As hardware technologies race ahead, grease formulations and test methods risk becoming stuck, mired in traditional but outdated grooves. At the Society of Tribologists and Lubrication Engineers annual meeting, formulators and equipment manufacturers highlighted a number of new developments helping grease move up the road.
Christian Specht, an engineer with German bearing manufacturer Schaeffler Technologies, reminded the late-spring gathering in Las Vegas that lubricating grease is a key factor in the performance and reliability of rolling bearings. But grease manufacturers are struggling with a conundrum: Grease performance is most critical under extreme operating conditions for which relevant properties and test methods often are not well defined.
For example, low ambient temperatures are common where wind turbines, offshore industries and railroads operate in areas of Canada, Scandinavia, Russia and China. Many current grease specifications were developed to evaluate grease suitability down to -20 degrees Celsius or possibly -30 C, but Specht pointed out that temperatures reach -40 C in these locations. Traditional tests for conveyability (DIN 51816-1), oil separation (ASTM D6184) and apparent viscosity (ASTM D 1092) provide limited information relevant to applications at extremely low temperatures.
Other tests such as ASTM D11478, Standard Test Method for Low-Temperature Torque of Ball Bearing Grease, use point-contact double-groove ball bearings, even though performance differs significantly between point-contact and line-contact bearings at low temperatures. Specht warned that friction and torque increase strongly as temperature sinks below -20 C. As an alternative, he recommended using the FE-8 Low Temperature Torque Test Rig with Climate Control to measure wear prevention and durability in axially loaded bearings, because the FE-8 can compare start-up and operating torques for various point- and line-contact bearings under wide ranges of temperature (down to -70 C), load, speed, and humidity.
Greases are also subjected to extreme operating conditions when there is little or no movement. Specht explained that a particular combination of conditions can cause false brinelling, which damages rolling bearings. While brinelling refers to permanent indentation due to high load (like a hammer blow), false brinelling is fretting wear caused by vibration in a relatively lightly loaded contact. For example, false brinelling can damage wheel bearings in cars that vibrate while fastened tightly to railroad cars during transport.
A traditional grease test for false brinelling involves measuring the depth of wear scars on bearings after 10 million load cycles in a test rig. Specht showed test data that clearly identified greases with poor performance across a wide temperature range, but the test failed to predict or explain an abrupt change from acceptable performance above -20 C to unacceptable performance at lower temperatures.
Though grease applications increasingly involve extreme operating conditions such as low ambient temperatures and false brinelling, data relevant to extreme conditions are scarce and can be difficult to understand in terms of traditional tribology models and tests. Formulating greases and testing for quality control to guarantee performance under these extreme conditions presents significant challenges. However, Specht declared his hopes that ongoing studies at Schaeffler will yield results as soon as the next STLE annual meeting in 2017, in Atlanta, Georgia.
Extending Service Life
Real progress is being made toward new additive packages that can prolong the service life of greases for automotive and electric motor applications, said Gareth Fish, technical fellow and strategic technology manager at Lubrizol.
Fish explained that bearing technology has advanced far faster than that of lubricating greases. Hardware now lasts 10 times longer than the grease within the vast majority of bearings. Equipment operators are placing high priority on reducing grease consumption by extending its useful life. More durable greases are in particularly high demand for sealed-for-life bearings in passenger cars and light trucks to enable longer equipment warranties.
Along with many other performance characteristics, the service life of grease relies on additives. However, performance in tests for service life depends on the combination of additives and base oil, as well as the test method. For example, Fish formulated an additive package with 4 percent zinc by weight in two simple lithium based greases. Four-ball wear scar diameters (ASTM D2266) and four-ball extreme pressure weld points (ASTM D2596) were comparable for both cases and met typical requirements. But one of the greases outlasted the other by 40 hours in ASTM D3527, Standard Test Method for Life Performance of Automotive Wheel Bearing Grease, while the other grease prevailed by 172 hours in the DIN 51821-2 test for roller bearing grease.
