Lubrication reduces friction between moving parts by substituting fluid friction for solid friction. Without lubrication, it is difficult to move a hundred-pound weight across a rough surface; with lubrication, and with proper attention to the design of bearing surfaces, it is possible to move a million-pound load with a motor that is small enough to be held in the hand. By reducing friction, thereby reducing the amount of energy that is dissipated as heat, lubrication reduces the amount of energy required to perform mechanical actions and also reduces the amount of energy that is dissipated as heat.
Lubrication is a matter of vital importance throughout industry. Moving surfaces must be steadily supplied with the proper kinds of lubricants. Lubricants must be maintained at specified standards of purity, and designed pressures and temperatures must be maintained in the lubrication systems. Without adequate lubrication, machinery would quite literally grind to a screeching halt.
The lubrication requirements of machinery are met in various ways, depending upon the nature of the machinery. This article examines the basics of lubrication as it applies to equipment used in industry today.
The two most general categories of friction are static friction and kinetic friction.
Static friction, which must be overcome to put any body in motion, is greater than kinetic friction, which must be overcome to keep the body in motion.
There are three types of kinetic friction:
1. Sliding friction exists when the surface of one solid body is moved across the surface of another solid body.
2. Rolling friction exists when a curved body, such as a cylinder or a sphere, rolls upon a flat or curved surface.
3. Fluid friction is the resistance to motion exhibited by a fluid. Fluid friction exists because of the cohesion between particles of the fluid and the adhesion of fluid particles to the object or medium that is tending to move the fluid.
EXAMPLE: If a paddle is used to stir a fluid, the cohesive forces between the molecules of the fluid tend to hold the molecules together and thus prevent motion of the fluid. At the same time, the adhesive forces of the molecules of the fluid cause the fluid to adhere to the paddle and, thus, create friction between the paddle and the fluid.
Cohesion is the molecular attraction between particles that tends to hold a substance or a body together. Adhesion is the molecular attraction between particles that tends to cause unlike surfaces to stick together.
From the point of view of lubrication, adhesion is the property of a lubricant that causes it to stick (or adhere) to the parts being lubricated; cohesion is the property that holds the lubricant together and enables it to resist breakdown under pressure.
Cohesion and adhesion are possessed by different materials in widely varying degrees. In general, solid bodies are highly cohesive but only slightly adhesive. Most fluids are quite highly adhesive but only slightly cohesive; however, the adhesive and cohesive properties of fluids vary considerably.
This section covers:
The lubrication of rubbing surfaces is based on the actual separation of surfaces so that metal-to-metal contact will not occur. As long as the lubricant film remains unbroken, sliding friction and rolling friction are replaced by fluid friction.
In any process involving friction, some power is consumed and some heat is produced. Overcoming sliding friction consumes the greatest amount of power and produces the greatest amount of heat. Overcoming rolling friction consumes less power and produces less heat. Overcoming fluid friction consumes the least power and produces the least amount of heat.
A presently accepted theory of lubrication is based on the Langmuir theory of the action of fluid films of oil between two surfaces, one or both of which are in motion. Theoretically, there are three or more layers or films of oil existing between two lubricated bearing surfaces. Two of the films are boundary films (indicated as I and V in part A of Figure 1), one of which clings to the surface of the rotating journal (the portion of the shaft supported by a radial bearing) and one of which clings to the stationary lining of the bearing. Between these two boundary films are one or more fluid films (indicated as II, III, and IV in part A of Figure 1). The number of fluid films shown in Figure 1 is arbitrarily selected for purposes of explanation.
Figure 1:Oil Film Lubrication
When the rotating journal is set in motion (part B of Figure 1), the relationship of the journal to the bearing lining is such that a wedge of oil is formed. The oil films II, III, and IV begin to slide between the two boundary films, thus continuously preventing contact between the two metal surfaces. The principle is again illustrated in part C of Figure 1, where the position of the oil wedge W is shown with respect to the position of the journal as it starts and continues in motion.
The views shown in part C of Figure 1 represent the journal of a shaft rotating in a solid bearing. The clearances are exaggerated in the drawing to illustrate the formation of the oil film. The shaded portion represents the clearance filled with oil. The film is in the process of being squeezed out while the journal is at rest, as shown in the stationary view. As the journal slowly starts to turn and as the speed increases, oil adhering to the surfaces of the journal is carried into the film, increasing the film thickness and tending to lift the journal as shown in the starting view. As the speed increases, the journal takes the position shown in the running view. Changes in temperature, with consequent changes in oil viscosity, cause changes in the film thickness and in the position of the journal.
If conditions are correct, the two surfaces are effectively separated, except for a possible momentary contact at the time the motion is started.
All metals are capable of retaining a film of lubricating oil to a greater or lesser degree. The ability of a material to retain a lubricant is referred to as wettability. Wettability depends on the molecular attraction between particles of various kinds of metal and oil. A material that has good wettability characteristics will be able to maintain a film of lubricant better than a material with poor wettability characteristics.
In all modes of lubrication, surfaces in contact are separated by a lubricating medium, which may be a solid, a semi-solid, a pressurized liquid, or a gaseous film.
Hydrodynamic lubrication occurs in a system due to the shapes and relative motion of the surfaces in contact. These two factors will work together to form a fluid film that will hold the surfaces apart under pressure.
Hydrostatic lubrication is the result of the lubricant being supplied at a pressure high enough to separate the surfaces.
Boundary lubrication and thin-film lubrication are two modes in which friction and wear are affected by the properties of the surfaces in contact, as well as the lubrication. In boundary lubrication, each surface is covered by a chemically bonded lubricant, which may or may not separate the surfaces, and viscosity (discussed below) of the lubricant is not a factor for determining wear. In thin-film lubrication, the lubricant is not bonded to the surface; therefore, the lubricant will not separate the surfaces. Wear determination for thin-film lubrication will require the study of the lubricants viscosity.
Mechanical devices often operate under several lubrication modes simultaneously or alternately. For example, when a hydrodynamic bearing starts turning from rest, it operates under boundary lubrication and then thin-film lubrication for a short time until a stable, thick oil film develops and the solid surfaces separate.
The process is reversed when rotation is slowed or stopped. Wear occurs during the initial and final boundary-lubricated periods. Gears experience both hydrodynamic and boundary lubrication at the same time. For example, during meshing of one tooth of a spur gear with a tooth of a mating gear, initial contact is sliding contact, which results in wear and scuffing at the tips and roots of the teeth. However, contact along the pitch-line is essentially rolling contact, and hydrodynamic conditions prevail. Pitch-line damage takes the form of pitting or spalling and is similar to rolling-contact fatigue found in ball and roller bearings.
This section describes viscosity index and viscosity measurement.
Viscosity is the most important property of a lubricating oil. This property is a measure of an oils resistance to flow. Oil with a high viscosity would be thick, heavy bodied, and slow flowing. It has a high resistance to motion within itself.
In high-viscosity oil, there is more internal friction than normal due to the oils molecules sliding over each other. When used with moving machine parts, high viscosity oil would be less efficient because of its resistance to motion. The advantage of high viscosity oil is the thick film developed during use, as shown in Figure 2. A plate is shown sliding over another surface with oil used as a lubricant. A high film thickness, or a greater separation of the two surfaces, is developed from high viscosity oil.
Figure 2: Film Thickness
Low viscosity oil has less internal friction and resistance to flow. Oil with low viscosity flows more easily and develops a thinner film thickness. These oils are used in high-speed parts where surfaces need to be close together.
The viscosity of an oil changes with temperature and pressure. Viscosity decreases with a rise in temperature. An oils viscosity index (VI) is a measure of how much the viscosity increases when the oil is cooled from 210 to 100F. The amount of increase is compared with two crude oils of known viscosity change; they are given VIs of 0 to 100. Figure 3 shows a chart of the change in viscosity with temperature for the two crude oils.
Figure 3: Viscosity Versus Temperature
Because of the effect of temperature on viscosity, the operating temperature is important in selecting lubricating oils. If the oil is heated to an overly high temperature under operating conditions, the viscosity of the oil may become too low to provide the required lubrication.
The viscosity of a liquid is measured with an instrument called a viscosimeter or viscometer. The type most commonly used by engineers is the Saybolt Universal Viscosimeter, shown in Figure 4.
Figure 4: Saybolt Viscosimeter
This instrument measures the number of seconds it takes for a fixed quantity of liquid (60 cubic centimeters) to flow through a small orifice of standard length and diameter at a specific temperature. The time of flow is taken in seconds, and the viscosity is expressed as Second, Saybolt Universal (SSU). For example, a certain liquid might have a viscosity of 80 SSU at 130F.
The saybolt viscosimeter consists of a container for the liquid surrounded by a bath heated by heating coils to bring the liquid to the temperature at which the viscosity is to be measured. There is a standard viscosimeter orifice located at the bottom of the container. Passage through the orifice is blocked with a cork.
Selecting a lubricant for operating machinery is no longer a simple task. With advances in technology, machines are running at higher speeds, pressures, and temperatures. This section examines the wide variety of lubricants used on process equipment and the reasons for selecting these lubricants. This section covers:
Lubricants can be most broadly classified as gas, liquid, semi-solid, or solid. Semi-solid lubricants are classified as greases and will be discussed later.
Lubricating oils can be specifically classified as mineral oils, synthetic oils, or fixed oils. Mineral oils are produced from hydrocarbon crude oil (petroleum) and are still the largest class of lubricants in common use. Synthetic oils, which are artificially made from organic chemicals, are continuing to be developed for replacement/substitution for typical crude oil applications. Each of these two oil types commonly contains various additives to inhibit oxidation, reduce corrosion, improve film strength, and disperse detergents.
