An Investigation About the Ageing Behaviour of Pre-Applied Threadlockers

Pre-applied threadlockers are widely adopted in industrial applications, due to their favorable features in terms of handling of the fastener and savings in terms of assembly-line dosing systems. In the practice, fasteners with the pre-applied adhesive may undergo storage and/or shipping in harsh environmental conditions, both in terms of temperature and humidity. Questions may arise as to whether such thermal-hygroscopic aging (prior to assembly): (i) affects the tribological properties of the fastener at tightening and/or (ii) the breakaway torque of the fastener after curing. By means of an experimental approach, this work primarily aims at answering the first question, indeed scarcely covered by the literature. As for the second question, measurement of the breakaway torque is carried out for the sake of comparison with the data provided by the adhesives datasheets, where available. Based on a common bolt and plate configuration, the study involves different pre-applied threadlockers in terms of strength grade, among several hygrothermal conditions, representative of frequent storage/shipping conditions. The experimental data have been analyzed by means of the ANOVA tools to assess the effectiveness of the input parameters (adhesive type and aging conditions) on the response in terms of both thread friction coefficient at tightening and breakaway torque of the fastener after tightening completion and complete polymerization. Guidelines useful to the design engineer are reported in the conclusions.