The Science of Bearing Lubrication: Boundary, Mixed, and Elastohydrodynamic Regimes
Lubrication is not merely 'adding oil.' The ratio of film thickness to surface roughness (λ) governs friction and life. Understanding bearing lubrication through the Stribeck curve and EHL theory.
The Purpose of Lubrication
A lubricant does far more than reduce friction. It forms an oil film between the rolling elements and raceways that prevents direct metal-to-metal contact, carries away frictional heat and wear debris, and protects surfaces from corrosion. A large share of premature bearing failures trace back to poor lubrication.
The Stribeck Curve and Three Regimes
Plot the friction coefficient against speed, viscosity, and load and a characteristic curve emerges. This Stribeck curve divides lubrication into three regimes:
- Boundary lubrication — almost no film; surface asperities touch directly. Friction and wear are highest, and additives (extreme-pressure agents, friction modifiers) chemically protect the surface.
- Mixed lubrication — film and asperity contact coexist. Part of the load rides on the film, part on metal contact.
- Full-film / EHL — the film fully separates the two surfaces. Metal contact disappears and wear is minimized.
The Film Parameter λ — The Key Metric
The regime is quantified by the film parameter λ:
λ = h / σ
where h is the minimum film thickness and σ is the composite RMS roughness of the two surfaces.
- λ < 1 : boundary lubrication; high wear risk.
- 1 ≤ λ < 3 : mixed lubrication; where most real bearings operate.
- λ ≥ 3 : full film; surface fatigue dominates life.
Designers typically aim for λ ≥ 2–3. With the same lubricant, low speed or high temperature lowers λ and pushes the contact into the mixed regime.
Why "Elastohydrodynamic"
Pressure in a rolling-element contact reaches several GPa. At that pressure two things happen at once:
- The surfaces deform elastically, widening the contact area.
- The lubricant's viscosity rises sharply with pressure (the piezoviscous effect), so the film resists being squeezed out.
The theory that accounts for both effects is Elastohydrodynamic Lubrication (EHL). Thanks to EHL a sub-micron film can keep surfaces apart even under GPa-level pressure.
Grease vs. Oil
Most rolling-element bearings are lubricated with grease. Grease combines a base oil with a thickener and releases the base oil slowly as needed. It is easy to seal and has long relubrication intervals, making maintenance simple.
Oil is used for high speed, high temperature, or where external cooling is required. Circulating systems actively remove frictional heat. The decisive variable is the base-oil viscosity at operating temperature, because that sets λ.
Practical Checklist
- Select the lubricant by viscosity at operating temperature (not at room temperature).
- Check the speed factor (n·dm) when choosing grease vs. oil.
- Contamination and water destroy the film and sharply shorten life — sealing and cleanliness matter as much as the lubricant itself.
Lubrication accounts for more than half of bearing reliability. Keeping the film parameter λ in mind while reviewing operating conditions is the habit that prevents premature failure.