In the 1970s, MIT Mechanical Engineering Professor Emeritus Ernest Rabinowicz, a true pioneer in the field of tribology, discovered that 70% of bearings lost their usefulness through surface degradation. Fifty percent of that degradation was due to mechanical wear; 20% was due to corrosion. Calculating the impact of this problem, Dr. Rabinowicz noted that 6% of the U.S. Gross Domestic Product (GDP) was lost annually through mechanical wear (an assertion that became known as the "Rabinowicz Law"). Interestingly, lubricant films are one of the first lines of defense in the war against such losses, which, today, equate to trillions of dollars.
The cause of surface degradation is both directly and indirectly attributed to ineffective lubrication practices that include under- and over-application of lubricant, incorrect lubricant choice (viscosity and additive package), particle contamination, moisture contamination and neglect. By placing the right lubricant, in the right place, in the right amount, at the right time, with the right level of cleanliness, surface degradation can be minimized to acceptable and often negligible levels. Here, we investigate how the amount of lubricant separating the two surfaces—i.e., the lubricating film—along with the correct lubricant choice affects and controls interacting surface degradation and resulting wear.
Lubricant film regimes
There are five lubricant film regimes, each describing a different relationship between two interacting surfaces as they slide over one another.
1. HDL (Hydrodynamic Lubrication) is often described as “full film” lubrication, wherein the moving surfaces are totally separated by the lubricant. In the Element 1 installment of this series (LMT, Jan./Feb. 2013), we examined the role friction plays and recognized that even a relatively smooth surface appears rough under a microscope, similar in cross-section to a range of craggy mountain peaks and valleys. Before any separation can take place, the lubricant must fill these valleys and up past the highest peaks, so there is no surface-to-surface interaction between the two moving surfaces. In sliding-friction bearings, this is the most desirable lubrication state. Any friction that's present is due to the fluid friction of the lubricant.
2. HSL (Hydrostatic Lubrication)is a state that occurs when a lubricant is used to hydraulically separate a loaded surface and “float” one surface over another. This film regime is typically set up on precision machines and machine tools like plunge grinders where a grindstone carriage is “floated” into the work piece to perform precision grinding on gears, etc. HSL is similar to HDL in that it also provides full film separation.
3. MF (Mixed Film Lubrication) is classified as an Intermediate regime: Lubricant is present between two sliding surfaces, but in not enough quantity to fully separate them, thus allowing intermittent contact between the highest surface points. This is also known as an “unstable” regime, generally caused by insufficient lubricant, heavy loads at rest or use of a lubricant with too low of a viscosity.
4. BL (Boundary Layer Lubrication) is the least-desirable regime with the highest coefficient of friction. Although a minimal amount of lubricant is present, the sliding surfaces are in full contact with one another at rest. With heavy-load, slow-moving machinery, boundary-layer to mixed-film regime may be the best condition achievable (requiring a lubricant with EP [extreme pressure] and
AW [anti wear] additives) to offset the extreme bearing-surface working condition. If insufficient lubricant is present or an incorrect viscosity is used, a normally loaded bearing can stay in a boundary layer state when in full motion in which the surfaces will interfere with one another and cause rapid wear.
5. EHDL (Elastohydrodynamic Lubrication) is unique to rolling-friction surfaces experienced in ball and roller-style bearings and in combination-friction circumstances such as when two gear teeth mate in a sliding and rolling action. When a ball is rolling in a race and comes under full load, the mating surfaces will momentarily deform, trapping the lubricant in the deformed area known as the Hertzian contact area. Under deformation pressure, the lubricant viscosity rapidly rises and the lubricant changes from a liquid to a solid, providing full protection to the rolling surfaces. As the ball moves out of the load zone, the lubricant returns back to its original viscosity. Because rolling surface contact is in a line and not over the entire surface area, a lot less lubricant is required to achieve full film lubrication.
