Blade pitch adjustment is one of the most critical functions for wind turbines, and the giant bearings in the adjustment mechanism must perform with the accuracy usually associated with small parts. Now, that degree of precision can be assured in the bearing manufacturing process with a vertical turn/grinding machine built by KMT Lidköping. It combines advanced motion systems with Renishaw’s SiGNUM optical and magnetic linear and angle encoders on critical axes to achieve form deviation of less than 1 µm on parts 4000 mm in diameter.

“This is hard turning and grinding, and is very demanding,” said Eive Johansson, KMT Lidköping’s chief designer. “Positioning accuracy is very important, with a direct effect on the quality of the finished bearings.” Prior to the VTG4000, the company’s largest machine could accommodate parts up to 600 mm (24 in) diameter, but the VTG4000 handles diameters in excess of 4000 mm (157 in), the size of the largest wind turbine bearings, while providing much greater accuracy.

Linear motors on the linear axes make the VTG4000 fast, and the rigidity of the hydrostatic guideways increases the accuracy. “A standard-sized machine, using ballscrews on the axes, maintains about a 3-µm form deviation,” said Johannson. “This machine maintains form deviation of less than 1 µm with feed resolution in 0.1 µm steps.”

Linear slides are at the core of Lidköping machines, and the combination of hydrostatic guideways, air seals, and linear motors creates a stiff and accurate, maintenance-free system. “To achieve dynamic stiffness we need high gain, which is linked to the quality of the encoder scales,” explained Johansson. “It also makes a big difference that the angle encoders have the scale integrated directly onto the ring.”