BEARING LUBRICATION 
Bearing lubrication presents no special difficulties in itself as long as an adequate supply of oil reaches the bearings; this is because the rotation of the journal tends mechanically to build up and maintain an oil film between the bearing and journal surfaces. Bearing failures are due generally, either to the restriction in the supply of oil as the result of mechanical faults or blocked oil-ways, or to break-up of the bearing metal under the repeated pounding to which it is subjected by the power strokes. This latter effect is most likely to occur if there is an abnormally high rate of pressure rise caused by faulty ignition, the use of low grade fuel, incorrect timing, etc.

Circulating systems i internal combustion engines are necessarily enclosed and the oil is exposed to air at moderately high temperatures in the crankcase. Heated air has a very definite oxidizing effect on all oils, an effect which is most pronounced when the oil is in a fine state of dispersion, as it is in the crankcase of an engine during operation. It is therefore desirable that the oil should have high resistance to oxidation, otherwise rapid deterioration of the oil may take place with consequent thickening, sludging and production of acids. With the wider spread use of certain alloy bearing metals which are more susceptible to corrosion than white metal, the need for oils having high oxidation stability has increased, especially in the case of engines running under heavy duty conditions. 

The more common troubles in the bearing lubrication systems of I.C. engines are that the oil thickens up, thins out, emulsifies or throws down black deposits. The last, by lodging in oil-ways and filters, may interfere with the circulation of the oil and in extreme cases, cause bearing failure. Troubles of this nature can occur, irrespective of the quality of oil used, through contamination of various kinds. Thus thinning of oil is caused by unburned fuel reaching the relatively cool cylinder walls and then findings its way into the crankcase. Partially burnt fuel and carbons, which are the results of imperfect combustion, increase the viscosity of the oil and in time are deposited as sludge. Water may be present as a result of a leak in a gasket or condensation of combustion products due to low operating temperatures; this may lead to sludging through formation of oil-and-water emulsions, particularly if  dust or other solid foreign matter is present, even in minute quantities. 

To get the maximum life out of a good lubricating oil, it is necessary to pay great attention to conditions of combustion and to prevent the entry of impurities.