Fiber reinforced composites belong to a new class of materials and allow great flexibility in flywheel design. The most efficient flywheel may no longer have the classic Stodola taper and indeed, may not even be round. Some of the flywheel designs that have been developed in the past are discussed. Although choice of material, mounts and service requirements often dictate the final design choice for a particular application, the composite flywheels in this paper are classified within a geometric framework, a simple stress analysis of a circular disk is carried out

Steele, R. S.

English

NASA Technical Reports Server

13,qG4
GENERIC COMPOSITE FLYWHEEL DESIGNS
R. $. S_eele
Greenville, Sou_h Carolina
Private flywheel consultanu.
PRECEDL'-'G PAGE BLANK NOT 7_...'.5_:'_
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Fiber reinforced composites belong to a new class of materials. They
allow such flexibility that free thinking individuals are encouraged to "let
themselves go" with their design. The ,,>st efficient flywheel may no longer
have the classic "Stodola n taper and indeed, may not even be round. Since
selection of an optimum design can be the subject of a sizeable study [refs. 1
and 2], I thought that it might be instructive to review some of the flywheel
designs that have been developed and comment on what NASA might learn from this
experience.
Although choice of material, mounts and service requirements often dictates
the flnal design choice for a particular application, I have chosen to classify
these composite flywheels within a geometric framework. The breakdown given in
figure I is probably not exhaustive but will suffice for today's discussion.
NOH-ROUHD ROTORS
Bar
Brush
ROOND ROTORS
Solid Disk
Flat disk (pseudo-isotroplc)
Alpha-ply layup
Without rim
With rim
Molded (sheet molding compound)
Plywood
Tapered disk (constant stress)
Stodols
ModiEied Stodola
Pierced Disk (rims)
Woven
Radial stringers
flelicsl weave
Thln Circ Wound
Rollers
Levitated
Thick Circ Wound
Single material (residual stress)
Si,.gle step winding
MuLti-step _inding
Multi-material rims
Circular
Sub-circular
!
T
"T
1
170
Figure i.- Classification of generic rotor designs.
, J
t
Let me start with a few co_nents regarding non-round rotors (fig. 2).
They are generally simple to construct, and in the case of the fan-brush type
leer. 31 possess • relatively benisn failure mode. A _aJor problem Is their
low volume efficiency. They do not fully occupy the volume In whlch they spin.
The bar type tends to split at the tips and take on a fan-brush character.
The splitti_ is the result of low transverse strength In the composlte. As we
will see, this is usually the liLitin8 characteristic of composltes.
_xT
Figure 2.- Non-round rotors.
171
Before discussing the various verturbations of circula_r flywheels, we
should look at a simple stress analysis of a circular disk [ref. 4]. (See
fig. 3.) This analysis shows that a constant thickness disk with a small central
hole can have a greater hoop stress at its center than if it did not have the
hole. On the other hand, the maxim,-_ radial stress will he lower.
Now consider the two types of construction used. The first has a fiber
orientation that macroscoplcally simulates the character of an isotroplc
material (maria1). These were called pseudo-lsotropic in the DOE program. They
lean to the solid disk designs because even though their overall composite
strength is lower than the unidirectional fiber composite, they can survive the
high radial stresses.
The other is characterized by a highly structured (usually orthogonal)
fiber orientation that pisces the strength of the fiber in the direction of the
major principal stress component. This allows the composite structure to
survive a very high major prlncipal stress so lone as the minor prlnclpal
stress does not exceed the transverse strength of the composite.
The favorite structured orientation is a hoop wound cylinder which
naturally possesses a central hole. The limiting factors have been either the
low strength transverse to the fibers or items independent of the rim. The
larger the diameter of the hole, the lower the transverse stress. Thus the
designer is faced with balancing his desire for a thick, high energy rim -alth
the maximum transverse stress his material can stand.
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172
Figure 3.- Stress analysis of spinning circular disk.
m _ im
Let me review some of our experience with actual rotors. I will rely upon
my own experience with the Department of Energy's Mechanical Energy program as
test director at the Oak Ridge Y-12 Plant's Flywheel Evaluation Laboratory.
