US3726158A

US3726158A - Speed-reducing coupling

Info

Publication number: US3726158A
Application number: US00112749A
Authority: US
Inventors: H Brown
Original assignee: Individual
Current assignee: Individual
Priority date: 1971-02-04
Filing date: 1971-02-04
Publication date: 1973-04-10
Anticipated expiration: 1990-04-10

Abstract

Disclosed is a device for coupling a rotary input of a first speed to a load which utilizes the input at a reduced speed, and wherein apparatus embodying the principles of a harmonic drive are combined with novel adjustable restraining elements and coupling means between the input and output of the apparatus to provide a speed-reducing coupling which has great strength, is simple, is inexpensive, and delivers smooth and continuous, vibrationless motion to the load.

Description

United States Patent 1 1 Brown [451 Apr. 10, 1973 SPEED-REDUCING COUPLING [76] Inventor: Henry C. Brown, 410 Crusader Drive, Dallas, Tex. 75217 [22] Filed: Feb. 4, 1971 [21] Appl. No.: 112,749

[52} U.S. Cl ..74/804, 74/243 R [51] Int. Cl ..F16h 1/28 [58] Field of Search ..74/804, 243 R [56] References Cited UNITED STATES PATENTS 3,307,434 3/1967 Kope ..74/804 FOREIGN PA'IEIITS OR APPLICATIONS 961,052 6/1964 Great Britain ..74/804 Primary ExaminerCv J. Husar Attorney-James D. Willborn [5 7] ABSTRACT Disclosed is a device for coupling a rotary input of a first speed to a load which utilizes the input at a reduced speed, and wherein apparatus embodying the principles of a harmonic drive are combined with novel adjustable restraining elements and coupling means between the input and output of the apparatus to provide a speed-reducing coupling which has great strength, is simple, is inexpensive, and delivers smooth and continuous, vibrationless motion to the load.

11 Claims, 8 Drawing Figures PATEEJTEU 3,726,158

FIG. 2

E CCENTRIC Q l SHAFT q INVENTOR HENRY C. BROWN ATTORNEY PATENTEU 0575 I 3,726,158

SHEET 2 BF 4 9 4 2, mmv 25 l3 FIG. 3

50A INVENTOR HENRY C. BROWN FIG. 4

ATTORNEY SHEET 3 [1F 4 FIG. 6

INVENTOR.

HENRY C. BROWN ATTORNEY PATEE'STEG 3.726.158

SHEET u 0F 4 ZlB-l 0 16 ECCENTRIC [NVENTOR HENRY C. BROWN 9,03%

ATTORNEY SPEED-REDUCING COUPLING This invention relates to mechanical movements and, more particularly, to harmonic drive speed-reducing couplings.

Harmonic drive speed-reducers have been known and have been in use for many years. Some typical examples (which are illustrative of the less complicated forms of this type of machine) are shown and discussed in the material found and discussed at page 340 of Volume I, Ingenious Mechanisms for Designers and Inventors, The Industrial Press, New York, N. Y., and at page 323 of Volume II of the same work. Through their simplicity, these speed-reducers appear to offer a high degree of reliability, low initial cost and low maintenance costs. Unfortunately, however, these machines, in their simplified form, have never proven practical for industrial applications.

The machines of the examples achieve speed reduction through the use of two planetary gears, rather than the four which would otherwise be required to have the output aligned with and rotating in the same direction as the input. The basic design suffers, however, because there is a high degree of wear concentrated on the teeth of the input. and output planetary gears, where they mesh. This wear results in an ever-increasing amount of backlash which appears in the output of the device as vibrating and pulsating motion, thus rendering the machine entirely unsuitable for many industrial applications.

Additionally, the driving teeth in any harmonic drive are heavily stressed, since only a relatively few teeth on the input planetary gear are normally in mesh with the output teeth at any one time, thus the load is concentrated. The constant and high degree of wear as sociated with the teeth, coupled with the high stress, tends to cause the teeth to break and shear off, thus resulting in increased maintenance. Further, as more parts are added to the structure in an attempt to overcome these problems, especially those relating to the restraining mechanism, the machine tends to become larger and less compact. Compactness is often a desirable or an essential requirement of a speed-reducing coupling.

Therefore, it is an object of this invention to provide an improved harmonic drive speed-reducer.

A further object is to provide an improved harmonic drive speed-reducer having a simplified structure for producing a smooth and continuous and relatively vibrationless output.

Another object is to provide a harmonic drive speedreducer of a design which provides for reduced wear at critical points.

Still another object is to provide a harmonic drive speed-reducer of a design which includes improved load-handling capabilities.

An additional object is to provide a compact harmonic drive speed-reducer through the use of im proved, compact restraining means.

A still further object is to provide a harmonic drive speed-reducer which is adaptable for use in either high stress or miniaturized applications.

Other objects and advantages will be apparent from the specificationsand claims and from the accompanying drawings illustrative of the invention.

In the drawing:

FIG. 1 is a partially sectioned side view of an embodiment of this invention.

FIG. 2 is a partially sectioned side view of the embodiment of FIG. 1, with the input shaft and sprocket rotated counter-clockwise.

FIG. 3 is a view in partial vertical section of a portion of the embodiment of FIG. 1, taken along the lines 3- 3.

FIG. 4 is a partially sectioned side view of another embodiment of this invention.

FIG. 5 is a view in partial vertical section of the embodiment of FIG. 4, taken along the lines 55.

FIG. 6 is a top view of a portion of the apparatus for coupling the input to the output of the embodiment of FIG. 1.

FIG. 7 is a partially sectioned side view of still another embodiment of this invention.

FIG. 8 is a view in partial vertical section of the embodiment of FIG. 7, taken along the lines 7-7.

Referring now to FIG. 1, a housing 10 is securely fastened to ground 11 so that it does not rotate or move with respect thereto. An input shaft mount 13 extends through a first wall 10-1 of the housing 10 and is securely fastened to the housing, as with bolts or clamps (not shown). The mount 13 has a first end 13-1 positioned outside the first wall 10-1, a second end 13-2 positioned inside the housing 10, and has a bore 14 communicating with the first and second ends. The bore 14 has a circular cross-section and has a first portion 14-1, adjacent the first end 13-1, which has a smaller cross-section than that of the portion 14-2, adjacent to the second end 13-2. The bore 14 has a longitudinal axis, as does each portion 14-1 and 14-2 thereof, and the longitudinal axis of each portion is coincident with the longitudinal axis of the bore. A wall 16 joins the respective portions of the bore 14 and is substantially perpendicular to the longitudinal axis thereof.

