The Gearbox Standard for Windturbines gets an Upgrade

The recently drafted ISO/IEC standard for wind-turbine gearboxes is significant for many reasons. First, several organizations collaborated to produce ANSI/AGMA/AWEA 6006-A03, which was adopted without change in 2005 as ISO 81400-4, an international standard. The new draft (IEC project 61400-4), expands on preceding standards so it is more encompassing. It provides a conduit through which manufacturers and users can specify and evaluate products. The new IEC standard is a bit overdue, but may have been worthy of the wait.

The present ISO standard’s introduction briefly tells why it was developed. It says, for instance, the operation and loading of a wind turbine, speed-increasing gearbox is unlike most other gear applications so the intent of the standard is to describe the differences. AGMA standards have relied heavily on the experience of gear system performance in related applica-tions, whereas other standards are based on theoretical and laboratory data. But much of the information in 6006-A03 is based on field experience. The standard is a tool that lets wind turbine and gearbox manufacturers understand each other’s needs in developing a gearbox specification for wind turbines. The annexes also include informative discussions of various wind turbine and gear design applications.

The committee responsible for ANSI/ AGMA/AWEA 6006-A03 was somewhat unique in that it was composed of wind-turbine manufacturers, users, researchers, consultants, gear and bearing manufacturers, along with lubricant and system suppliers from around the world who brought years of experience to meetings.

The range of disciplines was needed because a wind turbine is one of the most demanding applications for a gearbox. It requires a relatively small, compact, high-power-density gear drive and an electric generator to transmit fluctuating loads in a harsh environment of deflections, high vibration, and temperature extremes.

The present ISO standard applies to gearboxes with power capacities from 40kW to about 2MW. I say “about” because 2MW was a sort of sweet spot for OEMs at the time of the standard’s development. Technology, of course, marches on and larger units are on drawing boards and in production. So the standard is a snap shot in time of sorts. The new version addresses larger units and other concerns.

Still, the standard generally applies to parallel axis, one-stage epicyclic, and combinations of epicyclic and parallel-shaft designs. It provides requirements on specifying, designing, manufacturing, operating and monitoring reliable wind-turbine gearboxes. Designers should be interested in the more comprehensive application sections, such as:

• How to specify system loads and calculate gear capacity. Turbines  require special attention because their gearboxes have to be relatively light weight. They will operate in tall towers so every pound must be considered.

• Manufacturing, inspection, testing, and documentation requirements were rewritten to include items that are intended to achieve the reliability required of wind turbines.

• Advanced gear-tooth-contact analysis and verification received attention because it has been shown that the prediction of torsional and lateral deflections in a high-power gearbox is important to proper operation.

• Extensive information on the application and capacity of rollingelement bearings were deemed necessary because the service operation of some bearings are troublesome.

• Lubricant and lubrication equipment receive attention because the characteristics of a lubricant and its distribution can make or break a highpower-density gearbox.

In addition, annexes supply information on wind turbine architecture, load descriptions, quality assurance, maintenance, lubrication selections, condition monitoring, and minimum ordering information from purchaser to gearbox manufacturer.

A few other additions and modifications include sections on design life and reliability, design process, wind-turbine-load calculations, gearbox components, design verification validation, operation, service, and maintenance requirements.

It is hard to determine what will be retained after the three required ISO/IEC ballots complete in about three years. In the meantime, most engineers believe the ISO/IEC standard has improved gear reliability. Bearings, however, still seem to need additional work.

Lastly, even though developing standards is a consensus process, individual positions have been expressed that enhance the document. The content of these standards may provide other ideas for designers in many gearbox applications.

Synthetic gear oils cut wind power operating costs

A worldwide manufacturer of specialty lubricants will showcase three gear oils for the windpower industry at the CanWEA 2009 trade show September 20 to 23 in Toronto, Canada. The lubricants, from Kluger Lubrication, Londonderry, NH, (klubersolutions.com/wind) include these three synthetic high-performance, high-load gear oils:
• Klübersynth GEM 4 N (polyalphaolefin)
• Klübersynth GH 6 (polyglycol)
• Klübersynth GEM 2 (rapidly biodegradable ester)

Klübersynth gear oils meet the requirements for gear performance and offer wear protection, resistance to micro-pitting, foam and residue formation. Compared with standard gear oils, these lubricants show good ageing resistance, high load-carrying capacity, and low friction values. Consequently, the synthetics lengthen oil change intervals, reduce power loss, and turbines using them generate more power over their operating life
In addition, Klüber’s Added Value program makes it easy for manufacturers and operators to simplify their lubrication requirements. For more information about Klüber lubricants for windpower applications, visit booth 608 at CanWEA 2009

Bearings for wind turbines

Several bearing configurations for wind turbines come from NTN Bearing Co. of America, Mt. Prospect, Ill.

For instance, for main shafts, designs include spherical roller bearings. The company says these increase efficiency and service life. Credit for that goes to new materials and improved heat treatments that significantly increase overall bearing service life. The company manufactures all sizes of mainshaft bearings, from designs for common 1.5-MW turbines to the more recent 3.5-MW designs.

For generators, insulated bearings include the Megaohm Series. These are well insulated against electrical currents. Generators often produce stray currents that cause electrolytic corrosion which can shorten bearing life. To combat the problem, NTN’s ceramic-coated Megaohm ball bearings insulate themselves from electric current. A ceramic coating applied to the outer surface and sides of the outer ring prevent current from passing though the bearing. The ceramic provides an insulation resistance of at least 2,000 M-ohm under normal operating temperatures, alleviating electrical arcing and early bearing failure. Ceramic-coated bearings come with or without seals or shields, and are interchangeable with standard, non-insulated units.

For yaw gearboxes, tapered roller and angular contact bearings are compact designs with high-load capacities. A yaw gearbox rotates a turbine face to keep it into the wind. The gearbox here is generally small yet capable of transmitting high torque so its bearings must be compact and capable of high loads.

Lastly, main gearboxes, or speed increasers, turn the low-speed rotor into higher speeds for generators. These generally use cylindrical roller, tapered roller, and occasionally spherical roller and ball bearings. Cylindrical roller bearings for gearboxes have higher rigidity and lower friction than conventional bearings through reduced rolling element slippage.

Gearbox, generator, and controls in one package

GE Drivetrain Technologies, a unit of GE Transportation, Erie, Pa, says the company has launched a wind generator and control systems division to serve wind turbine owners around the globe.

The new company will immediately offer doubly-fed induction and permanent magnet generators in the 2 to 6 MW power range. The company produces more than 3,000, 2 MW and greater generators annually and has sold more than one million generators and motors globally. The company says these generators have been proven reliable in extreme environments in applications as diverse as railway locomotives, mining trucks, and off shore drilling rigs.

“Our objective is to apply GE Transportation’s capability to advance the state of the art in wind energy generation technology,” says GE Drivetrain Business Leader Prescott Logan. “GE Transportation’s global supply chain, including existing generator-manufacturing facilities in Erie, Pa. and Monterrey, Mexico creates near-term production capability.”