How long can a gearbox last? How long should it last? Because there are so many factors that can rob a gearbox of its true lifespan, these questions cannot be simply answered. To truly understand the full range of premature-failure issues – and do everything possible to minimize root causes – consider that each of the following can contribute to the premature failure:
- inadequate lubrication;
- contaminated lubrication;
- high speed bearing failures;
- improper assembly; and
- material defects.
Each of these points is significant enough to warrant its own focus, and industry resources are hard at work trying to understand the overall impact of these factors. A proactive approach and a well-followed maintenance protocol can keep the oil and lubrication system in top shape. But rectifying the other root causes can only be done through retrofits, up-tower repair or down-tower re-manufacture.
Up-tower repair is a hot topic in the wind industry, and it is not surprising since the cost of repair can be half or less than a full down-tower remanufacture. However, an up-tower repair is not a replacement for a complete down-tower remanufacture. Only select components can be replaced up-tower. These are generally limited to high-speed and intermediate shaft gears and bearings and, in very few cases, the sun pinion shaft and high-speed gear. This varies by make and model, and depends on how modular the housing design is. Cranes are generally still needed, albeit smaller in size, to move the generator so that technicians can perform the work.
The planetary section – the components that generally rate the gearbox – will ultimately reach its designed life, and it simply is not serviceable up-tower due to weight and design. This same failure would eventually occur even after an up-tower repair, but it may take 10 years depending on how strong of an influencer those specific loading conditions and application factors are for that specific gearbox.
In addition, when technicians perform up-tower repair, the internal sections of the gearbox are exposed to the elements. Care must be taken to ensure the gearbox is properly sealed and that the lubrication system is flushed of all contamination. This includes properly "running in" newly installed components because they will generate a slight amount of wear debris during the first few hours of operation.
To accomplish this, the turbine operators should consider using an in-line particle counter while ramping the turbine slowly. There is an extra inherent risk of shortening the remaining useful life of the planetary section with the up-tower repair even if extra care is shown. It may be a good idea to perform the up-tower repair in year four so that the gearbox can run additional years on the planetary section.
However, if the planetary condition is already deteriorating, it may make more sense to immediately schedule a full down-tower remanufacture. In down-tower remanufacture, this "run in" function is typically performed on a test stand in a controlled manner.
When the down tower remanufacture is performed, it is important that the technician has practical experience understanding gearbox component failures to ensure all possible improvements can be made. There is a limitation to how much can be done given the available envelope, but there are many features that can be implemented to address the controllable premature-failure issues. These could be as simple as the use of a coated bearing in certain positions more complicated bearing configuration changes and material selection.
It is the result of these factors that we believe the life-span of a given population of gearboxes at a site will likely be distributed in a bell curve. In other words, a wind farm with 100 turbines may see a handful of gearbox failures the first few years of operation, with the failure rate increasing over time. We acknowledge that this is an anecdotal statement for the wind industry since most of the installed base is young.
However, the bell curve distribution is consistent with other industries and applications and, therefore, is a fair assumption. Based on the gearboxes that have come through our facility, the majority of lifespan distribution for wind turbine gearboxes will fall between seven and 10 years.
Whether the bell curve should be shifted further to right or left, the aftermarket infrastructure for wind is in its infancy and will expand. The market for down tower gearbox remanufacture and up-tower repair is growing. Given the age of the installed base, and the 2630,000+ gearboxes currently operating in the U.S., the need for service companies will expand exponentially over the next few years, resulting in new jobs and economic growth.
As for the owners and operators, this should not be considered bad news. Many industries use gearboxes and have fine-tuned their maintenance practices to maximize their assets' life. Only by considering all the factors can the wind industry truly answer the question "how long will a gearbox last?"
Brian Hastings is chief financial officer and founding partner at Mukwonago, Wis.-based Gearbox Express. He can be reached at (262) 378 – 4303.
Caption: An example of fatigue failure on a planetary pinion that was used in a compound planetary gearbox.
Photo courtesy of Gearbox Express