Roller bearings for longer turbine life

Over the past decade, Thyssenkrupp Rothe Erde has developed between 40 and 60 prototype designs of tapered roller rotors or bearings, explains Jan-Peter Bochert.

It is part of the Bearing Engineering group, which is responsible for advanced simulations and calculations on the latest large-scale designs.

All Rothe Erde bearings are tailor-made solutions, although most feature a symmetrical arrangement with a 45 degree roller contact angle.

The largest units currently deployed – exceeding 3.5 meters in outer diameter – are incorporated into a series of 6-8 MW direct-drive offshore turbines.

“At the end of 2016, the company re-equipped the Lippstadt plant [in the north-west German state of North Rhine-Westphalia,] with new production machines for the next generation of offshore wind turbines over 10 MW that require corresponding single rotor bearings in the 6.5 meter range, ”said Bochert.

“However, we refocused our sizing strategy last year, and our goal now is to increase load capacity and life while reducing bearing dimensions at the same time.”

Asymmetric movement

This new approach was made possible by moving from a symmetrical baseline design to an asymmetric layout with an upstream roller contact angle (facing the rotor) reduced in parallel with the increase in roller length.

Bochert reports that this scaling and optimization process was done in two main steps, concluding with reducing the windward roll angle from a base value of 45 degrees to 35 degrees, while still increasing the length of the roll by 20%. The leeward track remained unchanged.

“The most remarkable result was a combined 50% increase in the life performance of the downwind and downwind raceways, with only a 15% increase in bearing mass,” says Bochert.

“We were also able to keep the diameter of the bolt circle of the original bearing, while only slightly increasing the height. This means that this asymmetrical arrangement with dimensions almost unchanged from the bearings fitted in the direct drive turbines of 8 MW can now accommodate the last 10 MW designs. “

A key factor contributing to the improvement in bearing life has been a sharp reduction in the maximum roller-track contact pressure in both tracks.

And because bearing calculations always take deformation into account, an in-depth knowledge of loads and load distribution models is crucial.

“This requires an optimal match of all interfaces between the bearing and rotor support structure, gearbox and / or generator, and all interface links combined between the bearings and the hub and blade blades. “, explains Bochert.

“All of these interacting aspects concerning stiffness and deformation behavior must be addressed and, if necessary, adapted during the various stages of turbine design.”

Bochert cites another example. The company supplied the rotor bearing for a 3 MW medium speed gear turbine with a 100 meter rotor.

Further development of this turbine concept resulted in a 145 meter rotor, thus doubling more the swept area, and a slightly improved power of 3.2 MW.

The original bearing (bolt circle) of about 2.5 meters could be retained by adapting the design for optimized load distribution and improved load capacity.

“An integral approach and optimal system integration are essential to achieve robust and durable bearing solutions,” says Bochert.

“It requires close cooperation and full transparency between us as a bearing supplier and OEMs to go through all the iterative design steps required during turbine and bearing seal development together. “

Complex conditions

A thorough understanding of bearing-roller behavior under complex dynamic operating conditions is a key factor in the development of increasingly larger bearings, emphasizes Bernd Lüneburg, head of research and development at the company.

To meet these challenges, the company has developed its ingenious in-house diagnostic roller measurement technology.

This is based on a single “smart” measuring roller per bearing, a principle suitable for any future new bearing under development.

“A double-row tapered roller bearing for a large-scale turbine contains rollers that each have a diameter of 60 to 100 mm,” Lüneburg explains.

“One of the key aspects of the concept of measuring roll technology is that it always uses rolls made in-house from the same batch.

Each individual measuring roller incorporates a sensor for real-time measurement of the deformation at two axial positions, as well as slip, tilt, temperature, speed and relative circumferential position.

Each roll additionally contains a tiny permanent magnet generator with internal power and data storage capability, supplemented by wireless signal transmission, Lüneburg adds.

The system is designed for long term operation and measures all of the above mentioned characteristics per row of rollers without losing any load bearing capacity.

The technology combines Rothe Erde’s expertise in bearing and measurement and is validated on an in-house test bench.

These tests include evaluating roller tilt and unsymmetrical roller pressure distributions, as well as measuring roller forces against calculated values ​​around the circumference of the raceway.

Hydrostatic support

“This is an exciting and very important project to enable precise measurements at a fundamental level inside bearings.

“The technology is still being optimized, but it is already integrated into our bearing development and prototyping processes,” says Lüneburg.

“This will allow a better understanding of the system, including on critical interfaces and bearing load distribution, speed up validation work, make more accurate life predictions and ultimately develop more compact bearings.” and less expensive. “

Rothe Erde’s latest semi-integrated hybrid bearing technology has reached an advanced design stage.

The principle combines a “classic” oil-lubricated dynamic bearing bolted to a hydrostatic thrust bearing at the front, facing the rotor.

The inner ring of the hydrostatic part of modular design contains several highly flexible and easy to exchange hydrostatic elements (stamps) incorporated throughout the circumference at an inclination of 45 degrees, for example.

Together, these apply constant oil pressure against the outer rings. The film of oil between the punches and the outer ring is retained by hydrostatic pressure.

“The main motivation behind this innovation is that future large-scale transmission designs beyond 10 to 12 MW require more flexible design approaches to avoid a large increase in head mass,” Lüneburg explains.

“The combination of higher power rating and larger rotors also inevitably results in increased loads and strain.

Tipping moments

Reducing the size of components, as well as optimizing stiffness, have therefore become key goals for the wind industry, he adds.

The company has already performed a “proof of concept” test bench comparison between a scale hybrid prototype and a conventional moment bearing, with promising results.

These included doubling the life of conventional bearings while at the same time allowing a 20% reduction in bearing diameter with unchanged bearing loads.

These favorable results are obtained thanks to very reduced rotor-induced tilt moments in the roller-track interfaces, which allows to reduce the pressure of the rollers, the forces of the rollers and the internal deflections of the bearings.

“The integration of the Rothe Erde hybrid main bearing into existing or new turbine designs provides improved total driveline stiffness with only minor design changes to the turbine, and it provides a fail-safe solution,” Lüneburg says.

“If oil pressure is lost, the turbine can continue to run at full power – only temporarily consuming more bearing life – until the problem is resolved.

And, because the oil pressure of the hydrostatic bearings can be regulated, the solution offers a unique additional control option – tailoring the stiffness or damping behavior to the turbine systems. “

The next steps envisioned are a full-size prototype of a hybrid main bearing, with a test bench verification starting before the end of the year.

It will be followed by a field test with a customer prototype, for which no start date has yet been set.

In the open – Outdoor test stand for large bearings ready for turbines over 12 MW

Thyssenkrupp Rothe Erde has commissioned its modular open-air construction test bench for pitch and rotor bearings up to six meters in diameter, suitable for turbines over 12 MW.

It was built on an industrial site about 10 km from Lippstadt and consists of a huge “three-legged” flat concrete structure with an elevated central mounting area.

The key principle is to fit original turbine hubs with a matching original pulse bearing, pitch bearing and original blade. The current test arrangement is a shortened 75 meter offshore blade extending over the long leg, but with enough room remaining to mount much longer blades in the future.

Steel tubes with comparable key characteristics are mounted on the other blade support positions.

The test stand is equipped with three full-scale hydraulic cylinders, each capable of pivoting and applying pre-programmed tensile forces to the blade and steel pipes to perform tests in real conditions.

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