Submitted to: Tribology Transactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/14/2009
Publication Date: N/A
Interpretive Summary: Traction is a measure of the resistance of oils to shearing under high pressure. It is an important property of lubricating fluids used in engines, gears, cams, bearings, valve trains, compressors, etc of cars, trucks, tractors, wind mills, aircraft, space vehicles, etc. For these applications, lubricants are formulated to have as low traction as possible in order to minimize energy loss and maximize fuel efficiency. The traction properties soybean and eight other seed oils, and two petroleum-based oils of varying viscosity and chemical structures were investigated and compared. The result showed that the biobased oils had a much lower traction value than the petroleum based fluids, even when their viscosities were almost 1-2 order of magnitudes higher. Thus, biobased lubricating fluids, besides being environmentally friendly, also have the potential to provide fuel efficiency and energy saving. Such improved performance will provide biobased oils an opportunity to gain share of the lubricating oil market, which currently is almost exclusively dominated by petroleum-based products.
Technical Abstract: The elastohydrodynamic traction coefficient (tc) properties of nine seed oils of varying chemical structures, PAO and hexadecane, were investigated using a ball-on disk traction apparatus. The seed oils were: castor oil, a triglyceride with hydroxyl functional group; jojoba, a monoglyceride; and seven triglyceride seed oils with varying fatty acid compositions. Two types of experiments were conducted atconstant temperature (40 or 100 degrees C) and constant load (10, 20, 30, or 40N): tc as a function of slide-to-roll ratio (srr) at 1 m/s entrainment speed (u); and tc as a function of u at 50% srr. In both types of experiments, tc increased with decreasing temperature, increasing load, and increasing srr. All u vs. tc experiments gave the familiar Stribeck-type profiles. A maxima in tc values were observed in some srr vs. tc experiments. Regression analysis showed excellent agreement between limiting tc (tc at 1 m/s u and 50% srr) values from these two types of experiments. Hexadecane and PAO displayed higher tc values than the seed oils, even though their viscosities were up to 80 and 7 times lower, respectively, than that of seed oills. This observation can not be rationalized using molecular structure arguments. The results were attributed to differences in polarity between the two groups of oils. Unlike PAO and hexadecane, seed oils are polar, adsorb on friction surfaces and lower boundary friction, which contributes to the lowering of tc in the ehd regime.