Gearboxes remain problematical, but promising technologies are under development
In "The Gearbox Guys," our
Those who've grown up in the world of electronics, often far removed from the "nuts and bolts" of machinery, find it easy to judge the renewable energy industry only on the electrical art involved. Thus, a recent enthusiastic trade magazine editorial offers these glowing comments about wind turbine technology from a spokesman for the
His full-page ode praised this source of "clean electricity," offering "system reliability and market efficiency" using "sophisticated doubly-fed induction motors or 'Type 4' full converters." These "marvels of engineering" involve "advanced controls" using "state-of-the-art power electronics." Modem wind turbines have a "myriad of offerings" including "volt and var control/regulation; fault ride-through capabilities ... ramping and curtailment, primary frequency regulation ... short-circuit-duty control" and so on. No mention of any mundane concerns about bearings, shafts, gears, or the nagging problem of maintenance.
Gearbox breakdown is an international concern.
Instead of the intended life of 20 years, gearboxes are giving only 3 to 11 years of service. An individual repair can cost
Why gearboxes fail
Why are gearbox failures so prevalent? Extreme speed changes at high power aren't new. They've successfully involved gear trains for generations. Gearing transmitting more than 50,000 horsepower was being used in marine steam turbine propulsion almost a century ago. Land transportation applications have been around even longer, although in smaller sizes.
In a wind turbine, however, all operating conditions were not fully understood, particularly as turbine size grew quickly from less than 1 megawatt to more than 5. Much testing and condition monitoring has been needed to fully discover the nature of the stresses involved. A recurring theme throughout the literature of the last decade is "underestimation of the severity of gearbox operating conditions."
Engineers create designs largely on the basis of what has been known to work in "similar" situations. Why they work is often not fully known. Although the load/ life relationships in rolling bearings were well-understood, wind turbine gearboxes imposed loads of a different sort. It's been estimated that 60% to 85% of all gearbox failures originate with bearings and lubrication. For the smaller bearings used in railcars and automotive vehicles, production quantities are large, and the stresses imposed in service are well understood. In a wind turbine gearbox, many much bigger bearings are subjected to a variety of extreme loading conditions that vary in ways difficult to predict.
Bearing design, materials, manufacturing, and assembly methods have undergone many changes since the mid-20th Century. For example, advances in technology have led to the general replacement of journal bearings by roller bearings in rail transport vehicles. Extensive industry standards have been written.
Nevertheless, the business is highly competitive. Improvements developed by any bearing manufacturer are closely held. Outsiders express concern about internally developed, proprietary design standards "that have the potential to introduce significant differences that can affect actual calculated bearing life without revealing the details to customers." (Translation: bearing life is calculated different ways for the same bearing depending upon each supplier's rules. So what do we believe?)
A second reason for persistent gearbox failure is that no immediate solutions existed for some problems caused by wind turbine operating conditions. "What to fix" was only a first step. "How to fix it" remained to be discovered.
A particular complication for wind power is: "Location, location, location"-supposedly the three most important criteria in evaluating real estate, they might also be cited as distinguishing the maintenance of a wind farm from any other electric generating facility. Combining many complexities of electromechanical energy conversion with electronic accessories, wind turbine generators are all outdoors, and also likely to be: 1 ) in remote areas, where the wind is unobstructed and settlement is sparse; 2) in deep water, sometimes miles offshore; 3) up to 300 feet in the air. Monitoring, servicing, and repairs can be daunting.
What researchers are doing about it
Recognizing the seriousness of concerns about wind turbine reliability and operating cost, in 2007 the
By 2012, the membership included 15 turbine and gearbox manufacturers, 16 turbine operators, 3 bearing suppliers, 7 condition monitoring firms, 4 national laboratories, 11 independent verification agencies and software developers, 7 lubrication experts, and 18 universities.
The continuing GRC program has included modeling and analysis, dynamometer and field testing, condition monitoring, and development of a failure database. Although much work has been done at other DoE locations, including
Testing began in 2007 with two identical 750 kilowatt gearboxes. In 2009, one of them was installed in a turbine at a nearby wind generating plant, where it experienced two damaging lubrication failures after which it was retested by NREL.
As the work proceeded, evidence accumulated that tram sient torque events caused gearbox bearing loadings beyond their ratings. Planet gear bearings were especially sensitive to bearing operating clearances and alignment.
Data from early testing was used to evaluate condition monitoring systems, especially vibration measurement, through analysis by 16 "project partners." That "Round Robin" was described early in 2012 in a 157-page Technical Report (one of many NREL issued by the GRC). One of its important conclusions: "There is room for the industry to improve vibration-based diagnostic algorithms." That's because "Most partners had more missed detections than false alarms [or successful detections of trouble]." Some partners correctly identified only one real problem for at least five that were missed.
