Page 158 - Gear Technology Solutions

to achieve equal values for the slot width at the toe p5 and the tooth thickness
s5. The slot width at the heel p6 is also equal to the tooth thickness at the heel
s6. This geometry delivers a balanced design with a tooth thickness and tooth
depth which changes from heel to toe proportionally with the diameter.

11.2 Can Zerol Gears have a Spiral Angle not Equal to Zero?

Some gear designers like to design their Zerol bevel gears with a spiral angle
which is slightly above zero. This provides a smoother rolling Zerol set and al-
so the lubrication is enhanced because of the inclined contact line and path of
contact direction. The first engagement of a pair of teeth positions the entire
topland length of the pinion along the flank root transition of the gear and vice
versa. This leads to a harsh first tooth line contact which also can interrupt the
separating lubrication film.

®
The Gleason GEMS software allows spiral angles up to 10° while maintaining
the tooth depth proportions. If a deviation from zero-degree spiral angle is de-
sired, then the spiral angle should be limited at 8°. With a small spiral angle in
a Zerol gearset, the extremely large Zerol slot depth taper is reduced. This
phenomenon is covered in the following section.

11.3 A more Desirable Slot Depth Taper

If a Zerol bevel gearset is designed with zero-degree spiral angle, then a large
slot depth taper is the result. The Zerol tooth proportions are not really very
different from spiral bevel gears, but the slot depth taper increases due to the
zero-degree spiral angle.

To achieve a balanced tooth thickness and slot width of pinion and gear and a
nearly equal topland between heel and toe, the slot depth taper for Zerols is
very large. In case of completing, this extreme slot depth taper is required to
achieve a tooth thickness which is equal to the slot width of the opposite mem-
ber. In order to achieve this along the entire face width, from toe to heel, the
depth of the slot changes between toe and heel depending on the pressure
angle sum. That also means that a Zerol with 25° pressure angle and an in-
cluded pressure angle of 50° has much less slot depth taper than it would be
with 20° pressure angle and an included pressure angle of 40° [3]. In many
cases, the face width of Zerol bevel gears is larger than the recommended
33% of the outer cone distance. The reason is that gear designers have to
compensate the missing effect from the face contact ratio, and increase the
face width to achieve the required root bending strength.

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