Principles Of Lubrication* FRICTION

Chapter 1 Principles Of Lubrication* FRICTION W hen one body slides across another a resistive force must be overcome. This force is called friction. If the bodies are rigid, it is called solid friction. Solid friction may be static or kinetic—the former encountered when initiating movement of a body at rest, the latter when a body is in motion. (Distinct from solid friction is fluid friction, a normally less resistive force that occurs between the molecules of a gas or liquid in motion. As will be seen in later discussions, lubrication generally involves the substitution of low fluid friction for high solid-to-solid friction.) Causes of Solid Friction Solid, or sliding, friction originates from two widely differing sources. The more obvious source is surface roughness; no machined surface, however polished, is ideally smooth. Though modern machinery is capable of producing finishes that approach perfection, microscopic irregularities inevitably exist. Minute protuberances on a surface are called asperities, and, when two solids rub together, interference between opposing asperities accounts for a considerable portion of the friction, especially if the surfaces are rough. The other cause of sliding friction is the tendency of the flatter areas of the opposing surfaces to weld together at the high temperatures that occur under heavy loads. Rupture of the tiny bonds created in this manner is responsible for much of the friction that can occur between machine parts. On finely ground surfaces, in fact, these minute welds constitute a major source of potential frictional resistance. Factors Influencing Friction For rigid bodies in direct contact, static friction is greater than kinetic friction, that is, frictional drag is lower once a body is in motion with respect to the opposing body. Sliding friction varies only with the force that presses the two surfaces together; it is independent of both speed and the apparent area of contact. *Contributed by Exxon Company U.S.A., Marketing Technical Services, Houston, Texas. Reference: Form DG-5A. 1 Copyright © 2000 The Fairmont Press, Inc. 2 Practical Lubrication for Industrial Facilities Effect of Friction In some respects, it is very fortunate that friction exists. Without friction, walking would be impossible, and an automobile or a brake or a grindstone would be useless. On the other hand, almost all mechanisms involve the sliding of one part against another, Figure 1-2. Here, friction is quite undesirable. Work is required to overcome this friction, and the energy thus wasted entails a loss of power and efficiency. Whenever friction is overcome, moreover, dislocation of the surface particles generates heat, and excessive temperatures developed in this way can be destructive. The same frictional heat that ignites a match is what “burns out” the bearings of an engine, Figure 1-3. Additionally, where there is solid Figure 1-1. Friction of a sliding body is equal to the force required to overcome it. friction, there is wear: a loss of material due to the cutting action of opposing asperities and to the shearing apart of infinitesimal welded surfaces. In extreme cases, welding may actually cause seizure of the moving parts. Whether a piston ring, gear tooth, or journal is involved, the harmful effects of friction can hardly be overemphasized. One of the tasks of the engineer is to control friction—to increase friction where friction is needed and to reduce it where it is objectionable. This discussion is concerned with the reduction of friction. It has long been recognized that if a pair of sliding bodies are separated by a fluid or fluid-like film, the friction between them is greatly diminished. A barge can be towed through a canal much more easily than it can be dragged over, say, a sandy beach. Figure 1-4 should remind us of this fact. Lubrication The principle of supporting a sliding load on a friction-reducing film is known as lubrication. The substance of which the film is composed is a lubricant, and to apply it is to lubricate. These are not new concepts, nor, in their essence, particularly involved ones. Farmers lubricated the axles of their ox carts with animal fat centuries ago. But modern machinery has become many times more complicated since the days of the ox cart, and the demands placed upon the lubricant have become proportionally Copyright © 2000 The Fairmont Press, Inc. Principles of Lubrication 3 more exacting. Though the basic principle still prevails—the prevention of metal-to-metal contact by means of an intervening layer of fluid or fluidlike material—modern lubrication has become a complex study. LUBRICANTS Figure 1-2. Friction can vary. All liquids will provide lubrication of a sort, but some do it a great deal bettor than others. The difference between one lubricating material and another is often the difference between successful operation of a machine and failure. Mercury, for example, lacks the adhesive and metal-wetting properties that are desirable to keep a lubricant in intimate contact with the metal surface that it must protect. Alcohol, on the other hand (Figure 1-5), wets the metal surface readily, but