An input shaft 12 having a circular cross-section extends through the bore 14 and has a first end 12-1 coupled to a prime mover (not shown) and has a second end 12-2 positioned inside the housing 10, beyond the second end 13-2 of mount 13. The longitudinal axis of the shaft 12 coincides with the longitudinal axis of the bore 14. A roller-bearing 15 is positioned in the bore 14 axially of the shaft 12, adjacent to the wall 16.

An eccentric member 18 is located on the shaft 12, opposite a thrust washer 17 from the mount 13. A first end portion of the eccentric member 18 is cylindrically shaped and is located adjacent to the thrust washer 17 and, in a plane perpendicular to the longitudinal axis of the shaft 12, has a circular cross-section. The second end portion of the eccentric member 18 is also cylindrically shaped and, similarly, has a circular cross-section which lies in a plane which is perpendicular to the longitudinal axis of the shaft 12; however, the cross-section at the first end is larger in diameter than that of the second end. The center of the second end is offset a distance d from the center of the first end-portion,

which center is in register with the longitudinal axis of the shaft 12, thus the portion of the member 18 adjacent the second end forms an eccentric with a throw of d, while the portion of the member nearest the first end thereof forms a cylindrically shaped spacer with the center coinciding with the longitudinal axis of the shaft 12. A wall joins the first end-portion to the second end-portion of the member 18 and is substantially perpendicular to the longitudinal axis of the shaft 12. The

wall extends around the periphery of the member 18, thus the radius of the second end (from its center) is greater than the distance d.

The first end-portion of member 18 fits into, in a spaced relationship with the side thereof, the second portion 14-2 of the bore of mount 13. The wall joining the respective portions of member 18 is spaced from the wall joining the respective portions of the bore 14 a distance such that as member 18 is positioned against the thrust washer 17 and the washer is positioned against the joining wall 16 of bore 14, the wall joining the respective portions of member 18 is spaced along the longitudinal axis of shaft 12 into the interior of housing further than is the distal end of the second end of mount 13. The shaft 12 is held in place with respect to mount 13 by a bearing 15 and rotates about .its longitudinal axis with respect to the mount.

A circular bearing 19 is positioned on an eccentric surface 26 of member 18, and an input sprocket 20 is similarly positioned onto the bearing, thus the bearing 19 is constructed such that the eccentric 26 rotates with the shaft 12 when motion of the sprocket 20 is restrained. The sprocket 20 does not rotate, but nutates only, in responseto the eceentrics motion, as will be explained the following material.

The input sprocket 20 functions as a planetary gear having a substantially circular cross-section in a plane perpendicular to the longitudinal axis of the shaft 12 and having a surface 20-1 parallel to the plane and facing the wall 10-1 of the housing 10. In this embodiment, the sprocket 20 has a finite thickness which is small with respect to the diameter thereof, and the sprocket has iterative teeth 21 located around its periphery. The ratio of the number of teeth 21 on the input sprocket 20 and the number of teeth 46 on the output sprocket (to be described) are determinative of the speed reduction to be obtained by this device, and there may be an even or an odd number of teeth on either sprocket.

Two cam followers 22 are fixed to the input sprocket 20. Each cam follower 22 is a cylindrically shaped, elongate member having first and second ends and each has a longitudinal axis which is substantially parallel to the longitudinal axis of the shaft 12. The first end of each cam follower 22 is secured to the sprocket 20 (as by threaded fasteners) and the second ends of the cam followers are each positioned between the sprocket 20 and first wall 10-1 of the housing 10. Additionally, the second end of each cam follower 22 has a roller bearing arranged adjacent to the second end, circumferentially of the longitudinal axis of the follower, the outer surface of the bearing being the surface to contact the cam race to be hereinafter described. The

cam followers 22 are spaced apart and, preferably, are

respectively positioned on opposite sides of the input shaft 12 and on a straight line lying on the surface 20-1 and intersecting the longitudinal axis of the shaft.

Referring now to FIGS. 1 and 3, a pair of cam-members 24 are fixed to the first wall 10-1 of housing 10, adjacent to and on opposite sides of the input shaft mount 13 and each in register with a respective cam follower 22. Each cam-member 24 has located therein, in register with a respective cam follower 22, a bore which forms a cam race 23. Each bore has a circular cross-section in a plane perpendicular to the longitudinal axis of the cam follower 22, thus the cam race is circular in configuration, with the wall of the bore forming the surface in contact with a respective camfollower 22. In this embodiment, the bore forming the cam race 23 does not extend completely through the cam member 24, but is bottomed at a point within the member which is spaced from the second end of the cam follower 22.

Adjusting screws 27 are shown in FIG. 3 and provide a means for adjusting the position of each cam race. As will be discussed in the material which follows, the point of contact of each cam follower 22 with its respective cam race 23 and the point of contact of the respective tooth 21 with the coupling means 50 forms a force triangle with respect to each tooth. The diameter of each cam race 23 is exactly twice the throw d of the eccentric 26, plus the diameter of the cam follower 22 and the throw and diameter of each race are related to the pitch of the teeth 21 such that each cam follower 22 is in contact with a respective cam race 23, while simultaneously at least one tooth 21 is in contact with a respective link of the coupling means 50 (to be described), thereby forming the force triangle. It is desirable for the purpose of having a smooth and continuous output of the speed-reducer 10 to have the force.triangle transferred smoothly to successive teeth on the input sprocket 20. The cam races and other components of the input mechanism are machined to tolerances which aid in the desired transfer; however, as a matter of convenience and, consistent with the objective of providing a low-cost, high-performance embodiment, the cam races 23 are made adjustable through the use of screws 27 which can be loosened to accommodate the precise positioning of the cam member 24. The mounting holes in cam member 24 are also made over-size to accommodate this adjustment. The cams 22-23 are always, therefore, in precise contact, no matter which tooth 21 is engaged, thus backlash in the restraining mechanism, and between the input and output of the coupling 10, is substantially eliminated.

In FIG. 1, the output shaft 28 extends through a bore 34 through a second wall 10-2 of the housing 10. The output shaft 28 has a longitudinal axis which is in register with and is aligned with the longitudinal axis of the input shaft 12.

