Page 165 - Gear Technology Solutions

11.4 Ratio of Involute to Outer Cone

The ratio of involute to outer cone controls the contact movement under load of
spiral bevel gears. This implies the spiral angle is 20° and larger. As the spiral
angle gets smaller, the effect of the ratio of involute to outer cone becomes
less meaningful or is not applicable anymore. For example, straight bevel
gears have an involute outer cone ratio which is infinite. The reason is that the
straight tooth with no curvature has a virtual cutter diameter which is infinitely
large. However, straight bevel gears also have a reasonable adjustability re-
garding V-H-deflections or changes.

The involute to outer cone ratio depends, for Zerol bevel gears, only on the
cutter diameter. The lock-in effect under load and deformation is higher with a
smaller cutter and grinding wheel diameter. However, the criteria here is not
the location of the involute point but only the fact that the lateral movement of
two mating teeth with small radii show more resistance to the movement as if
the radii where larger.

In the case of Zerol an involute to outer cone ratio between 1.5 and 2.5 is con-
sidered standard design practice, with no negative implication to the contact
movement under load.

11.5 Summary

Zerol bevel gears have zero-degree spiral angles at midface with certain de-
viations up to 10° spiral angle. It is recommended not to exceed 8° spiral an-
gle. One advantage of Zerol bevel gears is the absence of large axial forces as
they are present with spiral bevel gears. Certain axial forces still exist because
the pressure angle is the only factor contributing to the axial force Fz in Fig-
ure 9.

Another advantage of Zerol bevel gears is the fact that they can be ground
with dressable grinding wheels, using the same technology as used for spiral
bevel and hypoid gears. For many aircraft applications, Zerols have been cho-
sen versus straight bevel gears several decades ago when it was not possible
to grind Coniflex straight bevel gears. For new aircraft applications today, also
ground Coniflex gearsets are also applied because the advantages regarding
axial forces and grindability are both today given as well.

In cases when the lock-in effect of the convex and the concave flanks are im-
portant due to a deflecting gearbox housing, Zerol gears still have an ad-
vantage versus straight bevel gears. The curved teeth reduce contact move-
ment from toe to heel under load and with deflections effectively.

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