He explained that the integrity of greases at high temperatures can be determined by combining several existing techniques with some new ones. Whats more, improving the integrity of grease formulations at high temperatures leads to improved bearing fatigue life.

High-Temp Performance

At the same conference, Ren Westbroek, Axel Christiernsson International AB, Nol, Sweden, said that two standards describing grease classification are generally accepted in Europe: DIN 51502 and ISO 6743. The standards specify that if the grease does not pass the specified tests at 120 degrees C, dropping point should be used to estimate the upper operating temperature.

However, according to Westbroek, Not all products perform well in these types of bearing tests, but may perform perfectly at high temperature in the application for which they were designed. He noted that softer greases (NLGI 0 and lower) tend to leak out of bearings during these tests, causing a failure not because of grease degradation but merely because of leakage.

Other greases might have excellent high-temperature stability, but may not lubricate properly in these types of bearing tests, he added. Both Kaperick and Westbroek then described alternative methods of determining the effect of high temperature on grease performance and in setting the upper operating temperature.

Westbroek explained, The definition of high-temperature performance for lubricating grease needs to be divided into two parts; namely, the upper temperature limit – the temperature at which the grease can function for a very short period – and the upper operating temperature – the highest temperature at which lubrication can be maintained for a long period of time.

He noted that there is a wide variation in how manufacturers determine the upper operating temperature. Looking at product data sheets for similar lithium greases, we find upper operating temperatures ranging from 120 to 250 degrees C, he said, adding that a recent paper showed the inconsistency in how the industry reports upper operating temperature and that different tests may lead to different conclusions about the upper operating temperature.

The Effect of Heat

Westbroek said, To define an alternative test or combination of tests that might be used to determine the upper operating temperature for greases, we need to understand the parameters that play a role in grease degradation and the effect degradation has on grease properties. He cited several studies that identified oil loss through evaporation at high temperatures as a critical factor causing loss of lubricity and eventual bearing failure. Another study showed that antiwear and boundary properties also play important roles in bearing failure, he said.

In a similar way, Afton investigated different aspects of grease formulations in an effort to correlate them to a greases ability to protect bearings at high temperatures. The analysis identified three critical factors, each representing a different facet of understanding the high-temperature performance of grease.

The first factor was understanding the structural and thermal stability of grease at high temperatures, Kaperick said. The testing consisted of dropping point, thermal gravimetric analysis, oil separation and rheological strain sweeps.

Dropping point indicates the ability of a grease thickener to hold oil at high temperature, explained Kaperick. If oil is released and comes out of a bearing at high temperatures, it could lead to bearing failure.

Aftons rheometry tests measured the shear stress at which the grease structure loses its integrity (called the yield point) as well as grease stiffness after integrity is lost (known as the storage modulus at the flow point). Both parameters can be important measures of a greases ability to protect a bearing, said Kaperick.

Three greases were examined in the rheometry tests, which were run at 140 and 160 degrees C. The yield points showed significant differences among the greases. One grease showed a minor drop in the amount of stress needed to lose structural integrity, while a second maintained its original stability even at the higher temperature, Kaperick said.

The third grease needed considerably less stress to lose structural integrity at the higher temperature. Therefore, Kaperick said, a greases ability to maintain its structural integrity at the temperature of interest could contribute to its ability to protect a bearing at that temperature.

Likewise, the modulus at the flow point (the point at which the internal structure of the grease begins to behave less like a gel-like material and more like a viscous material) is a likely measure of how well the grease stays in the bearing under application conditions. Again, the three greases demonstrated wide differences in response, with two showing little difference between measurements at 140 and 160 degrees C. On the other hand, there was a relatively large change for the third grease at the two temperatures, said Kaperick.

This measurement, which is representative of grease stiffness, could be an indication of a greases ability to flow into the contact area. Therefore, if grease stiffness is high, there is less likelihood that the grease will move into the bearings contact areas to properly lubricate the metal surfaces, Kaperick said.

The second factor Afton evaluated was the friction and wear reducing properties of grease. The company measured boundary lubrication and thin-film friction coefficients, and analyzed bearing wear after testing in a FAG FE-9 rig. We used modified versions of standard test methods to measure friction and wear as well as the wear preventive characteristics of lubricating grease, Kaperick said. We used the results to study the friction and wear response of different formulations at high temperatures.

