German specialist foundry, mining machinery and gearbox supplier Eickhoff has been making gearboxes for wind turbines since 1990. The company's current product portfolio comprises conventional high-speed, three-stage wind-turbine gearboxes in the 2.0-3.4MW volume product class, but larger 3.5-3.8MW class gearboxes form a rapidly growing second segment.
Globally, Eickhoff employs around 1,800 employees, including 300 staff spread over eight international subsidiaries, such as wind turbine gearbox repair facilities in the US. Besides repair and reconditioning, Eickhoff conducts upgrades of competitor gearboxes, which Thomas Nahrath, product manager for wind turbine gearboxes, says is enabled by a combination of strong in-house engineering know-how and a modern materials analysis laboratory. The laboratory is also used for analysing all raw materials inflow.
The raw materials for gears and shafts are brought in from external suppliers, shaped as solid steel bars, large rings or otherwise. These materials are then processed into the various components. The actual gear cutting is an almost fully automated high-precision activity. Like all specialist gear suppliers, Eickhoff applies a blend of semi-standard gear profiles and know-how focused on ensuring optimised load transfer and lubrication performance, quieter running and enhanced lifetime. Nahrath says: "Our gearboxes are the quietest in the market, which is enabled by a decade of in-house experience, comprehensive gears and gearbox R&D, an integrated design approach, and continuous product manufacturing quality main focus. A good example is our gear-tooth surface treatment called vibratory finishing. This final gear manufacturing process step offers smoother running, and improves lubrication performance while offering elongated operating lifetime."
Inside a prototype assembly and testing hall Nahrath shows off helical gear with very smooth gear-tooth surfaces in several different colours. He explains that this colour pattern is part of prototype test runs aimed at gaining better insight into gear contact-area meshing and surface contact patterns.
Another helical gear has multiple strain gauges fitted equally spaced all along the base of a gear tooth, this time aimed at determining loads and load distribution during rotation when meshing teeth continuously engage and disengage.
In a separate prototype set-up, strain gauges are attached to a big 3MW-plus cast planet wheel carrier for measuring stiffness characteristics under load. These measurements are evaluated in relation to results from product design simulation.
For its gearbox testing, Eickhoff currently has one 5MW test bench at its headquarters in Bochum, where operating temperatures down to minus 40 degrees Celsius can be simulated, plus a 4MW and a 3MW unit for serial testing of up to 2.5MW-size gearboxes. A fourth 4MW test bench is located in Klipphausen, and all units enable highly accelerated life testing (HALT), whereby a full 20-year lifetime is compressed into just a few months.
Volume-class gearboxes with a power rating of up to 2MW typically comprise one planetary low-speed stage and two helical or parallel stages. From 2MW upward, but depending on rotor diameter, a measure for maximum torque loads, Eickhoff switches to gearboxes with two planetary stages and one high-speed parallel gear stage. In this drive technology strategy, the company is like most of its competitors. Planetary stages are known as compact gear systems for transmitting high torque loads, but also characterised by a more complex manufacturing process.
Eickhoff's 2011 global market share for wind-turbine gearboxes was 4.1%, and its two main megawatt-class gearbox clients are German turbine makers Nordex and Repower. Other gearbox clients include US turbine giant GE, Germany's Fuhrländer and project developer and wind turbine manufacturer ENO. Its current global installed base is around 10GW.
Larger 3MW-plus class high-speed gearboxes have already been supplied for several years as series products to Repower for their 3.XM-series, now comprising 3.0MW, 3.2MW, and 3.4MW sister model versions. Eickhoff has also been selected to supply gearboxes for the new Nordex Delta series and 3MW-plus gearboxes are being developed for ENO.
Regarding the next envisaged product development, Nahrath says Eickhoff is able to develop and manufacture 6-7MW class gearboxes. He adds that semi-integrated medium-speed geared drive trains are also being investigated, an ambitious development the company would prefer to tackle in close cooperation with a dedicated generator specialist.
"Gearbox design is always a close cooperation between turbine OEM and the gearbox supplier. OEMs usually deliver the main product specifications and a conceptual design, which our engineer team further develops into a final product design," Nahrath says. "It also happens that gearbox development advancement points to a need for main chassis design changes. This underlines the great value attached to regular exchange of ideas and continuous evaluation of progress for achieving technically and economically optimised end products. Increasingly, turbine and gearbox development starts to match industrial automotive design practice."
Eickhoff is also one of the largest highly specialised foundries in Germany today. The foundry currently has an annual production capacity of 13,000 tonnes, with plans to boost this to 20,000 tonnes in the next few years. The wide range of cast-iron and cast-steel products churned out are both for internal use in Eickhoff turbine-gearbox components and for use by third parties. About 10% of annual output, in terms of mass, is grey cast iron, 15% cast steel, and the remaining 75% is capacity spherical cast iron also known as nodular or ductile cast iron. The biggest castings produced weigh up to 18 tonnes. The foundry comprises four furnaces with a five-tonne capacity each and one much smaller unit of 1.5 tonnes.
Apart from their distinct smell, foundries are characterised by hot moulds with yellow flames climbing out, the grey-coloured rough finished castings, and piles of used casting sand. Eickhoff's foundry operates in a three-shift, seven-day schedule during busy periods. Nahrath says having its own foundry makes Eickhoff uniquely placed among leading wind-turbine gearbox suppliers. "In-house castings production not only adds to our supply-chain independence, but also offers us a key opportunity to cooperate and exchange vital design ideas and other information already at very early stages in design," he says. "That, in turn, reduces time to market, while simultaneously enhancing product optimisation and industrialisation with the highest possible overall efficiency."
Grey cast iron has been used since the late 1700s for structural applications, and is characterised by a graphitic microstructure, which causes fractures of the material to have a grey appearance. It is the most commonly used cast iron, but has less tensile strength and shock resistance than steel. It does, however, score well for compressive strength properties. This material is widely used for wind turbine gearbox housings, hatches and covers.
Spherical cast iron is a more recent development and a main characteristic is that tiny quantities of magnesium added to these alloys allows the carbon to separate as spheroidal particles as the material solidifies. This different carbon shape and distribution provides superior mechanical properties compared to grey cast iron. This makes it well suited for many different heavy-duty applications in industry, such as gearbox planet carriers in wind turbines as well as torque arms and complex highly stressed hydraulics components.
A third category of heavy-duty product is made in cast steel, by far the most difficult of the three to process because it has less favourable fluidity and castability characteristics compared to cast iron.