Draw transmission

A transmission system is provided for differential speed control. An endless, resilient member in the form of an endless belt is driven by an input pulley. The speed differential is derived by using the input speed taken from one effective radius of the resilient element and picking the output speed from a different effective radius of the resilient element, with the different effective radii being variably selectable. Thus an output pulley has its periphery in contact with the resilient member and a control carriage coupled to the resilient member is operative to vary the curvature of the resilient member with respect to the periphery of the output pulley. In this manner, the neutral axis of the resilient member is varied to control the speed of the surface of the resilient member that is contacting the periphery of the output pulley.

BACKGROUND OF THE INVENTION 
The present invention concerns a novel transmission system for differential 
speed control and, more particularly, a narrow range draw transmission 
that is suitable for servo control of rotating mechanisms requiring 
differential speed control up to the 10 percent range. 
Differential control of draw percentage is required during the processing 
of continuous strands or webs of a generally elastic substrate. Such 
differential control is utilized in the areas of the process line where it 
is essential to maintain tension or to attain and maintain a constant 
length, such as in the drawing, printing, coating or perforating of a 
paper substrate. In closed loop applications, the substrate is monitored 
at various points throughout the process line and the differential draw is 
adjusted to compensate for changes in elasticity. 
In the prior art, variable draw was obtained by either (a) using a wide 
range device over a small portion of its total range capability, or (b) 
using a wide range device in conjunction with a mechanical differential 
transmission. 
Wide-range devices which were used over a small portion of their total 
range capability included variable pitch "V" pulleys in conjunction with 
standard or specially formed "V" belts, variable pitch toothed pulleys 
used in conjunction with a special "profile conforming" chain, friction 
drives using balls or cones or discs, variable voltage DC drives, variable 
frequency AC drives, etc. Generally, these wide-range devices have a 
control range between 4:1 and 20:1. This creates extensive range 
capability problems when the device is used as the output of a narrow band 
servo system. Many of these devices are subject to localized wear 
seriously limiting their useful life when they are continuously used over 
a narrow portion of their normal range. 
As stated above, variable draw was also obtained in the prior art by 
utilizing the above devices in conjunction with a mechanical differential 
transmission. This could be an epicyclic gear arrangement, a harmonic 
drive set, a bevel or spur gear differential, or other related 
arrangements. While this approach reduces the total range of the device, 
it does not appreciably reduce wear grooving. Further, adding the 
mechanical differential system increases expense and adds to the 
complexity of the total machine. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a transmission system is provided 
for differential speed control. The system comprises input means connected 
to a driven input pulley and an endless resilient member driven by the 
input pulley. A tensioning pulley is spaced from the input pulley for 
providing tension for the endless resilient member. 
In accordance with the present invention, output means are connected to an 
output pulley. The output pulley is an idler pulley located intermediate 
the driven pulley and the tensioning pulley, with the output pulley having 
its periphery in contact with the resilient member. 
Means are provided for controlling the speed of the output pulley. The 
controlling means are coupled to the resilient member and are operative to 
vary the curvature of the resilient member with respect to the periphery 
of the output pulley. In this manner, the neutral axis of the resilient 
member will be varied to control the speed of the surface of the resilient 
member that is in contact with the periphery of the output pulley. 
In the illustrative embodiment, the controlling means comprises a control 
carriage having four pulleys. Means are provided for moving the control 
carriage toward and away from the output pulley to vary the curvature of 
the resilient member with respect to the periphery of the output pulley. A 
nip pulley is positioned adjacent the output pulley but on the opposite 
side of the resilient member. The periphery of the nip pulley contacts the 
surface of the resilient member that is opposite to the surface of the 
resilient member in contact with the periphery of the output pulley. 
A more detailed explanation of the invention is provided in the following 
description and claims, and is illustrated in the accompanying drawing.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT 
Referring to the drawing, the transmission system shown therein is useful 
for differential speed control, particularly in the range from 0 to 10 
percent. An input shaft 10 is keyed to an input pulley 12, with input 
shaft 10 being driven at a predetermined speed so that input pulley 12 
will operate as a drive pulley having such predetermined speed. Input 
pulley 12 drives an endless resilient member 14 which comprises a 
conventional endless belt preferably constructed with rubber-covered cords 
and/or other suitable resilient materials. 
A tensioning pulley 16 in the form of an idler pulley also supports endless 
belt 14 and is mounted to provide a variable center distance between the 
two pulleys. Tensioning pulley 16 is suitably loaded so as to provide a 
predetermined belt tension. In the illustrative embodiment, tensioning 
pulley 16 has an equal diameter to the diameter of input pulley 12. 
An output pulley 18 having an output shaft 20 keyed thereto comprises an 
output pulley and is located intermediate the driven input pulley 12 and 
the idler pulley 16. It is preferred that output pulley 18 be located at a 
point that is approximately equidistance between pulleys 12 and 16. In the 
illustrative embodiment, output pulley 18 has a diameter that is equal to 
the diameters of pulleys 12 and 16. It can be seen from the drawing that 
the inside of belt 14 contacts the peripheries of pulleys 12 and 16 while 
the outside of belt 14 contacts the periphery of pulley 18. 
