Page 80 - Gear Technology Solutions

5.7 Strength Advantages due to Hourglass Effect

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The hourglass effect in the root of straight bevel gears cut with Coniflex Plus cut-
ters provides the shape of a dam. The dam-effect is one of the reasons why the
convex side of spiral bevel ring gears is preferred as the side with the major load
(drive side).

Figure 11: Hourglass root width provides dam effect

The dam effect increases the root bending strength, which is proven with FEM
calculations and field tests with spiral bevel and hypoid gears.

5.8 Blade Radii Optimization

For straight bevel gears, more than for spiral bevel gears, the criteria of kinematic
undercut exists. The projected tooth in Figure 12 shows a gear tooth with the ac-
tive working area drawn in blue. Both the lost area above the pinion root and the
lost area above the gear root should be avoided. First the question of the nature of
the lost area must be answered. Possible causes are:

· Edge radius too large - causes interference

· Kinematic undercut - causes possible interference

· Physical undercut - weakens teeth but causes no interference

The lost areas indicate the three fundamentally different mesh deficiencies, listed
above. The first lost area between the topland of the gear and the red border line
exists because the gear topland corner will not mesh above the red line. The sec-
ond lost area between the end of the gear root fillet and the green line exists, be-

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