An output shaft mount 29 is positioned in the bore 34. The mount 29 has a bore 35 extending therethrough, and the bore has a longitudinal axis in register with the longitudinal axis of shaft 28. An output bearing 33, similar to the bearing 19, is positioned in the bore 35, and the output shaft 28 is positioned in the bearing 33 such that the shaft rotates with respect to the mount 29. A retaining plate 32 has a bore 32-1 at substantially the center thereof and the bore is large enough to accommodate the shaft 28 without interference. The plate 32 is fastened to the housing 10 with screws 31 which also pass through the mount 29'to position and hold the mount and thus the shaft 28 in place in the bore 34. additionally, the retainer 32 has an offset 36 machined in a face thereof and communicating with the bore extending through the center thereof. The offset is positioned adjacent to the bearing 33 as the mount 29, retainer 32 and bearing 33 are assembled and fastened to the housing 10. The edge of the offset 36, which is opposite the center bore of retainer 32 is spaced from the bore a distance which places the face of the retainer 32 in contact with the outside guide of bearing 33, thus preventing any movement of the bearing outwardly along shaft 28, while at the same time providing clearance between the retainer and other portions of the bearing such there is no interference with the normal operation of the bearing.

The output shaft 28 has a first end 38 and a second end 39. The shaft 28 has a circular cross-section at the first end 38, a larger circular cross-section at the second end 39, and a wall 40 joining the first end to the second end. The wall 40 is substantially perpendicular to the longitudinal axis of the shaft 28.

The wall 40 is positioned against the inner race of the bearing 33 and on a side opposite to the retainer 32 to prevent movement of the shaft 28 outwardly of the housing 10.

The second end 39 has a bore 41 which extends inwardly along the longitudinal axis of the shaft 28 toward the first end 38 thereof. The bore 41 has a circular cross-section which is larger in diameter than that of the input shaft 12 and extends into shaft 28 a distance which will accommodate a bearing 42 and shaft 12 without interference longitudinally. Bearing 42 is positioned in the bore 41 between the shaft 12 and the wall of the bore.

An output sprocket 30 is fixed to the output shaft second end 39. The sprocket 30 has teeth 46 extending outwardly from the longitudinal axis of the shaft 28. The sprocket 30 is secured to the shaft 28 by means of a standard spline and key arrangement 43. A lock screw 45 holds the sprocket 30 and key 43 in position to prevent their movement along the length of the shaft 28. The spline 43 also locks the sprocket 30 to the shaft 28 such that rotary motion imparted to the sprocket will correspondingly rotate the shaft. Additionally, the center of the sprocket 30 is in register with the longitudinal axis of the shaft 28 and the teeth 46 are concentric with and equally spaced therefrom.

The second end 12-2 of the input shaft 12 is positioned within the housing and within the bore 41 of the second end 39 of shaft 28. A thrust washer 44 is positioned on shaft 12, between the second end 39 of shaft 28 and the eccentric 18. Eccentric 18 is secured to shaft 12 by spline 25 and is spaced from the second end of shaft 12 such that movement of the shaft 12 iongitudinally into the housing 10 brings the eccentric into contact with the thrust washer 44, the washer 44 into contact with the second end 39 of shaft 28, wall 411 into contact with the bearing 33 and bearing 33 into contact with retainer 32 to prevent further movement of the shaft 12. When the shaft 12 is in this position, the end of the shaft is spaced from the bottom of the bore 41 to prevent interference therewith.

Movement of output shaft 28 longitudinally into the housing 10 brings the second end 39 thereof into contact with thrust washer 44, which, in turn, contacts eccentric 18, which contacts thrust washer 17, which contacts mount 13; thus, longitudinal movement of either shaft 12, 28 (and thus of the sprockets and 30) is restricted in either direction while the teeth of the

sprockets

20 and 30 are free to rotate about the

respective shafts

12, 28, substantially in their respective planes.

A coupling means 50 (see FIG. 6 also) having a first channel 48 and a second channel 47 is arranged around the periphery of the output sprocket 30, and each link of the first channel 48 is in mesh with a respective tooth 46 thereof. Each link of each channel includes pins 51 which traverse the length of the means 50 to form pivots which have rollers 52 positioned thereon. Each roller 52 is free to rotate about the axis of the pin 51. The second channel 47 extends outwardly from the output sprocket 30, toward the first wall 10-1 of the housing 10, and is arranged in register with the teeth 21 of the input sprocket 20. Each roller 52 and each corresponding

channel

47, 48 are wide enough to accommodate the

teeth

21, 46 of the respective sprockets, with which the links are in register.

The specific means of securing coupling means 50 to the output sprocket 30 is not material to this invention. It is only significant that the motion imparted to channel 47 by sprocket 20 be transmitted smoothly to the output shaft. Whatever means are used should be rigid and free of bending or backlash at the pressures applied. For instance, pins 51 could be secured to the face and around the periphery of an output sprocket, similar to 30, but having a circular cross-section with no teeth. Rollers, such as 52, could be mounted on the pins 51 and retaining means similar to the coupling linkage of coupling means 50 could be added to create a second channel similar to channel 47. Additionally, rollerbearings could be added to such a second channel to further reduce the friction if this were required for a particular application. Coupling means of this type,

while having certain advantages, would undoubtedly increase the cost of the unit and would be a more difficult unit to maintain with respect to the roller-chain embodiment described above.

Each tooth of the output sprocket 30 is in mesh with a respective link of channel 48 of chain 50 at all times, while at least one of the teeth 21 of the input sprocket 20 is in contact with a respective link of channel 47 of chain 50 at all times. While the at least one tooth 21 is in full contact with a link of channel 47, teeth 21 on the opposite side of the sprocket 20 from the at least one tooth are not touching the coupling means.

FIG. 2 illustrates the speed-reducing coupling of FIG. 1 with the input sprocket 20 rotated counterclockwise with respect to its position as shown in FIG. 1.

FIG. 3 is a sectioned view of FIG. 2, showing the positioning of the cams 22 within the races 23, as the shaft 12 has been rotated. Further, this FIG. provides a better view of the means for adjusting the position of the cam member 24. The bolts 27 extend through the cam number 24 and fasten to the wall 10-1 of the housing 10. Holes are provided at either end of the cam member 24 for accommodating the bolts 27. These holes are made slightly larger than the outside diameter of the body of a respective bolt 27, for instance, for a three-sixteenths inch bolt, a one-quarter inch hole would be used. The oversized holes provide a means for loosening the screws 27 and relocating the member 24 to compensate for errors in the machine tolerances and positioning of the cam races 23, as previously described.