A nip pulley 22, having a diameter that is smaller than the diameter of 
pulleys 12, 18 and 16, is mounted directly opposite to pulley 18 with its 
periphery in contact with the inside of belt 14. Pulley 22 is suitably 
loaded so as to provide an essentially no-slip nip point between pulley 
22, the belt 14 and output pulley 18. 
In accordance with the invention, the speed of output pulley 18 and thus 
output shaft 20 is controlled by varying the curvature of belt 14 with 
respect to the periphery of output pulley 18. In this manner, the neutral 
axis of belt 14 will be varied to control the speed of the outside surface 
of belt 14 which is in contact with the periphery of the output pulley 18. 
The speed control means of the present invention includes a control 
carriage 24a, 24b having a first pulley 26, a second pulley 28, a third 
pulley 30 and a fourth pulley 32. It can be seen that first pulley 26 is 
smaller in diameter than nip pulley 22 and is located intermediate output 
pulley 18 and input pulley 12 with the periphery of first pulley 26 being 
in contact with the inside surface of belt 14. Second, third and fourth 
pulleys 28, 30 and 32 have equal diameters to the diameter of pulley 26. 
Second pulley 28 is located between output pulley 18 and tensioning pulley 
16 with its periphery being in contact with the inside surface of belt 14. 
Third pulley 30 is located between output pulley 18 and input pulley 12 
with its periphery being in contact with the outside of belt 14. Fourth 
pulley 32 is located between output pulley 18 and tensioning pulley 16, 
with its periphery being in contact with the outside surface of belt 14. 
Pulleys 26, 28, 30 and 32 are mounted on fixed centers and turn freely on 
their internal bearings. 
Pulleys 26, 28, 30 and 32 are connected to move selectively toward and away 
from the output pulley 18 in synchronism. Each pulley 26, 28, 30 and 32 
has a shaft which terminates at the free end of a four-bar link 
arrangement and the shafts are tied together with a connector link. A 
force bias spring is provided to offset the force of the belt pull as the 
control pulleys are moved out of their center or neutral point. Thus the 
control carriage 24a, 24b is moved upward, away from output pulley 18 if 
the speed of output pulley 18 is to be increased with respect to the speed 
of input pulley 12. If the speed of output pulley 18 is to be decreased so 
that it becomes more equal to the speed of input pulley 12, control 
carriage 24a, 24b is lowered. 
To understand the operation of the system, it can be seen that rotating 
input shaft 10 will rotate input pulley 12 causing belt 14 and all of the 
other pulleys 16, 18, 22, 26, 28, 30 and 32 to turn. Since input pulley 12 
is wrapped by belt 14, input pulley 12 will impart its driving force to 
the inside surface of belt 14. However, the belt 14 leaving the input 
pulley 12 will have an outside surface speed that is greater than the 
speed of the periphery of the input pulley 12 due to the shift of the 
neutral axis of belt 14 as the input pulley 12 is unwrapped by belt 14. 
As belt 14 passes through the nip point between nip pulley 22 and output 
pulley 18, these pulleys will turn. Since the belt 14 is in a straight 
line at this point, it will drive output pulley 18 at the outside surface 
speed of the belt as it left input pulley 12. If input pulley 12 and 
output pulley 18 are the same diameter, output shaft 20 will rotate faster 
than input shaft 10, because the outside surface of belt 14 in contact 
with output pulley 18 is moving faster than the inside surface of belt 14 
in contact with pulley 12. 
If the control carriage 24a, 24b is moved downwardly, the outside surface 
of belt 14 will begin to wrap output pulley 18 and thus change the neutral 
axis of belt 14. This change will be in a direction corresponding to the 
neutral axis of belt 14 as it passes around input pulley 12. The greater 
the amount of wrap of belt 14 on output pulley 18, the closer the speed of 
the input shaft 10 and output shaft 20 will coincide because the speed of 
the outside surface of belt 14 will decrease as it forms an arc around the 
periphery of output pulley 18. Since the speed of output pulley 18 will 
correspond to the speed of the outside of belt 14, it can readily be seen 
that wrapping the output pulley 18 with belt 14 will decrease the speed of 
output pulley 18. 
Conversely, if control carriage 24a, 24b is raised so as to wrap nip pulley 
22, the speed of the outside surface of belt 14 at the nip point between 
nip pulley 22 and output pulley 18 will increase. As the wrap of the belt 
14 on pulley 22 is increased, the speed of the outside surface of belt 14 
is increased at the nip point. Since output pulley 18 rotates in 
accordance with the speed of the outside surface of belt 14 at the nip 
point, it can be seen that moving the control carriage 24a, 24b away from 
the output pulley 18 will increase the speed of output pulley 18 and thus 
increase the speed of the output shaft 20. 
Within limits, the ultimate range of the system can be controlled by 
selecting belt thickness and the relationship between the diameters of the 
pulleys. The useful range of any selected combination of diameters and 
thicknesses can be limited by controlling the excursion of the control 
carriage 24a, 24b. 
While in the illustrative embodiment the output pulley 18 has been shown on 
the outside of the belt, the output pulley could be on the inside of the 
belt in the position of pulley 22 and the nip pulley could be on the 
outside of the belt in the position of pulley 18. Although an illustrative 
embodiment of the invention has been shown and described, it is to be 
understood that various modifications and substitutions may be made by 
those skilled in the art without departing from the novel spirit and scope 
of the present invention.