In LMT’s January/February 2011 issue, we focused on the importance of certification for lubrication professionals, as well as the programs that offer it. In this issue, we launch a series on the major knowledge that’s required in the pursuit of certification. While these articles are based on the content of STLE- and ICML-suggested training modules, they’re intended only as an informational framework for individuals who wish to become certified. Candidates should plan to engage in substantial additional study to develop the degree of in-depth lubrication knowledge that’s necessary to pass a certification exam. Here, we look at the most important topic: lubrication fundamentals. Centered around tribology, these fundamentals serve as the foundation on which other elements build.
Tribology is the science of interacting surfaces in relative motion. The term comes from the Greek word “tribo” meaning “to rub.” The three major fields of study in this science are friction, wear and lubrication.
Friction is defined as the force resisting the relative motion of two contacting bodies or fluid layers. The surface of a solid body is classified by its roughness—which is called root mean square and expressed as the centerline average of the peaks and valleys, called asperities, in microinches. Fluid friction consists of molecular plates sliding over one another. The resistance to the sliding, which is related to the fluid type and thickness, is called fluid friction.
Solid friction is caused by contact of the asperities resulting in heat generation, welding and wear. This is minimized if a lubricant film prevents asperity contact. Fluid friction that’s not as severe—and which doesn’t cause wear—results from using a fluid that is too thick for the application. This situation means more energy is required to circulate the fluid (a condition related directly to viscosity). When choosing between a viscosity that is too high or too low for an application, always go with the higher one, which will prevent solid-surface contact at the expense of higher fluid friction.
Wear is the rubbing away of metal surfaces due to mechanical action. As Table I shows, there are several wear modes.
Table I. The Most Common Wear Modes
|Abrasive||Three-body abrasive wear is the most common mode. It is caused by clearance-size particles embedded between metal surfaces.|
|Adhesive||Asperity contact resulting from an insufficient lubricant film causes heat, welding and tearing away of metal surfaces. Scuffing, scoring and galling (the most severe form) describe this wear mode.|
|Fatigue||Fatigue is caused by particles bridging a clearance, which leads to stress risers resulting in the generation of micro-cracks that come together and spread, releasing material. Also, in non-conforming surfaces such as roller bearings, high loads can result in surface deformation producing surface cracks that spread and release material. “Spalling” is another name for fatigue wear.|
|Erosive||This wear mode is caused by small high-speed particles in a fluid stream that erode a metering edge or critical surface.|
|Corrosive||In the corrosive mode, an unprotected surface is attacked by either water or an acid fluid to produce rust.|
|Cavitation||A reduction in pressure in a pump fluid results in the production of vapor or dissolved air bubbles that collapse at the high-pressure part of the pump, causing severe surface damage. A closely related effect is air entrainment that enters the system through a leaky fitting under normal pressure. Collapse of these bubbles at high pressure also causes surface damage.|
The final—and largest—component of tribology is lubrication, which is defined as the principle of supporting a sliding load on a friction-reducing film. The film substance, or lubricant, has the following functions:
As shown in Fig. 1, sliding surfaces—such as plain bearings—go through three lubrication regimes: boundary, mixed and hydrodynamic. The following are the major variables relating to coefficient of friction and film (f) thickness:
The film thickness of a lubricant is directly related to viscosity and speed and indirectly related to load. This is known as the duty parameter ZN/P. At low speeds, such as startup of a rotating shaft, the speed is low; therefore the film thickness is thin, which results in metal-to-metal contact. This is a high-wear regime called “boundary lubrication” that usually requires anti-wear additives for protection.
The highest wear in an engine occurs during startup because the lubricant film is not thick enough to prevent contact between the piston rings and cylinder. Engine oils have anti-wear agents—as speed increases, the increasing film thickness offers greater protection. There still will be some metal-to-metal contact, however. At this point, we have entered the “mixed lubrication” regime.
When operating speed is finally reached, and if the correct viscosity has been selected for the load and speed, we enter the “hydrodynamic” regime, where there is no metal-to-metal contact. If load or speed changes, the viscosity needs to be adjusted to the new conditions. If the operating speed of a bearing increases, viscosity needs to be lowered to minimize fluid friction.
There are instances during equipment startup—such as with large turbine rotors in a power plant—when no metal-to-metal contact can be tolerated. In these cases, the only lubrication regime is hydrodynamic (by virtue of the use of lift pumps supplying high-pressure oil that totally separates the shaft and bearing). When running speed has been achieved, the pumps are shut off, and the shaft is supported by the natural hydrodynamic pressure exerted by the oil.
“Elastohydrodynamic” is another lubrication regime—one that occurs between non-conforming surfaces such as ball bearings. Point contact occurs between the ball and raceway, resulting in very high pressures due to the small surface area carrying the load. An extremely thin film of oil (less than one micron in size) is trapped between the two surfaces. The high pressure that’s exerted turns the oil film into a solid and causes the raceway to stretch, thus allowing greater surface area to carry the load. After the ball passes, the raceway returns to its original shape. The deformation of the raceway will eventually fatigue the bearing (if no other failure mode causes the bearing to fail). Few rolling-element bearings make it to the fatigue stage.
