In the fast growing market for wind power systems, quality assurance according to international industrial standards has become an important issue. Wind power plants are no longer a niche playground for environmental proponents, but a serious business where cost efficiency, life-cycle costs, reliability, and availability of the components must be considered. Coordinate measuring machines (CMMs) play a vital role in ensuring the accuracy and quality of components. This article describes how Carl Zeiss IMT has developed its series of MMZ machine measuring centers, which are tailored for the precise, economical measuring of large parts directly in the production environment, and in particular, looks at the MMZ G, which has established applications in quality assurance for gear boxes and other parts for large wind turbines.

Gear boxes are a major part of all wind turbines. The quality requirements are high: to withstand wind gusts, backlash-free operation is critical; mechanical efficiency is also important because it is directly related to economic efficiency; and noise reduction in the gear box is important to ensure the public’s acceptance of wind turbines. The wind energy industry has to meet all these requirements and more. Not many manufacturers are able to produce large gear boxes that meet these requirements (and with off-shore wind turbine systems, the gear box sizes are even larger), but now they must also deal with reliably producing these systems on a production basis rather than as custom built.

Gears (ring-gear, sun gear, and planet), bearings, and housings have to be produced with tolerances down to 0.01 mm—on components with 4 meters or more in diameter. It is no longer possible to machine these parts without specific real-time measures in place to ensure the quality. The parts are too expensive to produce rejects. The machining process needs to be controlled in real time to ensure that no defective parts are manufactured.

Figure 1: MMZ G measuring a wind turbine gear unit.

Carl Zeiss has been a manufacturer of precise measuring instruments for 90 years. Since 1973, the company has developed into one of the market leaders in 3-D coordinate measurement. For a traditional customer of CMMs, such as the automotive industry, the standard CMM measuring volume was around one cubic meter. But very early on, applications for printing machines, truck diesels, energy, and aircraft parts etc., made it necessary to develop a series of larger machines.

Today’s larger Zeiss models are the MMZ series: The MMZ E and B gantry-type machines, dedicated for composite parts and machined parts with standard accuracies; the MMZ T bridge-type machine with an integrated steel table for the high-accuracy “mid-size” class (up to 2,000 mm x 3,000 mm x 1,600 mm); and finally, the MMZ-G for large precision components. Sizes are available from X=2,000 mm, Y=3,000 mm, Z=2,000 mm up to very large systems able to measure gears and bearings up to 5 m in diameter, and housings or other machined parts up to 11 m in length, 5 m in width, and 3.5 m in height.

The machine incorporates a welded steel construction; everything is calculated according to the finite elements method. The machine moves on linear guideways, is driven by a ball screw spindle, is rigid and robust as a machine tool, and has no air consumption. The machine can be equipped with floor integrated rotary tables, holding up to 12,000 kg. Gears can be measured with or without the rotary table. For some helical or bevel gears, a rotary table would help to get full access to gear flanks.

Figure 2:  All mechanical components have been designed for high accuracy and resistance to ambient influences. This makes the MMZ G fast and accurate.

With the standard CALYPSO software, all geometric dimensions can be easily measured, including form and position tolerances. Adding the well-proven GEAR-PRO software allows the measurement of internal and external gears, straight or helical. Bevel gears and worms are also measurable.

With the ZEISS VAST probe head, the machine can take single points and is optimized for surface scanning. Navigator technology allows scanning using long, flexible styli at high speed without losing accuracy. The probe head can operate in all directions holding probes weighing up to 600 g and with lengths of up to 800 mm. This makes it possible to reach deep features in housings, and other difficult structures.

The typical accuracy varies from 2.8 µm + L/400 for the 2 m by 2 m bridge, to 8.4 µm + L/180 for the 5 m by 3.5 m bridge.

The MMZ G is not a dedicated gear checker; the advantage is that you can measure gears as well as housings, bearings, and many different precision parts on the same machine whose accuracy is best in class. MMZs are in use in the wind energy industry all over the world including North America, South Korea, China, India, and several European countries. The MMZ helps the wind energy industry to ensure the growing quality standards that have to be achieved—today and in the future.

Figure 3:  Measuring a gear with the VAST gold probe.  The active scanning standard probe, VAST gold, can handle very large probe systems and extensions.