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Figure 7: Visible root interference lines and steps

It appears as if the interference lines at the right-side flanks are within the starting
fillet radius, which the right-side blade was not able to finish because it was not
adjusted deep enough. This blade and machine setup error caused the typical in-
terference as shown in Figure 5.

5.5 The Consequences of Interference

It appears that straight bevel gears, manufactured with two-tool generators as
shown in Figures 4 and 7 have been designed with their blade edge radii at or
above the interference limit. Although the root fillet looks cleaner with the larger
blue fillet radius in Figure 5 and suggests higher bending strength, the reality is
different.

The result of an interference is often failure after many service hours of a straight
bevel gear transmission. Some small interferences can polish out and “heal them-
selves” during the first 20 to 100 hours of service. If the interference is more se-
vere, first scratches within the upper area of the root fillet become visible after a
break-in of the gearset. During some fraction of the calculated lifetime of the
gearset, the scratches will initiate the population of hair cracks in the case depth,
most commonly in profile direction. Over time, the load cycles make the hair
cracks grow which results in some cases in tooth fracture (see Figure 8).

What looks like a smooth and fully rounded root radius can reduce the strength
and lead to a catastrophic failure if the chosen radius was larger than the interfer-
ence limit radius.

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