Fixed oils are processed from animals and vegetables and are rarely used alone as lubricants because they tend to oxidize into a varnish-like layer at temperatures where mineral oils remain stable. Because of this, fixed oils are usually combined with mineral oils and greases. Examples of fixed oils include tallow, lard, degras, neats foot, whale, porpoise and dolphin oils, as well as vegetable oils extracted from fruits, or seeds from plants or trees.
As mentioned above, mineral oils are processed from crude oil, which is a complex mixture of individual hydrocarbons. An individual hydrocarbon is an organic compound of hydrogen, carbon, and one or more than one additional element, with these elements present in varying proportions in different hydrocarbons. Crude oils from different reservoirs display a wide range of properties. In fact, no two crude oils are exactly alike.
Hydrocarbons are organic compounds containing hydrogen, carbon, and one or more than one other element in various proportions. The carbon content of hydrocarbons ranges from 83-87 percent, and the hydrogen content ranges from 11-14 percent. These differences between hydrocarbons cause the properties (such as color, odor, density, viscosity, and chemical composition) of a crude oil from one reservoir to differ from the properties of a crude oil from another reservoir.
One basis for classifying a specific crude oil as lighter or heavier is the number of lighter or heavier hydrocarbons the crude oil contains. (An individual hydrocarbon is regarded as lighter or heavier according to the number of carbon atoms it possesses per molecule: the fewer the number of carbon atoms per molecule, the lighter the hydrocarbon.) Some crudes are light enough to be used as diesel fuel. Others are so heavy that they have a tar-like consistency and, even in the tropics, must be heated for transmission through a piping system.
In addition to hydrogen and carbon, crude oils contain varying amounts of sulfur, oxygen and nitrogen. These additional elements are regarded as impurities. In the refining of crude oil, sulfur and nitrogen in particular are often removed.
Additives are chemical compounds that are added to lubricating oils to improve lubrication properties. As equipment improves, there is a greater need for lubricants that can handle high bearing and gear loadings, greater speeds, and wide ranges of operating temperatures.
Additives improve lubricant properties in four ways:
Most lubricating oils usually have more than one additive. Oils used in turbines, electric motors, and hydraulic and circulating systems have the following additives:
Other additives reduce the change in viscosity with temperature, increase the solubility of the oil in water, or increase the flash point. Table 1 provides an overview of lubricant additive types, the reason for addition, and their applications.
For best performance results, buy and use only carefully selected lubricants. The basic steps for lubricant selection below establishes a list of lubricant types that is sufficiently extensive to cover essentially all the lubricant requirements for the entire plant; determine the desired characteristics for the lubricant types selected; and make a final selection on the basis of field testing results and lessons learned from past experience.
Lubricating oils under ideal conditions will last forever in operating machinery. It can be said that lubricating oils do not wear out, they simply become contaminated. The deterioration of oil occurs largely due to improper mechanical maintenance practices.
Proper machinery operation relies directly upon the personnel responsible to maintain the equipment within the manufacturers specifications. It cannot be stressed enough that without the proper selection and application of lubricants for operating machinery, the oil will deteriorate rapidly and cause extreme wear to rotating equipment.
Synthetic lubricants are artificially made, as opposed to naturally occurring petroleum fluids. Generally, the synthetics are organic chemicals. Certain groups have been found to have characteristics that make them suitable lubricants. They often have outstanding properties, such as high viscosity index or thermal stability. The favorable characteristic may be accompanied by one or more undesirable properties, such as low viscosity, high pour point, or low stability toward water. Some classes of synthetics are polyolefins (synthesized hydrocarbons), polyalkylene glycols, esters, silicones, polyphenyl ethers, and halogenated hydrocarbons.
Synthetic lubricants, because of high cost, are often used only where the particular property is essential. There are instances, however, where although petroleum oils may function reasonably well, the higher cost of synthetics is justified. For example, in applications where oil temperatures of about 100 to 150C (212 to 302F) exist, petroleum oils may require replacement of the charge on a relatively frequent basis to prevent excessive oxidation. Extension of oil life using synthetics may be quite economical. Synthesized hydrocarbons, organic esters, and polyglycols should be considered for such applications.
Table 2 gives a comparison of some pertinent properties of several synthetic fluids as well as relative cost.
In applications where accidental rupture of an oil line may cause fluid to splash on a very hot surface (in excess of about 310C or 600F), a degree of fire resistance well above that of petroleum oil is desirable.
Four classes of fluids, generally used in hydraulic systems operating in such an environment, are available:
Finally, there are conventional emulsions in which some 5 to 10 percent petroleum oil is dispersed in water. With water as the outside phase, the fluid is a rather poor lubricant, and equipment requiring lubrication must be designed and selected to operate with these emulsions. Steps must also be taken to ensure that problems such as rusting, spoilage, and microbial growth are controlled.
A solid lubricant is any solid material placed between two moving surfaces to reduce friction and wear. Two common solid lubricants are graphite and molybdenum disulfide. Solid lubricants are used where extreme pressure, temperature, or operating conditions, such as vacuum, dust or dirt, and corrosive contaminants prevent using mineral oils and greases.
Solid lubricants are added to oils and greases to carry them to the surfaces to be lubricated. During operation, the equipment oil or grease may be squeezed out from extreme pressure. However, the solid lubricant would remain to lubricate the part.
Solid lubricants can also be sprayed on surfaces. Solid particles can be mixed with a volatile solvent like alcohol. When sprayed on a surface, the alcohol evaporates, leaving a layer of solid lubricant.
Greases are semi-solid lubricants; they are used instead of oil when the lubricant has to stay in one place or stick to a part. Greases will not leak out as easily as oils. Greases are also used when the component cannot be lubricated often or are not accessible during operation. Greases are thick or viscous and, therefore, unlike oil, cannot be pumped continuously through equipment to remove heat. Greases are used for lubrication to prevent friction and wear, to protect against corrosion, to provide a seal from dirt and water, to provide lubrication that does not leak or drip off the surface to which it is applied, and to lubricate for a long time without breaking down.
Most greases are made from mineral oils. For some special uses, greases may be made of waxes, asphalt, or other substances. Greases can have four parts: fluid base, thickener, additives and fillers. All greases are made of a fluid base and thickener. Most of the properties of a grease are from the fluid base and the thickener, with small gains in the addition of additives.
For most greases, mineral oil is used as the fluid base. The mineral oil part of grease may have any viscosity and amount of refinement. High viscosity oils are used to make grease for high-temperature, low-speed service. Low viscosity oils are for lower temperature, higher speed service. Mineral oils require the addition of thickness to obtain the lubricating status of grease.
The most common thickener is soap. Soap is made from combining fatty material with an alkali. The fat may be from an animal or vegetable. Greases are often named from the type of thickener used. For example, lithium grease is a grease made from mineral oil and lithium soap. Clay and silica are other soaps commonly used as thickeners.
For high-speed devices, it is important to use grease that will not centrifuge. A grease centrifuges when, because of the high centrifugal force, the filler separates from the fluid base. In other words, a filler that has a high bond-ability with the mineral oil should be used.
Greases are classified by the National Lubricating Grease Institute (NLGI) from No.1 to No.5, depending on the stiffness of the greases. A soft grease would be a No.1 NLGI. A stiff grease may be No. 4 or No. 5 NLGI. A soft grease, say No.1 in the NLGI series, might run 90 percent oil, a water content ranging from a trace to 2 percent, and the remainder as thickness and additives. On the other end, a stiff grease such as No. 5 NLGI may run 30 percent soap and 3 percent water, the rest oil and additives.
Additives are chemical compounds added to greases to change or add properties. Additives in greases do the same as additives in oil. The most common additives are oxidation or rust inhibitors, pour point depressants, extreme pressure anti-wear agents and friction reducing agents.
Fillers make greases more solid and stable; graphite is the most commonly used filler. Other fillers are molybdenum and mica. Fillers form a solid film on moving surfaces; this helps grease withstand a heavier load. Fillers keep a grease from squeezing out under pressure. Fillers also reduce the coefficient of friction between moving parts by filling in the low points (valleys) on the surfaces.
Greases may be any combination and any amount of many different types of thickeners, additives and fillers. Multi-purpose greases combine the properties of two or more specialized greases to function over a broader range of conditions and applications.
The most popular multi-purpose greases contain a soap base of lithium or calcium. Table 3 lists the most common greases and their uses.
A major part of any maintenance program is the ability of the technician to determine the cause of component failure. By determining the cause of failure, measures can be taken to ensure that the new component will not fail again. Most component failures will show distinct signs of type of failure that occurred.
Wear is defined as "the removal of material from one or more solid surfaces in solid-state contact." When relative motion occurs between two objects in contact (i.e., sliding or rolling contact), wear products will always be produced. The type and amount of wear that results in operational systems depends on proper lubrication.
Understanding the wear behavior of various materials is a very complicated process. Many factors need to be understood to determine the mechanism that caused the wear to occur.
Abrasive wear occurs when two materials contact each other and one is harder than the other. This type of wear is sometimes called scratching, scoring, or gouging, depending on the severity of damage to the equipment.
Two separate considerations are examined when abrasive wear is discovered. The considerations that are made determine whether the abrasive wear was due to two-body abrasion or three-body abrasion.
Two-body abrasion occurs when two surfaces of different hardness are in contact with each other. The harder of the two materials will wear away the softer material when there is relative motion between the two objects in contact. This type of abrasive wear is commonly seen in lathes, milling machines, grinders, and cutting operations.
Three-body abrasion occurs when two surfaces of the same hardness are in contact with each other and a piece of foreign material (i.e., grit, metal, rust) becomes lodged between the two surfaces. The foreign material (third body) is typically harder than the two surfaces in contact. When this phenomena occurs, the two softer surfaces will undergo abrasive wear as they slide or roll over the foreign object.
