The use of roller bearings as crankshaft main bearings has shown potential in reducing the fuel consumption of internal combustion engines. An effective way to mount the roller bearing onto the crankshaft is to split the outer ring. However, this may lead to a severe out-of-roundness in the split region when the bearing is mounted, further implying increased noise, vibrations and contact stresses. In this work, a novel approach to study the plasto-elastohydrodynamic contact using commercial finite element software is developed. The modelling approach is based on the contact moving in space, allowing for the stress history based on the lubricant pressure to be studied in a straight-forward manner. The model is first utilised to study the influence of asperities on the lubricating conditions, indicating that stresses may exceed the yield strength of the material due to the transient effects taking place when the surface feature is over-rolled. Thereafter, the model is used to analyse the step in a mounted crankshaft roller bearing with the purpose of specifying a critical step height, which implies zero plasticity and thereby a reduced risk of accumulated damage in the vicinity of the step.

A crankshaft roller bearing internal combustion engine (ICE) offers a five percent or more improvement in overall engine efficiency and, thereby, a reduction in a five percent of CO₂ emissions, compared to a plain bearing supported crankshaft. Current forged crankshaft steels represent the limiting factor of the rolling component, therefore, a replacement of the crankshaft steel is required. Apart from this, the tribology of the rolling contacts has been shown to be detrimental when lubricated with current engine oils. Therefore, this paper investigates the tribological performance of potential crankshaft bearing steels, i.e. DIN C56E2 (G55); DIN 50CrMo4 (G50); and DIN 100Cr6 (G3), while utilizing a state-of-the-art low viscosity 0W20 engine oil and under conditions prevalent to ICE. For this, damage mode investigation was performed in a disc-on-disc setup. Based on the results, wear damage of DIN 100Cr6 discs was shown to be dependent on the steel grade of which the counterpart disc was made from and surface hardness difference between both discs. In addition, surface fatigue and wear damage can be completely eliminated by selecting a proper surface roughness and hardness combination. Also, while under an elevated roughness level, engine oil was shown to promote both surface fatigue and wear damage through the work of ZDDP additives, which under extreme conditions can act as an extreme pressure (EP) additive. The residual stress measurements using the XRD technique revealed relatively high compressive residual stresses for G55 and G50 in comparison to G3 steel after surface induction hardening. In addition, no significant changes in residual stress for G55 and G50 were observed after the test. In contrast, relatively high tensile stress was observed for G3 near the surface region. This suggests that the most commonly used 100Cr6 bearing steel, in this case, is the most susceptible to surface fatigue.

The current trend of increased power density and reduction of oil viscosity in machine components is forcing engineers and scientists to engineer surfaces to endure higher contact pressures and stresses induced from rolling/sliding motion. Typically, surface-initiated rolling contact fatigue represents the main failure mode and has been gaining more and more attention lately. In this paper, the extent and morphology of the surface damage of different bearing steels in disc-on-disc contact under reduced lubrication conditions are studied. For this, rough-on-smooth contact was selected to promote surface fatigue damage on the smoother surface. Special attention was given to study both the effect of the hardness difference of the discs and the influence of different heat treatments, i.e. surface induction hardening (SIH) or through hardening (TH), on micro-pitting and wear performance. It was demonstrated through tribo-testing that the faster, rough surface undergoes only mild wear and plastic deformation of asperities. However, three different damage modes were observed and assessed on the slower, smoother surface, where the damage mode showed a strong dependence on a surface hardness difference between the smooth and the rough surface. Furthermore, it was observed that a plain medium carbon steel (DIN 56E2) showed better surface fatigue resistance than a low alloy high carbon steel (DIN 100Cr6) at similar surface hardness levels if SIH is applied on the former.

Machine components are designed to endure increasingly severe operating conditions due to the strive for improved energy efficiency of mechanical systems. Consequently, lubricated non-conformal contacts must rely on thin lubricant films where the influence of surface topography on the lubricating conditions becomes significant. Due to the complexity of the multiphysical problem, approximate assumptions are often employed to facilitate numerical studies of elastohydrodynamically lubricated (EHL) contacts. In this work, the rough, time dependent, thermal EHL problem is solved with focus on two main analyses. The first analysis focuses on the influence of sinusoidal roughness and the difference between a thermal non-Newtonian approach and an isothermal Newtonian approach. The second analysis is focused on the lubricating mechanisms taking place when two-sided surface features overtake within the thermal EHL contact. The results indicate that the film thickness in the outlet of the contact may be significantly overestimated by an isothermal Newtonian approach and that differences in the high-pressure region may also occur due to viscosity variations in the inlet of the contact. Moreover, for the studied two-sided surface features, it became evident that not only the surface feature combination but also the overtaking position influence the film thickness and pressure variations significantly.

