Tribological and vibro-acoustic behaviour of a lubricated contact subjected to the stick-slip phenomenon: the case of the spring-brake system

The main objective of the PhD thesis is the analysis of stick-slip phenomena in lubricated contacts, by merging the physical, experimental and numerical points of view and proposing a novel methodological approach, applied here to an industrial case. Nowadays, in the field of applied mechanics and tribology, one of the most important challenges is the ability to predict and reduce surface damages, failure of machine components and undesirable frictional and dynamic characteristics. In particular, the appearance of friction-induced vibrations at the contact is hardly controllable and can result in high local contact pressure, elevated stresses, system oscillations, discontinuous motion and premature failure]. Understanding the conditions for which the system is more predisposed to the stick-slip phenomenon may allow preventing the appearance of such instabilities, and the related vibrations and noise emission. Friction-Induced Vibrations are a phenomenon that engages multiple scientific challenges, due to the complexity of their physics. Focusing the attention on stick-slip phenomena, these are generally characterized by a saw-tooth displacement-time evolution. Each change in the contact parameters influences directly the dynamic and frictional response of the system, due to the mutual influence of the local scale (contact) and the system scale (macroscopic frictional and vibrational response). Moreover, the presence of a lubricant, and in particular of grease, at the contact interface, increases the complexity of the phenomenon, from both a tribological and dynamic point of view. Lubricated systems are supposed to reduce the frictional losses and wear, but they can also collaborate in the appearance of dynamic contact instabilities, due to the friction-velocity characteristics when passing from boundary to mixed contact regimes. The complex rheology of a grease, function of both the matrix, additive and oil responses, becomes then a key point for the occurrence and evolution of stick-slip. Despite the great importance of this phenomenon, from both scientific and industrial points of view, a lack emerges into the literature about stick-slip of lubricated interfaces. The few works are manly focused on molecular dynamics simulations and numerical modelling of the dynamical response of the system. The different role of the grease components, during the sliding, is nowadays still not clear. Nevertheless, the complex rheology of a grease, function of both the thickener, the base oil and the additives, is a key point for understanding and controlling the occurrence and evolution of stick-slip. Moreover, a general approach is needed to account for the coupling between the local phenomena (e.g. lubricated contact response) and the system dynamic response. Aiming to improve the understanding of stick-slip in lubricated contacts, the present Ph.D. work proposes a novel methodological approach to the stick-slip problem of a lubricated contact, referring to a real industrial case, in order to deploy the obtained results in a more realistic and detailed manner. The subject of the investigation is a mechanical brake used in tubular electric actuators, which can present frictional instabilities originated at the lubricated contact between the two main brake components. The methodology used is twofold: i) on one hand, experimental tests are carried out to understand the local frictional response of the lubricated contact; ii) on the other hand, a lumped model is created in order to simulate and analyse the system dynamic response. Introducing the information about the local lubricated contact behaviour (friction law), achieved experimentally, into the numerical model, it is possible to investigate the parameters for which the system is more predisposed to the stick-slip phenomenon and recreate a representative scenario of its appearance. Particular attention has been placed on the analysis of the lubricant rheology, dealing with different types of lubricants and regimes of lubrication, with both oils and greases. The frictional response has been thus related to the different contributions of the grease components (i.e. thickener, base oil and additives) on the rheology at the interface. The obtained local information has been then integrated in the lumped model to evaluate the unstable dynamic response of the entire system (i.e. the stick-slip phenomena) and identify the lubrication parameters that most influence its appearance. The numerical analysis had the dual objective of understanding the role of the local contact response in the system instability and investigating the stick-slip occurrence as a function of the key system parameters. The obtained results allowed to identify the lubrication components, and the respective friction-velocity curves, more favourable for the stick-slip occurrence. Combining the grease rheology evolutions with the stick-slip dynamic response represents a further challenge in both the domains of research. The Ph.D. thesis has been developed in collaboration between the Sapienza University of Rome, Department of Mechanical and Aerospace Engineering (Rome, Italy), the Institut National des Sciences Appliquées (INSA) of Lyon, laboratory LaMCoS (Lyon, France), and the company SOMFY S.A. (Cluses, France).