Method for making an epicyclic speed reducer with two stage integral rotor

In the present invention, a two-stage integral epitrochoidal cyclic rotor is produced by a machining method which produces the desired tolerences in both stages of the rotor without removing the work-piece from the support that holds it throughout the manufacturing process. In the manufacturing process, the blank starting work-piece, which is preferably a circular disc, is secured to a support element, for example by a magnetic chuck. The overall outer surface of the work-piece is then rotated and orbited while it is accurately machined to produce the epitrochoidal contour of the first stage of the rotor. Then, without releasing the work-piece from the holding surface, a second accurate machining operation, again with rotation and orbiting, is carried out on the upper half of the work-piece to form the epitrochoidal contour of the second stage of the rotor. This arrangement provides an inexpensive method for making a two-stage integral epitrochoidal cyclic rotor in which the required precision of each contour is obtained while the tolerances between the epitrochoidal surfaces of the two stages are achieved and maintained. An epicyclic drive using the two-stage integral epitrochoidal cyclic rotor is also disclosed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a high-precision multiple stage speed reducer 
that incorporates a two-stage integral epicyclic orbiting rotor having 
significantly different diameters for each stage. 
2. Description of the Related Art 
Epicyclic power transmission systems have been known for many years. 
Important improvements have been made in manufacturing techniques and in 
the design of the epicyclic units, yet such transmission systems have 
never reached the breadth of application that was anticipated. One reason 
for this shortfall is relatively high backlash caused by the build-up of 
tolerances within the unit. In an effort to minimize the backlash 
increasingly expensive manufacturing procedures have been used, greatly 
increasing the costs of the units. Excessive friction also occurs in units 
that are not manufactured with the greatest precision. 
Epicyclic speed reducers using pinion gears orbitally rotating within ring 
gears have been used for limited applications. For example, such units 
have been used in hand-driven mechanisms for raising and lowering 
automobile windows and seats. Such a unit is described in U.S. Patent 
issued in 1962 to Loutron et al in which a pair of pinion gears of 
different diameter rotate orbitally within a pair of internal ring gears. 
Such gear units are not efficient in terms of the amount of torque that 
can be transmitted. The gears are expensive to make because each tooth of 
each toothed member must be precisely cut. Such devices can transmit only 
a limited torque because the only a few of the teeth are engaged at any 
particular time, and these few teeth must carry the entire transmitted 
torque. This is a defect that is inherent in the gear type speed reducer. 
Because of these limitations, such drives were not generally suitable for 
continuous operation particularly under high speed or heavy load 
conditions. Later, the teeth of the internal ring gear were replaced by a 
series of rollers or by a hypotrochoid surface. The pinion gear was 
replaced by an orbiting rotor having an epitrochoidal outer surface. 
Developments such as these are illustrated by U.S. Pat. Nos. 4,050,331 to 
Baren; 4,271,726 to Ryffel; and 4,487,091 to Pierrat. 
A major step forward is represented by the disclosure in U.S. Pat. No. 
4,584,904 to Distin which discloses a drive system in which a pair of 
conjugate epitrochoidal and hypotrochoidal surfaces are disposed 
respectively on driving and driven members, with a number of cylindrical 
rollers interposed between them. These rollers transmit the torque while 
remaining at all times engaged with the opposed trochoidal surfaces. When 
manufactured with sufficient precision, the Distin drive accomplishes 
intended purpose. However, the cost of manufacturing and assembling the 
drive with the necessary precision has so far precluded its wide 
industrial application. 
SUMMARY OF THE INVENTION 
The present invention is an improvement on the speed reducer described in 
the Distin patent mentioned above. A structure and method of fabrication 
is provided that materially reduces the cost and number of parts in the 
drive and results in a self-contained, ultra-precise, low-backlash 
speed-reduction mechanism with a high torque capacity relative to its 
size. The device is easily manufactured and is capable of being coupled 
directly to electric motors without special coupling devices 
In the Distin and similar units, it is desirable to use a two stage 
mechanism in which the first stage produces a lower speed reverse rotation 
of an eccentrically mounted orbiting rotor with an outer trochoidal 
surface. This rotor is secured, by bolts, welding or other means, to a 
second orbiting member that causes rotation of a trochoidal race at a 
still lower speed and in the same direction as the original driving force. 
