The specific film thickness: calculated minimum film thickness divided by the composite RMS roughness of the mating surfaces. Below about 1 the contact is in boundary lubrication, between 1 and 3 it is mixed, and above about 3 the surfaces are fully separated. Gear and bearing engineers use it to set the minimum operating viscosity, and bearing catalogs use a close cousin, the viscosity ratio kappa, for the same job.
Run the numbers at 90 C and the lambda ratio drops under one, so that gear set is basically living in boundary.
Also heard as
- lambda
- specific film thickness
Related terms
Elastohydrodynamic lubrication
Film regime in rolling contacts where huge pressure stiffens the oil and elastically flattens the metal.
Asperity
One of the microscopic peaks on a machined surface that makes first contact when the film gets too thin.
Boundary lubrication
Regime where surfaces touch through the oil and protection comes from chemical surface films, not fluid pressure.
Mixed lubrication
Transitional regime where the load is shared between a partial fluid film and asperity contact.