Referring to FIG. 4, a second embodiment of the harmonic drive of this invention is shown. In this embodiment the housing (FIG. 1) is not required and can be eliminated. In order to facilitate the description, the components of this embodiment are numbered corresponding to the same of similar parts of FIG. 1, followed by the suffix A (or suffix B, in the case of FIGS. 7 and 8). The configuration of the correspondingly numbered parts is substantially the same as that of the parts of FIG. 1, unless otherwise specified.

As with the embodiment of FIG. 1, the input shaft 12A is coupled to drive apparatus (not shown) for imparting rotational motion to the shaft, the speed of which is to be reduced at the output shaft 28A and applied to a load (not shown).

Eccentric member 18A has a flange 62 machined thereon and located between the eccentric surface 26A and the drive apparatus. A bearing 19A is fit onto the eccentric surface 26A, and the eccentric is secured to shaft 12A by a standard spline and key arrangement 25A. The bearing 19A fits tightly on the surface 26A such that at least the inner race of the bearing rotates with the eccentric. A sprocket 20A is then fit in a similar manner on the outer race of the bearing 19A, such that the shaft 12A and member 18A rotate in response to the rotational input motion applied to the shaft. The sprocket 20A nutates, but does not rotate, when rotational motion of the sprocket is restrained, as by the cam arrangement to be described. The flange 62 is in contact with and restrains movement of the bearing 19A along the axis of the shaft 12A in the direction of the drive apparatus. A face of the flange 62 extends outwardly and perpendicularly from the shaft 12A, beyond the eccentric surface 26A and contacts a side of the inner race of the bearing 19A to prevent such movement.

A spacer portion 60 is machined onto the shaft 12A, spaced from the second end 61. The spacer 60 has a first shoulder which is perpendicular to the longitudinal axis of the shaft 12A and is spaced from the second end 61 thereof. A bearing 33A is positioned on the shaft 12A, adjacent to the second end 61. The inner race of the bearing 33A is in contact with the first shoulder, thus movement of the bearing away from the second end 61 and along the shaft 12A is restrained. Additionally, the spacing between the first shoulder and the second end 61 corresponds to the width of the bearing 33A, thus the bearing does not extend outwardly from the shaft, beyond the second end. An eccentric 26A is positioned on the shaft 12A on the opposite side of the spacer 60 from the second end 61 and against a second shoulder thereof which is similar in configuration to the first. The eccentric 26A is secured to the shaft 12A by a spline and key arrangement A. The shape and configuration of eccentric member 18A is substantially the same as that of eccentric 26 of FIG. 1, except for the bearing retainer or flange 62 just described.

The output shaft 28A has a longitudinal axis which is in register with and is aligned with the longitudinal axis of shaft 12A. A first end of the output shaft 28A is coupled to a load, and the second end is positioned in close proximity to the second end 61 of the shaft 12A. An output sprocket 30A has a face 64 which is substantially perpendicular to the longitudinal axis of the shaft 28A and has an end 65, opposite the face 64.

The sprocket 30A has a bore with a circular crosssection located at the center thereof and communicating with the face 64 and the end 65. The bore has a first portion adjacent the end 65 which is substantially the same diameter as the shaft 28A and mates therewith, with the shaft being positioned in the bore and secured thereto by a spline and key arrangement 43A. The second portion of the bore extends from the first portion to the face 64 and has a slightly larger diameter. The wall connecting the first portion to the second portion is substantially perpendicular to the longitudinal axis of the shaft 28A. The diameter of the second portion corresponds to the outside diameter of the bearing 33A.

The second end of the shaft 28A does not extend into the second portion of the bore, but the bore second portion is of sufficient length to accomodate the full width of the bearing 33A. The bearing 33A is fit into this portion of the bore and has one edge positioned against the shoulder separating the bore first and second portions and has its opposite face in register with the face 64.

It can now be seen that as the

shafts

12A and 28A are placed into position, the inner race of bearing 33A rests on the shaft 12A, and the outer race of this bearing is in contact with the bore of sprocket 30A. In this position, the sprocket 30A and shaft 28A are free to rotate together with respect to the shaft 12A, but are held in a fixed axial alignment with respect thereto by means of the bearing 33A. Shaft 28A is positioned in the bore of sprocket 30A such that when the bearing 33A, sprocket 30A, shaft 12A and shaft 28A are in place, there is a nominal clearance between the end of shaft 28A and the end 61 of shaft 12A, such that there is no actual contact between these members.

The sprocket 20A has teeth 21A positioned around the periphery thereof. While this is not necessarily true of the embodiment of FIG. 1, the input sprocket of the device of FIG. 4 must have an even number of teeth on the input sprocket, for instance, in the embodiment shown, the input sprocket 20A has 18 teeth. Correspondingly, the output sprocket 30A of FIG. 4 must also have an even number of teeth, in this case 20.

A major difference between the embodiment of FIG. 4 and that of FIG. 1 exists in the cam arrangement. The cam follower 22A is substantially the same as that of cam follower 22 of FIG. 1, and the cam race 23A is substantially the same as cam race 23 of FIG. 1; how ever, cam race 23A is secured to a face 64 of the output sprocket 30A with screw fasteners, such as 63, rather than to a housing such as housing 10 as in FIG. 1. In FIG. 4, the cam follower 22A has its position reversed with respect to the position of cam 22 (FIG. 1), in that the cam 22A extends outwardly, away from a face of input sprocket 20A and toward the output sprocket 30A and the cam member 24A and cam race 23A are secured to the face 64 thereof. The spacing between the input sprocket 20A and output sprocket 30A, determined by the width of spacer 60, is such that the cam follower 22A engages the cam race 23A, but the distal end of the cam does not come into contact with any portion of sprocket 30A or cam member 24A, except the cam race.

A portion of each cam member 24A extends outwardly into the bore of sprocket 30A and into contact with the bearing 33A, thus securing the bearing 33A into place between the respective cam members 24A and the shoulder separating the first and second portions of the bore through sprocket 30A.

The output sprocket 30A in the embodiment shown has 20 teeth located around the periphery thereof. A coupling means 50A, similar in construction to the coupling means 50 of FIG. 1, is positioned on the output sprocket 30A, with one link of one channel corresponding and coupling to each tooth thereof. The second channel of the coupling means 50 is in register with the teeth 21A of the input sprocket 20A and in contact with at least one of these teeth.