A finished lubricant consists of a base stock and additives. (Table II reflects the API base stock classification system.) The right balance of high-quality base stocks blended with the right additives leads to high-quality finished lubricants. Most base stocks are mineral oils derived from crude oil. Lubricant base stocks are produced in oil refineries. A 42-gal. barrel of crude typically produces about 20 gallons of gasoline and about 0.5 gallons of base stock.
Table II. The API Base Stock Classification System
|Base Oil Category||Sulfur||Saturates||Viscosity Index|
|Group I||>0.03% and/or
||<90%||80 to 119|
|Group II||≤0.03% and
||≥90%||80 to 119|
|Group III||≤0.03% and
|Group IV||Polyalphaolefin (PAO)
|Group V||All stocks not included in Groups I to IV (Naphthenic oil and Non-PAO Synthetics)|
Groups I-III are paraffinic base stocks. Group I, made by solvent extraction, is being replaced in both engine and industrial oils by Group II, which reflects hydrocracked base stocks of a higher quality. Group III, the highest-quality base stocks, are used primarily to produce engine oils.
Additives impart special properties to finished lubricants. Table III illustrates some of the most common ones. A correct balancing of additives is critical.
Table III. The Most Common Additives
|Oxidation Inhibitor||Retards formation of sludge and varnish.||Included in all industrial & engine oil formulations.|
|Rust Inhibitor||Absorbs on metal surface to prevent water and acid attack.||Included in most oil
metals with a light
chemical fi lm.
|Included in systems
with yellow metals.
fi lm during boundary
|Primarily used in
hydraulic and engine
oils. Most common
additive is ZDDP.
fi lm due to heat
in heavily loaded
and greases. Usually
|Used in multi-grade
engine oils and
hydraulic oils operating
at low temperatures.
fl owability of oils at
by changing crystal
structure of wax
|Used in engine and
other oils operating at
|Demulsifiers||Promotes rapid separation of oil/water.||Used in turbine and
gear oils primarily.
NEVER used in
tension of air bubbles
to break foam.
|Used in many
air introduction is
|Dispersant||Keeps finely divided
|Used in engine oils.|
|Detergent||Keeps metal surfaces
clean, prevents rust,
and neutralizes acids.
|Used in engine and
characteristics of oil.
|Used in engine oils and
fluids. Common additive
in engine oils is molybdenum
Table IV lists typical additive treat rates.
Table IV. Additive Treat Rates by Application
|Lubricant Type||Additive Treat Rate, %|
|Automatic Transmission Fluid||10-18|
|Automotive Gear Oil||5-7|
|Industrial R&O Oil||0.1-1.0|
|Industrial Gear Oil||1.5-3.0|
Key lubricant properties
Viscosity, the most crucial lubricant property, is defined as resistance to flow—many people think of it as oil thickness. Kinematic viscosity is the most common viscosity system for industrial oils. Time to flow through a capillary tube is measured in seconds and converted to centistokes (cSt). Viscosities are measured at 40 C for industrial oils and 100 C for engine oils.
The ISO VG is a simplified system for classifying viscosity grades. As shown in Table V, an ISO Viscosity Grade is the midpoint of a range that is +/- 10%.
Table V. ISO Viscosity Grades
|Viscosity Grade ISO Standard 3448 ASTM D-2422||Mid-Point Viscosity mm2/s (cSt), @ 40 C||Kinematic Viscosity Limits, mm2/s (cSt), @ 40 C|
|ISO VG 2
|ISO VG 3
|ISO VG 5
|ISO VG 7
|ISO VG 10
|ISO VG 15
|ISO VG 22
|ISO VG 32
|ISO VG 46
|ISO VG 68
|ISO VG 100
|ISO VG 150
|ISO VG 220
|ISO VG 320
|ISO VG 460
|ISO VG 680
|ISO VG 1000
|ISO VG 1500
Viscosity Index (VI), another critical lubricant property, is defined as an arbitrary measure for the change of kinematic viscosity with temperature. Higher-quality paraffinic mineral oils have naturally high viscosity indexes before the addition of VI improvers. Group III has the highest viscosity index followed by Group II and Group I. This is especially important with multigrade engine oils that have to be light enough to flow at low temperatures and thick enough to protect at high engine temperatures. This is achieved by the use of VI improvers.
The Viscosity Index can be determined by plotting a lubricant’s viscosity versus temperature (at 40 C and 100 C). A line with a lower slope would have a higher VI.
High VI and multigrade oils have the advantage of flowing better at low temperatures and giving better protection with a higher viscosity at higher temperatures.
Synthetics (as listed by major type and applications in Table VI) result from chemical reactions of pure components that produce materials of a larger molecular weight. Containing no wax or sulfur, as a composite group, they have the following advantages. (Note: Not all synthetics offer these advantages.)
