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Fretful Situations


Wind turbine operators installed over 41 gigawatts of new capacity worldwide in 2011. That brought global capacity for wind energy to more than 238 GW at year end, and represented a 21 percent leap in total cumulative installed wind power, according to the Global Wind Energy Council.

The United States is in the thick of the action, with around 47 GW (19.7 percent) of global capacity. Although wind energy still contributes less than 3 percent of the U.S. electricity mix, the country added 2,900 turbines in 2010, gaining 5.0 GW of wind power. It followed that up in 2011 by adding another 6.8 GW.

These gains did not come cheap. The American Wind Energy Association estimates that installing a wind turbine and hooking it up to the electricity grid costs roughly $2 million per megawatt, so in two years the projects have swallowed more than $23 billion of investors money.

Meanwhile, some $40 billion of existing U.S. wind energy assets passed out of the manufacturers warranty period last year, leaving their operators to shoulder the burden of maintenance, the AWEA observes. Most turbines require servicing at least once or twice a year, meaning the drumbeat for equipment reliability is rising just as fast as the turbine towers.

Current drain intervals for wind energy lubricants average about three years for onshore wind turbines and five years for offshore units, according to a new study from Kline & Co. in Parsippany, N.J. It says global lubricant consumption for wind energy rose from about 5,000 metric tons in 2005 to more than 20,000 tons in 2011 – in near lock-step with the overall growth of wind capacity.

The study, Lubricants for Wind Turbines: 2011, also found 80 percent of the lubricants consumed in wind energy are synthetics, making this a dyanamic market in terms of value as well as volume, said Klines Milind Phadke, who recapped its research in a March 6 webinar. Global wind energy capacity over the next five years will grow at 17 percent a year, he said. Thats slower than the 27 percent annual growth of the last 11 years – but still an attractive and healthy market for premium lubricants.


At current pergigawatt rates of consumption, demand for wind energy lubricants would top 45,000 tons a year by 2016, Phadke remarked. However, better maintenance, new equipment designs and longer-life products will probably temper this to a bit more than 40,000 tons, he added.

Gear oils are the biggest lubricant type used in wind energy, at roughly two-thirds of the total, and have rigorous lubricant specifications, test regimens and approvals. Nevertheless, gearboxes have been a thorn in operators sides. The average wind turbine experiences a gearbox failure once every eight years, a collaborative study sponsored by the National Renewable Energy Laboratories in Colorado found. Worse, gearboxes tend to fail miserably, with average downtime lasting more than six days. (By comparison, the average hydraulic system outage lasted a little over a day.)

These gearbox woes spurred some OEMs, such as Siemens, to develop direct-drive turbines that eliminate the gearbox entirely, Klines Phadke said. These systems weigh more, take up more space and cost more, but they also dont need any gears or gear oil.

In response, gearbox manufacturers stepped up their game and greatly boosted their units reliability, he continued. On-board condition monitoring systems also proliferated, further improving reliability. These enhancements have helped undercut the selling proposition for direct-drive units, for now.


Gearbox headaches may take the lions share of their attention, but the nacelle holds other daunting challenges for lubricant researchers as well. One of these is false Brinelling, named after the Swedish engineer Johan August Brinell who developed the hardness scale for metals and other materials.

False Brinelling is a very specific type of wear, also known as fretting, explains Jaime Spagnoli, senior research associate for ExxonMobil Research & Engineering in Paulsboro, N.J. At the annual meeting of the European Lubricating Grease Institute, he described how false Brinelling can plague wind turbine pitch, yaw and rotor bearings.

Unlike a true Brinell (a mark or dent left in a bearing contact area caused by a one-time impact or load), false Brinelling occurs over time and is caused when parts repeatedly contact each other due to vibration. Since vibration is a fact of life in wind nacelle bearings, Spagnoli said, even when theyre at rest, you get wear.

The problem is exacerbated by the severe operating conditions in wind turbines, which are found anywhere from dusty deserts and plains to stormy offshore reefs. Ambient temperatures may range from -45 degrees C to over 55 C, and depending on the equipment, operating temperatures inside the nacelle can run from -30 C to over 100 C.

As Spagnoli noted in his May 1 presentation, You get vibration and fretting when the blades are stationary. These blades can be 10,000 kilos in weight, and youll also get shock loads as the blades are hit by gusts of wind. Humidity and saltwater corrosion complete the mix.


With the newest turbines now up to 7 megawatts each, Spagnoli told ELGI, this means bigger bearings, and requires better lubes. The towers pitch and yaw bearings are among the most critical. The yaw bearing holds and positions the entire nacelle (weighing 60 tons or more), while the pitch bearings control and absorb the actions of the blades.

These are slewing bearings, which dont turn completely around, the ExxonMobil researcher related. Instead, they [pivot] back and forth just 10 to 20 degrees, to angle the blades and move the nacelle so it points into the wind.

Assembled slewing bearings have inner and outer raceways that can hold either roller or ball bearings, and are lubricated with a pretty good amount of grease, which is why ExxonMobil is interested in this application, Spagnoli commented.

Because the slewing bearing does not make a full rotation like a carousel, but simply angles back and forth, he went on, the grease is never redistributed around the bearing, and that creates false Brinelling. It also can create a channel in the grease where water can get into the raceway, leading to further wear and corrosion issues.

Slewing bearings are designed to contain a lubricant or grease inside to separate the bearing and race surfaces, but when at rest, the units weight will squeeze out the grease and the ball or roller will come into contact with the race; next, it will vibrate and pound against its mate. If the bearing turned, the grease would be redistributed inside the bearing and could help avert the wear, Spagnoli pointed out.


How to discern which greases can best prevent false Brinelling? For answers, Germanys Rothe Erde, a division of Thyssen Krupps and the worlds largest maker of slewing bearings, turned not to the lubricants industry but to RWTH Aachen Universitys Institute for Machine Design. Besides wind turbines, Rothe Erde slewing bearings are used in mine equipment, shipyard cranes, tunneling machines, offshore platforms and other applications that vie with windmills for harshness if not remoteness.

IME and the bearing manufacturer created the Riffel test, a mechanical wear test to assure greases can stand up to vibration and protect slewing bearings from wear. As Spagnoli noted, General Electric and other leading OEMs now require the Riffel test for any grease to gain approval for use in their wind turbines.

The Riffel test apparatus includes 15 balls within a cage and a vibration device to shake them in place. It also injects saltwater, at 1 percent sodium chloride, into the bearing for the length of the test. The test runs for 1 million vibrations – about 28 hours – all the while that saltwater is being injected into the bearing, Spagnoli said. The volume of grease that the bearing must contain is not defined for the test, he added. You just pack in as much as you can.

Over the 28-hour test run, the sample and bearing will endure vibrations of 70 kiloNewton and be flushed with 4 liters of saltwater (about 6 ml per minute). At the end of the test, the bearing is cleaned and a profilometer used to gauge any wear on the races. With 15 ball bearings contained inside the upper and lower races, thats 15 scars to be checked on each race, each of which is measured three times for a total of 90 measurements. The races and bearings also are visually rated for corrosion.

The big question for grease makers is, if you have a good grease, with good corrosion properties, good water washout, good wear control, can you assume it will do well on this test?

The answer, unfortunately, is no said Spagnoli. We thought so too, but this test has a very severe appetite. The motion is up and down, with vibration at high frequency. It is not rotational.

However, he emphasized, this test does seem to correlate to the conditions of vibration and corrosion seen in wind turbine pitch and yaw bearings. And although its finicky, grease companies now are working with the IME at Aachen University to improve the tests precision.

And then its on to the next hurdle in wind turbine lubrication.

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