Grease and Additive Mysteries Unfold
From carousels to wheel bearings and turbines to electric vehicle motors, grease keeps the world rotating on its axis. But predictable grease performance under stress is essential. Innovative laboratory tests for high-temperature grease applications, wear scar evaluation and film thicknesses were all discussed at the 74th STLE meeting in 2019. Mary Moon shares the highlights.
Nuclear power plants command the highest expectations for reliability and safety, and the consequences of a system failure can be catastrophic.Choosing the right grease for such a high-stakes, high-temperature environment is critical.
As temperature rises, an oils viscosity decreases and lubricating films become thinner, providing less friction and wear protection. Heat also accelerates chemical reactions that oxidize base oils and additives.
A microscopic matrix of thickener holds base oil and additives in greases and gradually releases them to lubricate in a process referred to as bleed. A grease can fail if the thickener and oil separate, the base oil evaporates or chemical degradation occurs. Standard tests can indicate whether a product is likely to provide satisfactory performance at specific temperatures, but may not always represent real-world application conditions.
To make sure lubricating products used in his workplace are optimal, Bryan Johnson, lubrication engineer at Palo Verde nuclear power station, Arizona, explained how he uses innovative tests to determine greases future performance.
Johnson wanted to lubricate stems and stem nuts in motor-operated valves working in environments from 85 to 141 degrees Celsius, at speeds from 0.002 to 0.03 meters per second and over an 18-month service life. Friction management and retention of grease on stem threads were chosen as critical performance parameters.
There is a need for industry consensus and improved understanding about how to artificially age greases to allow comparison and their selection for high-temperature applications, said Johnson.
Johnson tested a variety of NLGI grade 2 greases – a clay-thickened API Group I base oil, clay-thickened ester base oil, lithium-thickened silicon oil and calcium-sulfonate-thickened Group II oil – using identical flat-panel samples with a thickness of 0.79 millimeters. Vertical and horizontal grease-covered plates were then aged in an oven.
Oven temperatures were set at 21, 85, 113 and 141 C for up to 168 hours with a sample of grease removed and weighed at each temperature. The greases weight loss ranked consistently at each temperature level, with losses of 4.24 to 5.48 percent for vertical samples and 4.34 to 7.18 percent for horizontal samples at 141 C.
The silicon-based grease performed best, followed by the ester grease, clay-thickened grease and calcium sulfonate grease. Base oil evaporation was greater than bleed loss in every case. A mesh cone test with samples heated to 141 C for periods of 24, 48 and 168 hours was conducted, and in this test condition bleed loss was more significant than for the flat samples but still less than loss due to evaporation.
Johnson coated cylindrical pins, measured grease weight loss over time for oven aging and then evaluated the coefficient of friction, or COF, using a pin and vee block test machine. Based on these test results, he selected a candidate grease for full-scale testing that included oven aging of greased stems followed by COF measurements as the nut turned on the stem threads.
Oven aging produced a seemingly failed grease that appeared dry and cracked on the stems. During friction testing, the COF decreased from 0.25 (first stroke) to 0.126 (15th stroke) as dry surface grease reconstituted with wet grease underneath and stored in the stem nut.
Johnson concluded that future work to develop methods for aging and testing greases for high-temperature applications should focus on oil-loss mechanisms rather than chemical degradation.
Looking Inside Wear Scars
Nicole St. Pierre, emerging technology innovation manager at Nye Lubricants, compared subtly different chemicals for potential use as antiwear additives.
St. Pierres experimental design used two amine phosphates with different nitrogen and phosphorus content, blended with three zinc dialkyldithiophosphate compounds of different alkyl chain lengths in three ratios – 25:75, 50:50 and 75:25 – at two treat levels in a polyalphaolefin base oil (6 centistokes at 100 C). In total, there were 36 samples – 18 samples with a 1 percent treat of each additive blend and 18 samples with a 3 percent treat.
St. Pierre used the well-established four-ball wear test, where three lubricated steel balls are clamped snugly by a ring inside a cup while a fourth ball rotates against them for an hour. At the end of the test, the average scar diameter (length and width) is measured on each ball using a microscope to characterize antiwear performance.
