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65
Table IV.4 Helical Gears on Parallel Shafts
To find: Formula
Center distance C
ψ
+
=
cos2
dn
Gp
P
NN
C
Pitch diameter D
ψ
==
cos
dnd
P
N
P
N
D
Normal diametral pitch P
dn
ψcos
d
P
Normal circular pitch p
n
ψ
cos
p
Pressure angle
φ
ψ
φ
−
cos
tan
tan
1
n
Contact ratio m
P
n
bppbGG
p
F
p
CDDDD
ψ
+
φ
φ+−+−
sin
cos2
sin2
22
0
22
0
Velocity ratio m
G
p
G
p
G
D
D
N
N
=
BEVEL GEARING - GENERAL
Bevel gears may be classified as follows:
• Straight bevel gears
• Spiral bevel gears
• Zerol bevel gears
• Hypoid gears
• Spiroid gears
A straight bevel gear was illustrated in Fig. III-3. These gears are
usually used for pitch-line velocities up to 1000 ft/min (5 m/s) when the noise
level is not an important consideration. They are available in many stock
sizes and are less expensive to produce, than other bevel gears, especially in
small quantities.
A spiral bevel gear is shown in Fig. IV-1; the definition of the spiral
angle is illustrated in Fig. IV-2. These gears are recommended for higher
speeds and where the noise level is an important consideration. Spiral bevel
gears are the bevel counterpart of the helical gear; it can be seen in Fig. IV-1
that the pitch surfaces and the nature of contact are the same as for straight
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