New guidelines promise reliable wind-turbine gearboxes: What they mandate

The wind-turbine industry has been plagued with gearbox failures, which cause repair costs, lost energy production, environmental pollution, and legal expenses.

These failures include events in which the gearbox:

• Jams and stops rotating, due to failed gears, shafts, bearings, or housings.
• Breaks and allows the rotor (blades) to turn without turning the generator. Some wind turbines have been heavily damaged after the gearbox broke and let the rotor overspeed.
• Exceeds sound or vibration limits, due to failed gears or bearings. Excessive sound often exceeds regulations that limit wind-turbine noise in the U.S. and Europe.
• Has excessive oil leakage, caused by failed shafts, seals, and housings. Severe leakage leads to gearbox failure. Also, ground contamination by oil may require costly cleanup.
• Requires excessive maintenance.

Beginning of a solution to wind-turbine gearbox failures

Three years ago, a group of individuals dedicated to wind power recognized that many gearbox failures were due to a lack of understanding of the severity of the wind-turbine operating environment. They saw a need to better define this environment and to help gear and wind-turbine manufacturers understand each other’s needs so that more reliable gearboxes could be obtained.

As a result, the American Gear Manufacturers Association (AGMA) and the American Wind Energy Association (AWEA) formed a committee of wind-turbine manufacturers and operators, researchers, consultants, and gear and lubricant manufacturers.

This committee developed AGMA/AWEA 921-A97, Recommended Practices for Design and Specification of Gearboxes for Wind Turbine Generator Systems, which is expected to be published in mid-1997.

This document reflects the latest knowledge about wind loads and failures, plus the insights of gear and wind-turbine manufacturers and operators. It describes wind-turbine configurations, operating conditions, and environmental factors that affect gearbox life. It offers guidelines for defining wind loads and specifying gears and bearings, as well as operating and maintaining the gearboxes. All of these guidelines focus on one goal: ensuring reliable gearboxes for wind-turbine service.

Wind-turbine configuration and operation
Two wind turbines with the same generator can experience much different gearbox loads due to the rotor orientation, and the method of controlling rotor pitch. AGMA/AWEA 921 explains the significance of these factors on gearbox loads.

Rotor orientation can be either horizontal, as in a horizontal-axis wind turbine (HAWT); or vertical, as in a verticalaxis wind turbine (VAWT). In the more common HAWT unit, the gearbox is located on a tower, and it transmits loads from a large rotor to a generator (see drawing). In a VAWT unit, the gearbox is on the ground, at the base of the rotor.

Some HAWT’s are integrated systems where the gearbox supports the rotor, and in some cases, the generator and other components such as yaw drives. With this configuration, the gearbox housing must transmit rotor loads to the supporting tower without causing excessive stresses or deflections.

By contrast, a modular system consists of rotor, rotor support shaft and bearings, gearbox, and generator, each mounted to a common support plate. In this case, the gearbox doesn’t need to support other loads.

Rotor torque is controlled either by adjusting blade pitch (active control), or by aerodynamic stall (passive control). Active pitch control increases gearbox complexity because, in many cases, the control mechanism passes through a hollow shaft in the gearbox to the blade actuators. Passive pitch control is simpler but causes higher peak torques than active pitch control.

Environmental considerations to extend wind-turbine life

Wind turbines must withstand aggressive environments that range from hot, dusty deserts to cold, wet marine environments (some units operate offshore).

Temperatures can vary widely, which affects lubrication. The oil sump temperature should be at least 5 C above the pour point of the lubricant during startup, and less than 95 C during operation. Otherwise, heaters or coolers may be necessary.

Most sites require high-capacity, filtered breathers and positive shaft seals to control contamination.

Defining gearbox loads

Many early wind-turbine gearboxes failed because designers were uncertain of the operating loads that affect the gearbox. These include:

• Long periods of small oscillations while the unit is stopped by the parking brake and the rotor is buffeted by wind.
• Long periods of low-speed, low loads during light winds.
• Long periods of high-speed, low loads when winds are below the cut-in speed (minimum speed at which a turbine can connect to the utility’s power grid and start generating power).
• High transient loads when the generator connects to the power grid.
• Rapid load fluctuations during normal operation.
• High transient loads during braking. Such loads, although infrequent, can be very damaging.

Defining the load spectrum for each of these conditions is a difficult task due to the uncertainty of predicting loads. But recent experience has made these predictions more reliable. Accordingly, AGMA/AWEA 921 tells how to assemble load spectra that include both wind loads and transient loads that occur during start-up (connection to the power grid), rapid blade pitch change, and braking.

Until recently, wind turbines were not classified according to their capabilities. But the International Electrotechnical Commission (IEC), through its IEC 1400 initiative, has established four classes based on wind severity. A Class 1 wind turbine, for example, can operate in the most severe conditions, with an average wind speed of 10 meters per second (m/sec), an extreme wind speed of 50 m/sec, and a high turbulence intensity.

Specifying gearbox components

Failure analyses have shown that certain types of gearbox components perform best in wind turbines. Here’s a summary of the recommendations for gears, bearings, and lubrication systems. These recommendations may also be worth considering for other severe load applications.

Gears. Wind-turbine gears are subjected to wind loads that vary from light to very heavy (wind gusts). Carburized, hardened, and ground gears have proven to be reliable for these widely varying loads, and compact enough for the application. Metallurgical quality should meet the requirements for Grade 2 material in accordance with ANSI/AGMA 2001.

