Threaded fasteners with applied medium or high strength threadlockers: effect of different tightening procedures on the tribological response

Threaded joints are widely used in the automotive industry, where threadlockers can be applied to prevent self-loosening and to possibly achieve a friction drop upon tightening. This study deals with this topic, investigating threadlocker tribological response before polymerization takes place. These lubricating properties are usually not considered in adhesive datasheets, but can be highly beneficial from the point of view of the achievable preload, thus preventing selfloosening. The topic was tackled experimentally, running an extensive campaign that involved a medium and a strong threadlocker (LOCTITE 243 and LOCTITE 270, respectively), used for tightening zinc-plated and black-oxidized screws. The testing procedure was inspired by a protocol being used by most manufacturers in the automotive field: in particular, the impacts of pre-tightening speed and of the time interval between pre-tightening and final tightening were investigated. The retrieved results indicate a lubricating effect of the medium threadlocker with achievable friction coefficients in the order of 0.11. Moreover, for both the threadlockers, a generally decreasing trend for the thread friction upon tightening for increasing time interval before tightening was observed. This is a particularly original outcome that could have remarkable implications for optimizing the assembly tasks of large sets of bolts.