Grinding large bearings to micron precision

Case study: This application describes the vital role of encoders in the finishing process of giant bearings.

10/28/2013


Eive Johansson, KMT Lidköping VTG chief designer, is shown with the VTG4000 vertical turning grinding machine. Courtesy: Renishaw plc.Encoders play a vital role in the finishing process of giant bearings. This application illustrates one of numerous industries where encoders are at work, although it’s not an example of the extreme length position measurement possible—as discussed in the article “Long-length linear encoders look lively.” Actual measurement lengths required are likewise application dependent. 

It takes an extraordinary machine-tool system to produce large as well as precise parts in an efficient manner. A case in point is KMT Lidköping’s new VTG4000 (vertical turning grinding) machine that enables the manufacture of massive bearings for utility-scale wind turbines and other large components. Three kinds of encoders from Renishaw plc serve as a critical complement to the machine’s accurate and rapid operation. 

Lidköping company, based in the town of the same name in central Sweden, has a long history as a grinding specialist. It has produced grinding machines since the 1920s, supplying major bearing manufacturers worldwide. More recently, the company became part of the KMT group.

VTG4000 accommodates parts exceeding 4,000 mm (13.1 ft) diameter—that includes the size of the largest wind turbine bearings. Eive Johansson, Lidköping’s VTG chief designer, referred to the manufacture of these components in terms of “hard turning and grinding, which can be very demanding.” In addition, the machine’s positioning accuracy relates directly to the finished bearings’ quality.

“VTG4000 has been proven to achieve an exceptional form deviation of less than 1 µm (0.00004 in.), with feed resolution in 0.1 µm steps.” Johansson said. This compares to about a 3 µm form deviation obtainable with a standard machine that’s larger and uses ballscrews on the axes, he explained.

Three types of bearings are involved in a utility-scale wind turbine. Pitch bearings at the base of the blades are the largest (up to 4,000 mm diameter) and probably the most important since they permit blade pitch adjustment to accommodate wind speed variability. Also important are the main shaft bearings for the shaft that connects the turbine rotor to the gearbox and generator in the nacelle and yaw bearings that allow the nacelle to swivel into the wind. (See Ref. 4 for an overview of wind turbine operation, including blade pitch, rotor shaft, and yaw axis functions.)

VTG4000 is built around a core of linear slides which combined with other structures and linear motors leads to a stiff machine system. Dynamic stiffness is a further requirement for high-accuracy grinding machines.

“To achieve dynamic stiffness we need high gain, and the gain is linked to the quality of the encoder scales,” Johansson continued.

Only one setup needed

VTG4000 features two machining heads able to perform both turning and grinding, as needed. Moreover, the design is such that the workpiece doesn’t need to be removed between machining operations. The two heads are positioned with the linear slides along the machine’s 4.5-m long x-axis, and when they’re placed at opposite sides of the workpiece, grinding and/or turning tools can access the outside of the part. This is where the linear encoders’ precise position measurements come into play.

Lidköping selected Renishaw linear scales based on testing and performance comparison of different scales on its in-house reference slide. Other important considerations that led to the selection were availability of the linear encoders with a continuous length of at least 4.5 m and superior resistance to dirt. The result was that Renishaw Signum optical encoders are fitted to all four linear slides of VTG4000. These noncontact encoders carry a total accuracy spec of better than ±4 µm over a 5-m length.

As discussed in the main article and Ref. 1, proper setup is vital to a successful encoder installation. Renishaw encoders come with an integral setup LED to speed installation and eliminate the need for complex external/separate setup equipment or oscilloscopes, according to the company.

Lidköping’s Johansson specifically noted the easy setup of the encoders.

“With the scale attached [to the machine structure] and the read head approximately located, the indicator lights make it very easy to see how well the two elements are aligned and then make the final adjustments,” he added.

For the rotary table portion of VTG4000, the same type of analysis was done, which led to the selection of Renishaw Signum optical rotary (angle) encoders. These units feature a one-piece stainless steel ring with the 20 µm measurement scale marked directly on the periphery.

The third kind of encoder used in the grinding machine is Renishaw’s LM10 magnetic encoder—fitted to the B-axes of the grinding heads. Inherent immunity to harsh machining conditions is one of the features of noncontact LM10 magnetic encoders (see main article), including IP68 sealing of the read head. Overall, LM10 allows up to 100 m travel and high-speed operation of up to 25 m/s (82 ft/s) or 4 m/s at 1 µm resolution.

As this application illustrates, linear encoders often work together with rotary models.

- Frank J. Bartos, PE, is a Control Engineering contributing content specialist. Reach him at braunbart(at)sbcglobal.net 

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Link to the following related articles below.

Ref. 1 - LLL encoder addendum (See link at bottom.)

Ref. 4 – Winds of change for power and control

Long-length linear encoders look lively (See link at bottom.)



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