Bending Fatigue Behavior of Carburized Gear Steels: Planetary Gear Test Development and Evaluation

Little information is available concerning the bending fatigue behavior of helical gears with tall thin teeth and high contact ratios, particularly for planetary pinions which are subjected to fully reversed loading. The most common methods to acquire gear bending fatigue data are either through a four-square recirculating power arrangement or unidirectional single tooth bending experiments on standardized spur gears. There are some advantages to these test methods, but they generally do not represent actual operating conditions of a planetary gear environment. The purpose of this study was to develop a bending fatigue test for planetary pinions in automatic transmissions which would better represent actual operating conditions. The new testing procedure was used to evaluate the bending fatigue behavior of three gear steel/processing combinations. The results from the planetary gear testing is compared with laboratory four-point bending experiments. A finite element analysis of the test gears was completed to determine the actual stress in the root fillet for the test gear at different loads. The finite elment results were compared with traditional approaches of stress determination. The test gear material evaluated showed about a 20% difference in performance which was also reflected in the 4-point bend testing. Finite element and traditional stress calculation approaches predicted different failure locations and stresses.