Overviews
To have a look over the matters we are going to talk about, it is here proposed an explanation of the
technologies and software we exploit:
– Filament Winding (FW)
– Regenerative Braking
– CADWIND
It is here assumed these overviews could not be exhaustive of the topics because of the extension of
the topics. To get a comprehensive view of the subjects, the reading of specialized literature [2][3][4][22]
is recommended.
1 Filament winding
Filament winding (FW) is a technology for composites materials born in 40's to build rocket motor
cases as high-strength as lightweight products. Nowadays it is one of the most diffused technologies
about composites materials for convex shapes making. Applications can be found as in aerospace
and marine industry as for commercial use. Examples of successfully applications are ship hulls,
oxygen bottle for fire men, pressure vessels, pipes, tows for wind turbines, filament wound rotor
cases for Space Shuttle and fuselage for aeroplanes by Beech Aircraft[2].
The interest in FW technology mainly depends on the low production cost, the easiness of the
process and the wide range of windable
shapes. Moreover, the development of the
automation brought the filament winding
machines to manage up to 6 axis, increasing
productivity and even allowing more shapes
making (such as T-joints). To have a scale
about costs, in 1991 Peters[2] evaluated
production costs for different production
methods (Figure 1) and in 2002 Ellyin asserted that in the composites industry, the process of
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Figure 1: Relative cost versus fabrication method
filament winding has evolved to be the preferred, and most cost effective method, for producing
pressure retaining structures from fibre reinforced polymeric composites[5]. Industrial process can
get several advantages from this technology[2]:
– high repetitiveness in fibre placement (from part to part and layer to layer)
– can use continuous fibre over whole component area (without joints); can orient fibres easily
in load direction
– avoid capital expenses of autoclave
– large structures can be built, larger than with any autoclave
– high fibre-volume obtainable
– costs are relatively low for material, since fibre and resin can be used in their lowest cost
form rather than as prepreg.
At the same time disadvantages stand:
– cannot wind reverse curvature
– cannot change path easily (in one lamina)
– need mandrel, which sometimes can be complex or expensive
– poor external surface, which can hamper aerodynamics
– shape of components should permit mandrel removal, otherwise mandrel has to become a
functional part of the wound.
FW manufacturing process can be generally described as the process in which a fibre tow comes out
from a reel, it comes tensioned and then it passes throughout a carriage (which eventually wets the
tow). After this path, the tow overcomes a delivery eye and winds on the mandrel. Later on, the
workpiece is cured. The fibre comes “as a whole” from the reel and it is cut after working.
At this stage we recognize the presence of 5 main sets:
– the reel, which provides the tow material. An industrial machine has a bookshelf of reels up
to 70 at once
– the tensioner. Each tow goes through a tensioner, acting like a brake (the tow is pulled out
by the mandrel) to keep the fibre as tight as prescribed
– the carriage, which generally do not carry any tensioner but the feed eye. Its main function is
to deliver the tow in the proper position assuring the desired deposition angle. In this
way it can move along one or more axes . Depending on the winding method it can carry
a resin bath to coat the fibre and a cross carriage
– the mandrel. It is the part on which the tow is going to wind up, it is not necessary to be
symmetric
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– the curing system. The resin needs to be cured to turn in a reliable matrix, so an oven can be
required, or a UV lamp (in this case curing can start also during winding) depending by
the resin
As like as aforementioned, the carriage can carry a resin bath to coat the fibre so 3 methods can be
distinguished depending on the fact that the fibre comes wet on the trolley. They calls:
– wet winding
– dry winding
– wet re-rolled winding
1.1 wet winding
In the wet winding method, the dry fibre is passed through a resin bath, causing resin
impregnation[6]. This is followed by the application of the wet fibre to the mandrel. It is a low cost
system widely used in commercial applications with thermoplastic resins, it allows the lowest
surface quality of the 3 methods. The main trouble is about the resin content of the final product,
which is affected by the measuring blade set and by winding tension. While this troubles stand,
restrictions for the resin also stand about viscosity, toxicity and pot life. In so doing, resin with less
than 2 Pa s viscosity are preferred because of the larger ability to wet the tow. If the viscosity value
is higher than 2 Pa s, resin bath have to be warmed because of the decreasing of viscosity at higher
temperatures: unfortunately pot life behave the same way.
Two kinds of wet winding systems are available: the fibre-dip path (the tow gets fully immersed,
method used if super wetting is needed) and the drum-type path (on which the tow goes upon a wet
roller).
The resin toxicity generally is related to inhalation, skin sensitization and eyes irritation.
1.2 Dry winding
In dry winding method a pre-impregnated fibre is directly applies to the mandrel[6]. This method do
not need any pot to wet the fibre prior or during the production, just curing after winding is needed.
