(^698) A Textbook of Engineering Mechanics
- Dedendum circle. The part of a gear inside the pitch circle is called dedendum, and the
circle (concentric with the pitch circle) drawn through the bottom of the teeth is known
as dedendum circle or root circle shown in Fig. 34.2. The diameter of the dedendum
circle is known as dedendum circle diameter. - Pitch. The centre to centre distance between any two teeth, measured along the arc of
the pitch circle, is called pitch of the toothed wheel. Mathematically, pitch,
p=
d
Tπwhere d= Diameter of the pitch circle, and
T= No. of teeth on the wheels.
A little consideration will show, that the two wheels will only gear together smoothly, if
the two wheels have the same pitch.- Clearance. When the two gears mesh together, the addendum of one gear projects inside
the dedendum of the other. For smooth working, a small space is left between the adden-
dum circle of one gear and the dedendum circle of the other. This space is known as
clearance. - Depth of the tooth. The radial distance between the addendum circle and dedendum
circle of a gear is known as depth of the tooth. - Face of the tooth. The surface of a tooth along its width, and above the pitch circle is
known as face of the tooth as shown in Fig. 34.2. - Flank of the tooth. The surface of a tooth along its width, and below the pitch circle is
known as flank of tooth as shown in Fig. 34.2. - Face width of the tooth. The width of a tooth, measured parallel to its axis, is known as
face width of the tooth as shown in Fig. 34.2.
34.5. TYPES OF GEARS
Though there are many types of gears, yet the following are important from the subject point
of view :
1.External gears. Sometimes, the gears of the two shafts mesh externally with each other
as shown in Fig. 34.3.
Fig. 34.3. External gearing.
Such a type of gear is called external gearing. The larger of these two wheels is called
spur wheel and the smaller one is called pinion. In an external gearing, the motion of the
two wheels is always unlike, as shown in Fig. 34.3.