Investigation of repeated tightening behavior of zinc-plated bolts under different torque levels

Abstract

Zinc-plated bolts are used widely in various industries to improve corrosion resistance in bolted joints. However, the repeated tightening commonly carried out during maintenance or daily inspections can change the frictional behavior of a bolt's contact surfaces, which may adversely affect tightening behavior. To ensure the reliability of reused zinc-plated bolts, this study investigated the effect of repeated tightening on the coefficient of friction, preload, wear, and stress distribution in bolts. Given the varying preload requirements across industries and are usually controlled by adjusting the tightening torque, repeated tightening experiments were conducted at three tightening torque levels. The lubrication and nut replacement were also considered in the most severe wear condition. The distribution of bolt stress during repeated tightening was investigated using finite element analysis (FEA). Results showed that repeated tightening of zinc-plated bolts was significantly affected by the torque level and tightening condition. Under the three torque levels, repeated tightening decreased the preload to varying degrees. In addition, the preload exhibited different trends under different levels of torque. When the nut was replaced before each retightening, the preload decreased and stabilized at 80 % of the initial value, while the preload of lubricated bolts remained nearly consistent during repeated tightening. The FEA results indicate that repeated tightening of bolts under dry conditions can lead to greater stress concentration, even if the preload level is similar to that of the first tightening. Conversely, when the bolt was repeatedly tightened after lubrication, the stress distribution was nearly identical to that of the initial tightening. These results provide useful guidance for the future design of bolted joints. © 2025 Institution of Structural Engineers