Adhesive wear failure modes are scoring, galling, seizing, and scuffing. These types of wear result when microscopic projections at the sliding interface between two mating parts weld together under high local pressures and temperature. After welding together, sliding forces tear the metal from one surface. The result is a minute cavity on one surface and a projection on the other, which will cause further damage. Adhesive wear initiates on a microscopic level but progresses steadily once it starts.
Adhesive wear, shown in Figure 5, can be eliminated by preventing metal-to-metal contact of the two sliding surfaces. This is accomplished through either a lubricant film or suitable coatings or thorough deposits such as Teflon infusion layers.
Figure 5: Adhesive Wear
All machines are subjected to stresses during operation. The stresses subjected to operating machinery are not constant, they periodically increase and decrease. All these repeating stresses in a rolling or sliding contact can give rise to fatigue failure. The effects of fatigue wear are based on the stresses in or below the surface without a need for direct physical contact of the surfaces under consideration. This theory is proved by journal bearings that have undergone fatigue failure even though the surfaces are separated by a thick film of lubricant.
A fatigue crack (Figure 6) requires a given number of stress cycles to form, which implies fatigue cracks are not normally observed until the machine has been in operation for a long period.
Figure 6: Fatigue Cracks
Fretting wear, or fretting corrosion, is a type of wear that has considerable practical significance. Fretting can be defined as "accelerated surface damage occurring at the interface of contacting materials that are subjected to small amounts of periodically occurring displacement." Fretting corrosion is found in all kinds of press fits, spline connections, bearings, and riveted and bolted joints, among other places. Fretting corrosion is also called friction oxidation, bleeding, red mud, and fit corrosion. One important effect of fretting wear is its contribution to fatigue failures. Examinations of surface fractures have shown that fatigue cracks originate in or at the edge of a fretted area. An example of the effect of fretting on fatigue of an aged Al-Cu-Mg alloy is shown in Figure 7.
Figure 7: Effect of Fretting on the Fatigue of an Aged Al-Cu-Mg Alloy
The following factors influence fretting wear:
The prevention of fretting corrosion and wear lies in the elimination of relative displacement. One way to achieve this is to decrease fit clearances. Troubleshooters encountering fretting corrosion would do well to see, for instance, that improved shaft tolerances in repair work are adopted. The tighter fits obtained by such measures will increase contact areas between shafts and bores and consequently increase frictional force.
Increasing the coefficient of friction is another preventive measure. This can be achieved by coating the contact surfaces with materials that have suitable frictional properties. These coatings could be metallic or non-metallic. Metallic coatings that have been successfully employed are cadmium, silver, gold, tin, lead, copper, and chromium. Non-metallic coatings result from chemical treatments such as phosphatizing, anodizing, and sulfudizing, or from bonding of materials such as polymer of MoS2 (molybdenum-disulfide) or teflon to the contacting surfaces. Diffusion coating techniques such as carburizing, nitriding, and cold working (for example, cold rolling, shot-peening, and roll peening) enhance the fatigue strength of the contacting members.
Finally, when looking at the factors that influence fretting wear, the analyst should always attempt to minimize their impact to economically prevent this failure mode.
Erosion of components caused by impingement of solid particles or water droplets can cause reduced life of machinery. Impact erosion of a surface, as shown in Figure 8, has become a serious concern in industry over the last several years.
Figure 8: Impact Erosion
Technology has developed to a point where pressures, temperatures, and velocities of material traveling through newer systems are of an extremely high nature. Under these conditions, the energy carried by a moving particle can cause excessive erosion in operational systems that must be accounted for.
Cavitation, or the formation of cavities in a fluid, is the occurrence and subsequent collapse of vapor bubbles in the flow of liquid. Vapor bubbles are formed when the static pressure in the liquid sinks so low that it attains the vapor pressure associated with the temperature of the liquid at that particular point. If the static pressure then rises above the vapor pressure along the flow path, the vapor bubble collapses quite suddenly, followed by sudden condensation in the form of an implosion. If the implosion occurs not in the body of flowing liquid but at the wall of a component containing the flowing liquid, cavitation will result in material erosion.
Recent research in the field of cavitation has indicated that the vapor bubble inverts at first, once the implosion begins. After that, a fluid microjet is formed, directed toward the interior of the bubble, which pierces the opposite wall of the bubble. Slow-motion pictures of the phenomenon indicate that where bubbles are close to a wall, liquid microjets are always directed against the wall, striking it at high speed. This causes material disintegration, which is, in turn, intensified by chemical action. The microjet entrains the dissolved oxygen in the liquid, which is then liberated in the vapor and forced at high pressure between the grain boundaries of the material at the wall surface. This process increases the corrosion of the wall material.
Pitting, the first sign of cavitation corrosion, is shown in Figure 9. In its progressed form, the eroded surface will have a honey-combed, spongy appearance and structure.
Figure 9: Cavitation Erosion
The amount of material removed by cavitation can be determined by:
Usually, the troubleshooter will attempt to curtail the effects of cavitation corrosion by design or operational changes. Quite often, it will be impossible to shift the collapse of the vapor bubbles away from the wall toward the center of the flow path. A change of materials of construction will be appropriate under these circumstances.
Materials resistant to cavitation are those with high fatigue strength, ductility, and corrosion resistance.
In certain applications, there is the possibility that electric current will pass through a bearing. Current that seeks ground through the bearing can be generated from stray magnetic fields in the machinery or can be caused by welding on some part of the machine with the ground attached so that the circuit is required to pass through the bearing.
An electric current can be generated by static electricity emanating from charged belts or from manufacturing processes involving leather, paper, cloth, or rubber. This current can pass through the shaft to the bearing and then to the ground. When the current is broken at the contact surfaces between rolling elements and raceways, arcing results, producing very localized high temperature and consequent damage. The overall damage to the bearing is in proportion to the number and size of individual damage points.
Figure 10 shows a series of electrical pits in a roller and in a raceway of a spherical roller bearing.
Figure 10: Electric Arc Induced Wear on Rolling Element Bearing
The pit was formed each time the current broke in its passage between raceway and roller. The bearing from which this roller was removed was not altogether damaged to the same degree as this roller. In fact, this specific bearing was returned to service and operated successfully for several additional years. Hence, moderate amounts of electrical pitting do not necessarily result in failure.
Another type of electrical damage occurs when current passes for prolonged periods and the number of individual pits accumulate astronomically. This condition can occur in ball or roller bearings. Flutes can develop considerable depth, producing noise and vibration during operation, and eventual fatigue from local overstressing. The formation of flutes rather than a homogeneous dispersion of pits are not clearly explained but are possibly related to initial synchronization of shocks or vibrations and the breaking of the current. Once the fluting has started, it is probably a self-perpetuating phenomenon.
Individual electric marks, pits, and fluting have been produced in bearings running in the laboratory. Both alternating and direct currents can cause the damage. Amperage rather than voltage governs the amount of damage. When a bearing is under radial load, greater internal looseness in the bearing appears to result in greater electrical damage for the same current. In a double-row bearing loaded in thrust, little, if any, damage results in the thrust-carrying row, although the opposite row may be damaged.
Thermal softening is a plastic flow phenomenon that typically occurs in rolling element bearings. This mode of failure is caused by a thermal imbalance resulting from more heat being generated in the bearing than is being removed. The maximum permitted temperature in bearings depends on the material, but is generally said to be approximately 250F. An increase in temperature above this limit may result in lubricant failure and bearing seizure. Softening of the bearing material may also occur, resulting in a reduction of the load-carrying capacity of the bearing. Gross overheating of bearings above the temperatures at which the rolling elements and rings were tempered during manufacture will result in rapid softening and subsequent failure.
Gross destruction of heat-softened bearings can sometimes be recognized by discoloration and a predominantly plastically deformed appearance. Therefore, it may be different in appearance from fracture that results from cracking. However, temperature imbalance failures are often so devastating that little is left of the bearing to identify a failure source.
Impact wear occurs during handling or mounting, resulting in depressions. The depressions that occur become the start of premature fatigue. As an example, if a bearing were to suffer severe impact during handling, spaced areas of flaking could develop corresponding to the distance between the balls. A bearing installed in this condition will exhibit noise and vibration during operation.
Corrosion is the unintended destructive chemical or electrochemical reaction of a material with its environment. Many forms of corrosion lead to failure of metal parts or render them susceptible to other forms of mechanical failure.
Several factors must be considered to determine whether corrosion caused or contributed to the failure. To implement effective protective measures, the analyst must examine the type of corrosion, corrosion rate, how the corrosion process was influenced by the nature of its environment, and uniformity or non-uniformity. Factors that affect corrosion are not always constant, as the factors affecting corrosion change, corrosion rate changes.
Analyzing the effects of corrosion wear can be very complex, but in most cases, a simple visual examination or study of events leading up to the failure provides adequate information as to the failure of the component.
Plain bearings, also called journal or sleeve bearings, are one of the simplest machine components. The type of motion between the bearing and shaft is pure sliding.
In plain bearings, the lubricant must reduce sliding friction, carry away any heat generated in the bearing, prevent rust and corrosion, and serve as a seal to prevent the entry of foreign material.
Barring any unusual operating conditions, plain bearings will operate satisfactorily with any lubricant of the correct viscosity. Special operating conditions may require the use of oils containing additives. Anti-wear and extreme pressure oils may be desirable for plain bearings operating intermittently or under very high loads. Rust-inhibited and corrosion-inhibited oils are generally preferred for humid operating environments.
Numerous mathematical models of plain-bearing lubrication have been used in attempts to accurately select the best oil viscosity for a plain bearing. Unfortunately, these models are complicated and expensive to develop. For this reason, except in special cases, lubricant viscosity selection is usually based on standard practices established through experience.
