Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

Chapter 2 Indirect Power Transfer Devices 99
Because of the elliptical shape of the wave generator, full tooth
engagement occurs only at the two areas directly in line with the major
axis of the ellipse (the vertical axis of the diagram). The teeth in line with
the minor axis are completely disengaged.
As the wave generator rotates 90° clockwise, as shown in Figure 2-24B,
the inside arrow is still pointing at the same flexspline tooth, which has
begun its counterclockwise rotation. Without full tooth disengagement at
the areas of the minor axis, this rotation would not be possible.
At the position shown in Figure 2-24C, the wave generator has made
one complete revolution and is back at its 12 o’clock position. The inside
arrow of the flexspline indicates a two-tooth per revolution displacement
counterclockwise. From this one revolution motion the reduction ratio
equation can be written as:
FS
GR =
CS − FS
where:
GR =gear ratio
FS = number of teeth on the flexspline
CS = number of teeth on the circular spline
Example:
FS = 200 teeth
CS = 202 teeth
GR =
200
202 − 200
= 100 : 1 reduction
As the wave generator rotates and flexes the thin-walled spline, the teeth
move in and out of engagement in a rotating wave motion. As might be
expected, any mechanical component that is flexed, such as the flexspline,
is subject to stress and strain.
Advantages and Disadvantages
The harmonic drive was accepted as a high-performance speed reducer
because of its ability to position moving elements precisely. Moreover,
there is no backlash in a harmonic drive reducer. Therefore, when positioning
inertial loads, repeatability and resolution are excellent (one arcminute
or less).
Because the harmonic drive has a concentric shaft arrangement, the
input and output shafts have the same centerline. This geometry contributes
to its compact form factor. The ability of the drive to provide
high reduction ratios in a single pass with high torque capacity recommends
it for many machine designs. The benefits of high mechanical