Bearings carry load on a film of lubricant. The Stribeck curve shows three regimes set by speed, viscosity and load: boundary (metal touching, wearing), mixed (partial film), and hydrodynamic (a full film, surfaces fully separated — where you want to live).
Rated life is the L10: L10 = (C/P)p × 106 revolutions. Load hurts hard — for a ball bearing, life goes with the cube of the load ratio, so doubling the load cuts life to one eighth.
But most bearings never reach their L10. Around 80% of premature failures are lubrication and contamination problems — wrong oil, too much or too little, dirt or water — not metal fatigue. Get the lubrication right and you fix most bearing failures before they start.
1 · What a bearing does
A bearing locates a rotating shaft and carries the loads on it — the weight of the rotor, the hydraulic side-thrust from the impeller (largest when you run far from the best efficiency point), and the axial thrust down the shaft — all while letting it spin with as little friction as possible. It does that by keeping the moving and stationary parts separated by a lubricant film. The film is the bearing. Everything else is there to support and feed it.
2 · Two families
- Rolling-element (anti-friction) bearings — balls or rollers run between two rings. Low starting friction, compact, take combined loads, and used in the great majority of process pumps. They have a finite fatigue life (the L10).
- Plain / journal (sleeve) bearings — the shaft rides directly on a film inside a bush. No rolling elements to fatigue, so given a healthy film they can last almost indefinitely; the standard for large, high-speed or heavily loaded machines. They depend entirely on building a hydrodynamic film.
Both live or die by the same physics — the lubricant film — so that is where we start.
3 · The three lubrication regimes
How well the film separates the surfaces depends on three things: how fast they move (speed drags lubricant into the gap), how thick the lubricant is (viscosity), and how hard they're pressed together (load squeezes it out). Combine them into one duty parameter and plot friction against it, and you get the famous Stribeck curve:
- Boundary (low duty) — almost no film; asperities touch and the surfaces wear. This is where a bearing lives at start-up, at very low speed, or under shock/overload.
- Mixed — a partial film carries some of the load while peaks still touch. Friction is at its minimum here, but there is still wear.
- Hydrodynamic / elastohydrodynamic (EHL) (high duty) — a continuous film fully separates the surfaces. No metal contact, no wear — the bearing can run for years. Friction rises gently as viscous drag increases.
Interactive 1 — The Stribeck curve
Live modelDrop the speed or the viscosity, or crank up the load, and watch the operating point slide left into the boundary regime where metal touches metal. Open it back up to reach the full hydrodynamic film.
Friction vs duty parameter
μ = 0.004 + 0.11·e−D/0.06 + 0.02·D, where the duty parameter D = η·N/P (viscosity in Pa·s, speed in rev/s, load in MPa, scaled for display). Indicative shapes — real regime boundaries depend on surface finish, geometry and the specific lubricant.This is the deep reason a centrifugal pump is never left running dead-headed and is started carefully: at the instant of start the shaft speed is zero, so the bearing (and the seal film) sits hard in the boundary regime. The film only builds once it is turning. Slow rolls, frequent starts, and shock loads all spend time on the dangerous left of this curve.
4 · How long should a bearing last? The L10
For rolling-element bearings, fatigue life is statistical — run a batch and they fail over a spread. The standard rating is the L10: the life that 90% of bearings will reach or exceed (10% fail by then). ISO 281 gives it as:
L10h = L10 / (60 · n).
The exponent is the whole story. Because p = 3 for ball bearings, life depends on the cube of the load ratio — so a bearing running at twice its design load lasts only one eighth as long. This is why imbalance, misalignment and off-BEP side-thrust — all of which raise the dynamic load — are such efficient bearing-killers.
Interactive 2 — L10 bearing life
Live modelL10 life vs load
L10 = (C/P)p·106 rev, C = 60 kN. This is fatigue life only; it assumes good lubrication and clean conditions — the modified life Lnm adds factors for exactly the contamination and lubrication effects below.5 · Why bearings really fail
Here is the twist the L10 hides: in the field, most bearings never fail by fatigue at all. Study after study puts roughly 80% of premature bearing failures down to causes that have nothing to do with the metal getting tired:
| Cause | What happens | Tell-tale |
|---|---|---|
| Lubrication | Wrong grade, too much or too little, wrong interval, mixed greases — the film never forms or breaks down | Discoloured/over-greased, skidding |
| Contamination | Dirt or water in the oil; particles dent the raceways and water destroys the film and corrodes | Dents, etching, rust, milky oil |
| Misalignment | Coupling or mounting misalignment loads the bearing off-axis | Wear path angled across the race |
| Imbalance / off-BEP load | Raises the dynamic load P — and the cube law does the rest | Even, heavy wear; short life |
| Fitting / handling | Brinelling from hammer fitting; passing current through the bearing (fluting) | Evenly spaced dents; washboard |
The reliability point: the bearing is almost always reporting a problem somewhere else — a dirty lube system, a misaligned coupling, an unbalanced rotor, a pump dragged off its BEP. That is exactly what condition monitoring is for: the misalignment, imbalance and bearing-defect signatures all show up in the vibration spectrum long before the bearing seizes — which is the subject of our new condition-based maintenance series.
6 · Getting lubrication right
- Right lubricant — the viscosity grade that puts the bearing in the hydrodynamic regime at operating temperature and speed (exactly the Stribeck trade-off above). Don't mix incompatible greases.
- Right amount — over-greasing is as damaging as under-greasing: it churns, overheats and blows seals. Grease to the calculated quantity, not "until it comes out."
- Right interval — relubricate on a schedule based on speed, size and temperature, or better, on condition.
- Keep it clean and dry — sealing, breathers and filtration matter more than almost anything. Most contamination is ingested through breathers and failed seals.
- Mount and align properly — correct fitting tools (not hammers), and align the coupling. Alignment is the cheapest reliability you can buy.
Key takeaways
- The film is the bearing — the Stribeck curve's three regimes (boundary / mixed / hydrodynamic) are set by speed, viscosity and load.
- Start-up, low speed and shock loads push the bearing into boundary lubrication where it wears.
- L10 life goes with the cube of load ratio (ball bearings) — small overloads cost huge life.
- ~80% of failures are lubrication and contamination, not fatigue — fix those and bearings reach their rated life.
- Bearings report upstream problems — misalignment, imbalance, off-BEP load — and condition monitoring catches them early.