The problem arises in actual construction of the unit because of the 
tolerance build-ups. The two rotor segments must be machined with great 
accuracy and then must be secured and locked together with equal accuracy. 
This procedure is expensive, slow and usually fails to meet the desired 
operating characteristics. 
In the present invention, an integral rotor is utilized which is machined 
to the desired tolerances without removing the rotor from the chuck that 
holds the rotor during manufacture. In order to permit machining of the 
two trochoidal surfaces, the minimum radius of one of the rotor segments 
is greater than the maximum radius of the other. In manufacture, the 
circular part that is to become the rotor is secured to a flat surface, 
for example by a magnetic chuck. The outer surface is then machined to 
correspond to the trochoidal contour of the larger segment of the rotor. 
Without releasing the rotor from the chuck, the machining operation is 
carried out on the upper half of the rotor to form the epitrochoidal 
contour of the smaller rotor segment. This arrangement provides an 
inexpensive method for making the rotor while the required precision of 
each contour and the precise relationships between the two epitrochoidal 
surfaces are achieved and maintained.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in FIGS. 1 and 2, a drive shaft 1 is provided to be driven from 
any desired source (not shown). The shaft 1 is rotatably supported by two 
spaced bearings 3 and 4 (FIG. 1) and extends through an eccentric bushing 
5. There is a significant amount of clearance between the shaft 1 and the 
inner surface of the bushing 5 as illustrated at 6. Two diametrically 
opposed keys 7 and 8 (FIG. 2) extend into slots in the shaft 1 and in the 
inner surface of the bushing 5. These keys provide a torsionally rigid 
coupling between the shaft 1 and the bushing 5, but allow radial movement 
between the shaft 1 and the bushing 5 in the direction of the line A--A in 
FIG. 2 which passes through the point of greatest eccentricity of the 
bushing 5. A two-stage rotor 9 is mounted on a bearing 17 surrounding the 
bushing 5. The eccentrically mounted bushing causes the rotor 9 to follow 
an orbital path within a reaction hypo ring 10 and an output hypo ring 11 
each of which has a hypotrochoidal inner surface. 
A first set of rollers 12 is interposed between the larger outer 
epitrochoidal surface of the first stage of the rotor 9 and the 
corresponding inner hypotrochoidal surface of the hypo ring 10. A second 
set of spaced rollers 13, of a number different from the rollers 12, is 
interposed between the smaller outer epitrochoidal surface of the second 
stage of the rotor 9 and the corresponding inner hypotrochoidal surface of 
the hypo ring 11. The spacing of the rollers 12 and 13 is maintained by a 
pair of retainers 21 and 22. The arrangement of these rollers 12 and 13 
are described in more detail in the Distin patent referenced above. 
A set of rolling elements 14 are mounted between the hypo ring 11 and the 
hypo ring 10, allowing the hypo ring 11 to rotate freely within the hypo 
ring 10. The elements 14 can be either balls or rollers. The bearing races 
15 and 16 for the elements 14 are integral with the hypo rings 10 and 11, 
respectively. An end plate 23 houses the bearing 3 and a shaft seal 24 
around the drive shaft 1. Two counterweights 25 and 26 provide dynamic 
balancing. In operation, the shaft 1 causes the rotation of the eccentric 
bushing 5 which in turn forces the rotor 9, which is free to rotate on the 
bushing 5, to orbit within the hypo rings 10 and 11. Since the rotor 9 is 
an integral structure, the hypo rings 10 and 11 are forced to rotate 
relative to each other as described in the Distin patent referenced above. 
