Page 303 - Gear Technology Solutions

of the center distance eliminate the basis of the kinematic coupling condition,
which then leads to transmission error. The larger the cross-axes angle, the
larger the transmission error caused by a center distance change. The cross-
axes angle helical pair is meshing in its theoretical position like a spur gear
with extremely large length crowning [4].

21.2 Unique Factors of Cross-Axes Helical Gears

The shaft angle error will cause edge contact and transmission error in spur
gears. In case of 90° shaft angled cross-axis helical gears, the effect of the
shaft angle error is minimized because of the located point contact. Cross-axis
helical gears have a virtual length crowning, which causes point contact and
reduces the sensitivity to shaft angle errors.

The following factors are unique to crossed axes helical gears:

Spur Gears
► Are special cases of cross-axes helical gears
► If the axes cross angle is zero then the gears are spur gears
► The flank surfaces have line contact
► In this case the path of contact is oriented in profile direction (Figure 2, top)
► The gearset is center distance insensitive, but highly shaft angle sensitive

Introduction of Cross-Axes Angle
► If the axes cross angle is not equal to zero, then in the standard case, both
members have the same hand of helix and a helix angle which is 50% of
the shaft angle
► The axes-cross angle is the sum of the two members’ helix angle value

Cross-Axes Angle 90°
► If the cross-axis angle is 90°, then the path of contact has the shape of a
quarter Circle (Figure 2, center)
► The flank surfaces have point contact
► The gearset is sensitive to center distance changes but has little sensitivity
to shaft angle misalignment

Small Cross-Axes Angle
► If the cross-axis angle is small (close to 0°), then the path of contact has the
shape of a slim ellipse (Figure 2 bottom)
► The flank surfaces have point contact which spread already under light load
to contact lines.
► The gearset is almost insensitive to center distance changes but the sensi-
tivity to shaft angle misalignment is medium to high

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