Figure 14 Gearbox, courtesy of Bosch Rexroth.
Function
Where used, a gearbox converts rotor torque at a speed of 4-8 rpm to a speed of up to about 600 rpm for a medium speed gearbox for conversion to electrical energy by the generator.
What it costs*
The gearbox cost varies significantly for different nacelle layouts.
Who supplies them
Lamond and Murray, Moventas, Renk and ZF Wind Power.
Key facts
The gearbox is a critical item in the wind turbine drive train, with much attention given to the long-term reliable operation given a history of variable quality and reliability, with costly consequences.
Typically, designs incorporate a planetary first stage followed by a higher-speed parallel (helical) stage. Normally, a brake disk is mounted at the rear (higher-speed) end.
Design drivers include peak torques coming from a range of load cases; also loads in storms and during braking or other abnormal events, and avoiding high speeds in the gearbox itself.
Careful consideration is given to the variation in bearing and gear contact points due to a wide variation in operating power levels during turbine life, including periods of standstill and operation under minimal loading, introducing the possibility of skidding within bearings.
Design methodologies combine empirical rules of thumb, detailed dynamic analysis and highly accelerated lifetime testing (HALT) using specialist wind turbine drive train test rigs.
Automatic heating is frequently applied before restart in cold conditions and cooling is designed typically to keep operating temperatures below about 70ËšC. Cooling systems may be combined with that of the generator.
Lubricants specific to wind turbine gearboxes have been developed, with reduced end of life environmental impact.
Typically, the gearbox has a bore in the central shaft (along the axis of the main shaft) to facilitate provision of control signals and electrical or hydraulic power to the hub for operation of the blade pitch system.
What’s in it
- SG iron castings (including higher grade (say EN-CJS-700-2U) for items such as planet carrier) and steel forgings
- Cylindrical, taper and spherical roller bearings; plain bearings
- Gears
- Lubricants
- Sensors