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Hansen - Restoring confidence in the future of gearboxes

BELGIUM: Hansen Transmissions of Belgium is one of the world's leading makers of wind turbine gearboxes. Eize de Vries visited the company's Lommel gearbox manufacturing plant to discuss the future of geared wind turbine drive systems with chief technical officer Stefan Lammens.

Hansen's Lommel factory. Gear box assembly takes place in increasingly clean production halls
Hansen's Lommel factory. Gear box assembly takes place in increasingly clean production halls

Founded in 1923, Hansen Transmissions International of Belgium is one of the world's leading manufacturers of wind turbine gearboxes. It had a 22% market share in 2009.

As early as 1979, Hansen made its first wind turbine gearbox. Following rapid expansion in both the size and number of products it manufactures, Hansen now has production facilities in Coimbatore, India, and Tianjin, China, for the Chinese and other Asian markets.

The Belgium-based Lommel plant, inaugurated in 2004, was expanded in 2007 to acquire its current distinctive U-shape, making the site a model factory for the other two. The modern, fully integrated facilities operate in a three-shift system, five days a week.

A second Belgian plant for industrial gearboxes is located in Edegem.

Hansen employs approximately 2,200 staff worldwide, of which 750 are based in Lommel. The company's headquarters and most of the research and development (R&D) staff are located in Antwerp, and a second R&D team operates from a site in Ghent.

Top rankings

Hansen has produced more than 20,000 gearboxes during the past 31 years. Its current annual output capacity is approximately 3,000 units.

The company ranks second behind Flender-Winergy of Germany in terms of annual output in megawatts. However, Hansen ranks number one in the world in series production of large wind turbine gearboxes up to 3MW. This includes exclusive deliveries of the 3MW Vestas V90-3.0MW gearbox, a joint product development with the Danish turbine maker. About 1,560 units of this turbine model are now operational.

Hansen's cumulative annual wind turbine gearbox manufacturing capacity amounted to 8,500MW by the end of March 2010. In anticipation of renewed strong market conditions, it plans to grow capacity to 14.3GW by 2013. However, like several wind turbine suppliers, Hansen suffered from weak market demand in the first half of this year (Windpower Monthly, September 2010).

The Lommel production plant comprises several physically separated areas for specific activities such as raw materials and finished components storage, as well as logistics, component machining, heat treatment, component finishing, assembly and testing. The facilities also incorporate an advanced component-measuring department and one of the world's largest dynamic gearbox test benches, rated at 13.2MW with peak power capability up to 16.8MW (see box, page 68).

Precision manufacture

Gearboxes are large and power-dense high-tech components, requiring high-precision manufacturing and an equally high level of care during assembly and pre-testing. Gear manufacturing, for instance, involves tolerances at micron level (1/1000th of a millimetre) - a tiny fraction of the thickness of a human hair.

Like its main competitors, Hansen has developed its own specific in-house gear technology. This carefully protected and continuously refined science-supported technology base also incorporates invaluable know-how on gear teeth shape and gear-meshing refinement accumulated during decades of operational experience. Improvements are concentrated especially in key areas such as longer service life, integration, compact design and noise reduction.

Core components such as gears, planet carriers and housings are brought in as raw materials and fully processed in-house. Covers are bought ready-made from third parties. Gears and shafts for Hansen gearboxes are manufactured in a variety of sizes and shapes, for either individual or combined functions.

They can, for instance, be made as individual shafts and gears, or as a shaft with an integrated gear wheel. A major trend in planet gear systems, of which Hansen was an early pioneer, is to integrate the bearing outer ring directly into the gear's inner body, instead of applying a separate bearing outer ring. This solution enables the application of larger and stronger bearings in a by-definition confined structural space.

Manufacturing area

The area where machining operations, such as metal turning and primary gear cutting, are performed is what is known - despite being surprisingly clean - as a "dirty area". This reflects the fact that these processes produce considerable quantities of metal particles, dust and other manufacturing-related residues. These impurities should never enter precision components like bearings, settle on polished gear surfaces or contaminate gearbox housings before or during assembly.

For this reason, each subsequent gearbox production step is conducted in a separate "cleaner" manufacturing or assembly area, with each new process cycle characterised by gradually stepped-up environmental cleanliness requirements. Because of tight manufacturing tolerances, there is a need for constant temperature in the clean areas, with fluctuation only allowed within a narrow, strictly controlled bandwidth.

The machines for semi-automated component manufacture sit on heavy foundations. This is to minimise the risk of dimensioning errors through excessive machine vibrations.

Before executing the final gear grinding, gears and shafts have to be hardened. This heat-treatment process is conducted in an area with 25 large hardening ovens. Component hardening is a process that builds upon three decades of dedicated in-house know-how, explains chief technical officer Stefan Lammens: "Many of our competitors outsource this part of their component manufacturing process, but we regard it as essential in-house technology. The bulk of our process technology is developed in-house."

The hardening process used at Hansen is known as gas carburising. First the part is heated to above 925 degrees Celsius in a gaseous environment. The carbon penetrates the steel surface to a predetermined depth. In the next step, the temperature of the components is lowered to 850 degrees Celsius and the parts are then quenched in an oil bath. This sudden temperature drop - known as the chill - gives the material the required hardness.

Finally, the parts are annealed to make the material a little less hard but more ductile. Once the hardening process is completed, the components have blackened surfaces, and in the heat treatment hall there is a distinct smell of burned oil.

For the final gear grinding and polishing stages, Hansen and its industry partners have jointly developed advanced semi-automated machines, a whole battery of which is lined up in a separate hall. The grinding operation involves a fast rotating tool with an extremely hard surface, shaped and sized to exactly match the final spacing between two adjoining gear teeth. After a few grinding cycles, the tool is re-measured, dressed again, and brought back to its exact initial shape and size before engaging into the next cycle.

