Stress and Failure Analysis of a Second Stage Sun Gear in Planetary Gearbox

Sun gear is one of the most important elements used in planetary gearbox. This paper describes the detailed bending and contact stress analysis and failure analysis on the failed second stage sun gear. The possible failure reasons were analyzed. Insufficient heat treatment of the sun gear could have also resulted in a lower endurance limit of the sun gear case region than that possible in the chosen sun gear material. Chemical analysis was found that the alloy is 15 Ni Cr1 Mo12. The hardness distribution across the sun gear tooth thickness at the pitch line was measured with 100 gm load. The carburized case depth was found to be about 0.7 mm where hardness reaches a value of 632.1 HV. The presence of retained austenite is higher near the surface. This retained austenite may weaken the surface. Scanning electron microscope (SEM) result indicates the critical pitting area is located in the surrounding area of the pitch line.