Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVED MOTION TRANSMITTING MECHANISM
The present invention relates to motion transmitting mechanisms
and particularly to gearing in which relative motion occurs between
an ellipsoidal wave generator and a ~lexible tubular gear or flexspline
S and a rigid gear or circular spline. The motion occurs by introducing
and advancing a strain wave in the flexspline by inserting and rotating
the wave generator into an area of contact or preferably a plurality of
areas of contact between the respective gears and advancement of the
area of contact. More specifically, the invention relates to an improved
10 flexspline diaphragm adapted to maintain an acceptable operational stress
level associated with the diaphragm tlexin~ and minimize the stress
produced in the flexspline diaphragm due to the application of an axial
force which occurs when the wave generator is inserted into the flexspline
bore .
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DFSCRIYTION OF TE~E ~'RIOR ART
Generally, flexsplines are formed of a tubular flexible member
with external teeth located near one of the ends, and a diaphragm extend-
5 ing radially inward, affixed to the opposite end of the tubular member.The diaphragm per~orm~ two functions, it attenuates the axial excursion
of the tubular member where it is re~trlcted into a circular shape by
the diaphragm and it provides a means to attach the flexspline to a structually
rigid member which can either be a rotary qutput or held ~tationary.
10 As presently u6ed in strain wave gearing devices, flexsplines are de9igned
with consideration being given to the operating stresses. These operating
stresses include the radial deflection stress in the tubular portion of
the flexspline and the axial de~lection stress in the diaphragm. The
axial deflection stress is caused by the diaphragm end of the tubular
15 member not remaining in plane, but rather scalloping when the tooth
end of the tubular member is deflected from a round condition to an ellipsoidal
shape. Along the major axis the scalloping moves towards the diaphragm
while along the minor axis the scalloping moves away from the diaphragm.
AB the ellipsoidal shape iR rotated, the scalloping pattern imposed
20 in the diaphrsgm by the tubular portion of the flexspline also rotates,
the maximum excur~ioD being between the major and minor axis. To
minimize the axial flexing stress in the diaphragm in the pr.ior art, its
thickness was held to a minimum, but was maintained thick enough to
transmit the peak output torque that the ~lexspline was designed to handle.
25 The thickness of the diaphragm previou:~ly used was a nomiDal 0. 896
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of the internal bore of the flexspline tube and, in general, that thickness
was adequate to withstand the imposed operating stres~es.
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SUMMARY OF TI-IE INVENTION
From time to time flexsplines have failed at the diaphragm end
during operation and in the region between the mounting portion and
S the flexing portion. Since the failures occurred during operation it
was ~uspected in the past that the axial scalloping stress caused the
failure and the most obvious way to achieve a reduction in this operating
stress would be to reduce the thickness of the diaphragm. However,
it has been found that at times the assembler of the system inadvertantly
10 applies excessive axial force to fit the tubular portion of the flexspline
to the ellipsoidal wave generator. Since such axial force is reacted
in the flexspline diaphragm, any excessive axial force that i9 applied
will cause the diaphragm to yield and dish and thus residual stresses
can be retained in the diaphragm which can cause it to Eail during subsequent
15 operation. It ha9 been determined that an improved diaphragm can be
obtained by increasing the thickness to a predetermined range which
will slightly increase the operating flexure stress over the present level
but which will significantly decrease the stress introduced during the
assembly operation thus minimizing the possibility of residual stresses
20 remaining after the assembly operation, which was not previously recognized.
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According to a broad aspect, the present
invention provides a motion transmitting mechanism which
has a ring gear, a tubular flexible gear with a diaphragm
disposed adjacent one end thereof. The diaphragm is
attached to a shaft and a wave generator for progressively
forcing the flexible gear into engagement with the ring
gear. In accordance with the invention, the thickness of
said diaphragm is between about 1.0 and 2.0% of the bore
diameter of the tubular flexible gear.
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DESCRIPTION OF TME DRAWINGS
Figure 1 is a side elevational view, partially in cross-section
of a motion transmitting device according to the present invention.
