Tribology of Rolling-Sliding Contacts under Mixed Lubrication : With focus on a Crankshaft Roller Bearing Application

• A continuous increase in environmental legislation to reduce CO2 emissions is forcing engineers and scientists to develop more efficient and durable mechanical components, i.e. bearings, crankshafts, gears, etc. Such components are forced to operate under more severe operating conditions, reduced lubrication conditions, and under increased power density. The main failure mode has switched from traditionally subsurface to surface-initiated fatigue, typically caused by surface distress or micro-pitting. In this work, the tribology of rolling/sliding contacts, mimicking rolling bearing contact kinematics, operating under mixed lubrication conditions was studied. A special focus was directed to investigate the feasibility of employing such contacts in a crankshaft rolling bearing application by studying the tribological interface between a crankshaft and a roller. Three bearing steels were identified, selected, and then test specimens were manufactured from these steels for this work to represent a possible crankshaft rolling bearing steel for a four-cylinder light-duty internal combustion engine (ICE), as the current crankshaft steel does not meet the requirements to represent the rolling bearing component. Three tribological screening techniques were selected for this work to understand and investigate the surface performance, i.e. micro-pitting and wear damage of the rolling/sliding contact. The effect of the surface roughness and hardness, steel, and lubricant on the surface performance was investigated. This work was purely experimental, utilizing two tribological test devices: a twin-disc machine and a ball-on-disc machine. The results from this work will allow us to select and optimize the tribology of crankshaft rolling bearing contacts and bearing contacts in general to maximize the surface performance and in turn, contribute to increased efficiency and reliability of mechanical components. Based on the results from this Ph.D. work, micro-pitting and wear damage presents the main damage mode that can be associated with the engine tribology for a crankshaft rolling bearing. Micro-pitting and wear damage were found to be strongly dependent on the surface roughness and hardness combination of both contacting surfaces. A critical hardness difference was found between both surfaces, which is dependent on steel and heat treatments, where wear mode changes from severe fatigue wear to mild wear. A fully formulated state-of-the-art engine oil showed an increased wear component and, in some cases, eliminated micro-pitting damage compared to using only the base oil. Furthermore, it was shown that the ZDDP additive, present in the engine oil, can function as an anti-wear (AW) additive, as intended, or an extreme pressure (EP) additive depending on the contact iv severity. The latter increases the wear component through mild corrosion and delamination of the surface tribolayer, which leads to surface-initiated fatigue.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Maskinteknik -- Tribologi (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Mechanical Engineering -- Tribology (hsv//eng)

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Maskinelement

Machine Elements

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