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Heading off gearbox failure

Heading off gearbox failure

Wind-turbine gearboxes have become more reliable, but they’re not perfect.

Wind turbines are more reliable than ever, as evidenced by the uptime and availability numbers for today’s wind farms. It’s due in large part to wind-turbine gearboxes that have become increasingly rugged and dependable over the last decade. Despite several notable retrofits that have given the gear industry a black eye, many manufacturers have been producing relatively efficient and robust turbines with little fanfare.

Reasons for these improvements are almost as numerous as the components in a gearbox. They include better materials, bearings, gears, seals, and lubricants; enhanced designs, thanks to lessons learned from older units and advanced modeling and analysis software; and more-stringent maintenance practices and the availability of more-economical condition-monitoring systems.

Even with continued improvements, however, there are still a few issues that result in accelerated component wear and early failure. Early recognition of these failure modes will help reduce downtime for turbines even further. It will also cut maintenance costs by heading off problems before secondary damage, usually from debris, increases repair expenses. Addressing these issues with manufacturers prior to purchasing turbines will increase gearbox life and reliability. Here’s a brief look at the common wear and failure modes we continue to see in the field today.

Bearing issues

Bearing inner-race micropitting. Fatigue failure of a race’s metal surface leads to micropitting. It is characterized by a dull gray frosted appearance, and under magnification the surface appears to be covered by very fine and shallow pits. Micropitting often progresses to macropitting (spalling) as shown in the accompanying photo of the inner race of a roller bearing from a planet gear.

It is caused by inadequate oil-film thickness that lets the asperities of the roller and raceway surfaces touch one another. It normally happens on the planetary bearings in a wind-turbine gearbox because low speeds result in a thin oil film.

This failure mode progresses slowly and usually takes years to reach the point where a bearing must be replaced. To prevent this type of failure, designers should pressure lubricate the planet bearings, increase the operating oil viscosity, and improve the surface finishes on rollers and races.

Planet bearing shaft wear. If the inner race of a bearing rotates on the planet bearing shaft, it can lead to severe abrasive wear of the shaft. This wear generates significant amounts of metal debris that can damage the bearings and, over time, lead to planet-gear misalignment.

To prevent this type of wear, the inner race of the bearing should be assembled to the shaft with an interference fit, and the shaft should be surface hardened to boost wear resistance.

Housing bearing bore wear. Standard practice is to use a loose fit with a small amount of clearance between the outer race of a bearing and the housing bore. When bearings see a constant and significant load, the outer race usually does not creep. However, wind-turbine gearboxes rotate a great deal under light loads and experience significant speed fluctuations. Under these conditions, bearing races with loose fits rotate a great deal which, over time, can generate significant amounts of debris and lead to shaft and gear misalignment.

The solution is to use interference fits between the outer race and bearing bore. If interference fits are not possible, the outer races can be pinned to prevent rotation.

Bearing race axial cracks. The inner races of roller bearings sometimes experience axial cracks that lead to failure. The cracks are not at roller spacings and not due to standstill marks or false brinelling. Once cracks develop, macropitting usually develops at the cracks on the raceway surface.

This is a new failure mode that has become quite common in the last few years in gearboxes on turbines greater than one megawatt in capacity. Failures are most common on the high-speed shaft bearings, but they have also been observed on intermediate pinion shaft bearings. The root cause and prevention of these failures is still under investigation.

Gear issues

Planet gear snap-ring wear. Snap rings are sometimes used to position the outer race of the planet bearing inside the planet gear. The bearing outer race often rotates relative to the bore and rubs against the snap ring, which causes the snap ring to rotate as well. Over time, this rotation can wear down the snap ring. For instance, the nearby photo shows a severely worn snap ring from the bore of a planet gear. The small fin on the top right edge is all that remains of the original snap ring body. Should the snap ring fail, it would most likely cause a catastrophic gearbox failure.

Snap rings should not be used as locating shoulders in planet gears. Most of the planet gears we have seen disassembled show evidence of outer-race rotation in the planet-gear bore. The preferred way to ensure long-life planet bearings is to make the bearing outer race integral with the planet gear.

Tooth bending failure from inclusions. Inclusions are minute impurities in steel, such as alumina or sulfides, which are introduced in the steel-making process. Steel used to manufacture wind-turbine gears is generally high quality and clean. However, an inclusion in an area subject to high stress can initiate failure. These types of failures can happen early in the life of the gearbox, especially in the high-speed gearing section.

To avoid inclusion-induced failures, gearbox manufacturers must have purchase specifications that require high-cleanliness steel. Some wind-turbine gearbox manufacturers demand steel cleanliness levels above those required in current gearbox standards.

Gear micropitting. Gear micropitting is basically the same failure mode as bearing micropitting discussed above. Micropitting can turn up anywhere on the active profile of a gear tooth; in wind-turbine gearboxes, it’s usually on the intermediate pinion and sun pinion. Micropitting may arise early in the life of a gearbox and progress no further, but it can lead to significant tooth wear and eventually to severe macropitting.

Steps to prevent micropitting include improving surface finishes, using a higher-viscosity oil, and reducing local contact stress with tooth modifications such as crowning and tip relief.

Gear surface-temper failure. If tooth-grinding operations raise the surface temperature of the teeth above the tempering temperature of the original heat-treating process, the material softens. This can lead to severe macropitting in operation. The steel is not hard enough to resist the contact stresses and a surface fatigue failure results.

Avoiding these types of failures requires that the gearbox manufacturer perform nondestructive tests that can detect grind temper after tooth-grinding operations. A Nital Etch inspection is one method many gear manufacturers use to detect grind temper.

Spline wear. Relative motion between the internal spline and mating sun-pinion shaft can produce abrasive wear. Over time, the spline may experience significant wear, though it generally will not directly result in failure and a loss of operation. But it generates a great deal of metal debris that can migrate to bearings and gears and lead to premature wear and failure.

Reduce this type of wear by surface hardening both the external and internal spline teeth, and pressure lubricating the joint to flush out any wear particles that are generated during operation.

Through-hardened ring gear wear. Most wind-turbine gearbox manufacturers now surface harden ring gears to prevent surface fatigue pitting on the ring gear. But many older turbines have through-hardened ring gears that are prone to this failure mode.

Nitriding the ring-gear teeth works well as long as no large debris gets trapped in the mesh. However, the hardened layer on a nitrided gear is thin and can spall off and destroy the gear if large debris enters the mesh. In our opinion, carburized or induction-hardened ring gears are much more debris tolerant than nitrided ring gears.

Regular inspection, maintenance, and analysis are critical to prevent small problems from becoming costly breakdowns. A combination of visual checks through inspection covers, video probe inspections, sump sweeps with a magnet, and oil and filter analysis performed up-tower can usually detect many of the wear and failure modes discussed here. However, wear of the planet-gear snap ring, planet-bearing shaft, housing bearing bore, and splines, as well as planet bearing inner-race micropitting, are usually only discovered when technicians completely disassemble a gearbox.

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