Innovative experimental studies of lubricating greases are furthering the understanding of grease performance inside bearings and providing formulation guidance for meeting future needs of bearing manufacturers.
Piet Lugt, senior scientist of lubrication and tribology with bearing manufacturer SKF, summarized lubrication targets of the bearing industry for attendees at the Society of Tribologists and Lubrication Engineers national meeting last spring. Lugt, who is based at SKFs Engineering and Research Centre in Nieuwegein, the Netherlands, explained that advances in bearing technology are driving the need for new formulations, as well as specifications for higher-performance greases. Bearing OEMs main goals, he said, are to broaden operating ranges for temperature, speed and load; increase service life of both greases and bearings; and extend relubrication intervals.
Grease performance limits critical aspects of bearing performance. Lugt explained that service lifetime, measured in hours, can be one to two orders of magnitude shorter for greases than bearings over a range of applied loads.
In a freshly greased rolling bearing, grease first distributes itself and forms reservoirs-a process called churning-then bleeds lubricant into the contacts. When the reservoirs are exhausted, lubrication films break down and bearings seize. Achieving longer service life for greases entails improving bleed rates, reducing or preventing grease starvation at contacts, providing protective surface chemistries, and optimizing mechanical stability, Lugt said.
Research to improve these subtle aspects of bearing lubrication demands advanced experimental methods to measure grease performance and properties in bearing contacts. Three speakers presented their experimental approaches to this challenge at the STLE meeting in Las Vegas.
Tracking Migration
Mark Franken, also based at SKFs facility in Nieuwegein, pointed out the need to measure grease movement and understand the behavior of reservoirs in order to improve the design of both greases and bearings.
To assess grease migration, Franken observed changes in the distribution of dyes inside high-performance angular bearings. Angular ball bearings support axial and radial loads in applications with demanding requirements for guidance, speed and accuracy, such as spindles in machine tools. Spindle bearings feature offset raceways where the inner and outer raceways are displaced relative to one another along the bearing axis. Spindles entail high speed, long service life and relubrication intervals, and low energy consumption.
Franken used fluorescence spectroscopy to observe pairs of greased ball bearings mounted back-to-back on a spindle in a test apparatus. In a typical fluorescence spectroscopy measurement, a narrow frequency band of ultraviolet light is applied to excite molecules in a sample. Certain chemicals absorb the incident radiation, then flouresce, emitting radiation at lower frequency than the incident radiation. The strength of the emitted radiation is directly proportional to the concentration of the fluorescing chemical.
Franken selected two dyes-chemicals that fluoresce or emit visible light-one red and one blue. He dissolved each dye in a blend of ester and polyalphaolefin base oils, then prepared SKF GMM high-speed lithium grease (NLGI grade 2 consistency). At less than 0.1 percent dye concentration, tests showed insignificant differences in the rheology of greases with and without the dyes.
Next, Franken loaded undyed grease to 10 percent of the free volume in SKF 7008 CE/HEP4A bearings, which contain 19 7-millimeter ceramic balls with a mean bearing diameter of 54 mm. He applied a 310 Newton load and used a standard procedure to run in pairs of bearings, gradually increasing the speed from 3,000 to 37,000 rpm over several hours to churn and form grease reservoirs. After the bearings cooled to ambient temperature, Franken injected small quantities of dyed grease at three spots 120 degrees apart around the bearing axis. Blue grease was injected on the front of the bearings and red grease on the back.
The bearings were run at 37,000 rpm, some for 10 hours and some for 50 hours. After each test, fluorescence spectroscopy was used to measure the dye distributions on the fronts and backs of the bearings. Peak emissions heights of around 500 and 700 nanometers corresponded to concentrations of blue and red base oils from dyed greases, respectively.
Spots of blue oil spread to form rings around the spindle on bearing fronts, and also migrated to form rings on the backs of bearings. In contrast, spots of red oil were localized on the backs of bearings, except for slight smearing from assembly.
Franken concluded that grease reservoirs on seals and raceway shoulders of bearing fronts were active and bled oil. Conversely, grease reservoirs on bearing backs were inactive and did not bleed. Results were consistent for tests that lasted 10 and 50 hours.
