Flexible gears bolster wind-turbine reliability

Novel design increases torque capacity of planetary gears up to 50%. From all outward appearances, wind turbines are sleek and elegant works of engineering. But inside, they're hell on gears and bearings. In most turbines, the rotors turn a mechanical transmission that drives a small, high-speed generator. In terms of capital investment, this is the most economical and preferred design — but large and unpredictable loads often push gearboxes beyond their limits. Downtime and unreliability are two of the main reasons electricity from wind is expensive. Low-speed, direct-drive generators that eliminate the gearbox are a more-dependable alternative for turbine designers, but today they can be pricey. Engineers at Canton, Ohio, may have an even better answer: a rugged and powerful planetary gearbox that can handle extreme loads. Reliability problems stem from the fact that the wind doesn't blow in a nice, steady stream, says Gerald Fox, chief technologist of mechanical technology at Timken. It's turbulent, changes speed and direction in an instant, and creates loading conditions that play havoc with mechanical systems. "Turbines have a giant rotor, in some cases as large in diameter as a football field, generating 1 to 2 million lb-ft of torque," explains Fox. "The gearboxes commonly have 75:1 to 100:1 step-up ratios, taking wind energy from the rotors at about 20 rpm up to 1,500 to 1,800 rpm at the generator." The trouble is, says Fox, when wind speed suddenly changes, a relatively small amount of acceleration and angular movement at the gearbox input gets multiplied 100 times at the output — building up massive amounts of torsional windup and strain energy in the gears. Turbine makers typically use planetary gears to divide torque along three paths and reduce individual loads on each gear. But torsional loads twist gears out of alignment, and slight dimensional variations