The condition in which the oil film is thinner than the surface roughness, so asperities carry most of the load and rub against each other. Viscosity does little here; protection comes from adsorbed layers and reaction films laid down by antiwear and extreme pressure additives. It is normal at start-up, shutdown and reversal points, and in slow, heavily loaded parts such as slideways, open gears and cam followers.
Every start-up that conveyor runs in boundary lubrication for the first few turns, so the AW package is doing all the work.
Slow, heavy machines such as kiln trunnions and open gears live in boundary most of the time, so additive chemistry and grease choice matter more there than viscosity.
Also heard as
- boundary regime
- thin-film lubrication
Do not mix it up with
- Mixed lubrication: In mixed lubrication the fluid film carries part of the load; in boundary lubrication it carries almost none.
Related terms
Mixed lubrication
Transitional regime where the load is shared between a partial fluid film and asperity contact.
Stribeck curve
Plot of friction against viscosity times speed over load, showing the boundary, mixed and full-film regimes.
AW
Antiwear: additives or oils that build protective films to cut wear under moderate boundary contact.
EP
Extreme pressure: additives or oils that react with metal under heavy load to prevent welding and scuffing.
Lambda ratio
Oil film thickness divided by the combined roughness of the two surfaces, used to judge the lubrication regime.