is too thin to maintain a lubricating film of adequate thickness in conventional applications. Gas, a fluid-like medium, offers lubricating possibilities— in fact, compressed air is used as a lubricant for very special purposes. But none of these fluids could be considered practical lubricants for the multitude of requirements ordinarily encountered. Petroleum Lubricants For almost every situation, petroleum products have been found to excel as lubricants. Petroleum lubricants stand high in metal-wetting ability, and they possess the body, or viscosity characteristics, Figure 1-3. Friction causes heat. that a substantial film requires. Though the subject is beyond the scope of this introductory chapter, these oils have many additional properties that are essential to modern lubrication, such as good water resistance, inherent rust-preventive Copyright © 2000 The Fairmont Press, Inc. 4 Figure 1-4. Fluid and solid friction. Figure 1-5. Petroleum oils make the best lubricating films. characteristics, natural adhesiveness, relatively good thermal stability, and the ability to transfer frictional heat away from lubricated parts. What is more, nearly all of these properties can be modified during manufacture to produce a suitable lubricant for each of a wide variety of applications. Oils have been developed hand-in- Copyright © 2000 The Fairmont Press, Inc. Practical Lubrication for Industrial Facilities Principles of Lubrication 5 Figure 1-6. Highviscosity liquid flow slower than lowviscosity liquids. hand with the modern machinery that they lubricate; indeed, the efficiency, if not the existence, of many of today’s industries and transportation facilities is dependent upon petroleum lubricants as well as petroleum fuels. The basic petroleum lubricant is lubricating oil, which is often referred to simply as “oil.” This complex mixture of hydrocarbon molecules represents one of the important classifications of products derived from the refining of crude petroleum oils, and is readily available in a great variety of types and grades. Viscosity To understand how oil enters a bearing and picks up and carries the bearing load requires an explanation of viscosity. With lubricating oils, viscosity is one of the most fundamental properties, and much of the story of lubrication is built around it. The viscosity of a fluid is its resistance to flow. Thick fluids, like molasses, have relatively high viscosities; they do not flow readily. Thinner fluids, such as water, flow very easily and have lower viscosities. Lubricating oils are available in a wide variety of viscosities, Figure 1-6. Effect of Temperature The viscosity of a particular fluid is not constant, however, but varies with temperature, Figure 1-7. As an oil is heated, its viscosity drops, and it becomes thinner. Conversely, an oil becomes thicker if its temperature is reduced, and it will not flow as rapidly. Therefore, a numerical figure for viscosity is meaningless unless accompanied by the temperature to which it applies. HYDRODYNAMIC LUBRICATION Basically, lubrication is governed by one of two principles: hydrodynamic lubrica- Copyright © 2000 The Fairmont Press, Inc. 6 Practical Lubrication for Industrial Facilities Figure 1-7. Oil is thicker at lower temperatures, thinner at higher temperatures. tion and boundary lubrication. In the former, a continuous full-fluid film separates the sliding surfaces. In the latter, the oil film is not sufficient to prevent metal-to-metal contact. Hydrodynamic lubrication is the more common, and it is applicable to nearly all types of continuous sliding action where extreme pressures are not involved. Whether the sliding occurs on flat surfaces, as it does in most thrust bearings, or whether the surfaces are cylindrical, as in the case of journal (plain or sleeve) bearings, the principle is essentially the same, Figure 1-8. Hydrodynamic Lubrication of Sliding Surfaces It would be reasonable to suppose that, when one part slides on another, the protective oil film between them would be scraped away. Except under some conditions of reciprocating motion, this is not necessarily true at all. With the proper design, in fact, this very sliding motion constitutes the means of creating and maintaining that film. Figure 1-8. Sliding load supported by a wedgeshaped lubricating film. Copyright © 2000 The Fairmont Press, Inc. Principles of Lubrication 7 Consider, for example, the case of a block that slides continuously on a flat surface. If hydrodynamic lubrication is to be effected, an oil of the correct viscosity must be applied at the leading edge of the block, and three design factors must be incorporated into the block: 1. The leading edge must not be sharp, but must be beveled or rounded to prevent scraping of the oil from the fixed surface. 2. The block must have a small degree of free motion to allow it to tilt and to lift slightly from the supporting surface, Figure 1-9. 