Lubrication regime is the operating state that decides whether two moving metal surfaces are separated by a fluid film or are grinding against each other through it. Get the regime wrong and a bearing or gear rated for years of service can wear out in weeks, and much of that damage happens in the seconds around startup and shutdown, not during steady running.
Every lubricated contact, a journal bearing, a rolling-element bearing, a gear mesh, sits in one of three regimes depending on speed, load, oil viscosity and surface roughness.
The Stribeck curve plots coefficient of friction against a duty parameter that combines viscosity, speed and load (often written as the Hersey number or a similar dimensionless group). Reading it left to right at increasing speed: friction starts high in the boundary regime, falls sharply through mixed lubrication as a partial film builds, reaches a minimum, then rises again slowly through the hydrodynamic regime as viscous drag in the thickening film increases. That minimum point is the transition from mixed to full hydrodynamic lubrication, and it is also close to the most efficient operating point for many bearings.
Engineers classify which regime a contact is in using the specific film thickness, or lambda ratio (λ): the minimum lubricant film thickness divided by the composite root-mean-square roughness of the two surfaces. As a general guide used across tribology and gear engineering:
| Lambda ratio (λ) | Regime |
|---|---|
| λ ≤ 1 | Boundary lubrication |
| 1 < λ < 3 | Mixed lubrication |
| λ ≥ 3 | Hydrodynamic / elastohydrodynamic (full-film) |
For rolling-element bearings, ISO 281 formalizes this relationship through the viscosity ratio kappa (κ), the ratio of the lubricant's actual operating viscosity to the minimum reference viscosity that bearing needs at that speed. Kappa feeds into the bearing life modification factor, so inadequate film thickness shows up directly as a shorter calculated L10 bearing life, not just as a qualitative "more wear" warning.
A rotating shaft only generates a hydrodynamic film once it reaches sufficient speed to drag enough oil into the converging wedge between the surfaces. Below that speed, during startup and during the coast-down at shutdown, the contact is unavoidably in the boundary regime: metal-to-metal contact with only a thin, additive-dependent film for protection. This is well documented in bearing literature: the short window before a full oil film establishes itself at startup is when the bulk of adhesive and abrasive wear takes place, because asperities are grabbing and tearing at each other with no fluid film to separate them. Frequent start-stop cycling multiplies this exposure. A machine that starts and stops ten times a day accumulates far more boundary-regime wear cycles than one that runs continuously at steady speed, even if total running hours are identical.
Bearing life calculations (L10, per ISO 281) assume adequate lubrication film as a baseline. When a bearing runs mostly in the mixed or boundary regime because of low viscosity, low speed, misalignment, or heavy start-stop duty, its actual life can fall well short of the rated calculation regardless of load rating. Practically, this means viscosity selection, oil cleanliness, and startup procedure matter as much as bearing selection itself. Symptoms of chronic under-lubrication (increased vibration, elevated temperature, characteristic frequencies) often show up well before catastrophic failure and are the same signatures picked up by vibration analysis; see bearing failure modes and symptoms and ISO 10816-3 vibration severity for how that shows up on the plant floor.
Gear teeth operate under even more demanding contact conditions than bearings because the contact pressure at the mesh line is high and the film has to reform on every revolution. Research correlating specific film thickness to gear pitting life found that L10 life in the mixed-lubrication regime was only about 11 percent of the life achieved in the full-film regime, a roughly ten-fold difference driven purely by which side of the lambda transition the gear mesh operates on. This result is consistent with the relative trend predicted by AGMA 925-A03 methodology, which is also built on specific-film-thickness logic and is used to assess scuffing and micropitting risk in gear design. In practice this means gearboxes running slightly under-viscosity, slightly hot, or slightly overloaded do not fail gradually and predictably, they can drop out of the full-film regime and into a much higher wear rate with only a modest change in operating condition.
Most plants cannot watch every bearing and gearbox continuously enough to catch the transition out of full-film lubrication before damage starts. Fabrico reads machine condition and OEE from the line and auto-routes a work order the moment a loss is detected (computer vision catches what sensors miss), built and hosted in the EU with EU data residency, and operated under ISO 27001, ISO 20000-1 and ISO 9001. Book a Fabrico demo.
In boundary lubrication the oil film is thinner than the surface roughness, so metal asperities touch directly and friction and wear are high. In hydrodynamic lubrication the film is thick enough that the surfaces never touch, motion alone generates the supporting pressure, and friction and wear drop to their lowest, most stable levels.
Mixed lubrication means the load is shared unpredictably between a partial fluid film and direct asperity contact. Small changes in speed, load, or oil temperature can push the contact further into boundary conditions, so components that spend significant time in mixed lubrication see accelerated and less predictable wear than those running in stable full-film conditions.
Engineers use the lambda ratio (specific film thickness): calculated minimum oil film thickness divided by the composite surface roughness of the mating parts. For rolling bearings, ISO 281 uses an equivalent viscosity ratio, kappa, which feeds directly into the bearing life modification factor.
Yes, in the sense that each start and stop forces the contact through the boundary lubrication regime where wear rates are highest, since a supporting fluid film has not yet developed at low speed. Equipment that cycles frequently accumulates more boundary-regime wear exposure than one running continuously for the same number of total hours.
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