After gearbox assembly, each unit is factory-tested for about ten hours - a process that involves an oil flushing cycle aimed at removing any particles and other remaining contaminants.

Industrial background

Reflecting upon Hansen's 90-year manufacturing history, Lammens explains that the company - like several of its competitors - became involved in wind turbine gearboxes from a background in industrial gearboxes. Hansen delivered its first gearbox to Vestas in 1979 for a 30kW wind turbine model. This tiny standard industrial product represents a huge contrast to the later megawatt and multi-megawatt products - and the current largest 6MW gearbox unit developed for German turbine maker Repower.

Lammens continues: "Another key difference with today is that the early days were characterised by manufacturing small batches of largely standard gearboxes. The current focus is on large series of custom-developed products but not in the quantities known in the automotive industry. At the same time, high-volume 1.5-2MW gearboxes are increasingly turning into commodity products, a segment where competition from Asia is getting stronger."

The wind industry still lags behind with product development tools, such as virtual prototyping. "These tools were introduced 10-15 years ago in the automotive industry, but are only now gaining ground in the wind industry and we are in general catching up," he says. Like wind turbine suppliers, Hansen often hires experts who have come from the automotive industry.

Unacceptable delays

Another observation is that ten years ago a twoto three-year gearbox development period was standard. "Such a long period is unacceptable today. For new products, time to market has become a key driver, together with enhanced reliability and minimised costs of energy demands," says Lammens.

"We are now capable of developing new gearboxes much faster by combining multiple strategies and processes. Design validation is already done in the factory, and multiple products are developed in parallel through product standardisation and by working with product platforms," he adds.

The bulk of Hansen gearboxes are still high-speed products featuring a combination of two planetary stages and one parallel gear stage, and a 1,500-1,800RPM gearbox output speed range.

This gearbox concept is popular for good reasons, explains Lammens: "The low-speed and medium-speed planetary stages together represent the bulk of gearbox costs. The fast-speed third stage is, by comparison, a relatively low-cost element. But these products can suffer from various issues including noise and vibrations. Making this stage slightly more robust and heavier usually provides an adequate remedy with only a minor cost increment attached."

Hansen has put a lot of effort into addressing the main causes of gearbox failure, such as reliability issues with the high-speed shaft. One of the available solutions is a modular design with the option to replace a complete, defective high-speed stage within a matter of hours.

Elaborating further on the topic, Lammens says that, as a rule of thumb, it takes five to seven years to resolve the majority of teething problems with a gearbox design.

The current direct-drive technology trend - and the concomitant focus on reliability issues for wind turbine gearboxes - are a concern. Lammens says: "Each individual gearbox failure contributes to poor reputation that reflects upon the entire industry. At the moment, many new Asian competitors are entering the market. Each new entrant and its products, by definition, have to go up a learning curve. But this rule applies to direct-drive turbine market entrants, too - nobody is excluded."

At the moment, reliability statistics of both geared and direct-drive wind systems show similar performance results, he adds, while stressing that turbine availability is the product of system reliability and service organisation performance. "Adequate service is responsible for keeping a wind turbine system functioning. By neglecting the latter aspect in the equation, wrong conclusions can be easily drawn."

Thinking big

Returning to the topic of wind turbine size, Lammens observes: "Ten years ago, some believed that 5MW was the absolute wind turbine size limit. Today, we talk about 10MW, while 6-8MW is becoming feasible in people's minds, especially for the offshore wind market. This ongoing upscaling trend is reinforced by multiple parallel innovations in key areas like rotor blades, new generators and slide bearings," Lammens says.

His final remarks are devoted to the growing market and developer interest in medium-speed geared systems. "Hansen firmly believes in the viability of these hybrid solutions, as these systems do strike an attractive balance between reliability and complexity. Our aim is to become a leading player in the offshore wind market for the upcoming 6MW and, over time, up to 8MW medium-speed wind turbines," he says.

Confident in its long history and strong credentials, Hansen Transmissions now wants to make its mark "in multiple roles as a systems integrator, integrated product developer and large series supplier of reliable, long-lasting drive systems", Lammens concludes.

DYNAMIC GEARBOX TESTING - HOW IT WORKS

Hansen's 13.2MW dynamic test bench was inaugurated in 2008 and enables the testing of different gearbox types with power ratings up to 12MW. The difference between the two figures represents cumulative mechanical and electrical system losses.

The massive test bench, which was developed in-house, comprises two "opposing" drive systems necessary for so-called back-to-back gearbox testing. At one side of the system, an externally powered electric motor acts as the wind turbine rotor and drives the gearbox to be tested.

The output shaft of this gearbox is connected to a second drive system incorporating an electric motor that now functions as a generator providing the load for the system.

The test bench can be adjusted for a wide variety of gearbox input speeds, power ratings and load characteristics, including low, medium and high-speed systems. Test engineers can also measure gearbox vibrations, acoustic noise, oil and component temperature in relation to load, and other factors.

Conducting a so-called highly accelerated life testing (HALT) cycle further enables Hansen to reduce the testing time from years to months.

This new method represents a big improvement on the past situation, where new gearbox prototypes would typically be tested for a one to two-year period in the field before being released for series production.

However, there would still be uncertainty and added lifetime expectancy risk during the remaining 18- to 19-year operational period.

With a second test bench in the same hall, Hansen is now able to load rotating gearbox shafts by means of radial and axial actuators, simulating dynamic field conditions.

The behaviour of the shaft and bearing configuration is observed under the applied dynamic loading conditions. By measuring bearing temperature and changes in sound levels, potential failures can be detected at an early design stage, leading to more robust configurations.

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