This process is the most expensive because of the high cost of the prepreg material, but also gives
the best results back because of the high uniformity. Its use is limited to high precision applications.
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1.3 Wet re- rolled winding
Wet re-rolled winding process runs almost like dry winding process, the only difference is that fibre
comes impregnated and re-spooled prior to work. In this way it is possible to better control the
absorbed quantity of resin to obtain a prepreg-like roving, improving final finish grade. Roving can
be frozen for storing or just used.
1.4 mandrel
The mandrel is the part on which tow wraps it on. Depending on the required tolerances and from
the fibre to be wound, it can be built in a whole piece rather than in a set of parts or in a dissolvable
material, mandrel material can also be suitable to better couple to a particular tow fibre or just
become part of the final product. By this considerations it is clear that mandrel material is an
important issue. Typically, wood mandrels are used if low cost and low accuracy are needed, while
for high precision (and especially for cure temperature above 300°C) graphite mandrels are
preferred. Anyway steel and aluminium mandrels are the most used, they can also become part of
the whole if metal liner is needed.
Many issues have to be considered in choosing a mandrel. It must be strong enough to bear its own
weight during the process, the stress due to winding tension and, later on, curing stage. Stiffness is
important as well to avoid uncertainty in deposition angle of fibre and fibre path.
The mandrel coefficient of thermal expansion (CTE) should be coupled in respect of fibre CTE,
otherwise residual stresses and/or waviness could show up, especially during cure[7].
Mandrel axes can be as vertical as horizontal while feed eye keeps itself on moving to draw fibre
path on the mandrel, according to this, machines calls lathe winder. Tumble winders are also
available, in this case tumble of the mandrel allows fibre to cover the mandrel surface while feed
eye is fixed.
1.5 geodesic and non-geodesic paths
Depending on fibre path we split 2 main working: geodesic and non-geodesic winding. The former
relates to courses of fibre that are the shortest between the extreme mandrel points to be wound
(extreme point is intended to be the point at which the fibre go to the opposite end of the mandrel).
Considering a vessels with open shaped ends, the extreme points can stand on the opposite end and
side of the vessel (Figure 2). This course has the main advantage to avoid issues due to fibre
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adhesion to the surface to be covered, because the shortest course between 2 points on a surface has
the characteristic to stand on a curve without that cannot be shorted by pulling along its direction,
thus it cannot move. Using such a pattern, winding angles cannot be chosen but only depends on the
geometry of the vessel. Anyway hoop winding plies commonly interpose between geodesic wound
plies to improve strength performance.
Non-geodesic winding rely upon friction between tow and surface to be wound (Figure 3). In such a
way it is possible to turn the fibre on itself to change winding angle, or just run a path with several
rounds upon the surface only relying on the friction factor to let the tow to follow the path during
winding. This condition allows to wind within a range of allowed winding angles and it is the basis
to realize the stacking sequence of the plies. This fact let us understand the importance of the
friction factor (and its assessment) to define the tow path. Paths are allowed by friction of resin
versus mandrel for the first layer covering (it is generally the hardest condition) and friction of resin
versus resin for subsequent layers.
2 Regenerative braking
Trends in oil prices and relative tendency to fuel cost growth together with continuous decrease of
oil resources let us understand the meaning and importance of diminishing fuel consumption or just
to maximize usable effects of primary energy unit. This concept can be applied as for big plants for
electric energy production as well as for automotive and truck transportation. In such a looking
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Figure 2: Geodesic path on a sphere
Figure 3: Non-Geodesic paths on a semi-sphere,
each path relates to a different friction factor
several technologies about energy saving and/or energy recovery are being investigated in many
fields:
– in heat plants we talk about co-generation
– in processing industry, such as refineries, it is about heat exchange nets
– in automotive engineering, it is about kinetic energy storage for on-demand use
The latter is the matter of our interest.
It consists in absorb kinetic energy during braking, energy that should otherwise be dissipated by
brakes, and then give it back to the vehicle when speeding up. This technology should mainly apply
to large size means of transport, such as trucks or buses, because the bigger the system is, the bigger
the energy consumption is, thus the bigger the saving energy quantity could be (and also the bigger
the place to house the system is). The energy available in mechanical form is pretty valued because
it allows an higher saving quote but the storing systems are quite a few. Among them we call
– Flywheel Energy Storage (FES)
– Superconductor Magnet Energy Storage (SMES)
– Hydropneumatic Energy Storage (HES)
2.1 Flywheel Energy Storage
In FES systems energy gathers in kinetic form by a rotating flywheel:
E FES=
1
2
I 2
while the amount of specific energy to be stored (stored energy per mass unit) is limited by the
expression:
E
sp−FES=
1
2
Thus, if a material both lightweight and high-strength is employed, the solution is among the best
solution we can get. Examples of successful applications use carbon fibre (CF) and allows a specific
energy storage up to 532
W h
kg
at 66000 RPM. Available solution can be fully mechanical
(FES-Mechanic) or use an electric motor in interposition (FES-Electromechanic).