Additionally, she carried out a proprietary experimental scuffing test with a mini-traction machine, or MTM, which evaluates additives under speed, load and sliding conditions where lubricants can fail and where scuffing, pitting, seizure and early onset fatigue can occur. MTM results are determined by the wear scars volume divided by the distance travelled by the contact, with 1 cubic micron per millimeter as the standard unit, St. Pierre explained.
She also used an optical profilometer to generate three-dimensional digital images of rough surfaces and analyzed the depth and other features of the wear scars from the four-ball and scuffing tests.
Wear scars are rarely symmetric and often have jagged edges. By looking at the total wear volume of a scar, we can accurately account for the jagged edges and get a more representative picture of how well the additives protect the surface from wear, St. Pierre said.
Amine phosphate two gave the best overall performance and was more effective than amine phosphate one at reducing wear when used in a 25:75 ratio with the majority of the ZDDP compounds studied at a 3 percent total treat rate. Profilometer data revealed 7 percent smaller four-ball wear scar volume for 25:75 versus 50:50 blends of amine phosphate two and a ZDDP with C4 and C8 primary alkyl chain substituents.
MTM data for traction, or friction, and electrical contact resistance – a characteristic of the ball and disc interaction and additive absorption – showed no significant difference in additive film formation and stability for these two formulations.
Its important to consider data from more than one complementary test to fully understand additive performance, St. Pierre said.
Imaging analysis of test specimens will give formulators a better understanding of how additives perform in each lubrication regime – boundary, mixed hydrodynamic. This will allow them to better select appropriate additives for specific applications and meet needs of their customers, she concluded.
Measuring Films in Bearings
Most tribology tests measure friction and wear in a single contact, such as a ball moving across a disk or another ball. But many mechanical devices rely on ball bearings where a number of balls, separated inside a wire cage, rotate and slide on two tracks, or races.
Formulators and engineers need to understand how grease performs inside bearings to develop more efficient greases and bearings. Lubrication depends on film thickness. Engineers assumed that enough oil bled from grease and pulled into contacts by the movement of balls and races to fully flood contacts and provide elasto-hydrodynamic lubrication. Bearings were designed based on calculations of film thickness from oil viscosity. However, tribology tests showed that operating conditions could affect thickness and composition of films in a single contact.
Piet Lugt, a senior research and technology development scientist based in Utrecht at bearing company SKF, measured film thicknesses in greased ball bearings. Lugt used a laboratory test rig developed by the University of Twente in the Netherlands and a Lubcheck Mk3 instrument to measure the capacitance of films in operating bearings. Capacitance is the response of a dielectric material to an applied voltage or potential difference, and in the case of a grease, depends on chemical composition, thickness, viscosity and density of oil, thickener and additives.
Lugt compared three NLGI grade 2 and 3 greases – a polyurea thickener in an ester oil (PU/E), a lithium thickener in a mineral oil and a lithium complex thickener in a PAO. During the first 10 hours of each test, the spinning motion of the bearing churned the grease, causing friction and heat, which decreased oil viscosity and film thickness. Then, grease arranged itself into channels, oil bled into contacts and film thicknesses were semi-constant with occasional spikes such as thickener entering a contact or chaotic fluctuations. Film thickness data were adjusted to eliminate the effects of temperature changes.
Greased bearings were operated for 20 hours at speeds between 1,000 and 4,000 revolutions per minute and an applied axial load of 513 newtons.
Results showed that this PU/E grease gave thicker films than these lithium mineral oil and lithium complex PAO greases. Lugt attributed the difference to mechanical instability of the PU/E, which released oil more readily under shear than the other two greases, although its base oil viscosity was lower.
Film thickness was almost constant at slow speeds and decreased as speed increased where less oil was drawn into contacts due to starvation (as in single contacts). Applied loads between 500 N and 900 N had a relatively small effect on film thickness.
Our measurements show that a grease-lubricated bearing is running in starved conditions where lubricating films are thinner than expected for fully flooded contacts. It is so important to use a full bearing test rig for measuring the film thickness in grease lubrication because a single contact does not capture the effects of multiple balls rolling over the grease and, for example, centrifugal forces and/or side flow that affect the ability of oil to replenish the contacts, Lugt concluded.