Surface durability of injection-moulded carbon nanotube–polypropylene spur gears

Short fibre reinforced thermoplastics are being considered for light- and medium-duty engineering applications because of their improved mechanical strength combined with cost-effective advantages. In recent years, the carbon nanotube reinforced thermoplastics are being preferred over the short fibre reinforced thermoplastics because of the absence of directional shrinkage characteristics, directional mechanical and tribological properties. In this work, 1 wt% carbon nanotube–polypropylene material was injection-moulded to spur gears and evaluated for the durability using in-house developed power absorption gear test rig. During testing, the net surface temperature of the test gears and in-line torque available at the driver and driven shafts were continuously measured. The measured torque was used to evaluate the transmission efficiency of the test-steel gear pair. The measured net surface temperature was correlated with the gear failure mode. Test gears were run up to failure or up to 8.6 × 10⁵ cycles, whichever occurred first. Worn-out gear tooth surfaces were observed using an optical and scanning electron microscope to understand the wear mechanism. In the initial stage of service, test gears exhibited less wear near the pitch region compared to the tip and root regions. This behaviour is due to the maximum sliding velocities at the tip and root regions compared to the pitch region. The carbon nanotube–polypropylene gears exhibited lower surface temperature (5−10 ℃), improved service life (30%−80%) and higher transmission efficiency (1−1.5%) compared to the polypropylene gear.