Some time ago, Andy Horsfall from AH Gears in Auckland attended a gear failure analysis course in Montana. The course was run by Robert Errichello, one of the world’s leading experts in gear failure analysis. During the course Mr Errichello explained the gearbox issues involved in the wind industry and recommended that anyone invited to get involved in that aspect of the gear industry should run a mile.
A week after Andy’s return, he was approached by Windflow Technology to design and build the torque limiting gearbox at the heart of the WTL 500 Wind turbine. Andy ignored the expert’s advice and took up the challenge.
It took around 18 months from the initial design to build the first gearbox, and, although the gearbox continued to evolve, one of the very early gearboxes continues to operate on the Port Hills in Christchurch. AH Gears has now built 97 of those gearboxes and the design and manufacturing processes have received International certification to the highest standard - approved to IEC class 1 edition 3 certification.
The final design of the WTL 500 gearbox incorporated three planetary stages, and one parallel stage, using flex pin technology that allows better load sharing between planet gears. This allows eight planets in stage one, four planets in stage two, and six planets in the output stage. The lubrication system had to deal with low speed high torque and high speed 'wheels within wheels' that challenged conventional lube systems and needed AH Gears to find an innovative solution using centrifugal force to get lubricant into the 4th stage planet bearings. The 4th stage planetary was used for the torque limiting system which allows the gearbox to effectively change ratio to cope with excessive torque generated during sudden wind gusts yet maintain continuous output speed allowing the use of a synchronous generator.
“We have also used our gear design software to predict deflections in the cantilevered yaw pinions and then ground lead corrections onto existing yaw pinions to extend the life of the yaw gears,” said Andy.
“To make wind turbines economical in NZ, we have to run them in extreme conditions, which places very high stresses on the turbines. Local companies are best placed to develop understanding of those stresses and come up with cost effective solutions,” said Andy.
“We are now involved in retrofitting and repair of wind turbine gearboxes, working on Trustpower’s 660kW turbines, where we incorporate all the latest modifications normally carried out in Europe to bring the rebuilt gearboxes up to the latest specifications. Once gearboxes are out of warranty we recommend improved bearing arrangements and lubrication modifications to extend the gearbox’s life. Trustpower has indicated we will be considered for the retrofit of their 3MW turbines when they come out of warranty.”
According to Andy, wind has saved AH Gears from shrinking, which is rare among engineering shops these days as manufacturing in New Zealand continues to decline. AH Gears has up to 4 of their 16 employees working on wind-based projects at any one time utilising various precision CNC turning milling and gear manufacturing machines.