Effects of frequency on hysteretic heating and fatigue life of unreinforced injection molded polyamide 66 spur gears

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Gears are subjected to different rotational speeds/frequencies during their service life. The effect of the rotational speed on the performance of a metal gear is insignificant; however, it affects the thickness of the lubricant film. Polymer gears generate hysteretic self-heating because of the viscoelastic behavior of the material, thereby limiting their performance and usage in applications. The injection molded polyamide 66 spur gears were loaded by ground steel gear at different torques and frequencies using in-house realized servo motor driven gear test rig. Bidirectional loads at frequencies 2, 5, and 7.5 Hz and unidirectional loads at double the frequencies (4, 10, and 15 Hz) were applied on the polymer gears. The surface temperature of the gear due to the material hysteretic self-heating was continuously monitored and was recorded using an infrared thermal camera. Torque applied and angular displacement of the gear mesh were acquired to plot a hysteresis loop. The hysteresis loop area and surface temperature increase with the increase in the torque. Moreover, the bidirectional loads induce higher temperature than the unidirectional loads. This is because the gear tooth deflection increases under the bidirectional loads compared to that under the unidirectional loads for the tested frequencies. The fatigue life of the polymer gears was evaluated at higher frequency for different torques and was compared with that obtained at lower frequency. The gears tested at frequencies 15 and 7.5 Hz under unidirectional and bidirectional loads, respectively, exhibited inferior fatigue life compared to that at 10 and 5 Hz under unidirectional and bidirectional loads, respectively, because the temperature of the gear increases (30.6% and 43.7% for unidirectional and bidirectional loads, respectively) at higher frequencies. Both thermomechanical and root crack failures were observed under the bidirectional loads, whereas the gears exhibited only the root crack failures under the unidirectional loads. The failure morphology studied using the scanning electron microscope indicated straight root crack with overlapping fractured surfaces under both the bidirectional and unidirectional loads.