3. The bottom of the block must have sufficient area and width to “float” on the oil. Figure 1-9. Shoe-type thrust bearing. Full-fluid Film Before the block is put in motion, it is in direct contact with the supporting surface. Initial friction is high, since there is no fluid film between the moving parts. As the block starts to slide, however, the leading edge encounters the supply of oil, and it is at this point that the significance of viscosity becomes apparent. Because the oil offers resistance to flow, it is not wholly displaced by the block. Instead, a thin layer of oil remains on the surface under the block, and the block, because of its rounded edge, rides up over it. Effect of Viscosity As the sliding block rises from the surface, more oil accumulates under it, until the oil film reaches equilibrium thickness. At this point, the oil is squeezed out from under the block as fast as it enters. Again, it is the viscosity of the oil that prevents excessive loss due to the squeezing action of the block’s weight. Copyright © 2000 The Fairmont Press, Inc. 8 Practical Lubrication for Industrial Facilities With the two surfaces completely separated, a full-fluid lubricating film has been established, and friction has dropped to a low value. Under these conditions, the block assumes of its own accord an inclined position, with the leading edge slightly higher than the trailing edge. Fluid Wedge This fortunate situation permits the formation of a wedge-shaped film, a condition essential to fluid-film lubrication. The convergent flow of oil under the block develops a pressure—hydrodynamic pressure—that supports the block. It can thus be said that fluid-film lubrication involves the “floating” of a sliding load on a body of oil created by the “pumping” action of the sliding motion, Figure 1-10. BEARING LUBRICATION Shoe-type Thrust Bearings As was illustrated in Figure 1-9, many heavily loaded thrust bearings are designed in accordance with the principle illustrated by the sliding block. A disk, or thrust collar, rotates on a series of stationary blocks, or shoes, arranged in a circle beneath it. Each shoe is mounted on a pivot, rocker, or springs, so that it is free to tilt and to assume an angle favorable to efficient operation. The leading edge of each contact surface is slightly rounded, and oil is supplied to it from a reservoir. Bearings of the type described serve to carry the tremendous axial loads imposed by vertically mounted hydro-electric generators. Rotation of the thrust collar produces a flow of oil between it and the shoes, so that the entire weight of the turbine and generator rotors and shaft is borne by the oil film. So closely does this design agree with theory, that it is said that the babbitt facing of the shoes may be crushed before the oil film fails. Journal Bearings The hydrodynamic principle is equally applicable to the lubrication of journal bearings. Here, the load is radial, and a slight clearance must be provided between the journal and its bearing to permit the formation of a wedge-shaped film. Let it be assumed, for example, that a journal supports its bearing, as it does in the case of a plain-bearing railroad truck. The journal is an extension of the axle and, by means of the bearing, it carries its share of the load represented by the car. All of the force exerted by the bearing against the journal is applied at the top of the journal—none against the bottom. When the car is at rest, the oil film between the bearing and the top of the journal has been squeezed out, leaving a thin residual coating that is probably not sufficient to prevent some metal-to-metal contact. As in the case of the sliding block, lack of an adequate lubricating film gives rise to a high initial friction. As the journal begins to rotate, however, oil seeps into the bearing at the bottom, where the absence of load provides the greatest clearance. Some of the oil clings to the journal and is carried around to the upper side, dragging additional oil around with it. Copyright © 2000 The Fairmont Press, Inc. Principles of Lubrication 9 In this manner, oil is “pumped” into the narrowing clearance at the top of the journal, where there is greatest need. The consequent flow of oil from an area of low pressure through a converging channel to an area of high pressure, as shown in Figure 1-10, produces a fluid wedge that lifts the bearing from the top of the journal, eliminating metal-to-metal contact. When a state of equilibrium is reached, the magnitude of the entering flow displaces the bearing to one side, while the load on the bearing reduces the thickness of the film at the top. The situation is analogous to that of the inclined thrust-bearing shoe; in either case, the tapered channel essential to hydrodynamic lubrication is achieved automatically. The resulting distribution of hydrodynamic pressure is shown in Figure 1-11. Figure 1-10. Rotation of journal “pumps” oil into the area of high pressure to carry the load. Figure 1-11. Oil pressure distribution diagrams. Copyright © 2000 The Fairmont Press, Inc. 10 Practical Lubrication for Industrial Facilities If the load were reversed, that is, if the bearing supported the journal, as is more generally the case, the relative position of the journal would be inverted. The low-pressure region would be at the top of the journal, and the protective film would be at the bottom. Journal Bearing Design Requirements The