August 28, 2008--Toshiba GE Turbine Components (TGTC; Yokohama, Japan) has reduced the time required to inspect and measure steam turbine blades from 280 minutes to 45 minutes by using a MAXOS noncontact measurement system from Steintek GmbH (Greding, Germany) and their distributor NVision Inc. (Southlake, TX and Wixom, MI). The coordinate-measuring machine (CMM) used in the past to inspect the blades was not only slow but was unable to access hard-to-reach areas such as fillets or dovetail hooks (which are part of the mechanism that holds turbine blades onto the central rotor). The MAXOS, which scans the object under test with a point of white light, uses five axes to reach every point on the blades and also generates specific measurements of critical areas, eliminating requirements for additional manual inspection.
In the past, it took longer to inspect blades than it took to make them. A turbine bucket is comprised of an airfoil and a root. The long blades built at Yokohama have mid-span geometry that provides support for the midsection of the airfoil.
The complex geometry of these blades means that many cross-sections must be examined. The conventional touch-probe 3-D CMM used in the past to inspect the blades was unable to reach many points on the root. In addition, many fillet radii were inaccessible to the CMM or too small to be measured by a touch-probe ball.
The MAXOS collects individual points at a rate of 100 per second, many times faster than a CMM. Unlike laser or white light fringe scanners, the MAXOS can inspect turbine blades without having to apply a matte coating that introduces dimensional inaccuracy. The MAXOS provides accuracy of +/- 0.0004 in. and a resolution between measured points down to 0.0002 in. on this project. The system can be provided with a resolution as low as 0.0001 in. The MAXOS software is configured with an overall best fit of the measured geometry to allow a part with some error to fit within the overall tolerance envelope of the reference data. Win3DS Blade inspection software is configured to give fast results and different kind of evaluations on mid-span, fillets, gaps, and airfoils. Different best-fits are available, including Gauss and Chebyshev.
In this application, the MAXOS was configured with a five-axis horizontal arm with three linear axes and two rotary axes. One rotary axis is for the sensor, and the other is for positioning the blade. The rotary axes are servo-controlled and are not indexed. This configuration makes it possible to measure every part of the blade even when it is mounted vertically. Vertical mounting is preferred for the long blades because it prevents them from bending under their own weight. The MAXOS does not require highly accurate fixturing of the blade because it scans the roots as an alignment procedure.
Toshiba GE Turbine Components is a joint venture by Toshiba and General Electric to produce large blades ranging from 660 mm to 1320 mm for steam turbines at manufacturing facilities. The company's annual production of blades is sufficient to generate the equivalent of eight to ten gigawatts of electricity. TGTC is looking to implement the MAXOS at its other facilities around the world.