1 March 2017 by Eize de Vries

GERMANY: Thyssenkrupp Rothe Erde is a world-leading supplier of high-grade steel rings, and manufacturer of very large bearings and ring gears. Eize de Vries reports from the company's design, production and testing facilities in Germany.

Rothe Erde has a global footprint, with 17 factories worldwide employing about 8,000 staff, while its involvement with the wind industry dates back to the late 1980s.

It started with the delivery of what by today's standards was a tiny three-row single rotor bearing for a new 250kW HSW turbine, believed to be the first turbine fitted with a single rotor bearing. A few years later, the firm was the first to supply three-roller type pitch bearings.

"We think big in products and in their design processes," says Rothe Erde's head of research and development (R&D), Bernd Lüneburg, as we enter the company's roller mill in Dortmund, in western Germany's Ruhr Area. "We are dedicated to the large-scale single rotor bearing because we believe in it."

"Large-scale" takes on greater meaning when you reflect on the facility's output — around 18,000 rings ranging from small to very large leave the roller mill every month (see box).

Each one represents the high-grade base element for the manufacture of pitch, yaw or main bearings, or the ring gear for planetary gearboxes or tower flanges. These components are made in-house, although some rings are sold to third-party bearing and gearbox suppliers lacking such roller-mill facilities.

The company's latest and largest gear-cutting equipment can handle diameters up to ten metres, although the biggest ring gears used in wind turbine gearboxes today are just over three metres.

The biggest bearing Rothe Erde has manufactured was for the oil and gas industry; the segmented design had a record 18.3-metre outer diameter. Lüneburg points to another application: the ten-metre-plus bearings fitted around jack-up installation vessel legs for crane mounting.

The largest rotor bearings it produces for wind turbines are double-row roller-type units with an outer diameter of around four metres.

But new turning, milling, grinding and hardening equipment was being set up during my visit that is capable of handling next-generation single rotor bearings up to 6.5-metre outer diameter.

Rothe Erde is a pioneer in steel hardening, dating right back to the 1930s. "All the necessary machinery for flame-hardening bearings was in place by 1962," says Lüneburg.

"The main process characteristic is that the entire bearing cross-section is heated up to the required temperature, and, after hardening, the material properties are uniform. But in the early 1980s Rothe Erde became the first bearing manufacturer to introduce induction hardening, which offers two main benefits over flame hardening.

"First, only the roller/ball surface contact area is hardened to pre-defined depth. Second, the remaining cross-sectional areas retain the original material properties, and the combination offers superior overall product characteristics."

He explains the initial bottleneck of classic induction hardening for main bearings, a "soft spot" at the running surface. This is a location at the circumference where the hardening process involving slow element rotation starts and ends. This induction-hardening method could therefore only be applied for pitch bearings because these do not make full rotations.

Rothe Erde supplied the first main bearings without a soft spot in 2002 for a European direct-drive turbine. Later, the hardening process was optimised and a new so-called scan hardening process with three inductors was developed.

"A major wind-industry player approached us in 2011 asking if we could supply induction-hardened single rotor bearings," says Lüneburg. "We transferred the new technology to meet the demands of the requested concept. This effectively resolved the soft-spot issue, and induction hardening is now a main windindustry trend, also adopted by our main competitors."

The manufacturing halls in Lippstadt, about 70km from Dortmund, contained different slewing-bearing types and sizes, inner and outer rings and cages in various stages of completion during the visit.

Bearings range from the three-roller type to classic four-point pitch bearings and double-row roller type main bearings.

An interesting design variation within four-point blade bearings is that some incorporate a plastic-coated steel (holding-separation) cage instead of the commonly used blank-steel cages.

"This was initially introduced to minimise the wear in offshore bearings, but plastic-coated steel cages are now becoming a semi-standard fitting for main bearings," says Lüneburg.

Bearing manufacture requires a uniform level of material purity. However, a 1% failure rate for the manufacture of small bearings from a single batch of steel is common. (Bearings affected by insufficient material purity are simply thrown away.)

"The impact of insufficient material purity is far greater with very large bearings," says Lüneburg. "A similar batch of steel is now perhaps sufficient to produce only one bearing, so failure is not an option."

Rothe Erde's new R&D-centre in Lippstadt opened in 2014. One recent innovation, shown at WindEnergy Hamburg 2016, involves a surprisingly simple mechanical solution without moving parts, which minimises the quantity of grease that could leak away through the seals, and which enhances seal performance substantially.

The R&D centre houses a variety of mostly in-house-developed component testing and validation equipment.

A full-scale test facility was specially built for highly accelerated lifetime testing (HALT) of the latest and largest wind turbine single-row rotor bearings. Research is being undertaken on magnetic bearings (zero contact) used in medical equipment, and on advanced lubricants and materials testing over multiple areas.

"The R&D facility is an integral part of our quality culture, and continuous monitoring also improves and optimises Rothe Erde products," says Lüneburg

Despite the crucial role of science and advanced technology during all processes and product stages from design to manufacturing, there is still a need for human skill and craftsmanship. One prime example is the run-out check for four-point contact bearing inner and outer rings following the induction hardening process.

Each individual ring is placed on a heavy-duty, slow-rotating turntable. If a deviation in component roundness is detected, the technician in charge applies local heat at such spots by a mobile inductor, relying entirely on their skill and experience.

This is repeated at other locations until the required roundness properties have been achieved.

Single rotor bearing assembly is conducted under clean-room conditions, a process that in the final stages includes coating, seals assembly, greasing and packaging.

Before entering the assembly area, all incoming parts are thoroughly washed in a separate, sealed-off space.

Following precision assembly at specially designed structurally stiff and level tables, each bearing is flushed, first while stationary and then while being rotated slowly until the required oil cleanliness levels are achieved.

"For Rothe Erde there is no real maximum limit to size," says Lüneburg. "In fact, we scale down existing products to sizes required for wind turbines."

Rolled steel rings are characterised by their favourable grain structure orientation, making them well-suited for highly-stressed bearing and ring-gear applications.

Of the 150,000 tonnes of pre-processed steel that arrives annually at the roller mill, 90% comes by train. The long steel bars with various diameters - called ingots - are pre-cut in specific barrel-shape pieces that match their later ring materials requirement.

Self-moving vehicles transport the steel to ovens in one of the huge halls, each equipped with one or more roller mills of different sizes.

The vehicles then remove the sparking red/orange-hot (1,100-1,200ºC) pieces and transport them to a press, where in a single process they are step flattened into doughnut shape with a hole in the centre.

In the final semi-automated step, the roller-mill turns the doughnut into a still red-hot ring, which is then removed to cool down.

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