Based on what had been accomplished by 2012, a Phase 3 plan for further testing was presented to a GRC meeting in
Large-scale test programs are under way in
What the tests have shown
Between 2007 and 2011, several conclusions emerged in a series of analysis and test reports produced by the
I. "Dynamic torque." In other industrial gear drives, transmitted torque tends to be of a fairly steady value and always in the same direction. In the wind turbine, wind gusts load the turbine blades unevenly. Abrupt changes in wind force and direction, as well as a sudden turbine shutdown, cause severe torque reversals (see Figures 4 and 5). These "slam" gearbox shafts back and forth, applying shock loads to bearings and gears that repeatedly change direction. Gear teeth become slightly misaligned, begin to wear; misalignment then worsens; and so on.
2. Main shaft combined loads, including "gyroscopic precession" forces resulting from varying wind direction acting on the spinning rotor. Non-torque loads such as shaft bending were assumed to be uncoupled from the gearbox. But bending in the main shaft causes radial reaction forces passing through gearbox bearings. This can result in gear/ bearing misalignment. Such main shaft bending force has been found to be almost as great as the torque in that shaft. Shaft bending also affects tooth contact patterns within the low-speed gearing.
3. Wind force on the rotor blades also causes thrust on the main shaft. Transmitted to the main shaft support bearings, this leads to axial drive train motion and consequent wear.
4. The "slicing" effect. When a turbine encounters a local wind gust, each rotating blade in turn passes through (slices) the gust, causing a mechanical shock load on the gearing of a frequency determined by turbine RPM and number of blades.
5. Capacitor switching that may be involved in volt/var control. A step voltage change of only 3% or 4% can impose severe shock loads on the gearing. The effect of switching on and off the grid is similar.
6. Temperature extremes. Whether or not in full operation, turbines are entirely exposed to the outdoor climate. Many of the best wind farm locations in the U.S. are in mountain, desert, or plains areas where seasonal temperatures may range from -40 to +60 degrees C.
7. High gearratios, with a combination of high torque and low speed at the turbine rotor shaft. Maintaining a suitable oil film at only 10 RPM is a major lubrication challenge.
8. Severe vibration. So many separate components operating at different speeds generate many vibration frequencies and amplitudes, all subject to constant variation with shifting wind loads.
9. Difficulty in maintaining alignment of multiple shafts, subject to high variable loading in various directions. Long operating life requires that gearing shaft alignment be held within 20 to 30 microns-only about one-thousandth of an inch.
10. Uncertainty concerning transfer of loads from the drive assembly to its fixed gearbox mounting, contributing to the difficulty of maintaining shaft alignments, particularly in the high speed output stage.
11. Finally, manufacturing of both gears and bearings has shown several deficiencies leading to early gearbox failure. These include basic steel manufacture and heat treatment, finishes, and surface coatings.
As the NREL reported in 2007 to the
Causes of gearbox failure
What are the failure mechanisms? Metal fatigue is familiar to most of us. We usually think of it as resulting from repeated bending back and forth of some structural member, which eventually causes breakage even though the stress level of each bending cycle is well below the breaking strength of the material.
In gears and bearings, what's involved is Hertzian fatigue. This is what results from cyclic rolling pressure between rounded surfaces. In the contact between gear tooth faces, or between bearing races and rollers, local deformation of the material comes and goes with component movement. This combination of cyclic compressive and shear stress ultimately results in fatigue failure (Figure 6). It may be hastened by defects in the material itself or by heat treatment during manufacture.
The combination of Hertzian fatigue with the difficulty of maintaining adequate lubrication at low RPM has drawn attention to several failure modes that shorten gearbox life.
The first is micropitting; see Figure 7. This occurs when oil film thickness is comparable to average surface roughness. It's associated with bearing sliding or "skidding" during "unsteady" operation. This causes load between surfaces to be sustained by some combination of lubricant and surface asperities (Figure 8). Although the fundamental mechanism causing micropitting is still under discussion, surface roughness (which governs the height and distribution of the asperities) appears to have the greater influence.
Besides that contact stress, gear tooth sliding subjects asperities to shearing force. The consequent plastic flow leads to residual tensile stress, and eventually fatigue cracking. Wear along pitch lines can cause gear noise, impact loading, and "scuffing." In bearings, micropitting can cause stress concentration leading to "secondary failure modes" such as macropitting (spalling) at roller ends (Figure 7).
Scuffing (also called gray staining, frosting, or microspalling) is surface damage through sliding contact, involving serious adhesion between surfaces with rapid plastic deformation. This causes local frictional heating where the oil film is too thin. A scuffed surface typically has a gray, "frosted" appearance.
"Rolling contact fatigue" is nothing new.
The white appearance results from "microstructural alteration" in the material. That's commonly blamed on hy- drogen embrittlement of the steel, originating with lubricant breakdown. The complex metallurgical and chemical process is enhanced by water in the oil, causing corrosion that promotes diffusion of hydrogen into the metal. The chart of Figure 12 is a simplified version of what can be taking place.