Figure 2 i8 a cro6s-sectional view of a flexspline showing an
unstrained position and two deflections due to the revolution of the elliptical
wave generator.
Figure 3 i8 a cross-sectional view of a wave generator and a
flexspline showing the forces exerted as the flexspline and the wave
generator are being fitted together and Figure 3A is another embodiment
of the means for joining the output shaft to the flexspline.
Figure 4 are curves illustrating the deflection stress exerted
upon diaphragms of varying thicknes5es and also illustrating the axial
force stresses involved as the wave generator and flexspl;ne are be~ng
IS fitted together.
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DESCR.IPTION OF THE PREF`ERRED EMBODIMF:NT
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Referring now to E;igure 1, a motion transmitting mechanism i8
shown which has an input shaft 30 journalecl at 31 in a housing portion
32. Housing 32 is secured to housing 54 with bolts 55 after the device
has been assembled. A wave generator or strain inducer 35 iB locked
on the shaft by nut 33 engaged with threads 34. Desirably, the wave
generator is ellipsoidal in shape and flexible inner race 3fi i6 pressed
on to it with the inner race assuming the ellipsoidal shape of the wave
lO generator 35. The race 36 receives bearing ballfl 37 of uniform diameter
which engage within an outer race 38 that is deflectable and is fitted
on the inside of the tubular portion 44 of a flexspline gear 43 having
- exterior teeth 41 meshing with interior teeth 42. These interior teeth
42 form a ring gear which is internally disposed on the interior surface
IS of housing 54. The flexspline includes the deflectable tubular portion
44 attached to diaphragm 51 which in turn is connected to output shaft
45. Output shaft 45 is journaled in bearings 46 that are disposed in
a housing 47. The interior end of output shaft 45 is secured to the diaphragm
51 by means of a pair of collars 52 a and 52 b that engagre both sides
20 of diaphragm 51 and are bolted together by boltfl 53. The peripheries
of collars 52 a and b derine an inflexible portion of diaphragm 51 while
the remainder of diaphragm 51 is relatively f1exible.
In opersltion, as the input ~haft 30 turns it turns the wave generator
35 through bearings 3? and the race 38. ï'he flexspline 44 i~s elastically
25 deflected into enga~ement at two spaced point~ in the case of a two lobe
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unit or three in the cafie of a three lobe unit wil.h respect to the interior
teeth 42 attached to housing 5~. The motion of the input shaft 30 causes
relative rotation of the output shaft 45 throu~h the tubular portion 44,
flexspline 43 and diaphragm 51.
When the tubular member 44 is fitted around the outer race 38,
with the eLliptoidal snape im,posed in it by the wave generator 35, a
certain amount of ~lexing occur~3 in the flexible portion of diaphra~m
51. The ~pace into which the flexspline must fit is only slightly larger
than the flexspline itself and æince the wave generator 35 has an ellipsoidal
10 shape, the tubular portion 44 i8 required to a~3sume the same ellipsoidal
shape In other words, there is radial deflection at the open end of
the tubular portion 44 of the flexspline and this radial deflection is graduallyattenuated along the length until it becomes essentially circular at the
diaphragm end 51. F3ecause of the attenuation of this deflection within
15 the tubular portion of the flexspline, a scalloping condition occurs which
is transmitted to diaphragm 51 and causes diaphragm ~l to flex axially
and produce an a~ial stress.
In Figure 2, a cross-secti.on is shown of a member in three positions
relative to the position of the wave ~enerator . I he amount of deflection
20 is exaggerated, omewhat, to illustrate the deflection9. The section
rnidway between the major and mjnor axes is shown as A (which i8 the
sarne a~ a ~ection of the undei1.ected part) . The section through the
major axis B is radially deflected upwardly at the open end and axially
displaced towarcls the diaphragm 3. The sectic-n through the minor
~5 axis C is radially deflected inwards toward the center c-f the open end
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and axially displaced away from the diaphragm 3. As shown, the axial
deflection x at the open end of the flexspline is equivalent to the axial
deflection x at the flexible portion of the diaphragm.
