Helical
gears and spiral bevel gears have an overlap of tooth action in the axial
direction. This overlap adds to the contact ratio. This is in contrast to
spur gears which have no tooth action in the axial direction. Thus, for
the same tooth proportions in the plane of rotation, helical and spiral
bevel gears offer a significant increase in contact ratio. The magnitude
of axial contact ratio is a direct function of the gear width, as
illustrated in Figure 11-3. Equations for calculating axial contact
ratio are presented in Table 11-4.
It is obvious that contact ratio can be increased by either
increasing the gear width or increasing the helix angle.
