The Stribeck curve: the one chart behind every lubrication regime
Friction against speed makes a swoosh. Read it left to right and boundary, mixed and full-film lubrication, and why start-up is risky, all fall out.
Where it comes from
The curve was developed by the German engineer Richard Stribeck in 1902. It also explains how a journal bearing can run from boundary lubrication at one end to oil whirl at the other, a range described by the Sommerfeld number.
Reading it, speed by speed
- Low speed: journal and bearing touch and the high points carry the load. Friction is high and flat. This is boundary lubrication, the start-up and shutdown window.
- Rising speed: rotation drags oil into the gap faster than it escapes. A film builds and lifts some contact apart. This is the mixed regime and the steepest part of the curve. Friction falls fast over a small speed change, like a water-skier rising onto the surface.
- Transition: the surfaces separate completely and the film alone carries the load. A common rule of thumb puts this at a lambda ratio of about 3, meaning film thickness is roughly three times the combined roughness of both surfaces. A 1 micron film over 0.3 micron combined roughness gives lambda of about 3. This is also where friction is lowest.
- Higher speed: friction climbs again, now from the oil's viscosity resisting flow. Fluid friction is still friction. It makes heat, heat thins the oil, and a thinner oil gives a thinner film. This is a self-limiting phenomenon.
The minimum is not the sweet spot
The lowest point is only where surface contact has just cleared. Equipment does not run in a perfect state, because speed, load and temperature change. So engineers build in a safety factor on viscosity, expressed through the viscosity ratio kappa, and set the operating viscosity at around three times the point where contact ends. That is a small buffer without paying much in viscous drag.
What it means in practice
- Start-up and shutdown are the riskiest moments for any rotating machine. You pass through the boundary and mixed regimes every time, whether the machine then runs five minutes or five years.
- Running needlessly fast, or with oil thicker than the application calls for, is not automatically safer. It just moves you up the rising right-hand side of the curve with more drag and heat.
- Check kappa at the real running temperature, not at 40 degrees C.