Page 463 - Wind Energy Handbook

GEARBOX 437

damping or even just sand or bitumen layers added to the tower wall, for instance.
In some cases the treatment can have more than one effect. When blades are the
major source of radiation and damping material is added inside the blades then this
material can act as a stiffening material as well as a damping mechanism. Some-
times it is useful to add tuned absorbers to parts of the structure to damp out one
particular frequency. An alternative use of such tuned absorbers is to design them
to raise the impedance at the tuned frequency so that the offending vibration does
not pass that point on the structure.

7.4.9 Integrated gearboxes

As noted in Section 6.11.1, the cases of integrated gearboxes must be very robust, in
order to transmit the rotor loads to the nacelle structure without experiencing
deflections which would impair the proper functioning of the gears. In view of the
complex shape of the casing, stress distributions due to each load vector usually
have to be determined using finite-element (FE) analysis – these can then be
superposed in line with the different extreme load combinations. The fatigue
analysis will require the superposition of stress histories resulting from simultane-
ous time histories of rotor thrust, yaw moment and tilt moment derived from
simulations at different wind speeds.

7.4.10 Lubrication and cooling

The function of the lubrication system is to maintain an oil film on gear teeth and
the rolling elements of bearings, in order to minimize surface pitting and wear
(abrasion, adhesion and scuffing). Varying levels of the elastohydrodynamic lubri-
cation provided by the oil film can be identified, depending on oil film thickness.
These range from full hydrodynamic lubrication, which exists when the metal
surfaces are separated by a relatively thick oil film, to boundary lubrication when
the asperities of the metal surfaces may be separated by lubricant films only a few
molecular dimensions in thickness. Scuffing, which is a severe form of adhesive
wear involving localized welding and particle transfer from one gear to the other,
can occur under boundary lubrication conditions, which are promoted by high
loading and low pitch line velocity and oil viscosity.
Two alternative methods of lubrication are available: splash lubrication and
pressure fed. In the former, the low-speed gear dips into an oil bath and the oil
thrown up against the inside of the casing is channelled down to the bearings. In
the latter, oil is circulated by a shaft driven pump, filtered and delivered under
pressure to the gears and bearings. The advantage of splash lubrication is its
simplicity and hence reliability, but pressure fed lubrication is usually preferred for
the following reasons:

• oil can be positively directed to the locations where it is required by jets,
• wear particles are removed by filtration,