Improvement of crankshaft oil hole quality through surface integrity studies

With increasingly strict emission regulations, engine manufacturers are being forced to increase peak cylinder pressures and reduce the size of crankshaft bearings to increase specific power output and efficiency. This places greater stress on the drilled crankshaft oil hole and increases the probability of fatigue failure. The goal of this research is to gain an improved understanding of the surface integrity of drilled holes, to enable manufacturers to select better machining strategies, and to have greater confidence when applying safety factors to designs. Current research surrounding the surface integrity of drilled oil holes is limited due to the infrequent use of drilled holes in heavily loaded components in other applications and the prior dominance of failure at other locations within the crankshaft. This thesis addresses this problem by applying state-of-the-art surface and materials analysis techniques to characterise the evolution of hole quality under varied drilling conditions. Through the exploitation of X-ray diffraction and electron backscatter diffraction techniques, large microstructural changes within hole surfaces are identified without significant sample preparation. Using this new analysis method, a new thermally activated microstructural recovery phenomenon is identified after burnishing that has significant implications for industrial implementation. Thermomechanical testing of 4140 Steel reveals a strong link between deformation temperature and microstructural evolution. In light of this, the surface integrity of holes drilled with two lubrication strategies with different cooling capacities is analysed. The results demonstrate that, although there may be sustainability benefits, the increased cutting temperatures associated with alternative lubrication strategies may restrict hole quality. The results of this project will allow engine manufacturers to design the crankshaft with a better understanding of the expected robustness and fatigue life of the oil hole and make more informed decisions when selecting drilling strategies for industrial manufacturing.