An experimental analysis of the gear tooth bending strength predictions from ISO 6336 in low and high cycle fatigue

Abstract Gears are mission critical machine elements used for primary and accessory power transmission in many industrial, automotive and aerospace mechanical systems. In normal operation gear teeth experience torque dependent cyclic loading which can initiate and propagate fatigue cracks. One of the most prevalent manifestations of this fatigue action are fractures through the tooth root resulting in tooth separation, commonly called tooth bending fatigue. Probably the most cited and applied methodology for designing gears to resist these fractures is the ISO 6336 “Calculation of Load Carrying Capacity of Spur and Helical Gears” which provides analytical equations and tabulated reference data to estimate design safety factors against bending fatigue. Method A of the ISO standard considers experimental evaluations of fatigue resistance while Method B relies on tabulated reference data. It is important that gear designers understand potential differences in the resulting fatigue strength and location of failure on the tooth root (critical location) between different design methods to ensure designs are safe from fracture. However, there is little information available in English and with tabulated data that gives a comparison on the accuracy (critical location and fatigue life) between experimental methods and analytical results. Even less information is available in terms of failure due to cyclic loads exceeding the yield strength (Low Cycle Fatigue), even if such type of loading should be carefully analyzed in case of applications involving severe loading, such as aerospace applications, or to account for unexpected load peaks. This work explores bending fatigue failure in gears under cyclic plastic loading. A gear Single Tooth Bending (STB) fatigue methodology is used to perform Low Cycle Fatigue (< 10,000 cycles) tests to complement existing High Cycle Fatigue STB data on case-carburized gear specimens made from AISI 8620 (20NiCrMo2-2). The critical locations and the fatigue strengths are compared between the experimental methodology and the analytical results of ISO 6336 Method B, providing insight about the accuracy of ISO 6336 when cyclic plasticity is involved.