T-47
reduction ratio in one step, such as 1/100.
z2 – z1
Speed Ratio = –––––––
(5-11)
z1
In Figure 5-2 the gear train has a difference of numbers of teeth of only 1; z1 = 30
and z2 = 31. This results in a reduction ratio of 1/30.
Fig. 5-2 The Meshing of Internal Gear and External Gear
in which the Numbers of Teeth Difference is 1
(z2 – z1 = 1)
SECTION 6 HELICAL GEARS
The helical gear differs from the spur gear in that its teeth are twisted along a helical path
in the axial direction. It resembles the spur gear in the plane of rotation, but in the axial
direction it is as if there were a series of staggered spur gears. See Figure 6-1. This design
brings forth a number of different features relative to the spur gear, two of the most important
being as follows:
Tooth strength is improved because of the
elongated helical wraparound tooth base support.
Contact ratio is increased due to the axial tooth
overlap. Helical gears thus tend to have greater
load carrying capacity than spur gears of the same
size. Spur gears, on the other hand, have a
somewhat higher efficiency.
Helical gears are used in two forms:
Parallel shaft applications, which is the largest
usage.
Crossed-helicals (also called spiral or screw
gears) for connecting skew shafts, usually at right
angles.
Fig. 6-1 Helical Gear
1.
2.
1.
2.
ax
