In the frameworks of a correct identification and characterization of friction-induced vibrations problems and the related noise emissions, this research project is focused on the stick-slip phenomena in lubricated contacts. The noise caused by friction-induced vibration is a major issue encountered in many mechanical systems. Typical engineering examples are brakes systems, train wheel¿rail, frictional belts and machine tools. The existence of a coupling between contact local conditions and global system dynamics can induces the appearance of alternated phases of stick and slip during the sliding of two bodies in contact. The presence of a lubricant between the two surfaces can worsen the situation, adding uncontrolled boundary conditions and complicating the understanding of the conditions for for which stick-slip phenomenon appears. Usually the stick-slip problem can be solved analytically through a simple damper spring-mass system on a moving belt. However, in this way the frictional contact behaviour response is neglected despite it have a key role in the understanding of the stick-slip phenomenon. In this work, we propose a methodology for the analysis of stick slip that takes into account both the tribological contact behaviour and the dynamic response of the system. The friction response at the contact will be analysed thought experimental tests. Changing the boundary conditions (normal load, velocity, temperature, materials, lubricant type), the frictional response at the contact will be analysed. Subsequently, a numerical lumped model will be developed. The model will take as input the information previously obtained on the contact behaviour. With this methodology, the coupling between the contact behavior and the dynamic response at the origin of the stick-slip will be taken into considerations.
Friction-induced vibration problems have received considerable attention from a number of researchers, but there are still no general solving methods to completely eliminate or reduce instabilities. Solving the problem requires a complete understanding and appropriate analysis of the entire system to identify the effects of all physical parameters on system stability. The problem can become more complicated when lubricated interfaces are involved. Lubricated systems are supposed to reduce the frictional losses and improve the lifetime of mechanical components, they can also generate undesirable vibrations leading to contact deterioration and material fatigue. The possibility of understanding the conditions for which a system is more predisposed to the stick-slip phenomenon may allow preventing the appearance of such instabilities before its occurrence.
Despite the strong scientific and industrial interest, the bibliography on this phenomenon, specifically in the case of lubricated contacts, is poor and limited to specific single application cases. In this work, a methodology will be therefore proposed for the analysis of stick-slip in a lubricated contact, using the combination of numerical simulations and experimental tests. Thanks to this innovative approach, it is possible to study the local frictional contact and introduce this information into the global dynamics of the entire system. Finally, it is possible to analyze the appearance of the stick-slip varying the boundary conditions (materials, geometry of the components, type of lubrication, contact pressure, sliding velocity and temperature). In this way it will be possible understand the conditions for witch the system is more predisposed to the stick-slip phenomenon.