When selecting bearings, engineers should be concerned also about specifying the lubricants that will provide an optimal performance. In order to do that, the viscosity ratio is the parameter to defined in order to select the lubricant. Viscosity ratio is defined as:

$$ \kappa = \frac{\nu}{\nu_{1}} $$


  • $ \nu $ is the operating viscosity of the lubricant [$mm^2/s$]
  • $\nu_{1}$ is the rated viscosity depending on the bearing mean diameter and rotational speed [$mm^2/s$]

From literature, it is said that a viscosity ratio greater than one can extend the bearing's life. If $ 0.4 \lt \kappa \lt 1$ it is recommended to use additives. While if $ \kappa \lt 0.4$ it is compulsory to use them.

What are the reasons behind this?


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