Torque traces at 140 degrees C show that different greases have widely different responses at the temperature of interest. Grease with a high coefficient of friction will contribute to bearing failure by increasing the amount of energy needed to turn the bearing, Kaperick explained, and many bearing tests use increased torque as a contributing factor in determining that a bearing has failed.

The third factor Afton used to evaluate high-temperature performance was oxidative stability. This enabled researchers to determine the effectiveness of antioxidant combinations for high-temperature formulations. In addition, we screened antioxidant combinations for their effectiveness in the presence of extreme pressure agents, antiwear additives and corrosion inhibitors, Kaperick said.

Axel Christiernssons Westbroek explained that his companys testing compared the high-temperature performance of seven commercially available greases. We studied oil separation and evaporation as well as oxidation stability in standard tests. In addition, the greases were tested on a rheometer with a rolling bearing assembly (RBA test). After three weeks of aging (or at failure), the bearings were opened, the remaining grease was removed from the cage and tested with a Remaining Useful Life Evaluation Routine (Ruler) to determine the percentage of antioxidants remaining in the grease after the test.

The results of the oil separation and evaporation tests and the oil loss calculations from the RBA test show that the standard test method, ASTM D 6184, alone is not sufficient to provide insight into the amount of oil loss at elevated temperatures inside a bearing, Westbroek said. For example, one grease showed the highest oil loss in the ASTM D 6184 test, but lost significantly less oil than the other greases when heated to
120 and 140 degrees C inside a bearing.

Another grease showed the lowest oil loss in the ASTM D6184 test but exhibited significantly increased oil loss at 120 and 140 degrees C inside the bearing. Westbroek said, This can be easily understood from the fact that the dropping point of this grease is very close to 140 degrees C.

He also noted, Both the PDSC and Ruler measurements showed calcium sulfonate complex grease had the best performance. Even though the amount of antioxidant in this formulation is comparable to that in the other greases, as much as 49 percent of the original antioxidant is left after three weeks at 140 degrees C.

Lithium-thickened and clay-thickened greases showed the poorest oxidation stability in the PDSC test. However, these greases behaved completely differently in the RBA test, Westbroek said. The lithium-thickened grease failed after three weeks at 120 degrees C and after two weeks at 140 degrees C. The clay-thickened grease passed the test after three weeks at both temperatures.

Westbroek explained that the behavior of all greases in the RBA test can be divided into two stages. In the first part of the aging process, torque is equal to or lower than the torque for the fresh grease. The lower torque could be caused by the oil that has bled out of the grease during this time at elevated temperature, which might improve flow toward the raceway, he said.

In the second stage, torque starts to increase again, eventually leading to bearing failure. Increased torque in the second stage may have been caused by oxidation and polymerization of the base oil or oil loss from the bearing, leading to starvation, Westbroek said.

The results from the RBA tests at both 120 and 140 degrees C are in line with the temperature at which the greases pass the SKF R2F-B test, which is used to determine the upper operating limit in DIN 51502. Only one result did not confirm the SKF R2F-B test result, the reason for which is unclear and will have to be investigated further, said Westbroek.

Conclusions

Kaperick summed up, The importance of rheology in understanding various facets of grease performance is becoming clearer. Aftons research shows that several techniques allow the formulator to gain a better understanding of the behavior of both solid and fluid aspects under conditions that more closely approximate real world conditions.

He added that the adaptation of standard analytical and bench tests can offer insights into the determination of critical performance factors in grease formulations. The ability to look at structural and thermal stability, the effect of wear and friction on high-temperature torque and the oils oxidative stability can be invaluable to the grease formulator, he said. Segmenting the various aspects of grease performance in a given application provides a better understanding of which performance parameters are most critical, clearing the way to optimize a formulation in a more timely and cost-effective manner.

Westbroek concluded his presentation saying, This work showed that oil loss from the bearing plays a critical role in the process that causes failure. Also, the standard oil separation test does not always provide a good understanding of the oil loss at the actual operating temperature. Rather, the oil loss determined from statically aged bearings provided a more realistic way to determine the upper temperature limit of a grease.

In addition, rheological measurements with the rolling bearing assembly give a good understanding of how various grease technologies behave when exposed to elevated temperatures for a prolonged period. Although this research is in an early phase, the first results indicate that it may be possible to develop an alternative method of determining the upper operating temperature using rheological measurements with a rolling bearing assembly, Westbroek said.