Windtech: Increased torque-to-mass density for Moventas gearbox

FINLAND: Finnish gearbox manufacturer Moventas earlier this year announced a new medium and high-speed gearbox platform for 2MW and 3MW class turbines.

Cross section of Moventas Exceed gearbox… Mass reduction of 20%
Cross section of Moventas Exceed gearbox… Mass reduction of 20%

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The Exceed uses new gearbox technology that provides a high 135 Newton metres per kilogramme (Nm/kg) torque-to-mass density, compared with the roughly 60Nm/kg as supplied by Moventas in 2006. Torque density is a key focus for research and development for Moventas and its main gearbox competitors, as it is a key performance indicator (KPI) in the industry.

The Exceed gearbox uses a design option with two planetary stages. High-speed gearboxes of up to about 2MW with 90-metre rotor diameters typically comprise one planetary input stage and two parallel-gear stages. Turbines with larger rotor diameters and higher ratings tend to use gearboxes with two planetary and one high-speed parallel stages.

Absorb loads

Planetary stages by comparison can absorb higher torque loads, enable larger possible gear ratios and offer additional gearbox compactness and mass-reducing benefits.

"With the 3MW gearbox, we achieved over 20% mass reduction compared to similar rated state-of-the-art gearboxes by pursuing parallel advances in five dedicated areas of development," says Kari Uusitalo, Moventas' wind gears product manager. "The first area was to maximise the planet wheels number in the low-speed input stage for optimal torque-load distribution. We also use Flexspider flexible planet wheel shaft mounting technology to secure even load distribution."

Flexspider is applied in planetary gear stages with more than three planet wheels, but as an integral part of advanced torque-load distribution management.

The company says that the technology gives optimal tooth contact and load sharing between planet wheels, even in extreme load conditions. Flexspider technology is patented by Moventas, was tested and certified in 2007 and is already integrated in hundreds of planet gears.

The integrated double-row cylindrical planet wheel and bearings arrangement also plays a key role in transmitting high torque reliably in a confined space. Due to the double-row gear arrangement, almost perfect load sharing is achieved in the planet wheel bearings, minimising space requirement if separate bearing outer rings are eliminated too.

Optimise components

The second stage was to optimise the mass of all individual input stage drive components, says Uusitalo.

The combination of extra planet wheels and Flexspider technology integration enabled a substantial reduction in planetary stage ring wheel outer diameter.

"Favourable low-speed gear staging has a relatively high impact on the dimension and mass of other gear stages and of the total gearbox mass," says Uusitalo.

"Our third research area focused on switching to a higher-grade material with superior physical properties and fewer material texture imperfections. This improves the outcome potential and allows optimised component dimensions. We have applied this special-grade material to the sun wheel shaft and planet wheels."

The fourth area to focus on for Exceed involved the use of advanced software tools to reduce the mass of large cast components such as the planet carrier. The key was to optimise the shapes by maximising the component stiffness and strength, while removing non-functional material whenever possible but without sacrificing cast-ability performance.

"We started this process several years ago and there is still huge potential for further optimisation," says Uusitalo. "The fifth, and final area we focused on was to improve the load-distribution values for Moventas gearboxes compared with state-of-the-art values for gearbox calculation in the common IEC-61400-4 standard."

Ramp up

Bench testing for a 3MW prototype is planned for the end of this year, followed by commercial ramp-up in early 2015 with first the gearbox deliveries to Acciona's AW3000 series for both 50Hz for the EU and 60Hz versions for North America.

"Our next research and development goal for 2018/19 is to achieve up to 200 Nm/kg torque densities. This involves the introduction of new materials and manufacturing concepts, plus incorporating journal bearings, says Uusitalo.

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