Pseudo-isotropic flywheels demonstrated lower energy densities but also
had relatively low fabrication cost. They also demonstrated a higher level of
durability than their competition. One type was built using an alpha-ply [ref.
5] layup in which sheets of unldlrectlonal fibers in an epoxy =strlx were
stscked at various angles until the desired thickness was reached. Circular
disks were machined from the cured plates and_ in some cases, tapered to
simulate the constant stress shape [ref. 6] used in high performance metal
flywheels.
A pseudo-isotcopic flywheel (fig. 4) survived our only 10,000 cycle fatigue
test. The graphite version of the modified Stodola flywheel achieved close to
1000 m/s tip speed, giving it the top speed in our record book.
Two problems persisted [ref. 7J. An elastomerlc interface is required
between the the metallic arbor mount and the coRposlte surface. Unless the
metallic arbor and the disk mass centers both lie on the spin axis (a very
remote possibility), the distance between them will vary with speed. This adds
complexity to the suspension system. The fatigue stresses at the bonded
interface are also found to be detrimental to the llfe of the rotor.
The other problem is the severe failure modes. The constant thickness
disks usually failed at the drive shaft interface. This allowed the entire
disk to roll around inside our containment, setting up severe vibratory
stresses in the containment for several seconds. A "lightweight" containment
vessel would not stand the failure. The tapered disk performed such that it
disintegrated in an explosion that sen_ chunks of graphite composite into our
containment ring. _hat we must learn is that the pseudo-isotropic rotor can
produce the full range of failure problems, thus making contai._ment development
difflcult and expensive.
Figure 4.- Pseudo-lsotroplc disk.
173
."4
A second method of accomplishing isotropy was demonstrated by the molded,
chopped fibergZass rotors [ref. 8]. They required a hoop wound support ring to
achieve the highest energy density. A similar arbor attachment was used and
the same balance problems were encountered. The failure mode was consis__ent
[ref. 9]. The hoop supports usually burst, leaving the disk no longer able to
support Itself. Th.e entire rotor broke £nto small fibrous wads of mass, falrly
uniformly distributed in the chamber.
Finally, a very inexpensive flywheel was proposed and built from
hexagonal, birch plywood [re£. 10]. (See fiE. 5.) Successful testing was
reported with respectable enerEy densities for the cost involved.
However, since NASA will place mo=e importance upon performance than cost,
it is unlikely that a solld disk from any composite will be chosen for space
appllcatlous.
FiEure 5.- Lover cost pseudo-lsotroplc disk.
" 174
m1
If you are going to use a composite flywheel, it will be a disk with a
hole. Should it have a big hole or a little hole? i know of at least one
composite flywheel that was circumferentially wound around a 25---- spindle with
an outer diameter of about I mm. When tested, it failed at a very low speed due
to transverse cracking. (See fig. 3.) The smaller the hole relative to the
overall diameter, the greater the transverse stress. Thus the limiting materiel
factor in thick, orthogonal composite rings is the transverse composite
strength.
You have two problems: how to make the rim as thick as possible so it
will be volume efficient and how to attach it to the outside world.
Several attempts have been made to wind a residual stress state into the
flywheel so that the inner circumference is in compression et rest. This
allows a thicker rim at a given speed.
I am familiar with two flywheels that use this technique. In the one
pictured in figure 6(a), carbon fibers were used in a polysulfone matrix [ref. ]1]
that was cured durir winding. The design speed was not achieved because the
transverse strength _f the composite wlth this matrix was lower than the designer
had expected. The one in figure 6(b) used 14 w/rid-cure steps to produce the rim.
The band-wrap [ref. ]2] technique was used to attach the rim to a central shaft.
Pre-stressing did result in an improvement upon an earlier non-pre-stressed wheel.
However, the ultimate speed was llmited by the band-wraps.
Another method to increase the thickness of a rim is to place radial
reinforcements in the rotor [ref. 13]. (See fig. 6(c).) These rotors were fairly
successful but suffered interface problems with the drive shaft.
(a) Continuous winding with
residual stress.
