high speed harmonic drive gearing

8/13/2019 High Speed Harmonic Drive Gearing

1/3

High Speed Harmonic

Drive Gearing

Automation equipment is constantly seekingincreased 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 gearratio 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 showsthe 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 formulafor harmonic drive

gearing is;

FS

GR =

(CS-FS)

with the tooth

differential between the

circular spine (CS) andthe 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 flexsplineadvance for every 80 revolutions of the wave generator in this particular example.


2/3

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 flexstress 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 wererequired.

HD Systems introduced the "S"-tooth design in 1991. This profile, a departurefrom 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 flexsplineand 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 profilebrought 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 furtherprofile 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 of30:1 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 detailedfinite element analysis of the flexspline toothbed, identifying high stress areas and

allowing design modifications for reducing stress to safe and reliable limits.


3/3

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 drivegearing. 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 whereplanetary 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 ofapplications suitable for harmonic drive gearing and in addressing industry's need for

increased throughput and accuracy.

high speed harmonic drive gearing

Documents