Continued from page 16 Table 4. Test results for LF4 and LF9 additives


As seen in table 4, the package LF4 gave the best response in the lithium complex base and the LF9 in the urea-thickened greases. Most of the packages tested gave friction coefficients around 0.06 for the simple lithium grease. Adding additional MoDTC to both the LF4 and LF9 packages improved the running-in characteristics, but increased the treat level and net treat cost significantly. This work has shown that low friction can be achieved by formulating packages with combinations or molybdenum complexes, organic friction modifiers and a mixture of anti-wear and other performance additives.


However for complete energy efficiency other factors need to be considered, most of which were investigated at by both Harinarain (3) and Yamamoto (4) in their development of energy efficiency lubricants papers. These factors: lubricant traction; film thickness; and rheology and consistency; along with grease fill will now be considered in turn.


Lubricant Traction


One of two definitions of traction in use today is as a measure of the internal friction of a lubricant. It was reported by both Harinarain (3) and Yamamoto (4) that measuring the traction coefficient of the base fluid was an important step in determining the energy efficiency of the lubricant. The lower the traction coefficient the less heat generation that occurs in the application. Yamamoto (4) compared the traction coefficients of a number of base fluids and traction fluids including different types of mineral oil with increasing temperature. One challenge was that the viscosities of the fluids were all different. Comparing the two naphthenic mineral oils, the traction coefficient was determined to be higher for the higher viscosity fluid and similarly for the two synthetic oils tested. The work did however confirm that when comparing different types of base fluids of the same viscosity, polyalphaolefin (PAO) fluids give lower traction than paraffinic mineral oils which in turn are lower than naphthenic.


Some recent unpublished work looked at testing select novel base fluids with extremely low traction coefficients. The fluids gave traction coefficients as measured by The PCS Instruments MTM2™ as of the order of 0.02 which is half the typical value reported for PAO fluids (4). Optical measurements to determine their elastohydrodynamic (EHD) lubrication properties showed that they did not form films thick enough to be measured. High-pressure rheology measurements taken on the fluid showed that these types of fluids were largely incompressible and had no piezo-viscous characteristics. Without suitable piezo-viscosity, lubricants are not capable of generating EHD films and cannot separate the surfaces. A 4-ball wear test carried out on the same fluid, resulted in a wear scar of ~2 mm, compared to about 0.8 mm for a neat PAO fluid. This type of behaviour showed that going to a low traction fluid to improve energy efficiency, cannot be done at the expense of film forming. This is illustrated in the Stribeck curve in Figure 2, showing zone of optimum lubrication in which the film formed is thick enough to fully separate the surfaces, prevent wear but not so thick that churning losses predominate.


Also an attempt was made to determine the traction coefficients of some different greases. This proved to be a challenge. The first issue was controlling the temperature. As grease is an insulator with poor heat convection characteristics, heating up and controlling the temperature of the grease during the test was difficult. For greases running between room temperature and 80 °C, samples were pre-heated and incubated at temperature in an oven, prior to apply to the test pieces. Above this upper temperature, it was not possible to maintain stable temperature. The second issue was starvation. In both traction and optical EHD testing, it is very difficult to get the grease to behave like it would in a bearing. In the test, the contact pushes the grease away and as the grease is on a horizontal flat, it cannot flow back into the contact and has to be pushed back by a wiper. This creates artificial conditions, which need further development.


Figure 2. Stribeck curve showing zone of optimum lubrication


Film Thickness Effects Historically it has always been reported that with greases, it is the oil that does the lubrication and the thickener plays no role in the lubrication. Worked carried by Cann and Hurley (10) showed that under the conditions of their testing that their greases built up thicker films than when testing just base oils alone. This was also supported when looking at the lubrication properties of CVJ greases using the optical EHD tester (8). A recent publication by SKF (11) suggested that at higher speeds, the film thickness equates to the viscosity of the base oil. Looking at the SKF data (11) and that presented by Yamamoto (4), greases still form slightly thicker films than the base oil alone. The most interesting feature of both the SKF (11) and Yamamoto (4) work was that at low speed, greases formed much thicker films than was expected. Yamamoto (4) compared lithium soap greases with a urea-thickened grease and saw that at an entrainment velocity of 0.01 m/s, the base oil gave a central film thickness of 20 nm, the two lithium soaps about 60 nm and that of the urea about 120 nm. Similar thicker films were reported by SKF (11). This has important consideration when developing more energy efficient greases.


Rheology and Consistency How stiff the grease is and how easily it flows are important considerations for the development of energy efficient greases. Another important factor is how the grease behaves under shear. Most greases soften when subject to shear, either in by working Continued on page 20


18 LUBE MAGAZINE NO.129 OCTOBER 2015


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