Arcuate spring

An arcuate spring having a plurality of helical coils which are configured and dimensioned to provide an arcuate shape to the spring and being substantially free of internal stressses which would tend to urge the coils into linear alignment. The spring is designed to function under load conditions while maintaining its natural arcuate shape. The spring is adapted for use in a channel having an arcuate shape approximately matching that of the spring itself, for providing resilient force in a direction tangential to the arcuate centerline of the spring at its contact ends.

TECHNICAL FIELD 
The present invention relates to an arcuate spring, a method of making same 
and to the use of the arcuate spring in a torsional vibration damper 
assembly. 
BACKGROUND OF THE INVENTION 
Vibration in a vehicle drive train has been a long-standing problem, and a 
torsional vibration damper assembly is desirable to neutralize any 
torsional vibrations emanating from the vehicle engine which could result 
in undesirable impact loads, vibration, noise, etc. 
Heretofore, torsional vibration damper assemblies have usually comprised 
straight resilient means, such as coil springs, which were forcibly bowed 
through the use of clips, wedges, spring separators or dividers, or the 
like to obtain the desired arcuate shape. In addition, a plurality of 
shorter straight springs were sometimes substituted for the longer bowed 
springs along the path that would have been occupied by the longer bowed 
springs. Such configurations, however, were complicated, requiring a 
plurality of precise parts to complete the assembly. Thus, such assemblies 
were difficult to manufacture, maintain and operate, which translates into 
a higher product cost. 
Thus, there is a need for arcuate springs to improve the performance and 
lower the manufacturing cost of such assemblies. 
SUMMARY OF THE INVENTION 
The present invention relates to a helical spring comprising a plurality of 
coils which are configured and dimensioned to provide an arcuate shape to 
the spring in its free or natural state. Further, the coils of the arcuate 
spring are free of internal stresses which would tend to urge the coils 
into linear alignment, and the arcuate spring has a strength sufficient to 
resiliently absorb and release forces in either arcuate direction along an 
arcuate path. 
Preferably, the spring is made of a hardenable or hardened steel, and is 
capable of achieving a Rockwell C hardness of at least 20 to 60 and 
generally between about 45 and 55, with a tensile strength of at least 
100,000 psi. 
The invention also relates to a method for making an arcuate helical spring 
by forming a straight helical spring; prestressing the spring to an 
arcuate shape; heat treating the spring at elevated temperatures for a 
sufficient time to relieve stresses and obtain an arcuate spring; and 
recovering the arcuate spring at ambient temperatures. 
In this method, the straight spring is generally prestressed by use of a 
fixture, and the spring is heat treated subsequent to being prestressed by 
the fixture. Alternatively, the straight spring may be heated prior to the 
prestressing step and then prestressed by use of a the fixture. 
The heat treating step includes a step of cooling the spring to ambient 
temperature. In one embodiment, this cooling step comprises quenching the 
spring into a liquid which includes water optionally containing a soluble 
oil. Instead, the spring may be air cooled to ambient temperatures. For 
either embodiment, the spring is released from the fixture after being 
cooled to room temperature. 
When desired, the ends of the spring are ground to a flat condition. 
Preferably, the ends of the straight spring are ground prior to 
prestressing. 
This invention further relates to an application of the arcuate helical 
spring in a torsional vibration damper assembly. The arcuate configuration 
eliminates the need to forcibly bow straight springs so that they may be 
received in the damper housing, and this configuration also eliminates the 
need to replace a longer bowed spring with a plurality of shorter straight 
springs. 
Further objects of this invention are to provide a construction of maximum 
simplicity, efficiency, economy, and ease of assembly and operation, and 
such further objects, advantages and capabilities as will later more fully 
appear and are inherently possessed by this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1 and 2 disclose a known torsional vibration damper assembly 10 
adapted to be operatively connected to torque input means and to torque 
output means (not shown). While the arcuate helical spring of the present 
invention may be utilized in any number of torsional vibration damper 
assembly configurations, the following torsional vibration damper assembly 
configuration is provided for illustrative purposes only. In no way is 
this illustration intended to limit the scope of the application, in 
torsional vibration damper assemblies or otherwise, for the arcuate 
helical spring of the invention. 
