Method of winding a wind turbine blade using a filament reinforced mandrel

A hollow, stiff, one-piece filament-reinforced composite mandrel of nonuniform wall thickness for a wind turbine blade spar is comprised of bonded inner and outer filament-reinforced shells. The inner shell is fabricated by bonding together separately formed tubular halves. The outer shell is built up on the outer surface of the inner shell preferably by judicious winding of a plurality of turns of filament-reinforced epoxy resin material.

BACKGROUND OF THE INVENTION 
1. Field of Invention 
The present invention relates to wind turbine blades and more particularly 
to the production of a simple, stiff, lightweight filament-reinforced 
mandrel of nonuniform wall thickness for the spar of a wind turbine blade. 
2. Description of the Prior Art 
In the current press for new forms of energy sources, one attractive 
candidate has been the wind turbine. In general, a wind turbine comprises 
an arrangement of rotor blades, hub, pitch change mechanism, gear box, 
tower, generator and typically an electrical interface, all adapted to 
extract energy from atmospheric winds and convert it into electrical or 
other useful forms of energy. 
The large wind turbine (100 KW and over) is characterized by its extremely 
long rotor blades (typically at least fifty feet long each although they 
may be as long as one hundred feet or more each) which are subjected to 
severe bending and twisting at design loadings. The rotor blades are 
advantageously constructed in the spar/shell configuration, with the spar 
being fabricated by filament winding on a mandrel, as indicated for 
example in U.S. Pat. No. 4,081,220 issued on Mar. 28, 1978 to Andrews and 
owned by assignee common to the present invention, which disclosure is 
incorporated herein by reference. While the nature of the spar mandrel is 
not discussed in detail in the aforementioned Andrews' patent, it 
typically consists of a central load carrying structure in the nature of a 
steel channel box member upon which are carried a series of separate 
removable sections consisting of formers, stringers and sheet metal skin 
which are held in place on the central box beam with keys, pins, or the 
like. To facilitate mandrel removal since there is considerable taper from 
the butt to the tip end of the spar and since the former/stringer/skin 
assembly is carried on the outer surface of the central box beam, the load 
carrying capacity at the tip of the mandrel is very low. The limitation on 
the thickness of the tip of the mandrel requires that it be supported 
primarily as a cantilevered beam with a very small portion of the weight 
(up to a maximum of approximately three percent) carried on a steady rest 
at the tip. This type of mandrel construction is extremely heavy (on the 
order of twenty tons for a one hundred foot blade spar) and imposes a 
substantial overhung moment on the headstock of the machine which supports 
and rotates the mandrel during winding, resulting in massive structure 
bearings and foundation requirements. 
In addition to the weight, a mandrel constructed in several sections as 
heretofore described always has motions at the joints between the sections 
due to deflections in the structure as the mandrel rotates. These motions 
cause heavy wear on the mandrel sections and also result in distress in 
the composite material as it is being applied and cured. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to overcome the aforementioned 
deficiencies of the prior art and to provide a one-piece, high strength to 
weight ratio, hollow filament-reinforced composite mandrel suitable for 
use in fabricating a wind turbine blade spar, the mandrel being in the 
form of a stiff cantilevered structure which is extremely simple to 
manufacture. 
The present invention contemplates the production of a one-piece wind 
turbine rotor blade spar mandrel which tapers from base to tip and which 
comprises a hollow, cantilever structure of bonded inner and outer 
filament-reinforced layers and fitting means mounted on the ends of the 
structure for rotatably mounting the same in a blade fabricating 
mechanism, the mandrel being capable of carrying at its tip up to ten 
percent of the total of its own weight as well as the weight of the blade 
being fabricated thereon, and being sufficiently stiff to deflect less 
than one percent during blade fabrication. In one embodiment, the mandrel 
wall is provided with a plurality of perforations to facilitate, by 
passage of pressurized air, the removal of the blade spar fabricated 
thereon. 
