Page 462 - Wind Energy Handbook

436 COMPONENT DESIGN

However, if gearbox noise is expected to be more intrusive at low wind speeds,
when it is less likely to be masked by aerodynamic noise, then a low compensation
load level should be selected.
Helical gears are usually quieter than spur gears (with teeth parallel to the gear
axis) because the width of the tooth comes into mesh over a finite time interval
rather than all at once. Moreover, the peak tooth deflections of helical gears are less
than those of spur gears because there are always at least two teeth in contact rather
than one, and because the varying bending moment across the tooth width means
that the less heavily loaded portions of the tooth can provide restraint to the part
that is most heavily loaded. As a result, the tooth misalignments due to insuffi-
cient/excessive tip relief at a particular load level will be reduced.
Epicyclic gears are normally quieter than parallel shaft gears because the reduced
gear size results in lower pitch line velocities. However, this benefit is lost if spur
gears are used rather than helical gears, in order to avoid problems with planet
alignment. One way of maintaining the alignment of helical planet gears is to
provide thrust collars on the sun and annulus.
As the annulus of an epicyclic gear stage is often fixed, it would be convenient to
integrate it with the gearbox casing. However, this would enable annulus gear
meshing noise to be radiated directly from the casing, so it is preferable to make the
annulus a separate element, supported on resilient mountings. Similarly, resilient
gearbox mountings should be used to attenuate the transmission of gearbox noise
to the nacelle structure and tower.
The noise produced by gear tooth meshing can reach the environment outside
the wind turbine by a variety of routes:

• through the shaft directly to the blades, which may radiate efficiently,

• through the resilient mounts of the gearbox to the support structure and thereby
to the tower, which can radiate efficiently under some circumstances,
• through the resilient mounts of the gearbox to the support structure and thereby
to the nacelle structure, which can also radiate,

• through the casing wall to the nacelle air and then via air intake and exhaust
ducts,

• through the casing wall to the nacelle air and then via the nacelle structure.

All these paths are modally dense and it is virtually impossible to design out a
selected frequency. If noise is a problem then the options are to reduce the source
sound level, perhaps by improving the tip relief as described above, or to modify
the major path to reduce transmission. Identification of the major path is not
straightforward, but one way of doing so is to use Statistical Energy Analysis (SEA),
which combines a theoretical model with extensive field measurements. The path
may not be simple, as non-linearity in the system can make one path the predomi-
nant one at low wind speeds and another path critical at higher wind speeds.
Treatment of a radiating path can involve damping treatment such as shear layer