Power transmission system

A worm drive system includes a worm wheel having two sets of angularly arranged rollers. Each set of rollers mates with a separate screw thread on a worm screw associated with the worm wheel. The worm drive system may also include a second worm screw having two separate screw threads, each one mating with a corresponding set of rollers on the worm wheel. The worm drive system may be used as a speed increaser and/or a speed decreaser by suitably selecting the ratio between the worm wheel and the worm screws. The worm screws rotate in the same angular direction if the screw threads on one worm screw have the same hand, which is different from the hand of the screw threads on the other worm screw. The worm drive system also has a load-sharing capability and an anti-backlash capability.

FIELD OF THE INVENTION 
The present invention relates to a power transmission system, and, more 
particularly, to such a system of the worm drive type which employs an 
enveloping-type worm screw and a roller worm wheel. 
BACKGROUND OF THE INVENTION 
Conventional worm gear sets employ a worm screw and a fixed tooth gear 
wheel. Although such worm gear sets are successful drive mechanisms at low 
speeds, their efficiency is limited due to the rubbing or sliding contact 
between the fixed teeth of the gear wheel and the screw thread of the worm 
screw. 
By substituting rollers for the fixed teeth of the gear wheel of the 
conventional worm gear sets, the friction between the gear wheel (which 
because of such substitution would now be more appropriately referred to 
as a roller worm wheel) and the worm screw can be reduced, thereby 
improving the efficiency of the resulting worm drive system. Worm drive 
systems employing a worm screw and a roller worm wheel have been proposed 
in the past. Such systems can be divided into the following three 
categories. 
The first category is characterized by roller worm wheels which employ 
radially arranged rollers (i.e., the axis of rotation of each roller lies 
in a plane which is normal to the axis of rotation of the worm wheel). The 
drive systems diagnosed in U.S. Pat. Nos. 626,515; 715,973; 747,463; 
767,588 and 3,597,990 are exemplary of this first category. These worm 
drive systems have limited power transmitting capability and limited load 
carrying capacity because the radially arranged rollers make it difficult 
to employ the type of bearings (i.e., needle bearings) required to 
transmit high power and carry large loads. Because the rollers would have 
to extend into the worm wheel in order to be used in combination with 
needle bearings, the number of rollers which could be employed without 
causing interference between their associated bearings would be limited, 
thereby limiting power transmitting capability and load carrying capacity 
even if such bearings were used. 
The second category is characterized by roller worm wheels which employ a 
set of radially arranged rollers and two or more sets of angularly 
arranged rollers (i.e., the axis of rotation of each roller forms an 
inclined angle relative to a plane which is normal to the axis of rotation 
of the worm wheel). The worm drive systems disclosed in U.S. Pat. No. 
908,049; 1,060,933 and 3,820,413 exemplify this category. Due to their 
utilization of radially arranged rollers, these drive systems suffer from 
the same power transmitting and load carrying limitations as the first 
category discussed above. They also, however, suffer from a further 
limitation in that their worm wheels can only be used in combination with 
two worm screws of the same hand which, therefore, would rotate in 
opposite angular directions. Thus, the worm drive systems of the second 
category would have no utility in applications requiring two worm screws 
which rotate in the same angular direction. 
The third category is characterized by a worm wheel which employs angularly 
arranged rollers only and a worm screw having a single double-cut screw 
thread. More particularly, the worm wheel is provided with two sets of 
rollers, the rollers of one set having full tips and the rollers of the 
other set having stepped tips. The worm screw is provided with a single 
screw thread having a first helical path generated to accept the rollers 
with the full tips and a second helical path generated to accept the 
rollers with the stepped tips. Because all of the rollers are only rolling 
in half threads, the power transmitting capacity and the load carrying 
capability of such a worm drive system is limited. Further, the worm drive 
system will only run in one direction to full capacity because when the 
direction of rotation of the worm screw is reversed the rollers with the 
stepped tips will be out of engagement with a thread surface and the 
rollers with full tips will engage a half thread surface only. 
SUMMARY OF THE INVENTION 
The problems and disadvantages of the prior art devices described above are 
overcome in accordance with the present invention by providing a worm 
drive system which includes a worm wheel mounted for rotation about an 
axis or rotation. The worm wheel has two sets of angularly arranged 
rollers without any additional rollers being interposed therebetween. A 
first worm screw has a first screw thread which matingly engages one set 
of rollers of the worm wheel and a second screw thread which matingly 
engages the other set of rollers of the worm wheel. The first and second 
screw threads of the first worm screw are completely separate and distinct 
from each other. The worm wheel and the first worm screw have a 
predetermined ratio between them. 
