Effect of macroscopic wear on friction in lubricated concentrated contacts

Wear is considered one of the main challenges in the twenty-first century for engineers and designers of mechanical systems. The objective is to understand the wear mechanisms and to seek new solutions - materials, lubricants, additives - to extend the lifetime of components, enable scheduled maintenance and replacement intervals. Research conducted during the past decades shows that wear can be minimized but not eliminated from systems operation due to a large number of parameters which are influencing the evolution of this phenomenon: load, velocity, temperature, type of lubrication as well as the surface roughness. Depending on operation conditions, the occurrence of wear leads to a change in the macro contact geometry of the components. In time, this will affect the functioning of the components, for example: high friction for brakes, clutches and transmissions or low friction for cylinder-piston contact, cam-follower and gears. In this thesis the influence of macroscopic wear on friction in lubricated sliding concentrated contacts is investigated. Experimental wear and friction tests were conducted on different types of lubricated contacts: line, point and elliptical. These tests have shown a change in contact geometry and in operating regime of the systems, i.e. friction level. To understand this, a relation between wear, contact geometry and minimum film thickness is made. In a lubricated contact three zones are distinguished: inlet, contact and outlet zone. The inlet zone dictates the formation of the minimum film thickness between contacting surfaces. When wear is present the contact geometry changes, leading to a modification in pressure distribution. Changes in separation due to wear are modeled based on hydrodynamic theory and are incorporated in a deterministic mixed lubrication friction model. Using this model the transition between the lubrication regimes, as shown in a Stribeck curve, can be predicted. The experimentally obtained results are in agreement with the theoretical simulations. It is shown that increased wear leads to a decreased friction level resulting from the occurrence of the hydrodynamic effects due to a reduced contact pressure. This causes a change in the operating regime of the system. In the one case this changed regime can be considered to be the main cause of failure in a system, whereas in another system it means smooth operation after a period of running-in. The theoretical results are presented in a generalized Stribeck curve. From this, it is possible to select the parameters such that the components of a lubricated system operate in the preferred regime in order to control friction and minimize wear.