The New Zealand Railways Magazine, Volume 4, Issue 2 (June 1, 1929.)
The Lubrication of Bearings — Principles of the Oil Film and Wedge
In the application of power to useful work dependence must be placed upon bearings, without which machinery cannot operate. In the transmission of power by shafting, bearings play an important part, and upon their efficiency depends the efficient operation of every industrial plant.
Lubrication affects the power consumption of every bearing, and, as even in a fair sized plant, the number of bearings runs into thousands, a small individual power loss in each, multiplied by the total number of bearings, becomes a large power loss. The money loss involved in this power loss may mount to serious proportions. Shafting, when poorly lubricated, is a large waster of power, and although this loss may not be seen, the business feels the effect of it. Power is thus steadily wasted which could otherwise be converted into production.
At one time the common conception of lubrication was that of making metallic surfaces smooth or slippery by the application of an oily substance, attention being given to the modifying influence that the lubricant has upon the surface. It has been found that the metal constituting the bearing surfaces has little influence on friction except when lubrication is incomplete and boundary or greasy lubrication exists, i.e., when the quantity of lubricant present is insufficient to form a complete separating film between the surfaces.
Correct lubrication implies the complete separation of the moving part (the journal) and the stationary supporting part (the bearing) by a lubricating film consisting of the correct oil, possessing sufficient body and adhesiveness to support the pressure, and at the same time presenting the least possible resistance to motion consistent with absolute safety of operation. It also means that the oil film is maintained by a sufficient supply of oil, of lasting quality, without waste.
An oil film between the bearing surfaces can be maintained only when there is motion. When there is no motion, it is evident that any fluid would become squeezed out gradually from the pressure area. Due to the adhesive properties of a suitable lubricant, however, and also to the greater or less porosity of the journal and bearing metals, the surfaces remain oil-wet for a long time, thus facilitating initial motion when the machine is started. As soon as motion takes place, the moving part carries with it sufficient oil from the adjacent supply to rebuild the film on which it may ride.
It will be shown how this film is formed and maintained, what mechanical conditions must be observed to render it effective, and what characteristics must be possessed by the lubricant itself in order that the best results may be obtained.
As a fundamental example of an oil film, take a journal or shaft revolving in a solid bearing, consisting of a cylindrical hole through a solid block of metal, the hole being very slightly larger in diameter than the journal. Assume that the clearance space between the journal and the bearing is kept filled completely with oil, with the load of the journal acting downward. In Figs. 1—4 are shown end views of such a journal in various positions with respect to the bearing. The clearance space has been exaggerated for the purpose of better illustration.
Fig. 1 shows the journal resting directly on the metal of the bearing, the film having been squeezed out while the journal is at rest. If slow rotation of the journal begins in the direction indicated by the curved arrow in Fig. 2, there is at first a tendency for the journal to roll to the left. This would result in a new line of contact D between the journal and the bearing, except for the presence of oil which separates the journal and bearing surfaces and facilitates the starting motion.
Due to the downward pressure, the rotating journal will occupy a low position in the bearing on starting, and the oil film will be thin on the lower side. With increasing speed the quantity of oil carried into this supporting film, by its adhesion to the rotating shaft, becomes greater, thus producing a thicker film, which has a tendency to raise the journal, as in Fig. 3.
Experiments have shown that at normal load and high journal-speed, in a well designed and well lubricated bearing, the greatest pressure within the fluid film is in the part marked E in Fig. 4; while the centre of the journal A lies on a diagonal line; approximately in the direction of the letter F.page 52
In order to explain this position of the journal, let us bear in mind that the weight of the journal tends to locate it in its lowest position, as shown at C in Fig. 1. The oil film carried under the journal, due to its rotation at slow speed, will lift it; resulting in a position shown in Fig. 3. The motion of the rotating journal carries the oil to the left at the top, and to the right at the bottom. The larger clearance at the top favours this carrying action, and the contracted clearance at the bottom retards oil flow, resulting in a broadly-distributed and increasing high pressure at the lower left or descending side, in the area marked H in Fig. 4, a rapidly diminishing pressure in the area marked K, and a minimum pressure in the area marked G.
This excess of pressure on the left results in a change in the lateral position of the journal toward the right as well as a lifting of the journal, as shown in Fig. 4. The area E of greatest pressure also moves towards the right or rising side until equilibrium is established.
