Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

Chapter 2 Indirect Power Transfer Devices 97
Figure 2-22 Exploded view of a
typical harmonic drive showing
its principal parts. The flexspline
has a smaller outside diameter
than the inside diameter of the
circular spline, so the elliptical
wave generator distorts the flexspline
so that its teeth, 180º apart,
mesh.
tooth gears that coupled the power of water wheel to millstones for
grinding corn. Those gears offered about a 5 to 1 reduction. In about 300
B.C., Aristotle, the Greek philosopher and mathematician, wrote about
toothed gears made from bronze.
In 1556, the Saxon physician, Agricola, described geared, horsedrawn
windlasses for hauling heavy loads out of mines in Bohemia.
Heavy-duty cast-iron gear wheels were first introduced in the mideighteenth
century, but before that time gears made from brass and other
metals were included in small machines, clocks, and military equipment.
The harmonic drive is based on a principle called strain-wave gearing,
a name derived from the operation of its primary torque-transmitting
element, the flexspline. Figure 2-22 shows the three basic elements of
the harmonic drive: the rigid circular spline, the fliexible flexspline, and
the ellipse-shaped wave generator.
The circular spline is a nonrotating, thick-walled, solid ring with
internal teeth. By contrast, a flexspline is a thin-walled, flexible metal
cup with external teeth. Smaller in external diameter than the inside
diameter of the circular spline, the flexspline must be deformed by the
wave generator if its external teeth are to engage the internal teeth of the
circular spline.
When the elliptical cam wave generator is inserted into the bore of the
flexspline, it is formed into an elliptical shape. Because the major axis of
the wave generator is nearly equal to the inside diameter of the circular