The Stribeck Curve
In 1902, Professor Richard Stribeck was the first to graphically describe how the coefficient of friction changes for bearings experiencing different lubrication regimes. A typical example of this is found in normally loaded sliding-friction bearings such as those found in a shaft and sleeve-bearing setup. At rest, the bearing surfaces will be in a boundary layer or mixed film state prior to start-up or shut-down. As the shaft ramps up speed, it will begin to centrifugally center and move through a mixed-film regime to a full film HDL regime at operating speed. The Stribeck Curve diagram (Fig. 1) shows the typical change in lubrication regime as the shaft/bearing speed increases. Change in regime state is also dependent on load, speed and viscosity.
Fig. 1. The Stribeck Curve shows the typical change in lubrication regime as the shaft/bearing speed increases.
The Stribeck Curve clearly demonstrates that a hydrodynamic film regime—of the correct viscosity and Lambda thickness—results in the lowest coefficient of friction and least wear.
Common wear mechanisms
There are four common wear mechanisms that cause surface degradation and eventual loss of usefulness in bearing surfaces.
1.Abrasive wear occurs when bearing surfaces run in a MF or BL regime. Abrasive wear can occur as a result of a 2-body or 3-body surface interaction. In the 2-body example shown in Fig. 2, we see two surface points that touch and cut into the opposing sliding surface, resulting in a scratched, grooved or furrowed surface and a third-body metal cutting being released into the lubricant.
As reflected in the 3-body diagram, that third body is now free to get caught between two surface points and add to the surface degradation. A 3-body abrasion can also occur due to large particles (dirt) in the lubricant (introduced by the lubrication process or machine operation). Erosive wear is a form of abrasive wear caused by particles impacting a surface.
2.Adhesive wear typically occurs under highly loaded sliding friction, when an incorrect viscosity lubricant is used, EP and AW additives have been depleted or under heavy shock loading. As the surfaces come together, they weld under the heat and load pressure, and metal is transferred and torn apart under movement, leaving discreet, often jagged or smeared, surfaces. Adhesive wear is also described as scuffing, shearing or galling wear.
3.Fatigue wear usually occurs in rolling friction surfaces that have experienced repeated long-term load cycles and stress that causes elastic deformation to the surfaces, as in EHD lubrication. This long-term action eventually results in small surface and sub surface cracking, which eventually travels up to and across the bearing surface, resulting in sur-face delamination and pitting.
4.Corrosive wear leaves an acid-etched surface in the bearing-contact area due to an oxidative chemical reaction that is accelerated in the presence of moisture contamination and heat. This corrosive wear—also known as acidic pitting—is caused when a lubricant becomes moisture-contaminated or additive-depleted, or by one that contains no corrosion-inhibitor additive. LMT
Today, there are two main certifying programs for lubrication professionals: STLE (Society of Tribologists and Lubrication Engineers); and ICML (International Council for Machinery Lubrication). Originally designed for engineers, STLE's Certified Lubrication Specialist (CLS) program has been offered since 1993. ICML’s certification program was the basis for and is in accordance with ISO’s 18436 standard series, as it relates to lubricant-based condition monitoring professionals. ICML offers two certification paths for “hands-on” lubrication practitioners: MLT (Machine Lubrication Technician) and MLA (Machine Lubrication Analyst) designations. ICML also separates field functions from laboratory functions, with lab practitioners served by the LLA (Laboratory Lubricant Analyst) designation.
ICML programs have been offered since 2001. In 2008, its MLT and MLA certifications were pioneered into ISO 18436-4 for field practitioners. In 2012, its LLA certification became the basis of ISO 18436-5, for lab-based practitioners. Participants who attend the requisite formal preparatory training associated with ICML certification are also eligible to take exams as per the corresponding ISO standard (upon payment of the appropriate examination fee).
Of these all these programs, ICML’s (currently offered in 10 languages) has issued the most certificates around the world (over 9500 to date, with presence in more than 95 countries). One of the most recent ICML certification preparatory-training and exam opportunities in the U.S. was offered at the 2013 Maintenance & Reliability Technology Summit (MARTS) conference, in Chicago (Rosemont), IL. For more information on ICML certification, please visit: www.lubecouncil.org.