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HUGE Machines:
Optical and magnetic encoders make for high-accuracy production of giant wind turbine bearings

Blade pitch adjustment is one of the most critical functions of wind turbine operation, and the giant bearings in the adjustment mechanism can now be manufactured with accuracy typically reserved for small parts using a new vertical turn/grind machine from Swedish builder KMT Lidköping. The machine combines advanced motion systems with Renishaw optical and magnetic linear and angle encoders on critical axes to achieve form deviation of less than 1 μ on parts 4,000 mm (157 in.) in diameter.

Eive Johansson, Lidköping VTG chief designer, with the VTG4000.

 

 

"This is hard turning and grinding and is very demanding," says 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 13 ft, the size of the largest wind turbine bearings, while providing much greater accuracy.

Linear motors on the linear axes make the VTG4000 fast, and firm hydrostatic guideways increase accuracy.

"A standard-size machine, using ballscrews on the axes, maintains about a 3-μ form deviation," says Johannson. "This machine maintains form deviation of less than 1 μ with feed resolution in 0.1 μ steps."

Linear slides are at the heart of Lidköping machines, and the combination of hydrostatic guideways, air seals, and linear motors creates a stiff, accurate, maintenance-free system.

Hydrostatic radial and axial bearings, used with linear motors and Renishaw SiGNUM encoders, provide accuracy and high resolution on KMT Lidköping's new VTG4000 vertical turn/grind machine.

 

 

"To achieve dynamic stiffness we need high gain, which is linked to the quality of the encoder scales," says Johansson. "It also makes a big difference that the angle encoders have the scale integrated directly onto the ring."

High quality improves control of turbine blades
Pitch bearings allow wind turbine blades to optimize their angle according to wind speed or create a stall condition in high winds, so the reliability of these bearings is critical to efficient, safe operation.

The multi-tasking VTG4000 is designed to machine these large bearings in a single setup, performing both turning and grinding with two separate heads. The two heads are configured as required, usually grinding/turning or grinding/grinding. The machine's 4.5-m X-axis carries the two heads on opposite sides of the part, and allows both grinding and turning tools to access the outside of the part.

With some components weighing more than 35 tons, the VTG4000 is very resistant to distortion and thermal variation, which is aided by closely controlled hydrostatic oil and coolant temperatures.

"We've used hydrostatic guideways since the 1970s, and combining them with linear motors gives us a faster, more accurate machine capable of far greater acceleration and deceleration," says Plant Manager Henrik Jonsson. "When you see that you can move the 25,000-kg rotary axis with your finger, you realize how good the hydrostatic system is."

Renishaw SiGNUM linear optical encoder scale in place on a linear axis of the VTG4000.

 

 

Johansson first saw the Renishaw encoders that would end up on the VTG4000 at the EMO show, a major machine tool show held in Hannover, Germany, in 2007. Lidköping put the sensors through rigorous testing.

"We compared different scales assembled on our reference slide and selected the one with the best performance," says Johansson. "It was important that the linear encoders have a continuous length of at least 4.5 m, as well as high dirt resistance, and [Renishaw's] SiGNUM fit these requirements the best. We have fitted them to all four linear slides, and have had no problems. Our rotary table has the same design principle, with hydrostatic radial and axial bearings, air seals, and torque motors. We did the same analysis and chose the SiGNUM angle encoders."

Renishaw's LM10 magnetic angle encoders are fitted to the B-axes of the grinding heads. These encoders are exposed to the harsh conditions of the machine environment, but their non-contact, non-optical design, and sealing to IP68 protects them from the effects of coolant and swarf. The digital or analog output LM10 allows up to 100 m of travel and high-speed operation up to 25 m/s (4 m/s at 1-μ resolution).

Renishaw LM10 magnetic encoder on the B-axis of the grinding head is exposed to the machining environment. Note the green indicator lights on the readhead.

 

 

All Renishaw encoders feature an integral, patented set-up LED that speeds installation and removes the need for complex set-up equipment or oscilloscopes.

"One of the best features of the Renishaw encoders is how easy they are to set up," says Johnasson. "With the scale attached and the readhead approximately located, the indicator lights make it very easy to see how well the two elements are aligned."

To accurately set the part in the machine's work-coordinate system, the VTG4000 uses a Renishaw RMP60 radio signal transmission touch probe. After a raw part is placed on the bed, the RMP60 is loaded into the tool changer and used to locate the exact position of key features in a matter of seconds. This data is used to update the coordinate system in-cycle for quick, accurate machining.

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Renishaw

Source: Renishaw

Published March 2013

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