The Influence of Circumferential Waviness of the Journal on the Lubrication of Dynamically Loaded Journal Bearings

Current trends in automotive engine design are towards smaller, lighter components operating under higher specific loads. Consequently, engine bearings are expected to operate under highly stressed conditions, with minimum lubricant film thicknesses falling below 1μm. There is, however, insufficient understanding of acceptable tolerances on surface geometry of bearing shells and crankshaft pins. Measurement data suggest that some engine crankpins are machined with as many as 21 circumferential lobes. Some lobes have amplitudes in excess of 5 μm and are thought to be responsible for premature bearing damage. This study presents results from a theoretical analysis of dynamically loaded journal bearings with circumferential lobes on the journal. The Reynolds equation for a rigid journal bearing is solved for an incompressible, Newtonian, iso-viscous lubricant, with a flow conserving cavitation model accommodating oil film history. The influence of lobe number and size on operating parameters such as maximum lubricant pressure and minimum film thickness is examined. Theoretical results show that under heavy loads, lobed shafts are responsible for significantly increased maximum pressures and reduced minimum film thicknesses. The presence of lobes induces a series of fluctuating pressure waves in the lubricant film, thereby increasing maximum lubricant pressures.