23.11.2021: Join us at PRI King Engine Bearings Seminar is Back to PRI SHOW! You are welcome at our booth #601 (Green Hall) 9-11 December 2021 at the Indianapolis Convention Center. Our Seminar, titled ” Why traditional race bearings just can’t hack it? ” will take place on Friday, Dec 10...
See the section: Effect of Oil Viscosity in the first link. The effect of oil clearance on minimum oil film thickness for oils with different viscosity and at various rotation speeds is demonstrated in Figure 9. Also see the Conclusion section at bottom of the page.
It does indicate that there is a "sweet spot" of bearing clearance vs oil viscosity - clearance of 0.002-0.003" seems to work for about everything. Also, tighter bearing clearance results in a smaller MOFT, which also causes the shear rate to increase which causes the oil temperature inside the bearing to increase (3rd bullet below. and Figure 8). Most people think there is more heat rise in the bearing from thicker oil, but the shear rate and temperature rise inside the bearing is dependent on the MOFT too. Plus tighter bearings flow less oil, so that hurts the cooling too.
- There is a value of clearance at which minimum oil film thickness reaches the maximum value.
- Loose clearance results in lower temperature rise, but in greater oil leakage (risk of oil starvation), and in higher oil pressure peak (risk of material fatigue).
- Tight clearance results in less oil leakage and lower peak oil pressure, but in greater oil temperature rise and greater sensitivity to geometric distortions of the housing and crankshaft.
- The optimal range of clearance is 0.0015-0.003” (for a bearing with 2” diameter). Looser clearances are more suitable for highly loaded engines working at high rotation speeds and with thicker oils. Tighter clearances provide a better combination of hydrodynamic parameters in less loaded engines working at lower rotation speeds and using thinner oils.
- Higher oil viscosity produces greater minimum oil film thickness, more power loss due to friction and more uniform oil film pressure.
- King Engine Bearings developed a production technology - Bull’s Eye Tolerance™ - ensuring very accurate wall thicknesses.
- The consistent shell-to-shell thickness of King bearings results in a minor difference of oil flow rate and a more stable hydrodynamic lubrication.
I still haven't found any technical information that says thinner oil results in more MOFT and more bearing protection. Can anyone find that? The bigger concern is using the correct "W" rating for the lowest temperature environment you are in.