The Sommerfeld number can be written as

$$ S = \frac{6 \ \mu \ U \ b}{p \ c^2} $$


  • U: Velocity
  • b: Width
  • p: Pressure
  • c: Minimal Clearing
  • $\mu$: Absolute Viscosity

As far as I'm aware, it may be used for both thrust and journal bearings. It is also known as the Dimensionless Bearing Number or dimensionless surface pressure.

What I don't understand or differently stated, I couldn't find any resources on the significance of that dimensionless number.

For instance the Reynolds number is the ratio of the inertia and viscous forces. Depending on the situation, any value, approximately, above 1000 or 3000 is considered to be turbulent flow.

What is the ballpark for the Sommerfeld number?

When is it too small or too large to disrupt operation or damage the bearing?

Do several "zones" of values exist that exhibit similar behaviour? (such as between 1 and 3 it behaves as ....?)

In case there are huge differences between journal and thrust bearings in regard to the Sommerfeld number, I would be more interested in the thrust bearing case.


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