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2.2 Superconductor Magnet Energy Storage
In SMES systems energy is as stored as electric currents in a closed circuit made of a
superconductor material, which actually becomes a permanent magnet. Superconductor usage is
mandatory to curb energy dissipation due to Joule effect. Also in this case specific energy relates to
stress:
E sp− SMES=
but stress is now the one caused by currents running into the system, so it is about stresses due to
the magnetic forces. Successful applications allows storage up to 355W h
kg
.
2.3 Hydropneumatic Energy Storage
Hydropneumatic Energy Storage systems use gases and liquids properties to successful store
energy. Energy stores into pressurized gas and energy transmission lets a small bulk size. Specific
energy depends on stresses onto vessel material:
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Figure 4: Flywheel Energy Storage - Mechanic
Figure 5: Superconductor Magnet Energy
Storage System
E sp−HES=0,15
Specific energy values are smaller than previous systems, using composite materials maximum
energy stored is 160
W h
kg
. Main troubles come from the high temperature gradients (and from
high temperatures) due to impulsive-like pressure gradients. 2 technical solution are used, the first
uses a pump in interposition between pressure vessel and the drive shaft (HES Power Assisted),
while the second put a pneumatic subsystem between motor and drive (HES Hydrostatic
Transmission).
3 CADWIND software
CADWIND software is a product of MATERIAL S.A.. This software supplies the path
characteristics over the intended mandrel and the known machine motions. It accounts for both the
tow and the machine features. The software allows to customize the parameters about the fibre, the
resin, the mandrel dimensions, the machine dimensions and the motions up to 6 axes. It allows to
choose among the geodesic path, the non-geodesic path and the T-joint winding.
CADWIND (CW) split the work over 3 main stages[22]:
– mandrel modelling
– winding path definition
– postprocessing
3.1 mandrel modelling
Mandrel modelling allows to draw several mandrel shapes among the available cross sections. The
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Figure 6: Hydropneumatic Energy Storage
System – Hydrostatic Transmission
cross sections (circular, square, elliptical) and the mandrel ends (conical, sphere-shape, elliptical,
sphere) must be inputted. It is possible to shape T-joints and elbows. Once parameters are up, the
mandrel would show on the screen. It would be possible to save the mandrel into an *.MDR file and
lately to edit such file by the CADWIND editor or by a general *.txt editor. The mandrel is
modelled by frames. Each frame defines the coordinates among several axes, depending on the
shape different axes are necessary for the mandrel description. A symmetric mandrel such as a
cylindrical vessel needs the longitudinal coordinate of the frame and the respective diameter size.
The number of frames is limited. It is also possible to input tow parameters together with both fibre
and resin parameters, such as TEX, viscosity and fibre volume fraction.
3.2 winding path definition
On the winding window the software asks about winding angle α, friction factor, path number,
covering grade, frames management (turning frames, number of frames, starting frame) and dwell
angle.
Both winding angle and friction factor could be customized at each frame. Path number constitutes
of 1 sign and 2 numbers split by a slash. The sign defines the path to be a lagging path or a leading
path. The first number accounts for the fibre shifting over the mandrel. The turning frames are two,
they are the frames at which the fibre starts preparing the stroke back. At the starting frame
CADWIND begins the path.
The dwell angle is the extra-angle a mandrel runs at the end of the turning zones before to take the
stroke back. It was shown to be a reinforcement for the ends[8].
Once the set is up, CW starts looking for the path. The set is generally bad and it is necessary to
refine the data or to choose a different path number by the splash screen. It is also possible that
CADWIND asks to change some parameter (i.e. winding angle). Once the path is done, it is plotted
on the mandrel on the screen. Over this plot we can toggle colour images for thickness and winding
angle emphasizing. CW prints a colour-scale and reports the corresponding colour for each point of
the tow trajectory but, unfortunately, it does not compile any list.
An important limit of this stage is that CADWIND does not notify whether the path totally covers
the mandrel or not. This big limit lets the operator to check the path. Difficult stands because path
file (*.PTH) is not defined in CW manual, so the operator has to check the plot by sight on the
screen. The reason why CW avoids notifying the un-covered parts is that they stands beyond the
turning frames. Actually CW puts effort to assure thickness, low winding angle variations and other
parameters only for the frames within the turning frames. CADWIND seems to calculate the output
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