In one embodiment, the worm drive system further includes a second worm 
screw having a first screw thread which matingly engages the first set of 
rollers of the worm wheel and a second screw thread which matingly engages 
the second set of rollers of the worm wheel. The first and second screw 
threads of the second worm screw are completely separately and distinct 
from each other. By designing the second worm screw such that it and the 
worm wheel have a predetermined ratio between them which is different from 
the predetermined ratio between the worm wheel and the first worm screw, 
the worm drive system may be used as a speed increaser and/or a speed 
decreaser. 
Another embodiment of the present invention involves designing the first 
and second screw threads of the first worm screw such that they have the 
same hand, while designing the first and second screw threads of the 
second worm screw such that they have a hand opposite from that of the 
first and second screw threads of the first worm screw. Such a design 
permits the first and second worm screws to rotate in the same angular 
direction when they are driven by the worm wheel. 
In yet another embodiment of the present invention, the rollers of the 
first and second sets of rollers are received in the worm wheel such that 
the rollers are movable between a fully extended position and a retracted 
position. Each of the rollers is urged into its fully extended position by 
exerting a force on it equal to a predetermined percentage of the overall 
load on the worm wheel. When a roller is subjected to a load which exceeds 
its predetermined percentage of the overall load, the roller will 
automatically move from its fully extended position to its retracted 
position, to prevent the roller from carrying more than its predetermined 
percentage of the overall load. 
A further embodiment of the present invention involves dividing the worm 
wheel into a first segment, which is fixedly attached to a shaft, and a 
second segment, which is rotatable about the shaft. The first segment 
carries the first set of rollers, while the second segment carries the 
second set of rollers. The first segment is urged in one angular direction 
with respect to the shaft, while the second segment is simultaneously 
urged in an opposite angular direction with respect to the shaft, whereby 
the rollers of the first set of rollers are constantly urged toward a 
trailing thread surface on their corresponding screw thread or threads and 
the rollers of the second set of rollers are constantly urged towards a 
leading surface on their associated screw thread or threads.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
Referring to FIG. 1-7, there is shown a worm drive system 10 which includes 
a worm wheel 12 and two enveloping-type worm screws 14, 16. The worm wheel 
12 is attached to a rotatable shaft 17 such that the worm wheel 12 rotates 
conjointly with the shaft 17 about a central longitudinal axis 18 of the 
shaft 17. The worm screw 14 is attached to a rotatable shaft 19 such that 
the worm screw 14 rotates conjointly with the shaft 19 about a central 
longitudinal axis 20 of the shaft 19. The worm screw 16 is attached to a 
rotatable shaft 21 such that the worm screw 16 rotates conjointly with the 
shaft 21 about a central longitudinal axis 22 of the shaft 21. The worm 
wheel shaft 17 and the worm screw shafts 19, 21 are arranged such that the 
power transfer from the worm wheel shaft 17 to the worm screw shafts 19, 
21 is at a ninety degree turn. 
The worm wheel 12 has an outer circumferential surface 23, which is 
provided with an annular notch 24 sized and shaped so as to provide 
clearance for the worm screws 14, 16. The notch 24 forms two flat 
angularly opposed surfaces 26, 28. 
The surface 26 includes a number of bores 30, each of which is 
perpendicular to the surface 26 and incudes a large diameter section 32 
and a small diameter section 34. The large diameter section 32 receives a 
roller pin assembly 36, including a roller pin 38, a thrust bearing 40, a 
pair of radial needle bearings 42, 44 and a retaining ring 46. The radial 
needle bearings 42, 44 are press fitted or otherwise inserted into the 
large diameter section 32 of the bore 30 in rolling engagement with the 
roller pin 38. The radial needle bearings 42, 44 cooperate with the thrust 
bearing 40 to permit the roller pin 38 to be freely rotatable in the large 
diameter section 32 of the bore 30 about an axis of rotation 47. The 
retaining ring 46, which is received in an annular groove (not shown) in 
the roller pin 38, engages the radial needle bearing 44 to maintain the 
roller pin 38 in the large diameter section 32 of the bore 30. Of course, 
other techniques may be employed to maintain the roller pin 38 in the 
large diameter section 32 of the bore 30. The roller pin 38 has a head 48 
having an involute tooth shape. A disk spring 50 of a predetermined 
pressure is positioned between the thrust bearing 40 and the roller pin 
38. The small diameter section 34 of the bore 30 is provided so that the 
roller pin assembly 36 can be pushed out of the large diameter section 32 
of the bore 30 by inserting a suitable tool through the small diameter 
section 34. 
