More durable wind power Hot-rolled large gears last longer than milled ones

If something breaks in an offshore wind turbine gearbox, it gets expensive, say Fraunhofer researchers from Chemnitz. If you deviate from the usual manufacturing method, the susceptibility to repair decreases.

With a diameter of 500 millimeters and a module (tooth size) of 9.5 millimeters, the gear manufactured for the "GEARFORM" project is the largest gear ever produced by forming technology. And repairing gear damage on similarly sized offshore wind turbines is therefore also very expensive. Because if the plants stand still because of defective gearboxes, repair is often only possible with the help of special cranes. Often added difficulties are heavy seas, strong winds, and poor visibility, which do not exactly speed up the proceedings. As part of the project, the Fraunhofer Institute for Machine Tools and Forming Technology (IWU) therefore developed a hot rolling process for the manufacture of more durable, large-module gears with industry partners. Compared to the usual hobbing, hot rolling scores with high material efficiency and significantly shorter production times.

Hot-rolled giant gear in minutes

For this purpose, the project partners specially manufactured a hot rolling plant, which heats the rolling blank to 1,200 °C. The gearing is then formed into the component under rotary motion, it is further stated. No chips are produced in this forming process. And because the volume is not reduced during machining (volume constancy), it is possible to save as much as 20 percent of material. Even more impressive is the time savings, because the rolling process only takes 6.5 minutes — but hobbing would take about 60 minutes.

Forming technical effects are good for the metal structure

As mentioned before, forming also has a positive impact on the longevity of a gear. In the forming process, the crystal-like structure of the metal is not mechanically severed, but simply shaped into the geometric outer contour. This increases both the tooth root strength (lower area of the gearing) and the load-bearing capacity of the tooth flank (contact area with other gears), as the researchers from Chemnitz explain. Added to this are effects due to forming, such as the influence on dislocation densification and the fine grain influence. The plastic deformation as a result of the hot rolling process causes a significantly higher dislocation density in the crystal lattice of the metal, thereby increasing its strength and hardness. At the same time, a finer grain makes the material tougher. As a subsequent heat treatment also hardens it. Only the surfaces of the gear receive their final contour by subsequent grinding.

The "GEARFORM: Energy and Resource Efficient Forming of Large Module Gears" project was funded by the Federal Ministry of Economics and Climate Protection. In addition to Fraunhofer-IWU (coordination), the project partners are EMA-TEC GmbH, Härterei Reese Chemnitz GmbH & Co.KG, Dreiling Maschinenbau GmbH and Flender GmbH.