The gearbox challenge

More than any other single component, gearboxes have proved to be the Achilles heel of the wind turbine industry. As turbines rapidly grew in size during the nineties, so did the number and severity of gearbox failures. A veritable industry evolved around re-conditioning and replacing gearboxes. Specialist firms sprang up, advertising gearbox fixes to the hapless owners of turbines whose makers had long gone bust. The blame was spread far and wide.

Fingers were pointed at poor quality gearbox oil, poor quality filtration of debris in the oil, poor finishing of machined surfaces, and other imagined culprits. But these were symptoms and contributing factors, not the real problem, the cause of which lay buried deep in engineering departments. The dam burst in the late 1990s with a massive series failure of gearboxes that brought NEG Micon, at the time the company that had sold most wind megawatts, to the edge of bankruptcy. The rest, as they say, is history.

The wind industry appears to have been obstinately blind on the gearbox issue. As readers of this magazine know all too well, it was no secret that gearboxes were failing across the boards. It was no secret that gearbox development was not keeping up with increases in turbine size. At industry conferences, experts warned more than once that disaster was around the corner: gearboxes for bigger machines simply did not exist. Yet wind turbine marketing departments were busy selling the next size up.

With hindsight, the huge market pressure to deliver ever cheaper wind power led the industry to run before it could walk. As turbines got bigger, designers were shedding weight to bring down cost. Gearboxes were asked to bear loads never borne before. Prototype testing was too short to reveal defects. Not all manufacturers had reserves enough of cash to deal with major series retrofits when needed. The problems are not over yet, as gearbox or component replacement on Vestas 2 MW and Siemens 2.3 MW turbines bears witness to (page 58). But modern condition monitoring is allowing fast preventative action and repair.

Gearboxes for wind turbines present a unique challenge. Instead of step-down ratios, it is all about step-up ratios. In other uses, gearboxes reduce the speed of prime movers. Wind technology turns this convention on its head. These days, the input shaft -- the rotor -- rotates very slowly. Ten to 20 rpm is typical. Meantime the output shaft is turning at up to 1500 rpm. In squeezing the last ounce of power out of a turbine, the ratio of the two speeds can be as high as 100. Adding to the challenge, the input torque from the rotor is not constant, but variable. What's more, a modern 30 tonne rotor imposes a very high bending moment at its point of support and must be constrained from tipping forward. To further complicate matters, the wind pushes a large rotor backwards with a force of up to 20 tonnes -- and that fluctuates as well.

Early wind turbine designs had separate bearings to deal with each of the forces that had nothing to do with the gearbox. But pressure to reduce cost by reducing the weight and number of components led to either or both the separate bearings being incorporated into the gearbox. The approach is fine, provided the gear unit is still shielded from the massive forces. Gear units are precision instruments: they do not tolerate much misalignment from residual bending moments or thrust forces. That places a huge responsibility on the bearings, also high precision assemblies not tolerant of misalignment or stray forces.

And here we get to the heart of the matter. As an expert quoted in our gearbox feature this month says, most failures are secondary, due to problems with misalignment of bearings (pages 53-60). Debris from failed bearings gets into the oil, highly polished surfaces are damaged, leading to gearbox failure. Not that the wind turbine industry is saying as much out loud. With manufacturers sharing gearbox and bearing suppliers, commercial interests rule. Secrecy has been endemic on the issue of gearbox failures, to the point where it has clearly hindered a full understanding of exactly what goes wrong and why.

A new maturity

The good news is that in company boardrooms there appears to be a far better realisation of the scale of the challenge and a greater willingness to share knowledge across a common platform, in part thanks to an initiative in America. Gearbox suppliers and wind turbine manufacturers like Vestas and Siemens are confident they have a better understanding of the loads that gearboxes and bearings must be built to withstand. There will be no repeat of past failures, they say.

The entry of GE into the wind business is also setting new standards, not least in slowing the speed at which prototypes are pushed into the market, in demonstrating caution on price-cutting innovations, and in understanding when secrecy does not pay. GE's new and larger turbine series will not be ready for at least another year; it has chosen to employ a separate bearing to relieve the gearbox of thrust forces; and it is sharing data from simulated 20-year testing of various gearboxes among its gearbox and bearing suppliers (page 54).

Perhaps most importantly, the entire industry is these days basking in a seller's market. Demand for turbines is outstripping supply; competitive price pressure has eased. Never has there been a better opportunity to consolidate engineering experience. Slowing down is often a sign of maturity.