The surface 28 includes a number of bores 52, each of which is 
perpendicular to the surface 28 and includes a large diameter section 54 
and a small diameter section 56. The large diameter section 54 receives a 
roller pin assembly 58, including a roller pin 60, a thrust bearing 62, a 
pair of radial needle bearings 64, 66 and a retaining ring 68. The radial 
needle bearings 64 66 are press fitted or otherwise inserted into the 
large diameter section 54 of the bore 52 in rolling engagement with the 
roller pin 60. The radial needle bearings 64, 66 cooperate with the thrust 
bearing 62 to permit the roller pin 60 to be freely rotatable in the large 
diameter section 54 of the bore 52 about an axis of rotation 69. The 
retaining ring 68, which is received in an annular groove (not shown) in 
the roller pin 60, engages the radial needle bearing 66 to maintain the 
roller pin 60 in the large diameter section 54 of the bore 52. Of course, 
other techniques may be employed to maintain the roller pin 60 in the bore 
52. The roller pin 60 has a head 70 having an involute tooth shape. a disk 
spring 72 of a predetermined pressure is positioned between the thrust 
bearing 62 and the roller pin 60. The small diameter section 56 of the 
bore 52 is provided so that the roller pin assembly 58 can be pushed out 
of the large diameter section 54 of the bore 52 by inserting a suitable 
tool through the small diameter section 56. 
The roller pins 38, 60 are arranged at an angle (.alpha.) of thirty degrees 
relative to a plane (P) which is normal to the axis 18 of the shaft 17. 
The angle (.alpha.) is selected so as to provide ample space for mounting 
the roller pin assemblies 36, 58, while permitting proper engagement of 
the roller pins 38, 60 with the worm screws 14, 16, respectively. It 
should be understood that the angle (.alpha.) may be varied depending upon 
the size and/or number of the roller pin assemblies 36, 58. The roller pin 
assemblies 36, 58 are further arranged such that the axes 47, 69 of the 
roller pins 38, 60, respectively, intersect the axes 20, 22 of the worm 
screws 14, 16 only at points intermediate the worm screws 14, 16. Thus, 
roller pins 38, 60 trace paths which are parabolic relative to the axes 
20, 22 of the worm screws 14, 16. Moreover, the roller pins 38 are 
staggered in relationship to the roller pins 60. That is, each of the 
roller pins 38 is positioned between an adjacent pair of the roller pins 
60 (see FIGS. 1 and 7). The location of the roller pins 38 relative to the 
roller pins 60 is determined by the pitch angle of the worm screws 14, 16. 
The worm screw 14 has an hourglass shape. More particularly, the worm screw 
14 includes two cylindrical opposed ends 74, 76 and a parabolic midsection 
78. Further, the worm screw 14 includes two hourglass screw threads 
T.sup.1, T.sup.2 having the same hand. The screw thread T.sup.1 has a pair 
of screw thread surfaces 80, 81 extending around the worm screw 14 along a 
helical path which matches the path generated by each of the roller pins 
38 as the worm wheel 12 and the worm screw 14 are simultaneously rotated. 
The screw thread T.sup.2 has a pair of screw thread surfaces 82, 83 
extending around the worm screw 14 along a helical path which matches the 
path generated by each of the roller pins 60 as the worm wheel 12 and the 
worm screw 14 are simultaneously rotated. Because of the hourglass shape 
of the worm screw 14, at least two of the roller pins 38 are always 
engaged in the screw thread T.sup.1, while at least two of the roller pins 
60 are always engaged in the screw thread T.sup.2. Both of the screw 
threads T.sup.1, T.sup.2 have a pitch of twelve to one, whereby the worm 
wheel 12 rotates once for every twelve revolutions of the worm screw 14. 
The worm screw 14 can be manufactured in accordance with a unique method 
and apparatus described and illustrated in copending U.S. patent 
application Ser. No. 588,967 filed concurrently herewith, now U.S. Pat. 
No. 4,588,337 which copending application is owned by the assignee of the 
present application and is entitled "APATUS AND METHOD FOR MACHINING AN 
ENVELOPING-TYPE WORM SCREW", the specification of such copending 
application being incorporated herein by reference. 
