Page 199 - Analysis and Design of Machine Elements

Gear Drives
thickness. It is the arc length equal to the circumference of pitch circle divided by 177
the number of teeth, expressed as
d
p = (8.3)
c
z
9) Base pitch, p Base pitch is the arc length equal to the circumference of base circle
b.
divided by the number of teeth
d b
p = (8.4)
b
z
Combined with Eq. (8.1), the relation between base pitch p and circular pitch p is
b c
expressed as
p = p cos (8.5)
b c
Mating teeth must have the same base pitch and circular pitch.
10) Module, m. Module is the amount of pitch diameter per tooth. The module is the
index of tooth size in SI units. A higher module number denotes a larger tooth and
vice versa.
d
m = (8.6)
z
Combined with Eq. (8.3), circular pitch, p ,is
c
p = m (8.7)
c

11) The diametral pitch. The diametral pitch is the ratio of the number of teeth to the
pitch diameter, which is the reciprocal of module.
12) Pressure angle, . The pressure angle is the angle between the common tangent to
∘
the pitch circles and the pressure line. The preferred pressure angle is 20 .For the
proper meshing of a pair of gears, they must have same module and same pressure
angle.
13) Circular tooth thickness and width of tooth space. The circular tooth thickness
and the width of tooth space are measured along the pitch circle, each being
nominally equal to p /2. The tooth space is usually made slightly larger than
c
the tooth thickness in order to provide a small amount of backlash for smooth
operation.
14) Profile shift coefficient, x. To avoid undercutting, or to adapt to required centre dis-
tance, a gear cutter needs to move a distance xm away from standard assembly
position and x is the profile shift coefficient. Positive profile shift coefficient results
in a thicker and stronger tooth.
15) Backlash. Backlash is the difference between tooth space and tooth thickness. It
tends to eliminate tooth jamming, compensate for machining errors and heat expan-
sion and promote proper lubrication, but may also cause impact, vibration and
noise. Backlash is obtained by reducing the tooth thickness or by increasing the
centre distance between mating gears.
16) Face width, b. Face width is the length of tooth in the axial direction.
17) Top land and bottom land. The top land is the top surface of gear, while the bottom
land is the surface of gear between the flanks of adjacent teeth.