The diameter of the input sprocket 20A is less than that of the output sprocket 30A by an amount equal to the throw d of the eccentric 26A, thus at least one of the teeth of the input sprocket is always in full mesh with the second channel of the coupling means 50A and several other teeth are in partial mesh therewith.

In this embodiment there are two cams 22A and two cam races 23A. Refer now to FIG. wherein it is shown that the center of the circular cam races 23A are positioned with respect to the sprocket 30A on opposite sides of the shaft 12A and bearing 33A and on a straight line which passes through the longitudinal axis of the shaft 12A. Preferably, the center of the respective races 23A are aligned under a respective tooth 46A of the output sprocket 30A. Similarly, the center of cams 22A are arranged on the input sprocket 21A in a corresponding manner. Each cam member 24A is secured to the output sprocket 30A by means of bolts, such as 63. A hole is bored through the sprocket 30A to accommodate each bolt 63, and each is made larger in diameter than the bolt, as described in respect to the bolts 27 of FIG. 1, to allow room to adjust the position of the cam member 24A.

' Still another embodiment of this invention is shown in FIGS. 7 and 8. In this embodiment the input shaft 12B has a longitudinal axis and an eccentric 52B is machined on the input shaft and offset as described for other eccentrics (FIG. 1 and FIG. 4). Between the input shaft 128 and the eccentric 52B are threads 71, and on the opposite side of the eccentric from shaft 128 is a shoulder 72, which separates the eccentric 52B and a portion of the input shaft 82 which has a larger diameter than that of the eccentric 528. On the opposite side of the portion 82 from the eccentric 52 is a second portion of the input shaft 83 which has a shoulder 75 located on a side opposite the eccentric. The

shoulders

72 and 75 are plane surfaces which are substantially perpendicular to the longitudinal axis of the input shaft 12B, and shoulder 75 joins shaft portion 83 with shaft portion 84. Shaft portion 84 has a substantially smaller diameter than that of the eccentric shaft portion 82 or shaft portion 83. The shaft portion 84 terminates in an end 85, and adjacent to the end 85 are threads 79, which are located on the outer surface of the shaft. Tapered input bearings 198-1 and 1913-2 are positioned on the eccentric surface 528 and held in position between shoulder 72 and a retaining nut 70 which is fastened on the shaft 12B by threads 71.

In this embodiment, the input sprocket 21B is formed of two distinct portions 218-! and 218-2 which are held together with bolts 73. The first portion 218-! is cylindrically shaped and has a bore through the center and communicating with both ends thereof. The diameter of this portion 218-1 is larger than that of the eccentric 528. The width of the sprocket portion 218-1 is substantially the same as the width of the eccentric 52B. Near the center of the bore of the sprocket first portion 21B-1 is located a spacer 76 machined into the surface of the bore and located circumferentially around the bore. Each of the bearings 19B-1 and 19B-2 are tapered roller-bearings of the type which are identified in TABLE I. Bearings 158-1 and 19B-2 are placed on the surface of the eccentric 52B, and are spaced apart by the spacer 76, with the inner bore surface of the sprocket portion 21B-1 riding on each of the bearings 1913-1 and 198-2 are turned in opposite directions, each facing the spacer 76 and having its taper nearest the shaft 128 at the facing edges of the bearings.

The second portion 2lB-2 of the input sprocket is fastened to the first portion 2lB-l by bolts 73. The second portion 218-2 has a bore at the center thereof having a diameter large enough to accommodate, without interference, the shaft portion 82 as it nutates in response to motion of the eccentric 52B. Teeth 20B are located around the periphery of the input sprocket portion 2lB-2 which has a larger diameter than that of the input sprocket portion 218-1.

Each of the bolts 73 coupling the respective sprocket portions 21B-1 and 218-2 have located on the respective ends thereof, cam followers 228 which extend outwardly from the face of the portion 21B-2 toward the end 85 of the input shaft 128. The cam followers 228 are substantially the same in construction and configuration as those cam followers 22 of FIG. 1. The portion 83 of the input shaft 128 is slightly greater in width than the length of the cam follower 22B extending outwardly from the surface of the input sprocket second portion 21B-2.

The output sprocket 30B is of a slightly different configuration than that of the out sprockets shown in FIGS. 1 and 4, in that it has two parallel rows of teeth 468-1 and 46B-2 located around the periphery thereof. The output shaft 28B is secured, as by welding 80, to an output flange 81. The output flange 81 is secured to the output sprocket 30B by bolts 86. The longitudinal axis of the output shaft 28B is in register with the longitudinal axis of the input shaft 128. The output sprocket 308 has, at the center and along the longitudinal axis thereof, a bore 87, and the longitudinal of the output sprocket 30B is in register with the output shaft 288 when the two are positioned together as described.

A spacer 77, similar to spacer 76 associated with the input sprocket 21B, is located on the interioe of the bore 87. Bearings 338-1 and 338-2 are placed in the bore 87 similarly to the arrangement of bearings 198-1 ,and 19B-2 associated with the input sprocket 21B.

Specifically, the shaft portion 84 is in register with the on the shaft portion 84, with both bearings being held in position by a nut 78 which is fitted to the thread 79 at the end of the shaft portion 84. The bearings 338-1 and 338-2 are tapered in opposite directions, similar to All other parts are ASA Standard, as described, except the

