Certification Matters, Part I: The Fundamentals Of Lubrication

The foundation on which those who seek certification must begin building their lube bona fides starts here, with the basics of the craft.

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…
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…
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

Lubrication…
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:   

• To reduce friction and wear
• To reduce heat through circulation and cooling
• To avert rust by keeping air/moisture from contacting metal
• To remove contaminants by circulating fluid to filters or allowing settling in reservoir
• To seal out contaminants (a major function of grease)

Lubrication regimes
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.

Lubricant composition
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

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

Table IV lists typical additive treat rates.

Table IV. Additive Treat Rates by Application

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 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.

Synthetic lubricants
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.)

• Fire resistance
• Oxidation stability
• Thermal stability
• High Viscosity Index
• Enhanced lubricity
• High flashpoints
• Low pour points
• Good demulsibility and anti-foaming characteristics
• Natural detergency
• Wide operating temperature range

Table VI. Major Applications of Synthetic Lubricants

Grease lubricants
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:

• In applications where leakage and drippage are present
• In hard-to-reach places where lubricant circulation is impractical
• When sealing in a high-contaminant environment (i.e., water and particle contamination)
• When protecting metal surfaces from rust and corrosion
• In the lubrication of intermittently operated machines
• Where solid additives (such as moly) are suspended during slow-speed, high-load sliding conditions
• In the lubrication of sealed-for-life equipment like electric motors
• When lubricating under extreme or special operating conditions
• In the lubrication of badly worn machines
• When lubricating equipment where noise-reduction is important

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

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

Contributing Editor Ray Thibault is based in Cypress (Houston), TX. An STLE-Certified Lubrication Specialist and Oil Monitoring Analyst, he conducts extensive training in a number of industries. Telephone: (281) 257-1526; e-mail: rlthibault@msn.com.