Table VI. Major Applications of Synthetic Lubricants
|Polyalphaolefin (PAO)||Most versatile and most used synthetic. Used in gearboxes, screw compressors, oil mist, blowers, fans, motors and automotive.|
|Diesters||Primary use in high-temperature reciprocating compressors. Also used in oil-mist and non-hydrocarbon flooded screw compressors.|
|Polyalkylene Glycol (PAG)||Primary use of this versatile synthetic is in worm gears, compressor cylinder lubrication, flooded hydrocarbon gas screw compressors, rotary screw air compressors and food grade applications.|
|Polyol Esters||Used in land-based aviation gas turbines, air and refrigeration gas compressors and fire-resistant and biodegradable hydraulic fluids.|
|Alkylated Aromatics||Used in refrigeration compressors.|
|Phosphate Esters||Used as fire-resistant hydraulic fluid for steam turbines.|
Grease is a solid to semisolid product resulting from dispersion of a thickening agent in a liquid lubricant—some people use the analogy of a sponge. It consists of a network of thickener pores filled with oil that’s released through heat motion, mechanical agitation and other forces. The oil is reabsorbed into the thickener when the forces are removed. Figure 3 shows grease components.
Primarily soap-based, grease thickeners are produced through a saponification reaction between a fatty acid and a metallic hydroxide. The most common fatty acid is 12 hydroxy stearic acid and the most common base is lithium hydroxide. As shown in Fig. 4, there also are non-soap thickeners such as polyurea and clay.
A straight soap thickener results from the reaction of one high-molecular fatty acid and a base. A mixed soap (not very common) is formed by reacting one fatty acid with two bases. A complex soap consists of one fatty acid and a short chain Di-acid reacting with a base—this grease type is becoming the most popular due to its high-temperature properties. The most common thickener base is lithium: In 2008, it accounted for 66% of thickeners in North America. (Lithium complex, at 36%, was the most used.)
As shown in Table VII, greases are classified by the NLGI (National Lubricating Grease Institute) according to consistency.
The consistency of grease is determined by placing a funnel called a penetrometer on a smooth cup of grease at 77 F and, after five seconds, measuring its penetration in tenths of a millimeter. The greater the penetration, the softer the grease and the lower the NLGI Grade number. Most greases used today are NLGI 1, 2 or 3—the most common being NLGI 2. High-penetration greases (such as 00 and 0) are used in centralized lubrication systems in cold temperatures.
As a general rule, try to use oil—wherever possible—because it can be cooled and cleaned. There are many applications, though, where oil cannot be used. The following list reflects situations where grease should be used:
The oil in the grease does the lubricating, thus using a high-quality base stock with the right additives is crucial for good performance. Most greases are formulated with paraffinic base stocks; some high-performance products use synthetics like PAOs, diesters and PAGs. Since viscosity is the most important property, manufacturers select the right viscosity for the application (i.e., high-speed motor greases have a viscosity around 100 cSt).
Greases must have a number of properties to be effective—properties that are required based on the application. Table VIII describes some of them.
Table VIII. Key Grease Properties Based on Application
|Consistency||NLGI grade, which is based on amount of thickener, describes the stiffness of the grease. NLGI 2 is the most common grade.|
|Dropping||This is the temperature of grease where the first drop of liquid separates from the thickener in a perforated cup (or the point when the thickener breaks down and melts). Grease should be operated at temperatures no higher than 100-150 F below the dropping point. Complex soaps and polyureas have dropping points around 500 F.|
||Water washout test measures ability of a thickener to remain intact in a bearing when submerged in water. Water spray-off measures ability of a thickener to remain in a bearing in the presence of water spray. Both of these tests measure the percent of grease removed.|
|Base Oil Viscosity
||Since oil does the lubricating in a grease, and viscosity is the most important property of the lubricant, the viscosity of the base oil needs to be designed correctly for the application.|
|Load Carrying Ability
||Under high-load conditions, high-viscosity base stock is required and usually with EP additive or solid additive like molybdenum disulfide.|
|Shear Stability||Grease needs to maintain its consistency under high shear conditions. This test measures the softening of grease when sheared for 10,000 or 100,000 double strokes with a grease worker. Loss of less than one NLGI grease grade signifies a stable thickener under high shear conditions.|
|Compatibility||Compatibility is one of the most important grease properties. When two incompatible thickeners are mixed, grease usually becomes soft and runs out of the bearing. When mixing different thickener types, consult the supplier regarding compatibility. Some incompatible thickeners are aluminum and barium soaps, clay and some polyureas.|
|Pumpability||Pumpability is important when pumping grease in centralized systems at low temperatures. The most common test is Lincoln Ventmeter.|
|Oil Separation||For a grease to be effective, a small amount of oil must separate from the thickener (usually less than 3%).|
Ray’s advice to you
Having a firm grasp (and a full understanding) of these fundamentals is one of the keys to success on lubrications certification exams. This section is not only the largest—it’s the most important. For more information on the topics covered in this overview, please refer to previous articles I’ve authored over the years for LMT. You can also obtain study materials from the certifying organization(s) of your choice at www.stle.org and www.lubecouncil.org.
Coming in the May/June issue
The next article in this ongoing certification series will focus on bearings and their lubrication. LMT