shafts

12, 12A, etc. and their eccentrics, which are specially machined and prepared for the application described. By way of illustration, the throw of a the arrangement of the bearings 198-1 and 1913-2 asrespective eccentric can be easily computed as follows. sociated with the input spsocket 218. In FIG. 1, there are 19 teeth on the input sprocket 20 Cam races 24B are located on the surface of the outand 20 teeth on the output sprocket 30. The input put sprocket 30B, facing the input sprocket 21B. The sprocket 20 has a bottom diameter (the diameter of the input shaft portion 83 is wider than the maximum sprocket taken at the bottom of the teeth), B.I., which thickness of the cam races 24B such that there is no inis 2.725 inches. The bottom diameter of the output terference between a respective cam race 24B and the sprocket 30, B. 0., is 2.883 inches. The throw, thereinput sprocket portion 21B-2. Bolts 63B extend fore, is given by the equation d (B.O.B.l.l2) or through the output sprocket 308 to hold the cam races 0.079 inches. 24B in position. The cam races 23B are in register with The ideal outside diameter of the input sprocket (the the cams 22B, and are similar in construction to the diameter of the sprocket taken at the tip of the teeth), cam races 23 shown and described with respect to FIG. 0. I., is computed as follows. 1, The radius, I. N., of the input sprocket at the diame- The throw 1 0f the eccentric 52B corresponds to the ter B. 1. equals B.l./2. The radius, 1,, 0f the diameter 0. depth of the teeth 203 on th i t s rock t Th I. of the input sprocket should be the equal to the coupling means 508 is what is commonly known as a diameter of outside sprocket, B. 0., minus the radius 1,; triple roller-chain, having three channels extending since the outside diameter 0. I. of the input sprocket is around the periphery thereof and coupling back on ith, the Optimum diameter, a, the diameter of the self to encircle the output sprocket 30B. Each length of input Sprocket at the P of the teeth is the means 503 couples with a respective tooth on the 25 output sprocket 308, for instance 46B-l. As the chain is closed, the chain is effectively fixed to the output sprocket and will not move with respect thereto. One 1 HQ) Bl/2) X 2 or channel of the means 50B is in mesh with teeth 468-1 and the second channel thereof is in mesh with teeth 468-2. The third channel of the chain extends outwardly from the output sprocket 30B and is in register with the input sprocket 218-2. At least one tooth of the m our example input sprocket 218-2 is in contact with the third channel of the chain 50B at all times. Aside from having 5 three channels rather than two, the means 508 is constructed in the same manner as the means 50 of FIGS. 1 a 2-883 X 2 3-041 and 6. The bearing surface 523 of the third channel of the means 508 rotates on the connection pin linkage joining the three sections of the chain together.

In either of the embodiments described, the output sprocket and coupling means and their associated hard- Since the outside diameter 0. I. of the input sprocket is ware are considered as output means, in the broadest 3.30 inches (from mfg. catalog), 0.259 in. of material sense of the term, i.e., means for coupling the motion of would have to be removed from the input sprocket the input sprocket to the output shaft and the load. teeth of the example. Other standard sprockets of dif- For the embodiments of this device described herein, ferent sizes, or involving different input to output ratios the off-the-shelf components are of the type and mode or more or less teeth, may not have to be modified, but number described in the following table: the same clearance formula would apply.

TABLE I Desig- Part nator Model No. Manulnctun-r Manufacturer address Bearing 15 13-1210 'lhc Torrinuton (2o A 'Iorrington, Conn. 06700. Thrust washcr A North American Rockwell, Boston Gear Division 14 Hayward St., Quincy, Mass. 02171. Beaiigng Marlin-ltoukwell (30., Division of 'IRW, lnc J2II'IIFESS80WII, New York 14701.

MRC R-12-ZZ McGillMl'g. ()0 'Iimken Roller Bearing Company Martin Snrockctdz (lcars, lnc

The Torrington Co Canton, Ohio 14700.

3106 Sprocket Drive, Arlington, 'li-xas.

CF-1/2 McGill Mfg. Co., Bearing Division. Valpariso, Indiana 40383.

CF-1/2 .-d0 Do.

401320 Martin Sprockl-t & (icars, lne 3106 Sprocket Drivc, Arlington, Texas. 60820 (1 Do.

Double-601320.. Do

MGR R-20-ZZ Jarngstown, New York 14701.

Torrington. Conn. 06790.

TB-1225 North American Rockwell, Boston Gear Division. 14 Hayward St., Quincy, Mass. 021'71. ASA 40-2 Acme Chain Corporation Holyoke, Mass. ASA -2 do Do. Do. ASA 60-3 Cotter Boston Roller Chain 14 Hawyard St., Quincy, Mass. 02171. Sprocket fiflAl.) Acme Roller Chain Corporation IIolyoke, Mass.

D0. 60AM) Martin Sprockct K: Gears, Inc 3106 Sprocket Drive, Arlington, Texas. Cam follower. 2213 CF-1/2 McGill Mfg. Co. hearing Division Valpariso, Indiana 46383.

The operation of the basic nutating gear or harmonic drive is well-known and is described in Volume I of Ingenious Mechanism for Designers and Inventors at page 340 and in Volume III of the same work at page 323.

Fundamentally, the transfer of motion from the input to the output sprocket is accomplished by introducing wabble or nutating motion to the input sprocket and transferring this motion to the output shaft in the form of rotational motion.

Refer to FIG. 1. A prime mover is coupled to the first end of the input shaft 12 and provides torque and rotational motion of the shaft at that point. As the input shaft 12 rotates, the eccentric 26 rotates therewith and provides motion to the input sprocket 2]. Cam followers 22, in conjunction with the cam races 23, prevent the rotation of the input sprocket 20 about the shaft 12, while allowing it to nutate or wabble in response to the motion of the eccentric 26. The axis of the input sprocket 20 follows the motion of the eccentric 26, but does not rotate about its own axis because of the cam and cam race arrangement 2223.

As the axis of the eccentric 26 rotates about the shaft 12, the inner race of bearing 19 rotates therewith, while the outer race of bearing 19 tracks the motion of sprocket 20. The earns 22 coupled to the cam race support 24 prevent the rotation of sprocket 20, but allows the sprocket to move vertically and horizontally a distance which is equal to the difference between the diameter of the cam race 23 and the cam 22. Thus, this motion is imparted to each tooth around the periphery of the input sprocket and at least one tooth, in contact with the coupling means 50, meshes fully with a link of coupling means 50, backs out of mesh and re-meshes with the next adjacent link of the coupling means dur ing each revolution of the input sprocket. This occurs with respect to a particular tooth when the cam 22 is in contact with a point on the cam race 23 which corresponds .with the location of the respective tooth on the periphery of the sprocket 20. A tooth 21 directly opposite to the tooth in contact will be clear of the coupling means 50.

As the shaft 12 rotates each one-nineteenth of a revolution, another tooth engages the coupling means 50. During each one-nineteenth of the revolution, the motion of the sprocket 20 forces the respective tooth outwardly from the shaft 12, and into mesh with the respective link of the coupling means and, specifically, into contact with the surface 52 which is free to rotate on the axis 51 of the respective link, thus serving as a roller bearing between the tooth and the link. As the respective tooth 21 is brought into contact with a respective link, the coupling means 50 is rotated a finite degree. As this tooth is making contact, the next tooth is beginning to engage in a similar manner with its respective link, thus, it begins to impart motion to the coupling means 50 also. Successive teeth making contact in this manner in response to motion of the eccentric 26 provides rotary motion to the coupling means 50. The second channel 48 of the coupling means 50 does not move with respect to the output sprocket 30, since each tooth 46 of the output sprocket is directly coupled to a link thereof. As previously described, the output sprocket is coupled directly to the output shaft 28 which is in turn coupled to a load (not shown), thus the input is coupled directly to the output through the coupling means 50, and the rotary motion imparted to the coupling means 50 is transferred through the output shaft to the load.

