The New Zealand Railways Magazine, Volume 4, Issue 3 (July 1, 1929)
The Lubrication of Bearings: Part II. — Formation and Maintenance of the Lubricating Oil Film
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.
Influence of Construction on Wedge and Film Formation.
In general, the purpose of grooving is to facilitate oil distribution to the oil wedge and film. Oil grooves in bearings are as often harmful as beneficial. Incorrectly applied, they may destroy the oil film. Their function is to assist in spreading the oil over the full bearing surface. They should be cut from the point of oil entry in the top bearing brass diverging towards the bearing ends in the direction of rotation. In cases where the shaft may rotate in either direction the oil grooves should be cut in both directions from this point of entry. They should never be cut right to the ends of the bearing brass, otherwise oil will escape from the bearing resulting in insufficient supply to lubricate the surfaces, and necessitating the use of greater quantities of oil to prevent overheating. Such an arrangement of grooving as this is shown in Fig. 6, where it will be seen that the oil supplied to the top of the shaft is carried across the face of the journal and towards the chamfered edges of the brass where it is distributed over the entire surface of the revolving shaft.
Under normal conditions of service it is undesirable to employ grooving on the bottom brass because the surfaces of bearings made irregular by oil grooves tend to interfere with the formation of the wedge shaped film between the revolving shaft and the supporting bearing. Oil grooves are not necessary where the oil is forced under pressure to the bearing, but chamfered and rounded edges of the bearing brass must be used under all circumstances. A common use in bearings with small clearance is to provide a passage for the oil to spread, from a point of introduction, longitudinally along the bearing. For this purpose a longitudinal groove is cut through the oil inlet, as shown in Fig. 7, to points about half an inch (12 mm.) inside the bearing ends.
In large heavily loaded bearings, where the shaft speed is low, there is always a serious tendency for the oil to escape from the ends before it is carried into the pressure area. Some relief from this difficulty is obtained by the use of a groove as illustrated in Fig. 8, located about 30 deg. to 45 deg. in advance of the area of highest pressure. This groove extends longitudinally nearly to the ends of the bearing, and enables some of the oil near the ends of the bearing to return to the middle. Such a groove should not be employed except when necessary.
The form of the cross section through the groove is of importance. Deep, vertical-sided grooves will act as oil scrapers, removing the oil and breaking the film. Comparatively shallow grooves, with a flat-bevelled or rounded edge as shown at C Fig. 8, are effective in distributing the oil from the groove into the wedge. Such a flat bevel should be cut on the edge of all oil grooves.
Bearings made in two or more parts, common to large equipment, involve joints that may cause interference with the formation of an oil wedge and the maintenance of the oil film. The same is true of half bearings. Fig. 9 shows a half bearing in which the corners or edges have been left sharp. The result is that oil is scraped off from the journal, by the corner D and escapes.page 53
This results in oil waste and an impoverished oil film.
This objectionable feature is eliminated by chamfering the edges of the bearing parts as shown in Fig. 8. The chamfer at A, which should be as flat as possible, prevents scraping the oil from the journal, and thus forms the first part of the oil wedge. A similar chamfer on the opposite side is necessary if the direction of rotation is to be reversed.
In the foregoing it has been assumed that oil is present in a quantity sufficient to form the oil wedge. The production of the necessary oil wedge depends upon the introduction of oil to the bearings in quantities sufficient to overcome its loss by leakage at the bearing ends and partings. An insufficient supply would fail to maintain the oil wedge, and the oil film would not support the journal, resulting in retarded motion, increased friction losses and wear. An irregular supply, which might give acceptable results at times, would fail at other times. It is therefore, highly essential that the oil be introduced to the bearing with regularity by some reliable automatic means which will assure the maintenance of the oil wedge and film at all times.
If the oil wedge were always maintained, and there were not waste through leakage from the ends of the bearing, the selection of the correct oil would depend only on a correct interpretation of the factors which influence the formation and maintenance of the oil film. But lubricants are supplied to the bearings by many methods, and the method of supply affects the selection of the lubricant in four important ways:
1. The method may be one which will not supply, with reliable certainty at all times, a sufficient quantity to maintain the oil wedge. This would result in failure to the film through lack of oil, if the oil were selected from the standpoint of the lubricating film alone. Special staying and adhering characteristics of the oil are needed to compensate for the uncertainty of the supply.
2. The oil may be supplied automatically in quantities which just compensate for oil losses from the bearing. This is the normal case of film formation by oil-wedge effect. In this case the oil selected should be light in body—for the sake of minimum fluid friction—but must be heavy enough to provide a film that will support the pressure and prevent metallic contact. Oil body largely governs selection of oil for this case.
3. The method of supply may be one which imposes on the oil the duty of repeated service, requiring in the oil, in addition to the body requisite for film formation, certain characteristics that may be secured by selection of the crude and special treatment in its manufacture.
4. Where parts to be lubricated cannot be lubricated economically or advantageously by any available method of oil application, it may be necessary to use grease. Grease cups can sometimes be applied where an oiling device cannot.
It is therefore essential to have an understanding of the various in use and their influence on the selection of the lubricant. It is also important that the limitations of each method of supply should be understood, since it is impossible to secure the best results from even the correct oil, when the method of supply is not adapted to the type of bearing and conditions of operation.
(To be continued.)
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