These findings provide an improved understanding of lubricant flow in angular contact ball bearings running at high speed, and a guideline for improvements to increase service life of greased spindle bearings in a horizontal configuration.
Thickening Film
Febin Cyriac of the University of Twente in Enschede, the Netherlands, found that grease thickeners were present in contacts and enhanced lubricating film thickness, and that smaller thickener particles provided a more efficient way to form a thicker film.
Cyriac compared six greases formulated with a variety of thickeners and base oils. He measured film thickness in a rolling, elastohydrodynamically lubricated contact between a steel ball and a glass disc at medium speeds using optical interferometry. Greases formed lubricating films 3 to 40 percent thicker than films formed by base oils from these greases, he told the gathering in Las Vegas. His observations implied that the thickeners were present in contacts and enhanced film thickness, which was consistent with results from other published studies.
Atomic force microscopy, used to measure thickener particle volumes, revealed a direct, linear relationship between the fraction of thickener particles by volume and the percent enhancement of grease film thickness relative to base oil film. The increase in film thickness due to entrainment of thickener was proportional to the amount of thickener in the grease by volume, and was inversely proportional to thickener particle size. That is, smaller particles such as those in polyurea thickener provided a more efficient means to increase film thickness in this study.
Following Flow
Lars Westerberg of Lulea University of Technology in Sweden, reminded the audience that bearings are only partially loaded with grease. Free-surface effects at grease-to-air interfaces make important contributions to the lubrication of rolling bearings and open gears.
After the churning phase, centrifugal forces in bearings cause free surfaces of grease to flow and deform continuously, similar to waves on an ocean surface. These forces push much of the grease away from the bearing axis, leaving relatively thin layers in contacts. This flowing behavior influences friction torque and energy losses as well as lubrication.
Westerberg, who works in the Department of Engineering Sciences and Mathematics, devised a simple experiment to observe free surface flows of grease in the presence of centrifugal forces. He found the influence of temperature on flow was much more significant than the effect of surface roughness. He also observed that an oil-rich layer formed by oil separation from grease had much lower viscosity than the grease layer. This created a wall-slip effect, in which movement occurs at the grease-metal interface because cohesion within the grease is stronger than adhesion to the metal surface.
Westerbergs study focused on observing the effects of surface roughness and temperature on grease flow and determining whether base oil or thickener dominated the sliding of greases on steel. A lithium-thickened NLGI grade 2 grease and typical bearing steel were used.
Small cylindrical grease samples or pucks (5 mm diameter, 1 to 2 mm thick) were placed on metal squares mounted on a horizontal disc in a test apparatus. To study temperature effects, the disc was preheated for 10 to 20 minutes before starting rotation. The speed of rotation was increased gradually until the grease started to slide and spread. A high-speed camera was used to observe the movement of the grease pucks. Grease loss was measured from the mass change of each metal square.
For steels with average roughness of 10 nm (smooth) and 1.2 m (rough), critical speeds for grease sliding were comparable. However, sliding was faster for short pucks (1 mm high, 4.4 meters per second) than for tall pucks (2 mm high, 2.8 m/s). From these data, as well as the shapes of grease tracks and blobs on the steel, Westerberg concluded that surface roughness affected flow of a thin layer of grease on metal, while rheology governed flow of thicker layers.
Westerberg carried out experiments at 25, 50, 75 and 90 degrees Celsius and observed effects of temperature on critical speed, grease losses and grease tracks on steel. Oil separation was much more pronounced at higher temperatures, and affected critical speed for the onset of grease flow as well as grease loss. At 90 C, base oil bled on the steel, and a grease plug with no yield slid on a layer of oil. At lower temperatures, in the absence of bleeding, grease yielded and pucks slid on thin grease layers.
Westerberg was able to explain many features observed for tracks left by the sliding grease plugs in these experiments using a flow model based on Herschel-Bulkley rheology. As observed in the experiments, an oil-rich layer formed by oil separation from grease had much lower viscosity than the grease layer and was responsible for an apparent wall-slip effect at the grease-steel interface.
Mary Moon, Ph.D., is a physical chemist with R&D and management experience in the lubricating oil and grease and specialty chemicals industries. She is skilled in industrial applications of tribology, electrochemistry and spectroscopy. Contact her at mmmoon@ix.netcom.com or (267) 567-7234.