Many factors may enter into this condition of equilibrium; these include the effect of changes in the load and speed, the form of clearances, the bearing size, the quantity of oil supplied, and the body of the oil; which last factor, in turn, is influenced by its temperature.
Keeping other conditions the same, if the load is decreased on the journal illustrated in Fig. 4, the centre of the journal will rise towards the centre of the bearing; thus tending to make the clearance space more uniform throughout the circle. It has been found experimentally, however, that the centre or axis of the journal will not coincide with that of the bearing; and that the point of least clearance will remain in a diagonal position near the letter F.
If the load is increased, the journal centre moves downward, still in an angular position, until the film on the rising side has become extremely thin. As the load is further increased, the dragging effect of fluid friction in this very thin film tends to move the journal towards the centre vertical line.
If the load were increased to an amount entirely disproportionate to the supporting strength of the oil film, the journal would be brought down to actual contact with the bearing surface near the bottom point of the circle. When this takes place, complete lubrication ceases; and the result is excessive wear and friction.
The changes which take place due to decreased load would also result from greater speed, or increased body of the oil, which, in turn, may result from a lower temperature.
Reduced speed, decreased oil body and higher temperature produce the same changes in the location of the journal as have been described for increased load. In practice, very commonly, the clearance is only partly filled, as in Fig. 5, due to leakage at the bearing ends. This changes the distribution of pressures, and somewhat alters the position of the journal, which still, however, will take an angular position in the direction of the letter F for normal loads. Oil carried over or across the top, due to its adhesion to the journal, accumulates in the wedge-shaped space H. The carrying action of the journal builds up the pressure in the wedge-shaped space H until it reaches a maximum in the area E, slightly beyond the lowest point. After the point of least clearance F is passed, the oil pressure drops rapidly until, as the point K is passed, it ceases to fill the clearance.
The effectiveness of lubrication is due to the support of the journal by the pressure in the oil film. This pressure is due to the adhesive property of the oil and to the motion of the revolving journal, forcing the oil into a clearance space of decreasing thickness; thus forming an oil wedge (H—Fig. 5), which is one of the requisites in the lubrication of frictional bearings.
The presence of a lubricating film, between the journal and bearing surfaces, does away with friction between the solid parts and replaces it by page 53 fluid friction within the oil film, which is ordinarily far less.
This friction within the fluid is greater for a heavy-bodied or highly viscous oil than for oil of light body, emphasising the desirability of minimising frictional loss of power by the use of a light bodied oil. On the other hand, the greater the load, or the lower the speed, the heavier must be the body of the oil in order that a complete film may be maintained. High temperature also demands an oil of heavy body to compensate for the reducing effect that high temperatures have on oil body. To minimise fluid friction, the oil used should possess the lightest body that will maintain with safety a complete film under the existing conditions of load, speed, and temperature.
Wherever power is consumed in friction, the result is the generation of heat. The quantity of heat generated per minute is exactly proportional to the amount of power consumed in friction. Wherever metallic contact takes place in a bearing, the power consumed and the heat generated are greater than if a complete lubricating film were maintained. Even where a lubricating film completely separates the surfaces there is a certain amount of friction within the film; and, therefore, a proportional amount of heat generation. This heat is conducted away by the metal of the bearing and journal and dissipated.
Although the lubricating film is microscopic in its thickness, it may be regarded as composed of many layers; the outer layers being cooled by contact with the metal, while the inner layers are heated by the internal friction within the fluid. Although the temperature difference between these layers may be slight, the tendency is for the central layers to be of higher temperature, due to the heat generated therein. An increase in temperature always reduces viscosity or body, rendering this part of the film more fluid. Due to this greater fluidity, a greater part of the motion takes place between the central layers, localising the fluid friction in this part, and, therefore, tending to increase the temperature-difference between the layers until a stable condition has been reached.
A mental picture may now be formed of the movements of the lubricant within the bearing clearance. The oil directly adjacent to the metal surfaces is cooler than the central layers; and, being more viscous and adhesive, clings to the metal surfaces. Therefore, we may visualise this as the formation of a protective viscous coating on the bearing surface and a similar protective viscous coating on the surface of the journal, the latter coating moving with the journal, and the two coatings being separated by a film which is less viscous and in which the sliding motion takes place.
The foregoing discussion of the principles involved in the oil film and its maintenance by means of an oil-wedge suggests that the satisfactory action of this film may be upset completely if the characteristics of the oil itself are not as they should be. Experience confirms this conclusion.
(To be continued.)