When the elliptical wave generator is fitted into a flexspline bore
5 and the wave generator iq turned, the deflection within the flexspline
also turns and thus the deflection wave in the diaphragm is rotated too.
In motion transmitting mechanisms of this type, the axial flexure stress
in the diaphragm was used to establish the diaphragm thickness and
in order to minimize this stress, the thickness of the diaphragm was
10 held to a minimum yet thick enough to deliver the required output torque.
It has been found that the maximum stress which may occur is not necessarily
aseociated with the operating 6tresses in the unit but rather may be
caused by an excessively high axial stress being induced on the diaphragm
while the flexspline and wave generator are being assembled.
IS In Figure 3, the wave generator assembly 9 is shown being fitted
into the flexspline bore. The force shown as vector Fl is used to insert
the wave generator 9 and this force is transferred through ths flexspline
tubular portion S and to the flexing portion 3a of the diaphragm 3. The
force ie reacte~ by an opposing force F2 at the non-flexing portion of
20 the diaphragm 3 and is transferred through the diaphragm 3 to cause
it to "dish" producing a peak deflection stress in region 10.
The problem is critically present during assembly conditions
in which the wave generator may be slightly cocked or if thermal gradiants
are across the parts which produce an interference fit or if dirt or debris
25 is in the flexspline bore 6 or on the outer race 11. In some cases the
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l axial force applied during assembly produces over-stresses in the diaphragm
in region 10 thereby producing a high residual stress in the part. Then
when the wave generator is rotated during operation of the unit, the cyclic
alternating stresses are combined with the high residual stress and together,
S they can cause the part to fail in region 10. An arrangement sirnilar to
Figure 3 is shown in Figure 3A except that instead of using collars and
bolts to secure the diaphragm to the output shaft, the diaphragm is thickened
to provide an integral hub and may taper to a thinner thickness radially
outwardly of the hub, as shown in Figure 3A.
Figure 4 is a graph showing the relationship between the flexure
stress due to scalloping and the "dishing" stress caused by axial assembly
forces for various diaphragm thicknesses. In preparing this graph the
ratio of the mechanism between the wave generator input and the flexspline
output was 80 to 1 and the proportions of the flexspline length and diaphragm
lS clamping diameter, are the same as the prior art. It can be seen that
a very thin diaphragm produces a low deflection stress and generaly the
art has used diaphragms having thicknesses in the order of 0.8% of the
bore diameter of the flexspline. While a thin diaphragm will produce a
low deflection stress we have found that an extremely high axlal stress
20 may be produced because of an excessively high axial force at a.ssembly.
The axial force stress, we have found, is in the order of 90,000 pounds
per square inch or greater . When using diaphragms in the order 0 . 8%,
a deflection stress in the order of 12, 500 pounds per square inch is produced .
A The axial force used to compute these stress values is based on the formula
25 Force (8) D2 where D is the bore diameter of the flexspline 43, this force
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being the upper limit of the force used to seat the wave generator in the
CUp. According to the present invention, we found that if the thickness
of the diaphragm is increased to between about 1. 0 and 2 . 0% of the bore
diameter of the tubular flexspline 43, the axial force stress is reduced
5 dramatically to between about 15,000 and 55,000 pounds per square inch
and the deflection stress is only increased to between about 15,000 and
30,000 pounds per square inch.
In the new diaphragm proportions, the flexure stress is slightly
higher than the configuration of the prior art, but the diaphragm will
lO still be below the fatigue endurance limit of the material. We have found
that the deflection stress varies proportionally to the diaphragm thickness,
but the axial force stress varies inversely as the square of the thickness
and thus, a small increase in the axial diaphragm thickness increases
the deflection stress only a small amount, but significa.ntly lowers the
lS axial force stress.
It is apparent that modifications and changes can be made within
the spirit of the scope of the present invention but it is our intention, however,
only to be limited by the scope of the appended claims.
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