The Influence of Repeated Loading on Bolt Preload Under Multiple Lubrication Conditions

Bolts are core connecting components in engineering, and their preload directly affects equipment operation. Existing studies mostly focus on general mechanical scenarios, lacking adaptability and matching specifications for large-scale equipment. To address this issue, this paper designs bolt tightening experiments simulating actual working conditions to investigate the influence of repeated loading on bolt preload under four lubrication conditions: no lubrication, thread-only lubrication, bearing surface-only lubrication and full lubrication. Ten cycles of repeated loading tests were carried out on same-batch bolts from the same and different manufacturers. The results show that lubrication exerts a significant regulatory effect on preload: full lubrication results in the highest average preload (595.11 kN), while no lubrication leads to the lowest (426.89 kN) with the maximum dispersion. The effect of repeated loading on preload varies with lubrication conditions: preload drops sharply without lubrication, changes steadily under thread-only lubrication, decreases slightly under bearing surface-only lubrication, and increases notably under full lubrication. Bolt preload is jointly affected by individual differences, lubrication and repeated loading; same-batch bolts from different manufacturers exhibit consistent preload change trends but reverse fluctuations at specific stages. This study provides technical support for high-quality bolt assembly of large-scale equipment, as well as practical references for optimizing assembly processes, controlling preload and formulating matching specifications.