(b) Step winding with
residual stress.
(c) Radial stringers or
bidirectional weave.
Figure 6.- Thick rims.
175
• . ,._ "_._.. - ._
Ll .... I . I
Another successful and less expensive method is to wind the rim from
several materials. An inner material with a lower modulus/density ratio will
grow radially, with speed, into the higher modulus density rings [ref. 14]. This
creates compressive ra_lal stresses at the interface between the layers. To my
knowledge, no one has yet been able to create a hoop failure in such a
flywheel. Failures in the attachment system always come first.
Attaching a rim to the outside world is not simple. The primary problem is
to accommodate the radial growth of the rim without having such a "soft" system
that the flywheel would ha_e resonant frequencies in the operating range.
The most successful designwith the rim/shaft interface in tension used
rotational catenary spokes [ref. 15]. (See fig. 7(a).) These were carefully
designed to grow with the rim.
The other successful approach used compressive spokes and a sub-circular
rim [ref. 16]. (See fig. 7(b).) This required a rim made of multiple, concen-
_tic, thin rings held together at rest by a small interference fit. The spokes
remained in compression until the ri-: grew circular. The rim failures that have
occurred in this type resulted from overheating and fatigue. The outer layers are
most susceptible, and if the rest of the rim can be supported by the shaft and
spoke system, the containment problems are minimized. The designer of the flywheel
shown in figure 7(c) needed a very rugged flywheel [ref. 17] for his intended
application and used aluminum plates in the center of the rim with 16
slip-pins to ecco_date the differential radial growth. Unfortunately, the
tolerance on the slip-pins was insufficient to maintain adequate balance. We
were unable to confirm its 4 kWh potential energy storage capability.
(b) Compressive spokes
(a) Catenary spoke. (subcircular).
(c) Rim with aluminum plates.
Figure 7.- Multi-material rims.
176
Finally we come to the rim without a central shaft. I know of two basic
designs. The first is a thin ring supported on rollers. Power I/O was through
a rack and pinion system on its inside surface. The proposed rim was I meter
thick, 2 meters tall, and ] kilometer in diameter. The composite material was
steel reinforced concrete. That rim was never built.
The other is also a thin ring but is magnetically levitated and functions
as the rotor of a motor/generator. The test results I have seen regarding this
design involve control and power studies. I know of no test results concerning
t',e life or performance characteristics of these rotor designs• It strikes me,
though, that since a l-meter-diameter rotor will have about 5 mm radial growth
at operating speed (2% graphite operating with a 50_ knockdown factor), the
electrical coupling could be severely degraded at _ax_mum speed. An inefficient
coupling may lead to high temperatures at the highly stressed inner surface of
the rotor and thus reduce life.
Only testing under these specific conditions will provide the confidence
needed in the lifetime for one of these units•
A rim type rotor can be built that will have an energy density suitable to
NASA's requirements• The levitated rim type appears structurally attractive if
the electronic couplinR doesn't suffer too much from radial growth.
I am concerned that two areas might be neglected. The containment problems
are not trivial. A composite flywheel does not simply break up into fluff. It
loses a lot of energy as it stops. If it a_tempts to stop too suddenly, you
have a containment problem. For instance, a rim contained in a small cavity
will tend to jam suddenly, creating a tremendous torque [refs. 18, 19, and
20]. The severity of the problem is known to vary with materials_ but the test
results needed for accurate design optimization do not exist.
Neither does good data exist for my other conL-rn, which is the effect of
long term exposure to sustained high stress in vacu.,m. Composite properties,
and in particular matrix properties, will change wi_h time. Very little data
exists to estimate what the material characteristlcs will be after one to five
yearsof operation.
I have touched on many different topics that have been addressed in the
development of composite flywheels. My final words are to encourage NASA to
approach composite flywheels cautiously and with a healthy respect for the
energies involved (fig. 8).
*CoutainuentDevelopmentwill NOT beTrivial
• Remember total energy involved
Look for fast stops
* Long Term Effects Need Assessment
• Sustained stress
Continuous vacuum
Radiation
Figure 8.- Concerns needing study,
! 77
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