Lambda ratio
Lambda ratio turns up in 2 fields: tribology, gears. Here is what it means to the people in each one.
Lambda ratio in tribology
Oil film thickness divided by the combined roughness of the two surfaces, used to judge the lubrication regime.
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."
Lambda ratio in gears
Oil film thickness divided by combined surface roughness, showing how well the lubricant separates the teeth.
The specific film thickness: calculated minimum elastohydrodynamic film thickness divided by the combined roughness of the two flanks. Below about 1 the teeth run in boundary lubrication with heavy asperity contact, between 1 and 3 in mixed lubrication, and above about 3 in full film. Low lambda drives micropitting, wear and scuffing, and it is raised by higher viscosity, lower temperature, higher speed or smoother surfaces.
"Lambda ratio's about 0.6 at full load and low speed, no wonder it's micropitting, we need smoother flanks or thicker oil."