Bearing Clearance

Canonical Definition

The designed radial gap between a rotating journal and its bearing surface, typically between 1–100 µm depending on shaft diameter and bearing type. The clearance determines the minimum hydrodynamic oil film thickness required to prevent metal-to-metal contact at operating speed and load. Particles larger than approximately half the bearing clearance can contact both bearing surfaces simultaneously (three-body abrasion), causing accelerated wear and loss of clearance. Worn clearances increase lube oil consumption, reduce oil pressure, and accelerate further wear.

Engineering Context

Bearing clearances in high-speed engines (crankshaft main bearings, connecting rod bearings) typically range from 25–75 µm. Particle sizes in the 12–37 µm range are therefore the most damaging for these components, correlating with the ISO 16889 β₁₄(c) rating specification for lube filters. ISO cleanliness codes are calibrated to the most critical clearances in each system type.

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