Understanding the influence of surface roughness in elastohydrodynamically lubricated (EHL) contacts is essential to improve durability and friction performance of machine elements employing non-conformal contacting surfaces. In this work, the transient event of a surface feature passing through a thermal EHL line contact operating under different sliding conditions is investigated with the purpose of providing a deeper understanding of surface roughness influence. This is achieved by solving the EHL problem in space and time. It was seen that sliding influences the temperature rise in the contact significantly, especially in the vicinity of the asperity. However, due to the characteristic behaviour of EHL contacts, the local temperature rise mainly influence the film thickness during exiting of inlet perturbations and the asperity.

The urgent need to reduce greenhouse gas emissions in combination with an increased use of sustainable energy resources have led to immense efforts on improving energy efficiency of mechanical systems. Consequently, modern machine elements are typically lubricated with low viscosity oils and designed to endure an increasingly high power density. This typically implies that the surfaces in contact are separated by thinner lubricant films that must carry higher loads. Thereby, in order to improve the durability and efficiency of future machine elements, an improved fundamental understanding of the predominant lubrication mechanisms must be achieved. This thesis concerns numerical simulation of non-conformal lubricated contacts where significant elastic deformations in combination with hydrodynamic effects govern the lubricating film formation, typically found in e.g., rolling element bearings, gears, and cam-follower systems. Such contacts relate to a lubrication regime known as elastohydrodynamic lubrication (EHL), which is often studied using simplifying assumptions due to its multiphysical nature. Typical simplifications of the EHL problem may include e.g., steady-state conditions, perfectly elastic surfaces, isothermal conditions, and Newtonian lubricant behaviour. However, such simplifications may be progressively relieved because of a continuously improved computer performance and the use of increasingly efficient numerical methods. In this work, time dependent simulations of EHL contacts are conducted with the purpose of improving the fundamental understanding of the predominant lubrication mechanisms. This is achieved by the development and utilisation of novel models and modelling techniques, specifically developed for the conducted investigations focusing on e.g., temperature, plastic deformation, surface roughness, and contact geometry. The developed models are all an extension of a full-system finite element modelling approach that was relatively recently introduced to the EHL community. Important findings from this work relate to the influence of transient fluctuations in EHL contacts, which occur due to both transient loads and moving surface roughness. It is shown that in the case of transient loading of a finite line EHL contact, a steady state assumption may overestimate the minimum film thickness and underestimate the maximum pressure over time due to the formation of lubricant waves travelling through the contact. Furthermore, thermal and non-Newtonian effects on viscosity may be necessary to consider in both qualitative and quantitative studies of roughness within the EHL contact, especially under sliding conditions and short wavelength roughness. Lastly, transient fluctuations related to the over-rolling of surface features are shown to potentially increase the pressure within the EHL contact to such an extent that plastic deformations of the contacting surfaces occur. The models established in this work may provide researchers and engineers with ways to improve their studies of transient EHL contacts. The salient findings from the conducted studies also open up for interesting directions that may be extended to potentially improve the lubrication theories currently in use and thereby improve the design of future machine elements that employ non-conformal contacts.

In order to adapt to increasingly stringent CO2 regulations, the automotive industry must develop and evaluate low cost, low emission solutions in the powertrain technology. This often implies increased power density and the use of low viscosity oils, leading to additional challenges related to the durability of various machine elements. Therefore, an increased understanding of lubricated contacts becomes important where oil viscosity-pressure and compressibility-pressure behaviour have been shown to influence the film thickness and pressure distribution in EHL contacts, further influencing the durability. In this work, a finite line EHL contact is analysed with focus on the oil compressibility- and viscositypressure response, comparing two oils with relatively different behaviour and its influence on subsurface stress concentrations in the contacting bodies. Results indicate that increased pressure gradients and pressure spikes, and therefore increased localized stress concentrations, can be expected for stiffer, less compressible oils, which under transient loading conditions not only affect the outlet but also the edges of the roller

Current state-of-the-art engine oils tend to enhance micro-pitting damage in rolling contacts under certain operating conditions. ZDDP anti-wear additive was shown to promote such behavior. However, in order to optimize an engine oil formulation for rolling contacts, further studies are needed to assess engine oils in terms of micro-pitting and wear damage. This investigation studies the micro-pitting and wear performance of a number of engine oils for rolling contacts in a ball-on-disc configuration under conditions prevalent in crankshaft roller bearing applications. Based on the results it was concluded that an engine oil containing higher blend of PAO base oil compared to the oil mixture of Group III and PAO has a lower tendency towards micro-pitting and wear.