Page 320 - Wind Energy Handbook

294 DESIGN LOADS FOR HORIZONTAL-AXIS WIND TURBINES

the shaft. For fixed hub machines, the loading on the shaft will include a significant
moment arising from blade aerodynamic loads, but in the case of teetered two
bladed rotors this moment will be virtually eliminated. In either case, however, the
cantilevered low-speed shaft will experience large fluctuating moments due to rotor
weight as it rotates. Figure 5.39 shows a low-speed shaft and front bearing in a
factory prior to assembly.
The shaft moments due to out-of-plane loads on the blades can be expressed as
moments about a pair of rotating axes, one perpendicular to blade 1 and the other
parallel to it. In the case of a three-bladed rotor, these moments are respectively as
follows:
p ffiffiffi
3
1
M YS ¼ ˜M Y1 (˜M Y2 þ ˜M Y3 ) M ZS ¼ (˜M Y3 ˜M Y2 ) (5:118)
2 2
Here ˜M Y1 , ˜M Y2 and ˜M Y3 are the fluctuations of the blade out-of-plane moments
about the hub centre (M Y1 , M Y2 and M Y3 ) about the mean value (see Figure 5.40).

5.10.2 Deterministic aerodynamic loads

The deterministic aerodynamic loads on the rotor may be split up into a steady
component, equal for each blade, and a periodic component, also equal for each

Figure 5.39 Low-speed Shaft and Front Bearing Before Assembly. The hub mounting flange
at the right-hand end is bolted to a temporary support to allow the bearing to be threaded on
the shaft. (Reproduced by permission of NEG Micon)