Gear teeth should be designed to maximize load capacity as explained in AGMA 901-A92. For example:

• Use at least 20 teeth on each pinion to achieve a good balance between pitting resistance, bending strength, and scuffing resistance.
• Shift the tooth profile radially to obtain balanced specific sliding (a measure of friction loss) for a gearbox operating as a speed increaser, rather than a speed reducer. Running a speed reducer in reverse, as a speed increaser, causes rough operation and scuffing (See box, “Jargon leads to failure”).
• To achieve uniform load distribution on gear teeth, use a low aspect ratio (ratio of pinion face width to pitch diameter) and modify tooth profiles to compensate for deflections.
• To obtain quiet operation, use gears with high contact ratios (ratio of face width to axial pitch) and high accuracy (AGMA quality No. 11 or higher).

The preferred gear types for wind turbine gearboxes are spur, helical, and double helical.

Helical gears are quieter than spur gears, and have more load capacity. Double helical gears may also balance thrust loads. However, they require high accuracy to control dynamic loads, and shaft couplings must be designed and applied so they limit external thrust loads.

Spur gears are often used in epicyclic gearboxes because they avoid the tendency of some helical gears to misalign planet gears and interfere with floating sun gears. A combination of spur gears in the low-speed epicyclic stage, and helical gears in the high-speed stage provides a compact, quiet gearbox.

As wind turbines become larger, they create an incentive to use either epicyclic or splitpower- path gear arrangements to meet load capacity while limiting size and weight.

Bearings. Severe vibration in wind turbines precludes using standard industrial practice for fitting bearing outer races and housing bores. This vibration may cause outer races to spin within the bore, causing wear that misaligns gears, generates wear debris, contaminates lubricant, and damages gears and bearings. Therefore, bearing fits must be tight or races restrained from spinning.

Preferred bearing types for wind-turbine gearboxes are:
• Spherical roller.
• Double-row tapered roller.
• Cylindrical roller with retainer.
• Ball.

Single-row tapered roller bearings should only be used if endplay is held within acceptable limits. Otherwise, excessive endplay will misalign gears and may cause gear and bearing failures.

Full-complement cylindrical roller bearings offer high load capacity. But this type of bearing has no cage, which allows rollers to slide against each other, making them prone to scuffing. Therefore, use these bearings only on low-speed shafts where sliding speeds are low enough to avoid scuffing.

Wind turbines frequently rotate at full speed but with low loads. Under these conditions, bearing loads may be insufficient to maintain rolling contact between the rolling elements and raceways. As a result, skidding, overheating, and scuffing may cause bearings to fail. The highest risk occurs on high-speed shafts with four-point or angular-contact ball bearings. Therefore, specify preload if necessary to prevent skidding.

Braking to a stop subjects a gearbox to reversing loads. Therefore, minimize bearing endplay to reduce impact loads on bearings caused by reversing thrust loads from helical gears.

Other tips:
• Equip bearings with bronze retainers. Plastic retainers are susceptible to contamination by hard particles that abrade rolling elements.
• For shaft-mounted gearboxes, minimize radial clearance of the low-speed bearings to avoid misaligning the gears.
• Except for epicyclic gearboxes, where the sun pinion has no bearings, mount all pinions between bearings. Don’t use overhung pinions because they may cause gear misalignment and high bearing loads.

Lubrication. Scuffing, micropitting, and wear failures related to lubrication are widespread in wind turbine gearboxes. To avoid such failures, make sure the lubrication system — usually splash or pressure-fed — provides adequate lubricant to gears and bearings. Pressurefed systems should have a filter to clean the oil, and can have a heat exchanger to cool the oil.

The low pitch line velocities (up to 2,000 fpm) and high tooth loads (contact stress over 200,000 psi) of wind-turbine gears require a gear oil with antiscuff additives and the highest practical viscosity — at least ISO VG 320. Too low a viscosity causes micropitting, macropitting, adhesive wear, and scuffing.

Micropitting is especially prevalent in wind-turbine gearboxes. Though it does not always lead to catastrophic failure, it generally reduces gear tooth accuracy and can progress to full-scale macropitting and gear failure. In many cases, micropitting causes unacceptable gear noise.

To prevent micropitting, maximize the specific film thickness by using gears with smooth tooth surfaces and an adequate amount of high viscosity lubricant. You may need to try different lubricants to find one with adequate micropitting resistance.

Quality assurance

Making sure that gearboxes meet quality requirements is another important function covered by AGMA/AWEA 921. It offers guidelines for:

• Preparing a procurement specification, which includes quality assurance and testing requirements.
• Preparing a manufacturing plan, which includes plans for inspections and tests.
• Qualifying prototypes before manufacturing begins.

Operation and maintenance AGMA/AWEA 921 offers guidelines for on-site monitoring and laboratory analyses of lubricants for viscosity, water content, acids number, solid contaminants, and additive depletion. It also describes vibration and temperature monitoring methods.

Safety is a major concern for wind-turbine operators for several reasons: the risk of falling, getting trapped in rotating components, and being injured while maneuvering in restricted spaces. Make sure gearboxes have an adequate number of steps, handholds, and lanyard hook points. The housings should have rounded corners, and exposed rotating components should have safety guards. Because access is difficult and work space is limited, design the gearbox to ease maintenance.

For information on availability of AGMA/AWEA 921, contact AGMA at 703- 684-0211 or Suite 201, 1500 King St., Alexandria, Va. 22314.  Brian P. McNiff is a wind energy consultant and president of McNiff Light Industry, Blue Hill, Maine. Robert L. Errichello is a gear consultant and president of GEARTECH, Townsend, Mont.