Two metal surfaces sliding against each other should weld themselves together — the asperities (microscopic peaks) on each surface plastically deform and cold-weld under load. Lubrication is what prevents this. But not all lubrication works the same way. The Stribeck curve describes three distinct regimes, and knowing which one your machine operates in determines whether it lasts 10 hours or 10 years.

Boundary lubrication happens at startup, low speed, or high load. The oil film is thinner than the surface roughness, so asperities still touch. Wear depends entirely on additives — ZDDP, moly, EP agents — that form sacrificial chemical films on the metal. This is where engine cams and followers live during cold starts, and why oil chemistry matters more than viscosity for short-trip drivers.

Mixed lubrication is the transitional middle. Partial fluid film, partial asperity contact. Friction drops sharply as speed increases. Most plain bearings pass through this regime briefly during startup and shutdown — which is why those moments cause disproportionate wear.

Hydrodynamic (full-film) lubrication is the goal. The moving surface drags oil into a converging wedge, building enough pressure to fully separate the surfaces. Zero metal-to-metal contact, zero wear (in theory). A properly designed journal bearing in a steam turbine can run for decades because it never leaves this regime once spinning.

Rule of thumb — the Hersey number: H = (μ × N) / P, where μ is dynamic viscosity, N is rotational speed, and P is bearing pressure. Plot friction vs. H and you get the Stribeck curve. Higher viscosity, higher speed, or lower load pushes you toward full-film. Thinner oil, slower speed, or heavier load drops you into boundary.

Real-world example: A wind turbine main bearing fails not at rated speed but during low-wind feathering and yaw events — those slow-speed reversals operate in boundary or mixed regime, smearing the races. Operators now use synthetic greases with strong EP additives specifically because hydrodynamic separation isn't achievable at those speeds. Similarly, hypoid gears in truck differentials run at extreme contact pressures that crush ordinary oil films, which is why GL-5 gear oils contain sulfur-phosphorus EP additives that are actually corrosive to yellow metals — a tradeoff accepted because the alternative is welded gear teeth.

Quick check: If a bearing whines on startup but goes quiet at speed, you're transitioning correctly through Stribeck. If it stays noisy at speed, your oil is too thin, your load is too high, or your surface finish is shot.