Table 4 presents a general guide for viscosity selection for plain bearings subjected to average loading.
Plain bearings may be grease-lubricated if their operating speed does not exceed approximately 6 ft/s (2 m/s). At higher speeds, excessive temperature buildup could result.
In general, relatively soft greases are used for centralized systems and harder greases for compression cups and open journals. Each application should be considered on its own merits, taking into consideration the operating conditions. Temperature and water contamination require particular attention.
Journal bearings used on reciprocating shafts present additional problems when selecting a lubricant. The bearing will support the shaft radially during operation; however, because the shaft is reciprocating, two separate wear patterns will develop. These wear patterns will be created during the upstroke and downstroke.
It was explained earlier that the majority of bearing wear occurs at startup. In a bearing subjected to reciprocating load, the shaft will continually start and stop, creating an excessive amount of cyclic stress on the bearing. Under these conditions, a highly cohesive, high viscosity oil is necessary to prevent the lubricant from breaking down.
Whenever reference is made to a cylindrical journal bearing, the type usually referred to is the plain cylindrical bearing. This simple design has axial grooves or depressions at the horizontal splits to introduce lubricant to the bearing; however, there are no grooves in the loaded region of the bearing (lower half).
Because the loaded region of the bearing is unbroken by grooves, the load capacity of the bearing is fairly high. For this reason, the two-groove bearing is used where high loads are encountered. When this type of bearing is used and experiences a light load condition, it becomes very susceptible to oil whip, a condition in which the pressure exerted by the oil wedge is out of equilibrium with the centrifugal force exerted by the rotating shaft. This condition causes high levels of radial shaft vibration, which can strain the shaft and possibly damage the bearing.
Turbulence in lubrication is caused by velocities in excess of normal. When the lubricant delivery system is delivering the lubricant at high velocities, the turbulence that occurs will lead to an increase in material erosion. The erosion is due to impingement and cavitation erosion.
Extreme caution must be exercised if grease lubrication is to be used for plain bearings. The amount of friction created in plain bearings can lead to excessive temperatures without a continuous supply of lubricant. Plain bearings whose operational speed does not exceed 6 ft/sec may be grease-lubricated, but should be monitored closely for temperature increase.
In a steam turbine, the axial clearances between the rotating and stationary parts are quite close, in the order of a few thousandths of an inch. At the same time, the action of the steam on the rotating elements is such that considerable axial thrust may be produced. Therefore, it is essential that some means be provided for positioning the rotating element of a turbine while absorbing its axial thrust. In turbines and other equipment subjected to similar forces, this is accomplished by means of a thrust bearing.
The thrust bearing is a stationary element with a Babbitt surface adequately supported to withstand large loads. It is both a positioner and a load absorber. In a thrust assembly, the thrust plate with its Babbitt surface is located close to a flat rotating surface on the shaft known as a thrust collar or runner, as shown in Figure 11. Thrust loads are transmitted from the shaft by the thrust collar pushing against the thrust plate. The action of the rotating surface against the stationary thrust plate is a slider action, and hydrodynamic film pressures can be developed.
Figure 11: Fundamental Thrust Bearing
In the past, before the hydrodynamics of lubrication were understood, the importance of an oil wedge extending in the direction of relative motion was not recognized. Consequently, the journal bearing of that period was generally far superior to the thrust bearing because an oil wedge was inadvertently formed in the clearance space of the loaded journal bearing, while plain parallel surfaces were used to support the load in the thrust bearing. After the hydrodynamic theory of lubrication was understood, specially designed thrust bearings using the principle of the oil wedge were developed. The performance of the thrust bearing was then brought up to that of the journal bearing.
The thrust load on some machines may change direction under certain conditions. For this reason, two thrust plates are usually provided in thrust assemblies to absorb thrust in either direction. Normally, there will be a large load in one direction (the active thrust direction) and, at a different time, a lesser load in the opposite direction (the inactive thrust direction). The thrust plate absorbing load in the active thrust direction is called the active thrust plate, and the thrust plate absorbing load in the inactive thrust direction is called the inactive thrust plate. The inactive thrust plate can be somewhat smaller than the active one; but in many cases, both active and inactive thrust plates are made the same size.
As noted previously, the surface of the thrust plate is coated with Babbitt similar to a journal bearing. In most thrust plate designs, the actual thrust surface is broken by a series of radial grooves whose main function is to pass a quantity of lubricant. The grooves make the thrust surface appear as a series of separate pads. These pads are also often referred to as lands. Thrust bearing types are frequently designated by the type of land or pad design incorporated into them.
As in the journal bearing, the thrust bearing load is supported on a thin film of oil. Under proper conditions, there should not be any metal-to-metal contact during operation. Some Babbitt wear can be experienced during low-speed operation where hydrodynamic action cannot be produced and the bearing operates in the region of boundary lubrication. Wear can also occur during operation if the lubricant contains an excess of foreign material.
Three general classes of thrust bearings are:
The flat land thrust bearing is the least complex of thrust designs. Its simple, flat surface facilitates its manufacture and reduces its cost. The load capacity of this bearing is relatively low, making it more useful as a positioner than a thrust absorber.
The fact that it can carry any load at all is somewhat surprising since the all-important oil wedge is nonexistent. However, radial grooves in the thrust face pass a quantity of oil for lubricating and cooling the surface, and minute misalignments and radii on the oil grooves are apparently sufficient to produce some amount of wedge action to sustain small loads. Flat land thrusts have been used as the inactive thrust plate on many machinery designs.
In general appearance, the tapered land thrust bearing resembles the flat plate thrust bearing. Its surface is divided into a number of pads separated by an equal number of oil feed grooves, as shown in Figure 12. However, in the tapered land bearing, each pad is tapered in a circumferential and radial direction so that the motion of the runner will wipe oil into the contacting wedge-shaped area and build up load-carrying oil pressures. It has been applied in sizes up to 24 inches in diameter.
Figure 12: Tapered Land Thrust Bearing and Details
The total number of pads may vary from 6 to 14 depending on the bearing size. The number of pads varies in multiples of two (for example, 6, 8, 10) with the smaller number of pads being applied to the smaller diameter plates. The horizontal split can then be made through the oil groove. The dimensions of the grooves between pads are arbitrary.
The grooves are usually as deep as they are wide, although this may not be necessary in certain instances. However, the grooves must be large enough to pass the required amount of oil. At the outside end of the radial groove, a dam is provided to control the direct leakage of oil from the grooves to the outside of the plate. These dams are sized to pass sufficient oil to maintain a 30F temperature rise between inlet oil and drain oil.
Although tapered land thrust plates have a high load-carrying capacity, they are prone to misalignment, which can detract from their performance. Experiments have shown that thermal distortions contribute heavily to thrust failures in many cases. Uneven temperature distributions around the thrust plate result in distortion, which can destroy the oil film.
To alleviate distortion, copper-backed plates have been used. An excellent heat conductor, such as copper, provides a more uniform temperature distribution between lands. By reducing large temperature variations between lands, load-carrying capacity is doubled in some cases.
The Kingsbury thrust bearing, one version of a tilting pad type, has been widely adapted to rotating machinery. It differs from a tapered land or flat land thrust in that each pad is an individual plate which is free to tilt about a pivot (Figure 13).
Figure 13: Kingsbury Thrust Bearing
Normally, the pivot takes the form of a hardened spherical surface, which is inserted behind each pad. The pad is then free to tip in a radial or circumferential direction, or a combination of the two. This tilting feature compensates for any misalignment which may exist between thrust plate and thrust runner. When the runner is stationary, the pads lie with their surfaces parallel to the runner face. As the runner is started, an oil film is created between the pad and runner and each pad tilts to an angle that generates the proper distribution of film pressure.
In addition to the pivot feature of each pad, there are also a series of plates on which the pads sit, known as equalizing or leveling plates. This arrangement of plates is shown in Figure 13 (above). The equalizing plates are accurately machined, forged pieces that act to distribute the thrust load uniformly around the bearing.
The upper leveling plates are supported on the lower levelers and support the individual pads above. The lower leveling plates transmit the total load to the base ring. Hardened inserts are usually located in the base ring to support the lower leveling plates. A misaligned load requires the plates at the heaviest load point to tilt and push the remaining pads outward to equalize the load over all the pads. To keep the leveling plates in place, a series of set screws is provided in the outer diameter of the base ring to engage the upper leveling plates.
The close clearances that are necessary for the proper operation of bearings inhibit oil flow to all areas of the bearing. To allow oil to flow more readily to all bearing surfaces, grooves are machined into the surface of the bearing material.
An axial groove bearing has a bearing surface that is broken by a series of axial grooves running the length of the bearing parallel to the axis of the shaft. This bearing may have either a cylindrical or elliptical bore, but the cylindrical bore is the more common. The number of grooves varies from four to six over the circumference, usually spaced unequally.
Figure 14 shows a typical axial groove design of cylindrical bore. This four-groove design has unequal spacing between the grooving. Such spacing produces an unequal lobe pattern around the bearing. This pattern breaks up the hydraulic film and produces better stability during operation. This design has become popular for smaller shaft diameters of nine inches or less.
As shown in Figure 14, the axial grooves contain orifices through which oil is supplied to each groove individually. The location of the orifices is normally at the center of the groove both circumferentially and axially. However, the location of the orifice in the axial direction can vary from the center without affecting the performance of the bearing because the groove pressure is nearly constant for any location of the orifice.
Figure 14: Diagram of Axial Groove Bearing
Axial grooves run the entire length of the bearing except for a -inch length at each end. Small triangular chamfers located at these points (see end view of Figure 14, above) control approximately 30 to 60 percent of the total bearing oil flow. The absence of a chamfer at the groove ends results in rather high drain temperatures. Running the groove out to the ends without any restrictions results in very large oil flows.
