Mechanical Engineering Design
Spur gears are used to
transmit rotary motion
between parallel shafts.
Helical gears are used to
transmit motion between
parallel or nonparallel shafts.
This chapter addresses gear geometry, the kinematic relations, and the forces transmit-
ted by the four principal types of gears: spur, helical, bevel, and worm gears. The forces
transmitted between meshing gears supply torsional moments to shafts for motion and
power transmission and create forces and moments that affect the shaft and its bearings.
The next two chapters will address stress, strength, safety, and reliability of the four
types of gears.
Types of Gears
illustrated in Fig. 13–1, have teeth parallel to the axis of rotation and are
used to transmit motion from one shaft to another, parallel, shaft. Of all types, the spur
gear is the simplest and, for this reason, will be used to develop the primary kinematic
relationships of the tooth form.
shown in Fig. 13–2, have teeth inclined to the axis of rotation. Helical
gears can be used for the same applications as spur gears and, when so used, are not as
noisy, because of the more gradual engagement of the teeth during meshing. The inclined
tooth also develops thrust loads and bending couples, which are not present with spur
gearing. Sometimes helical gears are used to transmit motion between nonparallel shafts.
shown in Fig. 13–3, have teeth formed on conical surfaces and are
used mostly for transmitting motion between intersecting shafts. The ﬁgure actually
straight-tooth bevel gears. Spiral bevel gears
are cut so the tooth is no longer
straight, but forms a circular arc.
are quite similar to spiral bevel gears
except that the shafts are offset and nonintersecting.