Influence of Lubrication on High‐Strength Friction Grip Bolts: Bolt Tightening

Abstract

High‐Strength Friction Grip (HSFG) bolts of grade 8.8 are widely used in structural steel connections due to their high strength and ability to achieve significant clamping force through pre‐tensioning. However, the performance of these bolts is dependent on the tightening process. Among the key influencing factors, the type of lubricant used, and the method of its application play a role in the installation process. At present, New Zealand, European, and several other international standards do not provide explicit guidance on the selection of lubricants or on standardized lubrication procedures for HSFG bolts. In practice, contractors often adopt different lubrication approaches depending on project‐specific conditions, leading to inconsistencies in bolting quality control before and during installation. This study presents an experimental investigation into the influence of lubrication on the behaviour of grade 8.8 HSFG bolts tightened using the part‐turn method specified in NZS 3404. Three categories of lubricants were examined: factory‐applied lubricants, site‐applied lubricants, and other commonly used lubricants. In addition, three lubrication regimes were considered: nut‐only lubrication, thread‐only lubrication, and full lubrication covering both nut and thread. Based on the experimental results, this study identifies lubricants that are more consistent tightening behaviour and reduced torque scatter and proposes a practical lubrication procedure to improve bolting quality control during installation.