performance of a journal bearing is improved by certain elements of design. In addition to the allowance of sufficient clearance for a convergent flow of oil, the edges of the bearing face should be rounded somewhat, as shown in Figure 1-12, to prevent scraping of the oil from the journal. Like the leading edge of the thrust-bearing shoe, this edge should not be sharp. Oil can enter the clearance space only from the low-pressure side of the bearing. Whatever the lubrication system, it must supply oil at this point. If the bearing is grooved to facilitate the distribution of oil across the face, the grooves must be cut in the low-pressure side. Grooves in the high-pressure side promote the discharge of oil from the critical area. They also reduce the effective bearing area, which increases the unit bearing load. No groove should extend clear to the end of the face. Figure 1-12. Edges of bearing face are rounded to prevent scraping of oil from journal. FLUID FRICTION It has been pointed out that viscosity, a property possessed in a greater or lesser degree by all fluids, plays an essential role in hydrodynamic lubrication. The blessing is a mixed one, however, since viscosity is itself a source of friction—fluid friction. Fluid friction is ordinarily but a minute percentage of the solid friction encountered in the ab- Copyright © 2000 The Fairmont Press, Inc. Principles of Lubrication 11 sence of lubrication, and it does not cause wear. Nevertheless, fluid friction generates a certain amount of heat and drag, and it should be held to a minimum. Laminar Flow When two sliding surfaces are separated by a lubricating film of oil, the oil flows. Conditions are nearly always such that the flow is said to be laminar, that is, there is no turbulence. The film may be assumed to be composed of extremely thin layers, or laminae, each moving in the same direction but at a different velocity, as shown in Figure 1-13. Under these conditions, the lamina in contact with the fixed body is likewise motionless. Similarly, the lamina adjacent to the moving body travels at the speed of the moving body. Intermediate laminae move at speeds proportional to the distance from the fixed body, the lamina in the middle of the film moving at half the speed of the body in motion. This is roughly the average speed of the film. Shear Stress Since the laminae travel at different speeds, each lamina must slide upon another, and a certain force is required to make it do so. Specific resistance to this force is known as shear stress, and the cumulative effect of shear stress is fluid friction. Viscosity is a function of shear stress, i.e., viscosity equals shear stress divided by shear rate. Therefore, fluid friction is directly related to viscosity. Figure 1-13. Fluid bearing friction is drag imposed by one layer of oil sliding upon another. Copyright © 2000 The Fairmont Press, Inc. 12 Practical Lubrication for Industrial Facilities Effect of Speed and Bearing Area In a bearing, however, there are two additional factors that affect fluid friction, both elements of machine design. One is the relative velocity of the sliding surfaces, the other, their effective area. Unlike solid friction, which is independent of these factors, fluid friction is increased by greater speeds or areas of potential contact. Again, unlike solid friction, fluid friction is not affected by load, Figure 1-14. Other considerations being the same, a heavier load, though it may reduce film thickness, has no effect on fluid friction. Figure 14. Factors that affect bearing friction under full-fluid-film lubrication. BEARING EFFICIENCY Partial Lubrication This discussion of friction has so far been limited to full-fluid-film lubrication. However, formation of a full-fluid film may be precluded by a number of factors, such as insufficient viscosity, a journal speed too slow to provide the necessary hydrodynamic pressure, a bearing area too restricted to support the load, or insufficient lubricant supply. Only partial, or boundary, lubrication may be possible under these extreme conditions. The resulting high bearing friction is a combination of fluid and solid friction, the proportion depending on the severity of operating conditions. As in the case of full-fluid-film lubrication, friction occurring under conditions of partial lubrication, and characterized by varying degrees of metal-to-metal contact, is related to viscosity, speed, and area. The significant difference is that, in the absence of a full-fluid film, friction varies inversely with these three factors. Copyright © 2000 The Fairmont Press, Inc. Principles of Lubrication 13 Overall Bearing Friction It is thus possible to relate all bearing friction, regardless of lubricating conditions, to oil viscosity, speed, and bearing area. Engineers express the situation mathematically with the formula: where F is the frictional drag imposed by the bearing; Z is oil viscosity; N is journal speed; A is the load-carrying area of the bearing. (f) is a symbol indicating that an unspecified mathematical relationship exists between the two sides of the equation. Coefficient of Friction It is customary to express frictional characteristics in