Ball screw drives "Space-force wonder" for the gear cutting machine

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Kammerer Gewindetechnik GmbH & Co. KG

FAULHABER Drive Systems

For larger gearboxes in wind turbines, the gear cutting machine must also be adapted. Therefore, Kammerer has developed a ball screw drive for a larger milling unit that withstands higher loads—within the same installation space.

Ball screws are used in machine tools for positioning the axes: They convert a rotational movement into a linear movement without backlash and dynamically. High demands are placed on the ball screws, which must enable rapid axis movements while maintaining high precision. Such high-quality ball screws are manufactured by Kammerer Gewindetechnik GmbH from Hornberg in the Black Forest for the global machine tool industry.

Recently, a well-known manufacturer of machine tools approached Kammerer. The machinery manufacturer needed larger milling heads for the gear machines of a client who produces gearboxes for wind turbines. The ball screws previously used in the machines could not withstand the now higher weight force. For this problem, the Black Forest-based threading technology specialist found a clever solution.

Larger gearboxes require heavier milling units

The increasingly high demands in the generation of wind energy, involving ever-larger wind turbines, require larger gearboxes as well. A gear cutting machine, used to mill the gears of wind turbine gearboxes, needed to be adapted to changed conditions: A larger and heavier milling unit weighing 6 tons was to be installed without a weight compensation system, meaning without a technical device to offset the weight force.

The ball screw, responsible for moving the vertical axis of the tool carrier, had to withstand significantly higher weight forces under the new requirements, while fitting into the same installation space. The previously used standard ball screw from another manufacturer, measuring 100 millimeters × 30 millimeters, could not permanently withstand the increased weight force and thus had to be replaced after two years.

Another requirement for the redesign aimed at retrofitting was a higher load capacity while maintaining a constant no-load torque, as a non-constant torque negatively affects the quality of the surface being machined.

Doubled dynamic load rating in the same installation space

To meet these seemingly contradictory requirements, Kammerer's extensive experience in designing ball screws for machine tools was invaluable. "For the new application, the existing machine concept had to serve as the basis for the redesign of the ball screw," explains Marcel Wolber, a technician in Technical Sales at Kammerer, who oversees the retrofitting project. "This meant that the installation space was limited. Since a larger ball screw could not be used, we increased the pitch of the ball screw by 2 millimeters to 100 millimeters × 32 millimeters, which allowed for a double-threaded design. Through specially milled deflection pieces, the number of balls could be significantly increased while maintaining nearly the same nut length. Thanks to the second thread and hence many more supporting balls, the dynamic load rating could be increased from 465 to 679 kilonewtons, nearly a 50 percent increase. This extends the lifespan of the ball screw by 2.6 times to over 22,000 operating hours."

Increased lifespan achieved

The new ball screw, featuring an internally pre-tensioned single-flange nut made from the steel material 100 CrMo7-3, meets the precision class IT3. Its implementation has achieved a remarkable improvement in terms of dynamic load capacity and the lifespan of this crucial drive element. "The redesign is a true space-power marvel: minimal installation space, high dynamic load capacity while still maintaining smooth operation," expresses Andreas Rombach, a product manager in Hornberg who was also involved in the project.

So far, Kammerer has supplied five of these ball screw drives for the gear cutting machines. Gradually, the axes of additional machines are to be retrofitted with Kammerer’s "space-force wonder."