Starved lubrication is an important strategy for minimizing the amount of lubricant needed, and also inevitably occurs during idling and fail-safe lubrication. In this regime, however, the flow of the lubricant and the related friction coefficients are yet to be fully understood. This research aims to make fundamental contributions to the understanding of contact mechanics of partially lubricated contacts. Recent experiments with a pin-on-disk tribometer examined the microscopic behavior of partially filled gaps. Using a new experimental setup on a macroscale, new insights into partially filled gaps with rough surfaces were gained. This work presents the systematic analyses of the lubricant flow, friction coefficients, and other variables over a wide range of friction parameters. Distinct friction behaviors were observed, and similar effects occur on both the micro and macroscale. The experimental results show that a typical Stribeck characteristic is visible regarding not only the relative velocity, but also regarding the lubricant filling level in the gap. The fluid exhibits a variety of flow patterns for various velocities and viscosities. The patterns relate to different friction regimes, such as dry friction and mixed lubrication. It is concluded that the filling level is a valid parameter for regulating the transition from dry friction to hydrodynamic lubrication. These findings are quantified regarding the filling level and it is shown that for the identification of the friction regimes the filling level is an independent parameter in addition to the established parameters like speed, viscosity and pressure.
Currently, adhesive bonding processes are developed and optimised in a time-consuming trial & error procedure, which rarely leads to an optimal solution due to the high complexity of the adhesive flow behaviour during application. The ideal adhesive layer has precise geometric specifications; entrapped air bubbles or overfilling of the bond should be avoided. Numerical methods, such as Computational Fluid Dynamics (CFD), are only capable of calculating squeeze-flow processes to a limited extent. Apart from high computing times, mesh and convergence problems often occur due to the small ratio of adhesive layer thickness to adhesive layer length and width. The Generalised Partially Filled Gap (GPFG) model, published in a companion paper uses fundamental characteristics of every bonding process to derive clever assumptions, and thus provide an efficient simulation tool for adhesive squeeze-flow. The GPFG model simplifies the squeezeflow to a two-dimensional problem, as the flow in thickness direction can be neglected for most bonding processes-without significant loss of accuracy compared to analytical or CFD solutions. The experimental validation of the model is presented in this study. Both stresses and flow geometry were evaluated, and a very good agreement between experiments and model was proven.
In most bonding processes, an adhesive is applied to a substrate in a specific pattern before the second substrate is subsequently pressed against it. During this, the adhesive flows in such a way that, ideally, it completely fills the joint. In practice, however, areas with entrapped air frequently remain in the bonded adhesive layer. Within the scope of a research project, these flows are systematically analyzed in order to identify optimal initial application patterns for the adhesive and substrate geometry to minimise such risks. For this purpose, the authors use an efficient flow model, the partially filled gaps model (PFGM), extended in this study to include the functionality of trapped air pockets. Depending on the volume fractions of air and adhesive, the flow of both phases is computed. Therefore, the model is introduced and fully described, benchmarked with respect to its plausibility and functionality, and results obtained are compared with a CFD calculation. Thereafter, the functionality of openings and closings of the pockets are analyzed. Lastly, the model is then applied to a real scenario created with a Hele-Shaw cell measurement. The benchmark as well as the comparison with the measurement results show the high potential of this technique.
In grinding processes, heat is generated by the contact of the grains with the workpiece. In order to reduce damages on the workpiece and the grinding tool, cutting fluids are necessary for most grinding processes. They have the tasks of cooling and lubricating the contact zone and to remove the chips from the contact area. Different types of cutting fluids perform differently regarding these tasks, which can be investigated on a laboratory scale. However, the results of those experiments are limited to certain workpieces and processes and information about the contact mechanics are not available. The experimental investigation of contact mechanics under cutting fluid influence is hardly possible. For this reason, this paper uses a measurement strategy that uses scaled topographies and has already been successfully applied to contact mechanics problems. With such a setup, it is intended that at an early stage in the development of cutting fluids, their characteristics in terms of contact mechanics can be determined very efficiently. To demonstrate this approach, two different cutting fluids were tested with the help of the associated test rig—a water miscible emulsion and a non-water miscible grinding oil. The two fluids showed fundamentally different characteristics regarding their hydrodynamic load bearing effect, their influence on the friction behavior of the contact and their fluid flow in the gap. The properties analyzed here correspond to the practical application of cutting fluids. The results underline the potential of the presented setup for an integration into the development process of cutting fluids.
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