Rim Thickness Factor to Evaluate Fracture Resistance of Spur Gears

In thin rimmed gears running with sufficient bending stress resistance at the tooth-root, fatigue cracks with propagation through the rims, rather than at the tooth fillet, has been observed. Both the ISO and AGMA standards introduce stress-modifying factors (denominated rim thickness factors YB and KB) for the bending stress calculation where the rim thickness is not sufficient to provide full support of the tooth root. However, some new experiences and tests of gears have evidenced that the effects of rim thickness on fracture resistance in spur gear are not completely considered with the ISO and AGMA factors for rim thickness. In this paper, some studies published about the effects of rim thickness on spur gear bending stress are reviewed. Moreover, the influence in the rim thickness factor of both rim backup ratio and web thickness ration is confirmed based on statistical analysis and a better formula for calculation of rim thickness factor is proposed.