terms of coefficient of friction, rather than friction itself. Coefficient of friction is more broadly applicable. It is unit friction, the actual friction divided by the force (or load) that presses the two sliding surfaces together. Accordingly, if both sides of equation (1-1) are divided by the load L: Here, F/L is coefficient of friction and is represented by the symbol ␮. Also, A/L is the reciprocal of pressure; or A/L - 1/P, where P is pressure, the force per unit area that the bearing exerts upon the oil. By substitution, Equation (1-2) can therefore be written: This is the form that engineers customarily apply to bearing friction, the term ZN/P being known as a parameter—two or more variables combined in a single term. ZN/P Curve Equation (1-3) indicates only that a relationship exists; it does not define the relationship. Definition is accomplished by the curve in Figure 1-15. This ZN/P curve illustrates typical bearing performance under varying conditions of operation. The characteristics of a specific curve would depend on the bearing to which it is applied. The left portion of the ZN/P curve lies in the region of partial lubrication, where solid friction combines with fluid friction to yield generally high frictional values. The starting of a journal would be represented by the situation at extreme left, where friction is primarily solid and very high. As speed increases, however, the development of a fluid film reduces bearing fric- Copyright © 2000 The Fairmont Press, Inc. 14 Practical Lubrication for Industrial Facilities Figure 1-15. Bearing performance. tion. Correspondingly, greater speed increases the value of the parameter ZN/P, driving operating conditions to a point on the curve farther to the right. A similar result could be achieved by the use of a heavier oil or by reducing pressure. Pressure could be reduced by lightening the load or by increasing the area of the bearing. If these factors are further modified to increase the value of the parameter, the point of operation continues to the right, reaching the zone of perfect lubrication. This is an area in which a fluid film is fully established, and metal-to-metal contact is completely eliminated. Beyond this region, additional increases in viscosity, speed, or bearing area reverse the previous trend. The greater fluid friction that they impose drives the operating position again to a region of high unit friction—now on the right portion of the curve. Effect of Load on Fluid Friction Within the range of full-fluid-film lubrication, it would appear, from Figure 1-15, that bearing friction could be reduced by increasing the bearing load or pressure. Actually, as pointed out earlier, fluid friction is independent of pressure. Instead, the property illustrated by this curve is coefficient of friction—not friction itself. Since the coefficient of friction ␮ equals F/L, then F = ␮L, and any reduction of ␮ due to greater bearing load under fluid-film conditions is compensated by a corresponding increase in the load L. The value of the actual bearing friction F remains unchanged. In the region of partial lubrication, however, an increase in pressure obviously brings about an increase in ␮. Since both ␮ and L are greater, the bearing friction F is markedly higher. Efficiency Factors From this analysis, it is quite evident that proper bearing size is essential to good Copyright © 2000 The Fairmont Press, Inc. Principles of Lubrication 15 lubrication. For a given load and speed, the bearing should be large enough to permit the development of a full-fluid film, but not so large as to create excessive fluid friction (Figure 1-16). Clearance should be sufficient to prevent binding, but not so great as to allow excessive loss of oil from the area of high pressure. The relative position of the ZN/P curve for a loose-fitting bearing would be high and to the right, as shown in Figure 1-17, indicating the need for a relatively high-viscosity oil, with correspondingly high fluid friction. Figure 1-16. Bearing design should permit the development of a full-fluid film. Efficient operation also demands selection of an oil of the correct viscosity, an oil just heavy enough to provide bearing operation in the low-friction area of fluid-film lubrication. If speed is increased, a heavier oil is generally necessary. For a given application, moreover, a lighter oil would be indicated for lower ambient temperatures, while a heavier oil is more appropriate for high ambient temperatures. These relationships are indicated in Figure 1-18. Temperature-Viscosity Relationships To a certain extent, a lubricating oil has the ability to accommodate itself to variations in operating conditions. If speed is increased, the greater frictional heat reduces the operating viscosity of the oil, making it better suited to the new conditions. Similarly, an oil of excessive inherent viscosity induces higher operating temperatures and corresponding drops in operating viscosity. The equilibrium temperatures and viscosities reached in this way are higher, however, than if an oil of optimum viscosity had been applied. So the need for proper viscosity selection is by no means eliminated. Oils