The Inner Life of Bearings, Part 1: How Lubrication Really Works
EP Editorial Staff | February 18, 2015
What some personnel don’t know can hurt your equipment and processes. Expert advice bears repeating.
By Neville Sachs, P.E.
As a facility considers implementation of a sophisticated lubrication program, it’s not uncommon for someone to strongly insist that “oil’s oil,” and that “all our applications can be handled by one multi-purpose grease.” The numbers of mineral-based and synthetic lubricants in vendor catalogs run counter to those arguments. Manufacturers commonly list over 40 greases and lubricating oils, available in at least 10 viscosity ranges. Categories include aviation oils, automotive and light truck engine oils, gear oils, compressor oils, heavy-duty engine oils, gas engine oils, turbine oils and way oils, to name but a few.
Accounts of someone’s brother-in-law or friend who “never changed the oil in his truck,” or “used ATF (automatic transmission fluid) in his car engine,” may be more urban legend than truth—and they don’t reflect lasting solutions: Some oils will temporarily work in an incorrect application, but they won’t provide long, reliable service. Unproven theories and/or ill-informed theorists should carry no weight in a facility’s approach to lubrication, but often do.
Overcoming the harmful impact misinformation and flawed thinking can have in today’s industrial operations calls for continuous emphasis on correct information. This two-part article recaps lubrication fundamentals that have been covered in these pages before. But when it comes to the bearings in your plant’s critical equipment systems—and the ever-changing workforce that may be maintaining them—regular reinforcement of these principles is crucial.
Back to the basics
Friction, lubrication and wear (i.e., “tribology”) constitute a complex body of knowledge that involves, among other things, three basic types of bearings with very different “wear-prevention” mechanisms and critical point-of-contact temperatures.
For lubrication to be effective, a bearing’s mating pieces must be separated. In conventional plain bearings and rolling element designs, this separation depends on the lubricant’s viscosity. The success of a sliding application is governed by the lubricant’s additive package.
One of the most important aspects of lubrication is relative lubricant film thickness. Figure 1 illustrates this film thickness by depicting two pieces of metal as viewed through a microscope. Note that these pieces are not perfectly flat: R1 and R2 refer to their average roughness measurements. Between the two pieces, h is a measure of the separation resulting from the lubricant. Represented by the symbol λ, relative film thickness is calculated as:
λ = h/(R12 + R22)1/2
Within reason, the greater the λ value, the lower the wear rate.
Another important principle of lubrication can be seen in the Stribeck Curve in Fig. 2. Developed in 1902 by the German engineer and scientist Richard Stribeck, it shows how the coefficient of plain-bearing friction varies with surface speed and lubricant viscosity. Referring to the diagram, we can see that when a lubricant is supplied and the surface speed between two properly designed parts increases, the friction first rapidly drops off, then slowly increases. This curve is also helpful in that it shows the three lubrication zones—which basically equate to the three most common bearing types. Low-speed plain bearings and sliding applications fall into the boundary-friction zone; ball and roller bearings into the mixed-film (elastohydrodynamic) zone; and high-speed plain bearings into the full-film zone.
The plot in Fig. 3 uses somewhat different terminology than the Stribeck Curve for the three lubrication zones. It also shows the effect of relative film thickness on wear rates. Hydrodynamic lubrication typically is seen in plain bearings, i.e., in automobile engines and large turbines and generators. Elastohydrodynamic refers to the lubrication mechanisms seen in higher-speed rolling element bearings. Sliding (boundary-friction) lubrication occurs in applications like piston rings, wire ropes and slow-speed rolling element bearings.
How different bearing types operate
Lubrication occurs in the three categories of bearings by way of very different mechanisms. The diagram of a hydrodynamically lubricated plain bearing in Fig. 4 shows a journal that rotates inside the bearing. (The bearing can be made from any one of many materials, which will be discussed in Part 2 of this article.) Preferably, oil is fed into the gap at the unloaded area of the bearing, whereupon it is swept around the journal. In the process, the oil viscosity develops a wedge that separates the two pieces. The typical film thickness is in the order of 0.01 to 0.05mm (0.0004” to 0.002”). While this type of bearing can withstand tremendous pressures, as the load on it increases, internal shearing of the oil film increases the lubricant temperature, the viscosity drops and leakage increases.