Fish also tested a zinc-containing additive in four lithium complex greases. All four additized greases met the National Lubricating Grease Institutes LB standard for automotive chassis and GC standard for wheel bearing applications. Wheel bearing life was 80 hours for all four, yet DIN 51821-2 test results ranged from 94 to 343 hours.
Later, Fish told LubesnGreases that, Prior to the study, I had not made grease in an alkylated diphenyl ether base stock. It took several attempts to get the base oil, thickener and additive package to work together. Adding the gelled sulfonate to boost the EP and antiwear performance of the urea base proved to be the answer to overcoming this challenge.
Other greases formulated with a metal-free, ashless additive package also fully met GC-LB standards, but wheel bearing life results were 120 and 200 hours, versus 80 hours for greases with the zinc-containing additive package. This indicates that enhancing grease service life is more a matter of optimizing individual formulations than finding a universal magic bullet, he concluded.
Enhancing Dropping Point
In another presentation, Joseph Kaperick, customer technical service advisor at Afton Chemical Corp., noticed substantial improvements in dropping point for certain greases containing a boron amide additive. The borate chemistry shows great promise for high temperature applications, as well as having versatility in other performance areas, he explained.
Dropping point and consistency are key properties, used in many lubricating grease specifications and for quality control. However, Kaperick reminded his audience, dropping point has limited relevance to in-service performance, and NLGI cautions that it should not be used to determine a greases upper operating temperature.
The dropping point of a soap-thickened grease is measured by slowly raising the temperature of a small sample in a standard cup and watching for the temperature at which the grease changes from semi-solid to liquid-when a drop of grease falls through a hole in the bottom of the cup.
Kaperick and his team designed an experiment to investigate the effects of several borate chemistries, including a borate amide, a borated dispersant and a borated-phosphorylated dispersant, on dropping point. They also looked at the effects of grease alkalinity, water content, glycerin content and interactions with zinc dithiophosphate antiwear additives on dropping point.
Kaperick explained that lithium greases formulated with hydrogenated castor oil instead of 12-hydroxy stearic acid dont seem to respond well to additives with borate chemistry. He hypothesized that the reaction of HCO with inorganic base, such as lithium hydroxide (LiOH), to form soap thickener produces glycerin as a by-product; borates react preferentially with glycerin, leaving little or no borate available to complex with the thickener. In contrast, there is well-known synergy between borate additives and ZDDPs.
A screening study to identify effective treat rates of borates showed that glycerin clearly was a significant variable, and there was some indication that differences between borates might also be significant. Data from a second experiment confirmed that glycerin inhibited the effect of the borate amide on dropping point. The influence of glycerin on different borate chemistries provides a good explanation of why greases made from hydrogenated castor oil dont respond as well to these types of dropping point enhancers, Kaperick later told LubesnGreases.
Kaperick also reported that borate treat rate may have an important effect on dropping point, especially in the presence of excess LiOH or ZDDP. Higher levels of LiOH, ZDDP and even water could improve the effects of borate additives on grease dropping point.
While additives certainly have an effect on results, so could potential errors in dropping point tests. ASTM D2265 includes instructions to thoroughly clean the test cup and equipment, recommending that cups showing signs of wear on interior surfaces be discarded. Kaperick recalled anecdotes blaming inconsistent test results on the age of cups.
To investigate this claim, he performed seven consecutive dropping point tests on a particular grease with a single cup, cleaning the cup after each test. The first result (new cup) was 236 degrees Celsius, and subsequent results varied between 247 C and 253 C. While observations indicated there may be some chemical interactions going on at the cup surface that can affect results, Kaperick said, there was no evidence for a trend in dropping point in this set of seven measurements.
Mary Moon, Ph.D., is a physical chemist with hands-on R&D and management experience in the lubricating oil and grease and specialty chemicals industries. She has served on the board of the Philadelphia section of STLE in several roles. Contact her at mmmoon@ix.netcom.com or (267) 567-7234.