Reducing Gear Size for Compact Transmission Design By Selecting Steel Grade and Heat Treatment Process with Computer Modeling

Gears are the most important components for transmissions. In many situations, the size of a transmission design needs to be reduced without decreasing power density. One of the most effective methods is to reduce the gear size while keeping the same output torque capacity. In general, gears used in heavy load conditions are made of steel, and gear tooth residual surface stresses are critical to the fatigue performance. Compressive residual stresses in the critical region of a gear improve its fatigue performance. However, many steel gears are not processed to obtain residual surface compression, or the benefit of residual compression is not considered in the gear and transmission design. In this paper, virtual computer models using DANTE® software are used to assist with achieving gear size reduction by including steel grade hardenability and heat treatment in the design process. In this study, the original gear is made of AISI 4340 and oil quenched and tempered. The gear with reduced size is made of AISI 8620.  Carburization and oil quenching are used to introduce residual compression to the root fillet of the gear tooth; this is the most critical region concerning high cycle bending fatigue performance. By taking advantage of the residual compression at the root fillet, the gear with reduced size can deliver the same torque load while having better bending fatigue performance relative to the original gear.