Designing a hydrodynamically lubricated bearing primarily involves understanding operating temperatures and viscosities and the need to create a system that delivers more oil than can readily leak out from the edges of the bearing. Misalignment and excessive clearance will greatly reduce the bearing’s life. (As shown in Photo 1, the uneven wear pattern on a pair of gas-engine main bearing inserts contributed to their rapid degradation.)
As can be seen in Fig. 5, rolling element bearings, ball and roller bearings, have vastly different lubrication mechanisms.
In the operation of a ball or roller bearing, as the element rolls along and traps that easy-flowing oil, viscosity changes significantly (increasing by a factor of 10,000 or more and becoming stiff enough to actually separate the rolling element from the ring). As this occurs, the mating areas of the element and ring flatten elastically to distribute the load across the film and support continued operation. While the lubricant film separation isn’t great (less than a micron [≈0.00004”]) and the pressure is tremendous (typically more than 2GPa [150,000 psi]), the overall effect is substantial: Contact forces are distributed over a much greater area, fatigue stresses are reduced and bearing life is increased.
Two important factors in this process are lubricant temperature—i.e., the lower the viscosity the thinner the film—and lubricant cleanliness: Because the lubricant film is so thin and the pressures so high, solid particles and water have huge effects on component lives. (Photo 2 shows the inner ring of a spherical roller bearing and the fine-grained spalling that results from inadequate lubrication.)
With the third lubrication mechanism, i.e., in sliding bearings, additives are more critical than oil viscosity. Some additives, such as oxidation inhibitors, are designed to improve oil life. Others, such as anti-wear and high-pressure (EP) additives, are designed to improve oil performance. (The dark bands alongside the ball paths shown in Photo 3 are oxidized oil deposits.)
Although selection of the correct additive package is important for the lubrication mechanisms shown in Figures 4 and 5, with sliding applications (Fig. 6), the correct additive combination is the key to low wear rates and long component life.
The diagram of contacting metal pieces shown in Fig. 6 could represent piston rings or, alternately, rolling element bearings operating at a speed too low to generate a viscosity conversion. To reduce wear rates in these components, two general types of additives are used: anti-wear (AW) and, as they are known in North America, extreme pressure (EP). (Note: In the rest of the world, extreme pressure additives are characterized as “high pressure.”)
Anti-wear additives are almost always polar molecules—meaning they are compounds that have a positive charge on one end and a negative charge on the other. Because of their polar nature, they are attracted to the metals. An example of this is oleic acid, a fatty acid where one end of the molecule is attracted to the metal and the other end is repelled. With relatively low pressures and low contact temperatures below 100 C, these additives provide a cushion between the two sliding pieces. But at higher-point contact temperatures (and higher pressures), they lose strength and EP additives are needed.
There are two general types of EP additives: liquids and solids. Liquid additives in EP oils are generally compounds of sulfur and phosphorus, and sometimes chlorine, that, when heated, form hard semi-metallic coatings that provide the actual wear resistance. Solid additives found in greases commonly include molybdenum disulfide, graphite and other materials designed to slide between opposing metal parts to provide wear resistance. The proportion of solids varies with individual manufacturers. When using EP lubricants, keep these points in mind:
- When water is present, some additives will form extremely corrosive chemicals.
- Solid EP additives tend to disrupt the viscosity transformation that’s critical to higher-speed ball and roller bearing lubrication. (However, if those ball and/or roller bearings are in a gearbox, using EP additives to help preserve the gears is usually much more important than the life of bearings that can be easily monitored and replaced.)
Part 2 of this article will focus on oil and grease selection for an application; why speeds and temperatures are important; and why operating environments are critical in determining lubrication frequency.
Neville Sachs has extensive experience in machinery reliability and lubrication. The author of two books on failure analysis and a contributor of sections to other books, he has also written more than 40 articles. A Professional Engineer, Sachs holds STLE’s CLS certification, among others. Contact him at firstname.lastname@example.org.