CA1126540A - Motion transmitting mechanism - Google Patents

Motion transmitting mechanism

Info

Publication number
CA1126540A
CA1126540A CA348,048A CA348048A CA1126540A CA 1126540 A CA1126540 A CA 1126540A CA 348048 A CA348048 A CA 348048A CA 1126540 A CA1126540 A CA 1126540A
Authority
CA
Canada
Prior art keywords
diaphragm
gear
flexible
flexspline
stress
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired
Application number
CA348,048A
Other languages
French (fr)
Inventor
John H. Carlson
Daniel C. Seluk
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
USM Corp
Original Assignee
USM Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by USM Corp filed Critical USM Corp
Application granted granted Critical
Publication of CA1126540A publication Critical patent/CA1126540A/en
Expired legal-status Critical Current

Links

Classifications

    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16HGEARING
    • F16H49/00Other gearings
    • F16H49/001Wave gearings, e.g. harmonic drive transmissions

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  • Engineering & Computer Science (AREA)
  • General Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Retarders (AREA)
  • Flexible Shafts (AREA)

Abstract

ABSTRACT OF THE INVENTION
A Harmonic Drive unit which includes a tubular flexspline gear attached to a flexible diaphragm and in which the diaphragm has a thickness between about 1.0 and 2.0% of the bore diameter of the tubular portion of the gear.

Description

;S ~) 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 ~! 11~54~

of the internal bore of the flexspline tube and, in general, that thickness was adequate to withstand the imposed operating stres~es.

,, i5~

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.

5~) - 4a -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.

' ~ ~ Z. 65 4~) 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.

.65~0 DESCR.IPTION OF THE PREF`ERRED EMBODIMF:NT
. . . _ 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 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 ll'Z.65~0 ., , 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 .

. . , 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 J

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.

~,'

Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
1. In a motion transmitting mechanism having a ring gear, a tubular flexible gear having an internal bore and with at least a partially flexible diaphragm integrally disposed therewith adjacent one end thereof, said diaphragm being attachable to a shaft, and said mechanism including a wave generator for progressively forcing the flexible gear into engagement with the ring gear, the improvement comprising:
the thickness of the flexible portion of said diaphragm being between about 1.0 and 2.0% of the diameter of the bore of said tubular flexible gear.
2. The mechanism according to claim 1 wherein said diaphragm is divided into both a flexible and an inflexible portion, the inflexible portion being attachable to and disposable about a shaft and the flexible portion being disposed between the end of said tubular flexible gear and the inflexible portion.
3. The mechanism according to claim 2 wherein at least one collar is disposed about the inflexible portion of the diaphragm.
4. A gear comprising a flexible tube having an internal bore, said gear also having teeth radially disposed about the periphery of one end thereof and a diaphragm integrally disposed adjacent the other end thereof, the thickness of said diaphragm being between about 1.0 and 2.0%
of the diameter of the bore of said tube.
5. The gear according to claim 4 in which said diaphragm has an inflexible portion which tapers to a thinner flexible portion which comprises said diaphragm which is of a thickness of between about 1.0 and 2.0% of the diameter of the bore of said tube.
CA348,048A 1979-10-10 1980-03-20 Motion transmitting mechanism Expired CA1126540A (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US8333979A 1979-10-10 1979-10-10
US83,339 1979-10-10

Publications (1)

Publication Number Publication Date
CA1126540A true CA1126540A (en) 1982-06-29

Family

ID=22177684

Family Applications (1)

Application Number Title Priority Date Filing Date
CA348,048A Expired CA1126540A (en) 1979-10-10 1980-03-20 Motion transmitting mechanism

Country Status (13)

Country Link
JP (1) JPS5655742A (en)
AU (1) AU538778B2 (en)
BE (1) BE885610A (en)
CA (1) CA1126540A (en)
CH (1) CH649139A5 (en)
DE (1) DE3037758A1 (en)
DK (1) DK426780A (en)
FR (1) FR2467326A1 (en)
GB (1) GB2060123B (en)
IL (1) IL61230A (en)
IT (1) IT1133833B (en)
NL (1) NL8005555A (en)
SE (1) SE8006972L (en)

Families Citing this family (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4524639A (en) * 1983-06-23 1985-06-25 Usm Corporation Extended flexspline arrangement
US4491033A (en) * 1983-06-23 1985-01-01 Usm Corporation Double eccentric wave generator arrangement
EP0130763A1 (en) * 1983-06-29 1985-01-09 USM Corporation Diaphragm protection arrangement for harmonic drive mechanisms
JPS6095235A (en) * 1983-10-26 1985-05-28 Mitsubishi Electric Corp Harmonic gear device
CA1251242A (en) * 1984-06-19 1989-03-14 Roger L. Swensrud Improved wrist and post for welding robots
US4817457A (en) * 1986-08-18 1989-04-04 Quincy Technologies, Inc. Uniform wall flexspline
JPH04116358U (en) * 1991-03-29 1992-10-16 ソニー株式会社 battery storage case
JP2000009191A (en) * 1998-06-19 2000-01-11 Harmonic Drive Syst Ind Co Ltd Cup type wave gear device

Family Cites Families (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS515790Y2 (en) * 1971-03-05 1976-02-18

Also Published As

Publication number Publication date
DK426780A (en) 1981-04-11
IT8025209A0 (en) 1980-10-09
SE8006972L (en) 1981-04-11
IL61230A (en) 1984-01-31
AU538778B2 (en) 1984-08-30
GB2060123B (en) 1983-06-08
FR2467326B1 (en) 1984-11-16
IL61230A0 (en) 1980-12-31
AU6311680A (en) 1981-04-16
JPS5655742A (en) 1981-05-16
CH649139A5 (en) 1985-04-30
FR2467326A1 (en) 1981-04-17
BE885610A (en) 1981-02-02
DE3037758A1 (en) 1981-04-23
IT1133833B (en) 1986-07-24
GB2060123A (en) 1981-04-29
NL8005555A (en) 1981-04-14

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