Predicting the Fatigue Performance of Fiber-Reinforced Plastic Gears

Plastic gears are lighter, quieter and cheaper to produce than steel, but their fatigue life has been hard to predict: injection molding aligns the glass fibers anisotropically, and standard ISO 527 tensile-bar data overstates real-tooth strength. Using Stanyl TW271F6 (PA46 with 30% glass fiber), the white paper replaces the empirical 0.6-0.8 correction factor with a method that accounts for the actual failure drivers — weld lines forming at tooth roots, local friction heat measured at 20-30 C above ambient and modeled with the Arrhenius relationship, and crack growth captured by a Paris-law master curve fed into KISSsoft. Predicted S-N curves matched physical tests to within a factor of three on lifetime, compared with errors up to a factor of 100 using the older approach. For designers, this turns a rule-of-thumb estimate into managed risk and produces trustworthy lifetime predictions from a material card, reducing expensive mold iterations, design risk and time-to-market. The model is validated for fatigue (root cracking); at very high temperatures the failure mode can shift to wear, which it does not capture.

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