Hydrodynamic Lubrication

Canonical Definition

The lubrication regime in which two surfaces in relative motion are completely separated by a pressurised oil film generated by the geometry of the bearing and the relative motion of the surfaces, with no direct metal-to-metal contact. Load is supported entirely by the hydrodynamic pressure of the oil film. This regime requires sufficient viscosity, shaft speed, and load to sustain full film separation. Oil film thickness in engine crankshaft journal bearings is typically 5–50 µm depending on speed and load. Hydrodynamic lubrication produces the lowest friction and, when sustained, results in negligible abrasive wear of bearing surfaces.

Engineering Context

Oil cleanliness must be maintained so that suspended particle size does not exceed the minimum oil film thickness. In a crankshaft bearing with a 20 µm minimum film, particles >20 µm cause direct surface penetration and abrasive scoring. This is why ISO cleanliness targets for engine lube systems (16/14/11) limit particles >14 µm to 10–20 per mL — larger particles disrupt the hydrodynamic film and initiate abrasive damage sequences. Viscosity breakdown from overextended oil drain intervals reduces film thickness, increasing the risk of mixed-film or boundary lubrication conditions where wear rates increase by 100–1,000×.

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