Development of high precision and strength sintered ferrous alloy gear by rolling -3rd report improvement of tooth root bending fatigue strength of sintered ferrous alloy gear by modified tool

Surface finish rolling experiments were carried out using P/M spur gears of 0.6Mo-0.2Mn pre-alloyed sintered steel and of 0.8Mo-0.2Mn partially alloyed sintered steel with the density of 7.10 Mg/m³. A CNC form rolling machine of axis intersection type was employed, and a modified screw-shaped rolling tool was prepared: the modified concave tooth profile optimized by numerical tooth profile analysis for simultaneous rolling of tooth and fillet surface. Then, porosity distributions in the surface layer of rolled gear teeth were determined and the effects of the amount of tool radial feed Ao on the selective surface densification behaviors were evaluated. Besides, single tooth bending fatigue test was conducted. Gears with good tooth profiles were obtained by using the modified tools. As a result, the porosities in both the tooth flank and the fillet at the root were reduced to the target level at R ₀ = 600μn in simultaneous rolling process. Furthermore, the single tooth bending fatigue durability was much improved to about 60% of the case hardened wrought steel gears of SCM415.