Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

96 Chapter 2 Indirect Power Transfer Devices
Figure 2-21 Power flow from
two motors combine in a planetary
that drives the cable drum.
the output, will also revolve at the same speed (and in the same direction).
It is as if the entire inner works of the planetary were fused
together. There would be no relative motion. Then, if one motor fails, the
cage will revolve at half its original speed, and the other motor can still
lift with undiminished capacity. The same principle holds true when the
ring gear rotates more slowly than the sun gear.
No need to shift gears. Another advantage is that two working speeds
are available as a result of a simple switching arrangement. This makes is
unnecessary to shift gears to obtain either speed.
The diagram shows an installation for a steel mill crane.
HARMONIC-DRIVE SPEED REDUCERS
The harmonic-drive speed reducer was invented in the 1950s at the
Harmonic Drive Division of the United Shoe Machinery Corporation,
Beverly, Massachusetts. These drives have been specified in many highperformance
motion-control applications. Although the Harmonic Drive
Division no longer exists, the manufacturing rights to the drive have been
sold to several Japanese manufacturers, so they are still made and sold.
Most recently, the drives have been installed in industrial robots, semiconductor
manufacturing equipment, and motion controllers in military
and aerospace equipment.
The history of speed-reducing drives dates back more than 2000
years. The first record of reducing gears appeared in the writings of the
Roman engineer Vitruvius in the first century B.C. He described wooden-