Using the IEC Simple Load Model for Small Wind Turbines - Small Wind Turbines: Analysis, Design, and Application - page 180

where the two masses in parentheses are, in order, that of the nacelle and rotor and
that between the lifting point and tower top, and L lt is the distance between the
lifting point and the tower top. Loads occurring during tower raising and lowering
are discussed in Chap. 11 where the importance of only raising and lowering the
turbine on calm days is emphasised.

9.3 Stress Calculations and Safety Factors

The loads calculated in Sect. 9.2 must be converted into equivalent component
stress levels to compare with the allowable material stress limit. Stress levels are
calculated by the same procedure regardless of whether the SLM or aero-elastic
analysis was used.

9.3.1 Equivalent Component Stresses

Following the SLM calculations, the individual forces and moments are combined
to obtain the final equivalent stress level on the key load carrying components, the
main shaft and the blade root using the formulae in Table 9.6 . The calculation of
the equivalent stress levels has to take into account a number of key factors:

• Stress variations within the component
• Stress concentrations
• The size and direction of the resulting load or stress
• Variations in component dimensions and thickness
• Component surface treatment
• The type of loading on the component
• Any manufacturing effects on the components such as welding, machining etc.

Table 9.6 Equivalent stress formulae from IEC 61400-2

Circular blade root Rectangular

blade root

Rotor shaft

Axial load

r zB ¼ F zB
A B

r zB ¼ F zB
A B

r x shaft ¼ F x shaft
A shaft

p

M xB þ M yB Bending

r MB ¼ M xB

W xB þ M yB

r M shaft ¼ M shaft
W shaft

r MB ¼

W yB

W B

Shear Negligible Negligible

s M shaft ¼ M x shaft
2W shaft

q

r x shaft þ r M shaft

Combined
(axial ?
bending)

r eqB = r zB ? r MB

Þ 2 þ 3s 2 M shaft

r eq ¼ ð