Optimized gearbox design - Wind Power Generation and Wind Turbine Design - page 523

Nacelle

TDO brg

Rotor Fully floating

torque reaction

Figure 11: Independent rotor support arrangement.

The most critical aspects affecting reliability are the mounting of the gearbox
and the coupling of the turbine shaft to its input. Hitherto, a majority have had the
turbine shaft supported at its front by a single bearing mounted on the nacelle bed
plate while its rear end has been supported by the gearbox input shaft to which it
is rigidly coupled via a shrink fi t coupling. The rear end of the turbine shaft is
therefore, supported by the gear case and its resilient mounts via the input shaft
bearings. This has created detrimental parasitic loads on the gearbox due to the
pitching and yawing couples and associated shaft bending defl ections plus defl ec-
tions in the mounts due to torque fl uctuations. In the light of the problems that
have arisen from this situation, as powers have increased, most recent designs have
featured large back to back taper roller bearings in TDO confi guration to indepen-
dently support the turbine rotor in a mounting frame. The gearbox input shaft is
rigidly coupled to the rotor hub while the gear case torque reaction is supported by
a suitable mechanism designed to impose only pure torque (see Fig. 11). In effect
the rotor hub supports the gearbox, not vice versa.

6 T orque limitation

In its simplest form, the differential properties of an epicyclic gear can be exploited
by allowing what would otherwise be its fi xed reaction member to rotate in the
direction imposed by its torque. This is effected by gearing it to a fi xed stroke posi-
tive displacement pump with its delivery bypassed to its inlet via a pressure relief
valve to give a limited slip at a controlled torque. Such a gear is best used on the
fi nal low torque high speed stage where the component sizes are much smaller and
more manageable. This has been used very successfully by the Windfl ow company
in New Zealand for 500 kW turbines driving synchronous generators. They have