TVL (Toeleveringsbedrijf van Landuyt) in Wetteren, Belgium has been finishing transmission housings for wind turbines for the past several years. When they began, production numbered around 2,000 housings annually. Today, their production has doubled to 4,000, while future planning calls for an annual capacity of 6,000 housings per year.

The company produces transmission housings with high-precision machined bores having diameters of up to 1640mm (64.5″). The tolerance for a 650mm (25.6″) bore is only ± 25µm (±.001″), and this must be measured and verified on every housing. All in all, the bores are drilled at 0.08% of nominal diameter size. Managing director Geert Van Landuyt states: “We could not measure the big transmission housings in-house. So we were working together with a service provider with the necessary equipment in Germany.” As production quantities grew, transportation to the service provider became a costly proposition. “Ultimately, we decided we needed our own coordinate measuring machine (CMM).”

Temperature sensors enable measuring under shop floor conditions

Today, each of the 4-ton transmission housings are now measured on a Leitz PMM-F high precision CMM manufactured by Hexagon Metrology GmbH (Wetzlar, Germany) The housings are measured in a room with identical temperature conditions as the shop floor. This environment eliminates the usual waiting times required when parts must adjust to ambient temperatures in a thermally-controlled metrology lab. Currently, TVL operates in two shifts, and in some production areas three shifts, with the measuring cell running around-the-clock for “just in time” production.

The CMM’s monolithic gantry design has a length of 4.5m (14.6ft), width of 4.36m(14.2 ft), and a height of 5.09m (16.5 ft). The machine has a measuring volume of 3000mm x 2000mm x 1600mm (118″ x 78.7″ x 63″). The U-formed CMM body is made of gray granite, which does not absorb as much humidity as black granite, contributing to better overall stability of the measurement results.

TVL transmission housings are transported with a crane to the CMM. After the automated alignment of the part, and when the part’s temperature is read by the CMM, the measuring process begins. The actual temperature of the transmission housing is applied to the measuring program’s points and evaluations, ensuring the same measured points are taken at arbitrary temperatures each time. The entire measurement process takes about 30 minutes and includes some points in nearly inaccessible areas of the housing.

The tolerance on the axle bore of a 4-ton transmission case used in a wind turbine is just 50µ.

High Precision, Fast Measurement

Once the decision was made to buy a CMM, Van Landuyt provided a benchmark for select manufacturers with the task of measuring the axle and bearing bushings in a transmission case for parallelism, straightness and precision of the diameter. The measurement of the large axle openings of the transmission housing were determined by 20 single measured points each. The Leitz CMM results were precise as compared to known values, with a high repeatability and measurement uncertainty of only 2.5µm.

Each Leitz CMM is assembled to order in Hexagon Metrology’s production hall, and the assembly technician makes adjustments and tests the machine’s measuring results and repeatability. The CMM is delivered to the customer site completely pre-assembled, except the Y-Axis that is remounted on site. After installation, the geometric errors of the CMM are corrected using a laser measuring device. Test measurements are carried out to confirm compliance with the stated accuracy specification.

Innovation of a Retractable Tool Changer

The implementation process of a high precision measurement machine involves extensive consultation between the manufacturer and the end-user. Many questions needed to be addressed. How precise must the measurement be? Which measuring points should be scanned? Are there special application issues that should be taken into account? The close cooperation between TVL and Hexagon Metrology led to another special feature: a retractable tool change rack.

As a rule, complex inspections of large parts require several styli changes during the measuring process because no single stylus can reach all the points to be measured. A conventional tool rack placed at the end of the measuring volume can restrict the range of the styli system, especially if particularly long styli configurations are required. Much of the active measuring range can be lost using a standard stationary tool change rack. As a result of the collaboration, Hexagon Metrology developed a retractable stylus change range, which swings itself into the measuring volume for the change, and then out of the measuring volume after a styli change is completed. This innovation keeps the complete measuring area available and accessible. This capability was especially important due to difficult to access features on TVL’s parts.

Sourced and published by Henry Sapiecha 23rd June 2009