The New Zealand Railways Magazine, Volume 4, Issue 3 (July 1, 1929)
Recognising the importance of the oil wedge as the fundamental basis of film formation, it is desirable to examine and compare the various methods by which it may be formed and maintained, and to determine what physical conditions are favourable to its effectiveness. This calls for a discussion of the distribution of the distribution of the oil within the clearance to form an oil wedge, the features of construction which influence oil-wedge and film formation and the methods of supplying lubricants to the bearing.
The diameter of the shaft is always slightly smaller than that of the bearing surface. The difference between the two diameters is called the bearing clearance. In new bearings the amount of this clearance varies according to the service intended, a common rule being one-thousandth of an inch per inch of diameter: rather less than this for large bearings is good practice. Small bearing clearance is generally favourable to the formation of a strong oil wedge and film.
The correct construction of bearings requires such length and diameter as will produce the area necessary for supporting the shaft load without excessive pressures. For high-speed journals with small clearance, greater unit-pressures are generally allowable.
Irregularities (even microscopic) in the surfaces of the journal and the bearing will tend to cause striking or interlocking of high points. These irregularities under high pressures and slow speeds break through the oil film and cause heating, due to metallic friction. The perfection of the surfaces, therefore, influences the distribution of the oil and the amount of wear that will take place.
The point at which oil is introduced into a bearing is of importance, as it influences the distribution of oil in the clearance space where the oil wedge is formed. The point of introduction should be located correctly, both circumferentially and longitudinally, and it is often desirable that more than one point of introduction should be provided.
The journal load may be constant or variable, in a single or changing direction; or the direction of the load may be reversed alternately, as in the main bearings of a double- acting engine.
It has been explained that the pressure of oil in the film varies in a circumferential direction around the surface of the bearing. Where the load is constantly in one direction, it is evident that the oil should be introduced in the bearing where the pressure is low. No oil can enter the clearance space if the oil hole is closed by the pressure of the journal, unless it be forced in by a pump at a pressure greater than that resulting from the weight or load of the journal—a method employed only rarely on large machines. For this reason, when the load of page 52 the journal is downward, oil should be introduced at or near the top of the bearing, where the oil pressure is low. An upward journal-load, due to belt pull or other cause, calls for the oil to be introduced near the bottom of the bearing. Where the load fluctuates rapidly in an alternately reversed direction, the rotating motion of the journal forms an oil film in advance of the constantly changing direction of pressure; and, therefore, the oil may be introduced into the clearance space at any point.
The distribution of oil uniformly along the length of the bearing demands its introduction at one or more points from which it will spread rapidly in the form of a wedge under the action of the rotating journal. For horizontal bearings the point of introduction should be at the middle of the length, except when the length is too great for uniform distribution.
The majority of bearings of small and medium size are provided with only one point of oil introduction. Bearings longer than eight inches (200 mm.) demand two or more points of introduction, unless longitudinal grooving is provided for spreading the oil towards the bearing ends. For vertical bearings, it is generally best to introduce the oil at or near the top of the bearing, on the low pressure side, so that it will be compelled to pass through the full length of the bearing before it can pass out the lower end.