The damper assembly 10 includes a hub assembly 11 in the form of an annular 
ring 12 having a central opening 13 comprising a barrel 14 with shoulders 
15. The barrel 14 has internal splines or teeth 16 on the inner periphery 
of the shoulder 15 which engage torque output means (not shown). The hub 
assembly 11 further comprises a base portion 17 having three equally 
spaced elongated grooves 18 and three circumferentially equally spaced hub 
fingers 19 which are formed on the outer ring periphery 20 and are aligned 
with the midpoint of the grooves 18. The outer ring periphery 20 is 
slightly offset away from the barrel 14 so as to position the hub fingers 
19 on a parallel horizontal plane slightly lower than the base portion 17. 
Therefore, the hub fingers 19 will be positioned deeper into the elongated 
arcuate recesses 29 when the hub assembly is positioned on the first 
retainer plate 23. Each hub finger has a pair of generally outwardly 
diverging sides 21 and a circumferential end 22 which is aligned with the 
outer edge of the elongated arcuate recesses 29. 
The torsional vibration damper housing 53 is operatively connected to 
torque input means (not shown), and comprises a first retainer plate 23 
and a second retainer plate 43. The hub assembly is located on the first 
retainer plate 23. 
The first retainer plate 23 comprises an annular ring 24 having a central 
opening 25 comprising a barrel 26 with shoulders 27. The circumference of 
the barrel 26 is positioned and configured to matingly engage the inner 
circumference of the barrel 14 of the hub assembly 11. First retainer 
plate 23 further comprises three equally spaced guiding pins 28 positioned 
and configured to cooperate with the three equally spaced elongated 
grooves 18. Three equally spaced elongated arcuate recesses 29, which are 
integral with the first retainer plate 23, are separated by three spring 
separators 30. The spring separators 30 are affixed to the first retainer 
plate 23 and are normally axially and radially aligned with the hub 
fingers 19. The spring separators have a generally trapezoidal body with 
the narrow end positioned towards the center of the first retainer plate 
23 and with outwardly diverging sides extending to the larger base which 
is circumferentially aligned with the outer edge of the recess 29. 
The arcuate recesses 29 are each defined by an outer inclined lip 31 having 
a plurality of circumferentially spaced openings 32 and an inner curved 
lip 33, and act to receive arcuate springs 34 therein. The radial outward 
end of the outer inclined lip 31 drops substantially perpendicularly into 
the annular ring 24. The other side of said annular ring 24 is adapted to 
frictionally engage the torque input means. The outer periphery of said 
annular ring 54 comprises a substantially perpendicular shoulder 35. 
The second retainer plate 43 is also in the form of an annular ring 44 with 
the outer ring periphery 47 offset from the inner periphery 50. A 
plurality of circumferentially equally spaced openings 46 formed on the 
outer ring periphery 47 are aligned with the plurality of openings 32 of 
the first retainer plate 23 to receive suitable securing means, such as 
rivets, to couple the first retainer plate 23 and second retainer plate 43 
together. When the first retainer plate 23 and second retainer plates 43 
are secured together, the inner periphery 50 of the second retainer plate 
will be offset away from the center of the vibration damper housing 53, 
thereby completing the top portion of the elongated arcuate recesses 29 in 
the first retainer plate 23. 
The second retainer plate further comprises a central opening 45 and three 
circumferentially equally spaced drive straps 48. The drive straps 48 are 
indentations in the inner periphery of the second retainer plate 43 and 
are operatively associated and aligned with the three hub fingers 19, the 
three spring separators 30 and three circumferentially equally spaced 
openings 49. Openings 49 are located along the inner periphery 50 of the 
second retainer plate 43, are aligned with the registration means 39, 139 
of washers 36, 136 and are operatively associated with the guiding pins 28 
of first retainer plate 23. 
Washers 36, 136 in the form of an annular ring 37, 137 having a central 
opening 38, 138 and three circumferentially equally spaced registration 
means 39, 139 which are positioned to operatively associate with guiding 
pins 28 of the first retainer plate 23, act as separation means between 
hub assembly 11, first retainer plate 23 and second retainer plate 43. A 
washer 40 in the form of an annular ring 41 having a central opening 42 
and sides 52 which are inclined with respect to a horizontal plane acts as 
separation means between washer 36 and second retainer plate 43. 
