Page 211 - Analysis and Design of Machine Elements

Gear Drives
their teeth. Spur gears have straight teeth aligned with the axis of the gear, while helical 189
gears have inclined teeth at a helix angle with respect to the axis of the gear. A spur
gear may be regarded as a special helical gear with a zero helix angle.
The teeth of a helical gear may be either right- or left-hand. When a helical gear is
lying on a flat surface, if the teeth lean to the right then it is a right-hand helical gear
and if the teeth lean to the left it is a left-hand helical gear. Two externally mating helical
gears on parallel shafts must have the same helix angle but with the opposite hand. An
internal helical gear and its pinion must be of same hands.

8.4.1 Geometry and Terminology
8.4.1.1 The Geometry of a Helical Gear
The basic terminology of helical gears is substantially the same as that of spur gears.
Additional variables are introduced to account for different aspects due to helix angle,
as showninFigure8.6.
The description of the geometry of a helical gear is related to three primary planes;
namely, the transverse plane, the normal plane and the tangential plane. The transverse
plane or rotation plane is perpendicular to the axis of rotation. The transverse circu-
lar pitch p and transverse pressure angle are measured in the transverse plane. The
t
t
transverse plane contains the involute feature and is mainly used for the calculation of
geometrical dimensions. The normal plane is perpendicular to the curved surface of
teeth. The normal circular pitch p and normal pressure angle aremeasuredinthe
n
n
normal plane. The normal plane is used for strength analysis and standard values of
module and pressure angle are defined in the normal plane; for example, normal pres-
∘
sure angle = 20 . The normal plane contains the geometry needed for cutting teeth,
n
pn

p t
Addendum circle p b Dedendum
Pitch circle
Addendum
Dedendum circle
Base circle

r a r r b
r f

O
Figure 8.6 Geometry and terminology of a helical gear.