The epitrochoidal surface of the first stage of the rotor 9 has 12 lobes 
which react, through the rollers 13, with the hypotrochoidal surface of 
the reaction hypo ring 10 which has 14 lobes. The reaction hypo ring 10 is 
maintained in a stationary position and is not free to rotate causing the 
rotor 9 to rotate on the bushing 5 in a direction opposite from rotation 
of the shaft 1 and at a lower angular speed. Because the two stages of the 
rotor are integral, the second epitrochoidal surface, which has, for 
example, 12 lobes or at least a different number of lobes than the hypo 
ring 11, rotates on the bushing 5 and reacts, through the rollers 13, with 
the output hypo ring 11 which has two more lobes than the second 
epitrochoidal surface This action causes the output hypo ring 11 to rotate 
at a still lower speed and in the same direction as the input shaft 1. The 
ring 11 has a precision flat surface 20 that provides the output 
connection. If a resisting torque is applied to one hypo ring and the 
other hypo ring is held stationary, the rotation of shaft 1 will create a 
substantial radial force which will be exerted between the hypo rings. 
This force will pass through the rolling elements 14 creating minimum 
deflection and power loss. If the input shaft 1 is not exactly concentric 
with the axis of rotation of the hypo rings 10 and 11, or if the amount of 
eccentricity of the bushing 5 is not exactly matched by the total 
excursion of the epitrochoidal contours of the rotor 9 and the 
hypotrochoidal contours of the hypo rings 10 and 11, the error will be 
absorbed by a sliding motion occurring between the input shaft 1 and the 
eccentric bushing 5 in the direction of the line A--A of FIG. 2. The 
excursion of the sliding motion will be equal to the potential error in 
concentricity or in the eccentricity of the bushing 5. 
To machine the rotor 9, a circular steel blank 9a (FIG. 3) is secured to a 
metal surface 22 of a table 24 that is mounted for rotation about a center 
line C--C. The preferred securing means is a magnetic chuck (not shown) 
which locks the steel blank in place without any structures that would 
interfere with the contouring functions. The blank 9a is secured with its 
transverse central axis as near the line C--C as possible. Once the 
machining operation is started, the blank 9a is not moved with respect to 
the surface 22 until all contouring and machining operations have been 
completed. 
While the table 24 is rotating and orbiting at a constant speed, a 
precision machining tool 26, which may be either a milling cutter or 
grinding wheel, engages the outer edge surface of the rotor blank 9a in 
the position indicated by the arrow "D". The machining tool 26 follows a 
pattern that in conjunction with the rotation and orbital motion of the 
table 24 traces the desired epitrochoidal path of the larger first stage 
contour of the rotor 9. The rotation and orbital movement of the table 24 
can be controlled by a mechanical tracing mechanism or by a computer 
programmed to provide the precise movement of the table 24 relative to the 
machining tool 26. 
When the contour for the first stage of the rotor 9 is completed, a radial 
slot 28 is milled around the outer perimeter of the blank 9a at the 
interface between the first and second races of the rotor. The machining 
tool 26 is then raised to the position indicated by the arrow "E" where it 
machines the second and smaller epitrochoidal surface of the rotor 9. 
After completion of the two epitrochoidal surfaces, the central opening 
through the rotor 9 is formed by conventional boring operations without 
moving the blank 9a from the table 24. The table 24 then rotates without 
orbital motion. 
The difference in the two diameters of the epitrochoidal segments of the 
rotor 9 permits the contoured races 19 and 20 to be milled and ground in 
one set up to insure near perfect concentricity between the two races. The 
marked increase in precision is achieved with a substantial reduction in 
machining time over that required by other methods. The difficult task of 
matching and securing two separate segments to form the rotor 9 has been 
eliminated. 
The same machining tool 26 may be used throughout the operation, but 
increased precision and production can be obtained economically by the use 
of more than one grinding wheel during the machining operation. The term 
machining tool as used here means either one or a number of machining 
tools.