By Tom Pawlak.
Back in the 1980s, Gougeon Brothers was one
of the largest producers of wind turbine blades
in the US. The blades were built of wood veneer
and epoxy, and varied in length from 10'
to 70'. They were built in halves and vacuum
laminated in female molds built with WEST
SYSTEM® Brand Epoxy. Tolerances were tight,
and every aspect of the tooling was critical,
from molding to assembly. If something wasn't
right when the two halves were glued together,
there wasn't much you could do to
make it right later.
strongback supports a
female mold for a 70'
Westinghouse wind turbine
blade. (The plug
that was used to build
the mold is in the mold.)
The strongback is
stiffened by 5
glued into 1½x1½"
one in each corner and
one along the top and
The most difficult tolerances to maintain were
span-wise straightness and twist. Early
strongbacks for molds and assembly jigs were
made of wood and were built like tall and skinny "I" beams. Initially, everything worked
well. But as the seasons changed and the wood
picked up moisture, and the summer temperatures
in the shop went up, we were pushing
the limits of the tolerances. The engineers figured
it was a combination of forces caused by
wood's volumetric change, with changes in
moisture content and difference in coefficient
of thermal expansion between the wood
strongback and epoxy/fiberglass molds. Our
short-term fix was to cut the mold free from
the strongback and isolate it with plywood
supports (3/8" thick) that were installed perpendicular
to the length and located every 16" to
24" apart. They ranged from 4" tall at the root
end to 12" tall at the tip end of the mold. The
differences in height were due to the fact that
the molds were tapered to mimic the blade
half-geometry (thick at the root end and tapered
to thin at the tip). The plywood supports
were attached to the top of the
strongback and to the backside of the mold
with thickened epoxy fillets. Separating the
mold from the strongback with the thin plywood
supports meant slight changes in length
between the two did not cause a problem.
This is because the supports easily deflected to
allow relative movement but had almost no effect
on span-wise straightness. Luckily the
problem was caught early, but we decided we
needed a better strongback design that would
be more stable.
The solution was to make a box beam out of
plywood and epoxy. This in itself provided
significant improvement, but the engineers
went one step further. They incorporated steel
rebar originally designed for reinforcing concrete
into the four corners of the box beam.
The rebar, welded to full lengths, was glued
into 2"x2" wood stringers that ran full length
along the four corners of the box beam. To
make room for the rebar, a slot was cut into
the wood stringers that was wide enough and
deep enough to accept the steel. Epoxy thickened
slightly with 403 Microfibers was used to
fill the gaps in the groove surrounding the
rebar after it was in position (Figure 1).
On molds up to 20' in length, we used
3/8" diameter rebar. On larger molds, we used ½" or
5/8" diameter. The 70' long mold strongbacks
were reinforced with 6 stringers made of
5/8"diameter rebar, one in each of the four corners
and an extra one in the middle between the
corners of the top and bottom.
When the strongbacks were used for blade assembly
jigs or for saw jigs, they were anchored
to the floor to keep things stable. If the
strongbacks were used to support molds,
wheels were mounted inside the strongback
(on the larger molds) to keep the center of
gravity low and to keep the height of the mold
reasonable. On smaller molds where the width
of the strongback was too narrow for a caster
to pivot inside the bow beam, the wheels were
mounted on the outside of the structure.
Mounting wheels under molds with
strongbacks can present some dilemmas. The
wheels tend to get in the way of your feet. If
you mount wheels under the mold strongback,
you need to shorten the height of the
strongback to make up for the wheel height.
Otherwise, the mold gets too high for people
to work on. Unfortunately, shortening the
height makes the strongback less stiff, which
can be a problem.
Mounting the wheels inside the strongback
works best as long as you allow room for
wheels to swivel without interference. Another
option is triangular shaped strongbacks. We
found this to be the best solution for 20' to 30'
mold lengths. By positioning one of the three
sides horizontally, wheels could be mounted
under the mold on angled brackets (Figure 2).
This allowed room for the wheels to pivot without getting in the way. After the blade halves cured, they were pulled from the molds, positioned in saw jigs, and trimmed along the leading and trailing edges. From there, the blade halves were moved to glue jigs where a shear web was installed on the inside of the high-pressure side. Eventually, the two halves were glued together with thickened epoxy.
By Brian L. Knight
Jon Staudacher, of Staudacher Hydroplanes and Aircraft, has been using a long, very flat, work table/strongback that is mounted on casters. The table was originally 32' long, but because of space considerations, Jon has since shortened it to 20' (Photo 1). Four rubber casters support it, one at each corner (Photo 2).
This table is used to
wings. It must not twist
or sag. The table was
originally 32' long, but
has been shortened to
20'. Four rubber casters,
one at each corner, support
it. Built as a
strongback, it spans 20'
(previously 32') without
Four rubber casters,
one at each corner, are
mounted to brackets at
the ends of the table to
give it portability.
Jon uses this table as a surface to assemble airplane wings, so it cannot have twist or sag. Because of its stiffness, the table is not dependent on having an extremely level floor. When the table is moved to a different location, it is easy to shim it level. It is stiff enough that it does not sag, and if there is a little twist, it is easily shimmed out. Since the tabletop is the reference point for all objects being built, the table does not have to be perfectly level, but it musthave no twists. If Jon expects to use weights for clamp pressure, as he often does when
building the frames for wings, he temporarily
shims the bottom rail of the table so it cannot
sag under the weight. The table doubles as a
strongback with grid lines drawn on the top
for locating frames.
A table like this has to be built carefully, but
the materials that go into the construction are
very light and readily available. The table is a
long box beam made with plywood, and all
pieces are glued together to make the table
very rigid. There are plywood bulkheads every
4' to keep the front and back of the table from
buckling. In turn, the front and back of the table
keep the top flat, without twist. The top is
supported on 16" centers to prevent any
drooping of the plywood top between bulkheads.
In order not to waste space, storage is
built into the front of the table. The access
holes are centered between the top and
bottom rails, and have oval ends. These are
important dimensions. If the access holes were
sawn too close to the bottom or top surface,
the table would lose its rigidity. Because square
corners can cause stress concentrations, the
holes have rounded ends (Photo 3).
Full or half sheets of plywood are used where
ever possible. Construction starts by
sandwiching the top and bottom of the front
and back panel between two 1x4s (Photo 4),
which were scarfed together to improve
straightness. A ¼" plywood bottom panel was
glued to the bottom rails. Then ¼" plywood
bulkheads and supports for the top were installed.
The top was glued to the top of the
side rails, bulkheads, and supports. Finally,
brackets of scrap OSB were bolted to the ends
of the table to support casters (Photo 2).
Access holes to storage
spaces are centered
between the top and
bottom rails, and have
The 1x4s that sandwich
the top and bottom
of the front and back
panel are scarfed to improve
Epoxyworks 23 / Spring 2006
Copyright © 2006, Gougeon Brothers, Inc. All rights reserved.
Reproduction in any form, in whole or in part, is expressly forbidden without the consent of the publisher. EPOXYWORKS, Gougeon Brothers, WEST SYSTEM, Episize, Scarffer and Microlight as used throughout this publication, are trademarks of Gougeon Brothers, Inc., Bay City, Michigan, USA.