Feedback Critical to Machining Precision

Vertical Turning/Grinding Machine Uses Optical and Magnetic Encoders to Aid High-Accuracy Production of Wind Turbine Bearings

By Per Junskog

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The accuracy required during the grinding of bearing raceways is the most important parameter to obtain a minimum of friction, and thereby a long component lifecycle and reduced energy consumption. A KMT Precision Grinding customer that manufactures bearings once told us that if the friction of all the world's bearings could be reduced by 10%, then energy consumption would decrease by what a country the size of Brazil consumes during a year.

The wheels on cars and trains, turbines in power stations, the compressor in your refrigerator, all contain components that need to rotate with a minimum of friction, year after year, and often under heavy external pressure. The principle is a simple one: less friction, less energy. The bearings play a considerable role at each rotation in order to reduce friction and guarantee stable rotation. The requirements regarding the production process are respectively high, not least due to the final shape of the bearings and the finish during grinding and polish.

SEE ALSO: Wind Generators Prove Bigger Is Always Better

KMT, a builder of precision grinding machines, knows the critical importance of these machining issues well. The company is a merger of UVA International AB and KMT Lidköping, which were founded in 1928 and 1875, respectively. It has sales and aftermarket subsidiaries in the U.S., China and Germany. Production sites for new machines, rebuilds and repair work are in Sweden, and rebuilds also are done in Gurgaon, India.

Adjust to the Customer
KMT machines has been grinding components for the bearing industry during the past century, and during this time has continuously developed its technology to adapt to different customer applications. Today, wind power is a growing industry and an area KMT is focusing on.

"We follow our customers' development in new markets," says Thomas Karlsson, CEO of KMT. "When our customer was expanding in the area of wind power stations, new bearing types were needed. We were there with them, and developed new machines covering their needs."

Bearings are found in different locations in a wind power station turbine, such as on the fitting of the rotor blade, the turbine axes and the gearbox. The better that bearing performs its job, says Karlsson, the smoother is the turbine rotation and the lower the energy loss of the mechanic parts.

The rotating rotor blades are connected to the turbine axes through a bearing with a diameter of as much as 4 meters. Bearing rings of that size are ground by a Lidköping VTG4000 vertical grinding machine, through which the material gets its shape with the help of the optional combination of grinding and turning.

Critical Processing
Karlsson stresses how critical this process is, saying, "It is the grinding process that determines how accurate the recess (in which the ball will be placed) has to be. The accuracy determines the friction and, consequently, the accuracy of the power generation. But well-ground bearings with high precision offer, not only the possibility to reduce energy consumption, but also to improve their reliability."

If the rotor blades are positioned in the wrong axes under the stress of high wind forces, this might lead to severe consequences, Karlsson explains. The enormous power could loosen the blades and, in the worst case, the entire wind power station might collapse.

"Blade pitch adjustment is one of the most critical functions of wind turbine operation, and the giant bearings in the adjustment mechanism now can be manufactured with accuracy typically reserved for small parts by using our new VTG4000," says Eive Johansson, KMT Lidköping's chief designer. "The machine combines advanced motion systems with optical and magnetic linear and angle encoders on critical axes to achieve form deviation of less than 1 µm on parts 4 m in diameter. This is hard turning and grinding, and it is very demanding." Form deviation refers to the variation from the desired profile. For example, in a ball bearing raceway, it is the variation from a perfect circle sector. But it could actually be the deviation from any form, including straight, circular, oval, etc.

The reputation of Lidköping in the bearing machinery business has always been on high-speed and high-metal-removal rates. These capabilities are realized by the rigidity of Lidköping machine designs, with recognition that the stiffness inherent in each element of a machine's structure, when combined, contribute to its productivity overall.

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