The elastohydrodynamic traction coefficient (tc) properties of nine seed oils of varying chemical structures, polyalphaolefin oil (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 at constant temperature (40 or 100°C) and constant load (10, 20, 30, or 40 N): 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 maximum in tc values was 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 oils. This observation cannot 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.
Keywords: Castor Oil, Jojoba, Hexadecane, Polyalphaolefins, Seed Oils, Slide-to-Roll Ratio, Limiting Traction Coefficient, Elastohydrodynamic Traction Coefficient
DOI Link: https://doi.org/10.1080/10402001003599769