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2011 Edition, October 1, 2011

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The Effects of Helix Angle on Root Stresses of Helical Gears

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Description / Abstract:

The ISO and AGMA Gear Rating Committees have for several years been comparing the results of different rating methods for several sets of gear pairs that have similar normal sections but different helix angles. The analysis presented here uses a finite element code that was developed specifically for gear and bearing contacts to analyze the example gear sets. Analyses are also performed using a more conventional load distribution analysis program. The results for the original gear sets show that the narrow face width gear teeth twist significantly, thus moving the load to one edge of the face width and essentially showing that the example gear sets are highly unrealistic. Yet when analyzed by the ISO and AGMA rating methods, the results do not reflect this twisting action. In an effort to come up with a valid comparison of stresses for different helix angles, three adjustments using wider face widths were attempted. The first uses a narrow load patch in the middle of the tooth pair and results in the stresses increasing with helix angle. The second scheme again uses a wider face width, but with perfect involutes. Edge effects result in the peak stresses being near the ends of the face width. The third method, which uses the wide face width with teeth that have some lead crown and tip relief, gives the most reasonable results, with the root stresses being at a maximum in the center region of the tooth face widths. The paper compares each of the results to earlier analyses performed by others using both the AGMA and ISO calculations.