I acknowledge your disagreement. Ultimately, I'm simplifying things a bit. There's many factors in this such as pressure-viscosity coefficient, dynamic viscosity, etc... plus a buffer. However, this isn't my opinion. In speaking with multiple tribologists and lubricant engineers currently in the field, lubricant company owners, engine builders in professional motorsports, and engineers from Cummins and GM, they all stated the same thing. The bearings decide the viscosity because they're carrying the load. Higher viscosity (dynamic) will carry a higher load and a wider clearance wants a thicker oil film to control crank journal eccentricity. If any other part of the engine wants something different, too bad. The bearings take priority.
Given a typical 220°F sump temp... (generalized)
.0016-.0021" = 20 grade
.0022-.0027" = 30 grade
.0028-.0033" = 40 grade
.0034-.0040" = 50 grade
This is a generalized guide. It's a starting point that will be accurate the vast majority of the time. It obviously will change with temperature which is why I noted temperature first. Specific applications can and do vary from this at times depending on the mountain of variables at play from bearing width and diameter to rpm to block/rod material and so on. Driven came up with their own chart for this as well.
Ah ... I think I see what you're getting at.
Again, I'll nit-pick, but not meant as offense to you, just the way you stated it. If this comes off rude then I certainly apologize in advance; not meant as an affront to you personally.
If I understand you correctly, it would be better stated from your point of view in that bearing clearances are part of the determining factor in the
selection of a viscosity grade. It's not that clearances determine the viscosity of a fluid, but to avoid damage, the clearances must be maintained by a lube which can provide a viscosity sufficient to hold the MOFT (often assured by grade or HTHS specs).
When you stated it previously (hence my initial objection), you said "
viscosity is determined by 3 factors". I took that as the lubricant viscosity is manipulated by those three things (I do agree that temp directly affects it; the other two, no).
But what you are really trying to convey is that bearing loads and clearances dictate a specific grade for the acceptable minimum film barrier, etc. And that, in turn, is how the OE determines the viscosity recommendation. Your point became more clear when you said "bearings decide the viscosity".
It's not that bearing clearances actually affect the viscosity of the lube. Rather, the bearing clearances need a MOFT barrier, and that is criteria used to select a lubricant of grade "X" to satisfy that requirement. Viscosity of the lube isn't "determined" by those characteristics. Viscosity desired to maintain those clearances results in a selection of the recommended grade (or HTHS value). Bigger clearances dictate the need for thicker vis.
My view is that viscosity of the lube is determined by chemical composition and temperature. However, I can import your points into my view by agreeing with you, in that bearing clearances need a lube with a MOFT, and that is satisfied by the selection of a sufficient lube viscosity at the expected operational temp range.
Is that a better interpretation of your information?
