The New Zealand Railways Magazine, Volume 4, Issue 12 (April 1, 1930)
The introduction and distribution of oil within the bearing, the formation of the film and the effectiveness of the film in reducing friction are all dependent upon the materials and physical form of the bearing as resulting from design, workmanship, adjustment and wear.
Unsuitable materials—too hard or too soft, too granular or too fibrous—are sometimes, though rarely, the cause of bearing troubles. The subject of material selection is well understood by machine builders to-day.
Insufficient bearing area, resulting from faulty design, is rare in modern machines. It results in an excessive unit pressure on the bearing surface and enhances the importance of correct formation of the surfaces to produce an effective oil wedge. It may also require the use of a heavier-bodied oil in spite of the greater fluid-frictional losses which result.
Ineffective or insufficient oil inlets prevent complete lubrication. In a long bearing, two or more inlets are usually necessary. If the inlet enters the bearing clearance on the pressure side, it will be closed by the journal; so that oil cannot enter except by applying it under high pressure. This trouble often occurs where the oil holes are on the top of the bearing and there is a strong upward belt pull on the shaft. The location of the oil inlets should be changed to a low pressure area.
Uneven bearing surfaces, due to poor workmanship, result in a reduction of the true bearing area; making it impossible to maintain a strong supporting oil film. Wear may correct this fault or make it worse. The exact formation of the bearing surfaces by good workmanship is important.
Insufficient clearance, either from making the bearing and journal too close a fit or from too tight an adjustment, prevents oil distribution and film formation. Excessive clearance—resulting from inaccurate workmanship, wear, or loose adjustment—reduces the effective pressure of the oil film, and promotes oil waste.
The lack of a wedge-shaped clearance, in which the film pressure can be built up in order to support the load, is the cause of many bearing troubles. This occurs in bearings with small clearance, and is relieved by bevelling the sharp edges of the bearing parts and “easing away” the surfaces of the bearing adjacent to the bevel. This may be done by scraping or machining. Where grooves are employed the edges should always be rounded or eased away.
Incorrect grooving may permit the oil to escape from the end of the bearing or may destroy the film pressure at the point where pressure is required. Unnecessary grooves are harmful. Wear of chamfers and grooves may in time destroy their effectiveness or create sharp edges that cut away the film. Poor oil distribution often occurs in ring, chain, or collar oiled bearings, due to the lack of suitable channels to spread the oil over the full bearing surface.
Without the physical correctness of the bearing, perfection of lubrication is impossible. Especially in the cases where heavy loads must be sustained, much improvement of lubrication can be accomplished by paying attention to the correctness of oil inlets, bearing clearance and bevelled edges; and, where necessary, to the grooving and “easing away” of the surfaces to accomplish oil distribution and oil-wedge formation.