The Stribeck Curve
In 1902, Richard Stribeck published his landmark study of bearing friction, revealing a characteristic curve that remains the cornerstone of lubrication engineering. By plotting friction coefficient against a parameter combining speed, viscosity, and load, Stribeck showed that friction passes through a minimum as conditions change — high in boundary lubrication, dropping through a minimum in the mixed regime, then rising again in the viscous hydrodynamic regime.
Three Regimes of Lubrication
Boundary lubrication occurs at low speeds, high loads, or low viscosity: the oil film is thinner than surface roughness, and asperities contact directly. Mixed lubrication is the transition zone where partial film supports some load while asperity contacts carry the rest. Full hydrodynamic lubrication occurs when the film is thick enough to completely separate surfaces — friction is purely viscous shear, and wear is essentially zero. Each regime demands different design strategies.
The Lambda Ratio
The lambda ratio Λ = h_min/R_q provides a quantitative criterion for lubrication regime. When the minimum film thickness h_min exceeds about three times the composite surface roughness R_q, surfaces are fully separated. Modern surface metrology and elastohydrodynamic (EHL) film thickness calculations allow engineers to predict Λ for any operating condition, enabling rational lubricant and surface finish selection.
Design Implications
Understanding the Stribeck curve is essential for machine design. Engine bearings must operate in the hydrodynamic regime during normal running but survive boundary conditions at startup. Gears often operate in mixed or EHL regimes, demanding robust additive packages. The trend toward lower-viscosity lubricants for fuel efficiency pushes systems closer to the mixed regime, requiring better surface engineering. This simulation lets you explore how each parameter shifts the operating point on the Stribeck curve.