High Speed Harmonic Drive Gearing - Electromate Industrial Sales ...

High Speed Harmonic
Drive Gearing
Automation equipment is constantly seeking
increased throughput-it's a never ending requirement.
For power transmission components, this translates
into either higher motor speeds or lower gear ratios.
Most applications desire lower gear ratios since life,
wear, and noise generally increase with higher speed
option.
Harmonic drive gearing is a popular choice for many
automation applications because of its superior power
transmission qualities, which include zero backlash, high torsional stiffness,high
positional accuracy, and torque-to-weight ratios. Until recently, however, the lowest gear
ratio available has been 50:1. New proprietary tooth design technology has substantially
reduced this old limit, making ratios of 30:1 possible.
To see how this breakthrough in harmonic drive gearing technology occurs, we'll first
need to review the fundamentals of harmonic drive gearing tooth action. Figure 1 shows
the three components of harmonic drive gearing: the circular spline, the flexspline, and
the elliptical wave generator. Figure 2 shows the operating principle for this mechanism
(flexspline deflection is greatly exaggerated for clarity).
The gear ratio formula
for harmonic drive
gearing is;
GR =
FS
(CS-FS)
with the tooth
differential between the
circular spine (CS) and
the flexspline (FS)
accounting for the gear
ratios (GR) achieved.
For example, to satisfy
the formula, an 80:1
ratio would have 160 teeth on the flexspline and 162 teeth on the circular spline. The
two-tooth advance for every revolution of the elliptical generator, or one flexspline
advance for every 80 revolutions of the wave generator in this particular example.
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Harmonic drive gears operate by engaging multiple teeth at the major axes while
disengaging them at the minor axes of the wave generator ellipse. To minimize flex
stress in the flexspline, our goal is to keep ellipicity to a minimum, which leads us to
small tooth height. Achieving a 50:1 ratio, for example, means producing a 100-tooth
circular spline. As the number of teeth decreases, their size increases; consequently,
ellipicity must increase to provide the required clearance at the minor axis. In older
designs, ratios below 80:1 more often than not made use of a four-tooth difference,
usually doubling the number of teeth to 200 for a 50:1 ratio in order to reduce the tooth
size. Achieving a ratio of approximately 30:1 meant changes in the tooth design were
required.
HD Systems introduced the "S"-tooth design in 1991. This profile, a departure
from the old involute-tooth form, brought many advantages to harmonic drive gearing,
such as increased torque capacity, stronger teeth, longer life, and higher torsional
stiffness. Figure 3 shows the "S"-tooth profile and the relative motions of the flexspline
and circular spline in operation. Developing the new tooth form required significant
design analysis, resulting in a profile that lets high numbers of teeth in simultaneous
contact share in carrying the load while keeping a low ellipticity. This "S"-tooth profile
brought another benefit to harmonic drive gearing by permitting a reduction in ratio down
to 50:1, using the two-tooth difference.
Nonetheless, getting 30:1 ratios with a two-tooth difference required further
profile modifications. In the past, the axisymmetric nature of harmonic drive gearing
permitted the use of two-dimensional modeling, which was sufficient to study the
kinematics and stress distribution for ratios of 50:1 and higher. However, the demands of
30:1 ratios required a degree of modeling sophistication that two-dimensional models
could not deliver. We developed a complex three-dimensional simulation of tooth
engagement so that parameters of the "S"-tooth profile could be studied, examined, and
modified for optimum engagement and minimum wear. This procedure included detailed
finite element analysis of the flexspline toothbed, identifying high stress areas and
allowing design modifications for reducing stress to safe and reliable limits.
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Figure 4 shows the new "S"-tooth (for 30:1) action in the circular-spline/flexspline
interface.
The result is a new profile that permits 30:1 gear ratios using the two-tooth
difference, minimizing ellipticity and maintaining low flexspline stresses. In addition,
this new "S"-tooth profile still provides all of the benefits of "S"-tooth harmonic drive
gearing. This technology can be incorporated into the complete into the complete range
of products, including component sets, gearheads, and servo actuators. With the new
lower ratios available, harmonic drive gearing may now be used in applications where
planetary or other low-ratio gears might have been used previously, thus allowing a
reduction in package size and weight while increasing positional accuracy and achieving
zero backlash. This new tooth profile is a major step forward in expanding the range of
applications suitable for harmonic drive gearing and in addressing industry's need for
increased throughput and accuracy.
Sold & Serviced By:
ELECTROMATE
Toll Free Phone (877) SERVO98
Toll Free Fax (877) SERV099
www.electromate.com
sales@electromate.com