The worm screw 16 has an hourglass shape. More particularly, the worm screw 
16 includes two cylindrical opposed ends 84, 86 and a parabolic midsection 
88. Further, the worm screw 16 includes four hourglass screw threads 
T.sub.1, T.sub.2, T.sub.3, T.sub.4 having the same hand which is, however, 
opposite to that of the screw threads T.sup.1, T.sup.2 of the worm screw 
14. The screw thread T.sub.1 has a pair of screw thread surfaces 89, 90 
extending around the worm screw 16 along a helical path which matches the 
path generated by each of the roller pins 38 as the worm wheel 12 and the 
worm screw 16 are simultaneously rotated. The screw thread T.sub.2 has a 
pair of screw thread surfaces 91, 92 extending around the worm screw 16 
along a helical path which matches the path generated by each of the 
roller pins 60 as the worm wheel 12 and the worm screw 16 are 
simultaneously rotated. The screw thread T.sub.3 has a pair of screw 
thread surfaces 93, 94 extending around the worm screw 16 along a helical 
path which matches the path generated by each of the roller pins 38 as the 
worm wheel 12 and the worm screw 16 are simultaneously rotated. The screw 
thread T.sub.4 has a pair of screw thread surfaces 95, 96 extending around 
the worm screw 16 along a helical path which matches the path generated by 
each of the roller pins 60 as the worm wheel 12 and the worm screw 16 are 
simultaneously rotated. Because of the hourglass shape of the worm screw 
16, at least one of the roller pins 38 is engaged in the screw thread 
T.sub.1, while another one of the roller pins 38 is simultaneously engaged 
in the screw thread T.sub.3. Similarly, at least one of the roller pins 60 
is engaged in the screw thread T.sub.2, while at least another one of the 
roller pins 60 is simultaneously engaged in the screw thread T.sub.4. 
Thus, at least four of the roller pins 38, 60 are in simultaneous 
engagement with the worm screw 16 at any point in time. The screw threads 
T.sub.1, T.sub.2, T.sub.3, T.sub.4 have a pitch of six to one, whereby the 
worm wheel 12 rotates once for every six revolutions of the worm screw 16. 
Because the screw threads T.sub.1, T.sub.2, T.sub.3, T.sub.4 of the worm 
screw 16 have a different hand from the screw threads T.sub.1, T.sup.2 of 
the worm screw 14, the worm screw shafts 19, 21 rotate in the same angular 
direction. By making the hand of the screw threads T.sub.1, T.sub.2, 
T.sub.3, T.sub.4 the same as the hand of the screw threads T.sup.1, 
T.sup.2, the worm screw shafts 19, 21 could be made to rotate in an 
opposite direction. Like the worm screw 14, the worm screw 16 can be 
manufactured in accordance with the unique method and apparatus described 
and illustrated in the copending patent application identified above. 
The worm drive system 10 lends itself to many unique applications. For 
instance, it can be utilized as a speed increaser and/or decreaser. 
As a speed increaser, the worm wheel shaft 17 would be employed as an input 
shaft and the worm screw shafts 19, 21 would be employed as output shafts. 
With the worm screws having the ratios specified above, the rotational 
speed of the worm screw shaft 19 would be twelve times the rotational 
speed of the worm wheel shaft 17, while the rotational speed of the worm 
screw shaft 21 would be six times the rotational speed of the worm wheel 
shaft 17. 
As a speed decreaser, the worm screw shaft 19 would be employed as an input 
shaft, while the worm wheel shaft 17 and the worm screw shaft 21 would be 
employed as output shafts. With the worm screws 14, 16 having the ratios 
specified above, the rotational speed of the worm wheel shaft 17 would be 
twelve times less than the rotational speed of the worm screw shaft 19, 
while the rotational speed of the worm screw shaft 21 would be two times 
less than the rotational speed of the worm screw shaft 19. 
As a speed increaser and decreaser, the worm screw shaft 21 would be 
employed as an input shaft, while the worm wheel shaft 17 and the worm 
screw shaft 19 would be employed as output shafts. With the worm screws 
14, 16 having the ratios specified above, the rotational speed of the worm 
wheel shaft 17 would be six times less than the rotational speed of the 
worm screw shaft 21, while the rotational speed of the worm screw shaft 19 
would be two times greater than the rotational speed of the worm screw 
shaft 21. 
In the preferred operation of the worm drive system 10, the total torque 
load is shared equally between all of the roller pins 38, 60 which are in 
engagement with the worm screws 14, 16. The resulting force applied to 
each such engaged roller pin has a radial component and an axial or thrust 
component. In order for the axial or thrust load to be shared equally by 
all of the engaged roller pins, the roller pins 38, 60 should, brom a 
theoretical standpoint, extend outwardly from the worm wheel 12 the same 
distance. It is, however, difficult to manufacture the worm drive system 
10 such that all of the roller pins 38, 60 extend outwardly from the worm 
wheel 12 the same distance. The disk springs 50, 72 employed by the roller 
pin assemblies 36, 58, respectively, are designed to ensure that the axial 
or thrust load is shared equally between all of the engaged roller pins 
even if one or more of the roller pins 38, 60 extends outwardly from the 
worm wheel 12 more than the other roller pins. 