Drain chamfers allow cool oil from the grooves and hotter oil flowing from the ends of the bearing to mix in the circumferential drain grooves at the ends of the bearing. This action results in a lower average drain temperature. In cases where axial groove bearings are found to be operating at high drain temperatures, it is usually because the chamfers are too small. The trouble can be corrected by enlarging them.
Although the chamfers actually control the magnitude of the oil flow and drain temperatures, they have little effect on the oil film temperature in the bearing itself. They may also become a collection point for dirt particles, which can score the journal during operation, under improper lubrication conditions.
Rolling element bearings are also called anti-friction bearings. This name comes from the fact that rolling surfaces in contact create less friction than sliding surfaces in contact. Without proper lubrication, anti-friction bearings can be expected to fail early and to possibly cause other equipment damage. This section examines how rolling element bearings are lubricated.
Generally, oil is the best lubricant to use because it is a pure lubricant, but oil is not always practical due to design requirements. Grease is a secondary lubricant and is nothing more than oil suspended within a base. It is important to realize that these bases, when exposed to moisture or heat, may turn into soap or carbon ash. For these reasons, it may be necessary to use synthetic additives in the base of greases.
Anti-friction bearings must be lubricated to prevent metal contact between the rolling elements, raceways, and cage. Additionally, lubrication protects the bearing against corrosion and wear, helps dissipate heat, aids in sealing, and reduces bearing noise.
The best operating temperature for a rolling element bearing is obtained when the minimum of lubricant necessary to ensure lubrication is used. The quantity of lubricant used will also depend on other functions required of the lubricant, such as cooling and sealing.
Anti-friction bearings can be lubricated with grease or oil. The choice of lubricant depends on conditions such as operating temperatures, rotating speeds, loads, and environmental conditions.
Grease has several advantages as compared to oil:
Under normal operating conditions, anti-friction bearings can be grease lubricated. The free space in the housing and bearing should only be partially filled with grease. Thirty to fifty percent grease in the bearing housing is considered adequate. Overfilling may cause a rapid rise in temperature, particularly at high speeds. Bearings operating at slow speeds and those that require corrosion protection may have their housings completely full of grease. Additionally, overfilling may prevent a bearing that is designed to float in a housing from operating properly.
The period during which a grease lubricated bearing will function satisfactorily without relubrication depends on bearing size, type, speed, operating temperature, and the grease used.
When operating conditions of equipment are such that relubrication can only be carried out at infrequent intervals, it is sufficient if the bearing housing can be opened to remove as much used grease a possible from the bearing. Then, repack fresh grease between all the rolling members from one side only.
If frequent re-lubrication can be performed on the equipment, some provision is made for re-greasing, usually in the form of a grease nipple fitted to the bearing housing. A grease gun adds fresh grease to the bearing and replaces the old grease.
The lubrication duct in the housing should either feed the grease adjacent to the outer ring face, or, preferably, into the bearing by means of the lubrication groove. After numerous re-lubrications, the bearing housing should be opened and the used greased removed before fresh grease is added.
Oil lubrication is used when high speeds or high operating temperatures prohibit the use of grease. Oil will transfer frictional heat away from a bearing or adjacent machine parts effectively.
Oil bath systems are suitable for low shaft speeds. Oil is picked up by the rotating bearing elements and after circulating through the bearing, it drains back to the oil reservoir. When the bearing is at rest, as shown in Figure 15, the level of the bath should come to just below the center of the bottom rolling element.
Figure 15: Oil Bath Lubrication
To avoid having many oil changes, because of high operating temperatures causing oil aging, lubrication can be provided to ball and roller bearings by an oil circulation system. A positive-displacement oil pump sends pressurized oil through the bearing housing and into the roller bearing. After the oils passage through the bearing, the oil is filtered and possibly cooled before being returned to the bearing.
At high shaft speeds, oil must penetrate the interior of the bearing to remove the excess heat. An effective method for doing this is injecting oil into the bearing. The speed of the oil being injected must be high enough to ensure that sufficient oil penetrates the air vortex created during bearing rotation. Figure 16 shows an oil injection unit.
Figure 16: Oil Injection Unit
How often the lubricating oil has to be changed depends upon the operating condition and the quality of the oil. For oil bath systems, the oil should be changed more often if operating temperatures of the oil exceeds 120F or if the machine operates in an environment where abrasive and fluid contamination is great.
With circulation, the time for an oil change can be determined best by inspecting the oil quality. An oil analysis can determine if:
It is important to monitor the lubricating oil performance because it can affect the bearings service and life.
Many different methods are used to apply oils and greases to machinery. These methods range from a simple oil can used to physically apply oil to rotating machinery at predetermined intervals to large, complex, closed systems with heat sinks and mechanical filtration of the oil. Oil system types are classified open, continuous, or closed.
An open system of lubrication supplies new lubricants to moving surfaces. The used lubricant is then discarded. This system is sometimes called the all loss method of lubrication.
Hand lubrication has limited applications (Figure 17). For most uses, however, this method has been replaced because of its disadvantages. When hand lubrication is used, at first the equipment is usually over-lubricated, causing leakage and throw-off of lubricant. The equipment then has enough lubricant for some time. At a later period, depending on the frequency of lubrication, the equipment will be under-lubricated, causing wear and friction.
Figure 17: Hand Lubrication Devices
Hand lubrication can become costly from lost material and the labor needed. Lubricant leaking is also a fire hazard. In addition, certain places requiring hand lubrication may be overlooked or dangerous to reach.
Several devices are used in plants that eliminate the need for hand lubrication. These devices, which supply a continuous amount of lubricant to the moving part, are called continuous lubricators.
There are continuous lubricators for oil and grease. Figure 18 shows a simple drop feed and wick feed oiler. These are used to continuously add small amounts of oil to bearings and gears.
Figure 18: Drop Feed and Wick Feed Oiler
Figure 19 is another example of a simple drop feed oiler. A needle valve in the drop feed oiler can be adjusted to change the rate of oil supplied. The wick absorbs oil in the cup and loses the oil on the moving part; changing the number of strands in the wick changes the oil flow. There are many types of wick oilers. The wicks must be kept clean because wicks can clog from dirty oil. In addition, wicks should be replaced regularly.
Figure 19: Simple Drop Feed Oiler
Figure 20 shows a type of continuous oiler that is referred to as a bottle oiler because of the glass bottle used to store oil.
Figure 20: Bottle Oiler
The bottle oiler that is shown above keeps a constant level of oil in a bearing housing. This provides an oil bath for the bearing to rotate through. The oil level in the bottle can be seen and the bottle can be refilled when needed.
Two types of continuous greasing devices are shown in Figure 21. The caps are removed to add grease. The spring keeps a constant pressure on the grease; this forces the grease into the bearing as needed.
Figure 21: Grease Cups
A closed system of lubrication uses the same lubricant over and over again. The lubricating system of a car engine is an example of a closed lubricating system. Two types of closed lubricating systems are non-forced lubrication and forced lubrication.
Figure 22 shows a non-forced lubrication system. It is a type of a wick oiler used to lubricate a bearing and shaft. The wick absorbs oil in the reservoir and then applies it. The oil then flows along the shaft, through the bearing. As the oil drips from the shaft, it is collected in the reservoir and reused.
Figure 22: Wick Oiler (Non-Forced Lubrication System)
The ring oiler is a commonly used non-forced lubricating system for forced draft, induced draft, and primary air fans (Figure 23). As the shaft rotates, so does the metal oil ring resting on the shaft. The lower portion of the ring travels through an oil reservoir. Some oil sticks to the ring and is carried up to the shaft.
Figure 23: Ring Oiler
Using the proper lubricant for a ring oiler is important. An oil with very low viscosity may not stick to the ring and travel up to the shaft. A very viscous oil, however, may not allow the ring to pass through it; thus, no oil would reach the shaft.
Figure 24 shows a forced system that is referred to as a splash oiling system, in which the gear teeth pick up the oil and splash it around the gear chamber.
Figure 24: Splash Oiling System
In this example, the oil is collected and used to lubricate the bearings before returning to the oil reservoir.
In forced lubrication systems, which use oil under pressure to lubricate moving parts, an oil pump pressurizes the oil (Figure 25). Examples of systems using forced lubrication are turbine generators, boiler feed pumps, compressors, and gearboxes. Two reasons for using forced lubrication systems are to use oil pressure to separate two surfaces, and to cool moving parts with a high flow of oil.
Figure 25: Simple Forced Lubrication System
Bearings under heavy loads or used with slow starting equipment may need pressurized oil to make a film between the shaft surface and the bearing.
Equipment operating at high speeds and heavy loads can develop high temperatures from friction. To protect the equipment from high temperatures, a high flow of oil is needed. The oil carries the heat away from the equipment.
Figure 26 shows a forced lubrication system for a gearbox with bearings. The system has three continuous steps of operation. Oil from the gearbox is collected and sent to the oil reservoir; the oil pump takes suction from the reservoir, and the pump discharges oil through an oil cooler and back to the gearbox. Also, notice the oil filter shown in this figure. When needed, a pump takes the oil from the reservoir, cleans it in the filter, and then returns it to the reservoir.
Figure 26: Forced Lubrication System
Oil reservoirs hold an oil supply for the system and remove water, dirt, and other contaminants (Figure 27). Because oil reservoirs are built large enough, the oil flow through them is slow. This keeps the oil from foaming. Slow moving oil lets dirt and water in the oil settle to the bottom. Oil reservoirs have sloping bottoms so dirt and water collect at one end. A drain line at this end should be checked for dirt and water.
