Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

100 Chapter 2 Indirect Power Transfer Devices
efficiency are high torque capacity per pound and unit of volume, both
attractive performance features.
One disadvantage of the harmonic drive reducer has been its wind-up or
torsional spring rate. The design of the drive’s tooth form necessary for the
proper meshing of the flexspline and the circular spline permits only one
tooth to be completely engaged at each end of the major elliptical axis of
the generator. This design condition is met only when there is no torsional
load. However, as torsional load increases, the teeth bend slightly and the
flexspline also distorts slightly, permitting adjacent teeth to engage.
Paradoxically, what could be a disadvantage is turned into an advantage
because more teeth share the load. Consequently, with many more
teeth engaged, torque capacity is higher, and there is still no backlash.
However, this bending and flexing causes torsional wind-up, the major
contributor to positional error in harmonic-drive reducers.
At least one manufacturer claims to have overcome this problem with
redesigned gear teeth. In a new design, one company replaced the original
involute teeth on the flexspline and circular spline with noninvolute
teeth. The new design is said to reduce stress concentration, double the
fatigue limit, and increase the permissible torque rating.
The new tooth design is a composite of convex and concave arcs that
match the loci of engagement points. The new tooth width is less than the
width of the tooth space and, as a result of these dimensions and proportions,
the root fillet radius is larger.
FLEXIBLE FACE-GEARS MAKE EFFICIENT
HIGH-REDUCTION DRIVES
A system of flexible face-gearing provides designers with a means for
obtaining high-ratio speed reductions in compact trains with concentric
input and output shafts.
With this approach, reduction ratios range from 10:1 to 200:1 for single-stage
reducers, whereas ratios of millions to one are possible for
multi-stage trains. Patents on the flexible face-gear reducers were held
by Clarence Slaughter of Grand Rapids, Michigan.
Building blocks. Single-stage gear reducers consist of three basic
parts: a flexible face-gear (Figure 2-25) made of plastic or thin metal; a
solid, non-flexing face-gear; and a wave former with one or more sliders
and rollers to force the flexible gear into mesh with the solid gear at
points where the teeth are in phase.
The high-speed input to the system usually drives the wave former.
Low-speed output can be derived from either the flexible or the solid
face gear; the gear not connected to the output is fixed to the housing.