Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

Chapter 2 Indirect Power Transfer Devices 101
Teeth make the difference. Motion between the two gears depends on a
slight difference in their number of teeth (usually one or two teeth). But
drives with gears that have up to a difference of 10 teeth have been devised.
On each revolution of the wave former, there is a relative motion
between the two gears that equals the difference in their numbers of
teeth. The reduction ratio equals the number of teeth in the output gear
divided by the difference in their numbers of teeth.
Two-stage (Figure 2-26) and four-stage (Figure 2-27) gear reducers
are made by combining flexible and solid gears with multiple rows of
teeth and driving the flexible gears with a common wave former.
Hermetic sealing is accomplished by making the flexible gear serve as
a full seal and by taking output rotation from the solid gear.
Figure 2-25 A flexible face-gear
is flexed by a rotating wave former
into contact with a solid gear
at point of mesh. The two gears
have slightly different numbers of
teeth.
Figure 2-26 A two-stage speed reducer is driven by a common-wave
former operating against an integral flexible gear
for both stages.
Figure 2-27 A four-stage speed reducer can, theoretically,
attain reductions of millions to one. The train is both compact
and simple.