HELICAL GEAR DESIGN FOR BENDING FATIGUE

Edward E. Osakue, Lucky Anetor

Abstract: A bending stress capacity model explicit in the representation of helical gear design parameters is developed. The model accounts for the bending stresses from both tangential and axial loads on the gear tooth as well as the direct shear stresses from these loads. A shear stress concentration factor is incorporated to account for the geometric influence of the fillet at the root of the gear. The model clearly reveals the direct influence of the base helix angle on the bending stress and normal module size estimates. Several design examples of bending stress estimates from different references are computed using the new bending stress capacity model and compared with American Gear Manufacturers Association (AGMA) estimates. The examples considered cover a wide range of the helix angles which spans 15o to 41.41o and different normal pressure angles of 20o and 25o . In the examples considered, the shear stress contribution to the total root bending stress varies from about 24% to 36%. This is quite significant, so that ignoring it may result in early failure of helical gears in bending fatigue. The percentage variances between the new model and AGMA bending stress estimates are within -7% to 10% in this study. Therefore the results show very favorable comparisons with AGMA model. The new model appears to give slightly higher bending stress values in general so that for preliminary design, it offers the advantage of providing conservative solutions. The bending stress capacity model was modified for design sizing and its application is demonstrated in Example 6. The model may be used for bending fatigue design of spur gears if the helix angle is taken as 0o .

Keywords: Service Load Factor, Bending Fatigue Strength, Helix Angle, Base Helix Angle

DOI: https://doi.org/10.15623/ijret.2017.0603002