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Bearing up.

Who: Engineers at Swedish machine tool firm KMT Lidkoping have developed a massive grind-turn machine to produce the large bearings for wind turbines.

Technology: Lidkoping's engineers developed its VTG4000 (vertical turning grinding) machine for the larger bearings in wind turbines. The machine uses a combination of Renishaw's SiGNUM optical linear and angle encoders to achieve accuracy, and robust Renishaw magnetic encoders on the exposed axes of the cutting heads.


Eive Johansson, Lidkoping's VTG chief designer, says: "At the heart of a Lidkoping machine are the linear slides. The combination of hydrostatic guideways, air seals and linear motors creates a stiff, accurate and maintenance-free system. To achieve dynamic stiffness we need high gain and the gain is linked to the quality of the encoder scales. It also makes a big difference that the angle encoders have the scale integrated directly on to the ring."

Application: There are three types of bearings in wind turbines. The pitch bearings at the base of the blades are the largest, up to [??]4000mm in many wind turbines, and probably the most important as they allow the blade pitch to be adjusted. During operation the pitch must be matched to the wind speed or the stress on the blades could cause them to collapse, so the reliability of high-quality bearings is paramount. If the wind speed becomes too high, usually over 25km/h, then it is necessary to create a stall condition to protect the blades. Also important are the main shaft bearings and the yaw bearings, which allow the turbine to swivel.

The largest grind-turn machine used to accommodate parts up to 600mm diameter, but the VTG4000 can handle diameters in excess of 4000mm, the size of the largest wind turbine bearings. This turning and grinding can be very demanding, and the positioning accuracy is important, with a direct effect on the quality of the finished bearings. A standard-size machine, using ballscrews on the axes, will manage to maintain a 3[micro]m form deviation, yet despite the considerable difference in relative size, the VTG4000 has been proven to achieve a deviation of less than 1[micro]m, with feed resolution in 0.1[micro]m steps.

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Title Annotation:Grind-turn machine
Publication:Professional Engineering Magazine
Date:Jul 7, 2010
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