Heretofore, the resilient means utilized in the above-described torsional 
vibration damper assembly comprised nine conventional straight springs and 
six steel balls. However, in one application of the arcuate helical spring 
of the present invention, the plurality of components comprising the 
above-described resilient means are replaced by only three arcuate helical 
springs, as shown in FIGS. 1 and 2. The arcuate helical springs in the 
present example are manufactured to be close-fitting and substantially 
congruent with the elongated arcuate recesses 29. Therefore, the helical 
springs 34 are adapted to easily fit in the elongated arcuate recesses 29 
in between the spring separators 30, hub fingers 19 and drive straps 48 
without the need for forced bowing, clips, wedges or the like. 
In operation, the vibration damper housing 53 which is connected to torque 
input means rotates upon the application of torque. This rotation causes 
the springs 34 to be compressed by the drive straps 48 and sides 51 of the 
spring separators 30 against the sides 21 of the hub fingers 19. This 
compression of the arcuate helical spring 34 is along the same arcuate 
line of action as the rotation of the drive straps 48 and sides 51 of the 
spring separators 30. Instantaneously after this compression, the side of 
the helical spring 34 being compressed against the side 21 of the hub 
finger 19 will push against that side 21 in the same direction as the 
rotation of the vibration damper housing 53 so as to align the hub finger 
19 with the drive straps 48 and spring separators 30. This expansion of 
the arcuate helical spring 34 is also along the same arcuate line of 
action as the rotation of the hub finger 19. Thus, the entire assembly 
will be aligned as it was before the application of torque and the springs 
34 will no longer be compressed. This operation acts to obviate the 
adverse effects of uneven or rapid positive and negative torques in a 
manual clutch or lock-up clutch in a torque converter. 
The operation of the torsional vibration damper assembly with the arcuate 
helical springs 34 will be smoother than the operation of a torsional 
vibration damper assembly which utilizes forcibly bowed straight springs. 
A forcibly bowed straight spring is constantly experiencing internal 
stresses which tend to straighten the spring. Thus, the forcibly bowed 
straight spring rubs against and interferes with the sides of the 
elongated arcuate recess 29, inhibiting smooth operation. However, the 
arcuate spring 34 is manufactured to conform to and fit within the 
elongated arcuate recess 29 of the first retainer plate 23. Thus, there 
are no internal residual stresses tending to straighten the spring and 
there is no interference between the arcuate helical spring 34 and the 
sides of the elongated recesses 29. Thus, the operation of the torsional 
vibration damper assembly 10 with the arcuate helical springs 34 is smooth 
and unobstructed. 
The arcuate helical springs 34 will also provide improved attenuation or 
damping of spring vibrations than the conventional vibration damper which 
utilizes straight springs for the same reasons as specified above. In 
addition, the compression of the arcuate spring to a "solid" 
configuration, i.e., where each coil contacts each adjacent coil, operates 
as a stop in the system independently of the use of other means, such as 
pins. 
Undesirable stressing in the arcuate helical spring will also be avoided 
due to the springs arcuate shape, thereby improving the efficient use of 
the vibration damper assembly. A straight spring forcibly bowed upon 
assembly experiences stresses due to the unnatural installation that are 
opposite in direction to the stresses which arise in the spring through 
use in the torsional vibration damper assembly. Springs installed in this 
manner experience stresses in one direction with the unit at rest. As 
torque is applied to the unit and increased, the springs deflect until at 
a point where these stresses diminish to zero. Further loading and 
deflection results in these stresses increasing in the opposite direction. 
This bi-directional stressing reduces the stress allowable to avoid 
excessive relaxation or breakage that can be experienced by the spring in 
service. In contrast, the body of the arcuate spring will experience only 
normal uni-directional stressing because the arcuate spring is received in 
the housing in its natural arcuate state. Thus, the arcuate spring will 
not be overly stressed, thereby increasing the useful capacity and service 
life of the vibration damper assembly. 
The durability of the vibration damper assembly is also increased due to 
the reduction in the number of springs required for operation. Spring ends 
have historically been subject to bending fatigue breakage near the tips 
of the ground end coils. In conventional torsional vibration damper 
assemblies, a plurality of straight springs were employed, thereby 
increasing the number of ground ends and providing greater opportunities 
for failure. However, since a single arcuate spring may replace a 
plurality of shorter straight springs, the number of spring ends is 
reduced. Also, those remaining spring ends may be reinforced by making use 
of the saved space that results from the minimization of components in the 
damper. Thus, the number of potential failure locations is reduced and the 
life and durability of the assembly is increased. 
