Sommerfeld number

The Sommerfeld number is the most important parameter for plain bearings. It relates the bearing load (p) and bearing clearance (Ψ) to the viscosity (η) and angular velocity (ω).

Theoretically, the Sommerfeld number is between 0 and ∞, although in practical application it is limited to the range between 0.05 and 20. Bearings with a low Sommerfeld number are insufficiently stressed and tend toward instability. Bearings with a high Sommerfeld number are heavily loaded and tend toward direct material contact due to breaking down of the fluid film.  

So = (p* ψ²) / (η ω)

The lubricant viscosity at the maximum temperature is used with the Sommerfeld number.