vary, however, in the extent to which their viscosities change with temperature. Copyright © 2000 The Fairmont Press, Inc. 16 Practical Lubrication for Industrial Facilities Figure 1-17. Loosefitting bearings require high-viscosity oils. Figure 1-18. Relationships between oil viscosity, load, speed, and temperature. An oil that thins out less at higher temperatures and that thickens less at lower temperatures is said to have a higher V.I. (viscosity index). For applications subject to wide variations in ambient temperature, a high-V.I. oil may be desirable, Figure 1-19. This is true, for example, of motor oils, which may operate over a 100°F temperature range. With an automobile engine, there is an obvious advantage in an oil that does not become sluggishly thick at low starting temperatures or dangerously thin at high operating temperatures. So good lubrication practices include consideration of the V.I. of the oil as well as its inherent viscosity. Copyright © 2000 The Fairmont Press, Inc. Principles of Lubrication 17 Figure 1-19. For the same temperature change, the viscosityof oil ”B” changes much less than that of oil “A.” As stated before, all of the factors that make hydrodynamic lubrication possible are not always present. Sometimes journal speeds are so slow or pressures so great that even a heavy oil will not prevent metal-to-metal contact. Or an oil heavy enough to resist certain shock loads might be unnecessarily heavy for normal loads. In other cases, stopand-start operation or reversals of direction cause the collapse of any fluid film that may have been established. Also, the lubrication of certain heavily loaded gears—because of the small areas of tooth contact and the combined sliding and rolling action of the teeth—cannot be satisfied by ordinary viscosity provisions. Since the various conditions described here are not conducive to hydrodynamic lubrication, they must be met with boundary lubrication, a method that is effective in the absence of a full-fluid film, Figure 1-20. Additives for Heavier Loads There are different degrees of severity under which boundary lubrication conditions prevail. Some are only moderate, others extreme. Boundary conditions are met by a variety of special lubricants with properties corresponding to the severity of the particular application. These properties are derived from various additives contained in the oil, some singly, some in combination with other additives. Their effect is to increase the load-carrying ability of the oil. Where loads are only mildly severe, an additive of the class known as oiliness agents or film-strength additives is applicable. Worm-gear and pneumatic-tool lubricants are often fortified with these types of agents. Where loads are moderately severe, anti-wear agents or mild EP additives, are used. These additives are particularly desirable in hydraulic oils and engine oils. For more heavily loaded parts, a more potent class of additives is required; these are called extreme pressure (EP) agents, Figure 1-21. Copyright © 2000 The Fairmont Press, Inc. 18 Practical Lubrication for Industrial Facilities Figure 1-20. Sliding surfaces separated by a boundary lubricant of the polar type. Figure 1-21. ExtremePressure conditions. Oiliness Agents The reason for referring to oiliness agents as film-strength additives is that they increase the oil film’s resistance to rupture. These additives are usually oils of animal or vegetable origin that have certain polar characteristics. A polar molecule of the oiliness type has a strong affinity both for the petroleum oil and for the metal surface with which it comes in contact. Such a molecule is not easily dislodged, even by heavy loads. In action, these molecules appear to attach themselves securely, by their ends, to the sliding surfaces. Here they stand in erect alignment, like the nap of a rug, linking a minute layer of oil to the metal. Such an array serves as a buffer between the moving parts so that the surfaces, though close, do not actually touch one another. For mild boundary conditions, damage of the sliding parts can be effectively avoided in this way. Copyright © 2000 The Fairmont Press, Inc. Principles of Lubrication 19 Lubricity is another term for oiliness, and both apply to a property of an oil that is wholly apart from viscosity. Oiliness and lubricity manifest themselves only under conditions of boundary lubrication, when they reduce friction by preventing breakdown of the film. Anti-wear Agents Anti-wear agents, also called mild EP additives, protect against friction and wear under moderate boundary conditions. These additives typically are organic phosphate materials such as zinc dithiophosphate and tricresyl phosphate. Unlike oiliness additives, which physically plate out on metal surfaces, anti-wear agents react chemically with the metal to form a protective coating that allows the moving parts to slide across each other with low friction and minimum loss of metal. These agents sometimes are called “anti-scuff” additives. Extreme-pressure Agents Under the extreme-pressure conditions created by very high loads, scoring and pitting of metal surfaces is a greater problem than