A previous study of engine lubrication revealed that oil is pumped under pressure to the various bearings in the engine. However, to get this oil into the bearing and lubricate it, clearance for an oil film must exist.
The one most important thing to keep in mind in this connection is that the steel crankshaft journal MUST be separated from the bearing metal when the engine is running or the bearing will melt. The heat generated by friction when steel moves rapidly on soft, dry metal WILL melt the soft metal. Therefore, an automobile engine uses a film of oil between the journal and the bearing. SPACE MUST BE PROVIDED FOR THAT FILM.
The oil film serves to hold the two metals apart and also circulates to carry away the heat generated by friction. The space is not great (measured in thousandths), but those thousandths are all important.
This film thickness will vary with the design of the engine and the type of lubrication system used. In general, a splash lubrication system is less critical of oil clearances than a pressure lubrication system. In the splash system, the oil is churned up by internal parts of the engine into a combination of liquid and mist, which is sprayed over the entire interior of the engine.
In the pressure lubricated engine, the oil is pumped under pressure to the bearings. In this case, the flow of oil must be controlled by maintaining limited clearance all around a ROUND bearing and a ROUND shaft. If there are unequal clearances in the circulation system, too much oil will collect in one place, and not enough in other places. This is because oil under pressure will go through the largest clearance space in the greatest quantity.