The effect of the mating gear surface over the durability of injection-molded polypropylene spur gears

Extensive investigations have been carried out to understand the effect of gear material, gear parameters and service conditions over polymer gear durability. However, the effect of the mating gear surface condition over test gear performance has not been completely understood. In this study, injection-molded polypropylene gears were paired with stainless steel gears and evaluated in the power absorption gear test rig. This study considered steel gears manufactured through the wire-cut electric discharge machining with different surface roughness (3.8–4.1 µm, 2.5–2.8 µm and 1.9–2.2 µm). The bearing ratio curves of the steel gear surfaces were obtained with the aid of non-contact profiler. During testing, the surface temperature of polymer gear increases due to the gear material hysteresis and surface interaction. The surface temperature of the gear increased by 5–15℃ due to the increase of surface roughness of the mating steel gear tooth (1.9–4.1 µm). The worn-out gear tooth surface also confirmed the significance of the mating gear surface condition. Further, stainless steel discs with different surface roughness (4.7–5.4 µm, 2.6–3.2 µm) were manufactured through the wire-cut electric discharge machining process. Injection-molded polypropylene pins were slid against these discs. Due to the increase in surface roughness, coefficient of friction was found to increase from 0.38 to 0.45 for the chosen test condition. The measured net surface temperature of the test specimen also increased from 52 to 59℃ due to the increase in surface roughness.