Page 197 - Analysis and Design of Machine Elements

Gear Drives
shafts that are perpendicular to each other. A worm resembles a screw, in meshing with 175
a wormgear, whose teeth are similar to those of helical gear, except that they are con-
toured to envelop the worm. Both radial and thrust loads are imposed on the supporting
bearings of worm and wormgear shafts. Worm drives accomplish a large speed reduc-
tion ratio compared with other types of gear drives. The details about worm gearing will
be discussed in Chapter 9.
According to the hardness of gear tooth surface, we have soft tooth surface gears,
whose hardness is less than 350HBW and hard tooth surface gears, whose hardness is
greater than 350HBW.
Gears can also be classified by their working conditions. If gears and shafts are
enclosed in a housing, they are enclosed gearings; otherwise, they are open. Enclosed
gearings predominate in engineering practice, as in such arrangement, gears are in a
sealed environment that provides protection against contamination.

8.1.3 Geometry and Terminology
While transmitting power and motion between rotating shafts, the teeth of the driving
pinion mesh accurately in the spaces between the teeth on the driven gear. To make
further discussion more meaningful, the assigned terminology and defined geometric
variables of spur gears are shown in Figure 8.1 and introduced next.
1) Reference circle and reference diameter, d. Reference circles are circles that have a
standard module and pressure angle.
2) Pitch circle and pitch diameter. Pitch circles are two imaginary circles at a tangent
to pitch point P. The diameter of a pitch circle may be different from the reference
diameter if meshing gears are required to respond to centre distance variations. This
book is limited to discussion on standard installation where the pitch circle and
reference circle coincide with each other. Therefore, pitch diameter equals reference
diameter.
3) Base circle and base diameter, d The base circle is the circle from which an involute
b.
tooth curve is developed. The base diameter never changes. From Figure 8.1b, we
show the relationship between pitch diameter d and base diameter d as
b
d = d cos (8.1)
b
4) Addendum circle, addendum circle diameter, d , and addendum, h Addendum h a
a
a.
is the radial distance between the top land and pitch circle. Addendum circle is
obtained by adding the addendum to the pitch radius r.
5) Dedendum circle, dedendum circle diameter, d and dedendum, h f . Deden-
f
dum h is the radial distance from the bottom land to the pitch circle. The
f
dedendum circle is obtained by subtracting the dedendum from the pitch
radius r. The involute tooth profile extends only as far as the base circle. The
portion of the tooth inside the base circle cannot participate in conjugate action
and therefore must be shaped to provide tip clearance for the mating teeth. The
sum of addendum and dedendum is tooth height h.
6) Clearance circle and clearance, c. The clearance circle is the circle tangent to the
addendum circle of the mating gear. The dedendum is typically made slightly larger
than the addendum so as to provide clearance c between the tooth tip of one gear
and the bottom land of the mating gear. A root fillet is used to combine the tooth
flank and the bottom land.