frictional power losses, and seizure is the primary concern. These conditions require extreme-pressure (EP) agents, which are usually composed of active chemicals, such as derivatives of sulfur, phosphorus, or chlorine. The function of the EP agent is to prevent the welding of mating surfaces that occurs at the exceedingly high local temperatures developed when opposing bodies are rubbed together under sufficient load. In EP lubrication, excessive temperatures initiate, on a minute scale, a chemical reaction between the additive and the metal surface. The new metallic compound is resistant to welding, thereby minimizing the friction that results from repeated formation and rupturing of tiny metallic bonds between the surfaces. This form of protection is effective only under conditions of high local temperature. So an extreme-pressure agent is essentially an extreme-temperature additive. Multiple Boundary Lubrication Some operations cover not one but a range of boundary conditions. Of these conditions, the most severe may require an oil with a chemically active agent that is not operative in the milder boundary service. Local temperatures, though high, may not always be sufficient for chemical reaction. To cover certain multiple lubrication requirements, therefore, it is sometimes necessary to include more than a single additive: one for the more severe, another for the less severe service. Incidental Effects of Boundary Lubricants The question logically presents itself as to why all lubricating oils are not formulated with boundary-type additives. The basic reason is that this formulation is usually unnecessary; there is no justification for the additional expense of blending. Additionally, the polar characteristics of oiliness agents may increase the emulsibility of the oil, making it undesirable for applications requiring rapid oil-water separation. Some of the more potent EP additives, moreover, have a tendency to react with certain structural metals, a feature that might limit their applicability. Copyright © 2000 The Fairmont Press, Inc. 20 Practical Lubrication for Industrial Facilities Stick-slip Lubrication A special case of boundary lubrication occurs in connection with stick-slip motion. It will be remembered that a slow or reciprocating action, such as that of a machine way, is destructive to a full-fluid film. Unless corrective measures are taken, the result is metal-to-metal contact, and the friction is solid, rather than fluid. It will also be remembered that solid static friction is greater than solid kinetic friction, i.e., frictional drag drops after the part has been put in motion. Machine carriages sometimes travel at very slow speeds. When the motive force is applied, the static friction must first be overcome, whereupon the carriage, encountering the lower kinetic friction, may jump ahead. Because of the slight resilience inherent in a machine, the carriage may then come to a stop, remaining at rest until the driving mechanism again brings sufficient force to bear. Continuation of this interrupted progress is known as stick-slip motion, and accurate machining may be difficult or impossible under these circumstances. To prevent this chattering action, the characteristics of the lubricant must be such that kinetic friction is greater than static friction. This is the reverse of the situation ordinarily associated with solid friction. With a way lubricant compounded with special oiliness agents, the drag is greater when the part is in motion. The carriage is thus prevented from jumping ahead to relieve its driving force, and it proceeds smoothly throughout its stroke. EHD LUBRICATION The foregoing discussion has covered what may be termed the classical cases of hydrodynamic and boundary lubrication. The former is characterized by very low friction and wear and dependence primarily on viscosity; the latter is characterized by contact of surface asperities, significantly greater friction and wear, and dependence on additives in the lubricant to supplement viscosity. In addition to these two basic types of lubrication, there is an intermediate lubrication mode that is considered to be an extension of the classical hydrodynamic process. It is called elasto-hydrodynamic (EHD) lubrication, also known as EHL. It occurs primarily in rolling-contact bearings and in gears where non-conforming surfaces are subjected to very high loads that must be borne by small areas. An example of nonconforming surfaces is a ball within the relatively much larger race of a bearing (see Figure 1-22). EHD lubrication is characterized by two phenomena: 1) the surfaces of the materials in contact momentarily deform elastically under pressure, thereby spreading the load over a greater area. 2) The viscosity of the lubricant momentarily increases dramatically at high pressure, thereby increasing load-carrying ability in the contact zone. The combined effect of greatly increased viscosity and the expanded load-carrying area is to trap a thin but very dense film of oil between the surfaces. As the viscosity increases under high pressures, sufficient hydrodynamic force is generated to form a full- Copyright © 2000 The Fairmont