Dr. Ana Trajkovski, univ. dipl. inž., Nejc Novak, mag., Jan Pustavrh, mag., prof. dr. Mitjan Kalin, univ. dipl. inž., izr. prof. dr. Franc Majdič, univ. dipl. inž., all Univerza v Ljubljani, Fakulteta za strojništvo

Abstract​:
Engineering polymers have proven to have very promising tribological properties and successfully follow high-performance thermoplastics. In our research, we tested and analyzed polyoxymethylene (POM) for the very demanding operating conditions of hydraulic on/off valves. We tested POM and POM reinforced with 30% carbon fiber (PO CF30) in standard ISO VG46 hydraulic oil, glycerol-water mixture and demineralized water at room and elevated temperatures.
The results showed a very low coefficient of friction (~ 0.027 – 0.033) and specific wear (~ 10-7 mm3/Nm) in the mixture of glycerol and water, comparable to the values measured in standard hydraulic oil and lower than those measured in water (coefficient of friction ~ 0.14, specific wear ~ 10-6 mm3/Nm), at room temperature. At elevated temperature, the coefficient of friction in hydraulic oil and glycerol-water mixture increased slightly, while in water decreased, measured values become comparable for all lubricants. The specific wear rate increased significantly in the hydraulic oil and glycerol-in-water mixture at elevated temperature and the values became comparable to the specific wear rates measured in water. Based on the results, both polyoxymethylene and glycerol-water mixture can be good and sustainable alternatives for standard hydraulic oil and steel tribological pairs, leading to excellent tribological properties with a significant reduction in component mass.

Keywords:
polymer, polyoxymethylene, glycerol, water, hydraulic oil, friction, wear, hydraulics