Progress from co-operation -- Big bearings

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An unusual rotor shaft leading to the biggest ever toroidal roller bearing, plus new bearing assembly techniques and a new method of axial fixation for large bearings, was the solution adopted by bearing manufacturer SKF for the Repower 5 MW wind turbine. SKF was asked to design, manufacture and install the bearing system for the 1.5 metre diameter rotor shaft that supports the turbine's 130 tonne, 125 metre rotor. The resulting roller bearing, with its 1.5 metre inside diameter, weighs 2,700 kilogram and is the largest of its type ever manufactured by SKF.

The new bearing system uses a hollow rotor shaft for keeping down weight and cost. To eliminate the influence of alignment and deflection related errors, SKF worked with Repower to develop a system with the shaft supported by two bearings. This arrangement offers high load carrying capacity and the lowest possible bearing-plus-housing weight for a two bearing arrangement. To help it meet all performance requirements, SKF made use of an in-house dynamic simulation tool dubbed "Beast," a virtual test rig in a computer.

Lubrication is a challenge. The behaviour of grease in 500 millimetre diameter bearings is well documented, but not for a bearing with a diameter of 1500 millimetre. SKF set out to discover if grease distribution in a 1.5 metre bearing would cover all relevant surfaces, or whether gravity would take it to the bottom of the housing. A housing was designed for oil as well as grease lubrication, but inspection showed the grease lubrication works well and the housing cover design was simplified.

Another challenge for SKF was to devise ways to overcome difficulties during assembly and installation of the two huge rotor shaft bearings. Bearings are shrunk onto shafts after heating, to ensure a tight fit, but there were no commercially available induction heaters that could uniformly heat the huge CARB bearing and the 3,320 kilogram spherical roller. This meant that when the bearings and their two 8000 kilogram housings came to be heated for correct assembly and installation, SKF had to design and build a new 1000 kVA induction heater.

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