5.7 Helical Tooth Proportions
These follow the same standards as those for spur gears. Addendum, dedendum, whole depth and clearance are the same regardless of whothor measured in tho piano of rotation er the normal piano. Pressure angle and pitch are usually specified as standard values in tho normal plane, but there are times when they are specified standard in the transverse plane.
5.8 Parallel Shaft Helical Gear Meshes
Fundamental information for the design of gear
meshes is as follows:
5.8.1 Helix angle — Both gears of a meshed
pair must have the same helix angle. However, the
helix directions must be opposite, i.e., a left-hand mates with a right-hand helix.
5.8.2 Pitch dIameter — This is given by the same expression
as for spur gears, but if the normal
pitch is involved it is a function of the helix angle. The expressions are:
D = N =
N
(30)
Pd Pdn cos y
5.8.3 Center distance — Utilizing equation 30, the
center distance of a helical gear mesh is:
C = ( N1+N2
)
(31)
2 Pdn cos y
Note that for standard parameters in the normal plane, the center distance will not be a standard value compared to standard spur gears. Further, by manipulating the helix angle (y) the center distance can be adjusted over a wide range of values. Conversely, it is possible
a. to compensate for significant center distance
changes (or erçors) without changing the speed ratio between parallel geared
shafts; and
b. to alter the speed ratio between parallel geared shafts without changing
center distance by manipulating helix angle along with tooth numbers.
5.8.4 Contact Ratio — The
contact ratio of helical gears is enhanced by the axial overlap of the teeth. Thus, the
contact ratio is the sum of the transverse contact ratio, calculated in the same manner as
for spur gears (equation 18), and a term involving the axial pitch.
(mp)total
= (mp)trans + (mp)axial
(32)
where
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