Optimized gearbox design - Wind Power Generation and Wind Turbine Design - page 509

CHAPTER 15
Optimized gearbox design
Ray Hicks
Ray Hicks Limited, UK.
Superfi cially, gearboxes for wind turbines are required for a low technology, low
speed and relatively low power application. However, their very high torque and
speed increasing ratio requirements coupled with the capricious nature of the
power source have created many problems which have had a detrimental effect
on reliability. In reality therefore, they have had to be manufactured to the highest
possible quality with corrections to gear teeth, etc. to compensate for the parasitic
loads and defl ections to which they are subjected. This chapter explains the basics
of gear design criteria and offers solutions to the various problems.
1 Introduction
Wind turbines in common with virtually all other rotary machinery are subject to
speed limits such that the product of rotor diameter and rotational speed is a con-
stant, i.e. blade tip diameter is inversely proportional to speed. Since the propor-
tions of the blade length and chord section tend to be constant, then given similar
materials, wind speeds, etc., the rotor weight and torque are proportional to the
linear dimension cubed.
Because of the inverse relationship of diameter and speed, the product of torque
and speed, i.e. power, is directly proportional to the rotor diameter squared and there-
fore, the swept area. Thus, the power to weight ratio diminishes as power increases.
For example, if power is increased from 750 to 3000 kW, i.e. by a factor of 4, the
rotor diameter is doubled and its rotational speed is halved. It follows that the weight
and torque of the rotor are increased by a factor of 8. Incidentally the moment of
inertia (related to the 5th power of the diameter) is increased by a factor of 32.
Unlike other methods of power generation such as gas turbines, the input energy
source of wind turbines is of an uncontrolled stochastic nature. Its velocity, direc-
tion and pressure distribution over the swept area are all subject to sudden changes