Short gearbox life now steadily lengthening as technology matures

Short gearbox life now steadily lengthening as technology matures

Wind turbine gearboxes once had a well deserved bad reputation for their short working lives. Not long ago, a two or three-year life was about normal.

Things are looking up, however. Two companies recently announced improvements to gearboxes that stretch their working life too much longer periods. First, Moventas took the wraps off its Extra Life 1.5-MW gearbox last year and more recently, produced simulations by Sentient Science that point to a four-fold improvement in life. Then at the Wind Energy Update O&M Dallas conference, Gearbox Express unveiled an upgraded version of its proprietary gearbox, Revolution 2.0.

Moventas says that premature failures in the GE 1.5-MW gearboxes have led to unexpected downtime and increased costs. Moventas developed upgrades for the gearboxes, now called Extra Life, that can reduce premature failures on all types of gearboxes. To qualify its modifications, the company enlisted Sentient Science to validate the technology and life-extension claims and quantify the improvements in performance, durability, and reliability.


Computational testing shows a four-fold overall gearbox life improvement because of improvements made to the case-carburized ring gear, integrated planet-gear bearings, high-speed-stage bearings, tooth surface roughness, and material upgrades in bearings and gears. Sentient Science used its DigitalClone, a material science-based program that predicts the earliest time when cracks initiate in the microstructure of rotating gearbox components. The key analysis factors used in DigitalClone computational modelling included material quality, surface roughness, and stresses based on a full gearbox model subjected to a real turbine operating conditions. These are not explicitly accounted in industry standards.

Take the case-carburized ring gear, for instance. The method is used instead of another, such as case hardening. To demonstrate improved durability, simulations considered case-carburized microstructure, geometry, operating conditions, lubricant properties, surface finish, and residual stresses. More than 1,000 contact and bending simulations were conducted in DigitalClone software. The results demonstrated an improved L10 life from seven to 20 years, mainly due to better surface finish and material quality without detrimental defects or inclusions.


Also, a two-row arrangement of cylindrical roller bearings is used in each planet of the new gearbox, instead of a four-row arrangement typical of conventional designs. About 2,000 fatigue-life simulations were conducted in DigitalClone to compare the two-bearing arrangement. The planet bearing showed better load-carrying capacity compared to conventional bearings, thanks in part to an 8% reduction in contact stresses. Moventas says its planet bearings now offer superior fatigue life and attribute it to cleaner material quality and relatively lower contact stress.

And speaking of cleaner material, a white-etching-resistance steel is used in the Extra Life gearbox instead of a black-oxide or a conventional bearing material. DigitalClone also validated that the white-etching resistance bearing material was superior to the black-oxide coating against white etch cracking due to heat treatment and improved microstructure. The black-oxide coating showed a 3% probability of failure in less than 20 years at the high-speed shaft and high-speed intermediate shaft positions due to non-metallic inclusions. DigitalClone verified the white-etching resistant bearing is superior to other bearing materials in these positions. Furthermore, Moventas can replace these bearings up-tower, which lowers O&M costs and downtime.


Moventas add that its Clean Steel Tech has also been showing good result against IMS tooth fractures. The company reports no IMS tooth fractures with the upgraded material spec. In conventional steels, non-metallic inclusions can be as large as 100μm on the intermediate-speed pinion. Contact fatigue life simulations in DigitalClone showed that the enhanced material doubles the life over the original design. Rolling contact fatigue simulations demonstrated that the sun pinion used in the new gearbox with an Ra (surface roughness) of 0.3 μm increases L10 fatigue life by a factor of 2.2, compared to the original Ra of 0.6 μm.

The company adds that the Extra Life gearbox showed a lower cost of ownership compared to the legacy platform. Also, the units come with five-year warranties. ExtraLife features can be applied to larger gearboxes as well says the company. One example is Siemens 2.3 MW platform where Moventas has upgraded the design by using own core tech such as case carburised ring gear and combined two row bearing on the planet gears.


The more recent introduction comes from Gearbox Express. “The wind industry has wrestled with gearbox failures since its inception,” said Gearbox Express CEO Bruce Neumiller at the conference. “That led us to create our company and meet these challenges head-on. We are succeeding.” The company unveiled its initial Revolution gearbox in 2013 to address frequently seen failure characteristics. Since then, the company has successfully installed more than 200 of the designs across the United States. Neumiller says all are running well with the oldest about five years old.

“For more than three years, we have has been researching why the planetary configurations in some gearboxes were not reaching half of their designed lifespan,” said Neumiller. “Those development efforts are now in the Revolution 2.0.”

He says the top features of the new gearbox include:

• A redesigned planetary gear and bearing interface in which the bearing outer races are machined into the gear. This reduces the number of components and opportunity for failure.

• Integral tapered rollers are used in lieu of cylindrical rollers. The tapers permit preloading, which increases system stiffness and improves load sharing. The bearings also reduce internal bearing stresses, improve life 170%, and reduce rim deflection by 460%, which reduces bending stress and the propensity to crack planet gears.

• The use of steel that is cleaner than ISO 6336-5 ME. Cleaner steel improves contact and bending-gear ratings, letting planet gears run with a higher safety factor and significantly reducing the risk of failing from material inclusions.

• In-house super finishing improves as-ground surface finishes by 50%, ensuring bearing life while improving the gear rating.

• Gearboxes are now outfitted with a metallic wear debris monitor from Poseidon Systems, letting GBX remotely monitor and proactively address oil cleanliness issues. In addition, the gearbox is backed by a five-year warranty, which includes crane and labour expenses.



Source:  Short gearbox life now steadily lengthening as technology matures

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