T-111
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.
Table 11-4 Equations for Axial Contact Ratio of Helical and Spiral Bevel Gears, eb
NOTE: The module m in spiral bevel gear equation is the normal module.
SECTION 12 GEAR TOOTH MODIFICATIONS
Intentional deviations from the involute tooth
profile are used to avoid excessive tooth load
deflection interference and thereby enhances load
capacity. Also, the elimination of tip interference
reduces meshing noise. Other modifications can
accommodate assembly misalignment and thus
preserve load capacity.
12.1 Tooth Tip Relief
There are two types of tooth tip relief. One
modifies the addendum, and the other the
dedendum. See Figure 12-1. Addendum relief is
much more popular than dedendum modification.
Type of Gear
Equation of Contact Ratio
bsinb
–––––
pmn
Re btanbm
––––––– –––––––
Re – 0.5b pm
Helical Gear
Spiral Bevel Gear
Example
b = 50, b
= 30°, mn = 3
eb = 2.6525
From Table 8-6: Re = 67.08204, b = 20,
bm = 35°, m = 3, eb = 1.7462
(b) Dedendum Modification
Fig. 12-1 Tip Relief
Fig. 11-3 Axial (Overlap) Contact Ratio
b
px
pn
b
