Highly-loaded lubricated rolling-sliding contacts, which occur in gears, are generally known as elastohy-drodynamically (EHL) or thermal elastohydrodynamically lubricated (TEHL) contacts (Figure 1). The elastic deformation of the rolling-sliding elements allows the transmission of Hertzian pressures of over 2000 N/mm² (20.000 bar) at lubricant film thicknesses of < 1 µm. The transmitted shear stress can even exceed the shear strength of aluminum.
Basic research on film thickness formation in TEHL contacts is carried out at optical EHL tribometers. Based on chromatic interferometry the local film thickness in TEHL contacts is visualized and quantitative-ly evaluated (Figure 2). Depending on load, temperature and velocity, film thickness formation and friction behavior of lubricants, surfaces and materials can be evaluated.
Basic research on rolling and sliding contacts of bearings and gears is frequently carried out at twin disk test rigs, which allow continuous variation of slide-to-roll ratios for different loads and temperatures. Thereby contact-integral measurements of the coefficient of friction and lubricant film thickness as well as contact-local measurements of the pressure, temperature and lubricant film thickness by means of thin film sensors (Figure 3) can be conducted. Tribofilms, which form by interaction of the lubricant and ambi-ent medium in tribological contacts, are systematically investigated in long-term collaboration with re-search partners by latest surface analyses techniques (Figure 4).
Tribosimulations offer the possibility to dissolve the very small scales in space and time of TEHL con-tacts. Thereby, at least the areas of fluid dynamics and contact mechanics have to be solved in parallel. By taking into account measured lubricant properties, the physically based simulation models are suitable to calculate inter alia the temperature and friction behavior along the path of contact of a DLC coated spur gear (Figure 5). Further optimization of tribosimulations is an important subject of research at FZG.
Figure 5: Spur gear geometry plot (left) and TEHL simulation results of the temperature distribution in a coated gear tooth contact (Ziegltrum et al.: doi.org/10.3390/lubricants6010017)