Friction and Wear of Self-Lubricating Materials for Hydropower Applications under Different Lubricating Conditions

Abstract

Self-lubricating bearings in hydropower applications are often lubricated with water under boundary lubricating conditions. Polyhydric alcohols replacing water have shown the potential to reduce both friction and wear. The objective of this work is, therefore, to evaluate the effect of a polyhydric alcohol-based environmentally-acceptable lubricant (EAL) on the friction and wear of self-lubricating materials for conformal contacts under boundary lubricating conditions. The lubricating properties of four commercially-available self-lubricating bearing materials were investigated under three different lubricating conditions: dry, water and a new polyhydric alcohol-based EAL. Bearing materials include one metallic composite and three polymer composites. A reciprocating motion test rig was used to evaluate the wear and friction properties. Surface analysis was performed by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and optical profilometry. Results show that the wear rate for the polymer composites is increased when water is present compared to dry operating conditions. The new polyhydric alcohol-based EAL substantially improves both friction and anti-wear performance of all four self-lubricating bearing materials compared to both dry and water conditions. Surface analysis indicates that the material transfer to the counter-surface is limited when the polyhydric alcohol-based EAL is used.

A Novel Reciprocating Tribometer for Friction and Wear Measurements with High Contact Pressure and Large Area Contact Configurations

“…At low contact pressure (26 MPa), the deviation in average height reduction between repeat tests is only 1.6%. The deviation increases at higher contact pressures with a maximum deviation of 9.6%, which is within the range of what has previously been reported for the thermoset material [15,18,19]. Based on the height reduction curves at different contact pressures (Figure 11), it can be stated that the repeatability of the wear measurements for the test rig is good.…”

“…It is clear from the enveloping curves that the long-term repeatability of friction measurements for the test rig is good at all contact pressures. Both the mean and maximum coefficients of friction have a maximum deviation of 5% between repeat tests, which is within the range shown previously for the thermoset material [15][16][17][18][19].…”

“…The average specific wear rates for the thermoset material sliding against stainless steel at different nominal contact pressures between 26 and 80 MPa are presented in Figure 13b. In contrast to the coefficient of friction, the wear rate of the thermoset is not The obtained mean and maximum coefficients of friction for the thermoset at the lowest contact pressure of 26 MPa, with averages of 0.067 ± 0.005 and 0.075 ± 0.005 respectively, are lower than the ones reported in previous studies at similar operating conditions [8,15,18,19]. This is attributed to the size of the thermoset pins used in this study, which has a 20 times larger nominal contact area in comparison to previous studies [8,15,18,19].…”

“…Instead, the rate of friction decrease slows down and stabilizes somewhere between 26 and 53 MPa. [8,15,18,19]. This gives a better representation of the thermoset material as seen in Figure 7a due to a proportionally higher content of solid lubricants.…”

“…The bearings (radial and axial) in hydropower applications operate at high nominal contact pressures up to 80 MPa (such as the lever/link bearings). In previous studies on the thermoset bearing material, nominal contact pressures between 20 and 28 MPa have been selected for flat-on-flat configurations using small pins with dimensions of 4 × 4 × 4 mm 3 [8,15,18,19] and nominal contact pressures of 20-30 MPa have been selected for journal bearing configurations [4,20,21]. For the flat-on-flat configurations, the maximum load is limiting the nominal contact pressures and the polymer pins are usually quite small to obtain higher contact pressures.…”