Oil reservoirs are also vented to prevent condensation of water on the inside walls. Filters in vent lines keep airborne dust from entering the reservoir. These filters should be checked for clogging. A glass level gage attached to the oil reservoir shows the oil level. Oil level checks are part of the routine maintenance for this system.
Oil reservoirs also have openings so you can inspect and clean the inside. Check the reservoirs for rust and sludge on the inside surfaces. When replacing the oil, use lint-free rags to wipe the reservoir clean.
Oil coolers remove heat from oil. Friction in bearings or other moving parts heats the oil. For example, the oil temperature leaving turbine bearings is approximately 160F. An oil cooler, a vessel with tubes (Figure 27), lowers the oil temperature to about 120F for reuse.
Figure 27: Lubrication Oil Cooler
Cold water is passed inside the tubes. The oil is pumped through the vessel shell and around the tubes. Heat in the oil is transferred through the tubes and is absorbed by the water. In other oil coolers, the oil may pass through the tubes with the water in the shell.
When selecting a lubricant for a given application, three considerations must be taken:
If any one of these three conditions is not met, the component can be in jeopardy of premature failure.
The major cause of lubrication-related failures in process equipment is due to the incorrect amount of lubrication being applied to the component. The effects of over-lubrication and under-lubrication of machinery used in the process industry are discussed below.
Contrary to popular belief, the old adage, the more oil the better, is not true in terms of proper lubrication. Manufacturers of specific components state the amount and type of lubricant for a given application. These requirements should be strictly adhered to.
When a bearing is discovered to be operating at an abnormally high temperature, the first instinct is to add more lubricant. This action should never be taken. Bearings, or machinery that is operating at abnormal temperatures, should be shut down so the cause can be investigated. If the bearing was over-lubricated, the temperature rise was due to churning of the lubricant. Under these conditions, the lubricant will break down and the bearing will eventually fail.
When lubrication is inadequate, surface damage will result. This damage will progress rapidly to failures that are often difficult to differentiate from primary fatigue failure. Spalling will occur and often destroy evidence of the effects of under-lubrication. This type of failure is the number one cause of lubrication-related failures in equipment today.
The correct lubrication of plant equipment is an important factor in sustaining production with reduced equipment outage and lower maintenance costs. When a well planned and coordinated lubrication program has been established, the production plant will operate at its highest efficiency.
As previously discussed, there are many types of lubricants that can be used in a production plant. The reason for the different types of lubricants are to satisfy the different operating conditions machines operate under. This section focuses on how to properly lubricate the different machines used in industry today.
Lubricants in gear units have two functions: To separate the tooth and bearing surfaces, and to cool these surfaces. On low-speed gear units, the primary function is lubrication; on high-speed units, the primary function is cooling. This statement does not imply that both functions are not important, but rather refers to the relative quantity of oil required to perform each function.
On low-speed gear units, the quantity of oil necessary is determined by the amount required to keep the gear tooth and bearing surfaces wet. On high-speed units, oil quantity required is generally determined by the amount of heat loss (or inefficiency) in the bearings and mesh. As a general rule, one gallon per minute must be circulated for each 100 horsepower transmitted; this quantity would result in a temperature rise of approximately 25F. Higher horsepower units use a 40-50F temperature rise and require 0.5-0.6 gallons per minute per 100 horsepower transmitted. These figures are based on the assumption of 98 percent gear unit efficiency.
Several different techniques of supplying lubricating oil to the gears and bearings in a gear unit are available to the gear manufacturer. The three primary methods in use today are splash lubrication, force-fed lubrication, and intermittent lubrication. Each of these methods has identifying characteristics that are described in the following paragraphs.
Splash lubrication is the most common and fool-proof method of gear lubrication. In this type of system, the gear dips in oil and, in turn, distributes that oil to the pinion and the bearings. Distribution to the bearings is usually obtained by throw-off to an oil gallery or is taken off the sides of the gear by oil wipers (or scrapers) that deliver the oil to oil troughs.
When using the throw-off system, care must be taken that the operating speed is high enough to lift and throw off the oil.
Oil wiper systems can operate at much lower speeds, which are usually determined by test or through experience.
The splash system can be used in gear units with up to 4,000 feet per minute pitch line velocity. Higher speed gear units can be splash lubricated with special care.
Force-fed lubrication is pressurized lubrication and is used on almost all high-speed gear drives, on spiral bevel drives, and on low-speed drives when splash lubrication cannot be used due to gear arrangement.
A simple force-fed system consists of a pump with a suction line and supply lines to deliver the oil; the gear housing serves as the reservoir. In contrast to this simple arrangement, more complicated lubrication supply systems for high-speed drives may include many of the following components:
Many of these lubrication systems are well designed and are constructed not only to lubricate the gears and bearings of the gear unit, but also to enhance performance of the driving machine, gear unit, and driven machine.
Intermittent lubrication provides lubricant periodically to gears, bearings, or both, however not continuously. This type of lubrication system is the least common and is primarily suited for low-speed applications. The following methods are used to apply the lubricant:
The oil furnished to high-speed gears has a dual purpose: lubrication of the teeth and bearings and cooling. Usually, only 10 to 30 percent of the oil is used for lubrication, and 70 to 90 percent is used for cooling.
For high-speed gear units, a turbine-type oil with rust and oxidation inhibitors is preferred. This oil must be kept clean (filtered to 40 microns maximum, preferably to 25 microns), must be cooled, and must have the correct viscosity. Synthetic oils should not be used without the manufacturers approval.
In new gear units shipped from the factory, the rush inhibitor adhering to internal exposed surfaces should prevent corrosion of interior parts for at least six months. Exterior preservatives should last at least six months, but this protection will depend on handling and exposure to the elements. A new gear unit should be stored inside if possible. If inside storage is not possible, outside storage, with the gear unit covered, can be used. It is sound practice to use a dry nitrogen purge during storage to prevent or minimize condensation inside the gear housing.
When the recommended lubricant is used and the reducer has been operating for a period of time, the lubricating oil should protect interior parts for inoperative periods of up to 30 days because most of these oils have rust and oxidation inhibitors added.
If additional downtime or storage time is required, one of the following methods can be used to protect the internal parts of the gear unit:
When a unit is inoperative, most gear manufacturers recommend that it be inspected every thirty days to six months depending on the method of protection. Any areas of the preservative not performing properly should be removed with solvent and recoated.
Six factors affecting lubricant selection for gear units are listed in Table 5, along with the lubricant properties that should be considered in relation to each. Viscosity is probably the single most important element in lubrication selection and is determined by load, speed, and temperature variations. All of these factors should be reviewed and evaluated to determine the exact lubricant properties necessary for satisfactory gear performance. Final selection of the lubrication oil for the gear unit should be based on the best combination of all of the required lubricant properties.
A good rule to follow when evaluating the type of lubricant to use for a specific situation is to consider the least expensive one available that will perform well in that situation. If a specially blended type of oil is to be tried, determine its stability by selective use before making major changes. Lubricant failures are expensive!
There are many brand name lubricants available on the market today, but all fall into five basic types. The following discussion is a brief summary of the characteristics, advantages, and disadvantages of each of the different categories.
Mineral oils are still the most commonly used type of gear lubricant. Containing rust and oxidation inhibitors, these oils are less expensive than the other types, readily available, and have very long life. When gear units operate at high enough speeds or low enough load intensities, a type of mineral oil is probably the best selection.
Extreme pressure (EP) additives of the lead-naphthenate or sulphur-phosphorous type are recommended for gear drives when a higher load capacity lubricant is required. As a rule, this type of oil should be used in low-speed, highly loaded drives with medium operating temperatures. EP oils have the disadvantages of being more expensive and of requiring replacement more often than straight mineral oils. Some of these EP oils have a very short life above a temperature of 160F.
Synthetic lubricants are not usually recommended by gear manufacturers for general gear applications due to high cost, limited availability, and lack of knowledge of their properties. Nevertheless, they are used with good success in applications with extremely high or low temperatures, where fire protection is required, or where very high speeds or high wear rates are encountered. The user must be careful when selecting these lubricants since some of them remove paint and attack rubber seals. The new synthesized hydrocarbons (SHC) have many desirable features such as compatibility with mineral oils and excellent high and low temperature properties. They are excellent selections when EP lubricants along with high temperature operation are required.
Compounded oils are available with many different additives. The most commonly available is a molybdenum disulfide compound that has been successfully used in some gear applications. It is very difficult for a gear manufacturer to recommend these oils at this time since some of these additives have a tendency to separate from the base stock. In many instances, however, compounded lubricants are the only solutions to gear lubrication problems. These oils can be blended for extremely high load-carrying capacity and high temperature operation. Most of these super properties can be obtained, but sacrifices must be made in other lubricant properties such as life or corrosion protection.
Viscosity improvers in gear drives should be used with great care. These polymer additives make great textbook improvements in the viscosity index and extend the operating temperature range of an oil. However, the viscosity of these fluids reduces with shearing. A gear drive is a very heavy shear application and, as a result, the viscosity is reduced rapidly if too much polymer is used. These lubricants are seldom recommended in long life gear drives.
Piston ring trouble is one of the worst problems that can be encountered with internal combustion engines. Engines operating with faulty rings are vulnerable to cracked pistons, cracked heads, worn liners, or piston seizure with subsequent crankcase explosions. Faulty rings will also reduce the life of lubricating oil. It is very important to be alert for any indicators that will show piston ring problems. In large two-cycle engines, ring trouble caused by problems in the port area will result in a distinct clicking noise at the base of the cylinder. Therefore, the piston sounds should be checked daily for any unusual sounds. Other indicators of ring or liner trouble are increased lube oil consumption, breakdown of lube oil, decreased life of lube oil filters, increased crankcase pressure, and high exhaust temperatures.
Proper piston ring lubrication from the beginning of machine operation will prevent piston ring wear, thereby increasing machine life.
