Sommerfeld Number
S = (η·N)/(P)·(R/c)² - dimensionless bearing operating parameter. Low S → thick film, hydrodynamic; high S → thin, boundary.
Sommerfeld number S
0.3750 -
Formula, assumptions, and limits
S > 0.15 typical for well-lubricated hydrodynamic bearings; 0.05 < S < 0.15 mixed regime; S < 0.05 boundary lube. Large slow-speed main-engine crank bearings run at S ≈ 0.03–0.1 - extreme boundary design requires high-film-strength oils.
Symbol legend
| Symbol | Meaning | Unit | Source |
|---|---|---|---|
| Dynamic viscosity | Pa·s | oil | |
| Shaft speed | rev/s | tachometer | |
| Specific bearing load | Pa | load / area | |
| Journal radius, radial clearance | mm | bearing |