Press, Inc. Principles of Lubrication 21 Figure 1-22. EHD lubrication in a rolling-contact bearing. fluid film and separate the surfaces. The repeated elastic deformation of bearing materials that occurs during EHD lubrication results in a far greater incidence of metal fatigue and eventual bearing failure than is seen in sliding, or plain, bearing operation. Even the best lubricant cannot prevent this type of failure. BREAK-IN Though modern tools are capable of producing parts with close tolerances and highly polished surfaces, many machine elements are too rough, when new, to sustain the loads and speeds that they will ultimately carry. Frictional heat resulting from the initial roughness of mating parts may be sufficient to damage these parts even to the point of failure. This is why a new machine, or a machine with new parts, is sometimes operated below its rated capacity until the opposing asperities have been gradually worn to the required smoothness. Under break-in conditions, it is sometimes necessary or advantageous to use a lubricant fortified with EP additives. The chemical interaction of these agents with the metal tends to remove asperities and leave a smoother, more polished surface. As the surface finish improves during initial run-in, the need for an EP lubricant may be reduced or eliminated, and it may then be appropriate to substitute a straight mineral oil or an EP oil with less chemical activity. Copyright © 2000 The Fairmont Press, Inc. 22 Practical Lubrication for Industrial Facilities BEARING METALS The break-in and operating characteristics of a journal bearing depend to a large extent upon the composition of the opposing surfaces. In the region of partial lubrication, friction is much less if the journal and bearing are of different metals. It is customary to mount a hard steel journal in a bearing lined with a softer material, such as bronze, silver, or babbitt. There are several advantages in a combination of this sort. The softer metal, being more plastic, conforms readily to any irregularities of the journal surface, so that breakin is quicker and more nearly perfect. Because of the consequent closeness of fit, soft bearing metals have excellent wear properties. Moreover, in the event of lubrication failure, there is less danger of destructive temperatures. Friction is lower than it would be if steel, for example, bore directly against steel. If temperature should rise excessively in spite of this protective feature, the bearing metal, with its lower melting point, would be the first to give way. Yielding of the bearing metal often prevents damage to the journal, and replacement of the bearing lining is a relatively simple matter. However, composition of bearing metals has no effect upon performance under full-film lubrication. WEAR Even with the most perfectly lubricated parts, some physical wear is to be expected. Sometimes wear is so slight as to be negligible, as in the case of many steam turbine bearings. Turbines used to generate power operate under relatively constant loads, speeds, and temperatures, a situation that leads to the most effective sort of lubrication. Many other machines, however, operate under less ideal conditions. If they stop and start frequently, there will be interruptions of the lubricating film. Also, in any lubricating process, there is always the possibility of abrasive wear due to such contaminants as dirt and metallic wear particles. Wear is further promoted by overloading, idling of internal combustion engines, and other departures from optimal operating conditions. Wear vs. Friction Though wear and friction generally go hand-in-hand, there are extreme situations in which this is not so. Some slow-speed bearings are so heavily loaded, that an oil of the highest viscosity is required for complete lubrication. Because of the greater fluid friction, this lubricant imposes more bearing friction than a lighter lubricant would. On the other hand, the lighter lubricant, since it would provide only partial lubrication, could not be considered suitable from the standpoint of protection of the metal surface. Some frictional advantage must be sacrificed in favor of an improvement in wear characteristics. Contrary to popular conception, therefore, less wear actually means more friction under extreme conditions such as this. Copyright © 2000 The Fairmont Press, Inc. Principles of Lubrication 23 GREASE LUBRICATION Many situations exist in which lubrication can be accomplished more advantageously with grease than with oil. Most lubricating greases consist of petroleum oils thickened with special soaps that give them an unusual ability to stay in place. Grease is often used, therefore, in applications for which it is not practical to provide a continuous supply of oil. Though the retentive properties of grease—also resistance to heat, water, extreme loads, and other adverse conditions—depend primarily on the proportion and type of soap, frictional characteristics themselves are related almost entirely to the oil content. Base-oil viscosity is a determining factor in the ability of the grease to provide a proper lubricating film. Copyright © 2000 The Fairmont Press, Inc.