Technical Innovation, Precision Critical in Making Gears for Wind Turbines

The
growth and promise of wind turbines is fueling technology development focused
on the manufacturing of larger, more precise and optimized gearing. The need
for better performance, quieter operation and higher efficiency means that the
primary goal is to limit losses, select proper sealing elements and control
other significant factors that reduce efficiency.

"What's happening right now will be the
future of the gear industry because of the number of units that will be
manufactured and installed with a main purpose of achieving longer life," says N.K.
Chinnusamy, president of Excel Gear Inc. "One way to achieve long life is new
technology and computer tools that optimize the geometry of the gears and, for
example, optimized root fillet geometry that improves both life and load
carrying capacity."

One objective is to make components
lighter, and new materials being developed will help achieve that goal. Developments
with surface finish such as REM technology and electro polishing, some of which
are being used in the racing industry, are having some real significant impact in
gear life.

Chinnusamy says there are many challenges
in machining gears for wind turbines. Gears for wind turbine applications are
typically large in diameter and have wide face widths, requiring very exacting
material composition and heat-treatment processing. The gear design must be optimized
to insure low rolling resistance and long life, to minimize costs of
maintenance, down time, and repair of the gear box assemblies once they have
been commissioned in the field. Every step in the manufacturing phase of these
gears must be carefully processed, documented and controlled to achieve the
high quality, consistency, accuracy and reliability that is demanded for
operation in these environments.

The use of carburized steel for these gears
is common and the associated heat treatments and stress-relief operations have
to be exacting to minimize part distortion and growth, as well as to achieve
the proper metallurgical properties required. Often, a preheat treatment of the
forging or bar stock is necessary on large gears to minimize part distortion.

Chinnusamy says heat treatment can
cause cracks, so careful processing with predetermination of stock allowance
for grinding and final case depth must be considered. Inspection for cracks
with magnetic particle inspection and for grinding burns utilizing nital
etching is an important inspection tool. Plus, off center crown grinding of the
tooth geometry may be needed to properly distribute the load on the gear teeth.

To efficiently make gears for this
application, Chinnusamy says there are often modifications needed in the
tooling. Rigid, heavy-duty hobbing machines are needed for the coarse pitch
gears, using roughing hobs or gear milling (gashing) cutters. Likewise, coarse
pitch diamond dressing rolls and special grinding wheel abrasives are required
for the large, high-accuracy gear grinders to produce efficient, accurate
results and to prevent grinding burns and cracks.

Cutting fluids used must have the
proper viscosity, the right amount of extreme pressure additives, and must be
directed to the exact location of the work piece and cutting tool interface to
maximize results. These fluids have to be routinely sampled and adjusted for
optimum results.

In building the gearbox, it's also
important to establish the correct bearing clearances/preloads and proper
gearbox operating temperature that is critical to long life. Sophisticated
measuring techniques with bearing inspection gages can only insure these
results. The type and method of lubrication and proper sealing weighs heavily
on the performance of a gearbox. The verification of gearbox performance
through computerized analysis and testing is a crucial step to insuring long
life.

"The critical factor here, as with all
similar power transmission applications, is that the gears are properly
designed and manufactured," says Chinnusamy. "The other mechanical components that make up
the assembly, along with the gearing, must be applied and designed so the
overall system performance does not have any shortcomings that could affect the
performance and life of the unit."

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