The invention additionally contemplates a method for making the mandrel, 
which method includes the steps of making longitudinal half sections, 
bonding the sections together to form an inner shell of the mandrel, 
forming an outer shell of the mandrel by applying filament-reinforced 
epoxy resin on the outer surface of the inner shell, and securing fitting 
menas to opposite ends of the inner and outer shells to adapt the mandrel 
for rotatable mounting on blade fabricating winding mechanism.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the drawings, FIG. 1 shows a pair of molds 10 and 12 representing 
longitudinally bisected halves which each form an elongated, tapered 
half-cylinder opening. The inner surfaces of the mold, of course, 
represent the desired external dimension and contour of the inner mandrel 
shell halves to be fabricated therein. A pair of mandrel inner shell 
halves 14 and 16 shown in FIG. 2 are fabricated by lay-up of sheets of 
filament-reinforced matrix material to the molds 10 and 12. The 
filament-reinforced matrix material may be selected from various available 
filamentary material-matrix combinations, but is preferably woven or 
chopped fiberglass such as high modulus, high strength S-glass, E-glass or 
the like in an eopxy resin so that reinforcement is provided along both 
the longitudinal and transverse axis of the spar mandrel. Other 
filamentary material such as Kevlar, boron or graphite is also suitable. 
To provide structural support to the shell halves 14 and 16, a plurality 
of lightweight ribs 18 made, for example, of styrofoam are bonded as shown 
and the shell halves are then assembled and bonded together to form inner 
shell 20 as illustrated in FIG. 3. Adaptors 22 and 24 are secured 
respectively to the butt and tip ends of the shell 20 to allow mounting of 
the shell 20 to the winding mechanism to be subsequently utilized. 
The shell 20 is mounted for rotation about its longitudinal axis as shown 
in FIG. 4, on floor-mounted mechanism such as headstock 26 and tailstock 
28. Adjacent the shell 20 is a filament applicator 30 which is movable in 
a direction parallel to the longitudinal axis of the shell 20. Although 
not shown, the applicator preferably contains a plurality of filament 
sources, a tensioner and a resin applicator and winding head means. As the 
shell 20 is rotated, the applicator 30 moves from the base toward the tip 
of the blade to provide filament-resin layers derived from overlapping 
helical windings. A preferred filament for this operation is the same as 
that utilized for the inner shell 20, i.e., a fiberglass filament of 
substantial modulus, such as S-glass or E-glass, or a filament made of 
boron, graphite, Kevlar or the like. Each filament is resin coated to form 
a solid outer layer of material surrounding the mandrel inner shell 20. 
The wound filaments are preferably precoated in the applicator 30 and 
generally applied as rovings of several filaments held together by the 
coating resin. In winding the mandrel, the successive layers of filaments 
are placed at selected angles to the mandrel axis and to one another to 
produce a maximum of bending and torsional strength. For example, 
successive layers oriented at 30.degree. to the mandrel axis is in many 
cases the best arrangement. The filaments are wound on the inner shell in 
a helical form until the inner shell is completely covered with the 
desired number of layers of filaments of the necessary strength. 
Typically, the mandrel has a nonuniform wall thickness with the thickness 
tapering from base to tip. As will be recognized by those skilled in the 
art, this may be accomplished by increased layers of filaments near the 
base or by inserting layers of cloth or other fillers or by the use of 
tapered rovings. 
As shown in FIGS. 5 and 6, after winding of the shell 20 is completed, an 
end fitting assembly 32 comprised of a steel hub and stub shaft 34 and 
inner and outer rings 36 and 38 may be secured to the butt end while an 
identical or similar end fitting assembly 40 may be secured to the tip 
end. The function of the assemblies 32 and 40 is to provide a durable 
interface with the winding machine. 
As can also be seen in FIG. 5, the mandrel may be provided with a plurality 
of openings 42 to allow for the use of pressurized air within the hollow 
mandrel in order to separate the mandrel from the spar to be subsequently 
wound thereover. 
It will be appreciated that the inventive composite material spar mandrel 
has the following advantages: 
(a) better use of available volume for the required structure; 
(b) no joints or hard points in the mandrel to wear or cause distress in 
the laminates; 
(c) reduced weight and increased stiffness associated with the placement of 
the structural material at the outer surface of the mandrel, resulting in 
minimum mandrel sag during blade spar and shell winding; 
(d) reduction of winding machine size and cost due to minimum overhung 
moment on the winding machine; 
(e) low basic mandrel costs; 
(f) ease of duplication; 
(g) reduced handling time since mandrel disassembly and reassembly not 
required; and 
(h) longer mandrel life. 
What has been set forth above is intended primarily as exemplary to enable 
those skilled in the art and the practice of the invention and it should 
therefore be understood that within the scope of the appended claims, the 
invention may be practiced in other ways than as specifically described. 
The invention is considered, for example, suitable for producing any long 
tapered structure similar to a wind turbine spar or blade such as a ship's 
mast, lighting stanchion, or the like which requires low deflection and 
little tip support during fabrication.