These defects have been observed in some other industries for a generation. But exactly how they form-and how they can be prevented-has not been fully understood. In gearbox bearings, said a British researcher in 2012, "severe transient and tribological operating conditions ... are still mostly unknown and thus bearings are not designed to sustain them." The consequence: bearing life can be reduced to from 1 to 20 percent of the theoretical L10 period.
According to an informal survey of 75 wind farm operators in the U.S. during 2008, "Most gearboxes fail as a direct result of improper lubrication and lack of routine maintenance." Lubrication presents difficulties for several reasons. Up on that tower, sometimes as high as the roof of a 30-story building, the machinery is not subject to frequent close scrutiny. Constant exposure to the elements (humidity, dust, temperature extremes) poses a contamination hazard. Changing the oil in a large gearbox can involve transporting as much as a quarter ton of oil up and down the tower.
Using the right oil, correctly
The oils that appear best-suited to gearbox lubrication are synthetics of the PAO (polyalphaolefin) type, with a number of complex additives. Like any other organic compound, oil is subject to deterioration by heat. (Figure 13 shows the result of moderate bearing overheating; a more severe consequence appears in Figure 14.) PAO oils exhibit a lower "oxidation rate" than petroleum compounds. That rate is influenced by temperature; in a typical Arrhenius reaction, oil temperature increase of 10 degrees C approximately doubles the oxidation rate. Viscosity must be adequate at low temperatures. An equally important property is the "traction coefficient" (a measure of the film shearing force). Unfortunately, because of the variation in speeds and loads throughout the gear train, any oil used in the entire gearbox must involve some compromise in properties.
For any oil, though, cleanliness is vital. Contaminant particles can be left behind during gearbox manufacture. Recommendations to minimize that hazard include:
1. Before placing a gearbox in service, remove the oil and filter it to 3 microns.
2. Paint the gearbox interior with a white epoxy that is easily cleaned and seals porosity.
3. Keep all gears and bearings covered in a dry place prior to assembly, and clean all components beforehand in a separate area.
4. Fill the assembled unit with oil that has been analyzed for cleanliness.
Unless the gearbox itself is thoroughly cleaned, any residual contamination can cause gear or bearing damage in as little as 3 minutes after startup, before the gearbox filtration system can become effective.
The latest international standard for oil cleanliness is ISO 4406, which classifies contamination according to the number of particles of various sizes per milliliter of oil. The limit consists of three numbers in this form: A/B/C, in which A is the number of particles larger than 4 microns; B. those exceeding 6 microns; and C, 14 microns. Typical ISO ratings are:
New oil: 16/14/11
At the factory: 17/15/12
In service: 18/16/13
Some manufacturers feel that a
Water in the oil tends to increase its oxidation rate as well as causing corrosion (Figure 16) and can also act to separate necessary additives out of the oil. Water contamination as low as 200 parts per million can reduce bearing life 20%.
A typical expectation is that gear oil last three to five years before replacement. Some gearbox manufacturers have recommended a complete oil change at only 500 hours of initial operation, but a "good" filtration system normally allows that to be extended. However, oil analysis is advisable every three to six months. That includes tests for Total Acid Number (a measure of oxidation rate); water content; viscosity (look for a change reaching plus/minus 15% of the original value); and cleanliness (presence of dirt and hard particles).
On-line oil contamination monitoring methods are being developed. One such system passes oil through a magnetic field to detect ferrous wear particles. Another approach involves a laser to count obstructive particles in flowing oil, checking particle size and quantity against the ISO 4406 criteria. In still another method, the pressure drop across oil passing through a fine-mesh screen indicates the extent of entrained contamination.
Close attention is also being paid to the surfaces being lubricated. Although asperities cannot be entirely eliminated, "superfinishing" or honing can yield a smoother surface than grinding alone. In gear tooth shaping, conventional grinding can cause localized "grinding bum," responsible for up to 15% of one manufacturer's gearbox failures. Wear of both gears and bearings during initial turbine "run-in" can be mitigated by application of surface coatings such as manganese phosphate, copper, or silver. A "black oxide" coating is claimed to help prevent surface cracking, perhaps by preventing diffusion of hydrogen into the steel.
Manufacture of the steel itself has shown a need for improvement. Tests have revealed "inclusions" of foreign material in the alloy, such as particles of highly abrasive aluminum oxide, that can form crack nuclei. One gearbox manufacturer has stated that "approximately 25%" of reported failures have originated with inclusions. Standards of the
What does the future hold? The wind energy industry has long agreed that "direct drive" is the way to go. Eliminate the vulnerable gearbox entirely. The permanent-magnet lowspeed generator with sophisticated electronic control is a viable technology. In 2009, an estimated 15% of new turbines were being built with direct drive-a fraction expected to double within three years.
As of 2010, however, a
Whatever the extent of direct drive adoption for new units, scores of thousands of existing geared drive turbines will remain in service for years to come. Maintenance, repairs, and replacements will go on. In 2012, the managing director of a British wind energy firm reported that gearbox repairs amounted to a
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