Reciprocating compressors require lubrication of the cylinder walls, packing, and bearings. Because temperatures at the cylinder wall are high, sufficient viscosity must be provided. Since water condensation may be encountered, rust inhibitor is indicated in addition to oxidation inhibitor. The principal operating problem with regard to the lubricant is the development of carbonaceous deposits on valves and in piping. This can seriously interfere with valve operation and can give rise to disastrous fires and explosions.
It is essential to choose an oil with a minimum tendency to form such deposits. It is incumbent upon the machine operator to institute maintenance procedures to ensure that any deposits are cleaned on a regular basis and are not allowed to accumulate over long periods of time. Also, in units calling for all-loss lubrication to the cylinders, feed rates should be reduced to the minimum recommended levels to minimize deposit-forming tendencies.
Rotary compressors present a unique lubrication problem in that large quantities of oil are sprayed into the air during compression. Exposure of large surfaces of oil droplets to hot air is an ideal environment for oxidation to occur. This is very troublesome with rotary compressors because lacquer deposition interferes with operation of the oil separator, which is essential to good performance.
Although general-purpose and oxidation-inhibited oils, as well as crankcase oils, are often used, they are not the best choice. Specially formulated petroleum oils are available to serve under these severe conditions. For enhanced service life, synthetic organic ester fluids of comparable viscosity are often used.
Hydrocarbon compressors vary in their lubricant requirements depending on the refrigerant gas in the system. Because ammonia is not miscible with oil to any degree, many lubricants may be suitable. The pour point of the lubricant must be somewhat below evaporator temperature, and it cannot contain additives that might react with ammonia. Many fluorinated refrigerants are miscible with oil; consequently, if any wax separates from the mixture on evaporator surfaces, performance is seriously impaired. For such systems, highly refined, low-pour, wax-free naphthenic oils are normally recommended. Wax-free synthesized hydrocarbon fluids are also used for this purpose.
Diesel engines used in marine, railroad, and stationary service employ crankcase oils that are not standardized as to performance levels, unlike automotive oils. They are products of developments of reputable oil suppliers working with major engine builders. In general, they are formulated along the same lines as their automotive counterparts. However, in marine service, where fuels often contain relatively high levels of sulfur, the detergent additive may be formulated with a high degree of alkalinity to neutralize sulfuric acid resulting from combustion.
Gas engines pose somewhat different lubrication requirements. They burn a clean fuel that gives rise to little soot, but conditions of operation are such that nitrogen oxides formed during combustion can have a detrimental effect on the oil. Suitable lubricants may contain less additive than those that must operate in a more sooty environment. However, the quality of the base oil itself assumes greater importance. Special selection of crude source and a high degree of refining must be observed to obtain good performance.
Testing of oil systems has become a vital concern in industry. Through technological advances, we have learned that the amount of contaminants in an oil directly relates to the life span of a piece of equipment. Wear particle analysis is the preferred method used to mark the trends of lubricating oil system contamination.
Wear particle analysis is a method of detecting abnormal wear trends in operating machinery. This, along with oil testing, will extend the life of machinery well beyond the limits imposed by preventive maintenance. Tribological testing, which is concerned with the mechanisms of friction, lubrication, and wear of surfaces in relative motion, is used in the proactive maintenance system. Proactive maintenance is designed to replace preventive maintenance through wear particle analysis and oil sampling.
This section examines the importance of the tests that are performed on oil samples to determine oil performance. Sampling and testing must be performed at regular intervals, using the proper methods to obtain results that are valid.
Viscosity is probably the single most important property of a lubricant and may be influenced by temperature, pressure, and fluid motion (shear).
Kinematic viscosity, the preferred determination method, can be obtained in a variety of ways. Kinematic viscosity is expressed in centistrokes, cSt (mm2/s). A large number of commercially developed devices are acceptable in the determination of kinematic viscosity.
The absolute viscosity (dynamic) can be measured directly at low temperatures. The usual unit of measurement is the centipoise, cP (mPa x s).
The relation between kinematic and dynamic viscosities is given by the following equation:
The SAE viscosity grades 20, 30, 40, and 50 for crankcase oils define viscosity ranges at 100C. A W suffix shows that the oil is designed for winter service; so, emphasis is placed on ability to flow properly at low temperatures. The viscosity specifications for 0W, 5W, 10W, 15W, 20W, and 25W crankcase oils listed in Table 6 were adopted in 1980 and differ significantly from those published earlier. Furthermore, a new concept has been introduced, that of maximum borderline pumping temperature. This measurement, made with a mini rotary viscometer, attempts to define the temperature below which adequate flow to the oil pump inlet of a passenger car engine cannot be assured. An example of a SAE grade is 10W-30 engine oil. The 10W refers to winter-grade with a viscosity of 165 SUS. The 30 is the SAE viscosity range at 100.
The International Standards Organization (ISO) has issued, and ASTM has accepted a system for designating the viscosity grades of industrial oils. As provided in ASTM D2422, each grade is specified by ISO VG followed by a number that is the nominal kinematic viscosity in cSt at 40C (104F). Eighteen viscosity grades covering the range from 2 to 1,500 cSt at 40C, in increments of approximately 50 percent.
Viscosity index (VI) is an empirical system for expressing the rate of change of viscosity of an oil with change in temperature. VI is based on comparison of viscosity measurements of fractions from crude oils ranging from light to heavy. The fractions were chosen because they seemed to possess the maximum and minimum limits of viscosity-temperature sensitivity and were accordingly assigned viscosity indexes of 0 and 100 as the presumed end points of a 100-point viscosity-index scale. While all other oils were expected to fall between these limits, subsequent experience has identified lubricants that are far outside the viscosity-index scale in both directions.
The procedure for calculating the VI of an oil is to determine its viscosity at 40C and at 100C (104F and 212F). The Standard Method and ASTM viscosity tables are used for viscosity-index calculations.
Oils with viscosity indexes above 100 can be made from a wide variety of crude oils by solvent refining, by selective blending of paraffin-base oils, by the addition of relatively small amounts of high-molecular-weight polymeric additives to base oils, or by combinations of these methods. Lubricants with good temperature-viscosity curves (high viscosity index) are desirable where service temperatures vary greatly.
When lubricating oils are subjected to high pressures, several thousand lb/in2, their viscosity increases. When oil-film pressures are in this order of magnitude, the influence on viscosity should be considered. A number of empirical equations to relate viscosity to pressure have been derived. In rolling-contact bearings, gears, and other machine elements, the high film pressures will influence viscosity with accompanying increase in frictional forces and load-carrying capacity.
The viscosity of a liquid lubricant that does not contain a polymeric additive is independent of shear rate. However, where such additives are used, as in multi-grade engine oils, increasing shear rate in lubricated machine elements causes temporary viscosity loss as the large polymer molecules momentarily align in the direction of flow. A standard procedure has not yet been adopted to measure the degree of such behavior. Furthermore, such oils suffer permanent viscosity loss in service because the large polymer molecules are split by shear stresses into smaller ones with less thickening power. This phenomenon is readily observed by comparing standard viscosity measurements on used oil versus new oil.
Petroleum oils, when cooled, may become plastic solids as a result either of partial separation of wax or of congealing of the hydrocarbon composition. With some oils, the separation of wax becomes visible at temperatures slightly above the solidification point. This temperature, when it is reached under prescribed conditions, is known as the cloud point. With oils in which wax does not separate prior to solidification, or in which the separation is invisible, the cloud point cannot be determined. The temperature at which the oil will just flow under prescribed conditions is known as the pour point.
The cloud point is significant in that it indicates the temperature below which clogging of filters may be expected in service. In addition, the cloud point indicates the limit below which the straight line on the viscosity-temperature chart (where is it) should not be extrapolated, as the separation of wax will lead to higher viscosity than predicted from the chart.
Pour point indicates the lowest temperature at which a lubricant can readily flow from its container. It is only a rough guide as to flow in machines. Pour depressant additives are often used to reduce pour point; however, they do not affect cloud point.
The specific gravity of an oil is the ratio of its weight to that of an equal volume of water, both measured at 60F (16C). The gravity of lubricating oils is of no value in predicting quality, although it gives a clue to the source of the crude-oil base. Values for the specific gravities of crude oils are assigned by the American Petroleum Institute (API).
Low-viscosity oils have higher API gravities than the higher-viscosity oils of the same crude-oil series. Paraffinic oils have the lightest densities or highest API gravities, naphthenic are intermediate, and animal and vegetable oils are the heaviest or lowest in API gravity.
The flash point of an oil is the temperature to which an oil has to be heated until sufficient flammable vapor is driven off to ignite (but not continuously burn) when brought into momentary contact with a flame. The fire point, which is higher than the flash point, is the temperature at which the oil vapors will continue to burn when ignited. The standard method for determining flash and fire points of lubricating oils is by means of open-cup tester.
Flash and fire points may vary with the nature of the original crude oil, the viscosity, and the method of refining. For the same viscosities and degree of refinement, the flash and fire points of paraffinic oils are higher than the flash and fire points of naphthenic oils. Although these values give some indication of fire hazard, they should be taken as only one element in fire risk assessment.
The color of a lubricating oil is obtained by reference to transmitted light; the color by reflected light is referred to as bloom. The color of an oil indicates the uniformity of a particular grade or brand and not its quality.
ASTM D1500 is for the visual determination of color of lubricating oils, heating oils, diesel fuel, and petroleum waxes using a standardized colorimeter. The method compares the samples with glass color standards and reports color in terms of the ASTM Color Scale; it also provides for comparison with the former ASTM Union Color.
