Page 457 - Wind Energy Handbook

GEARBOX 431

rated torque – significantly less than for bending, as expected from comparison of
the BS 436 torque–endurance curves in Figure 7.27.
Figure 7.27 also shows specimen torque–endurance curves derived from S – N
curves in the American National Standards Institute/AGMA 2001-C95 plotted in
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terms of the torque at 10 cycles. The torque–endurance curve for tooth bending
stress, which is based on a middle of the range Brinell Hardness value of 250 HB,
closely parallels the selected BS 436 curve, except that the curve continues with a
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very shallow slope beyond 3 3 10 cycles instead of displaying an endurance limit.
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The design torques at 10 cycles for tooth bending for the example 500 kW machines
featured in Figure 7.24 are similar to the design infinite life torques obtained using
the BS 436 torque–endurance curves.
The ANSI/AGMA (1995) torque–endurance curves for tooth contact stress are
significantly more conservative than the selected BS 436 curve. This is particularly
so in the case of the ANSI curve selected, which is the one recommended for wind
turbine applications, in view of the elimination of the lower knee. The absence of
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the lower knee increases the design torque at 10 cycles for tooth contact to 1.4 times
the rated torque for the stall regulated machine, but the figure for the pitch
regulated machine is only about 10 percent higher.
From the above discussion, the general conclusion can be drawn that tooth
bending fatigue usually governs the increased gearbox rating required to take care
of load excursions above rated.
The effect of braking loads on the design infinite life torque according to BS 436
can be illustrated with respect to the example machines discussed in Section 7.4.2.
Although the mechanical brake must be capable of decelerating an overspeeding
rotor unassisted, a shut-down under these conditions will be a very rare event.
Accordingly the typical emergency shut-down considered for fatigue design pur-
poses is deceleration from normal rotational speed under the action of mechanical
and aerodynamic braking combined, with an assumed stopping time of 3 s. An
emergency shut-down frequency of 20 per annum is assumed. Normal shut-downs
are assumed to occur on average twice a day, with a stopping time of 1.5 s, because
of the reduced rotational speed at which mechanical braking is initiated for parking.
In each case the braking torque is assumed to remain constant at three times rated
torque throughout the brake application for simplicity. Based on these assumptions,
the percentage increases in design infinite life torque for gear tooth bending in
fatigue, due to the inclusion of braking loads in the load spectrum, are shown in
Table 7.5 for emergency braking alone on the one hand and normal plus emergency
shut-downs on the other.
Also shown in the table are the percentage increases in the ANSI/AGMA design
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life for gear tooth bending at 10 cycles due to the inclusion of braking loads. It is
seen that the inclusion of emergency braking loads alone makes very little differ-
ence to design torques in the case of the pitch-regulated machine, but is significant
in the case of the stall-regulated machine. The addition of braking loads at normal
shut-downs incurs a much greater penalty in both cases because of the large
number of stops involved, indicating that provision for brake application at
reduced torque on these occasions would probably be worthwhile. Note that the
larger percentage increases in design torques due to braking indicated by BS 436
are a consequence of the assumption that there is an endurance limit.