Refinements in Root Stress Predictions for Edges of Helical Gear Teeth


Industry demands for higher power density gearboxes to improve product performance require utilization of the load carrying capability of every inch of available face width. As a result, load distributions and the resulting root stresses are of considerable importance. Current analytical methods include three dimensional finite element analysis (FEA) and combinations of Wellauer and Seireg's moment image method with two dimensional boundary element analysis or beam bending formulas. Three dimensional FEA, although reliable, is time consuming and of great expense in comparison to the use of classical techniques and approximations. In helical gears these approximations give reasonable estimates of root stress distributions along the face width but lack the accuracy to design to engineering limits. Based upon finite element and experimental results, a discrepancy in the approximate methods is the stiffness change in the normal plane associated with the ends of helical gear teeth. Tooth stiffness is lower at the acute edge, where the normal force associated with tooth contact protrudes beyond the transverse edge of the gear on the back side of the tooth. The opposite result occurs at the obtuse edge, where the normal force is not producing beyond the transverse edge of the tooth. Analytical and experimental studies for a limited number of cases have been completed. The experimental results are used to verify the simplified three dimensional FEA parametric study on the effect of helix angle. The parametric study results, when completed, will be used to determine a root stress correction factor. The focus of this paper is on the initial parametric results and experimental studies, with an introduction to possible correction techniques. The correction factors are currently being researched and will be the topic of future publications.
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