The color scale ranges from 0.5 to 8; oils darker than the 8 color may be diluted with kerosene as prescribed by the test method and then observed by the test methods used for the lighter oils. However, very often they are simply described by visual assessment of color (for example: brown and black). For determining the color of petroleum products lighter than 0.5, ASTM D156, Test for Saybolt Color of Petroleum Products, can be used.
Neutralization number or total acid number is a measure of acidic components in oils. This test was originally designed to indicate the degree of refining of new oils, as well as the development of oxidation in service with its accompanying effects on deposit formation and corrosion. However, many modern oils contain additives, which act in these tests in a similar manner to undesirable acids, and are indistinguishable from them. Caution must, therefore, be exercised in accepting results without knowledge of additive behavior.
Total base number is a measure of alkaline components in oils, especially those derived from additives introduced to combat corrosive acids which result from fuel combustion in engines.
Precipitation number is a measure of the suspended solids (carbon or ash residue) contained in the oil. It is determined by taking a predetermined amount of oil, diluting it with naphtha, and centrifuging.
The foam characteristics in crankcase, turbine, or circulating oils are checked by a foaming test apparatus. Certain additive oils tend to foam excessively in service. Only a minute quantity of anti-foam inhibitor is required to break the foam occurring in oil in service.
Lubricating oils may be subjected to relatively high temperatures in the presence of air and catalytically active metals or metallic compounds. The process of oxidation becomes critical when oil is operating above 150F (66C). It is not uncommon to find lubricating oil sump temperatures in excess of 250F (121C). The rate of oxidation approximately doubles for each 18F (10C) rise in temperature of the oil above 150F (66C). The resultant oxidation of the oil develops increased viscosity, acids, sludge, and lacquer.
Many oxidation tests exist. To obtain data in a reasonable time, they generally rely on temperatures elevated above the normal operating range, the presence of catalytic metals in large amounts, and easy access to oxygen.
Because test conditions in these, as well as other procedures, are somewhat removed from those encountered in service, care should be exercised in extra-polating test results to expected performance in service.
Lubricating oils are often expected to protect ferrous surfaces against rusting when modest amounts of water enter the lubricating system. Many oils contain additives that are specifically designed for that purpose. Tests are often applied to steam turbine and similar industrial oils to evaluate this property. Where a greater degree of protection is required, as when a thin film of oil must act as a barrier against a moisture-laden atmosphere, ASTM provides a useful method for evaluation.
In many lubricating systems, it is desirable for the oil to have good water-separating properties. This is to allow removal of water before excessive quantities accumulate and lead to rusting and lubrication problems.
When conditions of lubrication become so severe as to reduce oil film thickness, viscosity assumes less importance, and chemical composition becomes the paramount factor in determining the effectiveness of lubrication. Lubricants that can function under such conditions are said to possess lubricity (or oiliness) and anti-wear properties, which enable friction and wear to be held to reasonable levels.
When the severity of operation increases to a point where unadditized petroleum oils would permit massive destruction of rubbing surfaces to occur in a relatively short period, extreme-pressure (EP) properties are required to obtain acceptable life of the equipment. While there is no sharp demarcation between these regimes, an examination of gears illustrates the different areas. Operating near rated loading, spur gears perform quite well with petroleum oils containing little or no additive.
Worm gears introduce a sliding component that tends to scrape oil from the gear, and their performance is much improved when a fatty oil is added to the petroleum product. Finally, hypoid gears have load concentrated at virtually a single point with consequent very high pressures. These require oils containing reactive compounds, usually of sulfur, phosphorous, or chlorine.
A variety of methods has been devised to evaluate the load-carrying capacity of lubricants under heavy-duty conditions. Each apparatus tends to emphasize a particular characteristic, and a given lubricant will not necessarily show the same extreme-pressure characteristic when tested on different machines. The procedures use the Timken EP Lubricant Tester to characterize industrial-type gear lubricants. The Falex Tester is used for evaluating the load-carrying capacity of oils and gear lubricants.
Lubricating greases are formed when a thickening agent is dispersed in a liquid lubricant. Additional ingredients may be used to impart special properties. Soaps are the most common thickeners. Complex soaps, pigments, modified clays, chemicals (such as polyuria), and polymers are also used, alone or in combination.
Soaps are formed by reaction of animal or vegetable fats or fatty acids with strong alkalies such as calcium, sodium, or lithium hydroxide. In this reaction, water and/or glycerin will be formed as byproducts. Soaps of weak alkalies, such as aluminum, are formed indirectly through further reactions.
The lubricating liquid employed is usually a petroleum oil. For special applications, synthesized hydrocarbons, esters, polyglycols, silicones, fluorinated hydrocarbons, and other materials are available. Additives may be incorporated to provide or enhance tackiness, load-carrying capability, resistance to oxidation and rusting, to decrease wear, and to lessen sensitivity to water. These additives are sometimes solid lubricants such as graphite, molybdenum disulfide, metallic powders, or polymers. The viscosity of the liquid used in a grease should be the same as what would be selected if the lubricant were an oil.
The consistency or firmness of lubricating grease is the characteristic that causes grease to be chosen over oil in some applications. Consistency is determined by the depth to which a cone penetrates into a grease sample under different circumstances. In this test, a standardized cone is allowed to drop into the product for five seconds at 77F (25C).
The resulting depth of fall, or penetration, is measured in tenths of millimeters. If this test has been carried out on a sample that has simply been transferred to a standard container, the test results will be known as unworked penetration. This is not the most used or a standard penetration test.
Greases change in consistency when manipulated or worked. Thus, a worked penetration value is more useful than the unworked value, and has become the standard. Working for the test is carried out at 25C for 60 double strokes in a standard churn-like device. Variations on unworked and worked penetrations, such as prolonged working, undisturbed penetration, or block penetration are also described in ASTM standards.
The laboratory grease worker subjects the grease sample to manipulation at low shear rate. In service, greases operate at a wide range of shear rates, most of them much higher than is found in the grease worker. Thus, the worked penetration and prolonged worked penetrations reported may not resemble what is found in service.
As grease is heated, it may change gradually from a semi-solid to a liquid state, or its structure may weaken until oil is lost. Grease does not exhibit a true melting point, which implies a sharp change in state. A repeatable temperature for defining the behavior of grease when heated can be established by carrying out a carefully controlled heating program under well-defined conditions. This test, giving a temperature, is called the dropping point.
Typical dropping points are given in Table 7. At its dropping point, a grease already has deteriorated; thus, the maximum temperature of application must be significantly lower than the dropping points as shown in the table.
The oil constituent of grease is loosely held. This is necessary for some lubrication requirements, such as those of ball bearings. However, as a result, on opening a container of grease, free oil is often seen. The tendency of grease to bleed oil may also be measured. Test results are related to bleeding in storage, not in service or at elevated temperatures.
Texture of grease is determined by formulation and processing. Typical textures are shown in Table 6 (above). They are useful in identification of some products, but of only limited assistance for predicting behavior in service. In general, smooth, buttery, short-fibered greases are preferred for rolling-contact bearings; and stringy, fibrous products are preferred for sliding service. Stringiness or tackiness imparted by polymetric additives helps control leakage, but this property may be diminished or eliminated by the shearing action encountered in service.
Ferrography is a method of measuring the existing wear particles in lubricating oil. As the name implies, ferrography can separate out the ferrous particles from the non-ferrous particles. This is quite useful in wear particle analysis to determine which machine component is eroding at a rapid rate.
A ferrograph separates out particles having magnetic characteristics. Magnetic separation is nearly one hundred percent effective for ferromagnetic particles larger than 0.1 micrometers. The ferrographic separation technique causes all ferromagnetic debris larger than 5 micrometers to deposit out after entering the magnetic field.
The concentration and size distribution of wear particles reveal considerable information about the condition of lubricated wearing surfaces within a machine. As particle concentration and size increase, the wear process progresses from a normal operating condition, to incipient failure and, finally, to catastrophic failure.
It appears that all abnormal wear modes result in increased levels of wear particle concentration (WPC), and most failure modes result in a trend toward larger particles. However, there are a few well-known exceptions to the progression to larger particle size as failure proceeds.
Corrosive wear is characterized by a prompt increase in WPC while particle size remains quite constant. A similar situation occurs with fine abrasive wear, which is certainly abnormal and can result in premature failure.
Generally benign wear particles are less than 15 micrometers, while most wear particles associated with failure are larger than 15 micrometers. As the wear becomes more catastrophic, wear particles can reach a major dimension of 200 micrometers.
The results of wear particle analysis are no better than the oil sample itself, that is, the lubricant must contain a representative selection of wear particles. This is a cause of concern for sample takers, since in any lubrication system, wear particles and contaminants are usually not evenly distributed.
Particles larger than a few micrometers are especially susceptible to uneven distribution as they tend to be more easily removed from the lubricant through filtration and settling. These particles settle in pipe nipples and valves where, over a period of time, they may become oxidized or otherwise chemically changed. If sample lines and valves are not flushed properly, large numbers of these old particles will find their way into the oil sample. This condition will lead to a sample that yields invalid ferrographic results.
A typical lubricating oil system contains the following components: a machine, drain line, sump, lube oil pump, and a filter from which oil is fed to the machine.
Once a machine reaches particle equilibrium, the increase in particle concentration across the machine is equal to the decrease in particle concentration across the filter. In other words, production rate equals removal rate. Obviously, oil entering the machine has already passed through the filter. The filter is not 100 percent efficient and will allow some particles to pass. When the oil passes through the machine, particle concentration does not increase as drastically as one might assume; however, large particle concentration will rise faster than the small particle concentration since the filter is more efficient at removing large particles.
Confronted with these facts, an operator may assume that sampling at any point in the system is fine as long as you sample from the same point all the time. This assumption is incorrect. Samples should be taken from the drain line, sump, or prior to the filter to ensure the oil has reached equilibrium with particles of all sizes.