Thus, it is apparent that the use of arcuate helical springs offers several 
advantages and improvements over the use of straight helical springs, 
particularly with respect to employment in torsional vibration damper 
assemblies. Because dividers, steel balls, wedges, clips, etc. are 
eliminated, there are fewer working components in the vibration damper 
assembly. Such minimization of components provides for simplicity, economy 
and ease of assembly in the manufacture of the torsional vibration damper 
assembly, and also provides for ease of maintenance, reliability, 
performance, and durability in operation. Such benefits translate directly 
into a substantial reduction in overall costs for the vibration damper 
assembly. The elimination of the superfluous components also makes for 
reduced mass and space requirements, which provides for improved travel, 
capacity, compliance, rate characteristics, and natural frequency of the 
torsional vibration damper assembly. 
The arcuate spring of the present invention can be made by various 
processes. In one method, a conventionally coiled straight spring is 
formed by traditional helical spring manufacturing techniques. Such 
techniques include beginning with annealed or prehardened and tempered 
material of any required cross section. Current materials of choice 
include, but is not limited to, 1070, 6150, modified 6150, and 9254 
steels, as processed into suitable quality spring wire. Preferably, round 
cross-section, pre-hardened and tempered (Rc 45-55) 6150 steel would be 
employed. The straight helical spring is bent and forced into an arc and 
then has a close-fitting curved rod or pin of a different free angle and 
arc radius (FIG. 3) than the desired free angle and arc radius of the 
finished arcuate spring inserted into the spring. This fixtured spring is 
then heat treated in a manner typical of conventional processing 
appropriate for the material and condition. For example, in the case of 
pre-hardened and tempered 6150 steel, the heat treatment would be 
approximately fifteen to thirty minutes at 800.degree. F. The heated 
fixtured spring is then cooled either by air or liquid quench, whichever 
is appropriate for the material. The liquid quench would preferably be 
with water which has soluble oil added to it, and the temperature of the 
solution would be approximately room temperature. The fixture pins are 
then removed, at which time the helical spring will be in an arcuate 
configuration, free of any internal stresses which tend to straighten the 
spring. 
Grinding of the spring ends, if required by the spring design, may be done 
either before or after the straight spring is processed into the arcuate 
spring. It would presently be more economical to perform the grinding on 
the springs while they are straight as conventional, existing machines may 
be used. 
The fixture pin which is inserted into the spring may be of different free 
angle and arc radius (FIG. 3) than is desired for the finished free angle 
and arc radius of the arcuate spring so as to compensate for any 
"spring-back" which may be exhibited by the heat treated spring upon 
removal of the fixture pin. The properly sized fixture pin would typically 
be determined by routine experimentation. 
The steps enumerated in the above-described process may be modified or 
rearranged as circumstances dictate to achieve similar results. For 
instance, a straight helical spring may first be heat treated, fixtured 
with the rod or pin while hot, and then have the rod or pin removed before 
it is cooled. Likewise, the spring may first be heated, fixtured while 
hot, cooled, and then have the rod or pin removed. In another variation, 
the spring may be fixtured, heat treated, and have the pins removed before 
cooling. In each case, the same result of producing a stress-free arcuate 
helical spring from a straight helical spring will be achieved. 
Each of the above variations may be further modified by fixturing the 
springs externally with a bowed or curved tube, or dies, or a drum or 
mandrel about which the spring is wrapped and retained; rather than 
internally with a curved rod or pin. Regardless of the type of fixturing 
which is employed in conjunction with the above-specified steps, the final 
product will continue to be the arcuate helical spring which is free of 
any internal stresses which tend to straighten the spring. 
"Prestressing" is used to define any type of action upon the spring or its 
coils which would cause the spring to conform to an arcuate shape, with 
the preferred prestressing method being the bending operation. 
Another method of manufacturing the arcuate spring is to force a 
conventional straight spring into an arc of much exaggerated curvature by 
forming dies at room temperature. The resulting mechanically induced 
plastic deformation will produce the desired arcuate configuration. To 
relieve the resulting residual internal stresses due to the plastic 
deformation, the spring is heat treated and cooled in a manner typical of 
conventional heat treating techniques appropriate for the material 
condition. If necessary, the spring may be fixtured before, during, or 
after the heat treatment. 
The above specification is merely illustrative of one possible application 
of the arcuate spring of the present invention. As such, it is intended 
that the appended claims cover all such applications of said arcuate 
spring as fall within the true spirit and scope of the present invention.