In other nutating drive devices, the meshing of the input teeth with the output portion of the device subjects these teeth to intensely concentrated shearing forces and a very high wear factor resulting from sliding friction. This has caused many machine designers to divert to other less direct coupling between the input and output, resulting in more expensive, more com plex, less desirable drives. Through the addition of the roller bearing 52 at the critical point of contact, tooth wear from sliding friction is substantially eliminated, while at the same time, the basic simplicity of the direct drive device is retained in conjunction with all the inherent advantages of the less complicated device.

Since the axis of the input sprocket 20 follows the motion of the eccentric 26 and the sprocket does not rotate about its own axis, the motion imparted to the driven sprocket 20 will be uniform and equal to the speed of shaft 12 times (N-n/N), where N is the number of teeth in the output sprocket and n is the number of teeth in the input sprocket. The reduction ratio for this device may be expressed by the equation R (N/N-n). Thus, if N equals 20 teeth and n equals 19 teeth, as in FIG. 1, the speed reduction ratio will be 20 to I. If N equals 20 teeth and n equals 18 teeth, then the reduction ratio will be 10 to 1. Similarly, if N equals 20 teeth and n equals 16 teeth, the reduction ratio will be 5 to l.

The positioning of the bearing 26 between the input sprocket 21 and the input shaft 12 operates to reduce the amount of wear previously experienced in the nutating gear drive apparatus, thereby greatly extending the life of the present device. The cam 22 is constructed with a bearing between the sprocket 20 and the cam surface 22, again substantially reducing friction.

The cam races 24 are constructed to be adjusted by loosening the retaining screws 27 (FIG. 3). One of the major causes of vibration in harmonic drives is the operation of the restraining elements. In this invention, the diameter of the cam race is precisely coordinated with the throw d of the eccentric 26 and the desired motion of the teeth 21 to allow alternative meshing and disengagement of the teeth with the coupling means 50. The bearing associated with the cam 22 substantially eliminates undesired wear, and with a circular cam race, the cam and race are maintained in constant and uniform contact, thereby preventing the vibration of other units using vertical or lateral cam races in conjunction with the restraining members. The cam 22 tracks the motion of the axis of the eccentric 26, and the motion of the axis of the eccentric 26 is uniform. The contacting surface of the circular cam race 23 is precisely aligned and in contact with the cam 22 at all times, thus the uniform motion of the axis of eccentric 26 is passed through'a direct path from the eccentric 26, through the bearing 19, the sprocket 20, the cam 22, the cam race 23 to the fixed ground 11. When the cam race supports 24, and thus the cam races 23, are properly aligned, vibration resulting from the backlash found in other devices can be effectively tuned out of the present device.

The backlash is still further reduced in the embodiment described by the use of more than one cam and cam race arrangement 22-23, and by providing for adjustment of the position of the cam race 23 with respect to its ground and the motion of the axis of the eccentric. Because of this aspect of the machine, standard, off-the-shelf hardware can be used in the construction of this harmonic drive and superior results in performance can be obtained, while the cost of the unit is proportionately reduced. Expensive high-precision components are not required.

Since the wear associated with sliding friction between the input and output of the device has been substantially reduced, the gradual wear of the loadbearing teeth in the drive is reduced and the teeth lose their tendency to shear under heavy loads.

Additionally, the restraining means, such as cam arrangement 22-23, do not require levers and linked mechanism for proper operation, thus the present device is more compact than devices having such arrangements. Also, the present device is particularly suitable for miniature applications, since all components can be easily reduced in size, and the device is reversible for applications requiring that feature. In the present device the output always rotates in the same direction as the input, which is still an additional advantage for many applications.

The embodiment of this invention shown in FIGS. 4 and 5 operates in the same manner and has the same advantages as those of the embodiment of FIGS. 1 and 2, except that it has the added advantage of being more compact. In this embodiment, the

restraining elements

22A and 24A are effectively grounded to the output, i.e., the load, and the requirement for a housing with fixed ground is eliminated.

The input shaft 12A and eccentric 26A are free to rotate free of the output because of

bearings

19A and 33A. The input sprocket 20A nutates in response to the motion of eccentric 26A. The cam race support 24A and cam race 23A are secured to the output sprocket 30A, and the motion of the input sprocket 20A in response to the rotation of eccentric 26A is always relative to a fixed point on the output apparatus, thus the nutating input sprocket advances the coupling means 50A, the output sprocket 30A and the output shaft 28A in the same manner as the device of FIG. 1. The advancement of the sprocket 30A rotates the cam race support 24A, but sprocket 20A and cam follower 22A track the cam race 23A and the relative speed of the input sprocket to the output sprocket is not affected.

The cam race supports 24A can be adjusted to reduce backlash by means of the adjusting screws 63.

The embodiment shown in FIGS. 7 and 8 operates in substantially the same manner and has the same advantages as the embodiment of FIGS. 4 and 5, except that this embodiment is designed with tapered bearings 19B and 333. The bearings 19B-1 is grounded to the shaft 128 by nut 70 with prevents movement of the bearing along the axis of the shaft 12-B toward the input. Similarly, bearing 19B-2 is fixed to the shaft 12B and its motion toward the end 85 of that shaft is prohibited by the shoulder 72 of shaft portion 82. Both bearings 19B-l and 198-2 are retained in these respective positions and fixed with respect to each other by the spacer 76 of the input sprocket portion 218-1.

Similarly, bearings SIB-1 and 318-2 are arranged on the shaft 12B, but are held in position with respect to each other and with respect to the output shaft 288 by the shoulder 77 of the output sprocket 303.

With these bearings arranged in this manner the coupling 108 will take substantially higher laterally imposed loads, either from the input or the output, without being damaged. A lateral load imposed along the input shaft 128, into the coupling is impressed through shoulder of shaft 128 on bearing 338-1, through bearing 33B-1 on spacer 77 of sprocket 30B, from spacer 77 to bearing 338-2 and thence to nut 78. In this manner the internal spacing of the drive 10 is maintained, and the need for thrust washers, such as 17 and 44 of FIG. 1, is eliminated. In any application where the lateral forces are likely to be applied to either the input or output shafts, the thrust washers are a source of added wear and friction, thus the

bearings

19B and 33B arranged in this manner provide a means for eliminating wear from lateral forces to further improve the coupling.

