Investigation of the Effect of the Surface Treatment and Lubrication During Repeated Tightening on the Nut Coefficient of a Bolted Joint Using the Taguchi Method

Abstract

Bolted joints are used extensively in mechanical engineering due to their reusability, appearance, and strength advantages. In different industrial applications, the separated mechanical components are clamped together during the assembly using a tightening process. Estimating precisely the initially achieved clamping force is a persistent problem for providing a secure and reliable connection for a particular design. However, the nut coefficient is one of the exploratory methods used for this purpose. This paper aims to investigate how the nut coefficient of a bolted joint is influenced by factors such as bolt/nut surface treatment and the presence of lubrication at joint contact surfaces throughout the cyclic joint assembly/disassembly process, serving as a simulation for real-life maintenance operations where the same bolt/nut are reused. Torquing cycles were introduced to bolted joints with two surface finishes and four lubrication conditions. The experiments were conducted on bolts at room temperature according to ISO16047. The nut coefficient was calculated for bolt sizes M6*1, M8*1.5, and M10*1.5. The results were analyzed using the analysis of variance and Taguchi method L32 for estimating the contribution of the different factors on the nut coefficient. It was found that the lubrication was the most significant parameter affecting the nut factor regardless of the bolt size. Also, a case study was conducted to investigate the underlying reasons behind the significance of lubrication in influencing the nut coefficient across different bolt sizes.