Heading off gearbox failure

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.