olefud wrote:The 50wt was a generic reference to the old 50W-50 “racing oil” to (rather poorly) refute the thought that thicker oil provides better protection. When viscosity stabilizers were suspect, between greater clearances and thinning of the oil with temperature, higher viscosity was perhaps more appropriate. Now, with roller valve lifter, as riff-raff has discussed, perhaps the critical lube point is the hydrodynamic bearing wedge that doesn’t need particularly high viscosity.
The fluid mechanics/heat transfer/tribology/etc. principles underlying hydrodynamic contacts are similar, whether the hydrodynamic contact is in a journal bearing, a rolling element bearing, or between gear teeth. However, journal bearing contacts have two unique design characteristics that requires different lubricant properties from those used with gears and rolling element bearings. First of all, the hydrodynamic contact in journal bearings consists entirely of relative sliding motion between the bearing/journal surfaces, while the hydrodynamic contact in gear teeth and rolling element bearings consists mostly of conjugate/rolling motion with little sliding. Thus, journal bearings with the large amount of shear created in the oil film, are far more affected by higher oil viscosity in terms of efficiency loss. Gears and rolling element bearings can tolerate use of high viscosity lubricants without incurring large efficiency losses, while journal bearings can not.
The other issue with journal bearings is their much lower allowable film pressure limits, due to the much lower compressive surface fatigue stress capability of the bearing materials, versus the hardened steel materials used for gears and rolling element bearings. While the surface contact fatigue stress limit for a gear or rolling element bearing might typically be something like 180ksi, a typical engine journal bearing shell would have a contact fatigue stress limit of something like 45ksi or less.