By way of example, it is assumed that the total torque load on the worm 
drive system 10 is nine hundred and twenty four inch pounds and that there 
are a total of six engaged roller pins, resulting in a radial load on each 
engaged roller pin of one hundred and fifty four inch pounds and an axial 
or thrust load of nine inch pounds on each engaged roller pin. By 
designing the disk springs 50, 72 such that each one has a pressure of 
nine inch pounds, if, for whatever reason, the axial or thrust load on any 
of the engaged roller pins exceeds nine inch pounds, the disk spring 
associated with any such roller pin will be automatically depressed until 
the axial or thrust load is shared equally by all of the engaged roller 
pins. In addition to performing such a load-sharing function, the disk 
springs 50, 72 provide a practical manufacturing tolerance as well as a 
shock-absorbing feature. 
Another exemplary embodiment of the worm drive system of FIGS. 1-7 is 
illustrated in FIGS. 8-12. The various elements illustrated in FIGS. 8-12 
which correspond to elements described above with respect to FIGS. 1-7 
have been designated by corresponding reference numerals increased by one 
hundred. The embodiment of FIGS. 8-12 operates in the same manner as the 
embodiment of FIGS. 1-7, unless it is otherwise stated. 
Referring now to FIGS. 8-12, a worm drive system 110 includes a worm wheel 
112 having a first segment 111 and a second segment 113. The first segment 
111 carries roller pin assemblies 136, each of which includes a roller pin 
138. The second segment 113 carries roller pin assemblies 158, each of 
which includes a roller pin 160. 
The first segment 111 is fixedly attached by a key 115 to a shaft 117 for 
conjoint rotation therewith. An inner face 123 of the first segment 111 is 
provided with a lug 125. 
The second segment 113 is rotatably mounted on the shaft 117 and has an 
inner face 127 which is provided with an arcuate slot 129 sized and shaped 
so as to receive the lug 125 such that the lug 125 is adjacent to one end 
131 of the slot 129. A coil spring 133 or a similar device is positioned 
in the slot 129 between an opposite end 135 of the slot 129 and the lug 
125. The coil spring 133 constantly urges the lug 125 towards the end 131 
of the slot 129, whereby the first segment 111 is urged to rotate in one 
angular direction about the shaft 117 and the second segment 113 is urged 
to rotate in an opposite angular direction about the shaft 117. Thus, the 
coil spring 133 urges the roller pins 138 carried by the first segment 111 
against a trailing surface of their associated screw thread on a worm 
screw 137, while simultaneously urging the roller pins 160 carried by the 
second segment 113 against a leading surface of their associated screw 
thread on the worm screw 137, whereby the entire torque load will be 
carried by the roller pins 160 of the second segment 113 (see FIG. 11). 
Because the engaged roller pins 138 of the first segment 111 are urged 
against the trailing surface of their associated screw thread and the 
engaged roller pins 160 of the second segment 113 are urged against the 
leading surface of their associated screw thread, any rotary play between 
the worm wheel 112 and the worm screw 137 is eliminated, thereby ensuring 
that rotation of the worm wheel shaft 117 will result in the corresponding 
rotation of the worm screw 137 and vice versa. 
As long as the total torque load is less than the pressure of the coil 
spring 133, the engaged rollers of the second segment 113 will continue to 
carry all of the torque load. Once the torque load exceeds the pressure of 
the coil spring 133, the coil spring 133 is compressed to permit the 
engaged roller pins 138 of the first segment 111 to contact the leading 
surface of their associated screw thread, whereby the total torque load 
will be shared equally between the engaged roller pins 138 of the first 
segment 111 and the engaged roller pins 160 of the second segment 113 (see 
FIG. 12). By designing the coil spring 133 such that its pressure is equal 
to half of the total design torque load, the engaged roller pins 160 of 
the second segment 113 would never have to carry more than half of the 
total torque load. 
It will be understood that the embodiments described herein are merely 
exemplary and that a person skilled in the art may make many variations 
and modifications without departing from the spirit and scope of the 
invention. All such modifications and variations are intended to be 
included within the scope of the invention as defined in the appended 
claims.