Effect of friction, surface treatment and adhesive-reinforcement on the behavior of threaded connections.Effetto dell'attrito, dei trattamenti superficiali e dell'incollaggio sul comportamento dei collegamenti filettati

This study aims at analyzing the phenomena which characterize threaded connections. This type of joint has been known for thousands years (one of the first applications is attributable to Archimedes) allows transforming a torque into an axial force, typically used to lock two or more components thanks to the friction forces that arise between the connected bodies. One of the main strengths of this type of coupling is the possibility to assemble and disassemble the assembled parts; in addition, it can be applicable to almost any type of material. However, the joint system is characterized by a low efficiency: only a modest amount of the tightening torque is actually converted into axial force. In addition, the obtained axial force is typically affected by a large scatter. This behavior is due to the combined effect of two different friction effects that arise between the under-head of the screw (or nut) and the surface of the elements to be locked in the first case, and between the screw threads and female screw in the second one. The relationship between tightening torque and preload axial force is very sensitive to the effect of the various friction coefficients that rule the tribological behavior of the joint:, these coefficients have a significant variability due to the processes used to manufacture the components, and to the different tightening parameters, such as the presence of lubrication or the tightening speed. Threaded connections do have a wide array of applications, both in the field of civil engineering and in mechanics. For instance, in the automotive compartment, due to a steadily growing strive towards weight reduction and increased automation of assembly procedures, an accurate knowledge of the performance of threaded connections has recently become critical in order to achieve the safety of the junction at an adequate production cost. In order to control a threaded connection, it is necessary to focus on the three main factors that characterize its behavior: the coefficients of friction at tightening, the self-relaxation phenomenon (decrease of axial preload in the first few seconds after tightening, due to sinking of the load bearing area in the clamped parts), self-loosening (loss of axial preload, typically due to the action of external shearing forces). In the following, literature review will be presented first, then it will be shown an experimental investigation on the effect of solid lubrication on the friction coefficients, carried out for repeated tightenings and different surface treatments. Moreover, the effect of the number of engaged threads on the performance of an anaerobic threadlocking adhesive will be examined. The need for these analyses arose from the study of an in-field failure occurred in a pinion shaft of a transmission for earth moving machines, which, as it will be shown in the following, probably failed due to a combination of the aforementioned typical issues of threaded components. The research has the aim of increasing the knowledge about the effect of lubrication on the different phenomena that contribute to determining the performance of a bolted connection.