focuses on the impact to the k-factor when studs are subjected to multiple reuse cycles. It includes an analysis of how the k-factor changes on common coatings such as PTFE (Polytetrafluoroethylene), black oxide, and Zinc-Nickel plating. These coatings are widely used in bolted joints to protect against corrosion, improve friction control, and enhance the longevity of the fasteners. However, after repeated use, the effectiveness of the coatings and their impact on friction—and thus the k-factor—can change significantly.

 

As seen on a previous 2022 PVP paper for k-factor testing of PTFE coated studs with lubrication, this paper also shows the benefits of lubrication when reusing studs. Lubrication plays a crucial role in reducing friction between mating surfaces, which lowers the k-factor and leads to more predictable and consistent preload.

 

Following the evaluation of the k-factors for different coating and lubrication combinations over multiple reuse cycles, this paper provides insights to the limitations and inconsistencies of reusing bolts for maintaining joint integrity when new hardware is not available. These results are relevant for industries such as petrochemical and power generation, where flange joints are critical components of pressure vessels and piping systems.

Presenting Author: Tommie Bao LGC - LAMONS GASKET CO

Presenting Author Biography: Tommie Bao holds a Bachelor's Degree in Mechanical Engineering from the University of Houston. With experience in the telecommunications field and over five years in the oil and gas industry, his expertise centers on bolted joints and customized sealing solutions. He is currently an Applications Engineer II at Lamons, based in Houston, Texas.

Authors:

Tommie Bao LGC - LAMONS GASKET CO
Von Hugo LGC - LAMONS GASKET CO
Abdullah Qureshi LGC - LAMONS GASKET CO