The fine art of bearings lubrication
Lynne Peskoe-Yang, Contributing Editor | TLT Lubrication Fundamentals April 2020
Knowing which lubricant to use for the right application can maximize your equipment's service life.
To protect bearings in operating environments, end-users have to factor how lubricants function in various conditions controlled by temperature and speed.
From the driver’s perspective, choosing the right gear lubricant is a simple matter of reading the manual. The pairing of parts with their ideal lubricant formulations is an integral aspect of engine design, typically covered on the supply side. For most applications, engineers have already determined what kind of lubrication will result in the longest service life for the lubricated parts.
But an engine manual can only provide general guidelines. Choosing lubricants for engine parts is its own branch of science. The possibilities for bearings lubrication are as diverse as engines themselves, with variations that account for everything from the relative positioning of the bearings to the seal type to the feasibility of relubrication. Operating conditions such as extreme weather and heavy loads introduce another layer of complexity to the question.
Technical experts rely on their strong grasp of the fundamentals of lubrication science and fluid dynamics to solve this problem in their work. “There are three functions of lubricant, reducing friction between contacting surfaces, minimizing wear, and removing heat and contaminate particles from the contact zone,” says STLE-member Frank Uherek, principal engineer at Rexnord Corp., in Milwaukee, Wis. “For slow-speed applications, the driving functions are friction reduction and wear management.” A precise match between lubricant and application can be the difference between a long, healthy service life and engine failure.
Oil or grease?
According to Kyle Smith, principal application engineer at The Timken Co., in North Canton, Ohio, lubrication experts begin the selection process with a classic tribology dilemma: oil or grease? “How clean or dirty, how hot or cold, how wet or dry the surrounding environment is will drive selection of lubrication method,” Smith explains.
The structure and materials of the bearings themselves are the first variable to consider— bearings, gears, clutches, and other small components all affect how well the lubrication will work. Maintenance accessibility is another early determinant; technicians choosing lubrication should think ahead in the service life of a product in order to anticipate its needs. These key factors, Smith says, “need to be identified early on in the selection process to begin building the case for using grease or oil lubrication.”
Delivery methods and relubrication
The question of grease versus oil comes first, in part, because it determines many of the other circumstances that will affect bearings lubrication. For grease, which tends to stay in place over time despite operational wear, the initial application is key. Equipment seals, bearing type and size, and the way the engine will eventually operate can all make reapplication and filtration easy, challenging, or structurally impossible; in the latter case, lubricants with a long service life make more sense than products that must be filtered or replaced at service intervals.
Lubricating with oil is a simpler endeavor, thanks to its comparative flexibility and ease of movement within the hardware. Oil systems vary widely, but many are equipped to periodically replace themselves. Flingers, dams, and splash systems can help circulate the oil regularly, extending the life of the bearings without direct maintenance.
The ideal time interval between lubrication services depends on how effectively the system can regulate itself, as well as the service life of each component, including the oil. A bearing application, with controlled sump temperatures and advanced filtration systems, could theoretically control its own oil flow indefinitely, according to Smith. On the other hand, an application with “only its own grease fill, operating in a dirty environment with extreme fluctuations in ambient temperatures, and with basic labyrinth seals, may require daily relubrication intervals.”
To protect bearings in unusual operating environments, experts consider how lubricants respond to the combined conditions—and to the full range of their fluctuations. Low-speed applications, for example, tend to be temperature-agnostic, while, “as speeds increase, thermal considerations become more important,” Uherek explains. Increased component temperatures—due to friction—can lower oil viscosity, further reducing the effectiveness of the lubrication. Engine designers who anticipate those fluctuations typically “move away from dip and splash lubrication and into a pumped application of oil into the bearing or gear mesh.”
Oil mist systems are another example of a highly specific lubrication method suitable to a small subset of applications. These internal sprays are used to relubricate the bearings “when the lubricant supply is limited and cooling of components are critical for efficiency considerations,” says Uherek. Mist systems, thus, work best where ambient conditions are carefully controlled, away from extreme fluctuations in temperature and speed.
To deal with extreme fluctuations, Smith says he turns to the industry standards. A product’s current NLGI grade, for example, can help technicians predict how the changing temperatures may affect the grease’s distribution on the bearing during replenishment and operation. Other possible solutions include multigrade oil formulations, which let the system adapt to changing viscosity needs at various temperatures, as well as methods for pre-warming and pre-cooling the lubrication and bearing system before operation.
Problems with bearings lubrication
Properly lubricated bearings can add years of service life to an engine, while the wrong lubricant in the wrong application can summon disaster. The complexity of the bearings lubrication challenge comes down to the enormous array of variables that factor into each match. Selecting bearing lubricant, thus, requires real expertise, whether from a live technician or from standards and guidelines developed by teams of researchers. More advanced adaptations, such as the introduction of lubricant additives, are best left to producers, Uherek warns. He says tampering with a finished product can have unpredictable and harmful results and should not be attempted by anyone other than the lubricant supplier.
Even for experts, such a complex problem doesn’t necessarily come with a right answer, Smith confesses. “Each individual set of operating conditions has a different effect on the lubrication, bearings, and adjacent components, making each selection somewhat customized. For these reasons, the selection of a lubrication solution might be better considered as optimized rather than defined as correct,” he says.
Lynne Peskoe-Yang is a freelance writer based in Tarrytown, N.Y. You can contact her at firstname.lastname@example.org.
FROM TLT ARCHIVES
Want more on bearings? Check out these TLT Archives articles at www.stle.org.
1. McGuire, N. (2019), “Designing Better Bearings,” TLT, 75 (4), Available here.
2. Canter, N. (2012), “Key Trends in Bearing Lubrication,” TLT, 68 (4). Available here.
3. Gresham, R.M. (2012), “Bearings and their Lubrication,” TLT, 68 (10). Available here.