The double-sprocket 308 at the output of the device adds greater strength to the overall drive and provides a means for keeping the coupling means 50B in alignment and in register with the teeth of the input sprocket 218 when the drive 108 is subjected to lateral forces.

It is apparent that other variations and modifications may be made without departing from the present inven tion. Accordingly, it should be understood that the forms of the present invention described above and shown in the accompanying drawing are illustrative only and not intended to limit the scope of the invention.

What is claimed is:

l. A speed-changing coupling comprising:

a first rotatable shaft;

a first sprocket eccentrically connected to the first shaft and having means for driving movable orbitally about the axis of the shaft;

a second sprocket having a diameter larger than that of the first sprocket and rotatable about the same axis;

coupling means fixedly secured to the periphery of the second sprocket and having a portion thereof in driven engagement with the driving means of the first sprocket;

a pair of spaced, circular cam races secured to a surface of the first sprocket, adjacent to the second sprocket;

a pair of circular cam followers secured to the second sprocket, each disposed to mate with and follow a respective one of the cam races; and

a second rotatable shaft coupled to the second sprocket.

2. The speed-reducing coupling claimed in claim 1 wherein the first sprocket includes teeth at its periphery and the coupling means is a continuous double chain, having first and second channels, said first channel being fixedly coupled to the second sprocket and said second channel being in register with the teeth of the first sprocket and in driven contact with at least one tooth thereof.

3. The speed-changing coupling claimed in claim 2 wherein each link of at least the second channel of the double chain has associated therewith a roller bearing arranged to make contact with at least one tooth of the first sprocket as the first sprocket is rotated.

4. A speed-changing coupling comprising:

a rotatable input shaft;

an input sprocket eccentrically connected to the input shaft and having teeth arranged to be movable orbitally about the axis of the shaft;

an output sprocket having a diameter larger than that of the input sprocket and rotatable about the same axis;

a double-chain having one channel thereof secured to the periphery of the output sprocket and having a second channel thereof in driven engagement with at least a portion of the teeth of the input sprocket;

at least one cam race having a circular cross-section secured to a face of the second sprocket adjacent to the first sprocket;

at least one cam follower having a circular cross-section of a smaller diameter than that of the at least one cam race secured to the face of the first sprocket and disposed to mate with the at least one cam race; and

output means coupled to the output sprocket for attaching the coupling to a load.

5. The speed-changing coupling of claim 4 wherein the at least one cam race is formed as a part of the second sprocket.

6. The speed-changing coupling of claim 4 wherein the at least one cam race is secured to the first sprocket and the at least one cam follower is secured to the second sprocket.

7. A speed-changing coupling comprising:

a housing having an internal surface;

a first shaft rotatably mounted in the housing;

a first sprocket eccentrically connected to the first shaft and having teeth movable orbitally about the axis of the shaft, said shaft being disposed with respect to the internal surface;

coupling means fixedly secured to the periphery of the second sprocket and having a portion thereof in driven arrangement with the teeth of the first sprocket;

a pair of spaced cam races secured to the internal surface of the housing and each having a circular cross-section;

a pair of cam followers, each having a circular crosssection smaller than that of the respective cam races secured to a face of first sprocket and disposed to mate with a respective one of the cam races, and

output means coupled to the output sprocket for attaching the output sprocket to a load.

8. The speed-changing coupling claimed in claim 7 wherein the cam races are secured to the input sprocket and the cam followers are secured to the internal surface of the housing.

9. The speed-reducing coupling of claim 1 wherein the cam races are secured to the second sprocket and the cam followers are secured to the first sprocket.

10. The speed-reducing coupling of claim 1 wherein an eccentric havinfg a throw d is dis osed between the first shaft and the irst sprocket and he diameter of the cam race is equal to the diameter of the cam follower plus the throw d of the eccentric.

11. The speed-changing coupling of claim 1 wherein the cam races are formed as a part of the first sprocket.

Claims

1. A speed-changing coupling comprising: a first rotatable shaft; a first sprocket eccentrically connected to the first shaft and having means for driving movable orbitally about the axis of the shaft; a second sprocket having a diameter larger than that of the first sprocket and rotatable about the same axis; coupling means fixedly secured to the periphery of the second sprocket and having a portion thereof in driven engagement with the driving means of the first sprocket; a pair of spaced, circular cam races secured to a surface of the first sprocket, adjacent to the second sprocket; a pair of circular cam followers secured to the second sprocket, each disposed to mate with and follow a respective one of the cam races; and a second rotatable shaft coupled to the second sprocket.

4. A speed-changing coupling comprising: a rotatable input shaft; an input sprocket eccentrically connected to the input shaft and having teeth arranged to be movable orbitally about the axis of the shaft; an output sprocket having a diameter larger than that of the input sprocket and rotatable about the same axis; a double-chain having one channel thereof secured to the periphery of the output sprocket and having a second channel thereof in driven engagement with at least a portion of the teeth of the input sprocket; at least one cam race having a circular cross-section secured to a face of the second sprocket adjacent to the first sprocket; at least one cam follower having a circular cross-section of a smaller diameter than that of the at least one cam race secured to the face of the first sprocket and disposed to mate with the at least one cam race; and output means coupled to the output sprocket for attaching the coupling to a load.

7. A speed-changing coupling comprising: a housing having an internal surface; a first shaft rotatably mounted in the housing; a first sprocket eccentrically connected to the first shaft and having teeth movable orbitally about the axis of the shaft, said shaft being disposed with respect to the internal surface; a second sprocket having a diameter larger than that of the first sprocket and rotatable about the same axis; coupling means fixedly secured to the periphery of the second sprocket and having a portion thereof in driven arrangement with the teeth of the first sprocket; a pair of spaced cam races secured to the internal surface of the housing and each having a circular cross-section; a pair of cam followers, each having a circular cross-section smaller than that of the respective cam races secured to a face of first sprocket and disposed to mate with a respective one of the cam races, and output means coupled to the output sprocket for attaching the output sprocket to a load.

10. The speed-reducing coupling of claim 1 wherein an eccentric having a throw d is disposed between the first shaft and the first sprocket and the diameter of the cam race is equal to the diameter of the cam follower plus the throw d of the eccentric.